Morphology and distribution of fossil soils in the Permo

Morphology and distribution of fossil soils in thePermo-Pennsylvanian Wichita and Bowie Groups, north-centralTexas, USA: implications for western equatorial Pangeanpalaeoclimate during icehouse–greenhouse transition

NEIL JOHN TABOR* and ISABEL P. MONTANEZ�*Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USA(E-mail: [email protected])�Department of Geology, University of California, Davis, CA 95616, USA

ABSTRACT

Analysis of stacked Permo-Pennsylvanian palaeosols from north-central Texas

documents the influence of palaeolandscape position on pedogenesis in

aggradational depositional settings. Palaeosols of the Eastern shelf of the

Midland basin exhibit stratigraphic trends in the distribution of soil horizons,

structure, rooting density, clay mineralogy and colour that record long-term

changes in soil-forming conditions driven by both local processes and

regional climate. Palaeosols similar to modern histosols, ultisols, vertisols,

inceptisols and entisols, all bearing morphological, mineralogical and

chemical characteristics consistent with a tropical, humid climate, represent

the Late Pennsylvanian suite of palaeosol orders. Palaeosols similar to modern

alfisols, vertisols, inceptisols, aridisols and entisols preserve characteristics

indicative of a drier and seasonal tropical climate throughout the Lower

Permian strata. The changes in palaeosol morphology are interpreted as being

a result of an overall climatic trend from relatively humid and tropical, moist

conditions characterized by high rainfall in the Late Pennsylvanian to

progressively drier, semi-arid to arid tropical climate characterized by

seasonal rainfall in Early Permian time. Based on known Late Palaeozoic

palaeogeography and current hypotheses for atmospheric circulation over

western equatorial Pangea, the Pennsylvanian palaeosols in this study may be

recording a climate that is the result of an orographic control over regional-

scale atmospheric circulation. The trend towards a drier climate interpreted

from the Permian palaeosols may be recording the breakdown of this pre-

existing orographic effect and the onset of a monsoonal atmospheric

circulation system over this region.

Keywords Late Palaeozoic palaeoclimate, palaeosols, Permo-Carboniferous.

INTRODUCTION

Outcrop and numerically based modelling stud-ies of the Late Palaeozoic geological record indi-cate that Earth’s climate system underwentsignificant evolutionary change during this per-iod. It is generally believed that climate changeoccurred in response to the formation of thesupercontinent Pangea, an accompanying trans-

ition from icehouse to greenhouse states and aprobable global-scale reorganization of atmo-spheric circulation systems (Rowley et al., 1985;Cecil, 1990; Parrish, 1993; West et al., 1997; Gibbset al., 2002; and references therein). This plethoraof studies indicates that the mid-latitude andequatorial regions of Pangea became progres-sively more arid, with more seasonal preci-pitation, throughout the Late Palaeozoic,

Sedimentology (2004) 51, 851–884 doi: 10.1111/j.1365-3091.2004.00655.x

� 2004 International Association of Sedimentologists 851

strongly supporting the development of northernhemisphere monsoonal circulation during thisinterval. Furthermore, it has been hypothesizedthat monsoonal circulation began in the LateCarboniferous and reached its peak in the MiddleTriassic based primarily upon lithological andpalaeontological studies of North American andEuropean terrestrial strata (Parrish, 1993).

Geological proxy records (e.g. coals, evaporites,aeolianites, tillites) and numerical models canprovide an excellent framework for the long-termdevelopment of climate systems over Pangea. Fewdetailed records exist, however, that offer insightinto higher resolution spatial and temporal varia-tions in climate regimes that accompanied theevolution of Pangean monsoonal circulation.Significantly, recent studies of terrestrial depositsand their associated palaeosols in the south-westUSA suggest that monsoonal circulation was well-established over western equatorial Pangeaby earliest Permian time (Kessler et al., 2001;Soreghan G.S. et al., 2002; Soreghan M.J. et al.,2002; Tabor & Montanez, 2002). These studies,coupled with continuing discrepancies inmodel-data comparisons for low-latitude Pangea(Patzkowsky et al., 1991; Rees et al., 2001; Gibbset al., 2002), highlight the necessity of understand-ing the evolution of specific Pangean climatezones.

The palaeosol-rich, Permo-Pennsylvanian ter-restrial strata of north-central Texas hold thepotential to provide insight into climate changeover low-latitude Pangea, given that climatevariation would have been most pronounced inwestern equatorial Pangea because of the confi-guration of the supercontinent and its attendanteffects on atmospheric circulation and precipita-tion patterns (Dubiel et al., 1991; Parrish, 1993).Furthermore, rich faunal and floral collectionsand interstratified regionally correlatable fluvialsandstone bodies and fusulinid-bearing lime-stones provide a chronostratigraphic frameworkin which to define spatial and temporal trends inpalaeosol morphology, mineralogy and geochem-istry and the degree of palaeopedogenic devel-opment. This study evaluates the influence oflocal- to regional-scale autogenic and larger scaleallogenic processes on the stratigraphic distribu-tion of palaeosol morphologies and compositionsthrough detailed field, petrographic, mineralogi-cal and geochemical analyses of � 200 Permo-Pennsylvanian palaeosols. These palaeosolsprovide a detailed data set for comparison withpreviously published palaeoclimate trends basedon geological data and numerical models. The

results of this study help to refine the nature andtiming of climate change in western equatorialPangea during the early stages of supercontinentformation, major change in global atmosphericcirculation and the onset of deglaciation.

GEOLOGICAL SETTING

The study area in north-central Texas (Fig. 1) waslocated in the western coastal zone of equatorialPangea during the Late Palaeozoic (Ziegler et al.,1997)1 and remained between 0 and 5�N of theequator throughout this time period (Golonkaet al., 1994; Scotese, 1999; Loope et al., 2004).Major tectonic elements such as the MuensterArch and the Wichita, Arbuckle and OuachitaMountains (Fig. 1) had developed by latestPennsylvanian time (Oriel et al., 1967), and tec-tonic quiescence has characterized the regionsince (Donovan et al., 2001)2 . Late Pennsylvanian(Virgilian) and Early Permian (Wolfcampian andLeonardian) terrestrial strata of north-centralTexas, along with subordinate marine sediments,were deposited upon the broad, gently slopingEastern shelf of the Midland basin as it subsidedthroughout the Late Palaeozoic (Fig. 1; Brownet al., 1987; Hentz, 1988). A minor component ofdetrital sediments representing the Upper Penn-sylvanian and Lower Permian strata of the EasternMidland basin may have been derived from EarlyPalaeozoic sediments and metamorphic rocks ofthe distal Ouachita and Arbuckle Mountainchains (Hentz, 1988). However, the principalsource of detrital sediments during this timewas probably derived from second-cycle terrigen-ous-clastic sediments from Middle Pennsylva-nian fan-delta and fluvial facies that wereeroded from the proximal Ouachita foldbeltand Muenster Highlands (Fig. 1; Brown, 1973).Although there is no evidence for a major changein the lithology of source lands during Permo-Pennsylvanian deposition, a progressive decreaseupsection in grain size and thickness of fluvialsandstones, coupled with a decrease in strati-graphic frequency of thick, multistorey sand-stones, is interpreted as recording significantdenudation of the source areas by the close ofthe Early Permian (Hentz, 1988).

The primarily terrestrial deposits of the Bowieand Wichita Groups on the north-eastern portionof the Eastern shelf reach a thickness of up to530 m in the east and thin westward towardsmarine-dominated strata (Fig. 2). Three terrestrialfacies are recognized in the Late Pennsylvanian

852 N. J. Tabor & I. P. Montanez

� 2004 International Association of Sedimentologists, Sedimentology, 51, 851–884

and Early Permian terrestrial strata of theEastern Midland basin: (1) sand- and gravel-richchannel-bar facies; (2) point-bar facies; and (3)floodplain facies (Hentz, 1988). Sand and gravel-rich, channel-bar facies were deposited by brai-ded stream systems fringing the north-easternhighlands, whereas the point-bar facies recordpenecontemporaneous meandering stream sys-tems that developed on the Eastern shelf to thesouth and west of the braided stream system.

Alluvial floodplain deposits formed primarily inassociation with meandering streams given theirstratigraphic predominance in the western andsouthern portions of the study area. Based onregional facies distributions, Hentz (1988) definedthree physiographic provinces for the latestPennsylvanian to Early Permian landscape ofthe Eastern shelf: (1) the piedmont provincedominated by sand- and gravel-rich channel-barfacies and, to a lesser degree, point-bar facies; (2)

Fig. 1. Regional distribution of source terranes and physiographic provinces in the latest Pennsylvanian to EarlyPermian, Eastern Shelf of the Midland Basin. Province boundaries are defined for the period of maximum regressionin the earliest Permian. Structural and tectonic elements shown are for the pre-Virgilian palaeogeography of north-central Texas; the Muenster and Ouachita highlands remained significant topographic features in the Early Permian.Encircled numbers delineate locations of palaeosol-bearing stratigraphic sections used in this study. Diagrammodified after Brown et al. (1987) and Hentz (1988).

Permo-Carboniferous palaeoclimate from palaeosols 853


the upper coastal plain province characterized bypoint-bar facies; and (3) the lower coastal plainprovince dominated by floodplain facies and, to alesser degree, point-bar facies.

A high-resolution chronostratigraphic frame-work for Permo-Pennsylvanian terrestrial strataof north-central Texas has been developed

through correlation of multiple fusulinid-bear-ing limestones and intercalated multistorey flu-vial sandstone deposits, which were described(ss units in Fig. 2) and regionally correlated byHentz (1988). The Pennsylvanian–Permianboundary (301 ± 2 Ma; Rasbury et al., 1998)occurs within a few metres of the contact

Fig. 2. Regional correlation of Late Pennsylvanian and Early Permian strata in north-central Texas. Light grey bedsdelineate regionally extensive fluvial sandstone sets; black horizons are coals; white regions represent alluvialmudstones and intercalated siltstones and thin-bedded, fine-grained sandstones; dark grey beds are marine lime-stones. Encircled numbers show the stratigraphic distribution of studied palaeosols – numbers correspond to sites inFig. 1. Diagram modified after Hentz (1988).



between the Markley and Archer City Forma-tions of the Bowie Group (Dunbar & Committeeon Stratigraphy, 1960)3 . The Wolfcampian–Leo-nardian boundary (283 ± 2 Ma; Ross et al.,1994) is defined by the fusulinid-bearing ElkCity Limestone at the base of the PetroliaFormation of the Wichita Group (Fig. 2).

BURIAL HISTORY

The entire stratigraphic thickness of the LatePennsylvanian and Early Permian rocks in thestudy area is � 1250 m, with the lower 1000 mdominated by sedimentary rocks characteristicof terrestrial depositional facies (Hentz, 1988;Nelson et al., 2001; Tabor & Montanez, 2002)4 .Deposition in this area during the Triassic andJurassic was insignificant (Oriel et al., 1967;McGowen et al., 1979). However, Cretaceousstrata were probably deposited in the area, asshown by outliers of Cretaceous marine rocks inthe extreme western portion of the study area,but these probably did not exceed 330 m inthickness (Barnes et al., 1987). Tertiary and earlyQuaternary burial was insignificant in this area(Barnes et al., 1987). Therefore, the maximumburial depth of Virgilian strata in the EasternMidland basin probably did not exceed 1600 min thickness. This shallow burial history isreflected by relatively mild burial temperaturesthat never exceeded 40–45 �C in the EarlyPermian strata along the flanks of the Midlandbasin (Bein & Land, 1983).

METHODS

Thirty-one stratigraphic sections were dug orpicked back 30–60 cm to provide a fresh surfacefor description and sampling. The geographic andstratigraphic positions of these 31 sites are shownin Figs 1 and 2. Palaeosols (n ¼ 190) from the 31stratigraphic sections of the Bowie and WichitaGroups were logged and described in detailwithin the chronostratigraphic and sedimentolog-ical framework defined by Hentz (1988). Palaeo-sol profiles were recognized and described usingestablished criteria (Retallack, 1988; Kraus &Aslan, 1993; Kraus, 1999). Profile tops wereidentified on the basis of a marked change ingrain size and/or colour, as well as preservation ofprimary sedimentary structures. Profile baseswere delineated at the lowest occurrence ofunaltered parent material. Palaeosol and sedi-

ment colours were identified from dry samplesusing Munsell colour charts (Munsell Color,1975). Palaeosol classification primarily followsthe USDA Soil Taxonomy system (Soil SurveyStaff, 1975, 1998).

Palaeosol matrix was sampled at 0Æ1–0Æ2 mvertical spacings; rhizoliths and nodules weresampled where present in the profile. Palaeosolmatrix samples were disaggregated overnight byultrasonic agitation in a dilute Na2CO3 solutionand analysed for grain-size distribution. Percent-age sand, silt and clay was determined bycentrifugation and filtration. Thin sections(n ¼ 78) of representative samples were exam-ined for their micromorphology and mineralogyaccording to the approaches and terminology ofBrewer (1976) and Wright (1990).

X-ray diffraction of the < 2 lm size fraction wascarried out for identification of clay mineralogy.Samples were exchange saturated with K or Mgon filter membranes and transferred to glass slidesas oriented aggregates. A split of the Mg-saturatedclays was also treated with glycerol. Orientedaggregates of all Mg-treated samples were ana-lysed at 25 �C. K-treated samples were analysed at25 �C, 300 �C and 500 �C after 2 h of initialheating at their respective temperatures. Stepscan analyses were performed on a Diano 8500X-ray diffractometer with CuKa radiation be-tween 2 and 30�2h with a step size of 0Æ04�2hand a 1 s count time.

PALAEOSOL TYPES ANDPALAEOENVIRONMENTALIMPLICATIONS

Field-scale indicators of pedogenesis in the stud-ied Permo-Pennsylvanian terrestrial strata includecolour, mottling, structure, slickensides, accumu-lation of oxides, clays and carbonate and fossilroot traces. Palaeosol profiles were subdividedinto horizons on the basis of downprofile changesin macro- and micromorphological features, claymineralogy and abundance and geochemicalcharacteristics. Eight major pedotypes (sensuRetallack, 1994; Table 1; Figs 3 and 4) are recog-nized based on distinctive characteristics thatinclude the aforementioned macromorphologicalfeatures as well as profile thickness, degree ofdevelopment (maturity), relationship to strati-graphically adjacent palaeosols (i.e. composite,compound or cumulate palaeosols; sensu Marriott& Wright, 1993; Kraus, 1999) and clay mineral-ogy. Palaeosols assigned to each pedotype are



Table

1.

Morp

holo

gy,

min

era

logy

an

dch

em

ical

ind

ices

of

rep

rese

nta

tive

pro

file

sof

ped

oty

pes.

Ped

oty

pe

an

dst

rati

gra

ph

icoccu

rren

ce

Hori

zon

dep

th(m

)*M

acro

morp

holo

gy

Typ

ep

rofi

levari

ati

on

Nod

ule

an

drh

izoli

thm

inera

logy�

£2

lmC

lay

min

era

logy�

Pro

vin

ce

A:

His

toso

lM

ark

ley

Fm

.O

i(0

–0Æ2

0)

Dark

gre

yto

bla

ck,

mass

ive

an

dorg

an

ic-r

ich

cla

yst

on

e.

Wh

ole

Neu

rop

teri

sle

aves.

Non

-calc

are

ou

s.

Org

an

icla

yers

vari

able

thic

kn

ess

(3–70

cm

);th

incla

stic

part

ings

com

mon

.R

are

jaro

site

rep

lacem

en

tof

rhiz

o-

lith

s.

––

Low

er

coast

al

Aj

(0Æ2

0–0Æ2

7)

Lig

ht

gre

y(5

Y7/1

)cla

yst

on

ew

ith

fin

egra

nu

lar

stru

ctu

re.

Com

mon

med

ium

tocoars

eyell

ow

(5Y

8/8

)m

ott

les.

Den

sen

etw

ork

of

thin

(<1

cm

),ta

bu

lar

yell

ow

(5Y

8/8

)rh

izoli

ths.

Non

-calc

are

ou

s.

Jaro

site

(40%

)K

,I

(62%

)

C(0Æ2

7–0Æ3

5)

Gre

y(5

Y6/1

),m

ass

ive

silt

ycla

yst

on

e.

Non

-calc

are

ou

s.–

K,

I(5

9%

)

B:

Ult

isol

Mark

ley

Fm

.A

Bt1

(0–0Æ1

5)

Red

(10R

4/4

)cla

yst

on

ew

ith

gra

nu

lar

tosu

ban

gu

lar

blo

cky

stru

ctu

re.

Com

mon

dis

cre

te,

fin

e-s

cale

yell

ow

(5Y

8/6

)verm

icu

lar

mott

les.

Abu

nd

an

tF

e-

(less

soM

n-)

oxid

ecoati

ngs

an

dori

en

ted

cla

ysk

ins

on

matr

ixaggre

-gate

s.N

on

-calc

are

ou

s.

Cla

yaccu

mu

lati

on

inu

pp

er

pro

file

svari

es

from

mod

era

teto

sig-

nifi

can

t.

–K

,I

(67%

)L

ow

er

coast

al

Bt2

(0Æ1

5–0Æ6

0)

Red

(10R

4/3

)cla

yst

on

ew

ith

suban

gu

lar

blo

cky

stru

ctu

re.

Few

dis

cre

te,

fin

e-s

cale

yell

ow

(10Y

R8/8

)su

bsp

her-

ical

mott

les.

Abu

nd

an

tF

e-

(less

soM

n-)

oxid

ecoati

ngs

an

dori

en

ted

cla

ysk

ins.

Non

-calc

are

ou

s.

Haem

ati

te,

goeth

ite

(10%

)K

,I

(78%

)

BC

g(0Æ6

0–1Æ0

2)

Mott

led

gre

y(5

Y6/1

)an

dre

d(1

0R

4/4

)si

lty

cla

yst

on

ew

ith

suban

gu

lar

blo

cky

stru

ctu

re.

Dis

cre

te,

bro

wn

-yell

ow

(10Y

R6/8

)to

red

(5R

4/8

)vert

icall

yori

en

ted

tubu

lar

mott

les

an

dn

od

ule

s.N

on

-calc

are

ou

s.

Haem

ati

te,

goeth

ite

(15%

)K

,I

(42%

)



Table

1.

(Con

tin

ued

).

Ped

oty

pe

an

dst

rati

gra

ph

icoccu

rren

ce

Hori

zon

dep

th(m

)*M

acro

morp

holo

gy

Typ

ep

rofi

levari

ati

on

Nod

ule

an

drh

izoli

thm

inera

logy�

£2

lmC

lay

min

era

logy�

Pro

vin

ce

Cgv

(1Æ0

2–1Æ2

0)

Gre

y(5

/N)

mass

ive

silt

ycla

yst

on

e.

Larg

e(�

10

cm

);h

ori

zon

tal,

red

(5R

4/8

)F

e-

an

dM

n-o

xid

en

od

ule

sth

at

coale

sce

tofo

rma

part

i-all

yin

du

rate

dp

oly

gon

al

netw

ork

inp

lan

vie

w(F

ig.

5B

).N

on

-calc

are

ou

s.

Haem

ati

te(6

0%

)K

,I

(43%

)

C:

Incep

tiso

lM

ark

ley

Fm

.A

g(0

–0Æ2

8)

Oli

ve

gre

y(5

Y5/2

)m

ud

ston

ew

ith

fin

egra

nu

lar

stru

ctu

re.

Abu

nd

an

tfi

ne-

tom

ed

ium

-scale

,su

bsp

heri

cal

wh

ite

(5Y

2Æ5

/1)

mott

les;

fin

e-s

cale

Fe-o

xid

en

od

ule

s.N

on

-calc

are

ou

s.

Vari

able

hori

zon

develo

pm

en

tfr

om

poor

tom

od

era

te.

Ped

stru

ctu

revari

es

from

mod

era

tely

tow

ell

-d

evelo

ped

,an

dm

ed

-iu

mto

coars

ean

gu

lar

an

dw

ed

ge-s

hap

ed

.

Haem

ati

te(1

0%

)K

,I

(22%

)L

ow

er

an

du

pp

er

coast

al;

pie

dm

on

t

Bv

(0Æ2

8–0Æ4

9)

Oli

ve

(5Y

4/4

)m

ud

ston

ew

ith

med

ium

an

gu

lar

blo

cky

stru

ctu

re.

Fin

e-

tom

ed

ium

-sc

ale

,d

usk

yre

d(1

0R

4/4

)h

aem

ati

ten

od

ule

scoale

sced

tofo

rma

sem

i-in

du

rate

dla

yer.

Non

-calc

are

ou

s.

Haem

ati

te(2

5%

)K

,I

(25%

)

BC

ssg(0Æ4

9–1Æ3

1)

Dark

oli

ve

gre

y(5

Y3/2

)m

ud

ston

ew

ith

wed

ge-

tole

nti

cu

lar-

shap

ed

stru

ctu

ral

aggre

gate

sd

efi

ned

by

slic

ken

sid

es.

Com

mon

,m

ed

ium

-to

coars

e-s

cale

yell

ow

(10Y

R7/6

)to

du

sky

red

(10R

4/4

)m

ott

les.

Non

-calc

are

ou

s.

–K

,I

(32%

)

Cg

(131-c

over)

Dark

oli

ve

gre

ym

ass

ive

top

oorl

yla

min

ate

dm

ud

ston

e.

Abu

nd

an

tm

ed

ium

-scale

,sp

heri

cal

tola

min

ar

ligh

tre

dd

ish

bro

wn

(5Y

R6/4

)m

ott

les.

Non

-calc

are

ou

s.

–K

,I

(29%

)



Table

1.

(Con

tin

ued

).

Ped

oty

pe

an

dst

rati

gra

ph

icoccu

rren

ce

Hori

zon

dep

th(m

)*M

acro

morp

holo

gy

Typ

ep

rofi

levari

ati

on

Nod

ule

an

drh

izoli

thm

inera

logy�

£2

lmC

lay

min

era

logy�

Pro

vin

ce

D:

Vert

isol

Mark

ley

Fm

.A

Bw

(0–0Æ1

5)

Pale

red

(5R

4/2

)m

ud

ston

ew

ith

med

ium

rhom

boh

ed

ral

stru

ctu

re;

sup

eri

mp

ose

dfi

ne

gra

nu

lar

stru

ctu

re.A

bu

nd

an

tfi

ne-

tom

ed

ium

-scale

,li

gh

tgre

en

-gre

y(8

/5B

G)

mott

les.

Cla

stic

-fill

ed

dykes

exte

nd

thro

ugh

hori

zon

.N

on

-cal-

care

ou

s.

Haem

ati

ten

od

ule

sm

ay

be

lackin

g.

–I,

I/S

,K

(28%

)U

pp

er

coast

al;

pie

dm

on

t

Bgc

(0Æ1

5–0Æ3

5)

Gre

y(6

/N)

mu

dst

on

ew

ith

med

ium

rhom

boh

ed

ral

stru

ctu

re.C

om

mon

med

ium

-sc

ale

du

sky

red

(10R

3/4

)m

ott

les

an

dh

aem

ati

ten

od

-u

les.

Cla

stic

dykes

exte

nd

tobott

om

of

hori

zon

.N

on

-cal-

care

ou

s.

Haem

ati

te(1

0%

)I/

S,

I,K

(33%

)

Bss

(0Æ3

5–0Æ7

5)

Red

dis

h-b

row

n(5

YR

5/4

)m

ud

ston

ew

ith

wed

ge-

shap

ed

stru

ctu

ral

aggre

gate

sd

efi

ned

by

slic

ken

sid

es.

Com

mon

med

ium

-to

coars

e-

scale

,li

gh

tgre

en

-gre

ym

ot-

tles

(8/5

BG

).N

on

-calc

ar-

eou

s.

–I,

K(3

9%

)

2C

(0Æ7

5–1Æ0

7)

Lig

ht

gre

en

-gre

y(8

/10Y

)p

oorl

yla

min

ate

dm

ud

ston

e.

Non

-calc

are

ou

s.

–I,

K(3

6%

)

E:

En

tiso

lM

ark

ley

Fm

.A

rch

er

Cit

yF

m.

Nocon

aF

m.

Petr

oli

aF

m.

Waggon

er

Ran

ch

Fm

.

AC

(0–0Æ9

1)

Du

sky

red

(7Æ5

R3/2

)m

ass

ive

tow

eakly

bed

ded

fin

e-

gra

ined

mu

dd

ysa

nd

ston

e.

Bra

nch

ing

netw

ork

sof

fin

e-

scale

mott

lin

gan

dth

ick,

vert

ical,

tubu

lar

ligh

tgre

y(7

/N

)m

ott

les.

Non

-calc

are

ou

s.

Vari

able

develo

pm

en

tof

red

oxim

orp

hic

fea-

ture

sin

clu

din

ggle

yed

matr

ix,

mott

lin

gan

d/

or

Fe-o

xid

en

od

ule

s.W

eak

develo

pm

en

tof

slic

ken

sid

es

–K

,I

(31%

)L

ow

er

an

du

pp

er

coast

al;

pie

dm

on

t



Table

1.

(Con

tin

ued

).

Ped

oty

pe

an

dst

rati

gra

ph

icoccu

rren

ce

Hori

zon

dep

th(m

)*M

acro

morp

holo

gy

Typ

ep

rofi

levari

ati

on

Nod

ule

an

drh

izoli

thm

inera

logy�

£2

lmC

lay

min

era

logy�

Pro

vin

ce

Cc

(0Æ9

1–1Æ4

0)

Weak

red

(5R

3/3

)fi

ne-

gra

ined

mu

dd

ysa

nd

ston

e.

Com

mon

,fi

ne

tocoars

ed

us-

ky

red

(7Æ5

R4/3

)m

ott

les

an

dh

aem

ati

ten

od

ule

s.N

on

-cal-

care

ou

s.

Haem

ati

te(5

%)

K,

I(2

9%

)

F:

Alfi

sol

Arc

her

Cit

yF

m.

Nocon

aF

m.

Petr

oli

aF

m.

Waggon

er

Ran

ch

Fm

.

AB

(0–0Æ0

8)

Dark

yell

ow

-bro

wn

(10Y

R6/

4)

mu

dst

on

ew

ith

suban

gu

lar

blo

cky

stru

ctu

re.F

ew

fin

e-

tom

ed

ium

-scale

,p

ale

red

(10Y

R4/4

)verm

icu

lar

mot-

tles.

Non

-calc

are

ou

s.

Vari

able

thic

kn

ess

pro

file

s(0Æ7

mto

2m

).In

som

ety

pe

Fp

ala

eo-

sols

,carb

on

ate

isn

ot

pre

sen

tor

occu

rsover-

lyin

gor

sup

eri

mp

ose

don

cla

y-r

ich

hori

zon

s(i

.e.

poly

gen

eti

cso

ils)

.

–S

,K

(30%

)L

ow

er

an

du

pp

er

coast

al;

pie

d-

mon

t

Bt1

(0Æ0

8–0Æ2

3)

Pale

red

(10R

6/4

)si

lty

cla

y-

ston

ew

ith

med

ium

an

gu

lar

blo

cky

stru

ctu

re.

Com

mon

,m

ed

ium

-to

coars

e-s

cale

very

pale

red

(10R

4/3

)vert

ical

tubu

lar

mott

les.

Abu

nd

an

tcla

ysk

ins

on

ped

surf

aces.

Non

-calc

are

ou

s.

–S

,K

(56%

)

Bt2

(0Æ2

3–0Æ5

3)

Very

pale

red

(10Y

R4/3

)si

lty

cla

yst

on

ew

ith

med

ium

an

gu

lar

blo

cky

stru

ctu

re.

Com

mon

,m

ed

ium

-to

coars

e-s

cale

red

dis

h-y

ell

ow

(7Æ5

YR

6/6

)vert

ical

tubu

lar

mott

les.

Abu

nd

an

tcla

ysk

ins

on

ped

surf

aces.

Non

-calc

ar-

eou

s.

–S

,K

(61%

)

Btk

(0Æ5

3–0Æ7

9)

Bro

wn

-yell

ow

(10Y

R5/3

)si

lty

cla

yst

on

ew

ith

med

ium

an

gu

lar

blo

cky

stru

ctu

re.

Abu

nd

an

tcla

ysk

ins

on

ped

surf

aces.

Abu

nd

an

tst

age

IIcarb

on

ate

nod

ule

s.

Calc

ite

(10%

)S

,K

,I/

S(5

7%

)

BC

ss(0Æ7

9–1Æ0

6)

Red

(7Æ5

R4/6

)m

ud

ston

ew

ith

wed

ge-s

hap

ed

stru

c-

tura

laggre

gate

sd

efi

ned

by

slic

ken

sid

es.

Very

weakly

calc

are

ou

s.

–S

,I/

S,

K(3

3%

)



Table

1.

(Con

tin

ued

).

Ped

oty

pe

an

dst

rati

gra

ph

icoccu

rren

ce

Hori

zon

dep

th(m

)*M

acro

morp

holo

gy

Typ

ep

rofi

levari

ati

on

Nod

ule

an

drh

izoli

thm

inera

logy�

£2

lm

Cla

ym

inera

logy�

Pro

vin

ce

C(1Æ0

6–1Æ7

6)

Dark

yell

ow

ish

-bro

wn

(10Y

R4/4

),m

ass

ive

tow

eakly

lam

inate

dm

ud

ston

e.

Non

-calc

are

ou

s.

–I/

S,

S,

K(3

3%

)

G:

Vert

isol

Arc

her

Cit

yF

m.

Nocon

aF

m.

Petr

oli

aF

m.

Waggon

er

Ran

ch

Fm

.

AB

k(0

–0Æ2

7)

Red

dis

h-b

row

n(2Æ5

YR

4/4

)m

ud

-st

on

ew

ith

med

ium

pri

smati

cst

ructu

re;

sup

eri

mp

ose

dm

ed

ium

an

gu

lar

blo

cky

stru

ctu

re.

Abu

n-

dan

tst

age

IIcarb

on

ate

nod

ule

s(F

ig.

8G

).

Carb

on

ate

devel-

op

men

tla

ckin

gin

som

ety

pe

Gp

ala

eoso

ls.

Calc

ite

(10%

)S

,K

(33%

)L

ow

er

an

du

pp

er

coast

al;

pie

dm

on

t

Bkss

1(0Æ2

7–0Æ5

7)

Red

dis

h-b

row

n(2Æ5

YR

4/4

)m

ud

ston

ew

ith

wed

ge-s

hap

ed

stru

ctu

ral

aggre

gate

sd

efi

ned

by

slic

ken

sid

es.

Tra

ce

red

dis

h-b

row

n(5

YR

5/3

)sp

heri

cal

mott

les.

Cla

stic

dykes

exte

nd

thro

ugh

hori

zon

.A

bu

nd

an

tst

age

IIcarb

on

ate

nod

ule

s.

Calc

ite

(15%

)S

,K

(30%

)

Bkss

2(0Æ5

7–0Æ9

8)

Red

dis

h-b

row

n(2Æ5

YR

4/3

)m

ud

ston

ew

ith

wed

ge-s

hap

ed

stru

ctu

ral

aggre

gate

sd

efi

ned

by

slic

ken

sid

es.

Cla

stic

dykes

exte

nd

thro

ugh

hori

zon

.A

bu

nd

an

tst

age

IIcarb

on

ate

nod

ule

s.

Calc

ite

(10%

)S

,K

(30%

)

BC

ss(0Æ9

8–1Æ4

0)

Red

dis

h-b

row

n(2Æ5

YR

4/3

)m

ud

ston

ew

ith

wed

ge-s

hap

ed

stru

ctu

ral

aggre

gate

sd

efi

ned

by

slic

ken

sid

es.

Few

toabu

nd

an

tm

ed

ium

-scale

,re

dd

ish

-bro

wn

(2Æ5

YR

5/3

)m

ott

les.

Cla

stic

dykes

exte

nd

tobase

.V

ery

weakly

calc

are

ou

s.

–S

,I/

S,

K(2

7%

)

C(1Æ4

0–2Æ1

1)

Du

sky

red

(2Æ5

YR

3/2

)p

oorl

yla

min

ate

dm

ud

ston

e.F

ew

med

ium

gre

y(6

/5B

G)

sph

eri

cal

mott

les.

Very

weakly

calc

are

ou

s.

–S

,I/

S(2

3%

)



Table

1.

(Con

tin

ued

).

Ped

oty

pe

an

dst

rati

gra

ph

icoccu

rren

ce

Hori

zon

dep

th(m

)*M

acro

morp

holo

gy

Typ

ep

rofi

levari

ati

on

Nod

ule

an

drh

izoli

thm

inera

logy�

£2

lmC

lay

min

era

logy�

Pro

vin

ce

H:

Ari

dis

ol

Arc

her

Cit

yF

m.

Nocon

aF

m.

Petr

oli

aF

m.

Waggon

er

Ran

ch

Fm

.

Bkm

(0–0Æ2

2)

Red

dis

h-g

rey

(2Æ5

YR

6/1

)m

ud

ston

e;

stage

III

carb

on

-ate

s.

Degre

eof

carb

on

ate

develo

pm

en

tvari

es;

morp

holo

gie

scan

be

nod

ula

rto

tubif

orm

(e.g

.F

ig.

8E

).M

atr

ixst

ructu

revari

es

be-

tween

pri

smati

c,

an

gu

-la

rblo

cky

an

dsu

ban

gu

lar

blo

cky.

Som

ety

pe

Hp

ala

eo-

sols

inW

aggon

er

Ran

ch

Fm

.h

ave

slic

k-

en

sid

ep

lan

es

fill

ed

by

calc

ium

carb

on

ate

at

dep

th.

Calc

ite

(95%

)I,

KL

ow

er

an

du

pp

er

coast

al;

pie

dm

on

t

Bk1

(0Æ2

2–0Æ3

9)

Yell

ow

(10Y

R7/6

)m

ud

ston

ew

ith

med

ium

suban

gu

lar

blo

cky

stru

ctu

re.

Abu

nd

an

tst

age

IIcarb

on

ate

nod

ule

s.

Calc

ite

(35%

)I,

K(2

2%

)

Bk2

(0Æ3

9–0Æ5

8)

Pale

red

(5R

4/3

)m

ud

ston

ew

ith

med

ium

tocoars

ean

gu

lar

blo

cky

stru

ctu

re.

Abu

nd

an

tst

age

IIcarb

on

ate

nod

ule

s.

Calc

ite

(10%

)I,

I/S

,K

(25%

)

2B

ss(0Æ5

8–0Æ7

6)

Pale

red

(10R

5/3

)sa

nd

ym

ud

ston

ew

ith

wed

ge-

shap

ed

aggre

gate

stru

ctu

red

efi

ned

by

weakly

develo

ped

slic

ken

sid

es.

Calc

are

ou

s.

–I,

I/S

,K

(22%

)

3C

s(0Æ7

6–1Æ3

0)

Red

dis

h-g

rey

(10R

5/1

)sa

n-

dy

mu

dst

on

ew

ith

wed

ge-

shap

ed

aggre

gate

stru

ctu

red

efi

ned

by

weakly

develo

ped

slic

ken

sid

es.

Few

fin

e-

tom

ed

ium

-scale

gre

y-b

row

n(1

0Y

R5/2

)su

bsp

heri

cal

mott

les.

Very

weakly

calc

are

ou

s.

–I,

I/S

,K

(25%

)

4C

(1Æ3

0–1Æ6

0)

Gre

y-b

row

n(1

0Y

R5/2

)m

ass

ive

tow

eakly

bed

ded

fin

e-g

rain

ed

san

dst

on

e.

Mod

era

ted

evelo

pm

en

tof

fin

e-

tom

ed

ium

-scale

yell

ow

-bro

wn

(10Y

R5/6

)verm

icu

lar

mott

les.

Very

weakly

calc

are

ou

s.

––

*D

ow

n-p

rofi

led

ep

thfr

om

the

inte

rpre

ted

pala

eosu

rface.

�P

erc

en

tage

of

the

matr

ixre

pre

sen

ted

by

Fe-o

xid

e,

jaro

site

or

carb

on

ate

nod

ule

san

drh

izoli

ths.

�C

lay

min

era

logy

pre

sen

ted

inre

lati

ve

ord

er

of

abu

nd

an

ce

in<

2lm

size

fracti

on

;n

um

bers

inp

are

nth

ese

sare

perc

en

tage

of

matr

ixre

pre

sen

ted

by

this

size

fracti

on

:K

¼kaoli

nit

e,

I¼

illi

te,

I/S¼

inte

rlayere

dil

lite

–sm

ecti

te,

S¼

smecti

te.

Cla

ys

refe

rred

toin

this

stu

dy

as

illi

teare

form

all

ycla

ssifi

ed

as

mic

a-l

ike

min

era

lsor

seri

cit

e.



interpreted as being the product of similar soil-forming environments in which local and regionalprocesses combined to produce a distinctiveprofile. Pedotypes are thus analogous to a soilseries in a modern landscape (Retallack, 1990;Bestland et al., 1997).

In the following section, the pedotypes aredescribed based, in part, on individual measuredprofiles judged to be most representative of allpalaeosols assigned to a given group. Majorvariations in morphology or composition fromthe type profile are discussed and summarized inTable 1. The pedotypes are related to the USDAsoil taxonomy classification (Soil Survey Staff,1975, 1998), the spatial and stratigraphic distri-bution of the associated palaeosols delineatedand the palaeoenvironmental implicationsassessed.

Type A palaeosols

DescriptionThe type profile of pedotype A is composed ofthree non-calcareous layers that define a thinprofile (0Æ4 m; Figs 3 and 4A, Table 1). Theuppermost dark grey to black, massive layer isorganic rich and dominates the profile (Fig. 5A).The underlying light grey claystone layer exhibitsangular structure, is composed of quartz siltand low cation-exchangeable clays (kaoliniteand subordinate amounts of illite; Table 1) andlacks labile minerals (biotite, muscovite, feld-spars) present in the unaltered parent material.The basal grey claystone layer contains asmall amount of labile minerals and lacks clearpedogenic features. Redoximorphic features(e.g. Vepraskas, 1994) are abundant in type A

Fig. 3. Key of symbols and horizon nomenclature used in Figs 4, 7 and 9–13.



palaeosols and include overall low chroma matrixcolours (gleying), tabular networks of downward-bifurcating rhizoliths and medium to coarseyellow mottling. Rhizoliths in the type palaeosolare replaced by jarosite.

Type A palaeosols are developed within siltymudstones of the floodplain facies; all type Apalaeosols are laterally discontinuous over tens tohundreds of metres (e.g. see sections 1 and 2 inFig. 6). These palaeosols occur as thin (averaging< 0Æ5 m) composite profiles (sensu Marriott &Wright, 1993; Kraus, 1999) with common thinclastic partings. They are stratigraphically restric-ted to sites 1–3 in the lower half of the UpperPennsylvanian Markley Formation (below ss11 inFig. 2).

Interpretation and classificationLaterally discontinuous, organic matter-richlayers in type A palaeosols are interpreted to besurficial accumulations (O horizons) that formedby in situ accumulation of plant material (Figs 4Aand 5A). Angular blocky structure in the under-lying claystone layer is interpreted as being pedstructure that formed by differential shearing ofplastic materials (clays) in the profile. The gleyedmatrix, lack of weatherable labile minerals, loworganic content and abundance of kaolinite in the£2 lm size fraction suggest that the claystonesdirectly underlying O horizons formed as eluvialA horizons in response to intense hydrolysis oracidolysis in poorly drained profiles (Fig. 4A; cf.van Breeman & Harmsen, 1975)5 .

Fig. 4. Measured sections of palaeosol profiles representative of each of the eight pedotypes. (A) Type A palaeosolfrom the Late Pennsylvanian Markley Fm., site 1 (Fig. 1). (B) Type B palaeosol, Markley Fm., site 1. (C) Type Cpalaeosol, Markley Fm., site 4. (D) Type D palaeosol, Markley Fm., site 5. (E) Type E palaeosol, Markley Fm., site 6.(F) Type F palaeosol from the Archer City Fm., site 11. (G) Type G palaeosol from the Waggoner Ranch Fm., site 26.(H) Type H palaeosol, Waggoner Ranch Fm., site 28.



Type A palaeosols are interpreted as beinghistosols (Soil Survey Staff, 1998) given therelative thickness and concentration of organicmatter in the O horizons of the palaeosol profiles(Fig. 4A). Modern histosols form in low-lying,waterlogged regions characterized by always wet

conditions and anoxic pore waters that promotein situ accumulation of organic matter. Thepredominance of gley matrix colours and lowchroma mottles and root traces in type A palae-osols, as well as the paucity of discrete Fe-oxidenodules, supports prolonged saturation of these

Fig. 5. Macromorphologies of Permo-Pennsylvanian palaeosols. (A) Couplets of dark–light banding are type Apalaeosols; dark layers are O horizons, light layers are A horizons. Approximately 1Æ7 m thick dark horizon at base ofoutcrop is a type B palaeosol. Palaeosols are from the Upper Pennsylvanian Markley Fm., site 2. (B) Haematite redoxconcentrations, oriented into a polygonal pattern (1Æ5 m across), in the Cgv horizon of a type B palaeosol, MarkleyFm., site 1. (C) Slickensides in a type G palaeosol, Lower Permian Nocona Fm., locality 15. Knife (0Æ25 m long) forscale; blade points in the direction of mineral orientation along the slickensurface. (D) Type G palaeosol showingpedogenic slickensides in cross-section, Lower Permian Waggoner Ranch Fm., site 26. White ‘blebs’ are pedogeniccarbonate nodules. Field of view is 1Æ5 m. (E) Calcareous rhizoliths from a type H palaeosol, Waggoner Ranch Fm.,site 31. 35 mm lens cap for scale. (F) Type G palaeosol, upper Waggoner Ranch Fm., site 31. Overlying light-colouredbeds are marine calcareous silts and limestones of the Maybelle Limestone.



profiles (Simonson & Boersma, 1972; Wildinget al., 1983; Schwertmann, 1985). The shallowand tabular distribution of jarosite-replaced rootsin fossil A horizons further reflects the poorlydrained palaeosoil conditions (cf. Retallack, 1988,1990). Jarosite, however, may represent post-pedogenic oxidation of Fe-sulphides despite itsneomorphic replacement of roots (cf. Kraus,1998).

Late Pennsylvanian type A palaeosols areinterpreted as having formed in poorly drained,shallow depressions on the regionally extensivecoastal plain given their morphological charac-teristics and laterally discontinuous nature. Thepredominance of stacked, thin, composite profiles(Fig. 6) reflects slow, episodic sediment accumu-lation in geomorphic lows on the latest Pennsyl-vanian coastal plain of western equatorial Pangea.

Fig. 6. Palaeosol-bearing sections ofUpper Pennsylvanian Markley For-mation. Palaeosol tops are delinea-ted by wavy ‘lines’ to the left of eachmeasured section. Capital lettersrefer to palaeosols of a given pedo-type; circles delineate pedotypeprofiles discussed in the text. Sec-tions 1 and 2 are from two outcropsseparated by 8Æ5 km; correlation be-tween outcrops based on sandstoneset 10 (ss10) in the middle of theMarkley Fm. Measured section 4underlies sandstone set 12 (ss12) inthe upper portion of the MarkleyFm. Measured section 5 underliesss14 from the uppermost MarkleyFm., proximal to the Pennsylva-nian–Permian boundary. See Fig. 2for stratigraphic position of sand-stone sets.



Type B palaeosols

DescriptionThe type profile of pedotype B (Figs 4B and 6) iscomposed of four distinct units: (1 and 2) twoupper, red claystones that exhibit fine angular tosubangular blocky structure and oriented clayand Fe-oxide coatings along matrix aggregatesand lining channel voids; (3) an underlying,mottled red/grey silty claystone that exhibitssubangular blocky structure; and (4) a basal,

massive grey silty claystone (Fig. 4B; Table 1).Redoximorphic features are present throughoutthe profile as redox depletions (i.e. low chromamottles and matrix colours) and as redox concen-trations (i.e. Fe- and, less commonly, Mn-oxidecoatings on matrix aggregates and Fe-oxide nod-ules). Oblate Fe-oxide nodules, ranging from 0Æ05to 0Æ2 m in length, are laterally oriented and forma partially indurated horizon at the base of thetype profile (Fig. 5B). Type B palaeosols with lesswell-differentiated horizons commonly lack thickaccumulations of Fe-oxides.

Kaolinite and trace amounts of illite dominate the£2 lm size fraction of all claystones; labile miner-als are generally lacking (Fig. 7A). The abundanceof the clay-dominated, £2 lm size fraction decrea-ses downprofile (e.g. from 78% to 43%; Table 1).

Type B palaeosols occur primarily as thin tomoderately thick (< 1Æ5 m), strongly developedprofiles within mudstones of the floodplain facies(Figs 5A and 6). Type B palaeosols are strati-graphically associated with type A palaeosols.Type B palaeosols are primarily developed uponmudstones and are stratigraphically limited to thelower half of the Upper Pennsylvanian MarkleyFormation (below ss11 in Fig. 2).

Interpretation and classificationRedoximorphic colouration and the abundance ofmoderate to large Fe-oxide nodules in the lowerportion of profiles (BCg and Cgv horizons inFig. 4B) indicate that type B palaeosols werewaterlogged for prolonged periods (i.e. hydro-morphic soils), given that the degree of chemicalweathering of parent material and the size andabundance of Fe-oxide nodules commonly in-crease with increased duration of waterlogging(Simonson & Boersma, 1972; Sobecki & Wilding,1983; Retallack, 1990). However, the predomin-ance of kaolinite in the clay fraction indicates thatthese hydromorphic soils were characterized byepisodic or seasonal changes in soil moistureconditions (Wilding et al., 1983; Schwertmann,1985; Dixon & Skinner, 1992).

Clay-rich horizons in type B palaeosols areinterpreted as being zones of clay accumulationor argillic horizons (ABt1 and Bt2 in Fig. 4B)that formed by downward translocation of clayminerals (i.e. illuviation). Concentration of clayminerals, in particular kaolinite, in the inferredBt horizon and the presence of continuous, orien-ted clay coatings (argillans of Brewer, 1976) onpeds and lining channel voids suggest extensivepedogenic kaolinite formation. The presence ofFe-oxide (and less commonly Mn-oxide;

Fig. 7. XRD patterns of clay minerals from the £ 2 lmsize fraction in select type palaeosols. Clay mineralogyof: (A) a Bt horizon; type B palaeosol, Upper Pennsyl-vanian Markley Fm.; (B) Bt horizon, type G palaeosol,Lower Permian (lower Leonardian) Petrolia Fm.; (C) Bkhorizon, type H palaeosol, Lower Permian (mid-Leonardian) Waggoner Ranch Fm. K ¼ kaolinite;I ¼ illite; S ¼ smectite; C ¼ hydroxy-interlayeredminerals; Q ¼ quartz.



ferri-mangans of Brewer, 1976) lining peds andvoids, in addition to the downprofile increase inFe-oxide nodules, records downward transloca-tion of Fe and Mn in addition to clay minerals.Translocation and mineral accumulation also indi-cate that type B hydromorphic palaeosols under-went periods of free drainage (Birkeland, 1999).

The haematite- and goethite-cemented layers atthe base of type B palaeosols are analogous tomodern plinthites that form in soil horizonscharacterized by a seasonally fluctuating watertable (Duchaufour, 1982; Soil Survey Staff, 1998).The thickness of the palaeoplinthite horizon(� 1 m) and its depth below the palaeosol surfacesuggest that the local palaeowater table probablyfluctuated within 1–2 m of the land surface(cf. Duchaufour, 1982).

Type B palaeosols are interpreted as ultisolsbased on morphological evidence for argillic hori-zons, prolonged saturation and extensive hydro-lysis interrupted by episodes of free drainage.Modern ultisols form in warm, humid environ-ments (e.g. modern tropical rainforests) character-ized by high rates of chemical weathering. Thesesoils minimally require periodic drainage inorder to form argillic horizons through illuviation(Retallack & German-Heins, 1994; Gill & Yemane,1996). Although Late Pennsylvanian ultisolsformed penecontemporaneously with type A pala-eosols, ultisols are interpreted as having formed onstable landscapes such as interchannel highs thatmay have developed during minor episodes offluvial downcutting. This interpretation is basedon evidence for their strong pedogenic develop-ment and for their having been at least episodicallywell-drained in the upper argillic horizons ofthe profile (cf. Markewich & Pavich, 1991;Bestland et al., 1997), and on the inferred lowsedimentation rates recorded by their thin,composite profiles (cf. Marriott & Wright, 1993).

Type C palaeosols

DescriptionThe type profile of pedotype C consists of fourpoorly to moderately developed horizons (Table 1)that exhibit fine to medium angular blocky struc-ture. Larger scale wedge- to lenticular-shapedstructural units are defined by randomly orientedslickensides that overprint the finer scale angularblocky structure in the lower mudstone (BCssg inFig. 4C). Although all type C palaeosols exhibitsome structure that is indicative of pedogenicdevel-opment, most profiles only have weak horizonation.Abundant redoximorphic concentrations (yellow

to reddish-brown mottles and Fe-oxide nodules)and reductions (gleyed matrix colours) occurthroughout type C palaeosols (Fig. 8A). A semi-indurated layer of coalesced submillimetre-sizedhaematite nodules (Fig. 8D) occurs near (28–49 cmdepth) the interpreted palaeosurface of the typeprofile (Fig. 4C); analogous Fe-oxide layers arelacking in some type C palaeosols. The < 2 lm sizefraction contains a moderate amount of clayminerals (22–32%) that are dominated by illiteand subordinate amounts of kaolinite and hydro-xy-interlayered mineral (HIM) clays.

Type C palaeosols occur as thick compoundand composite profiles (Fig. 6) developed inintercalated thinly bedded mudstones, siltstonesand fine-grained sandstones. They are strati-graphically restricted to overbank mudstonedeposits associated with channel and crevasse-splay sandstone deposits in the Upper Pennsyl-vanian Markley Formation above fluviogeniccycle ss11 (sites 4 and 5; Figs 2 and 6).

Interpretation and classificationThe laterally continuous, semi-indurated Fe-oxidelayer in the upper part of the type profile isinterpreted as being a palaeoplinthite (Bv horizonin Fig. 4C). The development of a plinthite proxi-mal to the palaeosol surface records a shallow,fluctuating palaeogroundwater table (Table 1; cf.Duchaufour, 1982). Further evidence for fluctu-ating soil moisture conditions is the occurrenceof large-scale wedge- to lenticular-shaped pedsdefined by slickensides in basal mudstones.Pedogenic slickensides form in clay-rich modernsoils as a result of soil mass movement and struc-tural modification driven by wetting and dryingcycles (Dudal & Eswaran, 1988; Wilding & Tessier,1988). Limited development of large-scale pedsand slickensides in type C palaeosols, however,suggests modest fluctuations in soil moisturecontent in the lower portions of the profile and/or profile immaturity (Brewer, 1976; Jim, 1990),given that these features form rapidly in soilscharacterized by repeated wetting and drying(Yaalon & Kalmar, 1978; Birkeland, 1999).

Type C palaeosols are classified as inceptisolsgiven their moderate development of ped structureand horizonation. These probably immature pro-files formed in floodplain deposits proximal tofluvial channels, as evidenced by their close lateraland stratigraphic proximity to channel-filling andcrevasse-splay sandstones; their compound andcumulate profiles record steady, and sometimesrapid, sedimentation in these environments (cf.Marriott & Wright, 1993; Kraus, 1999).



Type D palaeosols

DescriptionThe type profile of pedotype D is composed offour red to grey mudstone units that exhibitabundant redoximorphic features including small(� 10 mm) haematite nodules, manganese oxidecoatings and low chroma soil matrix (or gley)colours (Fig. 4D; Table 1). The top two unitsexhibit medium-scale (50–200 mm long) rhombo-hedral structural aggregates; the uppermost clay-stone exhibits superimposed angular blockystructure defined by clay-lined microslicken-sides. Clay-rich mudstones in the lower half ofthe profile exhibit wedge-shaped structural aggre-gates defined by larger scale slickensides. Clayand Mn-oxides typically coat slickensides. Sand-filled V-shaped dykes (Figs 3D and 6) cross-cutthe upper half of the type profile. The < 2 lm size

fraction in the type profile is dominated byhydroxy-interlayered minerals and smectite andlesser amounts of kaolinite (Table 1).

Type D palaeosols are developed within mud-stones of the sand- and gravel-rich channel-barfacies (Fig. 6) of the uppermost Upper Pennsyl-vanian Markley Formation (above ss13 in Fig. 2).Their distribution is limited to the eastern portionof the study area (sites 5 and 6 in Fig. 1).

Interpretation and classificationWell-developed, medium-scale rhombohedral-and wedge-shaped structure and associated ped-ogenic slickensides in type D palaeosols areanalogous to ‘parallelepipeds’ in Holocene verti-sols (Dudal & Eswaran, 1988). In modern soils,vertic features develop during mass movementand shearing in smectite-rich soils that becomesaturated during seasonal rainfall (Yaalon &

Fig. 8. Photomicrographs of redoximorphic features in Upper Pennsylvanian palaeosols. (A) Haematite nodules (e.g.dark spots in area marked by white ovoids) and ferrans (dark area) in a claystone (Bv horizon) with angular blockystructure; type C palaeosol. Scale bar ¼ 1 mm. (B) Redoximorphic depletions (light grey) and concentrations (darkgrey) in an AC horizon; type E palaeosol. Scale bar ¼ 1 mm. (C) Redoximorphic depletions (light grey) and con-centrations (black) of both diffuse and nodular haematite in a Cc horizon; type E palaeosol. Scale bar ¼ 1 mm. (D) Aplinthite horizon altered to an ironstone; the ironstone is composed predominantly of haematite nodules (black) anda minor amount of clay minerals; type C palaeosol. Scale bar ¼ 1 mm.



Kalmar, 1978; Dudal & Eswaran, 1988; Wilding &Tessier, 1988). The thin clay coatings on super-imposed angular blocky peds probably formed as

stress cutans (Jim, 1990) due to differentialwetting, and shrinking and swelling forces drivenby repeated wetting and drying of the soil matrix.Decreasing soil moisture and shrinking of soilclays resulted in cracking of the soil matrixduring drier intervals. The V-shaped dykes thatcross-cut the upper portions of the profilesresulted from back-filling of these surficial cracksfrom sediment-laden water during subsequentflood events.

Type D palaeosols are classified as vertisolsbased on abundant and well-developed verticfeatures, their clay content (28–39%) with asignificant percentage of smectite and the pres-ence of deep V-shaped cracks developed withinthe upper horizons (Soil Survey Staff, 1998).Modern vertisols form in seasonally moist andtypically tropical to warm-temperate climates,with four to eight dry months yearly (Dudal &Eswaran, 1988; Buol et al., 1997). The relativelylow kaolinite content and preservation of expand-able 2:1 layer-lattice clays (smectite) in type Dpalaeosols coupled with the occurrence of clasticdykes indicate periods of drying that were suffi-ciently long to preserve expandable clay mineralsand to allow for significant shrinkage of the clay-rich matrix (Duchaufour, 1982; Retallack, 1990).However, the presence of gley matrix colours andcommon to abundant redox depletions requiresthat periods of palaeosoil moisture deficiencyin type D palaeosols were temporally limited(Daniels et al., 1971; Duchaufour, 19826 ; Buolet al., 1997). Based on their stratigraphic associ-ation with overbank mudstones as well as themorphological and chemical characteristics ofthese palaeovertisols, they are interpreted ashaving formed in interfluve muds on the LatePennsylvanian upper coastal plain and piedmont.

Type E palaeosols

DescriptionThe representative profile of pedotype E con-sists of two units that are very weakly devel-oped in muddy fine-grained sandstone and are

Fig. 9. Palaeosol-bearing sections of Lower PermianArcher City Formation. Palaeosol tops are delineatedby the wavy lines to the left of each measured section.Capital letters refer to palaeosols of a given pedotype;circle delineates pedotype profile discussed in the text.Correlation of three sites (sites 9, 10 and 11 in Fig. 2)based on sandstone set 8 (ss8). Sites 9 and 10 are sep-arated by 0Æ3 km, and sites 10 and 11 are separated by0Æ2 km. See Fig. 3 for key to symbols.



distinguished by their colour and nature of theredoximorphic features (Fig. 4E; Table 1). Theupper red unit exhibits a vertically oriented,branching network of light-grey, fine- (< 10 mmwide) to coarse-scale (10–25 mm wide) tubular

mottles (Figs 6 and 8B); superposition of differentgenerations of mottles is observed. The lower palered unit contains fine to coarse dark red mottlesand haematite nodules (Fig. 8C). The < 2 lm sizefraction in the profile is dominated by kaoliniteand illite (Table 1). In addition, some type Epalaeosols also exhibit weakly developed slic-kensides.

Type E palaeosols occur as thick (1–2 m)cumulate and compound and composite palaeo-sols throughout the Permo-Pennsylvanian terrest-rial strata in the study area (sites 1–3, 5, 6, 9–11,12–15, 19–22, 24, 25 and 29 in Fig. 1). They aredeveloped within or stratigraphically adjacent tofinely laminated to slightly bioturbated silty

Fig. 10. Palaeosol-bearing sections of Lower PermianNocona Formation. Palaeosol tops are delineated by thewavy lines to the left of each measured section. Capitalletters refer to palaeosols of a given pedotype. Corre-lation of the siltstone- and claystone-dominated sectionfrom site 16 with the sandstone-dominated section atsite 15 (80 km apart) based on contemporaneity of ElkCity Limestone and ss11 defined by Hentz (1988). Keyto symbols as in Fig. 3.

Fig. 11. Palaeosol-bearing sections of Lower PermianPetrolia Formation at sites 17 and 18. Palaeosol tops aredelineated by the wavy lines to the left of each meas-ured section. Capital letters refer to palaeosols of agiven pedotype. Key to symbols as in Fig. 3.



mudstones and siltstones and fine- to medium-grained sandstones of the channel bar and pointbar facies (Figs 6, 9, 10, 12 and 13).

Interpretation and classificationType E palaeosols are interpreted as entisolsbased on the very weak development of pedogen-ic structure and horizonation (Soil Survey Staff,1998). Modern entisols exhibit little to no devel-opment of horizonation, and typically form uponunstable, wet landscapes (Buol et al., 1997).Entisols record stunted horizon developmentresulting from prolonged saturation and/or lim-ited periods of pedogenesis (tens to hundreds ofyears) caused by rapid sedimentation rates (Buolet al., 1997). The occurrence of redoximorphic

depletions and concentrations in many type Epalaeosols indicates a certain degree of hydro-morphy (waterlogging; cf. Simonson & Boersma,1972; Fanning & Fanning, 1989).

In contrast, type E palaeosols that formed insandy siltstones typically lack redoximorphicfeatures. Given that many type E palaeosolscommonly occur as weakly developed, com-pound or cumulate profiles in proximal positionsto fluvial siltstones and sandstones and estuarinesiltstones and mudstones (south-western portionof study area), they are interpreted as havingformed on unstable landscapes proximal tofluvial channels (e.g. levees and crevasse splays)on the coastal plain and piedmont and wetlandsof the lower coastal plain.

Fig. 12. Correlated transect (67 kmlong) of four palaeosol-bearing sec-tions of the Lower Permian PetroliaFormation beneath the regionallytraceable Beaverburk Limestone.The two sections (site 22) on the leftare siltstone and claystone domin-ated, whereas sections from sites 20and 21 contain abundant sandstonesof the point-bar facies. Palaeosoltops are delineated by the wavylines to the left of each measuredsection. Capital letters refer to pal-aeosols of a given pedotype. Key tosymbols as in Fig. 3.



Type F palaeosols

DescriptionThe type profile of pedotype F is composed of sixwell-developed horizons (Fig. 4F; Table 1) thatare distinguished by their clay content, structure,mottling and mineral accumulations. The upper-most horizon is defined by a dark yellow-brown,non-calcareous mudstone with fine to medium

subangular blocky structure. Three significantlymore clay-rich layers (56–61% clay vs. 30–33% inother units) underlie the uppermost mudstone.These are red to brown, silty claystones thatexhibit fine to medium subangular blocky struc-ture. Thick, continuous and oriented clay coat-ings (argillans) occur around detrital grains andline aggregates in all three claystones. The uppertwo claystone layers are non-calcareous, whereas

Fig. 13. Correlated transect (31 km long) of three palaeosol-bearing sections (from sites 27, 29 and 30) of the LowerPermian Waggoner Ranch Formation beneath the Maybelle Limestone. Palaeosol tops are delineated by the wavylines to the left of each measured section. Capital letters refer to palaeosols of a given pedotype; circle delineatespedotype profile discussed in text. Key to symbols as in Fig. 3.



the lowermost claystone contains discrete, cm-scale stage II carbonate nodules (sensu Machette,1985) and weakly developed calcareous rhizo-liths (Figs 9–12) that exhibit pedogenic fabrics(cf. Deutz et al., 2002). Faint tubiform red andreddish to yellow mottles in the mudstone andclaystone layers define a pattern similar tobranching root structures (cf. Retallack, 1988,1990) and probably record deeply penetratingroot systems. Smectite dominates the < 2 lm sizefraction of the claystone layers, with traceamounts of kaolinite and illite (Table 1). Thelower two horizons in the type profiles arered and yellowish-brown mudstones (Table 1);the upper horizon exhibits wedge-shaped struc-tural aggregates defined by randomly orientedslickensides, whereas the lower horizon pre-serves disturbed primary sedimentary lamina-tions (Fig. 4).

Some variants of the type profile lack thehorizon of stage II carbonate nodules. Moreover,some type F palaeosols contain carbonate-richhorizons that overlie or are superimposed on theclay-rich horizons with clay skins (cutans). Thesedominantly strongly developed palaeosols mayoccur as composite profiles and are found inLower Permian mudstones of the floodplainfacies (Figs 9–12) throughout the lower and uppercoastal plain and piedmont physiographic prov-inces (sites 9, 16 and 19 in Fig. 1). Given thatthese palaeosols are developed in fine-grainedmudstones and require good drainage, thesepalaeosols probably formed upon areas of thefloodplains that were removed from frequentflooding and depositional events associated withmajor stream systems (e.g. Buol et al., 1997).

Interpretation and classificationThe clay enrichment, abundant continuous argil-lans on detrital grains and peds and the non-calcareous matrix in the upper portions of type Fprofiles are evidence of clay translocation. Trans-location is facilitated through leaching of Ca2+

from clay exchange sites leading to the develop-ment of argillic (Bt) horizons (Franzmeier et al.,1985; Birkeland, 1999). Pedogenic carbonate canprecipitate in lower horizons of the profile ifsoluble minerals are incompletely leached fromthe soil horizon (Buol et al., 1997). The occur-rence of pedogenic carbonates in lower portionsof type F palaeosols, coupled with limiteddevelopment of redoximorphic features and illu-viated clays deep within the profile, indicate thatthese palaeosols formed well above the palaeo-water table and were well-drained (cf. Schwert-

mann, 1985, e.g. Kraus & Aslan, 1993). The darkyellowish-brown mudstones with fine-scalebranching root structures that cap most type Fprofiles are interpreted to be organic-rich ABhorizons.

The presence of well-differentiated profileswith argillic horizons suggests that type F palaeo-sols were alfisols (Soil Survey Staff, 1998).Modern alfisols form in humid to subhumid,mid-latitude to subtropical regions that have> 75–100 cm of annual precipitation (Strahler &Strahler, 1983; Buol et al., 1997). Given thatpedogenic carbonate typically accumulates insoils receiving < 70 cm mean annual precipitationin low-latitude regions (McFadden, 1988;Birkeland, 1999; Royer, 1999), carbonate-bearingtype F palaeosols may record formation at thesubhumid to semi-arid climatic boundary. Thepredominance of smectite and trace amounts ofkaolinite in type F palaeosols is consistent with asubhumid to semi-arid palaeoclimate. In addition,some type F palaeosols are characterized by thedevelopment of pedogenic carbonates above orwithin argillic horizons (polygenetic palaeosols).This superposition of calcic-over-argillic horizonsis climatically ‘out of phase’ and suggests thatthese type F palaeosols formed under two dis-tinctly different climatic regimes: an earlier, wet-ter climate that leached carbonate and formedargillic horizons in the profile and a subsequentdrier climate that accumulated carbonate andformed calcic horizons in the profile (cf. SoilSurvey Staff, 1975; Buol et al., 1997). Thesepolygenetic alfisols are stratigraphically limitedto the upper Archer City and Nocona Formationsas well as the uppermost Waggoner Ranch For-mation between the Maybelle and Lake KempLimestones (Fig. 2).

The development of multiple, well-definedhorizons, which commonly include superim-posed argillic horizons, and the composite natureof these palaeosols record prolonged periods (tensto hundreds of ky) of pedogenesis (cf. Bestlandet al., 1997). These characteristics coupled withevidence for well-drained profiles suggest thattype F palaeosols developed upon stable land-scapes distributed throughout the Early PermianEastern shelf for which the local groundwatertable was relatively deep. Evidence of large, deepfossil roots in type F palaeosols in conjunctionwith the presence of well-developed argillichorizons and blocky ped structure suggest thatwoodlands may have developed on such stableareas of the Early Permian alluvial landscape (cf.Mack, 1992).



Type G palaeosols

DescriptionThe type G profile is composed of five well-developed mudstone horizons (Fig. 4G; Table 1)that exhibit wedge-shaped structure defined byclay-coated slickensides (middle three horizons;Fig. 5C and D) and contain stage II carbonatenodules and rhizoliths (upper three horizons;Fig. 5F), although a few other type G palaeosolsin the upper Waggoner Ranch Formation mayexhibit 50–100 mm thick stage III carbonateaccumulations near the bases of the profiles.Redoximorphic features are generally lackingwith red hues dominating matrix colours. Sand-filled clastic dykes extend throughout most ofthe profile (to a depth of 1Æ4 m). The < 2 lm sizefraction in the type profile is dominated bysmectite and lesser amounts of kaolinite andmica-like minerals (sericite; Table 1; Fig. 7B).

Type G palaeosols developed in mudstones ofthe floodplain facies throughout the studiedLower Permian strata (Figs 9, 10 and 12). Theirdevelopment at numerous sites (sites 7–9, 11–14,16, 18, 23, 24 and 26–31 in Fig. 1) reflects theirwidespread distribution in the study area.

Interpretation and classificationAnalogous to type D palaeosols, type G palaeosolsare interpreted as vertisols based on theirwell-developed vertic features, their smectite-dominated clay composition and the presence ofV-shaped cracks developed to significant profiledepths. Type G palaeosols, however, exhibit twomajor differences from type D palaeosols: (1)accumulation of pedogenic carbonate; and (2) apaucity of redoximorphic features. The lack ofredoximorphic features in type G palaeosolsindicates that these well-drained profiles formedwell above the palaeowater table. Furthermore,the significant carbonate accumulation, includingstage II and III carbonate horizons in the profiles,and the presence of deeply penetrating V-shapedclastic-filled dykes require prolonged periods ofdrying of the profiles and prolonged duration ofpedogenesis.

Type H palaeosols

DescriptionThe type profile of pedotype H consists of sixmoderately to well-developed horizons (Fig. 4H;Table 1) that exhibit medium subangular tocoarse angular blocky structure in the upperhorizons and wedge-shaped structural aggregates

defined by weakly developed slickensides in thelower half of the profile. Mottling is limited to thelowermost horizons and typically exhibits highchroma. A thick, indurated layer (‡ 150 mm) ofstage III carbonate accumulation (¼ petrocalcichorizon; Soil Survey Staff, 1975, 1998; Machette,1985) near the top of the profile and extensivedevelopment of stage II carbonate nodules andcalcareous rhizoliths in lower horizons of theprofile (Fig. 5H) help to distinguish the type Hpalaeosol and its pedotypes from all other car-bonate-bearing palaeosols. Hydroxy-interlayeredminerals and mica-like minerals with traceamounts of kaolinite and chlorite make up the< 2 lm size fraction in the type profile (Fig. 7C).The type profile has lower clay contents (< 2 lmsize fraction in matrix of 22–25%) than all otherpedotypes.

Type H palaeosols may occur as compound,cumulate and composite profiles developed inLower Permian mudstones of the floodplainfacies (Figs 9–13). Their occurrence at manysites (sites 7–9, 16–22 and 24–31 in Fig. 1)reflects their widespread distribution in thestudy area.

Interpretation and classificationType H palaeosols are interpreted as having beensimilar to modern aridisols that formed undersemi-arid to arid climate conditions based ontheir extensive carbonate accumulation, in par-ticular as petrocalcic horizons, and relatively lowclay contents in comparison with all other ped-otypes (Soil Survey Staff, 1998). In reality, thesepalaeosols may only be classified as inceptisolsin the US Soil Taxonomy, as climate conditionsare not clearly understood during Early Permiantime (Soil Survey Staff, 1998). Nonetheless, thepresence of chlorite in the fine clay fractionsuggests that type H palaeosols formed underconditions of low soil moisture given that chlor-ite is not stable in humid environments (cf.Yemane et al., 1996).

Type H palaeosols are interpreted as havingformed on infrequently flooded, stable regions ofEarly Permian floodplains given that they recordwell-drained and oxidizing soil conditions andthat the development of thick petrocalcic hori-zons requires extended periods of soil formationand soil moisture deficiency (Machette, 1985;Wright & Marriott, 1996). The occurrence ofwedge-shaped aggregates defined by weaklydeveloped slickensides in horizons underlyingthe calcic horizons, however, records fluctuatingsoil moisture conditions.



DISCUSSION

Permo-Pennsylvanian palaeosols of north-centralTexas exhibit clear stratigraphic trends in pedo-type distribution and their mineralogical andgeochemical compositions. These superimposedtrends are interpreted here as recording theinterplay of autogenic factors inherent in thedepositional system (e.g. variability in sedimentaccumulation rate and parent material, localtopography, variations in floodplain hydrologyor soil drainage conditions) and basin- to regio-nal-scale allogenic processes such as climate,tectonic subsidence and eustasy. The overallstratigraphic distribution of the studied palaeo-sols delineates the time of appearance anddisappearance of pedotypes over the Permo-Pennsylvanian interval, as well as defininglong-term variations in the inferred degrees ofhydromorphy, free drainage and chemical weath-ering of the soil profiles and estimated soilmoisture conditions. These long-term variationsare interpreted here as recording the predominantrole that climate played in the development ofPermo-Pennsylvanian palaeosols associated withthe Eastern shelf of the Midland basin. Beforediscussing the application of these palaeosols toconstraining climate over western equatorial Pan-gea, the role of autogenic and basin-scale allo-genic processes other than climate in determiningthe development of the pedotypes and theirobserved spatial distribution is first assessed.

Superimposed upon the longer term strati-graphic trend are smaller scale patterns in pedo-type distribution defined by palaeosols withinthin stratigraphic intervals (metres to a few tensmetres; e.g. Fig. 14A and B). Lateral variations inpedotype distribution, degree of maturity of pal-aeosols and stratigraphic relationships betweenpalaeosols are also observed between age-equiv-alent deposits located hundreds of metres to tensof kilometres apart (Figs 6, 9, 10, 12 and 13).Reconstruction of palaeocatenas (e.g. fluvialchannels and their associated floodplains) in thisstudy was limited to a few laterally continuousoutcrops (hundreds of metres to 1Æ2 km long)exposed along Holocene stream valleys or inbadland exposures (e.g. Figs 9 and 13; Tabor,1999). The observed topography coupled withpalaeocatenary relationships inferred from metre-scale vertical stacking patterns of pedotypesindicates that geomorphic position on the aggrad-ing floodplain influenced soil developmentthrough differences in topographically controlleddrainage and sediment accumulation provided by

channel-margin deposits and shallow floodplainscours (cf. Bown & Kraus, 1987; Kraus, 1986)7 . Forthe latest Pennsylvanian period, the metre-scaleintercalation of type A and type B palaeosols infloodplain strata deposited on the lower palaeo-coastal plain (Figs 6 and 15A) necessitates thepresence of topography on the palaeolandscapeduring pedogenesis. Although both palaeosoltypes were hydromorphic, the presence of well-developed argillic horizons in type B palaeosolsindicates episodic free drainage of these profilesin comparison with the penecontemporaneouswater-logged type A palaeosols. However, theshallow depth to the palaeowater table inferredfrom palaeoplinthites in the ultisols, as well aslimited field relationships of channel-filling sand-stones and floodbasin strata, indicates that thepalaeotopography was probably not greater than afew metres.

Larger scale lateral variations (kilometres totens of km) defined by differences in the strati-graphic distribution of pedotypes observed with-in time-equivalent mudstone-dominated intervalsbracketed by laterally correlatable, major sandbodies (ss units in Figs 2, 6, 10, 12 and 13) furtherindicate that the geomorphic position in thePermo-Pennsylvanian alluvial basin influencedsoil development, probably through the influenceof river channel avulsion and broader patterns offloodplain drainage (cf. Kraus & Aslan, 1993;Kraus, 2002). Overall, these inferred meso- andmacroscale palaeosol–landscape associationsindicate that differences in sediment accumula-tion rates and topographically (or parent material)controlled drainage clearly influenced soil devel-opment on the Eastern shelf alluvial basin (e.g.Fig. 14B; Sobecki & Wilding, 1983; Wilding et al.,1991; Arndorff, 1993; Bestland et al., 1997; Kraus,1999, 2002; Kraus & Aslan, 1999). In turn, theimprint of landscape variability is recorded inthe vertical stacking patterns of pedotypes at themetre to tens of metre scale and accounts for thestratigraphic ‘noise’ observed in the longer termtrend.

Larger scale (tens to hundreds of metres) strati-graphic variations in the Permo-Pennsylvanianpalaeosols define a long-term trend upon whichthe aforementioned smaller scale variations aresuperimposed. The long-term trend records anoverall decrease in the degree of hydromorphy andintensity of chemical weathering of the soil profilesconcomitant with an increase in evidence for freedrainage of profiles and lower soil moisturethroughout latest Pennsylvanian (Virgilian) toEarly Permian time. This trend is defined by



floodplain palaeosols with morphologies thatreflect conditions of chemical weathering and soilmoisture regime (types A, B, D, F, G and H); no

equivalent trend is defined by the distribution oftype C and E profiles (Fig. 15). Type C and E pro-files are immature, poorly developed palaeosols

A B

Fig. 14. (A) Schematic stratigraphic column of fluvio-alluvial sedimentary rocks from the Early Permian successionof the Eastern Midland Basin, north-central Texas. The distribution of pedotypes between major fluvial channeldeposits suggests that depositional rate, position of the palaeogroundwater table and duration of pedogenesis acrossthe Permo-Pennsylvanian landscape played an important role in the stratigraphic stacking pattern of palaeosolmorphologies. (B) Interpreted distribution of soil types across the Early Pennsylvanian landscape of the EasternMidland Basin, north-central Texas. The stacking pattern of pedotypes between major fluvial channel sandstonedeposits is interpreted as representing migration of fluvial systems from proximal (positions 0 and 1) to more distal(positions 4 and 6) and back to proximal locations (positions 8 and 9). See text.



found throughout the three physiographic prov-inces. Their cosmopolitan nature probably reflectsthe abundance of young, episodically ‘disturbed’and poorly drained environments proximal tofluvial channels and ponds upon the floodplain(Buol et al., 1997).

In particular, the longer term variation inpedotype development records a relatively rapidshift over tens of metres from Late Pennsylvanianhydromorphic floodplain palaeosols (histosol-like type A; ultisol-like type B) to Early Permianwell-drained and oxidized profiles (alfisol-liketype F; calcic vertisol-like type G; and aridisol-like type H). Vertisols (type D) with abundantredoximorphic features but morphological evi-dence for periods of drying (e.g. clastic dykes inthe upper portions of profiles) make up animportant component of latest Pennsylvanianmature palaeosols. These palaeosols are inter-preted as recording intermediate soil moistureconditions and the initiation of a drying trendduring the transition at the Pennsylvanian–Per-mian boundary interval.

The Early Permian palaeosols (types F, G andH) record the onset of significant pedogeniccarbonate accumulation throughout the region,which could reflect a decrease in average soilmoisture levels and/or an increase in Ca2+ dustflux to the region. Moreover, a continuum of soilmoisture conditions is defined by these threeEarly Permian pedotypes (alfisols ¼ wettest toaridisols ¼ driest) by the previously discusseddifferences in their morphologies, abundanceand distribution of redoximorphic features andclay composition. Indicators of seasonal or epi-sodic fluctuations in soil moisture conditionsoccur in all mature palaeosols in the studyarea except histosols. However, the degree towhich the morphological indicators of soil mois-ture fluctuations and, increasingly, better soildrainage are developed in the studied palaeo-sols increases significantly from weakly devel-oped features in ultisols of the lower half ofthe Virgilian strata to strongly developed featuresin earliest Early Permian alfisols and calcicvertisols.

The stratigraphic distribution of Early Permianpedotypes (F, G and H) does not define a strongtemporal trend given that all three pedotypesco-exist in contemporaneous intervals. Type Fpalaeosols (alfisols), however, become somewhatless abundant upwards through the EarlyPermian strata (Waggoner Ranch Fm.) with aconcomitant increase in the abundance of type Hpalaeosols (aridisols; Fig. 15).

The longer term trend defined by the studiedpalaeosols could reflect a change in regional baselevel governed by tectonic activity or eustasy thatmay not have been directly linked to climatechange (cf. Marriott & Wright, 1993; Wright &Marriott, 1993). The palaeoshoreline migratednorth-north-eastward throughout the latest Penn-sylvanian to Early Permian in response to arelative sea-level rise, as recorded by the incur-sion of marine deposits on to the Eastern Shelf(Figs 2 and 14; Hentz, 1988). This shift in baselevel is expressed in the basinward shift in thegeomorphic position of palaeosol-bearing inter-vals in the studied Permo-Pennsylvanian stratathrough time (Table 2; Fig. 15). This shift shouldbe recorded by a progressive increase in hydro-morphy in response to shallower regional ground-water tables or overall increased maturity offloodplain palaeosols with increasing distancefrom sediment source areas (Wright & Marriott,1993; Birkeland, 1999; Kraus, 1999). The ob-served long-term trend of decreasing degree ofhydromorphy and intensity of chemical weather-ing of the palaeosol profiles concomitant withincreasing evidence for free drainage of profilesand overall falling soil moisture levels, however,is contrary to the trends anticipated in responseto a rise in base level. This indicates the predom-inant influence of an allogenic process(es) otherthan base level on the observed long-term trend inpedotype distribution. However, the greater abun-dance of type G (calcic vertisols) and type F(alfisols) on landscapes proximal to the palaeo-shoreline (0 to � 30 km; Table 2) and a domin-ance of aridisols (type H) in regions of the alluvialbasin proximal to sediment sources (> 60 kmfrom base level; Table 2) does indicate that adecline in topographic relief and sediment accu-mulation rates down palaeoslope on the EasternShelf may have had a subordinate influence onthe observed larger scale stratigraphic variationsin palaeosol morphology and maturity. The weakmanifestation of these basin-wide trends, how-ever, attests to the very low palaeoslopes on theLate Palaeozoic Eastern shelf. Temporal variationin Permo-Pennsylvanian parent material in thestudy area as an alternative allogenic control onthe large-scale stratigraphic variations in pedo-type is not considered likely given the lack of anyobserved systematic long-term variation in parentmaterial derived from the Ouachita or Arbuckleuplands (Hentz, 1988).

It is suggested that climate was the predomin-ant control on the origin of large-scale strati-graphic variations in pedotype distribution in



Table

2.

Rela

tion

ship

of

Perm

o-P

en

nsy

lvan

ian

pala

eoso

lsi

tes

rela

tive

toest

imate

dbase

-level

on

the

East

ern

shelf

of

the

Mid

lan

dB

asi

n.

Form

ati

on

Dis

tan

ce

from

rela

tive

sea

level

an

dp

ala

eoso

lm

orp

holo

gy*

0–10

km

10–20

km

20–30

km

30–40

km

40–50

km

50–60

km

>60

km

Sit

e#

Typ

eS

ite

#T

yp

eS

ite

#T

yp

eS

ite

#T

yp

eS

ite

#T

yp

eS

ite

#T

yp

eS

ite

#T

yp

e

Waggon

er

Ran

ch

31

H,F

,G26

H,G

,I29

H,G

30

H¢,E

¢–

––

––

–27

H,G

,I–

––

––

––

––

–28

H,G

,E–

––

––

––

––

––

––

–25

H,E

––

––

––

––

––

––

24

H,E

,G–

––

––

––

–23

G¢

––

––

––

––

––

–P

etr

oli

a–

–22

H,F

,G,E

––

––

––

––

19

H,F

,E–

––

––

––

––

–20

H,E

––

––

––

––

––

21

H,E

––

18

H–

––

––

––

––

––

17

H,F

––

––

––

––

––

Nocon

a–

–16

H,G

,F,E

––

––

––

––

14

G,E

––

––

––

––

––

––

15

E13

G,E

12

G,E

Arc

her

Cit

y–

––

––

––

–9

G,H

,F,E

––

––

––

––

––

––

10

H,G

––

––

––

––

––

––

11

G,H

––

––

––

––

––

––

––

8G

,H–

––

––

––

––

––

–7

G,E

––

Mark

ley

––

––

––

––

––

––

1A

,B,C

––

––

––

––

––

––

2A

,B,C

––

––

––

––

––

––

3A

––

––

––

––

––

––

4C

––

––

––

––

––

––

5C

,E–

––

––

––

––

––

–6

C,D

,E

*T

he

dis

tan

ce

from

base

level

was

dete

rmin

ed

by

measu

rin

gth

ed

ista

nce

dow

nd

ep

osi

tion

al

dip

from

the

stu

dy

site

sto

the

vert

icall

yd

ash

ed

lin

es

inF

ig.

2th

at

rep

rese

nt

the

stra

tigra

ph

iccu

t-off

betw

een

mari

ne-d

om

inate

dan

dte

rrest

rial-

dom

inate

dse

dim

en

tary

stra

ta(H

en

tz,

1988).

Th

ese

are

likely

tobe

min

imu

mest

imate

s,as

soil

form

ati

on

occu

rred

on

the

flood

pla

ins

du

rin

gti

mes

of

base

level

fall

or

low

stan

d(e

.g.

mid

-con

tin

en

tcyclo

them

s).

Th

ep

osi

tion

of

each

of

the

stu

dy

site

san

dth

eir

ass

ocia

ted

pala

eoso

lm

orp

holo

gie

sare

giv

en

inre

lati

on

to(1

)d

ista

nce

from

base

level

(in

ferr

ed

from

dash

ed

lin

es

inF

ig.

2);

an

d(2

)st

rati

gra

ph

icoccu

rren

ce.T

he

pu

rpose

of

this

table

isto

ass

ess

the

dis

trib

uti

on

of

pala

eoso

lsre

lati

ve

tobase

level.

Alt

hou

gh

more

data

shou

ldbe

gath

ere

dto

test

rigoro

usl

yth

eeff

ects

of

base

level

(an

dbase

level

ch

an

ge)

on

the

morp

holo

gy

of

pala

eoso

lsin

the

stu

dy

are

a,it

isap

pare

nt

that

there

isn

oro

bu

stch

an

ge

inp

ala

eoso

lm

orp

holo

gy

as

afu

ncti

on

of

dis

tan

ce

from

base

level

for

an

yon

eti

me

peri

od

.



the study area. The Late Palaeozoic is hypothes-ized to have been characterized by major changesin climate in response to the assembly of thesupercontinent Pangea, a transition from ice-house to green-house states and major changes inatmospheric circulation (Veevers & Powell, 1987;Parrish, 1993; Crowley, 1994). Climatically sen-sitive depositional and palaeontological recordsfor low-latitude Pangea (Rowley et al., 1985;Cecil, 1990; Dubiel et al., 1991; DiMichele &Aronson, 1992; Parrish, 1993; West et al., 1997;Kessler et al., 2001; Rees et al., 2001) coupledwith numerical model results (Parrish et al.,1986; Kutzbach & Gallimore, 1989; Patzkowskyet al., 19918 ; Gibbs et al., 2002) suggest that themid-latitude and equatorial regions of Pangeabecame progressively more arid and seasonalthroughout the Late Palaeozoic. The long-termaridification trend has been attributed to severalprocesses including: (1) northward drift of

Pangea out of the Intertropical Convergence Zoneinto the more arid low-latitude belt; (2) rainshadow effects created by uplift of the Alleghe-nian and Ouachita mountains; and (3) northwarddiversion of moisture-laden easterlies by thedevelopment of northern hemisphere monsoonalcirculation.

Significantly, recent studies have suggestedthat monsoonal circulation and attendant west-erly winds were well-established over westernequatorial Pangea by earliest Permian time(Kessler et al., 2001; Soreghan, G.S. et al., 2002;Soreghan, M.J. et al., 2002; Tabor & Montanez,2002; Loope et al., 2004), despite model resultsindicating that continental-scale monsoonal cir-culation would have strengthened progressivelyto its peak in the Middle Triassic (Parrish, 1993).The results of these lithological and geochemicalstudies, along with other discrepancies in model-data comparisons for low-latitude western Pangea

Fig. 15. Schematic diagram of the Permo-Pennsylvanian landscape through time based on palaeosol morphologicalfeatures. Roughly contemporaneous study sites from Figs 1 and 2 are plotted on each time period, as well as thepalaeosol types associated with each study site. See text for discussion.



(Patzkowsky et al., 1991; Rees et al., 2001; Gibbset al., 2002), highlight the need for a betterunderstanding of specific climate zones over LatePalaeozoic Pangea. Pennsylvanian to Permianpalaeosols from north-central Texas provide addi-tional constraints on the nature and origin ofclimate change over low-latitude Pangea giventhat climate change would have been most pro-nounced in western equatorial Pangea (Dubielet al., 1991; Parrish, 1993).

The long-term variation in soil moisture condi-tions and, in turn, precipitation inferred from thelarge-scale stratigraphic variations in Permo-Pennsylvanian pedotypes confirms the previ-ously defined long-term aridification trend forwestern equatorial Pangea. Notably, however, thepalaeosol data in this study indicate that thelong-term drop in inferred soil moisture levelsaccelerated at end-Pennsylvanian time. Incipientmorphological indicators of seasonality preservedin type D palaeosols of the study area suggest anabrupt onset of seasonality in conjunction with arapid decline in net soil moisture budget in thelatest Pennsylvanian.

The long-term aridification trend inferred fromthe palaeosol data cannot be interpreted asreflecting northward drift of Pangea given thatpalaeomagnetic studies indicate that the arearemained within a few degrees of the equatorthroughout the Early Permian (Scotese, 1999;Loope et al., 2004). The observed temporal trendmay record the development of a rain shadow inwestern Pangea in response to uplift of theequatorial Alleghenian and Ouachita mountains(Rowley et al., 1985; Parrish, 1993; Ziegler et al.,1997). However, the degree to which long-termaridification through the latest Pennsylvanianand Early Permian can be explained by a rainshadow effect is strongly dependent on theelevation of the equatorial highlands, for whicha significant degree of uncertainty exists (Otto-Bliesner, 19939 , 1998; Fluteau et al., 2001; Gibbset al., 2002). Moreover, the orographic effect ofthese prominent equatorial highlands on tropicalprecipitation (i.e. onset of aridity) should havebeen well-established by Late Carboniferous timerather than in the Early Permian as the palaeosolsof the Eastern shelf suggest (cf. Kessler et al.,2001; Soreghan G.S. et al., 2002).

The overall long-term palaeopedogenic trendand its inferred rapid decline in precipitationlevels and intensification of seasonality in west-ern equatorial Pangea in the latest Pennsylvanianand earliest Permian are strong evidence fornorthern hemisphere monsoonal circulation. For

western equatorial Pangea, this rapid change inregional climatic conditions may record a tec-tonic moderation of monsoonal conditions. Up-lift of the equatorial mountain chain in the EarlyPennsylvanian (Chesterian) would have acted tointensify the normal low pressure area of theequatorial belt, thus inhibiting the developmentof fully monsoonal conditions until their erosionin the latest Pennsylvanian and Early Permiandampened their climatic influence (Rowleyet al., 1985; Otto-Bliesner, 1993). Notably, thedelay in the development of fully monsoonalconditions may have promoted relatively rapidintensification of monsoonal conditions over thestudy area in the Early Permian as has beeninvoked to explain the origin of rapid variationin lithofacies of mid-continent cyclothems in thelatest Pennsylvanian to earliest Permian (Westet al., 1997).

Lastly, pedogenic evidence for intermediate-scale (103)105 years) climate fluctuations inwestern equatorial Pangea occurs in the poly-genetic alfisols characterized by well-developedcalcic horizons overlying or superimposed onargillic horizons. These polygenetic palaeosolsare restricted stratigraphically to the lower half ofLower Permian (Wolfcampian) deposits of north-central Texas, and reappear for a few tens ofmetres in the uppermost Leonardian-ageWaggoner Ranch and lowermost Clear Fork For-mations. These polygenetic profiles are analogousto those that occur within slightly younger,Lower Permian mid-continent cyclothems thathave been interpreted to record orbitally forced� 20 ky climate oscillations associated with wax-ing and waning of Early Permian continentalglaciers (Miller et al., 1996). Such precessionalforcing of climate would probably have beendampened following deglaciation and a trans-ition to greenhouse states. If polygenetic palaeo-sols from the eastern Midland basin recordprecessional-scale climate fluctuations, then theabrupt shift to more stable climatic conditionsmarked by the loss of polygenetic palaeosols inthe Early Permian (mid-Leonardian) successionmay be a sensitive record of the onset of degla-ciation. The brief reappearance of polygeneticalfisols in the later part of the Early Permianmight record a brief renewed episode of glaci-ation or a climatic cooling analogous to the lateQuaternary Younger Dryas. This interpretation, ifcorrect, places additional constraints on mostrecent estimates of a late Early Permian age forthe onset of deglaciation (Veevers, 1994; Gibbset al., 2002).



CONCLUSIONS

Late Pennsylvanian and Early Permian sedimen-tary strata of the Eastern Midland basin exhibit achange in palaeosol morphological, mineralogicaland chemical characters across the Pennsylva-nian–Permian boundary. This change is revealedby palaeosols with well-developed redoximor-phic depletions and concentrations in the LatePennsylvanian that are replaced by palaeosolsthat generally lack redoximorphic indicators inthe Early Permian. Furthermore, many of theEarly Permian palaeosols are characterized bypedogenic carbonate accumulation that is consis-tent with a net soil moisture deficiency, whereasLate Pennsylvanian palaeosols exhibit no pedo-genic carbonate. The clay mineralogical compo-sition of these palaeosols indicates a change frommore highly weathered and probably humidconditions in the Late Pennsylvanian to lessextreme weathering and potentially semi-arid toarid conditions in the Early Permian. Thesechanges may be a response to changing soilmoisture regimes from humid, poorly drained todry, well-drained conditions in the Late Pennsyl-vanian to Early Permian.

The inferred change in Late Pennsylvanian–Early Permian soil moisture regimes of westernequatorial Pangea is likely to be related to climatechange. It is suggested, based on the preservedlithological, mineralogical and chemical charac-teristics of these palaeosols, that a changeoccurred from a humid tropical climate charac-terized by high rainfall in the Pennsylvanian to aprogressively drier tropical climate characterizedby seasonal precipitation throughout the EarlyPermian. Furthermore, these data indicate theonset of monsoonal-type precipitation patternsnearly coincident with the Pennsylvanian–Per-mian boundary. Although these strata preserve ageneral trend of progressively drier and seasonalprecipitation through the Permian, polygeneticsoils preserved within the lower half of the EarlyPermian (Wolfcampian) and uppermost strata ofthe lower Leonardian Waggoner Ranch Formationmay indicate an oscillatory climate patternbetween more humid and drier climate duringdeposition of these stratigraphic packages

ACKNOWLEDGEMENTS

We are deeply indebted to Bill DiMichele and DanChaney, Department of Palaeobiology, Museumof Natural History, Smithsonian Institution, for

providing an introduction to the study area. JohnNelson and Dan Chaney provided excellent fieldguidance. This project was funded by NSF grantEAR-9814640 to I. P. Montanez and an NSF-IGERTgraduate fellowship to N. J. Tabor. Funding wasalso provided by grants to N.J.T. from the Geolo-gical Society of America, American Association ofPetroleum Geologists, Society for SedimentaryGeology and Sigma Xi. We thank Peter Haughton,Paul McCarthy and Susan Marriott for theirthoughts and helpful comments on this manu-script. All these reviewers (but particularly S.M.)have contributed significantly towards improvingthe quality and clarity of this contribution.

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Manuscript received 2 July 2003;revision accepted 22 March 2004.



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Morphology and distribution of fossil soils in the Permo