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Geomorphology xxx (2009) xxx–xxx

GEOMOR-02923; No of Pages 15

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Geomorphology

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Late Quaternary Paleohydrology of the Madre de Dios River, southwestern AmazonBasin, Peru

Catherine A. Rigsby a,⁎, Erin M. Hemric a, Paul A. Baker b

a Department of Geological Sciences, East Carolina University, Greenville, NC 27858 USAb Division of Earth & Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27705 USA

⁎ Corresponding author.E-mail address: [email protected] (C.A. Rigsby).

0169-555X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.geomorph.2008.11.017

Please cite this article as: Rigsby, C.A., et al.Geomorphology (2009), doi:10.1016/j.geom

a b s t r a c t
a r t i c l e i n f o
Article history:Received 3 December 2007Accepted 11 November 2008Available online xxxx

Keywords:QuaternaryPeruAmazonRiverPaleohydrology

Late Quaternary climatic and hydrologic variability triggered changes in fluvial deposition and erosion alongthe course of the Madre de Dios River, Peru, the largest tributary basin of the Madeira basin, itself the largesttributary basin of the Amazon. Three laterally extensive, Quaternary-age, terrace tracts are present within theMadre de Dios basin. Analysis of sedimentary facies, present in the modern cut banks and terracedsequences, along with radiocarbon dates on fossil wood and leaf material preserved in the terraced strata,allow reconstruction of the Late Quaternary depositional history of the sedimentary sequences, includingdetermination of the approximate timing of aggradation and downcutting episodes and its relationship to thetiming of past climate change in this portion of the Amazon basin and beyond.The Quaternary sediments underlying the terraces most often recorded deposition in a coarse-grainedmeandering fluvial system. The T3 terrace, the highest terrace, is underlain by the Miocene (?) IpururiFormation, which is unconformably overlain by the late Miocene–Pleistocene (?) (N48,000 cal yrs BP) Madrede Dios Formation, a multistory coarse-sandy to gravelly channel and point bar complex. The latter wasdowncut before 29,850±100 cal yrs BP. This downcut landscape was infilled by meandering fluvial stratacharacterized by gravelly channel deposits in a sequence dominated by floodplain and lateral accretiondeposits. These strata were in turn downcut to form the T2 terrace before 11,970±100 cal yrs BP. A thirdepisode of aggradation resulted in the deposition of a sand-dominated meandering channel complex thatinfilled the T2 valley and was subsequently downcut after 3780±50 cal yrs BP. This most recent terrace isinfilled by the modern fluvial sediment, which has been actively aggrading since at least 870±50 cal yrs BP.Importantly, the Madre de Dios fluvial system actively aggraded between 30,000 and 25,000 cal yrs BP, (andlikely much younger, as dated samples were, thus far, only found near the base of the T2 sequence). Thisobservation implies that some combination of (1) increased precipitation and decreased temperature,(2) decreased evapotranspiration and increased runoff, (3) increased Andean glacial erosion and increasedsediment supply, and (4) decreased atmospheric CO2 (hence decreased rain-forest primary productivity andaltered rain-forest physiology/ecology), entering the last glacial maximum period brought about increasedfloodplain deposition in the southwestern Amazon. Elsewhere in the Amazon basin few, if any, fluvialsediments of this age range have been observed. The start of the next major phase of aggradation coincidedwith the Younger Dryas and suggested that floodplain sedimentation in the lowlands was again related tocold and wet conditions in the adjacent highlands (and perhaps in the lowlands as well) and that Madre deDios history was also tied to large-scale global climate. This aggradation may have continued throughout theearly and mid-Holocene, until at least 3,780 cal yr BP. If so (and this is uncertain), this episode ofsedimentation took place during a dry period.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The Amazon River drains approximately 38% of the area ofcontinental South America and accounts for over 18% of the totalfreshwater input to the oceans. From the Andes to the Atlantic, the riverruns through the largest tropical rainforest, a region of unsurpassed

ll rights reserved.

, Late Quaternary Paleohydroorph.2008.11.017

biodiversity that is undergoing rapid changes as a result of direct humanendeavor and climate change. Despite growing recognition of thistransformation, few studies exist of the paleohydrology of the AmazonBasin designed to determine the range andmechanisms of past changesof fluvial processes as well as to elucidate the future of this fluvialenvironment (e.g., Latrubesse, 2003; Latrubesse et al., 2005). And,paleohydrologic information is largely absent from most paleoenviron-mentaldiscussionsof theAmazon region, despite theAmazon landscapebeing dominantly of fluvial origin. Understanding the relatively sparseand geographically restricted Quaternary vegetation history of the

logy of the Madre de Dios River, southwestern Amazon Basin, Peru,

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http://dx.doi.org/10.1016/j.geomorph.2008.11.017

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2 C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx

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Amazon (e.g., Colinvaux et al., 2000; Mayle et al., 2007; Anhuf et al.,2006) is difficult, at best, without independent information aboutQuaternary climate change and attendantfluvial evolution in the region.Moreover, the paucity of long-lived andwell-preserved natural archivesof past climate and hydrology in the Amazon lowlands (e.g., Ledru et al.,1998; Bush et al., 2004; Anhuf et al., 2006) has largely precludeddefinitive conclusions regarding thehistoryofAmazonprecipitation andrunoff. As a result,wedonot know for sure if Amazon climate has shiftedin lockstep with climate in the neighboring highlands, where paleocli-mate and paleohydrologic records of the late Quaternary are more

Fig. 1.Map showing the study area in theMadre de Dios Department in southeastern Peru, soufor site abbreviations.

Please cite this article as: Rigsby, C.A., et al., Late Quaternary PaleohydroGeomorphology (2009), doi:10.1016/j.geomorph.2008.11.017

abundant. And opinions differ widely about the nature of Amazonclimate, particularly prior to the Holocene.

In this paper, we document fluvial history in the southwesternAmazon basin by examining the Late Quaternary sedimentarysequences exposed in terraced strata along a portion of the Madrede Dios River (Peru) and we compare these sequences with records ofclimate and fluvial history elsewhere in tropical South America. Ourreconstruction of the history of the Madre de Dios fluvial systemprovides landscape level evidence of past hydrologic change thatextends back to about 30,000 cal yrs BP and is an important step

thwestern Amazon Basin. Triangles indicatemeasured section locations. Refer toTable 1



3C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx

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toward achieving a broader paleohydrologic history of the entireAmazon basin.

Fig. 3. Longitudinal profile of the Madre de Dios River from the Upstream of Diamantesection on the Alto Madre de Dios River to the confluence of the Madre de Dios andTambopata Rivers at Puerto Maldonado. Locations of confluences with tributary riversare indicated by vertical dotted lines. All profile points (including measured sectionlocations) are plotted as triangles (see Fig. 1 for locations of measured sections; Table 1for site abbreviations). The anomaly in elevation upstream of the Inambari tributary isthe result of a rapid and short-lived increase in river level after precipitation the daybefore the measurement was taken.

2. Environmental setting

2.1. Geological setting

The Madeira River is the largest tributary of the Amazon in areaand discharge. Its watershed has an area of 1,380,000 km2, 20.1% of thetotal area of the Amazonwatershed, and an average discharge of about30,000 m3 s−1, approximately 15% of the total Amazon discharge(Goulding et al., 2003), twice the average annual discharge of theMississippi. The Madeira drains the entire southwestern Amazonregion and is itself comprised of several large tributary basinsincluding the Guaporé–Iténéz with headwaters in the Brazilian Shield,theMamore drainingmuch of lowland eastern and central Bolivia, andthe Beni basin of northwestern Bolivia and southeastern Peru. TheMadre de Dios River is the largest tributary to the Beni; the drainagebasin of the Madre de Dios roughly comprises the western two-thirdsof the Beni basin. Some of the tributaries of the Madre de Diosoriginate in the glaciated Cordillera Oriental of the Andes whereasother tributaries arise in the lowland rain forests of Peru and Bolivia.

The 650-km long reach of the Madre de Dios River (12–13° S, 69–71°W) discussed in the present study (Fig.1) stretches from upstreamof the confluence of the Alto Madre de Dios and Manu Rivers at Bocade Manu, Peru, downstream to the confluence of the Madre de Diosand Tambopata Rivers at Puerto Maldonado, Peru (including as well aportion of the lower Tambopata River). Throughout this region thecourse of theMadre de Dios is eroded into thick Tertiary and Quaternarysedimentary sequences of the Andean foreland basin (e.g., Baby et al.,1997; Galloso et al.,1996; Guyot et al., 1999; Horton and DeCelles,1997).The strata examined in this study are exposed in cut-banks and terracedreaches of themodern river valley. The strata are insetwithin deposits ofthe lateMiocene–Pleistocene (age uncertain)Madre de Dios Formation,which is composed of fluvial gravel, sand and mud deposits. The Madrede Dios Formation is separated from the underlying Miocene IpururoFormation by the Ucayali unconformity (Campbell et al., 2001, 2006).The Ipururo Formation is best seen in exposures at the base of some ofthe terraced sections, especially during periods of lower water.

Fig. 2.Madre deDios river levels for January–December 2005 recorded at CICRA (AmazonConservation Association, unpublished data). The region has a pronounced austral winterdry season (July–October) that corresponds to the period of low river levels.


2.2. Climatic setting

The modern climate of the lowland portions of the southwesternAmazon basin is humid tropical with mean monthly temperaturesvarying just a few degrees throughout the year. The diurnaltemperature range is several times the seasonal cycle. The meanmonthly low temperature of 16.8 °C occurs in July and the meanmonthly high of 32.0 °C occurs in October at Puerto Maldonado, Peru.Annual rainfall at Puerto Maldonado averages 2300 mm. Three-quarters of this precipitation occurs during the half-year fromNovember through April. During the austral winter (June, July, andAugust), monthly precipitation averages less than 65 mm. Most of thewet-season precipitation in this region results from circulationsassociated with the South American summer monsoon (SASM; e.g.,Zhou and Lau, 1998). Interannual-to-decadal variability of SASMprecipitation is partly forced by tropical Pacific (ENSO) and tropicalAtlantic variability (e.g., Zhou and Lau, 2001). These relationships areweak and non-stationary (e.g. Ronchail et al., 2005).

River stage and precipitation data have been collected at theCentro de Investigación y Capacitación del Río Los Amigos (CICRA,12°34'S, 70°06'W) from 2001 to 2003 by Goulding and colleagues(2003) and from 2004 until the present by CICRA staff (AmazonConservation Association, unpublished data). Mean annual precipita-tion at CICRA during the measurement period was 2653 mm. Thedriest year recorded at CICRA (2150 mm) was 2005. Throughout theAmazon basin, 2005 was the sixth driest year on record and thisdrought was most severe in the southwestern Amazon (Luis Aragão,personal communication, 2007). The origin of this particular droughtcan be ascribed to abnormally warm sea-surface temperaturespervading in the northern tropical Atlantic during much of 2005.

The stage of the Madre de Dios (Fig. 2) responds rapidly toprecipitation events — 2 to 3 m rises or falls of the river overnightfollowing an upstream deluge are common, as anyone camping andboating on the river will surely learn. Our field work was undertakenfromthe endof June throughearly July during the2005dryseasonwhenthe total monthly precipitation values were respectively 217.6 and29.9 mm, and river level was the lowest on record (−7.63 m below itsmean; Fig. 2, Amazon Conservation Association, unpublished data).




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2.3. Morphology of the modern Madre de Dios

The modern geomorphology of the studied portion of the Madrede Dios valley has recently been described by Hamilton and co-workers (2007). The longitudinal profile of the Madre de Dios valleywithin the study area drops about 131m in elevation over a horizontaldistance of about 280 km (Fig. 3). Thus, the studied reach of themodern Madre de Dios has a gradient of about 0.00047 and exhibits ahighly sinuous meandering morphology with an average sinuosityindex of 2.3. Because river level was very low during our study, themeasured gradient is likely a close approximation of average channelbed slope. The river valley contains numerous cutoffs and floodplainlakes, which predominantly form by channel abandonment as a resultof neck cutoff. Mean channel width of the rivers in this system, asmeasured by Puhakka and colleagues (1992), ranges from 500 m forthe Madre de Dios to 100 m for the Los Amigos. Much like the Madrede Dios, theManu and Los Amigos rivers have tortuousmeanders withmany oxbow (floodplain) lakes. The Tambopata is also a meanderingriver with a well defined floodplain, but it has a very irregularmeander pattern and few floodplain lakes. The Alto Madre de Dios,Colorado and Inambari rivers, with headwaters in the nearbyCordillera Oriental, have braided reaches but are characterized by

Table 1Sample name, numbers, location stratigraphic information, radiocarbon age and calibrated

Sample name Samplenumber

NOSAMSnumber

Location Materialdated

Radiocarbonage

Ca

Colorado II #1 CII-1 OS-51221 12°35'36.67260qS70°22'31.18108qW

Fossilwood

24,800±130 2

Diamante #1 D-1 OS-56254 12°35'36.67260qS70°22'31.18108qW

Fossilwood

N48,000 N

Laberinto I #1 L1-1 OS-56257 12°40'38.89800qS69°33'25.93595qW

Fossilwood

3500±35 3

Laberinto I #2 LI-2 OS-51222 12°40'38.89800qS69°33'25.93595qW

Leafymaterial

5720±40 6

Laberinto II #1 LII-1 OS-56256 12°33'11.06217qS69°22'11.15636qW

Fossilwood

23,400±100 2

Los Amigos I #1 LA I-1 OS-56257 12°34'00.45811qS70°06'18.58780qW

Fossilwood

N48,000 N

Los Amigos II #1 LA II-1 OS-51223 12°33'26.29793qS70°05'29.45067qW

Fossilwood

21,100±110 2

Manu #1 M-1 OS-56246 12°17'40.54654qS70°52'34.74911qW

Fossilwood

10,150±45 1

Manu #2 M-2 OS-56247 12°17'40.54654qS70°52'34.74911qW

Fossilwood

10,250±40 1

Playa Caceres #1 PC-1 OS-56248 12°43'27.87141qS69°38'10.80215qW

Fossilwood

9310±35 1

Playa Caceres #2 PC-2 OS-56134 12°43'27.87141qS69°38'10.80215qW

Fossilwood

9270±40 1

Puerto AzulBerowe #1

PAB-1 OS-56249 12°17'21.02900qS70°45'43.99088qW

Fossilwood

N48,000 N


PAB-2 OS-56136 12°17'21.02900qS70°45'43.99088qW

Fossilwood

N48,000 N


PAB-3 OS-56250 12°17'21.02900qS70°45'43.99088qW

Fossilwood

N48,000 N


PAB-5 OS-51224 12°17'21.02900qS70°45'43.99088qW

Fossilwood

21,500±110 2

San Juan #1 SJ-1 OS-56251 12°33'35.40583qS70°09'44.35239qW

Fossilwood

N48,000 N

San Juan #2 SJ-2 OS-51225 12°33'35.40583qS70°09'44.35239qW

Fossilwood

24,900±120 2

Tambopata #1 T-1 OS-56252 12°39'22.34034qS69°10'58.12604qW

Fossilwood

730±25 6

Tambopata #2 T-2 OS-56253 12°39'22.34034qS69°10'58.12604qW

Fossilwood

940±30 8

See Fig. 1 for sample locations. All calibrations were done with the CalPal Radiocarbon Prog


meandering planforms near the confluences with the Madre de Dios.The Inambari and the Colorado Rivers have high sediment loads,locally augmented by mining activities, that bring increased sedimentsupply to the Madre de Dios in the study area.

Several anastamosing reaches are present within the Madre de Diosvalley downstream of the confluence of the Alto Madre de Dios andManu Rivers. The largest of these is located just below the Inambaritributary where a large, vegetated island is bordered by two channels.The presumed older channel, whichwas not easily navigable during lowwater, is located to the north, and exhibits near-braided channelmorphology, whereas the newer channel to the south has a distinctlymeandering character. The near-braided morphology of the northernchannel probably resulted from infilling as the southern channel becamethe dominant means of water and sediment transport.

3. Methodology

Our interpretation of the fluvial history in theMadre de Dios valley isbased on detailed field analysis of the sedimentology and geomorphol-ogy of the river valley. Terraces and modern cutbank exposures weresurveyed, using differential GPS data (error ±10 cm) to create thelongitudinal profile (Fig. 3) and a Jacob staff to determine terrace

calendar age for all of the samples used in this study.

alibratedge

Depositional(cal yr BP)environment

Unit lithology Terracenumber

9,780±100 Point bar Light tan to orange fine to mediumcross bedded sand

T2

48,000 Channel Base of a matrix-supported pebble tocobble conglomerate

T3

780±50 Floodplain Dark gray thinly laminated clay T1

530±60 Floodplain Clay above silt in a fining-upwardsequence

T1

8,160±80 Channel Imbricated, clast-supported pebbleto cobble conglomerate

T2

48,000 Channel Base of fining-upward sequences atcontact between red and yellowpaleosol (below) and thinly beddedsilty sand

T3

5,040±130 Floodplain lake(paleo-cocha)

Flaser bedded blue gray clay with somewavy laminations and small-scale softsediment deformation

T2

1,830±120 Channel Sand just below imbricated clast-supported pebble to cobble conglomeratewith sandy matrix

T2

1,970±100 Channel Sand just below imbricated clast-supported pebble to cobbleconglomerate with sandy matrix

T1

0,510±60 Floodplain Laminated to wavy laminated silt clay T1

0,450±80 Floodplain Laminated to wavy laminated silt clay T1

48,000 Channel Clast-supported cobble to boulderconglomerate

T2


T2


T2

5,530±240 Point bar Laminated sand with small clay lensesand some ripples

T2

48,000 Channel Clay lense in a poorly imbricated clast-supported pebble to cobble conglomerate

T2

9,850±100 Channel Clay lense in a poorly imbricated clast-supported pebble to cobble conglomerate

T2

90±20 Floodplain Dark brown and gray thinly laminated clay T1

70±50 Floodplain Dark brown and gray thinly laminated clay T1

ram (Weninger and Jöris, 2004) using the CalPal-2007-Hulu calibration curve.



Table 2Summary of lithofacies used in this study and explanation of lithofacies symbols used in the stratigraphic section (Figs. 4–7).

Fm Clay (F); massive and/ or mottled (m) Fsl Silt (F s) with thin or wavy laminations (l) Spr Sand (S) with planar crossbeds (p) and ripples (r)

Fmc Mud (Fm) with clay rip up clasts Fsr Silt (Fs) with ripples (r) Sr Sand (S) with ripples (r)

Fm cl Mud (Fm) with clay rip up clasts (c) andthin laminations (l)

Ssl Silty sand (Ss) with laminations (l) St Sand (S) with trough crossbeds (t )

Fml Mud (Fm) with thin or wavy laminations (l) Sslc Silty sand (Ss) with laminations (l) andclay rip up clasts (c)

Gch Gravel, clast-supported(Gc); imbricated (h)

Fm lm Mud (Fm) with thin laminations (l) andmottling (m)

Sm Sand (S); massive (m) Gcm Gravel, clast-supported(Gc); massive (m)

Fm rl Mud (Fm) with ripples (r) and thin orwavy laminations (l)

Sp Sand (S) with planar cross beds (p) Gmh Gravel, matrix-supported(Gm); imbricated (h)

Fsc Silt (Fs) with clay rip up clasts (c) Spc Sand (S) with planar cross beds (p) andclay rip up clasts (c)

Gmm Gravel, matrix-supported(Gm); massive (m)

Fsm Silt (Fs); massive and/ or mottled (m) Spg Sand (S) with locally dispersed gravel orgravel stringers (g) and planar cross beds (p)


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heights. Analysis of these data, alongwith radiocarbon and stratigraphicdata, allowed for terrace classification and for a determination of thesequence of terrace formation. Sediments beneath the terrace surfaceswere measured and described using standard sedimentological meth-ods of facies analysis to generate stratigraphic columns from fifteenlocalities in the study area. The focus of these descriptionswas on lateraland vertical facies changes as indicated by variations in grain size,sedimentary structures, bedding morphology, and the nature of stratalcontacts. From these data, we were able to differentiate between majorfluvial depositional environments preserved in the terrace sediments, toidentify smaller-scale environments (such as channel, point bar,crevasse splay, wetland, and floodplain), and to reconstruct thedepositional history of the river valley.

Datable material, including wood from fossil log-jams (palisadas),is quite abundant within the fluvial sequences. Samples were dated byAMS radiocarbon analysis at the National Ocean Sciences AcceleratedMass Spectrometer (NOSAMS) Facility, Woods Hole OceanographicInstitute. The resulting 14C dates were converted to calendar yearsbefore present (cal yrs BP) using the CalPal radiocarbon calibrationprogram with the CalPal-2007-Hulu calibration curve (Table 1). Alldates referred to in the text are the calibrated (cal yrs BP) ages.

Because of the dynamic nature of fluvial systems, 14C dates onorganic material in these sediments must be interpreted with care. Theterraces in the Madre de Dios valley occur as extensive traceable(visually and in satellite images) surfaces throughout the valley and ourinterpretations of the terraces (and our use of the radiocarbon dates forthose interpretations) are aided by the physical correlation (in the fieldand via satellite imagery) of key terraces and stratal surfaces.

Table 3Descriptions, dominant lithofacies, and depositional environments of the facies associations

Facies association Dominant lithofacies Description

FA1 Gch, Gcm, Gmh, Gmm Non/poorly imbricated or well imbricated cFA2 Sm, Sp, Spg Massive and planar cross bedded sands thaFA2 Fm, Fml, Fsl Massive and/or mottled clays and laminate


4. Sedimentology

The studied reach of the modern Madre de Dios is a meanderingriver with well-developed channel, point bar, and floodplain environ-ments. Grain sizes in the modern river range from cobble to clay, withcobble and pebble gravels present in the channel (as well as the largerpoint bars) and sand, silt, and clay present in the point bars and in theadjacent floodplain regions. Coarse-grained meandering fluvialdeposits, such as those in the modern Madre de Dios and thosedescribed below from terraced sequences in the river valley, are foundwhere fluvial discharge is sufficient to transport coarser grain sizes orwhere coarser alluvium is directly deposited into the river system(Bluck, 1971; McDowell, 1983; Grams and Schmidt, 2002). Highdischarge and local influx of coarse-grained sediment (e.g., from theColorado and Inambari tributaries) are important factors in thesedimentology of the modern Madre de Dios.

The terraced strata in the studied reach of the Madre de DiosRiver contain 23 distinct lithofacies (Table 2), that record depositionin three broadly defined facies associations (FA) within a mean-dering river depositional system: channel (FA1), point bar (FA2), andfloodplain (FA3) environments. Specific sediment types and sedi-mentary structures characterize each lithofacies, and distinctassemblages and vertical/horizontal sequences of lithofacies char-acterize each of the meandering fluvial facies association (Table 3).Overall, the meandering depositional system is characterized byfining-upward assemblages of lithofacies and FAs. These fining-upward assemblages typically overlie sharp, scoured basal contactsand are capped by thick fine-grained sequences. They are

preserved beneath the Madre de Dios terraces.

Depositional environment

last- and matrix-supported gravels Channelt may contain locally dispersed gravel or gravel stringers Point bard muds and silts Floodplain




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characterized by well-developed, laterally discontinuous, fining-upward sequences of gravel (FA1) at the base of units that fineupward to thick accumulations of sand, silt, and clay which includegeomorphically distinct lateral accretion deposits (FA2) and thicksequences of fine-grained strata (FA3).

Although the meandering fluvial strata preserved in the terracedreaches of theMadre de Dios valley have analogs in themodernMadrede Dios system (allowing for convenient, detailed comparisons), someuncertainty always exists when classifying ancient sequences as end-member (meandering or braided) fluvial types. This is because of thecomplexity of both the modern and the ancient fluvial systems. Suchcomplexity suggests that the lithofacies described here may not be theproduct of an end-member system. Indeed, themodernMadre de Diosexhibits transitional reaches characterized by poorly developed(seasonal?) braid bars in dominantly meandering reaches. Never-theless, the presence of fining-upward, laterally discontinuous gravelsequences topped with thick, laterally continuous silt and clay units –coupled with the identification of distinct lateral accretion deposits(i.e., point bars and crevasse splay deposits) – provides strongcorroborating evidence that supports the interpretation of the studiedsedimentary deposits as part of a dominantly meandering fluvialsystem.

4.1. Channel deposits — FA1

Facies association 1 (FA1), althoughmost common in the upstreamreaches of the study area, is present within all of the studied sectionsexcept Colorado II, Los Amigos II, and Tambopata (Fig. 1). This FA ischaracterized by ~6 to 7m (in the T2 terrace; e.g., Laberinto II) or ~1 to2.5 m (T1 terrace; e.g., Manu) thick sedimentary packages of thecoarsest material available in the system, is dominated by gravels andcoarse gravelly sands (the least common lithofacies in the Madre deDios strata), and is typically located at the base of fining-upwardsequences that are capped by sand or silt of FA2 or FA3. Deposits of FA1contain more fossil logs (palisadas) than any other depositionalenvironment in the studied sequences (an observation consistentwith what is observed in the modern river system) and most of theradiocarbon dates used in this study are from samples from this FA(Table 1).

4.1.1. Description of lithofaciesThe gravelly lithofacies in FA1 are generally massive, but are locally

cross-bedded and/or imbricated and may be either clast- or matrix-supported. The gravelly sequences typically fine upward from clast-supported (Gcm and Gch) cobble conglomerates to sand-supported(Gmm and Gmh) pebble conglomerates (Fig. 4A) and are overlain byeither Fsl (laminated silt), Sp (planar laminated sand), or Spg (planar-laminated gravelly sand). The massive lithofacies, such as Gcm(massive clast-supported gravels) and Gmm (matrix-supportedgravels), typically grade upward to imbricated lithofacies, such asGch (Fig. 4B) and Gmh (imbricated clast- and matrix-supportedgravels). Whereas poorly developed planar and trough cross-beddingis present rarely (e.g., at Downstream Los Amigos and UpstreamDiamante), these gravelly lithofacies do contain local, discontinuousstringers of massive (Sm) or trough (St) or planar cross-bedded sand(Sp), as well as planar cross-bedded sand with dispersed gravel (Spg).

The gravelly FA1 lithofacies are commonly conformably overlain byplanar cross-bedded point bar sediments (Sp, Spg). In some locations,especially where they occur near the top of a terraced section, thegravelly lithofacies are directly overlain by Fsl (laminated silt) and/orby a poorly developed modern soil horizon. Where exposed, the basesof the gravelly units are scoured surfaces.

Sandy FA1 lithofacies are most common in the highest (oldest; T3)terraced sequences, but are also present in the middle (T2) terracedsequence (e.g., at Laberinto I). They are characterized by thickintervals of stacked, planar cross-bedded or laminated units of


medium to coarse to gravelly sand (Spc and Sslc) with basal scoursurfaces and abundant mud clasts (Fig. 4C). These complex sandyintervals occur immediately above floodplain deposits and in associa-tion with thick lateral accretion (point bar) deposits.

4.1.2. InterpretationFA1 was deposited by vertical aggradation in active river channels.

Most of these deposits occur in well-defined fining-upwardsequences, are laterally discontinuous, and have scoured baseswhich suggests that the channels were part of a large rapidlyaggrading, sediment-rich meandering fluvial system. It is likely thatthis system contained braided and transitional reaches (as does themodern Madre de Dios), but the clear dominance of fining-upwardsequences, the scoured bases and lack of internal organization withinmany of the gravelly lithofacies, and the lateral discontinuity of thegravel deposits (as well as the FA2 sequences) suggests that thesystem was dominated by lateral migration.

Gravels overlain by point bar deposits are common in meanderingrivers (e.g., the River Endrick in Scotland Bluck, 1971, the Ramis Riverin Peru Farabaugh and Rigsby, 2005, and the Lower Wabash River ofIllinois Jackson, 1976) and channel abandonment caused by upstreamavulsion or events such as flash floods of increased sediment loadusually lead to vertical aggradation that results in fining-upwardsequences (Miall, 1996). Vertical changes in the texture of deposits,such as those seen here in the change from clast- to matrix-supportgravels, can occur during events such as channel migration, changes indischarge, and avulsion (Miall, 1996).

Clast-supportedgravels, suchas thoseof lithofaciesGchandGcm, aretypical in thick channel deposits of meandering river systems (Miall,1996). Matrix-supported gravels that overlie clast-supported gravels,such as is seen in the Madre de Dios deposits, are also common inchannel deposits and are usually indicative of deposition duringwaningflow. The thickness of the gravel deposits in these strata (beds N6 mthick in highest terraced sections at Downstream Los Amigos and UpperDiamante) likely records aggradation of established channel bars. Thesandy cross-bedded, mud-clast-rich intervals (lithofacies Spc and Sslc)are typical of multistory channel macroforms (Bridge, 1993) anddocument vertical accretion of the channel surface (Friend, 1983).

Finally, the high concentration of fossil wood in these deposits isconsistent with our own observations in the modern river channel.Rivers such as the Madre de Dios typically contain large amounts ofplant debris, including floating and submerged logs (Archer, 2005);the latter are commonly deposited in mid-channel bars and, duringlow flow periods, create log jams that are not complete removed bythe next flood.

4.2. Lateral accretion (point bar) deposits — FA2

Sediments of FA2 are characterized by sandy, cross-beddedlithofacies (Sp, Spg, Spc, and St) that generally occur within fining-upward sequences (between the coarser grained FA1 channellithofacies and the finer grained FA3 overbank lithofacies) and locallyexhibit large-scale epsilon cross-stratification. They are present in allbut three (Diamante, UpstreamDiamante, andManu) of themeasuredsections in the study area.

4.2.1. Description of lithofaciesThe most common lithofacies of FA2 are coarse- to fine-grained

planar cross-bedded (Sp) sand (Fig. 4D) and planar cross-bedded sandwith dispersed gravel and/or gravel stringers (Spg). These lithofaciesare common in terraced sequences from the T2 and T1 terraces, wherethey range from ~1 to 5 m thick. Trough cross-bedded sand (St) ispresent only in the Colorado II section, where it is overlain byoverbank deposits. Stacked sequences of planar cross-bedded sandwith angular clay clasts (Spc) are found in the Los Amigos I section.Inclined bedding and large-scale (N1 m) epsilon cross-stratification



Fig. 4. Examples of meandering fluvial lithofacies from the Madre de Dios outcrops discussed in this paper. A. Channel gravels in the Madre de Dios outcrops typical fining-upward.The sequence shown here fines from clast-supported cobble conglomerate (Gcm) at the base to matrix-supported pebble conglomerate (Gmm) at the top and contains a string ofmassive sand (Sm). B. Imbricated clast-supported conglomerate (Gch) at the top of a massive conglomerate bed. C. Close-up of a bed in a stacked sequence of planar cross-bedded tolaminatedmedium- to coarse-grained sandwith thin layers and lenses of subangular to roundedmud clasts (Sslc). D. Planar cross-bedded sand (Sp) in the basal portion of a point bardeposit at Tambopata. E. Ripple laminated sandy to silty floodplain clay (Fmrl) in the floodplain lake sequence at Los Amigos II. F. Interlayers beds of rippled sand (Sr) and wavylaminated mud (Fml) in floodplain strata at Colorado II. G. A large fossil log (diameter ~1 m) typical of those found in the channel gravels preserved in the T2 and T3 terracedsequences. H. Wood fragments in sandy silt near the base of a point bar (FA2) deposit in the T1 terraced sequence at Laberinto I.


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(after Allen, 1963) are present in the Playa Caceres (Fig. 5) and thePuerto Azul Berowe (Fig. 6) sections. Rippled sand (Sr) is common atthe base of these wedge-shaped macroforms and, locally (e.g., atLaberinto I) 10 to 20 cm thick intervals of laminated towavy laminatedto massive mud is present between the sandier beds (Fig. 7).

Organic material is rare in this facies association, but fossil wood ispresent in the FA2 sequences at Colorado I and Puerto Azul Beroweand leaf and wood debris are present in the associated muddyintervals.

4.2.2. InterpretationThe lithofacies of FA2were deposited by lateral accretion processes

on or adjacent to sandy point bars. As the primary depositional


environment in a meandering river system (Walker and Cant, 1979;Reineck and Singh,1980, andmany others), point bars producemacro-forms that record lateral migration of the river. They are typicallypreserved within fining-upward sequences, above channel depositsand below floodplain deposits (Walker and Cant, 1979; Miall, 1996;Leeder, 1999, and many others) and may be found in association withfiner grained oblique accretion deposits (Page et al., 2003).

Planar cross-bedding, which forms from mega-ripple migration, isthe most abundant type of cross-bedding in point bars — especially inthe lower portions of point bar sequences (e.g., Fig. 4D). Preservationof a sequence of scroll bars (or ridges) produced as a point barmigrates results in large, lenticular or sigmoidal packets of sandysediment that typify many ancient point bar deposits (Allen, 1963;



Fig. 5. Photographs and outcrop sketch of the meandering fluvial sequence preserved in the Playa Caceres terraced sequence (T1). The sequence fines upward and laterally andincludes point bar (FA2), channel (FA1), and floodplain (FA3) depositional environments.


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Reineck and Singh, 1980; Bridge, 1993) and are present in the Madrede Dios at the Playa Caceres, Puerto Azul Berowe, and Laberinto Isections. Trough cross-bedding, such as that in the Colorado II section(Fig. 8), is most common in the downstream ends of point bars wherewater depth and flow velocity are high enough to allow formation(Levey, 1978; Reineck and Singh, 1980). Inter-bedded woody or leafymuddy units, such as those present at Laberinto I (Fig. 7), may be theresult of oblique accretion of muddy floodplain sediments at the top ofthe point bar and migration of those sediments over the point barduring discrete floods (Page et al., 2003). Alternatively, these fine-

Fig. 6. Photographs and sketch from the point bar and floodplain facies associations (FA2developed inclined bedding surfaces and the associated lateral changes in grain size and sedimdiscussion.


grained inter-beds may have been deposited in swales or at theupstream end of the point bar during the waning phase of floods.

Dispersed gravels and gravel stringers, such as those seen atLaberinto I, Los Amigos II, and San Juan, are common in point bars incoarse-grained river systems. In such systems, stray pebbles or thinsheets of gravel (depending on the availability of gravelly material andthe strength of the current) may be incorporated in the otherwiseuniformly sandy deposits (Reineck and Singh, 1980). Where present,gravel sheets or stringers are typically inclined and intercalated withcross-bedded sands.

and 3) preserved in the Puerto Azul Bewore (PAB) section. Note the presence of well-entary structures. See Fig.1 for site location; Table 3 for lithofacies descriptions; text for



Fig. 7. Photographs and measured section of the downstream end of the Laberinto I terraced sequence. This vertical sequence (graphic and A) is characterized by stacked finingupward sand and laminated to massive mud deposited in a point bar environment (B and C) overlain by meter-think packets of fining-upward fine sandy silt to clay deposited in anoverbank (likely levee) environment.


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In addition to gravels, clay clasts (e.g., lithofacies Spc) may also beincorporated into cross-bedded point bar deposits. These rip-up clastsare typically formedwhen clayeymaterial is eroded from cut-banks or

Fig. 8. Photographs and measured section from the stacked fining-upward floodplain strataFig. 1 for site location; Table 3 for lithofacies descriptions; text for discussion.


adjacent muddy floodplain environments, deposited on the tops ofpoint bars, and subsequently reworked into the point bar sands(Bluck, 1971; Bridge, 1984).

(crevasse splay and/or levee depositions) preserved in the Colorado II (CII) section. See




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4.3. Floodplain deposits — FA3

Floodplain sediments, including levee, crevasse splay, and flood-plain lake (cocha) deposits, are common throughout the terracedstrata. They are composed of fine-grained lithofacies that may belaminated, rippled, or massive (Fig. 4E), are the most commonlypreserved deposits in the Madre de Dios strata, are typically located atthe top of fining-upward sequences, and may contain organicmaterial. Seven of our 19 radiocarbon samples are from samplescollected from floodplain deposits (Table 1).

4.3.1. Description of lithofaciesBy far, the most common floodplain lithofacies in these strata are

Fsl (laminated silt), Fml (laminated mud), and Fm (massive ormottled clay). Less commonly occurring floodplain lithofacies (e.g.,Fsr, Fmrl, Fmc, Fmcl, Fsc, Spc, and Sslc) may be mottled or containripples, angular clay clasts, or mud balls.

Wavy and rippled laminated mud (Fml and Fmrl) and silt (Fsl)lithofacies locally overlie point bar deposits (FA2) and/or clay-richfloodplain strata. At Los Amigos II, for example, faintly laminated blue-gray clay to silty clay is overlain by 5 to 10 cm thick fining-upwardpackets of cut-and-fill ripple drift lamination with clay clasts (Fig. 9).

Fig. 9. Photographs and sketch of the floodplain lake (cocha) and associated strata preserveB. Sketch of the outcrop showing lateral and vertical variations in lithofacies. C. Cut-and-fill rsilty clay and clay. D. Fossil wood typical of that preserved in the floodplain lithofacies of the Mdiscussion.


This sand is, in turn, overlain by ripple laminated and flaser beddedsandy to silty clay (Fmrl; Fig. 4E), and by laminated silt and silty clay.Small fossil logs and wood chips are present within the muddierportions of this unit.

Thin (b15 cm), intercalated beds of rippled laminated sand andsandy silt (Fsr) and wavy laminated (Fml) to massive mud (Fm) arepresent in the Los Amigos I (Fig. 4F) and the Colorado II (Fig. 8) sections.These couplets comprise horizontal to sub-horizontal beds that fineupward fromsharp, locally scoured, basal contacts. Theycommonlyhavelocally burrowed tops. The sandy basal intervals exhibit grain-size andsedimentary structure grading. The muddy intervals contain woodfragments.

At Laberinto I (Fig. 7) gently inclined, stacked, meter- to half meter-thick, fining-upward packets of laminated to wavy laminated silt andmassive mud are present above planar cross-bedded point bar sand.Similar, but thinner (10 to 20 cm), fining upward units are presentwithin the point bar unit. All of these fine-grained units contain woodfragments and local accumulations of leaf litter.

4.3.2. InterpretationAll of the FA3 lithofacies resulted from floodplain deposition.

Floodplain environments are highly complex and typically contain a

d in the Los Amigo II section. A. Overview of the right half of the Los Amigos II outcrop.ipple drift lamination overlying massive blue-gray clay and underlying wavy laminatedadre de Dios. Refer to Fig. 1 for site location; Table 2 for lithofacies descriptions; text for




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variety of sediment types and sub-environments, such as avulsiondeposits, single small crevasse splays, levees, oblique accretiondeposits, and floodplain lake (abandoned channel; cocha) deposits(Reineck and Singh,1980; Mjos et al., 1993; Brierley et al., 1997; Davis-Vollum and Kraus, 2001; Farrell, 2001; Page et al., 2003; and others).The floodplain strata, preserved in the Madre de Dios terracedsequences, include many of these sub-environments, but discontin-uous exposures and dense modern vegetation hinder precisedifferentiation among the various floodplain deposits.

Floodplain lake deposits are the most readily identifiable of thesub-environments. The vertical sequence of lithofacies in the LosAmigos II section records infilling of an abandoned channel on thefloodplain. Massive and wavy laminated mud inter-bedded withsandy, mud-clast-bearing cut-and-fill ripple drift laminations recordperiodic floods that resulted in traction deposition of sandy materialin the mud-dominated lake environment. The presence of clay rip-upclasts (likely derived from eroded cut bank and adjacent floodplaindeposits) and ripple drift laminations suggests rapid sedimentationwith high rates of suspended sediment fallout (Rubin and Hunter,1982; Farrell, 2001), as would be expected during an event thatbrought flood waters into the previously calm lake environment.

The sandmud couplets in the Colorado II and Laberinto I sequencesare likely the result of deposition in either levee or small crevassesplay environments. Distinguishing between ancient levee depositsand crevasse splay deposits is difficult. In general, discrete crevassesplay deposits are thinner and composed of finer-grained sedimentsthan levee deposits (Reineck and Singh, 1980), but large crevassesystems (such as those associated with major avulsion systems) bringextreme heterogeneity to the floodplain environments (Farrell, 2001),making it difficult to distinguish the sandier portions of floodplainenvironments from lateral accretion deposits. The sharp based sand/mud couplets in the Colorado I sequence (Fig. 8) are likely the result ofdeposition by a density current during a flood. Such deposits arecommon on trunk channel and crevasse channel levees. The scouredbases, grain-size and sedimentary structure grading, presence of woodfragments, and bioturbation and root mottling all suggest rapiddeposition by density currents followed by periods of relativequiescence.

Fig. 10. Photographs and satellite images showing the typical character of each of the threeabbreviations.


In the Laberinto I section (Fig. 7) the lowermost fining-upward silt/clay units are inter-bedded with lateral accretion deposits. The upper-most silt/clay units, however, form meter-thick fining-upward unitswith minimal sand. As stated earlier, these lower units could be theresult of oblique accretion adjacent to and above the point bar. Thepresence of leaf litter in the lower silt/clay units suggests deposition in awet, low energy but accreting environment that allows preservation ofplant material. The upper silt/clay units, which are thicker and lack leafmaterial, are more likely the result of traction current flow and leveedeposition.

5. Terrace morphology

The fluvial strata just described are preserved beneath 3 distinctterrace tracts (1 through 3) that ranged in height from 0.05 to 46.15 mabove modern river level at the time of study. The terraces arediscontinuous, but traceable (in the field and on satellite imagery;Fig.10) throughoutmost of the study area. Only the highest terrace (T3)appears to be paired. In this system no consistent, observable relation-ship exists between terrace height and distance downstream. In thissectionwe briefly describe each terrace, thenpresent a history of terraceformation based on terrace geomorphology, facies analysis, and ages ofthe terraced strata.

5.1. Terrace morphology and age

The terrace morphology and the stratigraphy of the terracedsediments preserve multiple episodes of late Quaternary fluvialaggradation and downcutting. T3, the oldest and highest terrace, rangesin height from 30 to 46.15 m above water level, and is present atColorado I, Diamante (note that the full terrace heightwasnotmeasuredat this location), and Los Amigos I (aswell asmany other locations alongthe studied reach). The highestmaximumelevation of this terrace in thestudy area (46.15 m) is at Los Amigos I, near the midpoint between theColorado and Inambari tributaries. The terrace is traceable on satelliteimages throughout much of the study area, but is only locally cut by themodern river. The late Miocene–Pleistocene (?) fluvial strata beneaththe T3 lie unconformablyabove theMiocene Ipururi Formation,which is

main terraces in the Madre de Dios valley. Refer to Fig. 1 for locations; Table 1 for site




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exposed in the lower fewmeters ofmany of the T3 exposures. The age ofthe fluvial strata underlying the T3 surface and above the IpururiFormation, is not well constrained (see Campbell et al., 2006). Samplesfrom fossil logs from the T3fluvial strata (at Los Amigos I andDiamante)yielded ages beyond the range of radiocarbon dating (N48,000 years)(Table 1). Similar results were reported by Antoine and others (2003),although the context of their sampling is unclear (for a discussion ofAntoineandothers, 2003;Hovikoski et al., 2005; Roddaz et al., 2006, andrelated studies, see Latrubesse et al., in review).

T2, themiddle terrace in age and height, wasmeasured at Colorado Iand II, Downstream Los Amigos, Laberinto II, Los Amigos II, Puerto AzulBerowe, San Juan, and Upstream Diamante. It ranges from 6.23 to20.05 m in height and is discontinuously present throughout the rivervalley. Locally, thin (b2 m) remnants of the Ipururi Formation arepresent at the base of the T2 terraced strata. Analysis of nine fossil woodsamples from the T2 terraced strata above the Ipururi Formationyieldedfive viable dates (the other four samples were radiocarbon-dead). Thedated strata yield ages of 29,850±100, 29,780±100, 28,160±80,25,530±240, and 25,040±130 cal yrs BP (Table 1). The radiocarbon-dead samples were from large fossil logs preserved in coarse gravelchannels near the base of the terraced sequences (Fig. 4G).

T1, the youngest and lowest terrace, is present at Manu (5.85 m,located on the Manu River near its confluence with the Alto Madre deDios), Playa Caceres (5.05 m), and Laberinto I (7.62 m). Wood fragments(Fig. 4H) and leaf material from sediments in the T1 terrace sequenceyield ages of 11,970±100,11,830±120,10,510±60,10,450±80, 6530±60, and 3780±50 cal yrs BP (Table 1).

Variable thicknesses of modern fill sequence are exposed anddowncut along the river valley. Although we did not study thesesections in most localities, we did examine an exposure along thelower Tambopata River, where several meters of sand-dominatedpoint bar and floodplain sediments were being actively eroded by themodern river (again, during an extreme low river level). Two samplesfrom fine-grained lithofacies near the base of a modern cut-bank inthe lower Tambopata River yielded radiocarbon ages of 870±50 and690±20 cal yrs BP (Table 1).

Fig. 11. Model of terrace development history along the studied reach of the Madre de Diossedimentation (arrows pointing up) and downcutting/erosion (arrows pointing down) eveterraced sequences; dates are in cal yrs BP. See text for discussion.


5.2. History of terrace development

Radiocarbon dating (Table 1) of fossil wood preserved in terracedfluvial sequences suggests that the terraces were formed by threedistinct aggradational periods, each followed by an episode of down-cutting. A simplified sketch of terrace development is shown in Fig. 11.

The late Miocene–Pleistocene (?) sediments in the ~50 m high T3terrace (above the Ucayali unconformity) are too old for radiocarbondating (N48,000 years BP), but record deposition in a sand-rich fluvialsystem characterized by multistory coarse sandy channel and pointbar complexes encased in thick (N6 m) sequences of silty floodplainstrata. The strata in this sequence were downcut sometime before29,780±100 cal yrs BP (the age of the oldest strata in the T2sequence). Although the principal focus of our study was on theQuaternary sediments found in the younger terrace sequences, inthose T3 sections that we did study (including some of the same onesdescribed by Campbell et al., 2006), we found no evidence supportingthe interpretation by Campbell and co-workers (2006) that the uppermembers of theMadre de Dios Formation (the upper portion of the T3terrace sequence) were deposited in an extensive deltaic environ-ment. Nor did we find any evidence supporting the interpretation ofthe same sequences as forming in a tidal environment (as suggestedby Antoine et al., 2003 and Hovikoski et al., 2005). Rather, oursedimentological observations support the deposition of T3 strata inour study area in a sand-rich, fluvial sequence, not much differentthan the modern river below.

Deposition of the T2 sedimentary sequence began before 29,780±100 cal yrs BP and lasted until at least 25,040±130 cal yrs BP —

probably much longer because the dated samples were found in thelower part of the sequence and the oldest sampled strata in the T1terrace is dated at 11,970±100 cal yrs BP. T2 strata contain thecoarsest deposits in the Madre de Dios system. These strata arecharacterized by gravelly channel deposits in a sequence that isdominated by floodplain and lateral accretion deposits. The Miocene“basement” is locally exposed in the lower portion of the T2 terracewalls and the overlying fluvial strata include basal gravels with fossil

River showing typical heights for each terrace, as well as the sequence of aggradation/nts. Dated samples are denoted by asterisks at the relative heights in the T3, T2, and T1




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logs that are beyond the range of radiocarbon dating (N48,000 yrs BP).These logs (all pieces of large trees; Fig. 4G) may have been reworkedfrom older (T3) deposits, as is probable in the San Juan section wheretwo samples of fossil wood from the same channel yielded twodifferent dates (N48,000 and 29,850±100). Alternatively, theN48,000 cal yrs BP fossil logs may be preserved within a remnant ofthe T3 sequence that is preserved at the base of the T2 sections, as ispossible in the PAB section where samples from three different fossillogs from the same channel conglomerate (FA1) are radiocarbon-dead.

The T2 fluvial sequence was downcut sometime after 25,040±130 cal yrs BP and before 11,970±100 cal yrs BP. Aggradation of the T1terraced sequence followed this downcutting: it started before11,970 cal yrs BP and lasted until at least 3780±50 cal yrs BP. Thestrata in the T1 terraced sequence record deposition in a sand-dominated meandering channel complex. The presence of modernsediments onlapping the T1 terrace, such as those in the Tambopatasection, suggest that the river system downcut the T1 terrace, thenbegan a new phase of aggradation sometime between 3780±50 and870±50 cal yrs BP.

6. Fluvial history and its relationship to climate

Deposition of the T3 sedimentary sequence (events 1 and 2, Fig. 9)began during the Miocene (Campbell et al., 2006) and ended before45,000 cal yr BP (Antoine et al., 2003). The oldest sample in the T2sequence (from San Juan) dates to 29,850±100 cal yrs BP. Therefore,incision of the T3 sediments (event 3) must have begun prior to29,850 cal yrs BP. Likewise, deposition of the T2 sediments (event 4)must have begun before this time. Deposition continued through atleast 25,040±130 cal yrs BP (the youngest sample recovered from T2)and probably much later. The T2 sediments (N30,000 to b25,000 calyrs BP) are the coarsest of any of the terrace sequences. This isconsistent with many previous studies that also found coarsestsediments deposited during the last glacial, if not the last glacialmaximum. For example, in the Bolivian Chaco at 19°S, May et al.(2008) dated a transition from coarse fluvial gravel to overlying sandsat a minimum of 22,000 cal yrs BP. May et al. (2008) summarizedother similar occurrences of the deposition of coarse fluvial sedimentsduring the last glacial in the southwestern Amazon basin: 40,000 to32,000 cal yrs BP in the Ucayali River at 5°S (Dumont et al., 1991),38,000 to 34,000 cal yrs BP in the Upper Jurua River at 8°S (Latrubesseand Rancy, 1998), 29,000 to 23,000 cal yrs BP in the Purus River at 9°S(Latrubesse and Kalicki, 2002). The paleoclimatic significance of thesefindings however, is not entirely clear.

Thewatershed of LakeTiticacaneighbors that of theMadre deDios—the headwaters of the Madre de Dios River arise within 100 km of theLake. During the entire duration of the last glacial, extending from atleast 60,000 to b20,000 cal yrsBP, the drill core record fromLakeTiticacashows unambiguously that the climate in the Andean headwaters of theMadre de Dios River was cold and wet relative to present (Fritz et al.,2007) — a positive glacial mass balance in the eastern cordillera of theAndes accompanied apositivewater balanceduring all of the last glacial.Glacial advance in the Andeanheadwaters of theMadre deDios, partly aresult of increased moisture availability, would have produced rapidrates of erosion, increased discharge and competence of the highlandtributaries, and likely, sediment aggradation in the lower reaches.

Does the coarse grain size (as well as reports of vegetation changeand increased eolian activity) of the last glacial signify that dryconditions prevailed in the southwestern Amazon during this time asconcluded byMay et al. (2008)? Bush and co-workers (2004) showedthat lowland Peru was forested during the last glacial. And Mayle(2007, personal communication) now believes that pollen studiesindicate that the last glacial climate of lowland Bolivia was relativelywet. The fluvial transport of coarser sediments necessitates wetterrather than drier conditions, although it is unclear if these wetter


conditions were limited to a shorter wet season. The Madre de DiosRiver surely remained an active fluvial system during the glacial withabundant floodplain deposition and abundant floodplain lakes. Insummary, nothing in the paleohydrologic record of the Madre de Diossuggests a dry climate in the southwestern Amazon during the lastglacial. Instead, discharge in the river was often much greater thanmodern (allowing transport of coarser sediment), but the fluvialrecord by itself is not conclusive regarding climate details such as thelength of the wet season and the seasonality of the discharge.

Aggradation of the T2 sequence may (or may not) have continuedthrough the end of the last glacial maximum, but at present we do nothave any dates from the upper part of the sequence. Incision of the T2terrace (event 5) may have begun as soon as deposition ceased. In anycase, downcutting was complete and aggradation (event 6) of the T1sequence commenced after 25,040 and before 11,970 cal yrs BP. Theriver aggraded during the Younger Dryas. On the Altiplano, evidenceexists that the Younger Dryas was an unusually wet period (e.g., Bakeret al., 2001a,b, 2005), consistent with the evidence for increaseddischarge and sediment aggradation on the floodplains of the Madrede Dios.

Aggradation of the T1 sequence began during the Younger Dryasand may have continued until at least 3,780 cal yrs BP. This is curiousbecause compelling evidence exists that the mid-Holocene was a verydry period on the Altiplano (e.g., Cross et al., 2000; Baker et al., 2001a)and in the lowlands (e.g., Mayle et al., 2007). Many alternativeexplanations exist, however, for the observations that can only beclarified by further study and further dating. For example, it is possiblethat, because mid- and late-Holocene dates (respectively, 6530 and3780 cal yr BP, and also reversed in sequence) are found in a terracethat does not have any early Holocene dates, this particular terracesequence is different from the other latest glacial (T1) sequences,continuous sedimentation did not occur during the early and middleHolocene (as portrayed in Fig. 11).

Downcutting of the T1 sequence (event 7) took place in the lateHolocene, after 3780 cal yr BP and before 870 cal yr BP. Presently, noevidence links this period of downcutting to known climate variationon the Altiplano (e.g, Ekdahl et al., 2008). Finally, deposition of themodern terrace sequence (event 8) of the Tambopata River beganbefore 870 cal yr BP and is ongoing in some reaches of the river.

7. Conclusions

Terraced strata outcrop along a significant portion of the Madre deDios River in the southwestern Amazon of Peru. Terraced sequencesrepresent four separate generations of aggradation and downcutting.Detailed sedimentological analysis of the aggraded sediments reveals23 distinct lithofacies. Despite previous studies of some of the sameoutcrops that propose deposition in sedimentary environmentsranging from marine tidal, to large lake, to extensive deltaic, wefound no evidence contradicting the parsimonious conclusion that allof the sediments that we studied were deposited in a meanderingriver environment quite similar to that of the modern Madre de DiosRiver. That said, variations in grain size and other sedimentologicalcharacteristics, as well as the presence of unconformities, demon-strate that several large changes occurred in paleohydrologic regime,likely driven by paleoclimatic variability. Although a great deal morestudy is required, particularly much additional dating to betterconstrain the timing of aggradation and downcutting, the paleohy-drologic evidence is generally consistent with the paleoclimate historyreconstructed from the fluvial and lacustrine sediments of theneighboring, high-altitude, Titicaca watershed. Thus, precipitationincrease, lake-level rise (e.g., Baker et al., 2001a), increased fluvialdischarge and floodplain aggradation on the Altiplano (e.g., Rigsbyet al., 2003; Farabaugh and Rigsby, 2005), and glacial advance in theAndes (Fritz et al., 2007) are generally concurrent with sedimentaggradation in the Madre de Dios basin. Importantly, it appears that




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sediment aggradation took place along the paleo-Madre de Dios riverthroughout much of the last glacial maximum (sensu lato, Peltier andFairbanks, 2006). Conversely, dry climates in the highlands seem tocoincide with downcutting events in the Madre de Dios River,although further study of the early Holocene is needed to determineif aggradation, downcutting, or both were coincident with thehighland climate of this period.

Acknowledgments

This project was supported by NSF grants to Rigsby (EAR-0227999)and Baker (EAR-0227550) and by a grant to Rigsby from the Divisionof Research at East Carolina University (2006–2007 Research Devel-opment Grant). We thank Professor Miles Silman for introducing us toresearch opportunities in the field area; Dr. Nigel Pitman and theACCA/ACA-funded Centro de Investigación y Capacitación del Río LosAmigos for their hospitality and data sharing; and Professors EdgardoLatrubesse and Sherilyn Fritz for their critical reviews of themanuscript.

References

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ARTICLE IN PRESS

Ronchail, J., Bourrel, L., Cochonneau, G., Vauchel, P., Phillips, L., Castro, A., Guyot, J.-L.,Oliveira, E., 2005. Inundations in the Mamoré basin (south-western Amazon—Bolivia) and sea-surface temperature in the Pacific and Atlantic Oceans. Journal ofHydrology 302, 223–238.

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Zhou, J., Lau, K.-M., 2001. Principal modes of interannual and decadal variability ofsummer rainfall over South America. International Journal of Climatology 21,1623–1644.


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ARTICLE IN PRESS - East Carolina Universitycore.ecu.edu/geology/rigsbyc/rigsby/Papers/Rigsby Hemric... · 2019. 11. 5. · The Ipururo Formation is best seen in exposures at the base