@@PERGAMON

Journil Journil ofof

South American

Earth SciencesJournal Journal of of Sotah Sotah American American F:rth F:rth Sciences Sciences 15 15 (2002) (2002) 525-512525-512

www.elsevicr.xrmllocatt.ds:unes

Paleosols and Andean uplift in Venezuela: assessing competing

hypotheses of relict tropical soils versus paleohydrogeochemical

variations

W.C. Mahanet'*, M.W. Milner", M. Bezadao, V. Kalm", R.G.V. Hancockd

^Geomorphokryy ^Geomorphokryy and and Pe&ilogy Pe&ilogy lahtrutrtry, lahtrutrtry, Atkin.utn Atkin.utn College, College, Yttrk Yttrk Univerity, Univerity, 47N 47N Keele Keele Street, Street, North North Yo*, Yo*, Ont., Ont., Canadt Canadt M3J M3J I I P3P3

bDewrtn'entof Earth*::#r::;;w"Hr;;x:;T"yw'r'caracasveneurela

oStOWpOKn-2 oStOWpOKn-2 Fucility, Fucility, Deytrtmcnt Deytrtmcnt of of Chcmistry Chcmistry atut atut Ctvftrbat Ctvftrbat Engincering, Engincering, Royat Royat Military Military College College of of Cmoda, Cmoda, Kingsut4 Kingsut4 Ont-, Ont-, Cann'ta Cann'ta K7K K7K 7M7M

Received Received I I February February 2tDl; 2tDl; accepted accepted l l May May 2Jff....22Jff....2

AbstractAbstract

Paleosols in northem Veoezuela, which have formed in residual regoliths of metaargillite rock complexes, have been considered to have

developed developed in in tropical, tropical, humid humid climates climates at at low low elevation. elevation. Their Their reddish reddish colons colons and and clay-enriched clay-enriched horizons horizons have have been been used used to to support support ttrettre

hypofresis hypofresis of of significant significant r€cent r€cent uplift uplift cf cf tbe tbe Andes Andes Mountains Mountains in in l.at€ l.at€ Quaternary Quaternary time- time- We We suggest suggest an an alternative alternative hypothesis hypothesis of of long-termlong-term

(+5 (+5 tvtyrl tvtyrl pedogenesis pedogenesis in in a a paleohydrogeoclremical paleohydrogeoclremical regime, regime, characterized characterized by by flucurating flucurating redox redox conditions conditions at at high high elevation elevation (2000-(2000-

2500 m a.s.l.); the evi&nce for this redox history is the presence of Mn-Fe nodules in the surface epipedon. The sites, though relatively dry

now, now, ars ars interprcted interprcted to to have have been been much much wetter wetter in in the the past, past, with with soil soil moisture moisture levels levels capable capable of of translocating translocating clay clay to to form form argillic argillic horizons.horizons.

The The presence presence of of thick thick Ah Ah lrcizonc" lrcizonc" mmpl€te mmpl€te with with large large (l (l -2 -2 mln) mln) l'{n-R l'{n-R rndules rndules and and approxinrately approxinrately 3O 3O crn crn &ick &ick 4gillic 4gillic horizons, horizons, suggestssuggests

long-term long-term pedogenesis. pedogenesis. Tlre Tlre prcvious prcvious classification classification of of these these paleosols paleosols as as Oxisols Oxisols is is untenable untenable because because of of the the presence presence of of a a mollic mollic epi@onepi@on

and and the the concentration concentration of of Mn-Fe Mn-Fe nodules nodules in in the the epipedon. epipedon. The The paleosols paleosols bear bear sorne sorne resembl,ance resembl,ance to to Placosols Placosols of of the the Canadian Canadian taxonomy,taxonomy,

prcviously prcviously described described for for the the maritime maritime cool, cool, moist moist climates climates of of the the Pacific Pacific and and North North Atlantic Atlantic areas areas of of Canada, Canada, but but in in Venezuela Venezuela tlrey tlrey occuroccur

wi$ont wi$ont an an irnpenreable irnpenreable zubsurface zubsurface Fe Fe oxide oxide layer. layer. [n [n the the US US taxonomy, taxonomy, they they are are classified classified as as Typic Typic or or Aridic Aridic Argiustolls. Argiustolls. @ @ 2W2 2W2 ElsevierElsevier

Scierrce Ltd. All rights reserved.

Keyworuls: Keyworuls: Mn Mn nodules; nodules; Typic Typic Argiustolls; Argiustolls; Paleopedology; Paleopedology; M6rida M6rida Andes; Andes; VenezuelaVenezuela

l.Introduc{kn

Relict paleosols on nearly flat, accordant interfluves in

the Cordillera de M6rida in the Venezuelan Andes

Morntains Morntains currently currently exist exist in in a a cml-temper,ate, cml-temper,ate, semi-aridsemi-arid

climate at ?ffi0-2500ma.s.1., at 8"N latitude and 7l"W

longitude (Fig. I )- These paleosols, first described by Vivas

(1974) and Giegengack and Grauch ( 1975) and later

classified as 'paleo'Oxisols

by \f'eingarten (.1977), Ghorih

and Fen'er (1981) ancl Giegengack (1984), have been used

as evidence for earlier warm, hopical forest environments at

low elevations (( 1000 m a.s.l.). According to this hypottr-

esis, esis, the the formation formation of of the the paleosols paleosols predates predates uplift uplift and and faultfault

displacement of the surface, and thus, they are primary

* Conesponding authr. Tel.: * l-4167F2|ffi; fax: $ l4l 6650-3876

E-mail E-mail address address : : bmalnney@yorkuca bmalnney@yorkuca (V/.C. (V/.C. Mahaney).Mahaney).

0895-981 WU$ - see front rnatter @ 2002E|aevier Science Ltd. All rights reserved.

P I t S O 8 9 5 - 9 8 r I ( 0 2 ) 0 0 0 7 8 - 0

evidence evidence to to support support a a structural structural model model forrecent forrecent uplift uplift in in thethe

northern Venezuelan Andes. According to Giegengack(1984, p. 355), rernnants of sirnilar paleosols have been

identified identified along along the the Santo Santo Domingo-Rio Domingo-Rio Chama Chama divide divide andand

in in several several localities localities in in and and adjacent adjacent to to the the Rio Rio Bocon6 Bocon6 (Fig.(Fig.

1) .

Giegengack (1984) referred to the red soils in the area of

Mesa Mesa del del Caballo Caballo (at (at the the head head of of Rfo Rfo Chama, Chama, 45 45 km km to to thethe

east-northeast of this site) and at the Mucubaji, Aracay,

Llano Llano Curedor, Curedor, Tuflame, Tuflame, Alto Alto del del Pajarito, Pajarito, and and Rio Rio BurateBurate

valleys as Oxisols. These areas lie along the Bocon6 Fault

zone, zone, a a 450 450 km km feature feature followed followed by by the the shearn shearn of of RfoRfo

BoconS BoconS that that extends extends west-southwest west-southwest through through the the headwatersheadwaters

of of Rio Rio Santo Santo Domingo, Domingo, both both Orinoco Orinoco tributrries, tributrries, andand

parallel to the valley of Rio Chama (Fig. l). The Fault is

active, active, with with rnore rnore than than 120 120 and and up up to to 250 250 m m of of righrlateralrighrlateral

displacement displacement of of moaines moaines in in the the headwaters headwaters (in (in the the order order ofof

W-C- W-C- Mcrfuiney Mcrfuiney et et aI- aI- / / Journal Journal of of ktuth ktuth American American Earth Earth Sciences Sciences 15 15 (2ffi2) (2ffi2) 525-542525-542

Fig. Fig. l- l- Location Location of of the the MPOSJA MPOSJA Fofile Fofile southwest southwest of of M6rida, M6rida, Venezuela, Venezuela, near near the the Bocon6 Bocon6 Fault Fault and and other other associated associated faulrg faulrg with with respecr respecr n n the the red red pale$ols-pale$ols-

Fault Fault narnes narnes are are shown shown in in italics. italics. Locations Locations are are abbreviated: abbreviated: LM, LM, La^s La^s Mesitas; Mesitas; SJ, SJ, San San Jose; Jose; MS, MS, Mucrrchies; Mucrrchies; CA, CA, Chiguara; Chiguara; MI" MI" Mrrcubaji; Mrrcubaji; PI-, PI-, h.reblo h.reblo Uano;Uano;

LC, LC, Llano Llano CorIEdq CorIEdq T'[J, T'[J, Tiriare- Tiriare- I-oc*ion I-oc*ion diagram diagram &om &om De De Tooj Tooj and and Keltcrgg Keltcrgg i1993). i1993). Fault Fault locarions locarions are are ftom ftom Schuben Schuben t t 1968, 1968, l969ab, l969ab, 1983), 1983), KovisarsKovisars( t 969, t 971, 1972 i. Ramirez et al. { I 972i irnd Carcia and Caraxrs i 197'h-b).

lcmlyr; Schubert, 1970, 1980, 1982). In some of these

areas, Gi egengack' s ( I 9t34 ) interpretation of paleo-Oxisols

has been dispted by Ocho.r arxl lV1aiag6n (i981), Malagdn(1982), Bezada and Schubert (1987i and Bezada (19$).

The The red red soils soils do do not not occur occur in in flights flights of of the the alluvial alluvial teiraces teiraces ofof

three three Tertiary Tertiary ages ages preserved preserved in in the the present present topographytopography(Ghosh and Ckkenran. I9B7).

To To test test the the assumption assumption that that the the paleosol paleosol at at these these sites sites isis

an an Oxisol, Oxisol, we we snrdied snrdied several several paleosol paleosol profiles profiles in in the the fieldfield

area and sampled one in considerable detail. The well-

studied profile, MPOSJA (Weingarten. 1977), was

described described and and then then studied studied in in the the laboratory- laboratory- The The preserrcepreserrce

of Mn-Fe nodules, previously overlooked at all ofWeingarren's (19?7) sites, focused our aftention on thepedogeochernical pedogeochernical processes processes needed needed to to produce produce them-them-

Because Mn-Fe nodules are unknown in Oxisols (Soil

Suney Staft, 2000), possibly as a result of low organic

matter matter and and high high permeability, permeability, we we searched searched the the literature literature forfor

soils with Mn-Fe concretions. In the US taxonomy, both

LHolls LHolls and and Ustolls Ustolls (Mollisol (Mollisol Order) Order) have have Fe-Mn Fe-Mn ooncre-ooncre-

tions, tions, but but Typic Typic Argiustolls Argiustolls are are Mollisols Mollisols with with Fe-MnFe-Mn

nodules nodules that that have have little little evidence evidence of of wetness wetness near near the the surfacesurface(0-40 cm), lack a shallow lithic contact, and have a loamy

or or clayey clayey texfirre texfirre in in the the argillic argillic hmizon-hmizon-

The US taxonomy (Soil Sun'ey Stafl2000) says nothing

ahut the genesis of the Mn-Fe nodules. [n contrast, the

Canadian system (Canada Soil Survey Ccmmittee, 1977)

has has an an order order Placosol Placosol that that has has f.e-Mn f.e-Mn nodules nodules that that areare

characterized characterized by by alternating alternating wet-dry wet-dry soil soil conditions, conditions, whichwhich

produces oxidizing-reducing microenvironments, such as

those described by .,{rshad ancl St Arnaucl (1980i. Thesupply supply of of Mn Mn appears appears to to be be ttrc ttrc l\dn l\dn garnet garnet spessartite, spessartite, u&ichu&ich

may may be be in in either either the the bedrock bedrock below below or or upslope. upslope. ConcretionsConcretionsof Mn-Fe, with P and glauconite, associated with

hydrommphic hydrommphic effects effects in in soils soils formed formed on on Cenozoic Cenozoic sand-sand-

stones stones of of eastern eastern Ausralia, Ausralia, have have been been studied studied and and discusseddiscussed

by Baker il962i. The red soils and Mn-Fe concretionary

nodules nodules described described tprein tprein may may be be entirely entirely hydromorphic,hydromorphic,

with with chemical chemical elements elements derived derived from from groundwater groundwater transporttransport

out out of of fractured fractured rocks rocks and and mineralization mineralization in in the the encompass-encompass-

ing fault zone.

At At least least three three hypotheses hypotheses can can be be developed developed for for the the redred

soils: soils: (A) (A) Pliocerrc, Pliocerrc, topical topical soils soils developod developod in in lowlandlowland

rainforests rainforests were were abruptly abruptly uplifted uplifted and and preserved preserved in in a a cool,cool,

rnontane environment; (B) Mn-Fe solutions were hydrG-

morphically morphically transported transported from from the the fractures fractures of of the the Bocon6Bocon6

Fault Fault zone zone to to drain drain and and dryosit dryosit on on an an erosion erosion surface surface gradedgraded

to to the the floor floor of of the the Bocon6 Bocon6 'trench' 'trench' in in Middte Middte to to EarlyEarly

Quaternary tirne. Concretions developed on locally topo-

graphic graphic flat flat surfaces, surfaces, with with restricted restricted bioturbation, bioturbation, setrling,setrling,

and and preservation preservation of of Mn Mn and and Fe Fe in in cool, cool, alternating alternating wet wet andand

dry dry climates climates in in Late Late Quaternary Quaternary tirne; tirne; or or (C) (C) in in situsitu

development development of of alpine alpine soils soils occured occured on on the the accordantaccordant

crests crests and and shoulders shoulders of of an an ufland ufland of of unknown unknown age age andand

complexity complexity dhring dhring Mid Mid to to Early Early Quaternary, Quaternary, with with a a paludicpaludic

environment environment from from Mn-Fe-bcaring Mn-Fe-bcaring argillaceous argillaceous netas€di-netas€di-

ments, ments, and and maturation maturation to to a a red red permeable permeable soil soil with with an an in in situsitu

growth growth of of concentric-shelled concentric-shelled Mn-Fc Mn-Fc concretionary concretionary nodulesnodules

during during alternating alternating wet-dry wet-dry climatic climatic cycles.cycles.

In In this this article, article, we we test test the the classic, classic, catashophic catashophic model model (A)(A)

initiated by Heirn {1951) and modified by }Veingarten

(1977', (1977', with with respect respect to to the the paleosol paleosol evidence evidence that that supportssupports

reoent reoent uplift uplift of of fre fre Andes Andes Mountains. Mountains. We We also also atternpt atternpt toto

establi$ establi$ the the origin origin of of the the Mn-Fe Mn-Fe concretionary concretionary nodules nodules (B(B

and and C) C) in in the the epipedons epipedons of of subalpire subalpire paleosols paleosols below below thethe

Iimit Iimit of of glrciation-glrciation-

2. Field area

The The field field site site is is located located on on a a prominent, prominent, flat flat interfluveinterfluve

south south of of Rio Rio Chama Chama 22ffi 22ffi m m above above sea sea level level (Fig- (Fig- 1).1).

Although Although relatively relatively dry dry now now with with a a mean mean annual annual precipi-precipi-

tation tation (MAP) (MAP) of of 750 750 mrn, mrn, these these sites sites support support a a drydry

woodland. woodland. Bedrock Bedrock at at the the field field site site belongs belongs to to the the TostosaTostosa

Formation Formation of of [,ate [,ate Paleozoic Paleozoic age, age, as as described described by by CanelixrCanelixr(1976) and Canel6n and tiarcfa (.1971\- It consists of slate,

phyllite, schist, gneiss, and amphibolite. The bedrock at the

field field site site is is phyllite, phyllite, with with a a mixnre mixnre of of locally locally derivedderived

material (2Cr,2Cox horizons) and aeolian- (Ah, Bt horizon

cornplex) cornplex) influxed influxed silt. silt. The The original original sediment sediment is is argillite,argillite,

with minor arenite, that includes both chemical and detrital

facies, facies, as as implied implied by by ttrc ttrc ruetamor$ic ruetamor$ic rninerals rninerals rich rich inin

elements elements inherited inherited from from chemical chemical sediments sediments that that containcontain

anomalous Mn, Fe, S, C, P, and Ti.In the field area (Fig. 1),

the the red red soils soils are are generally generally develo@ develo@ on on rocks rocks of of the the IglesiasIglesias

Group (Schubert 1968. l969a,h; Kovisars, 1969, 1971,

i972; Ramirez et al., 1972). The common lithologies,

originally aluminous clay-rich sedirnents like those of the

Tostosa Formation at the MPOSJA site, are now high

527

metamorphic metamorphic grade grade biotite-garnet-sillimanite biotite-garnet-sillimanite gneiss gneiss andand

quartz quartz mica mica schist. schist. The The cornnron cornnron initial initial chemistry chemistry of of thethe

parent parent materials materials of of the the red red soils soils and and the the ultimate ultimate weatheringweathering

product product has has much much to to do do with with the the clay clay component component of of the the redred

soils- soils- The The lglesias lglesias Group Group is is Precambrian Precambrian to to Paleozoic Paleozoic in in ageage

and and cut cut by by igneous igneous rocks rocks of of Permian Permian age age in in the the lowland lowland ofof

the the Rio Rio Mototan Mototan Valley Valley and and much much younger younger granites granites in in thethe

p6ramo p6ramo south south of of the the Tuframe Tuframe Fault. Fault. Neither Neither type type of of granitegranite

correlates correlates wi& wi& ftre ftre occurrenoe occurrenoe of of red red soil. soil. This This basementbasement

constitutes constitutes the the M6rida M6rida Arch, Arch, which which was was active active beneathbeneath

Cretaceous basin sediments from 150 to 100 Ma and from

56to50 Ma (Lugo, 1994; Pindell et al., 1998: Cordaniet al.,

2000)- 2000)- The The region region continues continues to to be be tectonically tectonically activeactive

(Audemard, 19991 Audemard et al., 20fn) and provides no

::n:ttt"t

eviderrce fm anomalous uplift in the Pleisto-

2-1- 2-1- Age Age and and paleogeography paleogeography of of the the Mdridn Mdridn AndcsAndcs

The The geologic geologic history history and and sedimentation sedimentation that that accompanyaccompany

this this uplift uplift relate relate to to drc drc preservation preservation of of paleoscils. paleoscils. They They alsoalso

place place constraints constraints on on the the time time cornlnnent cornlnnent of of the the developmentdevelopment

ard ard preservation preservation of of lowland lowland tropical tropical soils- soils- The The Msida Msida fuchfuch

basernent basernent complex complex emerged emerged as as tbe tbe southern southern and and centralcentral

Mdrida Mdrida And€s And€s in in Late Late Oligocene Oligocene time time (25 (25 Ma) Ma) (Higgs,(Higgs,

1993) 1993) heralded heralded by by conglomerates conglomerates in in adjacent adjacent sedirnentarysedirnentary

basins- basins- Much Much of of ttle ttle Mesozoic Mesozoic cover cover and and subsequent subsequent alluvialalluvial

4pron 4pron was was eroded eroded as as the the Andes Andes roee. roee. An An excqtion excqtion is is ttrettre

Mucujfn Fmmation, described by Ghosh and Orlreman

t1987), which exists as a renuunt in M6rida City at ttre

centerof centerof tk tk Bocod Bocod Fault Fault zone. zone. lt lt is is an an early early orogenic, orogenic, lowlow

energy energy sedirnent, sedirnent, correlative correlative with with the the l-oen l-oen and and PardngulaPardngula

Formations Formations and and the the Guayabo Guayabo Group, Group, and and it it occurs occurs sparinglysparingly

in in the the M6rida M6rida Andes Andes and and their their foothills, foothills, These These low low energyenergy

alluvial alluvial sdiments sdiments of of the the poto-M6rida poto-M6rida Ades Ades are are of of the the l-atel-ate

Oligocene (25 Ma; Pindell et al-, 1998). Uplift was

accelerated accelerated after after the the Middle Middle Miocene Miocene (12 (12 Ma). Ma). High High energyenergy

alluvial fans fringe tbe M6rida Andes, including the

Betijoque, Betijoque, Rfo Rfo Yuca, Yuca, and and La La Cope Cope Formations. Formations. The The lafterlafter

has been dated as Mo-Pliocene by M*cellari i1982) but is

probably Middle lvfiocene in age (I2-I4 Ma) (Pindell e;t al-,

1998, p. 79). Other authors have established uplift onset

according according to to differcnt differcnt criteria, criteria, for for example, example, the the seismicseismic

stratigraphy stratigraphy of of both both neighboring neighboring flexural flexural basins basins (P6rez (P6rez andand

Aggarn'al" 1981: Duerto, 1988: De Toni and Kellogg, 1993;

Freyrnueller et al., 1993: Duerto et al.- 199.3; Aritlenrard,

1999), fission-track dating (Kohn et al-, 1984), or regional

geological and geophysical data (Kellog,g and Bonini,

r982).Younger sedimentary rocks, including the Mucuchies

Formation (Grauch. l9?l; Schubert, 1984), occur on the

south-side south-side of of the the modern modern Bocon6 Bocon6 Fault. Fault. Miocene Miocene oror

younger, younger, this this sedirnentary sedirnentary deposit deposit records records a a younger younger phasephase

of of sedimentation sedimentation in in the the Bocon6 Bocon6 Fault Fault zone zone within within graben,graben,

half gnben, or sag basins, which are characteristic of such

W.C. W.C. Maluney Maluney et et al. al. / / lournal lournal qf qf South South Atneican Atneican Earth Earth Sciences Sciences 15 15 (2W2) (2W2) 525-y2525-y2

W-C. W-C. Mahaney Mahaney et et al- al- / / Journal Journal of of Soath Soath American American Earth Earth Sciences Sciences I5 I5 (20O2) (20O2) 521542521542

MPG'A (3$g)

faults, faults, and and with with drainage drainage subser1rrcnt subser1rrcnt to to the the active active faultfault

system-

A regional paleodrainage pattern southwest of the

M6rida Andes (Hcxrrn et al-, i995) indicates a paleo-

&inoco &inoco River River draining draining much much of of Colombia Colombia and and Venezuela.Venezuela.

This This drainage, drainage, nmthward nmthward through through the the Tachira Tachira Saddle Saddle intointo

Lake Lake Maracaibo, Maracaibo, continued continued during during the the Late Late Oligocene Oligocene toto

Early Early Miocene. Miocene. Uplift Uplift of of the the eastern eastern Cordillera Cordillera in in thethe

Middle Middle Mioene Mioene resnicted resnicted northward northward drainage drainage ard ard pro-pro-

moted moted eastward eastward captwe captwe by by neo-Orinoco neo-Orinoco River River drainage-drainage-

Initially, Initially, the the drainage drainage was was nortb nortb of of the the present present Orinoco,Orinoco,

close close to to the the proto-M6rida proto-M6rida Andes; Andes; later, later, it it shifted shifted southwardsouthward(Pindell et al., lq98). C-omomiant to this pattern of

evolution evolution is is a a high high gradient, gradient, proto'Rfo proto'Rfo Chama Chama that that extend€dextend€d

headward headward into into the the rising rising M6rida M6rida Andes Andes and and eastwarid eastwarid alongalong

the Bocon5 Fault zone (Fig. 1).

2-2. 2-2. Bocorui Bocorui Fault4.evelopmcnt, Fault4.evelopmcnt, drainnge, drainnge, andand

seditmentaionseditmentaion

The Bocon6 Fault system (Giegengack and Grauch.

1912) comprises a 20 km wide zone, with one+hird of its

l50km l50km displacement displacement occurtiug occurtiug betwecn betwecn 25 25 and and lzfuIalzfuIa(Pindell et al.. 1998, p. 79).The central, active Bocon6 Fault

valley ranges from 200 m (Kovisan, 1969. 197l, 1972) to 5

and l0km wide (Schubert, 1968, 1969a.b, 1982, 1984),

{o{ou,u,!c!ctrtrFF

zzvv

zz

EEuuoo

GUIDE FOF TSTURAL CTASSIFICATION

ttwloedttwloed

tod( tod( @4@4

r,tgt.reed

,Dd,Dd

Be&ock Be&ock (2R)(2R)

<#<#

gercert gercert sardsard

Fig. Fig. 2. 2. (A) (A) MPOSIA MPOSIA paleorcl paleorcl in in tlre tlre nonhern nonhern Venezuelan Venezuelan Ardes Ardes ard ard @) @) texnue texnue of of the the MPOSJA MPOSJA profile.profile.

(a )Nl1 a

A M OAh3 Ah3 II

8 t rCox Cox aa

including including local local normal normal displacements displacements related related to to pull-apartpull-apart

basin basin formation, formation, u&ich u&ich have have been been active active ttnoughout ttnoughout tbetbe

Quaternary (Schubert. l 970, 1980, 1982, 1 984: Audernard,

1993; Audemard and Giraldo, 199?). The Rfo Santo

Dorningo Dorningo drains drains the the Bocon5 Bocon5 affh affh to to the the north-side north-side of of thethe

water divide at Mucubajf (3550 m a.s.l-), nmtheast along the

fault, then southeast (Schutrert, 1968, l969a,b), whereas

the the Rio Rio Chama Chama drains drains to to the the southwest southwest and and then then north north toto

Lake Maracaibo (Grauch.l97l). Accuding to Pindell u al.(1998), for approximat€ly l00km in the Merida Andes, the

Bocon6 Fault is represented by two parallel faults 1-5 km

apart. apart. tn tn the the east, east, the the Momboy-Valera Momboy-Valera and and Tufiame Tufiame FaultsFaults

are are sinistral sinistral wrench wrench faults faults that that splay splay north north ftorn ftorn the the Bocon6Bocon6

Fault Fault and and app€ar app€ar to to merge merge with with the the Panuelos Panuelos and and GavalonGavalon

Faults ( Kovisars, 1969, 197 l, 197 2), respectively. The exact

location is uncertain (Audemard et al., 1999).

Thw Thw me me active active faults faults ad ad potential potential geological geological hazardshazards(Audernard et al., 2Un). The stmctures geographically

match match the the locus locus of of the the red red soils soils (Frg. (Frg. I I ), ), and and soiUpaleosolsoiUpaleosol

sites at Mesa del Caballo, Mucubaji, Aracay, Llano

Corredor, Corredor, and and Rio Rio Burate Burate are are in in the the Bocon6 Bocon6 Fault Fault zone.zone.

These These faults faults may may contain contain metasomatic metasomatic Fe Fe and and Mn, Mn, whichwhich

they they release release as as hydromuphic hydromuphic Fe Fe and and Mn Mn during during recurringrecurring

tectonic movements.They may also have either phyllic or

argillic argillic alteration alteration haloes haloes that that render render the the high-grade high-grade meta-meta-

morphic rocks fertile for pedogenesis- Hotsprings (Schuberr.

W-C- W-C- Mafutney Mafutney et et al- al- / / ltmrnal ltmrnal of of *uth *uth Ameicsn Ameicsn Earth Earth Sciences Sciences 15 15 (2N2) (2N2) 525-542525-542

1982) 1982) (Fig. (Fig. t) t) reflect reflect geoth€rnal geoth€rnal waters waters in in thesc thesc fatlts, fatlts, suplysuply

these these elements elements in in erly erly mogenesis, mogenesis, and and promote promote weatbering weatbering ofof

rrcks rrcks and and liberation liberation of of Fe Fe and and Mn Mn along along fault-conholledfault-conholled

groundwater groundwater systems. systems. Furthermse, Furthermse, evidence evidence of of verticalvertical

movsnent movsnent in in tb tb fanlt fanlt zone, zone, aocoming aocoming to to limitd limitd data data onon

half grabens anddragfolds (Schubert, 1968, 1969a,b; Grauch,

l97l ), is north-side down in the Rio Sana Domingo drainage

and and scnrth-side scnrth-side down down in in Rio Rio Chama. Chama. This This evidence evidence places places mostmost

of of ttp ttp rod rod soils soils in in either either down--faulted down--faulted blocks, blocks, preserved preserved therethere

from from erosion, erosion, or or rrnnre rrnnre topography topography related related to to paleovalleys,paleovalleys,

which which were were developed developed in in older older fault fault relicts, relicts, particularly particularly thethe

broad broad conidor conidor near near Uano Uano Cmredor Cmredor at at th th culmination culmination of of thethe

Rito Rito Prreblo Prreblo Llano Llano ard ard Rio Rio Aracay Aracay vallery vallery (Sc-hubert, (Sc-hubert, 1%8).1%8).

fttrer fttrer red red soils soils occur occur west west of of the the strdy strdy are4 are4 near near the the nufternnuftern

margin margin of of the the Gonzales Gonzales pull-apart pull-apart basin basin or or south south of of it, it, at at lowerlower

elevations.

At At odds odds with with this this interpretation interpretation is is th th work work of of SchubertSchubert

(198?), Soulas (1986), Soulas et al. (1986), Beltrfn (1993,

1994) and Audernard et al. (2OX)), wtrich places the faults

shown shown in in Fig. Fig. t t in in slightly slightly different different places places and and with with differentdifferent

orientations orientations and and senses senses of of movement. movement. The The acnral acnral faultfault

positions positions and and tlrc tlrc nature nature of of mineralization mineralization associated associated withwith

them them must must be be refined.refined.

Initiation of local orogenic activity began with intrusions

of Valera granite (Fig. l) between 2l andz4Maduring fu

Late Late Oligocene. Oligocene. The The onset onset of of basement basement cooling cooling and and uplift uplift ofof

the Mdrida Andes occurred 20-l8Ma (Farty Miocene)

(Kohu et al., 1984; Pinclell et al.. 1998, p- 76). Most

intrusions intrusions in in tbe tbe Mdrida Mdrida Andes Andes occurred occurred at at less less ttan ttan lO lO Ma,Ma,

which which suggests suggests that that basement basement uplift uplift accelerated accelerated during during ftefte

latter latter part part of of tlre tlre Late Late Oligocetre, Oligocetre, and and the the most most dramaticdramatic

uplift uplift and and erosion erosion of of the the M6rida M6rida Andes Andes was was from from LateLate

Miocene (10 Ma) to Recent (Ktrlur et al.. 1984; Pindell et ai.,

1998. p .5 l ) .

Drainage Drainage in in the the Bocon6 Bocon6 trench trench is is along along the the southern southern faultfault

trace, trace, with with alluvium alluvium extending extending foom foom tributaries tributaries on on the the northnorth

slope slope in in both both the the Cronsalvez Cronsalvez and and Muchuches-l:s Muchuches-l:s MesasMesas

basins basins ad ad the the graben graben of of hreblo hreblo Llano. Llano. This This reflects reflects thethe

wann wann asp€ct asp€ct of of the the south-facing south-facing slqrcs, slqrcs, the the favorablefavorable

temperatures temperatures for for pedogenesis, pedogenesis, and and the the alluvial alluvial contributioncontribution

of of distant distant northern northern glaciers glaciers and and is is in in conuas conuas with with tbe tbe glacialglacial

dorninance dorninance of of northern northern aspects aspects near near culminations culminations in in ftefte

Bocon6 Fault valley near Mucubajf (3550 m a.s.l.) and

Llano Corredor (3440 m a.s.l.).

3. 3. MethodsMethods

Tbe Tbe profile profile (Fig. (Fig. 2(A)) 2(A)) was was described described in in th€ th€ field field usingusing

the the standard standard nomenclature nomenclature and and terminolog5l terminolog5l of of Canada Canada SoilSoil

Sun,e_y Ccxnrnirtee (19771, Birkeland tI999) and Soil

Survey Staff (2000). Soil colors are fiom Oyanra and

Takehara { 1970).

Samples Samples from from each each horizon horizon were were air air dried dried in in thethe

Iaboratory Iaboratory and and sieved sieved to to separate separate pebblcs pebblcs from from sand, sand, silt,silt,

and and clay; clay; the the weathered weathered rock rock was was ground ground in in a a mortar mortar andand

pestle pestle for for chernical chernical analysis, analysis, but but particle particle size size analysis analysis waswas

not not viable. viable. The The silt/sand silt/sand boundary boundary of of 63 63 pm pm (see (see FolkFolk

(lq6S)) follows the Wentworth scale, and the silt/clay

boundary of 2 pm conforns with Soil Sun'ey Staff t2ti00i.

TIlc TIlc <2 <2 mm mm fraction fraction was was subsarnpled subsarnpled for for rnoi*ure rnoi*ure content,content,

and and ttre ttre air-&ied air-&ied equivalent equivalent of of 50 50 g g of of oven-dried oven-dried soil soil waswas

analyzed analyzed for for particle particle size. size. The The coarse coarse fraction fraction 2mm-2mm-

63 pm, (for particle sizes, see Folk (1968)) was fractionated

by by wet wet sieving sieving and and oven oven dryiog dryiog the the sands; sands; the the fine fine fractionfraction

(<63 +r.m) was analyzed by hydrometer (Day, 1965). The

Mn-Fe nodules were pulverized in a mortarard cennifuged

onto ceramic tiles. The clay fraEtion, also ceuEifuged onto

ceramic ceramic tileg tileg was was analyzed analyzed on on a a Philips Philips X-ray X-ray diftact-diftact-

onet€r onet€r fotlowing fotlowing procedures procedures established established by by Whittig Whittig ( ( 1965i1965i

and htahane.v (l-c)81)- A portion of the profile, including the

Ah2, Ah3, and 2Cox horizons, was examined for heavy

minerals minerals and and general general mineralqgy.mineralqgy.

Tbe Tbe scanning scanning electron electron microscope microscope (SEM) (SEM) ad ad energy-energy-

dispersive dispersive spectromety spectromety (EDS) (EDS) used used follow follow the the proceduresprocedures

established by Vortisch et al. {198?i and lvlahaney ii990).

Approximately Approximately lffi lffi individual individual nodules nodules were were selected selected forfor

study study from from the the surface surface horizons horizons of of the the MPOSJA MPOSJA proflle.proflle.

Some Some of of these these nodules nodules were were cut cut to to reveal reveal internal internal strucCIuestrucCIue

and compositional variations; others were ground for

mineral mineral and and cbmical cbmical analyses. analyses. Seminuantitative Seminuantitative differ-differ-

ences ences in in composition composition fuom fuom the the surface surface to to the the core core werewere

determined determined using using SEM SEM and and EDS EDS analysis- analysis- Individual Individual sandsand

grains grains (quartz (quartz and and heavy heavy minerals) minerals) were were analyzr.d analyzr.d fotfot

surfrce surfrce texhre texhre (Mahaney, (Mahaney, 1990) 1990) throtrghout throtrghout the the profile profile toto

deterrnine deterrnine the the relative relative degrees degrees of of weathering weathering in in eacheach

horizon.

Preparation Preparation of of the the samples samples for for instmmental instmmental neutronneutron

activation activation analysis analysis ([{AA) ([{AA) followed followed procedures procedures outlined outlined byby

Hancock {.i984i. Subsamples of approximately 80O mg

were were isolated isolated for for geochemical geochemical analysis analysis and and placed placed inin

Olympic Plastics Co. polyvials for INAu{. Elements, such

as U, Dy, Ba, Ti, Mg, Na, V, Al, M4 Ca, and Cl, $/hich

produce produce short-lived short-lived radioisotopes, radioisotopes, were were detected detected fromfrom

samples samples that that were were irradiated irradiated serially serially for for I I min min at at a a neutronneutron

flux of 5.0x 10rr ncm-2s-r in the SLOWPOKE-2

Reactor Reactor at at tle tle Royal Royal Mlitary Mlitary College College of of Canada. Canada. AfterAfter

19 19 min, min, with with the the decay decay of of 28Al 28Al

to to acceptable acceptable limits, limits, eacheach

sample was assayed using 5-min counts witrr on-site garuna

ray ray spectrometers. spectrometers. The The samples samples were were recounted recounted the the next next dayday

for for Erl Erl G4 G4 Na, Na, and and IL IL All All samfles samfles were were later later batchbatch

irradiated for 5 h at a neutron flux of 5 x l0ll n cm-2 s-t.

After7 After7 days, days, the the sarnples sarnples were were counted counted for for 10 10 min, min, and and thethe

concentrations of Sm, IJ, Yb, La, As, Sb, Fe, and Na were

determid- determid- After After 2 2 weeks, weeks, tb tb samples samples were were recountedrecounted

again, again, and and concentrations concentrations were were determined determined for for N4 N4 Ce, Ce, Lu,Lu,

Ba, Th, Cr, Hf, Sr, Cs, Ni, Th, Sc, Rb, Fe, Co, Ta, and Eu. Fe

and and Sc Sc were were used used as as cross-checks cross-checks for for the the third third phase phase of of thethe

analysis. analysis. Multi-elernental Multi-elernental calibrtions calibrtions were were confirmed confirmed usingusing

intemational intemational rock rock standards-standards-

Extractions Extractions of of Fe Fe and and Al Al were were made made using using Na-pyropho-Na-pyropho-

sphate and Na-dithionite, following the procedures of

W-C. W-C. Mafuney Mafuney et et al. al. / / Journsl Journsl of of South South Amcrican Amcrican Earth Earth Sciences Sciences 15 15 (2U2) (2U2) 525-542525-542

f)ornraar alld Lutwick (198-l) and McKeague atid Day

{ 1966), whereas acid ammoniurn oxalate extractions

followed Cotiin {1963). These extractions provide data onthe the concentrations concentrations of of organically organically bound bound Al Al (pyrophosphate)(pyrophosphate)

and and ferrihydrite ferrihydrite (oxalate) (oxalate) ad ad th€ th€ mtio mtio of of total total to to 'free' 'free'

Fe,Fe,

including including ferrihydrite, ferrihydrite, goethite, goethite, and and hematite hematite (dithionite(dithionite

extractable) (Daly, 1982; Paffirt and Childs, 1988). The ratio

of of dithionite-extractable dithionite-extractable Fe Fe to to total total Fe Fe in in each each horizon horizon givesgives

an an indication indication of of tlp tlp content content of of free free Fe Fe throughout throughout dre dre profiteprofite

(\,hhaney, 1999).

Organic C analysis followed the Walkley-Black method(Walkley. 1935). Total N was determined using a l-eco

CHN CHN analyzer- analyzer- Cation Cation exchange exchange capacity capacity was was deterndneddeterndned

with Na acetate (Rhoads. 1982).

4. 4. ResultsResults

4.1. The profile

The The MPOSJA MPOSJA profile profile consists consists of of a a raiduat raiduat regolithregolith

derived derived from from phyllite phyllite ad ad overlain overlain by by wind-blown wind-blown materialmaterial(FiS. ?(A)). It is 180 cm thick and contains an epipedon of

60 60 cm cm thick thick Ah Ah horizons horizons over over a a 30 30 cm cm thick thick argillic argillic horizon.horizon.

The The underlying underlying 2Cox 2Cox horizon horizon is is rrcarly rrcarly all all residual residual regolith,regolith,

with with a a minor minor amount amount of of locally locally derived derived rounded rounded particles,particles,

possibly possibly water water transported transported or or solution solution rounded.rounded.

The The Ah Ah horizon horizon complex complex is is suMivided suMivided on on the the basis basis ofofcoloc surhce Ahl is black (5YR l.7ll), Ah2 brownishblack black (5YR (5YR 2ll), 2ll), and and Ah3 Ah3 reddish reddish b'rown b'rown (5YR (5YR A3\, A3\, nncontrast contrast with with the the reddish reddish brown brown (5YR (5YR 4/6) 4/6) colors colors of of the the BtBt

horizon. horizon. Horizon Horizon boundaries boundaries are are sharp sharp and and planar.planar.

Wash-water Wash-water @lor, @lor, as as observed observed during during tpavy tpavy min€ralmin€ral

separation, separation, is is brown brown (7-5YR (7-5YR N4) N4) and and appears appears to to reflect reflect thetheMn content. Like the soil, it becomes lighter brown withdepth. depth. Although Although parent parent material material is is mostly mostly phyllite, phyllite, there there isissome some foreign foreign material, material, which which indicates indicates sonrc sonrc r€cycling r€cycling ofof

fluvial fluvial material- material- tn tn the the 2Cox 2Cox horizon, horizon, wash wash water water fromfromgravirnetric gravirnetric separation separation of of heavy heavy minerals minerals is is bright bright reddishreddish

bmwn (sYR 5/6 wet) to orange (5YR 716) ndfree of eitherlight light mottles mottles or or black black stains.stains.

Washed Washed lithic lithic fragments fragments from from the the paleosol paleosol are are rnostlyrnostlydark greenish gray (l0G 3/l) wben fresh and reddish brown(5YR 4/6) when weathered. Pyrite and graphite are

ubiquitous ubiquitous accessory accessory minerals- minerals- Relict Relict micaceous micaceous mineralsmineralsare are mostly mostly weathered weathered to to secondarSr secondarSr clays. clays. The The dry dry soil soil colmscolms

range from dull yellowish brown (l0YR 5l3) to brown(l0YR 414, n the surface epipedon to yellowish brown(IOYR (IOYR 5/8) 5/8) in in tlre tlre Bt Bt horizon horizon ad ad bright bright bnown bnown (?-SYR (?-SYR 5/8)5/8)in in the the 2Cox 2Cox horizon- horizon- The The 2Cox 2Cox horizon horizon sturry sturry is is ligltbnownligltbnown(5YR 5/6 wet) to orange (5YR 7/8). Below 90 crn, theprofile profile grades grades to to weathered weathered (2Cr) (2Cr) and and slightly slightly weatheredweatheredrock at lSocrn; fresh, unweathered bedrock (2R) isencountered ̂t215 cm (Fig- 2(A)).

In places, the phyllite is muscovite bearing witlr abundantquartz veinlets. The phyllite contains both Fe and Mn, as

indicated indicated by by the the presence presence of of both both fresh fresh pyrite pyrite and and goethitegoethite

pseudomorphous pseudomorphous after after pynte, pynte, as as well well as as Mn Mn garnet garnet (Mn(Mn

carbonate was not observed but may be present inunweathered rock). Both the concretions and the quartz

veinlets veinlets are are too too delicate delicate or or too too pristine pristine to to have have undergoneundergonernuch rnuch transport.transport.

Mn-Fe nodules, most abundant in the middle of the

epipedon (Ah2, Ah3 horizons), are between I and 2 mm indiarnerer. diarnerer. They They are are sptrcroidal sptrcroidal in in rnorphology, rnorphology, have have bothbothconcentric concentric and and asymrnetric asymrnetric shell shell structures, structures, and and contain contain FeFe

and Mn wittr some Si-rich shells (for chemical analysis, seeFig. 8). Detrital, angularquartz is incorporated in bottr softerMn-Fe Mn-Fe shells shells ad ad rnore rnore cornpetent cornpetent Si-rich Si-rich zones. zones. RootsRootspenetrate penetrate tbrory\ tbrory\ the the surface surface epipedon epipedon and and into into tlre tlre upperupperargillic horizon. The roots appearold and decayed relicts inplaces places adhering adhering to to the the outsides outsides of of the the nodules nodules and and cohesivecohesiveclay clay coatings.coatings.

The The presence presence of of Mn-Fe Mn-Fe nodules nodules indicates indicates the the formerformerocflurence ocflurence of of a a perched perched water water table table over over imperviousimperviousrnaterial, presumably bedrock (Fitzpatrick, t980). Thepresence presence of of a a ftick ftick Ah Ah horizon horizon cornplex cornplex over over a a well-leachedwell-leachedand and patially patially reduced reduced Bt Bt huizon huizon that that terminates terminates downwarddownwardtoward toward an an impenneable impenneable bedrock bedrock is is interpreted interpreted to to require require aaunique unique hydrogeochemical hydrogeochemical process process to to provide provide alternatealternatewetting wetting and and drying drying over over lcmg lcmg perids perids of of tinrc. tinrc. The The soil soil isisof of the the Mollisol Mollisol ordec ordec its its relict relict nature, nature, despite despite the the lack lack ofofburial, burial, might might wanant wanant the the term term Mollic Mollic paleosol- paleosol- The The surfacesurfaceepipedon epipedon moist moist colors colors are are shong shong enough enough to to yield yield coluscolusdarker darker than than 10YR 10YR 3ll- 3ll- Th Th textre textre shows shows an an upward upward finingfiningsequence sequence within within the the A A complex complex (Fig. (Fig. 2(B) 2(B) that, that, rogetherrogether

with the Bt, suggests a Typic Argiustoll. The Mollisoldesignation designation is is based based on on color color (darker (darker ttran ttran 10YR 10YR 3 3 whenwhenrnoist rnoist and and 5 5 wtrer wtrer dry), dry), higher higher organic organic carbon carbon in in thetheepipedon epipedon than than in in the the 2Cox 2Cox horizon, horizon, fine fine granular granular structure,structure,and and thickress thickress of of the the argillic argillic horizon. horizon. Vegetation Vegetation is is sparse,sparse,and and the the climate climate is is seasonally seasonally dry; dry; tbe tbe soil soil temperaturetemperature

requirernent requirernent is is rnesic rnesic u u thennic, thennic, ad ad ttr€ ttr€ moisture moisture regime regime isis

tonid (Soil Snney Statr" 2til0).

Very Very minor minor amounts amounts of of well-rounded well-rounded sard sard imply imply locatlylocatly

derived derived fluvial fluvial detrital detrital material material in in the the 2Cox 2Cox horizon, horizon, whichwhichcould could result result frrorn frrorn one one of of the the three three phases phases of of TertiaryTertiarysandstone sandstone sediments sediments introduced introduced in in the the Upper Upper Neogene.Neogene.

42. 42. Partich Partich sizesize

The texture of the MPOSJA profile (Fig. 2(B)) rangesfrom silty clay loam to clay. The Bt horizon contuns 527oclay clay compared compared with with to to 31, 31, 42, 42, ^trd ^trd M% M% downward downward in in thetheoverlying overlying mollic mollic epipedon. epipedon. Even Even the the 2Cox 2Cox horizonhas horizonhas 42Vo42Voclay. Sand in the mollic epi@on ranges from a low of 16%o

to a high of D%- In the Bt horizon, sand is llVo, increasingto to 28% 28% in in tbe tbe 2Cox 2Cox horizon. horizon. The The percentagp percentagp of of silt silt in in allallhorizons horizons ranges ranges from from 557" 557" in in the the surface surface Ahl Ahl to to between?7between?7

and and 30Vo 30Vo in in the the lower lower horizons- horizons- An An upward upward fining fining sequencesequencedominated by silt reflects a probable aeolian influence

W-C. W-C. Malwtel' Malwtel' et et al- al- / / Journal Journal cf cf South South Ame Ame rican rican Earth Earth Sciences Sciences 15 15 (2W2) (2W2) 525-542525-542

Tablc Tablc II

Chemical Chemical daa daa for for tbe tbe MPOSJA MPOSJA paleosolpaleosol

Horizon Horizon Depth Depth (crn) (crn) pHpH Conductiviry ( I :5) (S/crn-') Organic Organic niuogen niuogen (%) (%) Organic Organic carbon carbon (Vo)(Vo)

(%>(%> (cmol,/kg)(cmol,/kg)

Ahl

Ah?Ah?Ah3Ah3Bt

2C.ox2C.ox

o-3030-4530-4545-ffi60-90

90- 180

39.4

n5n56.06-0

31.3

2.19o.88

n.6'26.8

0.04

0.210o.840

o.M0.18

0.050

6.26-26-2

5.9

18.0

l l . 6

0.560 r8.l0.800 25.s

34.7

(Mahaney, 1990: blahane)r and Kalm. 1996)- The particle

size size data data depict depict a a weatherirg weatherirg profile profile partly partly fonned fonned in in aa

residual residual regolith regolith over over a a long long period period of of time time with with an an influx influx ofof

Pleistocerc Pleistocerc a€olian a€olian sediment sediment in in fu fu surface, surface, as as indicted indicted by by aa

texture texture that that shows shows an an upward upward fining fining sequenc€ sequenc€ within within the the AA

cornplex (F:ig. 2(B)), md possible late Tertiary fluvial

sediment in the subsurface, as indicated by minor,

anomalously anomalously round round sand sand graim graim in in the the 2Cox 2Cox horizon. horizon. TheThe

clay-rich clay-rich textures textures throughout throughout the the profile profile reflect reflect in in part part thethe

clayey clayey nature nature of of the the parent parent material material and and argue argue for for a a longlong

weathering weathering history, history, probably probably spanning spanning several several interglacialinterglacial

intervals intervals and and possibly possibly extending extending back back to to the the EarlyEarly

Quaternary Quaternary or or Late Late Neogene- Neogene- Even Even with with the the lutite lutite naturenature

of of the the parent parent material, material, conversion conversion of of clay clay from from 42% 42% in in thethe

2Cox to 52% in the Bt horizon represents slow but

substantial substantial weathering.weathering.

4.3. 4.3. Soil Soil chemistrychemistry

The The soil soil chemisury chemisury was was studied studied to to deterrnine deterrnine if if commoncommon

chemical chemical parameters parameters might might shed shed some some light light on on nodulenodule

genesis in tbe Ah horizon complex. The data appearin Table

1. 1. The The pH pH is is moderately moderately acidic acidic and and strows strows a a uniform uniform profileprofile

to to the the bottom bottom of of the the 2Cox 2Cox horizon. horizon. Condrctivity Condrctivity depicts depicts aa

well-leached profile with little total salt content- &ganic C

is highest in the Ahl horizon and slowly decreases into the

A3 A3 horizon horizon but but irrcreases irrcreases sofirewhat sofirewhat in in the the lower lower hodzons.hodzons.

Nitrogen Nitrogen shows shows a a similar similar trend trend with with depth. depth. Cation Cation exchangeexchange

Table Table 22

Extractable Extractable and and total total Fe Fe and and Al Al concentradonr concentradonr in in the the MFOSJA MFOSJA paleosol,paleosol,

Venezuelan Venezuelan AndesAndes

Extractable Extractable FeFe Extractable Extractable AlAl

capacity, capacity, though though relatively relatively low low in in the the Ah Ah hmizons, hmizons, irrcreasesirrcreases

slightly slightly down down profile. profile. Ttrc Ttrc CEC CEC values values are are low low and and indicateindicate

a clay mineral compmition of mixed l:1 (Si/Al : l:l) in

tb€ tb€ Ah Ah borizons, borizons, uihich uihich increases increases sligbtly sligbtly in in the the Btf2CoxBtf2Cox

horizons, where 2:1 (SilAl :2:l) clay minerals might be

expected (Birkelancl, 1999, p. 93). In general, the organic C

and and N N are are lower lower than than expected, expected, glven glven the the soil soil profile profile colors.colors.

4.4- 4.4- Fe Fe and and Al Al extroctionsextroctions

The Fe and Al extractions (1able 2) were analyzed to

dercrmine dercrmine if if either either elernent elernent rnight rnight provide provide usefirl usefirl relativerelative

estimates estimates of of the the time time of of weathering weathering andor andor fluctuation fluctuation of of tlretlre

water tables- As per the work of Childs (1q73) and Parlitt

ancl Cjhilds t1988), the extracts of considerable utility

irclude irclude Feo Feo (oxalate) (oxalate) X X constant constant 1.7 1.7 as as an an approxirnation approxirnation ofof

fenihydrite fenihydrite concentrations concentrations (fable (fable 3); 3); *re *re Ah Ah (pyro$o-(pyro$o-

sphate) sphate) is is taken taken to to approximate approximate the the arnount arnount of of organicallyorganically

complexed complexed Al Al and and the the Fea Fea is is used used as as a a nreasure nreasure of of free free ironiron(ferrihy&ite, (ferrihy&ite, goethite, goethite, and and bematite). bematite). The The Feo, Feo, formerlyformerly

considered considered to to r€present r€present amorphous amorphous plus plus organically organically corn-corn-

plexed plexed Fe, Fe, is is now now considered considered to to represent represent only only ferrihydrite-ferrihydrite-

Whereas Whereas Alo Alo is is generally generally higher higher than than Ah Ah in in some some profilesprofiles

(fulahaney and Fahey, 1988; Birkelaud et aL.. 1989;

Mahaney et al., lgX), in this profile, it is less than A16, aspredicted by h{cKeague and Da-v- (1966). Of Fea and Ald,

which which have have provided provided data data on on crystalline crystalline plus plus amorphous amorphous andand

organically complexed forms (il.lcKeague and Day, 1966;

l\{ahaney and Sanmugudas, 1985), only Fe6 is now

considered (Parfitt and Childs, 1988) reliable to provide

estimates estimates of of the the total total free free Fe.Fe.

Various Fe and Al extract concentrations, in ratio form,

may may be be used used to to calculate calculate the the increase increase or or movement movement of of oneone

fsrn fsrn of of the the trpo trpo elerents- elerents- The The activity activity ratio ratio of of Feo/Fe6Feo/Fe6

Table Table 33

Extractable Extractable Fe Fe and and Al Al weathering weathering indicesindices

Horizon Horizon (nratcrial) (nratcrial) A/Al A/Al Feo Feo x x 1.7 1.7 Feotre' Feotre' Fea-Feo Fea-Feo Fe.'Fe6Fe.'Fe6

Fee Fee F% F% Fea Fea Fe, Fe, Alp Alp AL AL Ald Ald AtAt

(%) (%r' (%> (%) (%, (%) (%) (%')

AhlAh2Ah2

Ah3Ah3

Bt2Cox2CoxSlightly Slightly weathered weathered rockrock

o.r4 0.42o.87 2.mQ.77 Q.77 A-82A-820.r3 0.35o.ffi o.ffi o.25o.250.12 0.15

0.65 1.680.86 2390.83 3.05o.1t 2.7 |o.70 3.33

AhlAh2Ah2AH}AH}

Bt2Cox2Cox

Wx Wx (Cr)(Cr)

2.104.393.87

3.06358

bb

3.24 o.LA 0.r3 0.28 s.91

5.11 0.06 0.37 0-66 6.U4.68 4.68 0-O7 0-O7 0-33 0-33 0-64 0-64 8.338.33

430 0.05 0.21 0.43 9.805.09 0.o2 0.r4 0-36 9.193.13 0.02 0.13 -b 4.n

" " Fe., Fe., concentration concentration multiplied multiplied by by 1.7 1.7 to to give give tnrc tnrc value value for for ferrihydriteferrihydrite

(Parfitt and Childs. 1988)-b b Insrfficient Insrfficient sample-sample-

0.020.ol

0.0r

0.010.0r

o.0l

0.20o.6o.2l0 .1 t

1'ot

o.7

3.4

1.4

0.60.4

0.3

W.C. W.C. Mahmey Mahmey et et aJ. aJ. / / lournal lournal of of South South hnertcan hnertcan Ectth Ectth Sciences Sciences 15 15 (2002) (2002) 525-542525-542

Fig. Fig. 3. 3. X-raY X-raY nace nace of of Mn Mn nodrles.nodrles.

estimates estimates the the buitd buitd up up of of fdhydrite fdhydrite (Feo) (Feo) relative relative to to Fe6, Fe6, 8s8s

discussed by Malianey and Falrey (t988)' In this shrdy, a

ratio ratio of of Feo Feo to to Fe6 Fe6 indicates indicates a a water water table table fluctuation fluctuation up up toto

and and irrcluding irrcluding the the Ah2 Ah2 horizon horizon level level to to a a depth depth ofof

approxirnately approxirnately 30 30 cm. cm. The The ratio ratio Fe/Fq Fe/Fq is is higb' higb' uthichuthich

indicates indicates a a large large degree degree of of conversion conversion of of ferrous ferrous to to ferricferric

iron. iron. The The function function Fe6-Feo, Fe6-Feo, which which estirnates estirnates the the accumu-accumu-

lation lation of of heuratite heuratite and and goethite goethite as as secoudry secoudry wea&erirgwea&erirg

products, products, is is particularly particularly hlgh hlgh in in the the 2C-ox 2C-ox horizon; horizon; it it isis

nearly nearly as as hig! hig! as as in in the the overlying overlying Bt Bt horizon. horizon. The The highhigh

E F

Fig. Fig. 4. 4. No&rles No&rles atrd atrd tbek tbek surface surface coatings coatings in in the the Ah3 Ah3 hodmn- hodmn- (A) (A) MnMn

no&rle no&rle with with a a diameter diameter close close to to 2 2 mm. mm. The The left left edge edge Ls Ls bulbous, bulbous, and and minorminor

protubiances protubiances are are pneseu pneseu in in the the uppet uppet right. right. (B) (B) Enlargement Enlargement of of the the surfacesurface

area area showing showing pronrberances pronrberances (anow) (anow) enlarged enlarged in in (D) (D) below. below. (C) (C) ClauconiteClauconite

on on Fe Fe nodule nodule srface. srface. (D) (D) Quartz Quartz particle particle imbedded imbedded in in no&rlc no&rlc and and coatedcoated

with with Fe. Fe. (E) (E) Mn Mn nodule nodule with with a a chemical chemical composition composition of of Fe' Fe' Ti' Ti' Mu Mu Si' Si' Al'Al'

ard ard K K in in the the central central area- area- (F) (F) Surface Surface area area of of noaub noaub in in (E) (E) with with ftre ftre silt-sizesilt-size

particles particles embedded embedded in in it.it.

valum valum of of Fe6-Feo Fe6-Feo in in the the Ah2/Ah3 Ah2/Ah3 horizon horizon complex complex indcateindcate

that that these these horizom horizom have have been been forning forning on on a a stable stable landland

surface surface for for a a ve4y ve4y long long tirne tirne {probably {probably ftrough ftrough several several majormajor

wetting wetting and and &ying &ying episodes, episodes, presumably presumably glacials glacials andand

interglacials)- interglacials)- The The pr€sent pr€sent envirrosment envirrosment could could not not pmducepmduce

the the rise rise in in the the watef watef table table necessary necessary to to satrxate satrxate the the soil-soil-

4-5- 4-5- P P mobilitymobility

Apatite, Apatite, tbe tbe primary primary min€ral min€ral that that contributes contributes P P to to soil" soil" isis

not not lresent lresent in in tte tte heavy heavy mirrcral mirrcral suite- suite- In In ofter ofter AndeanAndean

coucentrates, coucentrates, apatite apatite leaches leaches along along cleat'age cleat'age planes planes andand

appears appears very very soluble soluble (MahaneS' (MahaneS' and and Milner, Milner, 1998). 1998). OtherOther

humi4 humi4 alpine alpine soils soils show show depletion depletion in in P P wi*t wi*t time time throughthrough

the the dissolution dissolution of of apatite apatite (Firkeland (Firkeland et et al., al., 1989; 1989; sPalker'sPalker'

1964; 1964; Walker Walker and and Syers, Syers, 19?6). 19?6). The The scavenging scavenging effect effect ofof

soil soil Mn Mn on on P P and and Cr Cr (see (see ttre ttre positive positive Ce Ce anomaly anomaly inin

nodules, nodules, Fig- Fig- 10) 10) and and the the secondary secondary development development of of BaBa

phoryhat€ phoryhat€ grains grains ad ad Y Y phos$ate phos$ate (xenotire) (xenotire) coatings coatings onon

the the srrface srrface of of heavy heavy mineral mineral grains grains in in the the samples samples maymay

reflect reflect (1) (1) desiccation desiccation of of the the initially initially humi{ humi{ cold cold manga-manga-

niferous niferous soil; soil; (2) (2) concomitant concomitant nodularization nodularization of of the the Mn-FeMn-Fe

placon placon formpd formpd by by fte fte telease telease of of fefiic fefiic iron iron at at ttle ttle upper upper edgeedge

of of standing standing water; water; or or (3) (3) growth growth of of the the exotic exotic SosphateSosphate

nodules nodules from from the the loosely loosely bourd bourd P.P.

Alumino-phosphate Alumino-phosphate minerals minerals resembling resembling monazite, monazite, butbut

wi& wi& Fe, Fe, Mg, Mg, 84 84 asd asd Si Si as as tte tte dominant dominant cations cations andand

apparently apparently without without rare rare earth earth elernents elernents (Rffi's), (Rffi's), ale ale PresentPresent

io io the the Ah2 Ah2 and and Ah3 Ah3 hizons. hizons. These These miaerals miaerals have have aa

distinctive distinctive rmrnd rmrnd to to s$eroidal, s$eroidal, unwotrn unwotrn forrn forrn ad ad resinousresinous

red red to to brown brown colors. colors. They They appear appear to to be be a a mixnne mixnne ofof

dominant dominant wavellite, wavellite, an an alumino-phosphate, alumino-phosphate, wi& wi& lazulitelazulite

and and vivianite vivianite and and have have blue blue to to gr€en gr€en colcs. colcs. With With cations cations ofof

Fe Fe ad ad lrdg" lrdg" they they may may be be to to tb tb cradatlite cradatlite groupgroup

of of phosptrate phosptrate minerals, minerals, which which includes includes gorceixite gorceixite (Ba(Ba

alumino-phos@te) alumino-phos@te) and and florencite florencite tce tce atumino-pboqphale)-atumino-pboqphale)-

These These are are mre mre secondary secondary miaerals miaerals derived derived from from thethe

replacement replacement of of pimary pimary apatite, apatite, Ca Ca phoa$afe, phoa$afe, u u collo-collo-

pbarc, pbarc, ie ie concretionary concretionary sr sr colloform colloform variety- variety- They They rnayrnay

provide provide a a sourc€ sourc€ of of Fe Fe to to the the nodules nodules by by rapid rapid weathedngweathedng

when when sufficient sufficient water water is is available.available.

4.6- 4.6- Mn-Fe Mn-Fe nndulesnndules

The The Mn-Fe Mn-Fe nodules nodules in in the the Ah2 Ah2 and and Ah3 Ah3 horizons horizons reachreach

l-2 l-2 rnm rnm in in diaraeter diaraeter and and ane ane similar similar in in size size ts ts ten€srialten€srial

Fe*Mn Fe*Mn nodules nodules rcpst€d rcpst€d elseuihere elseuihere in in Japan Japan (Tnira (Tnira et et al-,al-,

1981) and Finland (Alhanen et a1., 1975) but arc smaller

than than those those r€eo{td r€eo{td by by Arshad Arshad and and St St fu'naud fu'naud (I980i (I980i inin

Saskatcbewan Saskatcbewan soils soils and and than than the the unique unique Ti-cernentedTi-cernented

nodules of \4trite (1p79). They are most commonly

spherical, spherical, layered layered Mn-Fe Mn-Fe conctptions conctptions but but also also occur occur asas

minor minor flattened flattened and and elongated elongated ellipsoidal ellipsoidal fmms. fmms. SpindleSpindle

fums fums that that occur occur mtre mtre cofirnonly cofirnonly lower lower in in the the pofile pofile (Cox(Cox

horizon) horizon) appear appear to to result result from from oxide oxide overgrowths overgrowths onon

elongate elongate lithic lithic fragments fragments of of the the parent parent matqial- matqial- An An )(RD)(RD

W-C- W-C- Mafuuey Mafuuey et et al- al- / / Jourrutl Jourrutl of of South South American American Earth Earth Sciences Sciences 15 15 Qnz) Qnz) 525-542525-542

Fig. Fig. 5. 5. (A) (A) Mn Mn nodrrle nodrrle with with several several embedded embedded puticles puticles (anows) (anows) ip ip the the Ah3Ah3

hdzoa- hdzoa- Tk Tk parti,clo parti,clo in in tle tle lower lower {igbt {igbt is is quare. quare. Prcfinlrting Prcfinlrting lnrticles lnrticles atat

right right are are &ied &ied and and c.oating c.oating is is cracked cracked @) @) Enlargement Enlargement of of gnnz gnnz gtain gtain withwith

multiflc multiflc adhering adhering paticles- paticles- Large Large adhoriqg adhoriqg patticles patticles ars ars quattz. quattz. (C) (C) NodttbNodttb

crac*d crac*d at at Ieft Ieft tasour) tasour) fxposiqg fxposiqg areaHo*r areaHo*r ibe ibe srfroe- srfroe- (D) (D) Iosi& Iosi& as€a as€a ofof

cracked cracked nodtle nodtle showing showing darker darker area area to to left left that that is is higbr higbr inMn inMn Area Area at at toptop

is is higher higher in in Fe- Fe- (E) (E) IUn IUn dule dule with with crrcked crrcked upper upper face. face. The The srrfrce srrfrce shwsshws

dativeiy dativeiy bigh bigh !vln, !vln, wkas wkas tbe tbe iurer iurer alea alea ie ie low. low. {F) {F) Ealargemed Ealargemed of of trytutrytu

crae.kod crae.kod arca arca in in @).@).

tace tace of of several several nodules nodules (F-ig. (F-ig. 3) 3) reveals reveals the the presence presence ofof

quartz and glauconite [(K, Na)(^Al, Fe, Mg)z(Al,

Si)4OldOH2)1, Si)4OldOH2)1, often often regatded regatded as as an an iron-rich iron-rich analqgue analqgue ofof

iltite iltite (fhough (fhough no no green green mineral mineral is is apprent apprent in in binocularbinocular

examination), examination), as as th th chief chief crystalline crystalline components. components. Quartz Quartz isis

very very abundant, abundant, as as indicated indicated by by the the 26.66 26.66 20 20 reflection,reflection,

wherpas wherpas glauconite glauconite is is a a rnere rnere hace- hace- This This is is connpatible connpatible withwith

the the SEM SEM and and EDS EDS results, results, which which shw shw that that both both quaazquaaz

particles particles and and rnatrix rnatrix Si Si rnake rnake up up a a high high percentage percentage ofof

crystalline crystalline material material in in some some shells shells of of several several nodules. nodules. InIn

eactr eactr rodule, rodule, near-spherical near-spherical laminations laminations form form ooncennicooncennic

shells shells of of fine-grained fine-grained earthy earthy oxides oxides cemented cemented with with large.rlarge.r

(5-2O pm) soil particles.

The The source source of of th th Fe Fe and and Mn Mn is is heavy heavy minerals, minerals, such such asas

pyrite, pyrite, conrmon conrmon in in bottt bottt the the matrix matrix and and veinlets veinlets of of thethe

phyllite, and the pale pink to colorless Mn garnet

(spessartite). (spessartite). The The Fe Fe and and hdn hdn concentrations concentrations in in fhe fhe ssil ssil afeafe

the the result result of of soil soil hydrommphic hydrommphic pmcesses pmcesses that that rnay rnay be be nono

longer longer active.active.

4-T 4-T Scam.ing Scam.ing electron electron microscoPymicroscoPy

The SEM and EDS results show in detail the

character character of of the the Mn-Fe Mn-Fe nodules nodules (Fig. (Fig. a(A)), a(A)), especiallyespecially

E F

Fig- Fig- 6. 6. (A) (A) San& San& in in the the Bthorizon. Bthorizon. (B) (B) T*oquaaz T*oquaaz grains grains (eftand (eftand center)center)

showing showing flat flat fracture fracture face face (right) (right) and and surface surface wi{ft wi{ft small small fractures fractures $eft). $eft). (C)(C)

Fracrrres Fracrrres on on quartz, quartz, possibly possibly of of terionic terionic origin. origin. (D) (D) Arnpbibote Arnpbibote in in centercenter

with with quartz quartz grain grain to to right right showing showing ftesh ftesh fracore fracore face face in in tle tle Ahl Ahl horizonhorizon

(E) (E) Qlarz Qlarz $ain $ain with with fracore fracore face face and and ft,esh ft,esh fracarres. fracarres. (F)Qnrtz (F)Qnrtz gnin gnin withwith

old old preweathcrd preweathcrd ad ad &esh &esh srfrces. srfrces. Srrfacc Srrfacc on on left left consists consists of of fieshfiesh

fracnrres fracnrres &at &at eould eould be be teconic.teconic.

ttre ttre granular/pebbled granular/pebbled surface surface texture texture (Fig. (Fig. a(B) a(B) ancl ancl CI)).ACI)).A

vaiety vaiety of of grains, grains, consisting consisting of of glauconite glauconite (iltite?) (iltite?) (Fig.(Fig.

a(O) a(O) andquat andquat (FiS. (FiS. a{D}} a{D}} arc arc shoqm shoqm in in each each nodule. nodule. TheThe

central central area area of of the the nodule nodule (Fig. (Fig. atE)) atE)) contains contains a a chemicalchemical

composition composition of of Ti, Ti, Si, Si, Al, Al, and and Il Il in in addition addition to to Fe Fe and and IVfn,IVfn,

which which is is ttre ttre glauconite. glauconite. Silt-size Silt-size particles particles dominate dominate on on thethe

surface surface (Fie. (Fie. a(F)).a(F)).

In In these these concretionary concretionary nodules, nodules, many many particles particles of of quartzquartz

are are present present (Fig. (Fig. 5{A}). 5{A}). Some Some quartz quartz grains grains carry carry adtrcringadtrcring

mineral particles (Fig. 5(B)). A spht nodrle (Fig. 5iC))

sbows sbows a a datker datker internal internal atea atea high high in in lv{n; lv{n; the the outer outer rid rid isis

higha higha in in Fe Fe (Fig. (Fig. 5{D). 5{D). Another Another cracked cracked no&ile no&ile rcveals rcveals aa

reversod reversod distribution distribution (FiS. (FiS. 5(E)), 5(E)), contrarJ/ contrarJ/ to to the the electronelectron

microprobe microprobe analyses analyses of of soil soil concretions concretions by by Gallaher Gallaher et et ai.ai.

ll973j, ll973j, who who showed showed a a near-uniforrn near-uniforrn concentration concentration of of MnMn

with with fluctuating fluctuating Fe. Fe. An An enlargement enlargement of of this this nodule nodule (Fig.(Fig.

5(F)) 5(F)) shows shows th th various various sizes sizes of of particles particles on on the the surface.surface.

Lower Lower in in the the soil soil profile, profile, in in the the Bt Bt horizon, horizon, the the sandssands

(Fig. (Fig. 6{Ai) 6{Ai) demonstrate demonstrate a a range range of of angular angular to to subangularsubangular

particles particles in in various various stages stages of of dissolution- dissolution- We We did did notnot

en€ount€ranyqua{tz en€ount€ranyqua{tz grains grains wi*r wi*r miaotextures miaotextures to to indicate indicate aa

fonns glacial environment (Mahaney et al.. 1988; Maha-

ney, ney, 1990), 1990), despite despite the the nearness nearness of of the the site site to to the the AndeanAndean

glacial limit (< 15 hn) (Mahaney and K;rlm, 1996). The

W-C. W-C. Mahaney Mahaney et et al- al- / / Journal Journal af af South South Aneian Aneian Earth Earth Sciences Sciences 15 15 (2W2) (2W2) 525-542525-542

E F

Fig. Fig. 7. 7. (A) (A) auartz auartz grain grain in in tlr tlr Ahl Ahl horizon horizon showing showing dissolution dissolution bur bur nonocoating. coating. (B) (B) Ihesh Ihesh grain grain of of biotitc biotitc with with minor minor fractures fractures in in rhe rhe Ahl Ahl horizonhorizon(C) (C) Ceneral Ceneral frame frame of of grains grains in in the the Ahl Ahl horizon horizon (D) (D) Ellipsoidal Ellipsoidal spirel spirel withwithminor minor disdution disdution (and (and organic organic u/reckage u/reckage clinging clinging to to surface surface and and replacedreplacedwith with Si, Si, AI, AI, Fe, Fe, C4 C4 arxd arxd Mg). Mg). (E) (E) Garnet Garnet with with &inMn &inMn coating coating in in the the Ah2Ah2himn. himn. $) $) a6p616o1u a6p616o1u in in tle tle Btlorizoo Btlorizoo mongfy mongfy urcahd urcahd wi& wi& mall mall IvfqIvfqpetlers petlers adlrcdng adlrcdng to to itit

two two quartz quartz grains grains in in Fig. Fig. 6(8) 6(8) are are representative representative of of quartz quartz inin

tbe tbe 2C-ox 2C-ox horizu-fte$ horizu-fte$ fuarrc fuarrc free free Gigbt) Gigbt) andquarb andquarb witrwitrshallow shallow surface surface 'crackles' 'crackles' (eft).(eft).

l,ess l,ess than than lATo lATo of of all all grains grains in in the the 2Cox 2Cox horizon horizon areare

fractured fractured (ng. (ng. 6(C)), 6(C)), and and therefore, therefore, some some may may have have had had aa

fluvial fluvial and/m and/m bctonic bctonic histnry histnry (possibly (possibly related related to to {looveinent{looveinent

on the Bocon6 Fault (Schutnrt, 1982, 1984). The sawtooth

fractures fractures on on part part of of the the surface surface are are similarto similarto quarlzgpains quarlzgpains inin

Swedish Swedish cavqs, cavqs, which which have have been been undiqputedly undiqputedly linked linked toto

neotectonic neotectonic processes processes (Mahaney (Mahaney and and Sjoberg, Sjoberg, 1993). 1993). AA

weathered weathered amphibole amphibole (FiS. (FiS. ap)), ap)), next next to to a a quartz quartz grain, grain, isispatly patly wearhered wearhered ad ad bears bears a a ftesh ftesh fracture fracture fzce fzce or or parting,parting,

possibly possibly from from tectonic tectonic lrocesses. lrocesses. Fractres, Fractres, fracture fracture facesfaces(Fig. (Fig. 6(E)), 6(E)), and and a a complex complex Feweathered Feweathered surface surface on on thethe

quartz quartz (Fig. (Fig. 6G)) 6G)) may may have have been been caused caused by by faulting faulting (for(for

SEM SEM microtexftrres microtexftrres used used to to discriminate discriminate sedimentarysedimentary

envirounents, see BulL (1981); for a definition of pre-

weathering, weathering, see see Mahane..y Mahane..y a&l a&l Kaftu Kaftu (2000)).(2000)).

At At the the base base of of the the profile, profile, in in the the 2Cox 2Cox horizon, horizon, angulmangulmquartz quartz ad ad lithic lithic ftagrrents ftagrrents {Fig. {Fig. 7€}} 7€}} dorninate dorninate the the aoarseaoarse

size size fraction, fraction, thougb thougb one one quartz quartz grain grain (Fig. (Fig. ?(A)) ?(A)) showsshows

some some evidence evidence of of solution solution rounding. rounding. Representative Representative grainsgrains

iu iu the the 2Cox 2Cox horizon horizon include include platy platy phyllosilicates phyllosilicates (Fig. (Fig. 7(B)),7(B)),

srnooth srnooth flattened flattened ellipsoids ellipsoids (Fig. (Fig. 7{D}, 7{D}, spheroidal spheroidal t5'pest5'pes

with with dimfles dimfles and and surface surface crackla crackla (Hg. (Hg. TiEi), TiEi), and and irregularirregular

tabular tabular amphiboles amphiboles (Fig. (Fig. XF)) XF)) with with a a coilrse, coilrse, pitred pitred texfiuetexfiue

{Fig- {Fig- ?tF})- ?tF})- The The SEM SEM data data support support tb tb field field interpretati,on interpretati,on ofof

a a profile profile furmed furmed in in a a residual residual regolith regolith that that mixed mixed partiallypartially

with with water water tranryorted tranryorted material material in in ifs ifs lower lower part part butbut

contained contained locally locally &rived &rived aeolian aeolian sedirnents sedirnents in in its its A A ad ad BB

hmizons hmizons that that had had a a composition composition similar similar to to that that of of the the parentparent

material.material.

4.8. 4.8. Energy4ispe Energy4ispe rs rs ive ive spectrvmet spectrvmet ryry

X-ray X-ray microanalysis microanalysis of of tlre tlre nodules nodules studied studied by by SEMSEM

reveal reveal a a nodule nodule surface surface with with a a high high SilAl SilAl ratio ratio (close (close to to 2:l),2:l),along with high K,IvIn, ad Fe (Fig. 8(A)) concent?tions,though though the the Fe Fe is is higher higher than than Mn, Mn, Sornc Sornc quarrz, quarrz, paticlespaticles

contain contain relatively relatively litde litde Fe Fe (Fie. (Fie. 8(B)). 8(B)). Cracked Cracked nodulesnodulesshw shw internal internal ratios ratios of of fie fie to to ldn ldn of of 2:1 2:1 (Fie- (Fie- 8{C}), 8{C}), which which isisrepesentative repesentative of of the the poprlation poprlation of of nodules nodules studied. studied. In In somesomecas€,r, cas€,r, tbe tbe intemal intemal dule dule strucfure strucfure is is reversed, reversed, witb witb Mn/FeMn/Fe

tatios of 2:l €re.8iD)).

4.9- 4.9- MineralogyMineralogy

Sad- Sad- atd atd silt-sizpd silt-sizpd minerals minerals throughout throughout the the profileprofile

contain contain predominantly predominantly quare quare and and a a complex complex suite suite of of lreavylreavy

minerals. minerals. Quartz Quartz dominates dominates the the coarse coarse fraction, fraction, withwithcoliform coliform to to amoeboid amoeboid surface surface textures, textures, vfrich vfrich implies implies aasolution or precipitation p'rocess (Fig. 9tA). Brownlimonitic limonitic grains, grains, secondary secondary after after pyrite, pyrite, also also are are commoncommonin in this this frrction.frrction.

The The finer finer sand sand from from the the nodnle nodnle zone, zone, otber otber than than lowlow

density density Mn-Fe Mn-Fe crurnbs crurnbs and and clots, clots, is is mainly mainly quartz, quartz, bothbothresidual residual and and secondary, secondary, as as well well as as mfuicr, mfuicr, very very fine-grainedfine-grained

beary beary minerals. minerals. The The semndary semndary min€rals min€rals are are ford ford on on andand

around around coarse coarse relict relict fragnents fragnents of of botb botb phyllite phyllite and and quartzquartz

veinlets, veinlets, which which becornes becornes more more apparent apparent with with depth.depth.

Heavy Heavy minerals minerals with with some some quartz quartz dominate dominate the the fine fine sandsand

fraction fraction The The healy healy rnineral rnineral fraction fraction consists consists mainly mainly ofof

zircon, zircon, wift wift prominent prominent pale pale tcnrmaline tcnrmaline arxd arxd black black Fe-TiFe-Ti

alumino-silicafes alumino-silicafes that that compose compose a a significant significant black black sand sand withwith

a a minor, minor, magnetic magnetic ilmenite ilmenite component.component.

The The non-magnetic non-magnetic black black fraction fraction is is dominated dominated byby

titanium titanium minerals, minerals, including including both both itmenite itmenite and and Ti-Fe-MnTi-Fe-Mn

alumiilo-.silicates. alumiilo-.silicates. These These appear appear to to be be or or astro-astro-

phyilite, phyilite, rare rare minerals minerals associated associated with with alkaline alkaline intrusives.intrusives.

The The magnetic magnetic fiaction fiaction consists consists mainly mainly of of ilmenite ilmenite (F{*e.(F{*e.

9(B). 9(B). The The downward downward disribution disribution of of these these minerals minerals isis

randorn randorn ad ad cannat cannat be be used used to to dlffwntiate dlffwntiate the the weatheredweathered

regolith regolith frorn frorn the the aeolian aeolian component-component-

Rare Rare pyrite pyrite is is present present in in the the Ah3 Ah3 horizon, horizon, and and secondarysecondary

Fe Fe pseudomorphous pseudomorphous of of limonite limonite after after pyrite pyrite are are common common inin

the the soil- soil- Colsless Colsless to to pale pale rose rose garret garret fragmens, fragmens, sirnilar sirnilar toto

the the iron-staired iron-staired quaru, quaru, ild ild rare rare colorless colorless to to pale pale pinkpink

dodecahdral dodecahdral forms forms of of a a Mn-Fe Mn-Fe garnet garnet occur occur in in both both thethe

Ah2 and Atr3 horizons (Fig.9(C) and (D)). The presence ofgarnet. garnet. a a relatively relatively unstable unstable ndrleN:al ndrleN:al in in fu fu weathering weathering indexindex

AA

W.C. W.C. Mahaney Mahaney et et al. al. / / Iournal Iournal of of South South Anerican Anerican Earth Earth Sciences Sciences I5 I5 (m02) (m02) 525-542525-542

o,o,FF

zz

oo()()

ooFFzzff

oo()()

6 0 -

aaFF

zzl o -oo()()

aaj-j-

zz))oo()()

ito o.o

E N E R G Y ( I ( E V }

OO 2.O il.O C.O Ag

E N E B G Y ( K E V )

t .

a.o a.o 6,06,0

E N E R G Y ( I G U

l l r a

2.O ..0 C.O t.0 ro.o

.o-

II

ro.ott

ooE N E R O Y ( K € V }

Fig- Fig- 8. 8. Chemical Chemical data data oo oo Mn-Fc Mn-Fc no&des. no&des. (A) (A) Ccntec Ccntec aea aea of of Fig. Fig. dA) dA) dro*'ing dro*'ing high high Si/Al Si/Al (2: (2: l) l) wi$ wi$ K K l,ln, l,ln, ud ud Fe. Fe. This This chemist4r chemist4r is is represenative represenative of of thethe

nodule nodule sgrface. sgrface. (B) (B) Chemistry Chemistry of of quartz quartz panicle panicle with with Fe Fe coating- coating- (C) (C) Chemktry Chemktry of of cracked cracked area area of of tk tk nodule nodule shown shown in in Fig. Fig. 5{D) 5{D) with with Fe/Mn Fe/Mn ratio ratio of of 2:l- 2:l- (D)(D)

Chemisuy Chemisuy inside inside the the 6acl@d 6acl@d upper upper face face sbwing sbwing an an MnlFe MnlFe ratio ratio of of 21.21.

Table Table 44

Elernental Elernental concentrations concentrations in in ttre ttre MFOSJA MFOSJA paleosolpaleosol

Horimn Horimn Al Al (%) (%) Cs Cs (%) (%) Tb Tb (ppm) (ppm) v v {ppm) {ppm) Mn Mn {ppn) {ppn) Na Na (pp) (pp) K K (%) (%) fi fi (ppm) (ppm) Fe Fe t%) t%) Hf Hf (ppm) (ppm) Sc Sc (ppm)(ppm)

Mnd I 6.3

Mn nod 2 5.9

Ahl 5.9 #.16

Ah2 6.2 #.34

Aht 8.3 *.&

Bt 9-8 #.09

ZCox ZCox 92 92 #-O4#-O4

Weatlrered Weatlrered rock rock (2Cr) (2Cr) 4.2 4.2 #.O3#.O3

Slighdy Slighdy weatlrered weatlrered rock rock 9.E 9.E #.4#.4

Phyllite 12.3 #.15

4343

nnt 2l9

L6t2

l5

t0r9t l

t77t7719377

tt7

150119

123123

4949131

LM

33m3m790790

630650650724

630630

400400

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W.C- W.C- Mahaney Mahaney et et al. al. / / lournal lournal of of South South Ameican Ameican Earth Earth Sciences Sciences 15 15 (2U2) (2U2) 525-542525-542

Frg- Frg- 9- 9- (A) (A) Arncboid Arncboid qtaru qtaru grain grain fflnwing fflnwing a a srrfrce srrfrce texflrq texflrq which which impliesimplies

solutioo solutioo and/or and/or precipitation- precipitation- (B) (B) Magneric Magneric fraction fraction showing showing predomimntlypredomimntly

ilrneoite ilrneoite gBins. gBins. (C) (C) Garnet Garnet grain grain in in ttte ttte Ab2 Ab2 funtj/tr funtj/tr showing showing solutionsolution

rundiog rundiog and and an an ached ached sdace sdace closcly closcly lollw'ing lollw'ing fre fre crystd crystd plmes- plmes- (D)(D)

Gam€t Gam€t ftomthc ftomthc Ah3 Ah3 horizon horizon $ows $ows sharp sharp crystal crystal edges edges and and etched etched furrowsfurrows

(left (left and and boaom), boaom), possibly possibly following following root root coffaclcoffacl

of of Oxisols, Oxisols, does does not not support support a a tropical tropical lowland lowland origin origin for for thethe

MPOSJA MPOSJA profile.profile.

4. 4. I0- I0- Gmeral Gmeral geochcmistrygeochcmistry

Geochemical Geochemical tends tends of of macru- macru- and and microelenrents microelenrents werewere

alrrrlyzed alrrrlyzed to to search search for for sedirrrnt sedirrrnt uniforrnity uniforrnity and and study study thethe

nodules nodules relative relative to to the the individual individual horizons horizons (Table (Table 4)-4)-

Elernental Elernental losses losses between between ttrc ttrc phyllite phyllite and and the the weattsedweattsed

rock rock (2Cr) (2Cr) appear appear to to be be a a rnaximum rnaximum of of a a factor factor of of three three (Al,(Al,

K K Mn, Mn, Sc, Sc, Ti, Ti, V), V), with with thc thc exception exception of of Na, Na, whichwhich

decreases decreases neady neady by by a a factor factor of of twenty. twenty. Other Other elements elements areare

relatively unchangpd (Hf) or little changed (Fe, Th)- All the

rnaterial is low in Ca.

The The upward upward trend trend of of elemental elemental losses losses in in the the paleosol paleosol isis

masked masked by by the the presence presence of of higber higber free free silica silica and and mganicmganic

contents in the Ah horizons, which dilute elemental

concentrations concentrations (At, (At, K K and and Sc) Sc) to to sorre sorre degree degree (for (for dilutioudilutiou

effects, see b{ahaney et al. (i992)). The increased silica

content, the residual mineral in the light mineral suite, is

probably probably mirrored mirrored by by increased increased Hf Hf content, content, a a reflection reflection ofof

the the mme mme inert, inert, residual, residual, Hf-bearing Hf-bearing zircon zircon in in fu fu heavyheavy

mineral mineral suite suite of of tbe tbe Ah Ah horizons. horizons. Alternatively, Alternatively, lffhas lffhas beenbeen

correlated correlated with with quartz quartz ard ard may may reflect reflect aeolian aeolian contributions,contributions,

as in results reported by Childs {fcr75} for Fe-Mn

concretions concretions in in silica-rich silica-rich New New Z'<nland Z'<nland soils- soils- In In waterloggedwaterlogged

soils, Mn is more soluble and mobile than Fe (Childs' 1973)-

In a study of various sandy soils of Alberta, Arcrena et al-

( 1992i discussed nodular microstmctures. Micromoqphol-

ogy ogy and and lysirnerer lysirnerer studies studies were were used used to to evaluate evaluate tb tb nafiuenafiue

and formation of microstructures, which vary from

relatively relatively simple simple monic monic or or orthogranic orthogranic (grains (grains uncoated)uncoated)

in Ae (E) horizons to chitonic m chlamydic (grains fully or

ta ta Cs Cs iel iel Srn Srn Eu Eu Tb Tb Dy Dy Yb Yb LuLu

Fig. Fig. 10. 10. Chondrite-nonnalized Chondrite-nonnalized rare-eardr rare-eardr profile profile for for the the MPOSJA MPOSJA profile.profile.

partially partially coated) coated) in in the the matrix matrix of of the the B B horizons horizons to to porphyricporphyric

(grains (grains embedded embedded in in fine fine mass) mass) in in the the nodules nodules of of the the BB

horizons. horizons. The The main main genetic genetic transport transport Process Process was was inter-inter-

pretod pretod to to be be podzolization" podzolization" to to include include the the eluviation eluviation ofof

organic organic natter natter and and organo.metallic organo.metallic complexes complexes of of Al Al and and FeFe

(lessivage), as well as the mobilization of phyllosilicates,

particulate particulate Fe Fe oxides, oxides, quartz, quartz, and and feldspar feldspar and and possiblepossible

oxidation oxidation reduction reduction prccesses prccesses accountable accountable fa fa tbe tbe move-move-

ment ment of of Fe+z. Fe+z. As As in in tbe tbe Venezuelan Venezuelan Andes, Andes, there there areare

indications indications that that iron iron oxides, oxides, like like goethite goethite and and ferrihydrite,ferrihydrite,

are are active active in in the the adsorption adsorption of of organic organic matter matter and and organo-organo-

retallic retallic complexes. complexes. Colloidal Colloidal minerals minerals in in the the lercbateslercbates

include include globular globular ferrihydrite, ferrihydrite, g@thite, g@thite, opal opal (exactly (exactly analo-analo-

gous to the MFOSJA silica-rich shells), phyllosilicates, very

fine fine quartz, quartz, Chrysophycean Chrysophycean stomatocysts stomatocysts (plant (plant spores), spores), andand

possibly possibly arnoqphous arnoqphous alurninous alurninous materials-materials-

The The Mn-Fe Mn-Fe nodules, nodules, relative relative to to the the Ah2 Ah2 and and Ah3Ah3

horizons, horizons, are are much much higher higher in in Mn Mn and and Na Na and, and, to to a a lesserlesser

extent, in Th, V and Fe. The geochemistry confirms the

- Alrl- - ' . . . . - ' . - . . H----,-z0ox

2Cr 2Cr Wealhered Wealhered rockrock

- - - - -2RSlightfy Slightfy w€attered w€attered rockrockPtryllitePtryllite

II

W.C- W.C- Mahanel Mahanel et et al. al. / / Joumal Joumal of of South South Anterican Anterican Earth Earth Sciences Sciences 15 15 Qm2) Qm2) 525-542525-542

wealt?€tdwealt?€tdb&db&d

,€{iod-,€{iod-

Fig. Fig. I I l. l. Sketch Sketch of of a a hydrogeochemical hydrogeochemical model model slnwing slnwing flucnration flucnration of of soilsoil

water water over over an an impcnneablc impcnneablc bedrock bedrock with with water water sanration sanration reaching reaching as as highhigh

as as the the base base of of the the Ahl Ahl huizon. huizon. Thc Thc genesis genesis of of Mn-Fe Mn-Fe nodrles nodrles depends depends onon

changes changes in in Eh Eh and and pH pH with with alternating alternating reduction reduction and and oxidation.oxidation.

XRD XRD results results of of the the nodules, nodules, which which show show the the presence presence ofofquartz quartz and and glauconite, glauconite, ess€ntially ess€ntially a a hy&ous hy&ous K-Fe K-Fe silicate-silicate-

4.11. Rare eanh elemcnt geochemistry

Rare Rare earth earth geochemistry, geochemistry, rarely rarely studied studied in in soils soils andandpaleosols (lv{c[,ennan, [989), shows some interesting andimportant important data data on on translocations translocations in in ancient ancient weatheringweatheringsystems.

The The REEs REEs are are chondrite chondrite normalized normalized relative relative to to thethedistribution distribution in in the the parcnt parcnt material. material. The The profiles profiles in in Fig. Fig. l0l0reveal reveal a a slight slight and and uniform uniform endchment endchment upward upward into into slightlyslightlyweathered weathered phyllite, phyllite, followed followed by by a a dramatic dramatic decrease decrease totoweathered rock and overlying paleosol. The Ahl, Bt, andZCax ZCax hcldzon hcldzon profiles profiles are are \ieq/ \ieq/ simila simila to to weatherod weatherod rrckrrckand and exhibit exhibit similar similar pofiles, pofiles, which which suggesrs suggesrs little little leaching leaching ofofREEs REEs in in the the paleosol paleosol itself itself but but greater greater movernent movernent below.below.The Ah2 and Ah3 horizons, wbere tbe Mn-Fe nodulesreside, reside, have have similar similar rarc rarc earth earth profiles, profiles, wi& wi& the the excepion excepion ofofa strong positive Ce anomaly (Fig. l0).

The The Mn Mn nodules nodules show show a a much much higher higher Ce Ce concentrationconcentrationthan than the the Ah2 Ah2 and and Ah3 Ah3 hmizons, hmizons, probably probably relafed relafed to to thethebuild-up build-up of of Mn Mn and and Fe Fe and and the the afrnity afrnity of of Ce Ce fu fu ttrcirttrciroxyhydroxides. oxyhydroxides. In In soil soil conditions conditions of of moderately moderately low low pH, pH, PPbecomes becomes fixed fixed as as very very complex complex and and insoluble insoluble cornpoundscornpoundswith Fe, Al, andMn (Bncknran andBrady, 1969). As well asbeing being adssbed adssbed onto onto th th Mn-Fe Mn-Fe nodule nodule surface, surface, Ce Ce cancanoccur occur either either in in a a pnirnary pnirnary phosphate phosphate residual residual mineral mineral or or aasecondary mineral to form either in polycrystalline

phosphate phosphate aggregates aggregates or or pseudomorphous pseudomorphous after after apatiteapatite(Duncnn and Willett. 1990) in tbe dule complex. Thisexplanation explanation is is problematic, problematic, in in that that it it does does not not accommodateaccommodateLa, La, which which can can occur occur in in the the monazite monazite lattice lattice or or relatedrelatedphosphates.

The The presence presence of of Y Y in in polycrystalline polycrystalline phosphate phosphate (second-(second-

ary ary xenotirne) xenotirne) on on derital derital zircons zircons in in the the nodules nodules is is intriguingintriguingand and begs begs frrrther frrrther geochemical geochemical study. study. Similarly, Similarly, Mn Mn and and Fe Fe inin

soils adsorb rnetal ions into their structures (Taylor, lg68;

Clracr aiiti Theohald, l97ti).

5. 5. DiscussionDiscussion

On On the the basis basis of of the the results, results, we we tested tested three three workingworkinghypotheses hypotheses that that could could be be used used to to explain explain the the origin origin of of thethenodules: nodules: (l) (l) a a chilled chilled u u cold cold tropical tropical soil soil model model with with a a longlongperiod period of of morphogenesis morphogenesis in in the the tropical tropical lowlands lowlands followedfollowedby by uplift; uplift; (2) (2) hydromorphic hydromorphic transport transport of of Mn-Fe Mn-Fe sotutionssotutionsfrrom frrom fractures fractures in in the the Bocon6 Bocon6 Fault Fault to to erosion erosion surfaces surfaces in in thethefloor floor of of the the Bocon6 Bocon6 trench trench in in Early Early Quatemary Quatemary time,time,followed followed by by alternating alternating wet wet and and dry dry cycles; cycles; and and (3) (3) in in situsitugenesis of alpine soils from Mn-Fe-bearing argillaceousmetasediments and genesis of Mn-Fe nodules occurringduring the alternating wet and dry climatic cycles of the

Quaternary-Quaternary-

5.1. 5.1. Chillcd Chillcd tropical tropical soil soil moful--hyqnthesis moful--hyqnthesis II

A Plio-Pleistocene tropical paleosol (soil) develo@ inlowland rainforest, abruptly uplifted in [,ate Pleistocenetirne, tirne, and and preserved preserved in in a a cool, cool, montane montane environrnent environrnent formsformsthe the crux crux of of this this argument.argument.

Paleosols Paleosols forrned forrned in in Late Late Paleozoic Paleozoic metamorphic metamorphic rocksrocksnear Lagunillas, Venezuela, at elevations betrveen 15m and3lS rn as.l, areconsideredby Vivas {lgl4rtohave fwn€dclose close to to sea sea level level in in a a tropical tropical rain rain forest forest environment.environment.Formed Formed on on old old erosion erosion surfaces, surfaces, ttrese ttrese paleosols paleosols have have beenbeenassigned assigned to to the the Oxisol Oxisol order order (Soil (Soil Survey Survey Staft, Staft, ?fXO) ?fXO) andandterrned latosols by Weingarten i197?) on the basis of theirlow CEC, Iowbase saturation, and pFIs between 4.2 and5.7 -Weingarten il977i reported that the mineralogy of the clayfraction fraction consists consists principally principally of of kaolinite, kaolinite, illite, illite, muscovite,muscovite,gibbsite, gibbsite, and and guartz. guartz. The The presewation presewation of of quaru quaru veins veins in in thetheweathered weathered rock, rock, which which happens happens in in other other weathered weathered soilsoilorders, orders, such such a^s a^s Ultisols Ultisols and and Alfisols, Alfisols, was was consideredconsideredcornpatible cornpatible with with the the development development of of Oxisols. Oxisols. No No Mn-FeMn-Fenodrles nodrles were were reported reported by by Weingarten Weingarten {1977)- {1977)- The The tectonic/tectonic/climatic implication of assigning the paleosols to the Oxisolorder order is is that that the the Andes Andes uplift uplift ftom ftom lower lower @ical @ical rainforestrainforestto to higher higher priramo priramo environments environments occurrod occurrod relatively relatively recently,recently,less than l00ka- This interpetation is considered compa-

tible with SchuberJ's (1994) glacial model, which limitedthe the age age of of the the oldest oldest Pleistocene Pleistocene glrcial glrcial deposits deposits to to the the LastLastGlacial Maximum (ca. 25,00 yean). Thus, a precise

analysis of the age and taxonomic placement of paleosols,such such as as MPCISJA, MPCISJA, on on ancient ancient erosion erosion surfaces surfaces below below thethelower limit of Pleistocene glaciation (<2800 rn a.s-I.;I'lahaney and Kalm, 1996) is crucial to an assessment of

the the age age of of the the uplift. uplift. Gibbsite, Gibbsite, as as a a weathering weathering product, product, isis

not necessarily indicative of a low elevation origin; it isfound at high elevations (3500 m a-s.I.) in the tropicalmountains (Mahaney, 1990. 1991).

Furthermore, Furthermore, garnets garnets that that are are prominent prominent in in these these soils soils areare

II

IIII

ffiffifluc{rrtionfluc{rrtion

IIII

II

W.C. W.C. Muhaney Muhaney et et al. al. / / Joumol Joumol tf tf South South American American Earth Earth Sciences Sciences I5 I5 (2002) (2002) 525-542525-542

urtlikely urtlikely to to survive survive fie fie intense intense weathering weathering of of lowlandlowland

tropical tropical soils- soils- Excessive Excessive Fe Fe and and Mn Mn in in tropical tropical soils soils tend tend totobe be converted converted to to lateritic lateritic duricrust duricrust and and supergene supergene Mn Mn oxideoxideore. ore. Strong Strong purple purple colors colors and and deep deep saprolite saprolite tongues tongues andand

corestones corestones re re characteristic characteristic of of such such soils- soils- Iron Iron concretionsconcretionsor or pisolites pisolites oocur oocur but but as as dense dense stnrctures stnrctures without without MnMnbanding.

5-2- 5-2- Hydrogeochcmical Hydrogeochcmical and and microbial microbial mdels mdels of of Mn-FeMn-Fe

rcdule rcdule genesis-4ypothesis genesis-4ypothesis 22

The The hydromorphic hydromorphic transport transport of of Mn-Fe Mn-Fe solutions solutions fromfromthe the fractures fractures of of tb tb Bocon6 Bocon6 Fault Fault zone zone to to drainage drainage on on ananerosion erosion surface surface graded graded to to the the floor floor of of the the Bocon6 Bocon6 trench trench ininEady Eady Quatemary Quatemary tirne tirne could could provide provide an an environrnent environrnent ofofnodrle nodrle develqment develqment ard ard preservation, preservation, wtrich wtrich might might pre-pre-serve serve red red soils soils with with Mn-Fe Mn-Fe nodiles nodiles in in cool, cool, alternating alternating wetwetand and dry dry climates climates in in Late Late Quaternary Quaternary time. time. This This is is partiallypartially

sup'ported sup'ported by by the the available available data.data.

Mn-Fe Mn-Fe nodules nodules form form in in cool, cool, humid humid climates climates withwithseasonal rain (Birkeland, 1999, see his references on theforrnation forrnation of of Mn Mn nodules). nodules). Soils Soils beconre beconre water water logged logged andanddense, decaying vegetation seates acidic conditions, inwhich which B B hmizons hmizons becorre becorre leachd leachd and and oxidized oxidized &ring&ringtimes when the water table is low (Fig. 1l). Wirh a highwater water table, table, the the soil soil becomes becomes wat€r wat€r saturated saturated above above ananimpermeable impermeable bedrock bedrock layer layer at at less less than than 2 2 m m depttr. depttr. WaterWatersaturation saturation promotes promotes anaerobic, anaerobic, reducing reducing conditions, conditions, aud aud Fe3Fe3is reduced to Fe2 (Garrels arxl Christ. fq65). These are thetenets tenets of of hypothesis hypothesis 3, 3, which which make make it it the the most most likelylikelyexplanation explanation of of Mn-Fe Mn-Fe nodule nodule gencsis gencsis in in An&an An&an paleosols.paleosols.

Unlike Placosols (Tilsley. 1971), which are abundant ineastern eastern and and western western maritime maritime areas areas of of Canada Canada with with moistmoistconditions conditions at at the the surface, surface, these these Andean Andean Mollisols Mollisols areareperiodically periodically wet wet above above the the impermeable impermeable bedrock.bedrock.

In In the the aforementioned aforementioned corrceptual corrceptual model, model, the the entire entire soilsoilprofile profile is is saturated saturated and and slowly slowly dries dries urt urt from from the the top top down-down-As As this this occurs, occurs, organic organic matter matter is is lost lost ttrrough ttrrough oxidation,oxidation,whicb whicb accompanies accompanies a a transition transition from from peaty peaty surface surface horizonshorizonsto to the the A A or or Ah Ah horizons horizons currently currently in in place. place. It It would would bebedifficult difficult to to drain drain these these soils soils laterally laterally bocausc bocausc &e &e lardlardsurface surface is is nearly nearly flat- flat- The The Mn-Fe Mn-Fe nodules, nodules, formed formed duingduingwet periods (dring either seasonal changes orlong climaticcycles), produced anaerobic conditions and remainedbehind behind as as relict relict features features rnodifi€d rnodifi€d in in a a fteely fteely drainingdrainingMollisol. Mollisol. Theh Theh diameters diameters may may be be related related to to the the length length ofoftirne tirne and/or and/or number number of of times times anaerobic anaerobic reducing reducing corditionscorditionsexisted in the past- The observations support severaloxidizing oxidizing ad ad reducing reducing episodes-episodes-

Although Although we we prefer prefer an an inorganic inorganic oxidation oxidation and and reductionreductionprocess similar to that described by Alhonen et al. (1975),

microbial microbial processes processes may may have have played played a a role role in in the the genesis genesis ofofsome some uodules, uodules, though though pnevious pnevious studies studies show show an an apparentapparentabsence of bacteria in Andean soil nodrles (h{ilner et al..1996). Because Mn and Fe are insoluble in the same generalEh and pH conditions (Henr, 1989), dissolution and

precipitation precipitation of of Mn Mn rnay rnay result result from from microbial microbial action action ratherratherthan inorganic redox phenomena (Ghiorse. 1980, I9ft4:Chiorse ami Llirsch. l9&2; Bunlige and Kepkay. l9fJ3;l.ovley and Phillips, 1988; Robbins et al., 1992; Goldeuet al., 1992\. In these environrnents, Mn oxidation isperformed by Fe-fixing bacteria (Clrukror, et a!.. i97-1).Rohbins et al. (1992i found that the most common bacteriaassociated with Mn precipitation in soils is lzptothrixdiscoplwra, discoplwra, which which has has been been identifie"d identifie"d in in Mn Mn deposits deposits bybyBougerd ancl De Vrinct {1987} and Ghiorse (1980, 1984).hevious hevious mineralogical mineralogical studies studies have have shown shown that that the the main main FeFeoxide oxide precipitated precipitated by by these these bacteria bacteria is is ferrihydrite ferrihydrite with with MnMnoxide (Chapnick et al., 1982; Nealson. 19S3)- The ability ofsome some microuganisms microuganisms to to oxidize oxidize dissolved dissolved Mn Mn (mainly (mainly fromfromMn2 Mn2 to to less less soluble soluble Mn3 Mn3 and and Mn4; Mn4; is is well well docunenteddocunented(lvlann arxl Quarsel, 1946). However, at high altitudes, themicrobial microbial poprlations poprlations in in the the Venezuelan Venezuelan Andes Andes may may bebeimpoverishe{ impoverishe{ as as was was demonsbated demonsbated at at high high altitudes altitudes ononMount Kenya by ltfahaney (1990). If this is the case in theAndean Mollisols, m reported by Milner et al. (1996), themicrobial microbial hypothesis hypothesis of of Mn-Fe Mn-Fe nodule nodule genesis genesis would would notnotapply.apply.

5 5 -3 -3 - - Bog Bog iron iron Placoliths Placoliths and and cul cul dehydmtion--hypottusis dehydmtion--hypottusis 33

This This hypothesis hypothesis postulates postulates in in siru siru development development of of alpinealpinesoils soils on on flat flat crests crests and and shoulders shoulders in in Mid Mid to to Early Early QuaternaryQuaternarypaludic environrnents frorn Fe-Mn-bearing argillaceousmetasediments, metasediments, as as well well as as maturation maturation to to red red perrneable perrneable soilsoilwith in sina growth of concentric-shelled Mn-Fe nodulesduring during alternating alternating wet wet and and dty dty alpine alpine climatic climatic cycles.cycles.

The The supply supply of of Mn Mn is is critical critical to to this this model model and and explainedexplainedby by the the presence presence of of Mn Mn garrrct garrrct (and (and possible possible siderite siderite * * MnMncarbonate in frresh rock). Analogous situations in coticulehorizons horizons in in slate slate belts belts of of subhumid subhumid temperato temperato climatesclimatesmincide with growth of Mn-Fe-Si nodules (Schiller andTaylor. 1965; Beals, 1966), albeit witb nnicrobial mediationin very humid environments (Ghiorse, 1980, 1984; Ghiorseand Hirsch. 1982; Chapnick et al-, i982). The primary Mnfs fs the the Mn-Fe Mn-Fe garnets garnets observed observed in in the the heavy heavy mineral mineral suitesuiteextract€d extract€d from from the the soil soil presrmably presrmably rnanifests rnanifests itself itself at at lolerlolermetamorphic metamorphic grades grades as as siderite siderite and and rhodochrosite, rhodochrosite, ufrichufrichwould would weather weather more more readily readily than than the the garnet garnet to to yield yield Mn Mn andandFe Fe for for the the soil soil processes processes (Golden (Golden et et al.. al.. 1992).1992).

The The pneserce pneserce of of a a thick thick Ah Ah horizon horizon complex complex over over a a well-well-leachod leachod and and partially partially redrced redrced Bt Bt horizon horizon and and terminatingterminatingdowuward toward an impermeable bedrock allows acomrnon comrnon hydrogeochemical hydrogeochemical process process to to provide provide altematingaltematingw€tting w€tting and and drying drying over over long long periods periods of of tirrrc tirrrc (Tilsley,(Tilsley,

L977\- L977\- Relict Relict paleoaols paleoaols of of this this type type show show evidence evidence ofofalternating alternating anaerobic anaerobic to to aerobic aerobic conditions, conditions, as as reflected reflected ininthe the genesis genesis of of concentric concentric growth growth shells shells and and the the orientationorientationof of the the 'dripstone' 'dripstone'

axis, axis, or or progressive progressive drying drying out out of of thethesurface surface horizons, horizons, which which resulted resulted in in increased increased aeration aeration and and aachange from ferrous Fe to a reddish or brown (5YR 414,416)ferric state. This condition helps destroy all evidence of a

furner furner perched perched water water table, table, as as well well as as ttle ttle placic placic horizonhorizonitseH. itseH. Tbe Tbe abundance abundance of of nodules nodules represents represents an an accumulationaccumulationof Mn of up to 4Vo in some cases. The MPOSJA profile,though though currently currently d4r, d4r, was was probably probably water water soaked soaked and and unableunable

to to accornrnodate accornrnodate increased increased soil soil moisture moisture at at some some times times in in fteftepast, past, which which pro&ced pro&ced the the anaerobic anaerobic conditions conditions necessa4r necessa4r forforMn-Fe nodules to form-

The The nodules nodules may may have have $own $own dwing dwing wet wet episodes episodes andandremained remained quiescent quiescent dning dning dry dry ones. ones. The The surface, surface, mganic-mganic-rich rich horizons horizons may may act act as as Fe Fe and and Mn Mn collectors, collectors, urhich urhich wouldwouldenable enable ttre ttre nodules nodules to to form, form, possibly possibly assisted assisted by by microbialmicrobialprocesses, processes, though, though, as as already already noted, noted, bacteria bacteria are are not not presentpresent

in the noduls (lvfilner et al., 1996)- Alternatively, bogmarcasite marcasite spheres spheres of of approxirnately approxirnately 50 50 pm pm diameter diameter arearecharacteristic characteristic of of reducing reducing acid acid conditions conditions and and would would act act asasgrowth growth centers centers in in oxidizing oxidizing conditions. conditions. Such Such flat flat to to slightlyslightlyundilating undilating interfluves, interfluves, on on which which the the profiles profiles are are frmed" frmed" andand

the the nahrre nahrre of of the the parent parent rock rock provide provide ideal ideal settings settings in in uAichuAichto Fduce Mn-Fe nodules in Mollisols because of thecollected rnoisturc, reducing conditions, and build up of Ahorizons-horizons-

A A problem problem wittr wittr this this hypottrcsis hypottrcsis is is the the amount amount of of metalmetalrequired; required; in in addition, addition, a a hydromorphic hydromorphic supply supply upslope upslope of of thethesite site of of deposition deposition is is probably probably rcquired.rcquired.

For For the the ftrmation ftrmation of of soil soil illfn illfn nodules, nodules, Mn Mn must must bebepresent present in in the the rock, rock, and and organic organic matter matter rnust rnust be be abundant.abundant.Robbins et al. (199?) found that Mr nodrles correlate withhigh high organic organic C C contents contents in in soils soils formed formed in in many many differentdifferentand high latitude environments, most commonly inGleysols, Luvisols, and Podsols (Orel'skaya, t974; Arshadand and St St Arnaud, Arnaud, 1980). 1980). However, However, other other reports reports noention noention aanodrle nodrle population population much much lower lower Sran Sran those those reeffied reeffied here. here. AtAtfte fte MPOSJA MPOSJA site, site, nodules nodules make make up up rnore rnore than than 507o 507o of of thethematerial material in in tb tb paleosol paleosol epipedon, epipedon, a a phenomenally phenomenally highhighpopulation population of of a a sediment sediment species.species.

Robbins et ai. (1992) also showed ttrat ttre fluctuation of{he {he water water table table is is an an iaputant iaputant facts facts in in the the growth growth ofofnodtles. nodtles. In In the the MPOSJA MPOSJA profile, profile, redrrcing redrrcing conditions conditions do do notnotexist exist currently, currently, but but during during Qraternary Qraternary glaciations, glaciations, thetheperiglacial zone reached ?A$ma.s.l. (Schutrerr, 1974) inthe the soutbast soutbast Cmdillera Cmdillera de de Mffda" Mffda" l-ower l-ower temperaturcstemperaturcscoupled coupled witb witb lowerevapotranspiration lowerevapotranspiration and and higher higher soil soil nraternratercould could have have produced produced a a grcater grcater arnount arnount of of organic organic matter, matter, asascan can be be obssved obssved in in the the p6ramo p6ramo zone zone (Mahaney (Mahaney and and Kalm,Kalm,1996). 1996). There There is is also also a a cloee cloee correlation correlation with with Mqrnt Mqrnt Kenya Kenya inineast€rn east€rn Africa, Africa, wh€re wh€re glacially glacially forced forced lower lower timberlinetimberlineproduced produced thich thich mganic-rich mganic-rich prcfiles prcfiles formed formed under under Afroal-Afroal-pine vegetation as low as 2500 m a.s.l. (ivlahaney, 1990)-

6. 6. C-odusioosC-odusioos

Mn-Fe nodules in high altitude Mollisols in theVenezuelan Venezuelan Andcs Andcs are are inteqp'reted inteqp'reted to to be be relict relict featuresfeatures

inherited inherited from from wet wet paleoclimates. paleoclimates. Relatively Relatively dry dry today today andandwith with spane spane vegetation, vegetation, the the thick thick Ah Ah hmizons hmizons (60cm) (60cm) andand

s39

prevalent prevalent Mn-Fe Mn-Fe nodules nodules in in the the lower lower Ah Ah horizons horizons supportsupporta a hypothesis hypothesis of of water-saturated water-saturated soils soils at at some some time time in in thethepast. past. The The development development of of anaerobic anaerobic reducing reducing conditions,conditions,with with water water above above bedrock bedrock on on an an undulating undulating interfluve,interfluve,provides provides the the paleohydrogeochemical paleohydrogeochemical setting setting for for the the genesisgenesis

of Mn-Fe nodules.

The hypothesis that these paleosols are paleo-Oxisols

fmmed fmmed in in a a tropical tropical forest forest at at considerably considerably lower lower elevationselevationspnor pnor to to uptft uptft is is not not zubstantiated zubstantiated by by the the analysis- analysis- TheThepaleosol paleosol is is a a Typic Typic or or Aridic Aridic Argiustoll, Argiustoll, one one of of a a group group ofofgrassland soils with a thick mollic epipedon, argillicsubsurface subsurface horizon horizon formed formed at at times times of of free free drainage, drainage, andandMn-Fe Mn-Fe nodrles nodrles tht tht dercloped dercloped during during tirnes tirnes of of saturation-saturation-The The geochemistry geochemistry of of this this paleosol paleosol indicates indicates higher higher Mn Mn andandslightly slightly higher higher Fe Fe in in the the Ah2 Ah2 and and Ah3 Ah3 horizons, horizons, where where thethenodules nodules were were obseryed obseryed The The nodules nodules are are interpreted interpreted to to havehavegrown grown very very slowly slowly through through rnany rnany periods periods of of waterwatersaturation. saturation. The The high high prodrrction prodrrction of of fenihy&ite fenihy&ite (Feo (Feo content)content)in in the the Ah2 Ah2 horizon horizon supports supports a a model model of of periodic periodic fluctuationsfluctuationsof of the the water water table table at at various various times times to to a a level level that that nearlynearlysaturat€d saturat€d the the entire entire profile. profile. Likewise, Likewise, the the high high Fe6/Fe,Fe6/Fe,indicates indicates that that nearly nearly all all Fe Fe has has been been convenpdto convenpdto free free forms,forms,which which trgues trgues for for an an Early Early Quatenary Quatenary or or possibly possibly older older age-age-

AcknowledgmentsAcknowledgments

The The field field work work was was zupportedby zupportedby a a grant grant ro ro MB MB from from thetheAssociaci6n Associaci6n Venezolana Venezolana de de Estudioa Estudioa Canadienses Canadienses and and thetheUniversidad Universidad Pedag6gica Pedag6gica Experirnental Experirnental Libertador. Libertador. We We arcarcindeh€d indeh€d to to the the Natural Natural Sciences Sciences and and Engineering Engineering ResearchResearchC-,otntcil C-,otntcil of of C-anada C-anada for for an an Infrastnrctrne Infrastnrctrne Grant Grant to to thetheSLOWPOKE SLOWPOKE Reactm Reactm Facility. Facility. WCM's WCM's research research is is fun&dfun&dby by a a minor minor research research grant grant from from York York University University and and gantsgantsfrom from Quaternary Quaternary Surveys- Surveys- Caitlin Caitlin Mahaney Mahaney assisted assisted with with ttrettrelabmatory labmatory analyses analyses in in the the Gmmorphology Gmmorphology and and PedologyPedologyI"aboratory I"aboratory at at York York University. University. The The pryer pryer benefited benefited fromfromcritical critical reviews reviews by by Ron Ron Dorn Dorn (Arizona (Arizona State State University),University),Peter Peter Birkeland Birkeland (University (University of of Colorado), Colorado), R-M. R-M. JoeckelJoeckel(Bellevue (Bellevue University), University), Robert Robert Giegengack Giegengack (University (University ofofPennsylvania), Pennsylvania), Jorge Jorge Rabassa Rabassa (Universidad (Universidad Nacional Nacional deldelComahue, Comahue, Argentina), Argentina), and and Franck Franck Audemard Audemard (FUIr[V$IS,(FUIr[V$IS,

Caracas)- Caracas)- John John Dawson Dawson (ITC, (ITC, York York University) University) prcpared prcpared thetheimager]' imager]' and and illustnations-illustnations-

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