Late Quaternary Paleohydrology of the Madre De Dios River, Southwestern Amazon Basin, Peru

ARTICLE IN PRESS GEOMOR-02923; No of Pages 15 Geomorphology xxx (2009) xxx–xxx

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Geomorphology

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

Catherine A. Rigsby a,⁎, Erin M. Hemric a, Paul A. Baker b a Department of Geological Sciences, East Carolina University, Greenville, NC 27858 USA b Division of Earth & Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27705 USA article info abstract

Article history: Late Quaternary climatic and hydrologic variability triggered changes in fluvial deposition and erosion along Received 3 December 2007 the course of the Madre de Dios River, Peru, the largest tributary basin of the Madeira basin, itself the largest Accepted 11 November 2008 tributary basin of the Amazon. Three laterally extensive, Quaternary-age, terrace tracts are present within the Available online xxxx Madre de Dios basin. Analysis of sedimentary facies, present in the modern cut banks and terraced sequences, along with radiocarbon dates on fossil wood and leaf material preserved in the terraced strata, Keywords: allow reconstruction of the Late Quaternary depositional history of the sedimentary sequences, including Quaternary Peru determination of the approximate timing of aggradation and downcutting episodes and its relationship to the Amazon timing of past climate change in this portion of the Amazon basin and beyond. River The Quaternary sediments underlying the terraces most often recorded deposition in a coarse-grained Paleohydrology meandering fluvial system. The T3 terrace, the highest terrace, is underlain by the Miocene (?) Ipururi Formation, which is unconformably overlain by the late Miocene–Pleistocene (?) (N48,000 cal yrs BP) Madre de Dios Formation, a multistory coarse-sandy to gravelly channel and point bar complex. The latter was downcut before 29,850±100 cal yrs BP. This downcut landscape was infilled by meandering fluvial strata characterized by gravelly channel deposits in a sequence dominated by floodplain and lateral accretion deposits. These strata were in turn downcut to form the T2 terrace before 11,970±100 cal yrs BP. A third episode of aggradation resulted in the deposition of a sand-dominated meandering channel complex that infilled the T2 valley and was subsequently downcut after 3780±50 cal yrs BP. This most recent terrace is infilled 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, (and likely much younger, as dated samples were, thus far, only found near the base of the T2 sequence). This observation implies that some combination of (1) increased precipitation and decreased temperature, (2) decreased evapotranspiration and increased runoff, (3) increased Andean glacial erosion and increased

sediment supply, and (4) decreased atmospheric CO2 (hence decreased rain-forest primary productivity and altered rain-forest physiology/ecology), entering the last glacial maximum period brought about increased floodplain deposition in the southwestern Amazon. Elsewhere in the Amazon basin few, if any, fluvial sediments of this age range have been observed. The start of the next major phase of aggradation coincided with the Younger Dryas and suggested that floodplain sedimentation in the lowlands was again related to cold and wet conditions in the adjacent highlands (and perhaps in the lowlands as well) and that Madre de Dios history was also tied to large-scale global climate. This aggradation may have continued throughout the early and mid-Holocene, until at least 3,780 cal yr BP. If so (and this is uncertain), this episode of sedimentation took place during a dry period. © 2009 Elsevier B.V. All rights reserved.

1. Introduction biodiversity that is undergoing rapid changes as a result of direct human endeavor and climate change. Despite growing recognition of this The Amazon River drains approximately 38% of the area of transformation, few studies exist of the paleohydrology of the Amazon continental South America and accounts for over 18% of the total Basin designed to determine the range and mechanisms of past changes freshwater input to the oceans. From the Andes to the Atlantic, the river of fluvial processes as well as to elucidate the future of this fluvial runs through the largest tropical rainforest, a region of unsurpassed environment (e.g., Latrubesse, 2003; Latrubesse et al., 2005). And, paleohydrologic information is largely absent from most paleoenvironmental discussions of the Amazon region, despite the Amazon landscape fl ⁎ Corresponding author. being dominantly of uvial origin. Understanding the relatively sparse E-mail address: [email protected] (C.A. Rigsby). and geographically restricted Quaternary vegetation history of the

Please cite this article as: Rigsby, C.A., et al., Late Quaternary Paleohydrology of the Madre de Dios River, southwestern Amazon Basin, Peru, Geomorphology (2009), doi:10.1016/j.geomorph.2008.11.017 ARTICLE IN PRESS

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Amazon (e.g., Colinvaux et al., 2000; Mayle et al., 2007; Anhuf et al., abundant. And opinions differ widely about the nature of Amazon 2006)isdifficult, at best, without independent information about climate, particularly prior to the Holocene. Quaternary climate change and attendant fluvial evolution in the region. In this paper, we document fluvial history in the southwestern Moreover, the paucity of long-lived and well-preserved natural archives Amazon basin by examining the Late Quaternary sedimentary of past climate and hydrology in the Amazon lowlands (e.g., Ledru et al., sequences exposed in terraced strata along a portion of the Madre 1998; Bush et al., 2004; Anhuf et al., 2006) has largely precluded de Dios River (Peru) and we compare these sequences with records of definitive conclusions regarding the history of Amazon precipitation and climate and fluvial history elsewhere in tropical South America. Our runoff. As a result, we do not know for sure if Amazon climate has shifted reconstruction of the history of the Madre de Dios fluvial system in lockstep with climate in the neighboring highlands, where paleocli- provides landscape level evidence of past hydrologic change that mate and paleohydrologic records of the late Quaternary are more extends back to about 30,000 cal yrs BP and is an important step

Fig. 1. Map showing the study area in the Madre de Dios Department in southeastern Peru, southwestern Amazon Basin. Triangles indicate measured section locations. Refer to Table 1 for site abbreviations.

C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx 3 toward achieving a broader paleohydrologic history of the entire Amazon basin.

2. Environmental setting

2.1. Geological setting

The Madeira River is the largest tributary of the Amazon in area and discharge. Its watershed has an area of 1,380,000 km2, 20.1% of the total area of the Amazon watershed, and an average discharge of about 30,000 m3 s− 1, approximately 15% of the total Amazon discharge (Goulding et al., 2003), twice the average annual discharge of the Mississippi. The Madeira drains the entire southwestern Amazon region and is itself comprised of several large tributary basins including the Guaporé–Iténéz with headwaters in the Brazilian Shield, the Mamore draining much of lowland eastern and central Bolivia, and the Beni basin of northwestern Bolivia and southeastern Peru. The Fig. 3. Longitudinal profile of the Madre de Dios River from the Upstream of Diamante Madre de Dios River is the largest tributary to the Beni; the drainage section on the Alto Madre de Dios River to the confluence of the Madre de Dios and basin of the Madre de Dios roughly comprises the western two-thirds Tambopata Rivers at Puerto Maldonado. Locations of confluences with tributary rivers of the Beni basin. Some of the tributaries of the Madre de Dios are indicated by vertical dotted lines. All profile points (including measured section originate in the glaciated Cordillera Oriental of the Andes whereas locations) are plotted as triangles (see Fig. 1 for locations of measured sections; Table 1 other tributaries arise in the lowland rain forests of Peru and Bolivia. for site abbreviations). The anomaly in elevation upstream of the Inambari tributary is – – the result of a rapid and short-lived increase in river level after precipitation the day The 650-km long reach of the Madre de Dios River (12 13° S, 69 before the measurement was taken. 71° W) discussed in the present study (Fig. 1) stretches from upstream of the confluence of the Alto Madre de Dios and Manu Rivers at Boca de Manu, Peru, downstream to the confluence of the Madre de Dios and Tambopata Rivers at Puerto Maldonado, Peru (including as well a portion of the lower Tambopata River). Throughout this region the 2.2. Climatic setting course of the Madre de Dios is eroded into thick Tertiary and Quaternary sedimentary sequences of the Andean foreland basin (e.g., Baby et al., The modern climate of the lowland portions of the southwestern 1997; Galloso et al.,1996; Guyot et al.,1999; Horton and DeCelles,1997). Amazon basin is humid tropical with mean monthly temperatures The strata examined in this study are exposed in cut-banks and terraced varying just a few degrees throughout the year. The diurnal reaches of the modern river valley. The strata are inset within deposits of temperature range is several times the seasonal cycle. The mean the late Miocene–Pleistocene (age uncertain) Madre de Dios Formation, monthly low temperature of 16.8 °C occurs in July and the mean which is composed of fluvial gravel, sand and mud deposits. The Madre monthly high of 32.0 °C occurs in October at Puerto Maldonado, Peru. de Dios Formation is separated from the underlying Miocene Ipururo Annual rainfall at Puerto Maldonado averages 2300 mm. Three- Formation by the Ucayali unconformity (Campbell et al., 2001, 2006). quarters of this precipitation occurs during the half-year from The Ipururo Formation is best seen in exposures at the base of some of November through April. During the austral winter (June, July, and the terraced sections, especially during periods of lower water. August), monthly precipitation averages less than 65 mm. Most of the wet-season precipitation in this region results from circulations associated with the South American summer monsoon (SASM; e.g., Zhou and Lau, 1998). Interannual-to-decadal variability of SASM precipitation is partly forced by tropical Pacific (ENSO) and tropical Atlantic variability (e.g., Zhou and Lau, 2001). These relationships are weak and non-stationary (e.g. Ronchail et al., 2005). River stage and precipitation data have been collected at the Centro 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 (Amazon Conservation Association, unpublished data). Mean annual precipitation at CICRA during the measurement period was 2653 mm. The driest year recorded at CICRA (2150 mm) was 2005. Throughout the Amazon basin, 2005 was the sixth driest year on record and this drought was most severe in the southwestern Amazon (Luis Aragão, personal communication, 2007). The origin of this particular drought can be ascribed to abnormally warm sea-surface temperatures pervading in the northern tropical Atlantic during much of 2005. The stage of the Madre de Dios (Fig. 2) responds rapidly to precipitation events — 2 to 3 m rises or falls of the river overnight following an upstream deluge are common, as anyone camping and boating on the river will surely learn. Our field work was undertaken from the end of June through early July during the 2005 dry season when the total monthly precipitation values were respectively 217.6 and Fig. 2. Madre de Dios river levels for January–December 2005 recorded at CICRA (Amazon − Conservation Association, unpublished data). The region has a pronounced austral winter 29.9 mm, and river level was the lowest on record ( 7.63 m below its dry season (July–October) that corresponds to the period of low river levels. mean; Fig. 2, Amazon Conservation Association, unpublished data).

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2.3. Morphology of the modern Madre de Dios meandering planforms near the confluences with the Madre de Dios. The Inambari and the Colorado Rivers have high sediment loads, The modern geomorphology of the studied portion of the Madre locally augmented by mining activities, that bring increased sediment de Dios valley has recently been described by Hamilton and co- supply to the Madre de Dios in the study area. workers (2007). The longitudinal profile of the Madre de Dios valley Several anastamosing reaches are present within the Madre de Dios within the study area drops about 131 m in elevation over a horizontal valley downstream of the confluence of the Alto Madre de Dios and distance of about 280 km (Fig. 3). Thus, the studied reach of the Manu Rivers. The largest of these is located just below the Inambari modern Madre de Dios has a gradient of about 0.00047 and exhibits a tributary where a large, vegetated island is bordered by two channels. highly sinuous meandering morphology with an average sinuosity The presumed older channel, which was not easily navigable during low index of 2.3. Because river level was very low during our study, the water, is located to the north, and exhibits near-braided channel measured gradient is likely a close approximation of average channel morphology, whereas the newer channel to the south has a distinctly bed slope. The river valley contains numerous cutoffs and floodplain meandering character. The near-braided morphology of the northern lakes, which predominantly form by channel abandonment as a result channel probably resulted from infilling as the southern channel became of neck cutoff. Mean channel width of the rivers in this system, as the dominant means of water and sediment transport. measured by Puhakka and colleagues (1992), ranges from 500 m for the Madre de Dios to 100 m for the Los Amigos. Much like the Madre 3. Methodology de Dios, the Manu and Los Amigos rivers have tortuous meanders with many oxbow (floodplain) lakes. The Tambopata is also a meandering Our interpretation of the fluvial history in the Madre de Dios valley is river with a well defined floodplain, but it has a very irregular based on detailed field analysis of the sedimentology and geomorphol- meander pattern and few floodplain lakes. The Alto Madre de Dios, ogy of the river valley. Terraces and modern cutbank exposures were Colorado and Inambari rivers, with headwaters in the nearby surveyed, using differential GPS data (error ±10 cm) to create the Cordillera Oriental, have braided reaches but are characterized by longitudinal profile (Fig. 3) and a Jacob staff to determine terrace

Table 1 Sample name, numbers, location stratigraphic information, radiocarbon age and calibrated calendar age for all of the samples used in this study.

Sample name Sample NOSAMS Location Material Radiocarbon Calibrated Depositional Unit lithology Terrace number number dated age age (cal yr BP) number environment Colorado II #1 CII-1 OS-51221 12°35'36.67260qS Fossil 24,800±130 29,780±100 Point bar Light tan to orange fine to medium T2 70°22'31.18108qW wood cross bedded sand Diamante #1 D-1 OS-56254 12°35'36.67260qS Fossil N48,000 N48,000 Channel Base of a matrix-supported pebble to T3 70°22'31.18108qW wood cobble conglomerate Laberinto I #1 L1-1 OS-56257 12°40'38.89800qS Fossil 3500±35 3780±50 Floodplain Dark gray thinly laminated clay T1 69°33'25.93595qW wood Laberinto I #2 LI-2 OS-51222 12°40'38.89800qS Leafy 5720±40 6530±60 Floodplain Clay above silt in a fining-upward T1 69°33'25.93595qW material sequence Laberinto II #1 LII-1 OS-56256 12°33'11.06217qS Fossil 23,400±100 28,160±80 Channel Imbricated, clast-supported pebble T2 69°22'11.15636qW wood to cobble conglomerate Los Amigos I #1 LA I-1 OS-56257 12°34'00.45811qS Fossil N48,000 N48,000 Channel Base of fining-upward sequences at T3 70°06'18.58780qW wood contact between red and yellow paleosol (below) and thinly bedded silty sand Los Amigos II #1 LA II-1 OS-51223 12°33'26.29793qS Fossil 21,100±110 25,040 ±130 Floodplain lake Flaser bedded blue gray clay with some T2 70°05'29.45067qW wood (paleo-cocha) wavy laminations and small-scale soft sediment deformation Manu #1 M-1 OS-56246 12°17'40.54654qS Fossil 10,150±45 11,830±120 Channel Sand just below imbricated clast- T2 70°52'34.74911qW wood supported pebble to cobble conglomerate with sandy matrix Manu #2 M-2 OS-56247 12°17'40.54654qS Fossil 10,250±40 11,970±100 Channel Sand just below imbricated clast- T1 70°52'34.74911qW wood supported pebble to cobble conglomerate with sandy matrix Playa Caceres #1 PC-1 OS-56248 12°43'27.87141qS Fossil 9310±35 10,510±60 Floodplain Laminated to wavy laminated silt clay T1 69°38'10.80215qW wood Playa Caceres #2 PC-2 OS-56134 12°43'27.87141qS Fossil 9270±40 10,450±80 Floodplain Laminated to wavy laminated silt clay T1 69°38'10.80215qW wood Puerto Azul PAB-1 OS-56249 12°17'21.02900qS Fossil N48,000 N48,000 Channel Clast-supported cobble to boulder T2 Berowe #1 70°45'43.99088qW wood conglomerate Puerto Azul PAB-2 OS-56136 12°17'21.02900qS Fossil N48,000 N48,000 Channel Clast-supported cobble to boulder T2 Berowe #2 70°45'43.99088qW wood conglomerate Puerto Azul PAB-3 OS-56250 12°17'21.02900qS Fossil N48,000 N48,000 Channel Clast-supported cobble to boulder T2 Berowe #3 70°45'43.99088qW wood conglomerate Puerto Azul PAB-5 OS-51224 12°17'21.02900qS Fossil 21,500±110 25,530±240 Point bar Laminated sand with small clay lenses T2 Berowe #5 70°45'43.99088qW wood and some ripples San Juan #1 SJ-1 OS-56251 12°33'35.40583qS Fossil N48,000 N48,000 Channel Clay lense in a poorly imbricated clast- T2 70°09'44.35239qW wood supported pebble to cobble conglomerate San Juan #2 SJ-2 OS-51225 12°33'35.40583qS Fossil 24,900±120 29,850±100 Channel Clay lense in a poorly imbricated clast- T2 70°09'44.35239qW wood supported pebble to cobble conglomerate Tambopata #1 T-1 OS-56252 12°39'22.34034qS Fossil 730±25 690±20 Floodplain Dark brown and gray thinly laminated clay T1 69°10'58.12604qW wood Tambopata #2 T-2 OS-56253 12°39'22.34034qS Fossil 940±30 870±50 Floodplain Dark brown and gray thinly laminated clay T1 69°10'58.12604qW wood

See Fig. 1 for sample locations. All calibrations were done with the CalPal Radiocarbon Program (Weninger and Jöris, 2004) using the CalPal-2007-Hulu calibration curve.

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Table 2 Summary 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 cross beds (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) and Ssl Silty sand (Ss) with laminations (l) St Sand (S) with trough cross thin laminations (l) beds (t )

Fml Mud (Fm) with thin or wavy laminations (l) Sslc Silty sand (Ss) with laminations (l) and Gch Gravel, clast-supported clay rip up clasts (c) (Gc); imbricated (h)

Fm lm Mud (Fm) with thin laminations (l) and Sm Sand (S); massive (m) Gcm Gravel, clast-supported mottling (m) (Gc); massive (m)

Fm rl Mud (Fm) with ripples (r) and thin or Sp Sand (S) with planar cross beds (p) Gmh Gravel, matrix-supported wavy laminations (l) (Gm); imbricated (h)

Fsc Silt (Fs) with clay rip up clasts (c) Spc Sand (S) with planar cross beds (p) and Gmm Gravel, matrix-supported clay rip up clasts (c) (Gm); massive (m)

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

heights. Analysis of these data, along with radiocarbon and stratigraphic 4. Sedimentology data, allowed for terrace classification and for a determination of the sequence of terrace formation. Sediments beneath the terrace surfaces The studied reach of the modern Madre de Dios is a meandering were measured and described using standard sedimentological meth- river with well-developed channel, point bar, and floodplain environ- ods of facies analysis to generate stratigraphic columns from fifteen ments. Grain sizes in the modern river range from cobble to clay, with localities in the study area. The focus of these descriptions was on lateral cobble and pebble gravels present in the channel (as well as the larger and vertical facies changes as indicated by variations in grain size, point bars) and sand, silt, and clay present in the point bars and in the sedimentary structures, bedding morphology, and the nature of stratal adjacent floodplain regions. Coarse-grained meandering fluvial contacts. From these data, we were able to differentiate between major deposits, such as those in the modern Madre de Dios and those fluvial depositional environments preserved in the terrace sediments, to described below from terraced sequences in the river valley, are found identify smaller-scale environments (such as channel, point bar, where fluvial discharge is sufficient to transport coarser grain sizes or crevasse splay, wetland, and floodplain), and to reconstruct the where coarser alluvium is directly deposited into the river system depositional history of the river valley. (Bluck, 1971; McDowell, 1983; Grams and Schmidt, 2002). High Datable material, including wood from fossil log-jams (palisadas), discharge and local influx of coarse-grained sediment (e.g., from the is quite abundant within the fluvial sequences. Samples were dated by Colorado and Inambari tributaries) are important factors in the AMS radiocarbon analysis at the National Ocean Sciences Accelerated sedimentology of the modern Madre de Dios. Mass Spectrometer (NOSAMS) Facility, Woods Hole Oceanographic The terraced strata in the studied reach of the Madre de Dios Institute. The resulting 14C dates were converted to calendar years River contain 23 distinct lithofacies (Table 2), that record deposition before present (cal yrs BP) using the CalPal radiocarbon calibration in three broadly defined facies associations (FA) within a mean- program with the CalPal-2007-Hulu calibration curve (Table 1). All dering river depositional system: channel (FA1), point bar (FA2), and dates referred to in the text are the calibrated (cal yrs BP) ages. floodplain (FA3) environments. Specific sediment types and sedi- Because of the dynamic nature of fluvial systems, 14C dates on mentary structures characterize each lithofacies, and distinct organic material in these sediments must be interpreted with care. The assemblages and vertical/horizontal sequences of lithofacies char- terraces in the Madre de Dios valley occur as extensive traceable acterize each of the meandering fluvial facies association (Table 3). (visually and in satellite images) surfaces throughout the valley and our Overall, the meandering depositional system is characterized by interpretations of the terraces (and our use of the radiocarbon dates for fining-upward assemblages of lithofacies and FAs. These fining- those interpretations) are aided by the physical correlation (in the field upward assemblages typically overlie sharp, scoured basal contacts and via satellite imagery) of key terraces and stratal surfaces. and are capped by thick fine-grained sequences. They are

Table 3 Descriptions, dominant lithofacies, and depositional environments of the facies associations preserved beneath the Madre de Dios terraces.

Facies association Dominant lithofacies Description Depositional environment FA1 Gch, Gcm, Gmh, Gmm Non/poorly imbricated or well imbricated clast- and matrix-supported gravels Channel FA2 Sm, Sp, Spg Massive and planar cross bedded sands that may contain locally dispersed gravel or gravel stringers Point bar FA2 Fm, Fml, Fsl Massive and/or mottled clays and laminated muds and silts Floodplain

6 C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx characterized by well-developed, laterally discontinuous, fining- medium to coarse to gravelly sand (Spc and Sslc) with basal scour upward sequences of gravel (FA1) at the base of units that fine surfaces and abundant mud clasts (Fig. 4C). These complex sandy upward to thick accumulations of sand, silt, and clay which include intervals occur immediately above floodplain deposits and in associa- geomorphically distinct lateral accretion deposits (FA2) and thick tion with thick lateral accretion (point bar) deposits. sequences of fine-grained strata (FA3). Although the meandering fluvial strata preserved in the terraced 4.1.2. Interpretation reaches of the Madre de Dios valley have analogs in the modern Madre FA1 was deposited by vertical aggradation in active river channels. de Dios system (allowing for convenient, detailed comparisons), some Most of these deposits occur in well-defined fining-upward uncertainty always exists when classifying ancient sequences as end- sequences, are laterally discontinuous, and have scoured bases member (meandering or braided) fluvial types. This is because of the which suggests that the channels were part of a large rapidly complexity of both the modern and the ancient fluvial systems. Such aggrading, sediment-rich meandering fluvial system. It is likely that complexity suggests that the lithofacies described here may not be the this system contained braided and transitional reaches (as does the product of an end-member system. Indeed, the modern Madre de Dios modern Madre de Dios), but the clear dominance of fining-upward exhibits transitional reaches characterized by poorly developed sequences, the scoured bases and lack of internal organization within (seasonal?) braid bars in dominantly meandering reaches. Never- many of the gravelly lithofacies, and the lateral discontinuity of the theless, the presence of fining-upward, laterally discontinuous gravel gravel deposits (as well as the FA2 sequences) suggests that the sequences topped with thick, laterally continuous silt and clay units – system was dominated by lateral migration. coupled with the identification of distinct lateral accretion deposits Gravels overlain by point bar deposits are common in meandering (i.e., point bars and crevasse splay deposits) – provides strong rivers (e.g., the River Endrick in Scotland Bluck, 1971, the Ramis River corroborating evidence that supports the interpretation of the studied in Peru Farabaugh and Rigsby, 2005, and the Lower Wabash River of sedimentary deposits as part of a dominantly meandering fluvial Illinois Jackson, 1976) and channel abandonment caused by upstream system. avulsion or events such as flash floods of increased sediment load usually lead to vertical aggradation that results in fining-upward 4.1. Channel deposits — FA1 sequences (Miall, 1996). Vertical changes in the texture of deposits, such as those seen here in the change from clast- to matrix-support Facies association 1 (FA1), although most common in the upstream gravels, can occur during events such as channel migration, changes in reaches of the study area, is present within all of the studied sections discharge, and avulsion (Miall, 1996). except Colorado II, Los Amigos II, and Tambopata (Fig. 1). This FA is Clast-supported gravels, such as those of lithofacies Gch and Gcm, are characterized by ~6 to 7 m (in the T2 terrace; e.g., Laberinto II) or ~1 to typical in thick channel deposits of meandering river systems (Miall, 2.5 m (T1 terrace; e.g., Manu) thick sedimentary packages of the 1996). Matrix-supported gravels that overlie clast-supported gravels, coarsest material available in the system, is dominated by gravels and such as is seen in the Madre de Dios deposits, are also common in coarse gravelly sands (the least common lithofacies in the Madre de channel deposits and are usually indicative of deposition during waning Dios strata), and is typically located at the base of fining-upward flow. The thickness of the gravel deposits in these strata (beds N6m sequences that are capped by sand or silt of FA2 or FA3. Deposits of FA1 thick in highest terraced sections at Downstream Los Amigos and Upper contain more fossil logs (palisadas) than any other depositional Diamante) likely records aggradation of established channel bars. The environment in the studied sequences (an observation consistent sandy cross-bedded, mud-clast-rich intervals (lithofacies Spc and Sslc) with what is observed in the modern river system) and most of the are typical of multistory channel macroforms (Bridge, 1993)and radiocarbon dates used in this study are from samples from this FA document vertical accretion of the channel surface (Friend, 1983). (Table 1). Finally, the high concentration of fossil wood in these deposits is consistent with our own observations in the modern river channel. 4.1.1. Description of lithofacies Rivers such as the Madre de Dios typically contain large amounts of The gravelly lithofacies in FA1 are generally massive, but are locally plant debris, including floating and submerged logs (Archer, 2005); cross-bedded and/or imbricated and may be either clast- or matrix- the latter are commonly deposited in mid-channel bars and, during supported. The gravelly sequences typically fine upward from clast- low flow periods, create log jams that are not complete removed by supported (Gcm and Gch) cobble conglomerates to sand-supported the next flood. (Gmm and Gmh) pebble conglomerates (Fig. 4A) and are overlain by either Fsl (laminated silt), Sp (planar laminated sand), or Spg (planar- 4.2. Lateral accretion (point bar) deposits — FA2 laminated gravelly sand). The massive lithofacies, such as Gcm (massive clast-supported gravels) and Gmm (matrix-supported Sediments of FA2 are characterized by sandy, cross-bedded gravels), typically grade upward to imbricated lithofacies, such as lithofacies (Sp, Spg, Spc, and St) that generally occur within fining- Gch (Fig. 4B) and Gmh (imbricated clast- and matrix-supported upward sequences (between the coarser grained FA1 channel gravels). Whereas poorly developed planar and trough cross-bedding lithofacies and the finer grained FA3 overbank lithofacies) and locally is present rarely (e.g., at Downstream Los Amigos and Upstream exhibit large-scale epsilon cross-stratification. They are present in all Diamante), these gravelly lithofacies do contain local, discontinuous but three (Diamante, Upstream Diamante, and Manu) of the measured stringers of massive (Sm) or trough (St) or planar cross-bedded sand sections in the study area. (Sp), as well as planar cross-bedded sand with dispersed gravel (Spg). The gravelly FA1 lithofacies are commonly conformably overlain by 4.2.1. Description of lithofacies planar cross-bedded point bar sediments (Sp, Spg). In some locations, The most common lithofacies of FA2 are coarse- to fine-grained especially where they occur near the top of a terraced section, the planar cross-bedded (Sp) sand (Fig. 4D) and planar cross-bedded sand gravelly lithofacies are directly overlain by Fsl (laminated silt) and/or with dispersed gravel and/or gravel stringers (Spg). These lithofacies by a poorly developed modern soil horizon. Where exposed, the bases are common in terraced sequences from the T2 and T1 terraces, where of the gravelly units are scoured surfaces. they range from ~1 to 5 m thick. Trough cross-bedded sand (St) is Sandy FA1 lithofacies are most common in the highest (oldest; T3) present only in the Colorado II section, where it is overlain by terraced sequences, but are also present in the middle (T2) terraced overbank deposits. Stacked sequences of planar cross-bedded sand sequence (e.g., at Laberinto I). They are characterized by thick with angular clay clasts (Spc) are found in the Los Amigos I section. intervals of stacked, planar cross-bedded or laminated units of Inclined bedding and large-scale (N1 m) epsilon cross-stratification

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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 of massive 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 to laminated medium- to coarse-grained sand with thin layers and lenses of subangular to rounded mud clasts (Sslc). D. Planar cross-bedded sand (Sp) in the basal portion of a point bar deposit 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 wavy laminated 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 terraced sequences. H. Wood fragments in sandy silt near the base of a point bar (FA2) deposit in the T1 terraced sequence at Laberinto I.

(after Allen, 1963) are present in the Playa Caceres (Fig. 5) and the environment in a meandering river system (Walker and Cant, 1979; Puerto Azul Berowe (Fig. 6) sections. Rippled sand (Sr) is common at Reineck and Singh,1980, and many others), point bars produce macro- the base of these wedge-shaped macroforms and, locally (e.g., at forms that record lateral migration of the river. They are typically Laberinto I) 10 to 20 cm thick intervals of laminated to wavy laminated preserved within fining-upward sequences, above channel deposits to massive mud is present between the sandier beds (Fig. 7). and below floodplain deposits (Walker and Cant, 1979; Miall, 1996; Organic material is rare in this facies association, but fossil wood is Leeder, 1999, and many others) and may be found in association with present in the FA2 sequences at Colorado I and Puerto Azul Berowe finer grained oblique accretion deposits (Page et al., 2003). and leaf and wood debris are present in the associated muddy Planar cross-bedding, which forms from mega-ripple migration, is intervals. the most abundant type of cross-bedding in point bars — especially in the lower portions of point bar sequences (e.g., Fig. 4D). Preservation 4.2.2. Interpretation of a sequence of scroll bars (or ridges) produced as a point bar The lithofacies of FA2 were deposited by lateral accretion processes migrates results in large, lenticular or sigmoidal packets of sandy on or adjacent to sandy point bars. As the primary depositional sediment that typify many ancient point bar deposits (Allen, 1963;

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

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 and includes point bar (FA2), channel (FA1), and floodplain (FA3) depositional environments.

Reineck and Singh, 1980; Bridge, 1993) and are present in the Madre grained inter-beds may have been deposited in swales or at the de Dios at the Playa Caceres, Puerto Azul Berowe, and Laberinto I upstream end of the point bar during the waning phase of floods. sections. Trough cross-bedding, such as that in the Colorado II section Dispersed gravels and gravel stringers, such as those seen at (Fig. 8), is most common in the downstream ends of point bars where Laberinto I, Los Amigos II, and San Juan, are common in point bars in water depth and flow velocity are high enough to allow formation coarse-grained river systems. In such systems, stray pebbles or thin (Levey, 1978; Reineck and Singh, 1980). Inter-bedded woody or leafy sheets of gravel (depending on the availability of gravelly material and muddy units, such as those present at Laberinto I (Fig. 7), may be the the strength of the current) may be incorporated in the otherwise result of oblique accretion of muddy floodplain sediments at the top of uniformly sandy deposits (Reineck and Singh, 1980). Where present, the point bar and migration of those sediments over the point bar gravel sheets or stringers are typically inclined and intercalated with during discrete floods (Page et al., 2003). Alternatively, these fine- cross-bedded sands.

Fig. 6. Photographs and sketch from the point bar and ﬂoodplain facies associations (FA2 and 3) preserved in the Puerto Azul Bewore (PAB) section. Note the presence of well- developed inclined bedding surfaces and the associated lateral changes in grain size and sedimentary structures. See Fig. 1 for site location; Table 3 for lithofacies descriptions; text for discussion.

C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx 9

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 fining upward 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 an overbank (likely levee) environment.

In addition to gravels, clay clasts (e.g., lithofacies Spc) may also be adjacent muddy ﬂoodplain environments, deposited on the tops of incorporated into cross-bedded point bar deposits. These rip-up clasts point bars, and subsequently reworked into the point bar sands are typically formed when clayey material is eroded from cut-banks or (Bluck, 1971; Bridge, 1984).

Fig. 8. Photographs and measured section from the stacked ﬁning-upward ﬂoodplain strata (crevasse splay and/or levee depositions) preserved in the Colorado II (CII) section. See Fig. 1 for site location; Table 3 for lithofacies descriptions; text for discussion.

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4.3. Floodplain deposits — FA3 This sand is, in turn, overlain by ripple laminated and flaser bedded sandy to silty clay (Fmrl; Fig. 4E), and by laminated silt and silty clay. Floodplain sediments, including levee, crevasse splay, and flood- Small fossil logs and wood chips are present within the muddier plain lake (cocha) deposits, are common throughout the terraced portions of this unit. strata. They are composed of fine-grained lithofacies that may be Thin (b15 cm), intercalated beds of rippled laminated sand and laminated, rippled, or massive (Fig. 4E), are the most commonly sandy silt (Fsr) and wavy laminated (Fml) to massive mud (Fm) are preserved deposits in the Madre de Dios strata, are typically located at present in the Los Amigos I (Fig. 4F) and the Colorado II (Fig. 8)sections. the top of fining-upward sequences, and may contain organic These couplets comprise horizontal to sub-horizontal beds that fine material. Seven of our 19 radiocarbon samples are from samples upward from sharp, locally scoured, basal contacts. They commonly have collected from floodplain deposits (Table 1). locally burrowed tops. The sandy basal intervals exhibit grain-size and sedimentary structure grading. The muddy intervals contain wood 4.3.1. Description of lithofacies fragments. By far, the most common floodplain lithofacies in these strata are At Laberinto I (Fig. 7) gently inclined, stacked, meter- to half meter- Fsl (laminated silt), Fml (laminated mud), and Fm (massive or thick, fining-upward packets of laminated to wavy laminated silt and mottled clay). Less commonly occurring floodplain lithofacies (e.g., massive mud are present above planar cross-bedded point bar sand. Fsr, Fmrl, Fmc, Fmcl, Fsc, Spc, and Sslc) may be mottled or contain Similar, but thinner (10 to 20 cm), fining upward units are present ripples, angular clay clasts, or mud balls. within the point bar unit. All of these fine-grained units contain wood Wavy and rippled laminated mud (Fml and Fmrl) and silt (Fsl) fragments and local accumulations of leaf litter. lithofacies locally overlie point bar deposits (FA2) and/or clay-rich floodplain strata. At Los Amigos II, for example, faintly laminated blue- 4.3.2. Interpretation gray clay to silty clay is overlain by 5 to 10 cm thick fining-upward All of the FA3 lithofacies resulted from floodplain deposition. packets of cut-and-fill ripple drift lamination with clay clasts (Fig. 9). Floodplain environments are highly complex and typically contain a

Fig. 9. Photographs and sketch of the floodplain lake (cocha) and associated strata preserved in the Los Amigo II section. A. Overview of the right half of the Los Amigos II outcrop. B. Sketch of the outcrop showing lateral and vertical variations in lithofacies. C. Cut-and-fill ripple drift lamination overlying massive blue-gray clay and underlying wavy laminated silty clay and clay. D. Fossil wood typical of that preserved in the floodplain lithofacies of the Madre de Dios. Refer to Fig. 1 for site location; Table 2 for lithofacies descriptions; text for discussion.

C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx 11 variety of sediment types and sub-environments, such as avulsion In the Laberinto I section (Fig. 7) the lowermost fining-upward silt/ deposits, single small crevasse splays, levees, oblique accretion clay units are inter-bedded with lateral accretion deposits. The upper- deposits, and floodplain lake (abandoned channel; cocha) deposits most silt/clay units, however, form meter-thick fining-upward units (Reineck and Singh, 1980; Mjos et al., 1993; Brierley et al., 1997; Davis- with minimal sand. As stated earlier, these lower units could be the Vollum and Kraus, 2001; Farrell, 2001; Page et al., 2003; and others). result of oblique accretion adjacent to and above the point bar. The The floodplain strata, preserved in the Madre de Dios terraced presence of leaf litter in the lower silt/clay units suggests deposition in a sequences, include many of these sub-environments, but discontin- wet, low energy but accreting environment that allows preservation of uous exposures and dense modern vegetation hinder precise plant material. The upper silt/clay units, which are thicker and lack leaf differentiation among the various floodplain deposits. material, are more likely the result of traction current flow and levee Floodplain lake deposits are the most readily identifiable of the deposition. sub-environments. The vertical sequence of lithofacies in the Los Amigos II section records infilling of an abandoned channel on the 5. Terrace morphology floodplain. Massive and wavy laminated mud inter-bedded with sandy, mud-clast-bearing cut-and-fill ripple drift laminations record The fluvial strata just described are preserved beneath 3 distinct periodic floods that resulted in traction deposition of sandy material terrace tracts (1 through 3) that ranged in height from 0.05 to 46.15 m in the mud-dominated lake environment. The presence of clay rip-up above modern river level at the time of study. The terraces are clasts (likely derived from eroded cut bank and adjacent floodplain discontinuous, but traceable (in the field and on satellite imagery; deposits) and ripple drift laminations suggests rapid sedimentation Fig. 10) throughout most of the study area. Only the highest terrace (T3) with high rates of suspended sediment fallout (Rubin and Hunter, appears to be paired. In this system no consistent, observable relation- 1982; Farrell, 2001), as would be expected during an event that ship exists between terrace height and distance downstream. In this brought flood waters into the previously calm lake environment. section we briefly describe each terrace, then present a history of terrace The sand mud couplets in the Colorado II and Laberinto I sequences formation based on terrace geomorphology, facies analysis, and ages of are likely the result of deposition in either levee or small crevasse the terraced strata. splay environments. Distinguishing between ancient levee deposits and crevasse splay deposits is difficult. In general, discrete crevasse 5.1. Terrace morphology and age splay deposits are thinner and composed of finer-grained sediments than levee deposits (Reineck and Singh, 1980), but large crevasse The terrace morphology and the stratigraphy of the terraced systems (such as those associated with major avulsion systems) bring sediments preserve multiple episodes of late Quaternary fluvial extreme heterogeneity to the floodplain environments (Farrell, 2001), aggradation and downcutting. T3, the oldest and highest terrace, ranges making it difficult to distinguish the sandier portions of floodplain in height from 30 to 46.15 m above water level, and is present at environments from lateral accretion deposits. The sharp based sand/ Colorado I, Diamante (note that the full terrace height was not measured mud couplets in the Colorado I sequence (Fig. 8) are likely the result of at this location), and Los Amigos I (as well as many other locations along deposition by a density current during a flood. Such deposits are the studied reach). The highest maximum elevation of this terrace in the common on trunk channel and crevasse channel levees. The scoured study area (46.15 m) is at Los Amigos I, near the midpoint between the bases, grain-size and sedimentary structure grading, presence of wood Colorado and Inambari tributaries. The terrace is traceable on satellite fragments, and bioturbation and root mottling all suggest rapid images throughout much of the study area, but is only locally cut by the deposition by density currents followed by periods of relative modern river. The late Miocene–Pleistocene (?) fluvial strata beneath quiescence. the T3 lie unconformably above the Miocene Ipururi Formation, which is

Fig. 10. Photographs and satellite images showing the typical character of each of the three main terraces in the Madre de Dios valley. Refer to Fig. 1 for locations; Table 1 for site abbreviations.

12 C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx exposed in the lower few meters of many of the T3 exposures. The age of 5.2. History of terrace development the fluvial strata underlying the T3 surface and above the Ipururi Formation, is not well constrained (see Campbell et al., 2006). Samples Radiocarbon dating (Table 1) of fossil wood preserved in terraced from fossil logs from the T3 fluvial strata (at Los Amigos I and Diamante) fluvial sequences suggests that the terraces were formed by three yielded ages beyond the range of radiocarbon dating (N48,000 years) distinct aggradational periods, each followed by an episode of down- (Table 1). Similar results were reported by Antoine and others (2003), cutting. A simplified sketch of terrace development is shown in Fig. 11. although the context of their sampling is unclear (for a discussion of The late Miocene–Pleistocene (?) sediments in the ~50 m high T3 Antoine and others, 2003; Hovikoski et al., 2005; Roddaz et al., 2006,and terrace (above the Ucayali unconformity) are too old for radiocarbon related studies, see Latrubesse et al., in review). dating (N48,000 years BP), but record deposition in a sand-rich fluvial T2, the middle terrace in age and height, was measured at Colorado I system characterized by multistory coarse sandy channel and point and II, Downstream Los Amigos, Laberinto II, Los Amigos II, Puerto Azul bar complexes encased in thick (N6 m) sequences of silty floodplain Berowe, San Juan, and Upstream Diamante. It ranges from 6.23 to strata. The strata in this sequence were downcut sometime before 20.05 m in height and is discontinuously present throughout the river 29,780±100 cal yrs BP (the age of the oldest strata in the T2 valley. Locally, thin (b2 m) remnants of the Ipururi Formation are sequence). Although the principal focus of our study was on the present at the base of the T2 terraced strata. Analysis of nine fossil wood Quaternary sediments found in the younger terrace sequences, in samples from the T2 terraced strata above the Ipururi Formation yielded those T3 sections that we did study (including some of the same ones five viable dates (the other four samples were radiocarbon-dead). The described by Campbell et al., 2006), we found no evidence supporting dated strata yield ages of 29,850±100, 29,780±100, 28,160±80, the interpretation by Campbell and co-workers (2006) that the upper 25,530±240, and 25,040±130 cal yrs BP (Table 1). The radiocarbon- members of the Madre de Dios Formation (the upper portion of the T3 dead samples were from large fossil logs preserved in coarse gravel terrace sequence) were deposited in an extensive deltaic environ- channels near the base of the terraced sequences (Fig. 4G). ment. Nor did we find any evidence supporting the interpretation of T1, the youngest and lowest terrace, is present at Manu (5.85 m, the same sequences as forming in a tidal environment (as suggested located on the Manu River near its confluence with the Alto Madre de by Antoine et al., 2003 and Hovikoski et al., 2005). Rather, our Dios), Playa Caceres (5.05 m), and Laberinto I (7.62 m). Wood fragments sedimentological observations support the deposition of T3 strata in (Fig. 4H) and leaf material from sediments in the T1 terrace sequence our study area in a sand-rich, fluvial sequence, not much different yield ages of 11,970±100,11,830±120,10,510±60,10,450±80, 6530± than the modern river below. 60, and 3780±50 cal yrs BP (Table 1). Deposition of the T2 sedimentary sequence began before 29,780± Variable thicknesses of modern fill sequence are exposed and 100 cal yrs BP and lasted until at least 25,040±130 cal yrs BP — downcut along the river valley. Although we did not study these probably much longer because the dated samples were found in the sections in most localities, we did examine an exposure along the lower part of the sequence and the oldest sampled strata in the T1 lower Tambopata River, where several meters of sand-dominated terrace is dated at 11,970±100 cal yrs BP. T2 strata contain the point bar and floodplain sediments were being actively eroded by the coarsest deposits in the Madre de Dios system. These strata are modern river (again, during an extreme low river level). Two samples characterized by gravelly channel deposits in a sequence that is from fine-grained lithofacies near the base of a modern cut-bank in dominated by floodplain and lateral accretion deposits. The Miocene the lower Tambopata River yielded radiocarbon ages of 870±50 and “basement” is locally exposed in the lower portion of the T2 terrace 690±20 cal yrs BP (Table 1). walls and the overlying fluvial strata include basal gravels with fossil

Fig. 11. Model of terrace development history along the studied reach of the Madre de Dios River showing typical heights for each terrace, as well as the sequence of aggradation/ sedimentation (arrows pointing up) and downcutting/erosion (arrows pointing down) events. Dated samples are denoted by asterisks at the relative heights in the T3, T2, and T1 terraced sequences; dates are in cal yrs BP. See text for discussion.

C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx 13 logs that are beyond the range of radiocarbon dating (N48,000 yrs BP). conditions were limited to a shorter wet season. The Madre de Dios These logs (all pieces of large trees; Fig. 4G) may have been reworked River surely remained an active fluvial system during the glacial with from older (T3) deposits, as is probable in the San Juan section where abundant floodplain deposition and abundant floodplain lakes. In two samples of fossil wood from the same channel yielded two summary, nothing in the paleohydrologic record of the Madre de Dios different dates (N48,000 and 29,850±100). Alternatively, the suggests a dry climate in the southwestern Amazon during the last N48,000 cal yrs BP fossil logs may be preserved within a remnant of glacial. Instead, discharge in the river was often much greater than the T3 sequence that is preserved at the base of the T2 sections, as is modern (allowing transport of coarser sediment), but the fluvial possible in the PAB section where samples from three different fossil record by itself is not conclusive regarding climate details such as the logs from the same channel conglomerate (FA1) are radiocarbon- length of the wet season and the seasonality of the discharge. dead. Aggradation of the T2 sequence may (or may not) have continued The T2 fluvial sequence was downcut sometime after 25,040± through the end of the last glacial maximum, but at present we do not 130 cal yrs BP and before 11,970±100 cal yrs BP. Aggradation of the T1 have any dates from the upper part of the sequence. Incision of the T2 terraced sequence followed this downcutting: it started before terrace (event 5) may have begun as soon as deposition ceased. In any 11,970 cal yrs BP and lasted until at least 3780±50 cal yrs BP. The case, downcutting was complete and aggradation (event 6) of the T1 strata in the T1 terraced sequence record deposition in a sand- sequence commenced after 25,040 and before 11,970 cal yrs BP. The dominated meandering channel complex. The presence of modern river aggraded during the Younger Dryas. On the Altiplano, evidence sediments onlapping the T1 terrace, such as those in the Tambopata exists that the Younger Dryas was an unusually wet period (e.g., Baker section, suggest that the river system downcut the T1 terrace, then et al., 2001a,b, 2005), consistent with the evidence for increased began a new phase of aggradation sometime between 3780±50 and discharge and sediment aggradation on the floodplains of the Madre 870±50 cal yrs BP. de Dios. Aggradation of the T1 sequence began during the Younger Dryas 6. Fluvial history and its relationship to climate and may have continued until at least 3,780 cal yrs BP. This is curious because compelling evidence exists that the mid-Holocene was a very Deposition of the T3 sedimentary sequence (events 1 and 2, Fig. 9) dry period on the Altiplano (e.g., Cross et al., 2000; Baker et al., 2001a) began during the Miocene (Campbell et al., 2006) and ended before and in the lowlands (e.g., Mayle et al., 2007). Many alternative 45,000 cal yr BP (Antoine et al., 2003). The oldest sample in the T2 explanations exist, however, for the observations that can only be sequence (from San Juan) dates to 29,850±100 cal yrs BP. Therefore, clarified by further study and further dating. For example, it is possible incision of the T3 sediments (event 3) must have begun prior to that, because mid- and late-Holocene dates (respectively, 6530 and 29,850 cal yrs BP. Likewise, deposition of the T2 sediments (event 4) 3780 cal yr BP, and also reversed in sequence) are found in a terrace must have begun before this time. Deposition continued through at that does not have any early Holocene dates, this particular terrace least 25,040±130 cal yrs BP (the youngest sample recovered from T2) sequence is different from the other latest glacial (T1) sequences, and probably much later. The T2 sediments (N30,000 to b25,000 cal continuous sedimentation did not occur during the early and middle yrs BP) are the coarsest of any of the terrace sequences. This is Holocene (as portrayed in Fig. 11). consistent with many previous studies that also found coarsest Downcutting of the T1 sequence (event 7) took place in the late sediments deposited during the last glacial, if not the last glacial Holocene, after 3780 cal yr BP and before 870 cal yr BP. Presently, no maximum. For example, in the Bolivian Chaco at 19°S, May et al. evidence links this period of downcutting to known climate variation (2008) dated a transition from coarse fluvial gravel to overlying sands on the Altiplano (e.g, Ekdahl et al., 2008). Finally, deposition of the at a minimum of 22,000 cal yrs BP. May et al. (2008) summarized modern terrace sequence (event 8) of the Tambopata River began other similar occurrences of the deposition of coarse fluvial sediments before 870 cal yr BP and is ongoing in some reaches of the river. during the last glacial in the southwestern Amazon basin: 40,000 to 32,000 cal yrs BP in the Ucayali River at 5°S (Dumont et al., 1991), 7. Conclusions 38,000 to 34,000 cal yrs BP in the Upper Jurua River at 8°S (Latrubesse and Rancy, 1998), 29,000 to 23,000 cal yrs BP in the Purus River at 9°S Terraced strata outcrop along a significant portion of the Madre de (Latrubesse and Kalicki, 2002). The paleoclimatic significance of these Dios River in the southwestern Amazon of Peru. Terraced sequences findings however, is not entirely clear. represent four separate generations of aggradation and downcutting. The watershed of Lake Titicaca neighbors that of the Madre de Dios — Detailed sedimentological analysis of the aggraded sediments reveals the headwaters of the Madre de Dios River arise within 100 km of the 23 distinct lithofacies. Despite previous studies of some of the same Lake. During the entire duration of the last glacial, extending from at outcrops that propose deposition in sedimentary environments least 60,000 to b20,000 cal yrs BP,the drill core record from Lake Titicaca ranging from marine tidal, to large lake, to extensive deltaic, we shows unambiguously that the climate in the Andean headwaters of the found no evidence contradicting the parsimonious conclusion that all Madre de Dios River was cold and wet relative to present (Fritz et al., of the sediments that we studied were deposited in a meandering 2007) — a positive glacial mass balance in the eastern cordillera of the river environment quite similar to that of the modern Madre de Dios Andes accompanied a positive water balance during all of the last glacial. River. That said, variations in grain size and other sedimentological Glacial advance in the Andean headwaters of the Madre de Dios, partly a characteristics, as well as the presence of unconformities, demon- result of increased moisture availability, would have produced rapid strate that several large changes occurred in paleohydrologic regime, rates of erosion, increased discharge and competence of the highland likely driven by paleoclimatic variability. Although a great deal more tributaries, and likely, sediment aggradation in the lower reaches. study is required, particularly much additional dating to better Does the coarse grain size (as well as reports of vegetation change constrain the timing of aggradation and downcutting, the paleohy- and increased eolian activity) of the last glacial signify that dry drologic evidence is generally consistent with the paleoclimate history conditions prevailed in the southwestern Amazon during this time as reconstructed from the fluvial and lacustrine sediments of the concluded by May et al. (2008)? Bush and co-workers (2004) showed neighboring, high-altitude, Titicaca watershed. Thus, precipitation that lowland Peru was forested during the last glacial. And Mayle increase, lake-level rise (e.g., Baker et al., 2001a), increased fluvial (2007, personal communication) now believes that pollen studies discharge and floodplain aggradation on the Altiplano (e.g., Rigsby indicate that the last glacial climate of lowland Bolivia was relatively et al., 2003; Farabaugh and Rigsby, 2005), and glacial advance in the wet. The fluvial transport of coarser sediments necessitates wetter Andes (Fritz et al., 2007) are generally concurrent with sediment rather than drier conditions, although it is unclear if these wetter aggradation in the Madre de Dios basin. Importantly, it appears that

14 C.A. Rigsby et al. / Geomorphology xxx (2009) xxx–xxx sediment aggradation took place along the paleo-Madre de Dios river Farabaugh, R.L., Rigsby, C.A., 2005. Climatic influence on sedimentology and geomorphology of the Rio Ramis Valley, Peru. Journal of Sedimentary Research throughout much of the last glacial maximum (sensu lato, Peltier and 75, 12–28. Fairbanks, 2006). Conversely, dry climates in the highlands seem to Farrell, K.M., 2001. Geomorphology, facies architecture, and high-resolution, non- coincide with downcutting events in the Madre de Dios River, marine sequence stratigraphy in avulsion deposits, Cumberland Marshes, Saskatchewan. Sedimentary Geology 139, 93-50. although further study of the early Holocene is needed to determine Fritz, S.C., Baker, P.A., Seltzer, G.O., Ballantyne, A., Tapia, P., Cheng, H., Edwards, L., 2007. if aggradation, downcutting, or both were coincident with the Quaternary glaciation and hydrologic variation in the South American tropics as highland climate of this period. reconstructed from the Lake Titicaca drilling project. Quaternary Research, in review. Friend, P.F., 1983. Towards the field classification of alluvial architecture or sequence. In: Acknowledgments Collinson, J.D., Lewin, J. (Eds.), Modern and Ancient Fluvial Systems. . Special Publication, vol. 6. International Association of Sedimentology, pp. 345–354. Galloso, A., Molina, O., Palacios, O., Reyna, C., 1996. Boletin de Puerto Luz (26-U), This project was supported by NSF grants to Rigsby (EAR-0227999) Colorado (26-V), Laberinto (26-X), Serie A: Carta Geológica Nacional, INGEMMET, and Baker (EAR-0227550) and by a grant to Rigsby from the Division Lima, Peru. 190 pp. Goulding, M., Barthem, R., Ferreira, E., 2003. The Smithsonian Atlas of the Amazon. of Research at East Carolina University (2006–2007 Research Devel- Smithsonian Books, Washington, D.C. 253 pp. opment Grant). We thank Professor Miles Silman for introducing us to Grams, P.E., Schmidt, J.C., 2002. Streamflow regulation and multi-level flood plain research opportunities in the field area; Dr. Nigel Pitman and the formation: channel narrowing on the aggrading Green River in the eastern Uinta Mountains, Colorado and Utah. Geomorphology 44, 337–360. ACCA/ACA-funded Centro de Investigación y Capacitación del Río Los Guyot, J.L., Jouanna, J.M., Wasson, J.G., 1999. 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Please cite this article as: Rigsby, C.A., et al., Late Quaternary Paleohydrology of the Madre de Dios River, southwestern Amazon Basin, Peru, Geomorphology (2009), doi:10.1016/j.geomorph.2008.11.017