Late Quaternary Sedimentation, Lower Colorado River, Gulf Coastal Plain of Texas

Late Quaternary sedimentation, lower Colorado River, Gulf Coastal Plain of Texas

MICHAEL D. BLUM Department of Geology, Southern Illinois University, Carbondale, Illinois 62901 SALVATORE VALASTRO, JR. Radiocarbon Laboratory, Balcones Research Center, University of Texas, Austin, Texas 78758

ABSTRACT and sediment supply. Basal unconformities for the ELA and CBA-1, which were emplaced Holocene valley fills, however, appear to be during the last full glacial lowstand and the Investigations in the lower Colorado River 1,000-2,000 yr younger in the upper Colorado transgression that followed. Valley, Gulf Coastal Plain of Texas, have re- drainage than they are in the lower Colo- sulted in the development of a spatially and rado valley. This time-transgressive episode of INTRODUCTION temporally controlled history of changes in bedrock valley cutting was initiated by climat- channel and flood-plain erosional and deposi- ically controlled reductions in sediment sup- Alluvial terraces and valley fill deposits of tional processes. When combined with paleo- ply, but conditioned by limits on rates of up- the lower Colorado River, Gulf Coastal Plain climatic and stratigraphie data from the upper stream propagation of incision through a large of Texas, have been the focus of geomorpho- Colorado River drainage and the record of drainage basin. By contrast, unconformities logical and sedimentological investigations glacio-eustasy in the Gulf of Mexico, this study within Holocene valley fills document time- for almost 100 yr. Early mapping efforts dif- permits evaluation of the relative influence of parallel episodes of flood-plain abandonment ferentiated a number of surfaces based on different external controls on channel and and soil formation, but little additional bed- landscape position (for example, Hill and flood-plain behavior, the development of allu- rock valley cutting, and indicate decreased Vaughan, 1897,1902; Duessan, 1924; Weeks, vial landforms, and the development of alluvial flood magnitudes following shifts to drier cli- 1945) and suggested initial correlations be- stratigraphie sequences. matic conditions. tween terraces within erosional, bedrock- Late Pleistocene and Holocene alluvial de- Flood-plain morphology and sedimentary confined valleys and the large-scale deposi- posits of the lower Colorado River have been facies changed through time in response to tional surfaces of the Quaternary alluvial subdivided into allostratigraphic units, with changes in climate coupled with a protracted plains (for example, Weeks, 1945; Doering, chronological control afforded by radiocarbon degradation of upland soil mantles, which al- 1956; Winker, 1979; DuBar and others, 1991). ages. In the bedrock-confined valley, up to 10 tered the rate at which precipitation was trans- More recently, Baker and Penteado-Orellana m of late Pleistocene (-20,000-14,000 yr B.P.) ferred to stream channels as runoff. During the (1977, 1978) and Looney and Baker (1977) sediments referred to as the Eagle Lake Allo- late Pleistocene through middle Holocene, run- examined morphological characteristics of formation (ELA) underlie a terrace at 17-20 m off was filtered through deep upland soils, younger surfaces, primarily from air photos, above the present-day channel. Deposition of floods were for the most part less flashy and then developed a model for fluvial response the ELA was followed by bedrock valley contained within channel perimeters, and to climatic change that is widely cited in both incision, then deposition of a complex Holo- flood plains were constructed by lateral migra- regional and topical literature (for example, cene valley fill referred to as the Columbus tion without significant vertical accretion; Knox, 1983; Bryant and HoUoway, 1985; Bend Alloformation (CBA). Columbus Bend hence, the ELA and CBA-1 contain few verti- Johnson and Holliday, 1986; Kochel, 1988; Allomembers 1 and 2 (CBA-1 and CBA-2) un- cal accretion facies. Exposure of bedrock sur- Schumm and Brackenridge, 1987; Walker derlie a terrace at 12-14 m above the present- faces during the late Holocene resulted in in- and Coleman, 1988; White and Weigand, day channel. CBA-1 was deposited -12,000- creased flood stages, deep overbank flooding, 1989; McDowell and others, 1990; Summer- 5,000 yr B.P., whereas CBA-2 was deposited and construction of flood plains by vertical ac- field, 1991). -5,000-1,000 yr B.P. Columbus Bend Allo- cretion; hence, CBA-2 and CBA-3 contain In spite of the abundance of studies, and member 3 (CBA-3) consists of channel and thick vertical accretion facies. the importance of Colorado River deposits in flood-plain deposits that represent the past 600 Allostratigraphic units and bounding un- regional correlation and topical literature, ex- yr of activity. conformities persist through the bedrock-con- amination of previous efforts shows they Allostratigraphic units within the lower Col- fined valley to the Quaternary alluvial plain, were based on limited documentation of geo- orado valley correlate with allostratigraphic but stratigraphic architecture changes sub- morphic and stratigraphic relations and were units in major valley axes of the upper Colo- stantially in the downstream direction as a re- primarily morphostratigraphic in approach, rado drainage and with records of climatic and sult of the last glacio-eustatic cycle. On the al- with terrace correlations based on landscape environmental change, suggesting that alluvial luvial plain, late Holocene CBA-2 and modern position alone. Moreover, numerical ages deposits record basinwide responses to climat- CBA-3, deposited contemporaneously with the that permit correlation of changes in fluvial ically controlled changes in discharge regimes present interglacial highstand, onlap and bury behavior with external controls were not

Data Repository item 9420 contains additional material related to this article.

Geological Society of America Bulletin, v. 106, p. 1002-1016, 12 figs., 3 tables, August 1994.

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available. Re-examination of alluvial deposits in the lower Colorado valley and comparison with radiocarbon-dated stratigraphic frameworks in the upper Colorado drainage (Blum and Valastro, 1989,1992; Blum and others, in press) suggest that previous mapping efforts miscorrelated and misinterpreted many deposits, that ages for alluvial deposits were greatly overestimated, and that genetic interpretations need substantial revision. This paper presents a chronologically controlled allostratigraphic framework for late Pleistocene and Holocene alluvial deposits of the lower Colorado valley. In doing so, it provides the first radiocarbon-controlled stratigraphic framework for large fluvial systems of the Gulf Coastal Plain. When combined with paleoclimatic and stratigraphic data from the upper Colorado drainage (for example,. Toomey and others, 1993; Blum and Valastro, 1989, 1992; Blum and others, in press) and the record of glacio-eustasy in the Gulf of Mexico (for example, Frazier, 1974), this study permits evaluation of the influence of external controls on alluvial channel and flood-plain behavior, the development of major alluvial landforms, and the development of alluvial stratigraphic sequences. Figure 1. Map of the Colorado River system, showing the upper part of the drainage above the Balcones Escarpment, the bedrock-confined lower Colorado valley of the Inner Coastal Plain, GEOLOGIC, CLIMATIC, AND the Quaternary alluvial plain (gray pattern), and the Gulf of Mexico shoreline. Key locations HYDROLOGIC SETTING mentioned in text as noted. The study area extends from Austin in the bedrock-confined valley to Wharton on the Quaternary alluvial plain. The Colorado River is a large fluvial system (drainage area of 110,000 km2; Tovar and Maldonado, 1981) with its upper reaches and and Bomar, 1983). Pronounced east-west Such data is crucial to interpretation of the all major tributaries (92% of total area) drain- precipitation gradients exist across the Colo- stratigraphic record, because previous inves- ing the geologically heterogeneous Southern rado drainage, with annual means of 510 mm tigators assumed that surfaces at elevations High Plains and Edwards Plateau regions of on the Southern High Plains and western- of 3-5 m above the low-water channel repre- West Texas. As the channel emerges from a most Edwards Plateau near the headwaters, sented the constructional flood plain, with deep canyon at the Balcones Escarpment, 810 mm where the Colorado channel emerges higher surfaces at 8-9 m above the low-water the drainage basin narrows considerably, and onto the coastal plain at the Balcones Escarp- channel interpreted as Pleistocene terraces the lower Colorado River transects the Gulf ment, and 1,100 mm on the alluvial plain near (for example, Weeks, 1945) or early Holo- Coastal Plain for 280 km until discharging into the coast (Larkin and Bomar, 1983). The sea- cene braided stream surfaces (for example, the Gulf of Mexico (Fig. 1). On the inner sonal distribution of precipitation is charac- Baker and Penteado-Orellana, 1977, 1978; coastal plain, the lower Colorado River flows teristically bimodal throughout the drainage, Looney and Baker, 1977). Instead, it is clear within a well-defined bedrock valley that with maxima in late spring and early fall, and that these surfaces were constructional flood transects Upper Cretaceous carbonates, then minima during the winter and summer plains prior to dam construction. progressively younger and less steeply dip- months. ping Tertiary siliciclastic rocks (Barnes, 1979, Since 1938, flood discharges of the modern METHODS 1981, 1982, 1987). Farther downstream, the lower Colorado River have been controlled Colorado River flows through a relatively un- by a series of dams constructed upstream Allostratigraphic units consist of map- dissected Quaternary alluvial plain consisting from the Balcones Escarpment. Analysis of pable, three-dimensional bodies of geneti- of the Lissie and Beaumont Formations (Ber- predam annual duration series data (1898- cally related lithofacies that are defined based nard and LeBlanc, 1965; DuBar and others, 1937), however, provides an indication of the on bounding discontinuities (North American 1991). scale of relatively frequent hydrological Commission on Stratigraphic Nomenclature, The modern climate of the Colorado drain- events under uncontrolled conditions. For 1983). Subdivision of Quaternary alluvial de- age ranges from continental-semiarid on the example, the mean annual flood was 2320 posits using allostratigraphic units provides Southern High Plains and western Edwards m3/s, with a stage of 9 m, whereas floods with an opportunity to move away from morpho- Plateau to subtropical-subhumid along the 5-yr recurrence intervals reached stages 10 m stratigraphic approaches traditionally em- present-day Gulf of Mexico coast (Larkin above the low-water channel (Blum, 1992). ployed along the Gulf Coastal Plain and else-

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TABLE 1. SUMMARY OF PREVIOUS WORK AND NOMENCLATURE

Elevation of terrace Investigator surface above Colorado • • -—j channel (m) Hill and Vaughan Weeks (1945) Mathis (1942) Weber (1968) Baker and Penteado-Orellana This paper (1897, 1902) (1977, 1978)

2-5 Flood plain Sand Beach Lower Flood plain channel assemblages 1, 2, 3 Columbus Bend 6-10 Riverview Terraces channel assemblages 4, 5 Allomember 3 10-15 Second Bottoms First Street First Street channel assemblages 6,6a, 6b Columbus Bend Allomembers 1 and 2 16-20 Sixth Street Sixth Street Eagle Lake Alloformation Sixth Street channel assemblage 6R 23-25 NR NR Montopolis Montopolis 40-44 Capital Capital Capital Capital channel assemblage 7 Capital 44-46 Hornsby 60-65 Asylum Asylum Asylum Asylum channel assemblage 8 Asylum 7 Uvalde Uvalde Delaney Manor high lag gravels high lag gravels

Note: The nomenclature includes named alluvial terrace and valley fill deposits of the lower Colorado River, Inner Gulf Coastal Plain of Texas (modified from Baker and Penteado-Orellana, 1977), compared with allostratigraphic units discussed herein. Elevation of ternce surface is given as elevation above present-day low water channel. NR signifies that the ten-ace is not recognized by that investigator.

where (see Autin, 1992). For the lower deposition of the host sediment (for example, races (Sorenson and others, 1976; Mandel Colorado River, allostratigraphic units were Matthews, 1985). Radiocarbon ages from and Caran, 1992). subdivided and mapped using air photos and similar materials provided chronological con- The age of high terraces in the lower Col- field documentation of cross-cutting geomor- trol for deposits in the upper Colorado drain- orado valley has been a source of confusion, phic and stratigraphic relations. Distinct sur- age that is both internally consistent and in as have correlations between terraces within faces of nondeposition and soil development good agreement with age estimates based on the bedrock-confined valley and the Lissie define the top of allostratigraphic units. Rep- diagnostic archaeological materials (Blum and Beaumont Formations of the Quaternary resentative soils from each allostratigraphic and Valastro, 1989,1992; Blum and others, in alluvial plain. An important marker bed, the unit were described, using terminology sum- press). All samples were processed accord- 0.62 Ma Lava Creek B volcanic ash noted by marized in Birkeland (1984), and analyzed for ing to methods outlined in White and Valas- Caran and Mandel (1988) and Mandel and trends in texture and percent carbonate, fol- tro (1984), with most samples corrected for Caran (1992), appears to be interbedded with lowing standard methods (for example, carbon isotope fractionation. deposits underlying a dissected part of the Singer and Janitzky, 1986). Basal and lateral Asylum terrace, and at elevations slightly boundaries of allostratigraphic units were de- OLDER PLEISTOCENE ALLUVIAL higher than soils developed on the Capital ter- fined based on laterally traceable erosional DEPOSITS race; hence, the Asylum terrace and under- unconformities with subjacent bedrock or lying alluvial deposits may be early Pleis- older alluvium and traceability to diagnostic Subdivision of alluvial deposits in the tocene in age, and the Capital terrace may be soils. Lithologic characteristics and facies lower Colorado valley has evolved substan- middle Pleistocene in age. Doering (1956) play no role in subdivision of allostratigraphic tially through time (Table 1). The oldest de- noted that the Asylum terrace can be traced units but are critical to interpretations of dep- posits identified by previous workers consist to the Lissie Formation—suggesting it, too, is ositional processes. Nomenclature for de- of gravelly lags on both sides of the valley at of early Pleistocene age—whereas the Capi- scription of sedimentary facies follows 80 m or more above modern channels (for tal terrace merges with the Beaumont For- McGowan and Garner (1970) and Miall example, Mathis, 1944; Urbanec, 1963; We- mation (Fig. 2), at least part of which is be- (1985). ber, 1968). Two partially dissected but rela- lieved to be last interglacial in age (see also Traditional materials for radiocarbon dat- tively continuous terraces occur at 50-65 and DuBar and others, 1991). The relatively un- ing—for example, wood, charcoal, or peat— 40-46 m above modern channels within the dissected surface morphology, clearly iden- are rarely preserved in the Colorado drain- bedrock-confined valley, above the apex of tifiable channel traces, and geomorphic and age; hence, samples for radiocarbon dating of the Quaternary alluvial plain. These corre- stratigraphic relationships with older and lower Colorado River deposits were, for the spond to the Asylum and Capital terraces as younger alluvial deposits suggest that the most part, collected from fine-grained sedi- originally defined by Hill and Vaughan (1897, Montopolis terrace is late Pleistocene in age. ments or buried soils. We assume that finely 1902) and as recognized by most later work- Correlations between Montopolis deposits divided organic detritus in fine-grained sedi- ers (for example, Weeks, 1945; Doering, and strata of the Quaternary alluvial plain re- ments accumulated syndepositionally but 1956; Urbanec, 1963; Weber, 1968; Baker main difficult to establish. may include a small fraction of reworked and Penteado-Orellana, 1977, 1978; Looney older organic matter; thus, radiocarbon ages and Baker, 1977). The Montopolis terrace LATE QUATERNARY STRATIGRAPHY on these samples represent an approximate was named by Weber (1968) and consists of time of deposition (for example, Haas and a widespread and relatively continuous sur- The remainder of this paper focuses on al- others, 1986) or, perhaps, a maximum age. In face at 23-25 m above modern channels. In luvial deposits younger than the Montopolis contrast, samples taken from buried soils in- contrast to older surfaces, primary deposi- terrace in the bedrock-confined part of the clude organic matter that accumulated post- tional topography and channel morphology lower Colorado valley. Farther downstream depositionally due to pedogenic processes, remains well preserved on the Montopolis on the Quaternary alluvial plain, these depos- and they represent a mean residence time for surface. Deeply weathered soils characterize its fill a valley incised below the Pleistocene the organic carbon and a minimum age for the Asylum, Capital, and Montopolis ter- Beaumont surface. Figures 3 and 4 summa-

1004 Geological Society of America Bulletin, August 1994

c have been leached to depths > 3 m, and calcic horizons are rare at any depth. Similar soils have developed in fine sandy facies, with an exception being that well-defined stage II calcic horizons occur at depths >1.8-2.2 m below the top of the profile (Fig. 5A). Soils developed in poorly drained muddy facies have well-developed mollic A horizons overlying noncalcareous Bt horizons, with stage 11+ to stage III calcic horizons at depths of 130-150 cm. Downstream from the type area, where the ELA is buried by younger deposits, the upper boundary still can be distinguished clearly based on its diagnostic soil profile. Channel facies dominate the ELA, with many sections consisting of 1-2 m of horizontally bedded and imbricated gravels over- lain by 5-7 m of crosscutting lenticular bodies Distance Downstream From Balcones Escarpment (km) of trough cross-stratified sand and gravel. Figure 2. Long profiles for older terraces and alluvial-plain surfaces of the lower Colorado Fine-grained swale-fill and channel-fill facies River, as compared with long profile of modern (pre-dam) flood plain. Also shown is the ap- occur within some sections and consist of proximate position of the 0.62 m.y. B.P. Lava Creek "B" volcanic ash near Smithville (marked lenticular bodies of interbedded fine sand and with "X") reported by Caran and Mandel (1988) and Mandel and Caran (1992). Long profiles mud that crosscut gravelly and sandy facies. based on Doering (19S6) and DuBar and others (1991) with additional mapping by M. D. Blum. By contrast, vertical accretion flood-plain facies are rarely >2-3 m in thickness, and in most cases are completely absent (Fig. 6A). rize geomorphic and stratigraphic relations ¡graphic relations with older and younger Examination of aerial photographs also for the bedrock-confined valley and farther deposits. shows excellent preservation of point-bar downstream on the Quaternary alluvial plain. Within the bedrock-confined valley, the and abandoned-channel morphology, with a Detailed maps illustrating locations of strato- ELA rests on Cretaceous or Tertiary strata at clear lack of ridge and swale flood-plain to- types and descriptions of measured sections elevations 5-8 m above the present-day low- pography (Fig. 6B). Abandoned channels on are available in the GSA Data Repository.1 water channel, is typically 8-10 m in thick- the ELA surface resemble the large meander ness, and is bounded at the top by a well- loops considered diagnostic of "Deweyville" Eagle Lake Alloformation defined terrace surface and soil profile at terraces elsewhere on the Gulf Coastal Plain, 16-18 m above the low-water channel (Figs. as does the stratigraphic position; the ELA is In the bedrock-confined lower Colorado 3 and 4A). Height of the basal unconformity younger than Pleistocene Beaumont strata valley, the oldest suite of deposits of interest with bedrock, measured with respect to but older than modern meander belts (for ex- to this discussion underlie the Sixth Street height of the present low-water channel, and ample, Bernard, 1950; Bernard and LeBlanc, terrace of Weeks (1945) and make up part of the degree of topographic differentiation be- 1965; Saucier and Fleetwood, 1970; Aten, channel assemblage 6R of Baker and Pen- tween the ELA and younger deposits remain 1983; Alford and Holmes, 1985; Autin and teado-Orellana (1977, 1978) and Looney and relatively constant in the downstream direc- others, 1991; DuBar and others, 1991; Baker (1977, Table 1). Exposures in this suite tion until the channel emerges onto the Qua- Gagliano, 1991). of deposits are rare and heavily biased ternary alluvial plain. Downstream from the Radiocarbon ages indicating the approxi- toward gravelly and sandy facies quarried for type area at Eagle Lake, the basal uncon- mate time of deposition for ELA range from road-building materials. Farther downstream formity with Lissie and Beaumont strata dips -18,600 yr B.P. to -15,600 yr B.P. (Table 2). on the Quaternary alluvial plain, natural and below the present low-water channel, In addition, minimum ages of 7200 ± 230 yr artificial exposures are widespread and dis- whereas the top of ELA is buried by younger B.P. (Tx-7229) and 6520 ± 140 yr B.P. (Tx- play a broader range of facies, but there is no deposits (Figs. 3, 4C, and 4D). Depth of 6808) were obtained from 2Btb soil horizons surface expression. Rather than use the term burial ranges from 10-20 cm to 5-6 m, in- developed in channel-fill facies at Garwood Sixth Street terrace, as defined on the basis of creasing in the downstream direction. and Wharton, where this unit is preserved in- morphostratigraphic criteria alone, these de- Soils developed in ELA deposits vary de- tact but buried by younger deposits. Thus, posits are referred to as the Eagle Lake Al- pending on primary depositional topography deposition of ELA occurred from —20,000 to loformation (ELA) after type localities, near and facies. In well-drained gravelly and 14,000 yr B.P., with soil formation after that the town of Eagle Lake, that display the full coarse sandy facies, profiles consist of pale time. range of characteristic geomorphic and strat- brown to light brownish gray (10YR 6/2 to 6/3) E horizons up to 50 cm thick, overlying Columbus Bend Alloformation red to yellowish red (2.5YR 5/6 to 5 YR 5/6) Bt 'GSA Data Repository item 9420 is available on horizons >2 m thick, with hematite and clay Deposits that make up the main valley fill request from Documents Secretary, GSA, P.O. occurring as continuous coats on framework in the bedrock-confined valley underlie geo- Box 9140, Boulder, CO 80301. grains. Primary carbonate rock fragments morphic surfaces defined as the First Street,

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E£agk Lake Alloformation Stak (km) "Mon topo] is Tcrrucc" Tertiary Bedri H. >. .11' Older Pleistocene

Colorado River.

Garwood Columbus Baid 3

Columhus Bend 1 and 2

Eagle Lake Alloformatitin

Be au mon! Formation Tertiary Bedrock or Older Plcjsloeene

Figure 3. Surficial geologic maps of the lower Colorado valley showing the distribution of allostratigraphic units for (A) the bedrock-confined valley between Smithville and LaGrange, Texas; and (B) the apex of the Quaternary alluvial-deltaic plain between Columbus and Garwood, Texas. Note that downstream from Eagle Lake, the late Pleistocene ELA and early through middle Holocene CBA-1 are buried by late Holocene CBA-2 and CBA-3.

Riverview, and Sand Beach terraces by tion has been subdivided further into three ing of deposition and subsequent soil forma- Weeks (1945), then subdivided further into distinct allomembers, referred to hereafter as tion. Valley fill deposits are considerably channel phases 6, 6a, 6b, and 5 through 1 by CBA-1, CBA-2, and CBA-3 (Figs. 3 and 4). younger than previous workers envisioned, Baker and Penteado-Orellana (1977, 1978) Radiocarbon ages from well-defined strat- with (1) deposition of CBA-1 from the latest and Looney and Baker (1977, see Table 1). In igraphie contexts (Fig. 4, Table 3) provide Pleistocene through middle Holocene time this paper, these deposits are referred to as chronological control on deposition of the (-13,000 yrB.P. until -5,000 yr B.P.), when the Columbus Bend Alloformation (CBA), Columbus Bend Alloformation. In addition, deposition ceased and soil formation was in- named after type localities near the town of minimum radiocarbon ages on buried soils itiated; (2) deposition of CBA-2 during the Columbus. The Columbus Bend Alloforma- from CBA-1 and CBA-2 further limit the tim- late Holocene (-5,000 yr B.P. or slightly be-

1006 Geological Society of America Bulletin, August 1994

ably on Cretaceous or Tertiary strata very ELA near the present low-water channel. Total Colorado River thickness ranges from 10 to 12 m, and the upper boundary consists of an erosional un- e mmmr^^ïïï^X conformity (separating CBA-1 from younger V}t/i deposits) or a distinct soil profile. Pre- c 5- m served soils make up part of the First u Street terrace of Weeks (1945), or they £ 0J may be buried by 20-200 cm of terrace veneer facies (terminology of Brackenridge, 1984) associated with CBA-2 or CBA-3 Scale (m) (Figs. 4A and 4B). The basal unconformity B. with bedrock is at or near the low-water 10 n Colorado River level upstream from the town of Eagle Lake, then dips below the channel farther 4490 ±120* downstream. Likewise, the upper bound- 5- CBA-2 : 7610 ±150 ary is at or near the surface until that point, but downstream is buried by younger over- low water channel" ¿3330 + 90, 12,950 ±640 bank facies. In the lowermost part of the study area, the upper boundaries of CBA-1 100 200 and ELA merge laterally until the two soils Scale (m) are welded together, and both units are buried by 2-5 m of younger deposits, with c burial depth increasing downstream 10. Caney Creek Colorado River (Fig. 4D). 1800 ±60t Soils developed in CBA-1 consist of dark c 5- 4030 ± no grayish brown to grayish brown (10YR 3/2 to M 960+ 1700 \ /1«I8,600±700 4/2) noncalcareous mollic A horizons, some 30-50 cm in thickness, overlying reddish brown (5YR 4/4 to 5/4) noncalcareous Bt horizons that extend to depths of 1-1.2 m or Scale (m) more. Stage II Bk horizons, dominated by D. nodules of CaC03 up to 1 cm in diameter, 10 n modern floodplain extend through depths of 1.2-2 m below the CBA-3 top of the soil profile (Fig. 5B). When buried 1 i 1 j i 1 1 ' 1 i 1 !r 111 '¡"i1 ivi ' r T T I1 Miii / '3490 ±70* • •WrTTI 11 by substantial thicknesses of CBA-2 terrace 2-3 m thick. Although modified by post- 600 yr or so. of the ELA or older units, and unconform- depositional bioturbation, weathering, and

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A. CaC03-Free pedogenesis, most exposures consist of Texture (%) CaC03 lenticular mud interbedded and inset 0 25 50 75 100 0 10 20 30 L, 1 within laterally extensive tabular bodies of silt and fine sand.

Columbus Bend Allomember 2

In the bedrock-confined valley, CBA-2 rests stratigraphically inset against, and in many cases partially buries, CBA-1 (Figs. 4A and 4B). Basal unconformities with Cretaceous or Tertiary strata typically occur slightly below present-day low-water channels, with total exposed thickness commonly >12 m. The upper boundary consists of soil profiles that make up part of the First Street terrace of Weeks (1945), but, in channel-proximal settings, soils may be buried by 20-50 cm of historic-age flood deposits. In the downstream direction, the basal unconformity with bedrock or older Pleistocene deposits remains be- Figure 5. Textural low the low-water level throughout the trends and percent car- study area, whereas soil profiles are no- CaC03-Free where buried by >20-100 cm of younger Texture (%) CaC03 bonate for soil profiles de- terrace veneer facies. 0 25 50 75 100 0 10 20 30 veloped in the Eagle Lake i i terrace veneer.-' and Columbus Bend Allo- Soils developed on CBA-2 typically con- 2 A b formations. (A) Soil pro- sist of mildly calcareous dark brown to -A file developed in silty to dark grayish brown (7.5YR 4/2 to 10YR \ sandy facies of the ELA at 4/2) mollic A horizons, most often cumulic : l r 2Btb ' Eagle Lake (location C in in nature and some 50-75 cm thick, which Fig. 3B). (B) Soil profile overlie calcareous reddish brown (5YR 4/4 • .. developed in sandy facies to 5/4) stage I Bk horizons that are also of CBA-1, with thin ter- 50-75 cm thick (Fig. 5C). Primary carbon- race veneer from CBA-2 ate rock fragments occur throughout the 2Kkb (location 5b in Fig. 3B). profile, although some leaching from the A !

spCbi CBA-2, buried by historic- and filaments precipitated on ped faces, 1 11 age terrace veneer facies of and small (<0.5 cm) nodules. CBA-3 (location B in CBA-2 possesses a range of channel fa- Fig. 3B). cies, with most exposures displaying CaC03-Free crosscutting bodies of trough cross-strati- Texture (%) •CaC03 fied sandy gravels and sands >5 m in thick- 0 25 50 75 100 0 10 20 30 -L——| ^J l i_ ness, or lenticular to undulatory bodies of interbedded fine sand and mud, also commonly >5 m in thickness. In sharp contrast to older allostratigraphic units, especially the ELA, vertical accretion facies within CBA-2 are commonly >5-6 m in thickness A (Fig. 7A). Primary characteristics of flood- plain environments and facies are also well preserved, with ridge and swale topography easily visible in aerial photographs (Fig. 7B), and lenticular to undulatory mud interbedded with silt and fine sand present in most outcrops. Terrace veneer facies from CBA-2 consist of 20-200 cm of massive fine sandy silt that truncate or bury soils developed in CBA-1.

1008 Geological Society of America Bulletin, August 1994

sion of the Colorado channel has separated modern depositional environments from the Caney Creek meander belt, named after the underfit stream that now occupies this abandoned channel course (Fig. 8A). Between Eagle Lake and Wharton, the abandoned Caney Creek meander belt and modern channel occur within a single valley that contains the ELA, CBA-1, and CBA-2, and flood- plain facies from the Caney Creek meander belt and the modern channel interlinger with each other. Downstream from Wharton, the Caney Creek and modern channel courses diverge and ultimately discharge into the Gulf of Mexico some 40 km from each other. In this lowermost part of the Colorado drainage, the ELA, CBA-1 and 2, and CBA-3 deposits of the Caney Creek meander belt occur within a valley that is physically separated from the modern depositional system by Pleistocene Beaumont strata. McGowan and Garner (1970) described modern chute-dominated point bars of the lower Colorado River and used component lithofacies to develop a model for coarse- \ Channel grained meandering streams. Typical point bar facies consist of 1-2 m of horizontally bedded and imbricated coarse gravel, over- lain by lenticular bodies of trough cross-stratified sandy gravel and coarse sand, and plane- bedded sand up to 5 m thick. Flood-plain Colorado River facies were not described, probably due to the perception that high constructional flood- plain surfaces at 8-9 m above low-water channels were Pleistocene or early Holocene terraces. In fact, flood-plain facies of coarse- grained meandering streams remain, as a whole, poorly understood (see Miall, 1985); however, they clearly represent major com- ponents of the lower Colorado depositional system, with most exposures consisting of laterally extensive, undulatory to lenticular, interbedded ripple-laminated sand and laminated to massive mud up to 5 m thick. The CBA-3 Channel scale and heterogeneity of the modern depositional system also went unappreciated Figure 6. (A) Photograph illustrating sandy and gravelly point bar facies of the ELA extending during previous geomorphological investiga- to top of section with a clear lack of fine-grained vertical accretion facies. (B) Air photo illustrating tions. For example, what Baker and Pen- large abandoned channels characteristic of ELA surfaces, as well as the lack of accretionary ridge teado-Orellana (1977, 1978) and Looney and and swale flood-plain topography (location 6b in Fig. 3A). Abandoned channel of the modern Baker (1977) interpreted to be early Holo- lower Colorado River (CBA-3) is shown for comparison. Air photo is a black and white repro- cene braided-stream surfaces actually repre- duction of color infrared original, so displays infrared spectral response. sent ridge and swale topography on modern flood-plain surfaces, whereas their late Hol- ocene high sinuosity channels correspond, Columbus Bend Allomember 3 is strat ¡graphically inset against CBA-2 for the most part, to recently abandoned and and/or CBA-1; in some cases historic terrace not yet completely filled meander loops Above the town of Eagle Lake, CBA-3 veneer facies bury soils developed in these (Blum, 1992). corresponds to predam and modern channel older units (Figs. 4A and 4B). Total thickness McGowen and others (1976) recognized and flood-plain-related depositional environ- of CBA-3 ranges from 1 m or less to >10 m. that the Caney Creek meander belt was sub- ments, rest on pre-Quaternaiy bedrock, and Downstream from Eagle Lake, recent avul- stantially different from the modern channel

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TABLE 2. RADIOCARBON AGES FROM THE EAGLE LAKE ALLOFORMATION

* Locality Lab # Depth Uncorrected ä13C Corrected Material Interpretation 14C Age "CAge Sampled

Wharton Tx-6808 1.5m 6520 ± 140 -16.4 6650 ± 140 2Btb horizon minimum age Garwood Tx-7229 2.5m 7200 ± 230 -22.2 7240 ± 230 2Btb horizon minimum age Columbus Tx-7230 7.5m 15,610 ± 1300 -25.2 15,610 ± 1300 fluvial sediment time of deposition Eagle Lake Tx-7011 1.9m 15,890 ± 810 -24.9 15,900 ± 810 fluvial sediment time of deposition Eagle Lake Tx-7010 1.9m 15,960 ± 1700 -24.8 15,970 ± 1700 fluvial sediment time of deposition Eagle Lake Tx-7013 3.9m 16,010 ± 450 -23.4 16,090 ± 450 fluvial sediment time of deposition West Point Tx-7225 7.0m 16,060 ± 1170 -17.9 16,180 ± 1170 fluvial sediment time of deposition Eagle Lake Tx-7012 7.3m 18,470 ± 3890 -31.0 18,380 ± 3890 fluvial sediment time of deposition Eagle Lake Tx-7011 4.0m 18,600 ± 700 -25.0 18,600 ± 700 fluvial sediment time of deposition

Note: Depth of sample indicates the depth below top of the unit.

and suggested that it was a highly sinuous, DISCUSSION Correlations with Alluvial Deposits of the mature, fully aggraded channel course prior Upper Colorado Drainage to abandonment. Indeed, examination of the Several characteristics of the allostrati- Caney Creek meander belt in air photos and graphic framework defined above deserve Late Pleistocene and Holocene alluvial de- in the fieldshow s that well-defined levee, cre- further discussion. These include: (1) corre- posits of the upper Colorado drainage have vasse splay, and flood basin depositional en- lations between alluvial deposits in the upper been subdivided into a series of allostrati- vironments were common to the lower Col- Colorado drainage and those in the bedrock- graphic units based on geomorphic and strat- orado River when it flowed through the confined lower Colorado valley; (2) the role igraphic relations, whereas radiocarbon ages Caney Creek course (Fig. 8B). Such features of climatic and environmental changes in provide chronological control for the last do not occur along the lower Colorado River controlling changes through time in the rela- 20,000 yr (Blum and Valastro, 1989, 1992; in the bedrock-confined portion of the valley, tive importance of flood-plainfacie s and proc- Blum and others, in press). Episodes of flu- upstream from the point of avulsion, or in the esses of flood-plain construction in the lower vial activity in large valley axes include: (1) recently occupied channel farther down- Colorado valley; and (3) downstream changes an extended period of channel aggradation stream until the lowermost reaches near the in stratigraphic architecture due to glacio- and flood-plain construction from —20,000 to present shoreline. eustatic controls. 14,000 yr B.P.; (2) incision of bedrock valleys

TABLE 3. RADIOCARBON AGES FROM THE COLUMBUS BEND ALLOFORMATION

Locality Lab # Depth Uncorrected ä13C Corrected Material Interpretation "CAge 14C Age Sampled

Columbus Bend Allomember 1 Eagle Lake Tx-7322 1.0m 3490 ± 70 -17.3 3610 ± 70 2Btb horizon minimum age Columbus Tx-7325 1.4m 4490 ± 120 -15.5 4640 ± 120 2Btb horizon minimum age West Point Tx-7226 1.5m 4780 ± 70 -14.6 4960 ± 70 2Btb horizon minimum age Eagle Lake Tx-7323 4.2m 5280 ± 180 -20.7 5350 ± 180 fluvial sediment time of deposition Columbus Tx-6811 5.2m 7610 ± 150 -17.8 7730 ± 150 fluvial sediment time of deposition Utley Tx-7328 5.5m 7940 ± 630 -24.2 7970 ± 630 fluvial sediment time of deposition Austin Tx-6532 3.3m 8960 ±220 fluvial sediment time of deposition Austin Tx-6531 4.0m 9030 ± 160 fluvial sediment time of deposition Austin Tx-6528 5.5m 9870 ± 220 fluvial sediment time of deposition West Point Tx-7224 2.8m 10,940 ± 600 -26.7 10,910 ± 600 fluvial sediment time of deposition Austin Tx-6527 6.1m 11,050 ± 220 fluvial sediment time of deposition Columbus Tx-7326 10.5m 12,950 ± 640 -23.4 12,970 ± 640 fluvial sediment time of deposition Columbus Bend Albmember 2 Columbus Tx-6813 0.3m 640 ± 70 -13.7 820 ± 70 2Ab horizon minimum age Austin Tx-6536 1.2m 1090 ± 80 fluvial sediment time of deposition Columbus Tx-6812 4.3m 1430 ± 70 -14.8 1590 ± 70 fluvial sediment time of deposition West Point Tx-7223 2.0m 1580 ± 60 -20.0 1660 ± 60 fluvial sediment time of deposition Webberville Tx-7331 1.5m 1760 ± 60 -16.1 1900 ± 60 fluvial sediment time of deposition Eagle Lake Tx-7007 1.1m 1800 ± 60 -20.8 1870 ± 60 fluvial sediment time of deposition Austin Tx-6535 5.5m 2610 ± 80 fluvial sediment time of deposition West Point Tx-7221 6.0m 2880 ± 60 -20.1 2950 ± 60 fluvial sediment time of deposition Webberville Tx-7330 5.5m 3180 ± 90 -20.6 3250 ± 90 fluvial sediment time of deposition West Point Tx-7222 9.8m 3280 ± 140 -18.9 3380 ± 140 fluvial sediment time of deposition Austin Tx-6534 1.2m 3320 ± 90 fluvial sediment time of deposition Webberville Tx-6809 10 m 3330 ± 90 -18.4 3440 ± 90 fluvial sediment time of deposition Columbus Tx-6810 10 m 3330 ± 90 -18.4 3440 ± 90 fluvial sediment time of deposition Austin Tx-6533 1.1m 3340 ± 90 fluvial sediment time of deposition Webberville Tx-7233 10.4m 3400 ± 60 -25.8 3390 ± 60 wood time of deposition Webberville Tx-7234 7.5m 3560 + 60 -19.7 3640 ± 60 fluvial sediment time of deposition West Point Tx-7220 10.6m 3950 ± 100 -21.2 4010 ± 100 fluvial sediment time of deposition Eagle Lake Tx-7008 4.3m 4030 ± 110 -18.9 4120 ± 110 fluvial sediment time of deposition Webberville Tx-7232 10.5m 4060 ± 70 -19.1 4160 ± 70 fluvial sediment time of deposition Webberville Tx-7231 10.4m 5060 ± 130 -21.6 5120 ± 130 fluvial sediment time of deposition Columbus Bend Allomember 3 Columbus Tx-7227 5.5m 110 ± 60 -27.9 70 ± 60 wood time of deposition West Point Tx-7321 6.2m 250 ± 60 -28.3 190 ± 60 wood time of deposition Columbus Tx-7335 8.0m 370 ± 60 -26.4 350 ± 60 wood time of deposition Columbus Tx-7334 8.5m 550 ± 60 -28.5 490 ± 60 wood time of deposition

Note: Depth of sample indicates the depth below top of the respective allomember.

1010 Geological Society of America Bulletin, August 1994

tro, 1989; Toomey and others, 1993; Blum and others, in press). The ELA and CBA within the bedrock- confined lower Colorado valley occupy the same geomorphic and stratigraphic positions as late Pleistocene and composite Holocene fills of the upper Colorado drainage, and are likewise separated from each other by a protracted period of bedrock valley incision. Hence, the two distinct parts of this large drainage basin record the same large-scale sediment storage and degradational events (Figs. 9 and 10). Although late Pleistocene fills of the upper Colorado drainage and the ELA in the lower Colorado valley have produced similar radiocarbon ages, they remain insufficiently dated to discuss the degree of temporal synchroneity in more detail. This is not the case with younger deposits. For example, basal unconformities for Holocene fills of the upper Colorado drainage appear to be some 1,000-2,000 yr younger than the base of CBA-1 in the lower Colorado valley, suggesting a time-transgressive but neverthe- less basinwide period of bedrock valley incision. By contrast, unconformities within the CBA, like counterparts within Holocene valley fills of the upper Colorado drainage, document flood-plain abandonment and soil formation accompanied by continued net storage of sediments, but little additional bedrock valley cutting. Episodes of flood-plain abandonment and soil formation appear to be time parallel in major valley axes of the Col- orado drainage, at least within the resolution limits of radiocarbon dating. A number of writers discuss problems of correlation of alluvial terrace and valley fill deposits within and between drainages, the relative influence of intrinsic versus extrinsic controls, and problems with interpretation of causality (for example, Schumm, 1977; Butzer, 1980; Patton and Schumm, 1981; Knox, 1983; Bull, 1991; Autin, 1993). We suggest that correlation of allostratigraphic Figure 7. (A) Photograph illustrating thick vertical accretion facies typical of CBA-2. (B) Air units through major valley axes of the upper photo illustrating accretionary ridge and swale, or scroll, flood-plain topography characteristic Colorado drainage to the lower Colorado of CBA-2 surfaces (location 7b in Fig. 3A). Air photo is a black and white reproduction of color River implies extrinsic rather than intrinsic infrared original, so displays infrared spectral response. controls. Moreover, since 92% of the drainage basin lies above the Balcones Escarp- ment within a tectonically stable and ungla- from -14,000 to 11,000 yr B.P.; and (3) accompanied by continued lateral migra- ciated continental interior where base-level lateral channel migration and valley wid- tion and net storage of sediments but little changes are not an issue, we argue the most ening with deposition of an extensive Hol- additional bedrock valley incision. Allu- likely extrinsic control would be climatic ocene valley fill since —11,000 yr B.P. vial deposits along major valley axes of the change. Yet, there is sufficient reason to ex- Deposition of the Holocene valley fill was upper Colorado drainage have been inter- pect differential responses of fluvial systems punctuated by periods of flood-plain aban- preted to record fluvial responses to to climate change, and both time-transgres- donment and soil formation —5,000-2,500 changes in sediment supply and discharge sive and time-parallel discontinuities within yr B.P. and -1,000 yr B.P. to the present. regimes that were driven by climatic and alluvial stratigraphic frameworks (Bull, Episodes of flood-plain abandonment were environmental changes (Blum and Valas- 1991). For the Colorado drainage, unconfor-

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mities that separate the ELA and CBA formed in response to a different suite of processes than unconformities that occur within the CBA (Fig. 10). The ELA and CBA rest on bedrock straths, as defined in Bull (1991), and are separated from each other by Peach Creek a major period of bedrock valley cutting. This time-transgressive episode of valley cutting probably was initiated by climatically controlled reductions in sediment supply, relative to stream power, but conditioned by limits on rates of propagation of bedrock incision through a large drainage basin. By contrast, time-parallel episodes of flood-plain abandonment and soil formation at —5,000 and —1,000 yr B.P., without additional bedrock valley cutting, probably indicate decreased flood magnitudes following shifts to relatively dry climatic conditions (see Toomey and others, 1993).

Processes of Flood-Plain Construction

Flood-plain morphology and sedimentary facies differ significantlybetween the allostratigraphic units defined above, which indicates changes through time in processes of flood- plain construction. The late Pleistocene ELA contains few vertical accretion facies, and channel-related gravel and sand extends to the top of many sections (see Fig. 6A). More- over, point bar and abandoned channel morphology is well preserved (see Fig. 6B). This suggests that floods during the late Pleis- tocene were, for the most part, contained Caney Creek ¿ within bankfull channel perimeters, and Meanderbelt \ flood-plain construction proceeded by lateral rather than vertical accretion. By contrast, vertical accretion facies occur throughout the Colorado River CBA, and increase in thickness and volumet- ric significance through time (see Fig. 7A). Moreover, flood-plain ridge and swale topography is ubiquitous on Holocene surfaces, especially those belonging to CBA-2 and CBA-3 (see Fig. 7B). Hence deep overbank Garwood floods and flood-plain construction by vertical accretion became more important through time, and was a most important process during the past 3,000-5,000 yr. Figure 8. (A) Map illustrating channel courses of the Caney Creek meander belt versus the Nanson and Croke (1991) summarize modern lower Colorado channel. Both channel courses occur within the same incised valley above flood-plain morphological types and suggest Wharton, whereas downstream from Wharton the courses diverge and discharge into the Gulf that changes through time in processes of of Mexico some 25 km from each other. Note that below Wharton, the ELA and CBA are flood-plain construction should reflect cli- contained within the same valley as the Caney Creek meander belt, except for the modern matic and environmental changes via their ef- channel, which flows through Pleistocene Beaumont strata. Adapted from McGowan and others fects on discharge regimes and sediment sup- (1976) and Barnes (1979). (B) Air photo illustrating contrasts in channel morphology and dep- ply. More specifically, Brackenridge (1988) ositional environments between abandoned Caney Creek meander belt and modern Colorado suggests that the thickness of vertical ac- channel (location 8b in Fig. 3B). Note clearly defined levees and crevasse splays along Caney cretion facies may be related to flashiness of Creek meander belt, as well as differences in channel sinuosity. Air photo is a black and white the hydrological regime. Indeed, changes reproduction of color infrared original, so displays infrared spectral response. through time in processes of flood-plain con-

1012 Geological Society of America Bulletin, August 1994

CLIMATES AND ENVIRONMENTS OF THE EDWARDS PLATEAU ALLUVIAL STRATIGRAPHY

Temperature Moisture Upland Soils U. Colorado L. Colorado low high low high Min. Max. Drainage River On m: SD CBA-3M; SD FA m FA SD SD LH : : : : : CBA-2 o SD SD o o 5" FA — FA

£3 EH CBA-1

10' OH \\\w\w:\\\\\ N Bedrock Valley v Cutting X X Bedrock Valley 0) ^ Cutting X> FA FA

S3 15-

LP ELA 20th Century Averages

20' Figure 9. Record of climatic change and alluvial stratigraphy from upper Colorado drainage, compared with stratigraphie framework from the lower Colorado valley. Record of climatic and environmental change is adapted from Toomey and others (1993), whereas alluvial stratigraphy of thé Upper Colorado drainage is adapted from Blum and others (in press). For alluvial stratigraphy of the upper Colorado drainage, LP = late Pleistocene allostratigraphic units, EH — early to middle Holocene allostratigraphic units, LH = late Holocene allostratigraphic units, and M = modern depositional systems. For the upper Colorado drainage and the lower Colorado River, FA signifies time periods of flood-plain abandonment, whereas SD signifies time periods of soil development on abandoned flood-plain surfaces.

struction in the lower Colorado valley may be in the frequency of floods that exceeded eyville" terraces elsewhere on the Gulf attributable to changes in hydrology that, in bankfull channel perimeters, and changes in Coastal Plain, discharge events were more this case, occurred as a result of the pro- the importance of flood-plain construction by protracted and flood peaks were actually tracted erosion of deep soils on upland land- vertical as opposed to lateral accretion. smaller during the glacial period, with flood scapes of the Edwards Plateau and upper The above interpretation represents an al- events contained within channel perimeters, Colorado drainage. Much of the upland land- ternative to traditional views of late Pleis- than they were during the late Holocene scape was covered by deep, red, clay-rich tocene hydrology and channel morphology when deep overbank flooding and vertical ac- soils during the late Pleistocene glacial pe- on the Gulf Coastal Plain. Large meander cretion were important processes and fluvial riod. Changes to Holocene interglacial cli- loops and abandoned channels of the "Dew- depositional systems had considerably more matic conditions favored the gradual degra- eyville" terraces, as compared to those of the relief. Moreover, the preservation of large dation of soil profiles such that by late Pleistocene Beaumont Formation or late Hol- meander loops and abandoned channels on Holocene time the upland landscape con- ocene meander belts, have been attributed to "Deweyville" surfaces may be related to vis- sisted of exposed limestone bedrock with lit- increased runoff and higher magnitude floods ibility due to a lack of burial by vertical ac- tle soil cover (Toomey and others, 1993). As during the cool and moist Pleistocene glacial cretion facies, rather than larger floodpeaks , a result, rates at which storm runoff was period (for example, Saucier and Fleetwood, whereas small meander loops typical of late routed to stream channels would have been 1970; Aten, 1983; Alford and Holmes, 1985; Holocene surfaces may reflect hydraulic ad- at a minimum during the late Pleistocene, Gagliano, 1991). Recent paleoclimatic and justments to bank-stabilizing vertical accre- when upland landscapes were covered by paleoenvironmental reconstructions confirm tion facies rather than decreases in flood deeply weathered soils, and reached a max- previous inferences of late Pleistocene cli- magnitude. imum in the late Holocene when the present mates with considerably more effective mois- bedrock landscape was exposed. Even with- ture (Toomey and others, 1993), but floodhy - Downstream Changes in Stratigraphic out changes in the magnitude or frequency of drographs and depositional processes in the Architecture precipitation inputs, this should have led to lower Colorado valley were conditioned by increases through time in the peakedness of deep soils then present on upland surfaces. Both the ELA and CBA persist through flood hydrographs, corresponding increases Hence, if the ELA is representative of ' 'Dew- the length of the lower Colorado valley, but

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Valley Walls / stratigraphic architecture, or stacking pat- terns, change substantially in far downstream reaches (Fig. 11). Upstream from the Qua- ternary alluvial plain, the ELA and CBA rest on bedrock straths, separated from each other by unconformities produced during bedrock valley cutting. Within the incised Valley Walls / valley on the Quaternary alluvial plain, however, the ELA is onlapped and buried by the CBA, and latest Pleistocene to middle Holo- cene CBA-1 is onlapped and buried by late Holocene CBA-2 and CBA-3 (Fig. 12). We attribute downstream changes in stratigraphic architecture to different styles of Valley Walls / base-level control superimposed on episodes of sediment storage and erosion that reflect climatic controls on discharge regimes and sediment supply. For the bedrock-confined lower Colorado valley, the base level of erosion is tectonically controlled, and over the time period of concern has been essentially Valley Walls / static. Hence, the ELA and CBA record episodes of lateral migration, valley widening, and sediment storage, followed by renewed valley incision, flood-plain abandonment, and sediment removal that were superimposed on a late Cenozoic trend of bedrock valley deepening. Under relatively static Valley Walls / base-level conditions, as is the case for many degrading, tectonically inactive continental interior settings, younger alluvial landforms and deposits rest inset against and at lower elevations than older alluvial landforms and deposits. By contrast, stratigraphic architecture on the Quaternary alluvial plain records Valley Walls / the influence of dynamic, glacio-eustatically controlled base-level change in the Gulf of Mexico. In this part of the valley, the ELA was deposited during the last sea-level lowstand and earliest stages of transgression, within a valley already incised below previous highstand aggradational alluvial plain surfaces, and extended across the shelf to a delta Figure 10. Schematic valley cross-sections illustrating evolution of Eagle Lake and Columbus some 80-100 km farther basinward (see Suter Bend Allofonnations: (A) —20,000-14,000 yr B.P., sediment supply exceeded transport capac- and Berryhill, 1985; Morton and Price, 1987; ity, resulting in deposition of ELA; (B) -14,000-12,000 yr B.P., sediment supply greatly di- DuBar and others, 1991). In contrast to the minished, resulting in abandonment of ELA flood plains and incision of bedrock valleys, with no bedrock-confined valley farther upstream, preserved depositional record; (C) -12,000-5,000 yr B.P., sediment supply exceeded transport this part of the valley records sediment by- capacity, resulting in deposition of CBA-1; (D) —5,000-2,500yr B.P., flood magnitudes decrease, pass from -14,000 to 12,000 yr B.P., but no resulting in abandonment of CBA-1 flood plains and soil formation, but sediment supply re- further valley cutting, because base level was mained high, promoting storage of sediments and production of unconformity by continued rising at the time. Deposition of CBA-1 also lateral migration of channels with initial deposition of CBA-2; (E) —2,500-1,000 yr B.P., sed- occurred within the deeper parts of the in- iment supply remained high, promoting continued lateral migration of channels and deposition cised valley during the postglacial transgres- of CBA-2, but increases in flood magnitudes resulted in burial of soils developed on previously sion, while the shoreline was still basinward stable surfaces of CBA-1; and (F) past 1,000 yr, decreases in flood magnitudes resulted in aban- and lower in elevation than present, whereas donment of CBA-2 flood plains, but continued lateral migration, storage of sediments, and CBA-2 and CBA-3 record valley filling and deposition of CBA-3. forced storage of sediments farther upstream during the late Holocene sea-level highstand. Avulsion and abandonment of the fully aggraded Caney Creek meander belt, with oc-

1014 Geological Society of America Bulletin, August 1994

level controls on stratigraphic architecture and preservation in the geologic record, rather than a strict one-to-one relationship with base-level change per se (Blum, 1990, 1993).

ACKNOWLEDGMENTS

We thank Karl W. Butzer, Stephen A. Hall, William E. Galloway, Thomas C. Gustavson, and Robert K. Holz (University of Texas at Austin) for insights provided during conduct of the research. We are grateful to GSA reviewers Charles G. Oviatt (Kansas State University) and Whitney J. Autin (Lou- Scale isiana Geological Survey), and GSA Associ- Figure 11. Schematic cross sections of the lower Colorado valley illustrating downstream ate Editor W. Burleigh Harris (University of persistence of allostratigraphic units and bounding unconformities with downstream changes in North Carolina-Wilmington), for useful com- stratigraphic architecture. (A) The bedrock-confined valley on the inner coastal plain; (B) the ments and suggestions that greatly improved incised valley on the Quaternary alluvial plain. the manuscript. We also thank James C. Durbin (Southern Illinois University), who performed soil analyses, and Alejandra cupation of the modern lower Colorado chan- cated by Fisk (1944) and used widely in the Varela (University of Texas at Austin), who nel, most likely occurred in response to near Gulf Coastal Plain and elsewhere. The influ- assisted with preparation of samples for ra- complete filling of the incised valley during ence of base-level change on stratigraphic ar- diocarbon dating. This research was sup- the present highstand. chitecture in the lower Colorado valley ex- ported by National Science Foundation Correlations between alluvial records and tended 90 km upstream from the present Grant SES-9001243 to M. D. Blum and K. W. late Quaternary climatic change, the down- highstand shoreline, but was superimposed Butzer, as well as Geological Society of stream continuity of allostratigraphic units on climatically driven episodes of sediment America and Gulf Coast Association of Ge- within the lower Colorado valley, and down- storage or removal. Thus, depositional se- ological Societies grants to M. D. Blum. stream changes in stratigraphic architecture quences of coastal plain rivers with large in- land drainage basins most likely record inter- call into question the deterministic relation- REFERENCES CITED ship between sea-level change and periods of actions between upstream controls on Alford, J. J., and Holmes, J. C., 1985, Meander scare as evidence fluvial deposition or erosion originally advo- discharge and sediment supply, and base- of major climate changes in southeast Louisiana: Association of American Geographers Annals, v. 75, p. 395-403. Aten, L. E., 1983, Indians of the upper Texas coast: New York, Academic Press, 370 p. Autin, W. J., 1992, Use of alloformations for definition of Holocene meanderbelts in the Middle Amite River, southeastern Lou- isiana: Geological Society of America Bulletin, v. 104, p. 233-241. Autin, W. J., 1993, Influences of relative sea-level rise and Missis- sippi delta plain evolution on the Holocene Middle Amite River, southeastern Louisiana: Quaternary Research, v. 39, p. 68-74. Autin, W. J., Bums, S. F., Miller, B. I., Saucier, R. T., and Snead, J. J., 1991, Quaternary geology of the Lower Mississippi Val- ley, in Morrison, R. B., ed., Quaternary non-glacial geology of the conterminous United States: Boulder, Colorado, Ge- ological Society of America, The Geology of North America, v. K-2, p. 547-582. Baker, V. R., and Penteado-Orellana, M. M., 1977, Adjustment to late Quaternary climate change by the Colorado River of central Texas: Journal of Geology, v. 85, p. 395-422. Baker, V. R., and Penteado-Orellana, M. M., 1978, Fluvial sedimentation conditioned by Quaternary climate change in central Texas: Journal of Sedimentaiy Petrology, v. 48, p. 433-461. Barnes, V. E., 1979, Geologic atlas of Texas—The Seguin sheet: Austin, Bureau of Economic Geology, University of Texas at Austin. Barnes, V. E., 1981, Geologic atlas of Texas—The Austin sheet: Austin, Bureau of Economic Geology, University of Texas at Austin. Barnes, V. E., 1982, Geologic atlas of Texas—The Houston sheet: Austin, Bureau of Economic Geology, University of Texas at Austin. Barnes, V. E., 1987, Geologic atlas of Texas—The Beeville-Bay Distance Upstream from Modern Shoreline (km) City sheet: Bureau of Economic Geology, University of Texas at Austin, Texas. Figure 12. Longitudinal profiles for the ELA and different members of the CBA. Upstream Bernard, H. A., 1950, Quaternary geology of southeast Texas [Ph.D. thesis]: Baton Rouge, Louisiana State University, from Eagle Lake, longitudinal profiles for CBA-1 and CBA-2 are essentially the same, and plotted 164 p. Bernard, H. A., and LeBlanc, R. J., 1965, Resume of the Quater- as such. Downstream from that point, longitudinal profiles for the ELA and CBA-1 are essentially nary geology of the northwestern Gulf of Mexico Province, in the same, and plotted as such. Also shown is the upstream limit of glacio-eustatic controls on Wright, H. E., and Frey, D. G„ eds., The Quaternary of the United States: Princeton, New Jersey, Princeton University stratigraphic architecture. Press, p. 137-185.

Geological Society of America Bulletin, August 1994 1015

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A., 1987, Late Quaternary sea-level the lower Mississippi River: Vicksburg, Mississippi, Missis- fluctuations and sedimentary phases of the Texas Coastal MANUSCRIPT RECEIVED BY THE SOCIETY MAY 12,1993 sippi River Commission, U.S. Army Corps of Engineers, Plain and shelf, in Nummedal, D., and Pilkey, O, H., eds., REVISED MANUSCRIPT RECEIVED OCTOBER 13,1993 78 p: Sea level fluctuations and coastal evolution: Tulsa, Okla- MANUSCRIPT ACCEPTED NOVEMBER 22, 1993

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1016 Geological Society of America Bulletin, August 1994

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