Signatures of climate vs. sea-level change within incised valley-fill successions: Quaternary examples from the Texas GULF Coast

Sedimentary Geology 190 (2006) 177 – 211 www.elsevier.com/locate/sedgeo

Signatures of climate vs. sea-level change within incised valley-fill successions: Quaternary examples from the Texas Gulf Coast Michael D. Blum a,⁎, Andres Aslan b a b

Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, United States Department of Physical and Environmental Sciences, Mesa State College, Grand Junction, CO 81501, United States

Abstract The passive margin Texas Gulf of Mexico Coastal Plain consists of coalescing late Pleistocene to Holocene alluvial–deltaic plains constructed by a series of medium to large fluvial systems. Alluvial–deltaic plains consist of the Pleistocene Beaumont Formation, and post-Beaumont coastal plain incised valleys. A variety of mapping, outcrop, core, and geochronological data from the extrabasinal Colorado River and the basin-fringe Trinity River show that Beaumont and post-Beaumont strata consist of a series of coastal plain incised valley fills that represent 100 kyr climatic and glacio-eustatic cycles. Valley fills contain a complex alluvial architecture. Falling stage to lowstand systems tracts consist of multiple laterally amalgamated sandy channelbelts that reflect deposition within a valley that was incised below highstand alluvial plains, and extended across a subaerially-exposed shelf. The lower boundary to falling stage and lowstand units comprises a composite valley fill unconformity that is time-transgressive in both cross- and down-valley directions. Coastal plain incised valleys began to fill with transgression and highstand, and landward translation of the shoreline: paleosols that define the top of falling stage and lowstand channelbelts were progressively onlapped and buried by heterolithic sandy channelbelt, sandy and silty crevasse channel and splay, and muddy floodbasin strata. Transgressive to highstand facies-scale architecture reflects changes through time in dominant styles of avulsion, and follows a predictable succession through different stages of valley filling. Complete valley filling promoted avulsion and the large-scale relocation of valley axes before the next sea-level fall, such that successive 100 kyr valley fills show a distributary pattern. Basic elements within coastal plain valleys can be correlated with the record offshore, where cross-shelf valleys have been described from seismic data. Falling stage to lowstand channelbelts within coastal plain valleys were feeder systems for shelf-phase and shelfmargin deltas, respectively, and demonstrate that falling stage fluvial deposits are important valley fill components. Signatures of both upstream climate change vs. downstream sea-level controls are therefore interpreted to be present within incised valley fills. Signatures of climate change consist of the downstream continuity of major stratigraphic units and component facies, which extends from the mixed bedrock–alluvial valley of the eroding continental interior to the distal reaches, wherever that may be at the time. This continuity suggests the development of stratigraphic units and facies is strongly coupled to upstream controls on sediment supply and climate conditions within hinterland source regions. Signatures of sea-level change are critical as well: sea-level fall below the elevation of highstand depositional shoreline breaks results in channel incision and extension across the newly emergent shelf, which in turn results in partitioning of the 100 kyr coastal plain valleys. Moreover, deposits and key surfaces can be traced from continental interiors to the coastal plain, but there are downstream changes in geometric relations that correspond to the transition between the mixed bedrock– alluvial valley and the coastal plain incised valley. Channel incision and extension during sea-level fall and lowstand, with channel shortening and delta backstepping during transgression, controls the architecture of coastal plain and cross-shelf incised valley fills. © 2006 Elsevier B.V. All rights reserved.

⁎ Corresponding author. Fax: +1 225 578 2302. E-mail addresses: [email protected] (M.D. Blum), [email protected] (A. Aslan). 0037-0738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2006.05.024

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1. Introduction Development of sequence stratigraphic models in the late 1970s and 1980s resulted in a flurry of interest in how fluvial and other continental systems fit within a mostly marine-derived conceptual framework (Shanley and McCabe, 1994; Blum and Tornqvist, 2000). “Incised valleys” emerged as a focus of attention, since they truncate older strata, commonly juxtapose fluvial or estuarine sandstone on marine deposits, and define a significant basinward shift of facies due to relative sealevel fall (Van Wagoner et al., 1990). Although widely recognized as a major step forward, a number of writers took issue with basic tenets of the original sequence stratigraphic and systems tract models, especially as presented in Posamentier et al. (1988) and Posamentier and Vail (1988). This includes Miall (1991), who noted that a number of criteria relevant to the role of fluvial systems and their response to sea-level change were problematic. Dalrymple et al. (1994) subsequently defined two types of “incised valleys”, those formed by relative sealevel fall, and those formed in response to some other mechanism. They argued the first type should have sequence boundaries at their base and will be filled by predictable successions of fluvial and estuarine facies due to relative sea-level rise (Zaitlin et al., 1994), whereas the second type will be difficult to recognize since they may occur within a succession of fluvial strata, and therefore have no significance in the traditional Exxon sequencestratigraphic sense. The nature of depositional sequences in non-marine successions has remained a topic of discussion and active research (e.g. Blum, 1990; Shanley and McCabe, 1991, 1993; Wright and Marriott, 1993; Blum, 1994; Gibling and Bird, 1994; Feldman et al., 1995; Aitken and Flint, 1995, 1996; Currie, 1997; Ethridge et al., 1998; Legarreta and Uliana, 1998; McCarthy and Plint, 1998; Martinsen et al., 1999; Zaitlin et al., 1999; Arnott et al., 2000; Törnqvist et al., 2000; Holbrook, 2001; Batson and Gibling, 2002; 2003; Amorosi and Colalongo, 2005; Feldman et al., 2005: Holbrook et al., 2006). Miall and Arush (2001) correctly noted that sequence boundaries in fully non-marine successions can be “cryptic”, and commonly difficult to distinguish from autogenic scour surfaces. Although perhaps unintended, one question that emerges from Zaitlin et al.s' (1994) initial distinction, and critiques such as that of Miall and Arush (2001), would relate to linkages between incised valleys that extend from the fully non-marine part of a basin where sea-level change is not an issue, to the highstand shoreline, and then across the shelf to a lowstand shoreline and beyond.

Such a question has been the focus of much attention since that time, and is especially relevant today, since fluvial systems are the conveyor belts that link sediment sources within tectonic hinterlands to sinks within depositional basins (Blum and Tornqvist, 2000). The view taken here is that such questions will never be fully resolved from studies of ancient successions alone, because (a) upstream source terrains are rarely preserved, (b) it is difficult to disentangle processes that control the initial accumulation vs. processes that control ultimate preservation in the stratigraphic record, (c) precise chronological controls remain challenging, and (d) it is difficult to develop records for external forcing mechanisms that are independent of the responses of the depositional systems themselves. Hence, the study of Quaternary systems within depositional basin settings will remain fundamental, because upstream reaches are still extant, and because chronologically-controlled Quaternary stratigraphic records can be compared with known system controls and boundary conditions, and with independently-identified records of external forcing. Quaternary systems of the passive margin Texas Gulf of Mexico coastal plain have long been used as a natural laboratory for the study of depositional systems, and the development or testing of new ideas in sedimentary geology. This paper continues that practice, and addresses a number of issues relevant to the stratigraphic organization and evolution of incised valley fills. The purpose of this paper is to (a) review current thinking on the stratigraphy, sedimentology, and geochronology of late Quaternary incised valley-fill successions, (b) summarize present understanding of fluvial responses to interacting climate and sea-level change, (c) correlate onshore fully non-marine records with the record offshore, and (d) suggest a conceptual model for incised valley evolution, one that emphasizes the signatures of climate vs. sea-level change over different time scales. 2. General setting for Texas coastal plain and shelf The Texas Gulf Coastal Plain is the updip component of the passive margin western Gulf of Mexico basin, and consists of a succession of progressively younger and less steeply dipping Cenozoic clastic sedimentary rocks that represent progradation of the coastal plain and shelf edge (Galloway, 1981; Winker, 1982; Galloway et al., 2000). The Texas Coastal Plain is traditionally separated into erosional Inner and depositional Outer portions at the updip limits of the relatively undissected Quaternary alluvial–deltaic plains, which includes the Pleistocene Lissie and Beaumont Formations, and younger unnamed strata of late Pleistocene and Holocene age (DuBar et al.,

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1991; Fig. 1). The Texas shelf ranges in width from <100 km to the west to >150 km to the east, and is underlain by a thick succession of Neogene and Quaternary strata (Galloway et al., 2000). 2.1. Sediment supply Sediment is delivered to the western Gulf of Mexico basin via fluvial systems with varying drainage areas and relief (Fig. 2A). Winker (1982) and Galloway (1981) initially described fluvial systems of the Gulf Coastal Plain as extrabasinal, basin fringe and intrabasinal in origin. Extrabasinal systems drain tectonic hinterlands and have large sediment supplies, whereas basin fringe systems cannibalize basin margins, and intrabasinal streams drain updip parts of the basin fill. The Colorado and Brazos Rivers are the largest systems of interest here, each with present-day drainage areas that exceed 100,000 km2, and total relief of >1000 m, and are considered to be extrabasinal. The Trinity River to the east presently drains some ∼60,000 km2, but has maximum relief of only 300–400 m, whereas the Nueces River to the west drains an area of ∼45,000 km2, but with relief exceeding 1000 m. Both the Trinity and Nueces would be basin-fringe systems, with intrabasinal systems not considered here. Global compilations of sediment discharge illustrate that, in unglaciated basins of the midlatitudes, relief and drainage basin area are, by far, the two most important factors (see Milliman and Syvitski, 1992; Hovius, 1998;

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Syvitski et al., 2003). From these criteria, it might be expected that the Brazos and Colorado Rivers have the highest sediment yields, followed by the Trinity and other basin-fringe river systems. However, all major rivers of the Texas coast are presently dammed, and pre-dam data on sediment supply is sparse: as an example, low dams have been in place on the Colorado River since 1898, whereas high dams have been in place since 1938, and trap sediments from ∼ 90% of the drainage basin, including all major tributaries (Kanes, 1970; Blum and Valastro, 1994). LeBlanc and Hodgson (1959) and Kanes (1970) published early data on suspended sediment loads for major streams, which show the Brazos River delivered an average of 31 Mt/yr (1924–1954) followed by the Colorado River at ∼12 Mt/yr (1931–1945), and the Trinity at ∼5.5 Mt/yr. Some of these data were subsequently used in global compilations (e.g. Hovius, 1998), whereas other global and regional compilations utilized more complete data sets that unfortunately over represent the post-dam period, and greatly underestimate natural sediment loads for some of these systems (e.g. Milliman and Syvitski, 1992; Anderson et al., 2004). As an example, Anderson et al. (2004) cite values of 16 Mt/yr for the Brazos River, and 1.9 Mt/yr for the Colorado River, as derived from Milliman and Syvitski (1992). However, for the Colorado River case, average sediment yields in the 7 years prior to completion of high dams (1931–1937) were measured at ∼20 Mt/yr (Kanes, 1970), an order of magnitude higher: pre-dam values such as these are likely more significant for interpreting the stratigraphic record.

Fig. 1. Simplified geological map of the Texas coastal plain, illustrating the spatial distribution of key stratigraphic units discussed herein (modified from Blum and Price, 1998).

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Fig. 2. (A) Digital elevation model illustrating drainage areas for major river systems discussed herein, with 250-, 500-, 750-, and 1000-m topographic contours, as well as the − 50, − 100, and − 150 bathymetric contours, as shown. (B) Long profile of the lower Colorado River on the coastal plain, and of the continental shelf and shelf margin extending to the lowstand delta of Colorado River. Key physiographic elements as labeled. Dashed line represents extension of the long profile from the mixed bedrock–alluvial valley to the shelf margin. DEM and long profiles derived from Coastal Relief Models available through the US National Geophysical Data Center.

2.2. Depositional systems Extrabasinal and basin-fringe rivers flow through the Inner Coastal Plain within mixed bedrock–alluvial valleys (sensu Howard et al., 1994), where modern channelbelts and floodplains occupy valley bottoms, and are inset within flights of Quaternary terraces (Blum and Valastro, 1994). Flights of terraces of this kind reflect long-term trends of bedrock valley incision, upon which are superimposed multiple episodes of channelbelt formation and lateral migration, then renewed valley incision

with terrace formation (Merritts et al., 1994; Blum and Tornqvist, 2000). In passive margin settings such as this, long-term trends of bedrock incision are likely driven by isostatic uplift, which has a component that is driven by erosion of the landscape itself, and a flexural component that represents the inland response to loading in the depositional basin (e.g. Pazzaglia and Gardner, 2000). Flights of terraces record storage of sediments, and can be preserved in the landscape for long periods of time (105– 106 yr or more). However, mixed bedrock–alluvial valleys are bypass zones, and terraces such as these

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have no preservation potential in the geologic sense. Nevertheless, mixed bedrock–alluvial valleys are the conveyor belts for sediment delivery to the basin margin. The Outer Coastal Plain represents the updip margin of the actively subsiding basin, and consists of Quaternary alluvial–deltaic plains that emanate from each major valley, then coalesce laterally. At a more detailed level, spatial variations in morphology and depositional systems reflect drainage basin size and relief, valley gradients, and the related volume of sediment delivered to the basin margin (Galloway, 1981). The Colorado channel is a coarse and steep end member, transporting significant gravel and coarse sand to the coastal plain, with medium to low sinuosity channels dominated by large chutemodified point bars (McGowen and Garner, 1970). By contrast, the Brazos and Trinity channels have lower gradients, and classic high-sinuosity channels with welldefined laterally accreting point bars, as initially described by Bernard et al. (1970). At a larger scale, as shown in Fig. 3, the extrabasinal Colorado and Brazos Rivers have filled valleys that were cut during the last glacial–interglacial

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cycle, and constructed open-marine deltas: over longer periods of time, these same rivers have constructed a laterally extensive alluvial–deltaic headland. By contrast, the basin-fringe Trinity flows into the basin at interdeltaic bights, with incised valleys from the last glacial cycle that remain unfilled, and which contain bay-head deltaic, estuarine, and barrier island/strandplain depositional environments (the wave-dominated estuarine systems of Boyd et al., 1992). Valleys on the Outer Coastal Plain are hereafter referred to as coastal-plain incised valleys, as distinct from mixed bedrock–alluvial valleys farther upstream. It has been known for some time that major river systems of the Texas coast cut through highstand depositional shoreline breaks, extended across the shelf during sea-level fall and lowstand, and incised valleys into pre-existing strata then, constructed a series of midshelf and shelf-margin deltas (e.g. Winker, 1982; Suter and Berryhill, 1985; Suter, 1987; Thomas and Anderson, 1994; Morton and Suter, 1996; Anderson et al., 1996; 2004; Abdullah et al., 2004). Gradients on the Texas

Fig. 3. Satellite image illustrating spatial variation in depositional systems, which in turn reflects spatial variation in rates of sediment delivery to the coastal plain. The Colorado–Brazos alluvial–deltaic headland is an example of large volumes of sediment delivered to the coastal plain, with complete filling of incised valleys and construction of open marine deltas. Note the Colorado River filled its incised valley, and avulsed to occupy a new channel course, and has constructed a smaller delta (labeled). The lower sediment supply Trinity is a classic wave-dominated estuarine system (sensu Boyd et al., 1992), with an unfilled incised valley that terminates in a bayhead delta, a central basin estuary (Galveston/Trinity Bay), and a fringing barrier island system (Galveston Island and Bolivar Peninsula). As labeled, the Colorado, Brazos, and Trinity valleys coincide with the postBeaumont valleys discussed herein. Also labeled are the Pleistocene barrier segments to the east of Galveston/Trinity Bay. In between these segments are highstand channelbelts from the ancestral Trinity River.

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shelf are similar to those of the coastal plain, and the shelf-slope break occurs at depths of ∼ 80 to 120 m below present sea level (Fig. 2B). The shelf is here considered to be the lowstand extension of the coastal plain, and valleys on the shelf are referred to herein as cross-shelf incised valleys. 2.3. Modern climate and late quaternary sea-level and climatic change The major controls on climate in source regions for Texas coastal plain rivers are: (a) latitudinal position, which extends from the midlatitudes to the subtropics; (b) the presence of the Rocky Mountains and Western Cordillera tectonic highlands to the west; (c) the primary Gulf of Mexico moisture source to the south and east; and (d) the secondary Pacific moisture source to the south and west, across the Western Cordillera of Mexico. Given this set of controls, precipitation values within contributing drainage basins decrease from east to west, and climate regimes range from subhumid in the east to semiarid in the west. Modern temperature regimes are continental midlatitude to the north, with lower coastal plain reaches of each river system more subtropical in character (Bomar, 1994). Flood regimes of modern rivers are dominated by the passage of midlatitude cyclonic storms in Spring and Fall, as well as the occasional inland penetration of tropical cyclones, and El Nino years have produced some of the largest floods within the period of historical monitoring (see Sylvia and Galloway, 2006-this volume). Due to strong climatic and vegetation gradients, mean discharges decrease from the subhumid east to the semiarid west (LeBlanc and Hodgson, 1959; Kanes, 1970). However, flashiness of discharge regimes and peak discharges increase to the west due to rapid runoff from steeper landscapes with thin soils and sparse vegetation (Blum et al., 1994; Blum and Valastro, 1994). Many discussions of Quaternary climate or sea-level change focus on end-members such as the full-glacial or interglacial. However, oxygen isotope curves (Imbrie et al., 1984; Chappell and Shackleton, 1986; Williams et al., 1988; Chappell et al., 1996; Waelbroeck et al., 2002; Fig. 4A) clearly show that 80% of any middle to late Pleistocene 100 kyr glacial cycle was intermediate in character, with global temperatures cooler than full interglacial conditions, but not as cold as a full-glacial, and with eustatic sea level at −40 to −85 m or more. For Texas coastal plain river systems, this would translate to cooler land temperatures, and a cooler and smaller Gulf precipitation source for the long glacial periods. Moreover, during the long glacial periods, rivers would have been extended to shorelines in mid-shelf or farther basinward

Fig. 4. (A) Oxygen isotope curve spanning the last 450 kyr (from Imbrie et al., 1984), illustrating major changes in global ice volume and sea level, and the ∼ 100-kyr cyclicity to major glacial–interglacial cycles. Small numbers on graph represent specific isotope stages and substages. (B) Cartoon illustrating concept of average shoreline positions over the last 450 kyr, based on isotope data in (A). Most of the time, river systems of the Texas coastal plain would have been extended to mid-shelf or farther basinward positions, and the shelf would have been a subaerially-exposed extension of the coastal plain.

positions, and much of the shelf was then a subaerial extension of the coastal plain (Fig. 4B). Such conditions represent the norm for each middle to late Pleistocene 100kyr glacial–interglacial cycle (see Porter, 1989), and the full interglacials, with a warm climate, a large and warm Gulf precipitation source, and rivers discharging to updip shoreline positions, are very short and therefore somewhat abnormal. Beyond this level of generalization, more detailed records of sea-level change for the western Gulf of Mexico basin are not readily available, with the exception of those that cover the last full-glacial maximum (OIS 2) to present (e.g. Frazier, 1974). However, there is little reason to believe that sea-level change in the Gulf of Mexico departed significantly from globally-coherent eustatic curves. Similarly, empirical records of climate change in continental interior source regions are poorly known for periods prior to OIS 4. Musgrove et al. (2001) examined isotopic composition of cave stalagmites on the Edwards Plateau, source terrain for the Colorado River, and suggest 3 distinct periods of increased effective moisture during OIS 4–2, with the most recent corresponding to the OIS 2 full-glacial maximum. Toomey et al. (1993) reconstruct

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the OIS 2 full- and late-glacial climate of the Edwards Plateau, source region for the Colorado and Nueces Rivers, and considered this reconstruction to be representative of the southcentral US as a whole. Temperatures were significantly cooler during the full-glacial period, with more effective moisture, but perhaps as important were the types of precipitation events, and the nature of upland soils. Tropical cyclones were probably rare to nonexistent when sea level was low and the Gulf was cooler, and most flood-producing precipitation events would have been derived from midlatitude cyclonic storms. Several lines of evidence converge to show that glacial period precipitation fell on uplands that were covered by relatively deep soil mantles that are no longer present in the area today (see also Cooke et al., 2003). Global climate changes that led to wastage of OIS 4–2 ice sheets resulted in changes in climate and vegetation in the southcentral United States as well. Toomey et al. (1993) suggest that post-glacial sea-level rise, coupled with increased surface temperatures, promoted frequent inland penetration of warm, moist tropical air, and corresponding increases in the frequency of tropical cyclones and convectional storms. On the Edwards Plateau, these changes triggered a period of landscape instability and soil erosion such that upland landscapes now consist of exposed bedrock (Toomey et al., 1993; Cooke et al., 2003). Details may differ, but Holocene landscape instability may have been widespread elsewhere in the southcentral United States due to the shift from glacial to interglacial climates. Blum et al. (1994) suggested this removal of soil covers, coupled with increases in the inland penetration of tropical moisture and tropical storms, resulted in increases in the magnitude of infrequent floods, relative to the glacial period. 3. Previous work 3.1. Large-scale alluvial–deltaic plains — the Beaumont formation Large-scale depositional units of the Texas coastal plain were initially mapped on the basis of elevation, slope, degree of dissection, and soil type (see Morton and Price, 1987; DuBar et al., 1991; Blum and Price, 1998 for reviews). Three “morphostratigraphic units”, presumed to be Pleistocene in age, were recognized in the early 20th century, and designated the Willis (oldest), Lissie, and Beaumont (youngest) Formations. Early workers recognized that: (1) each unit is bounded at the top by an oxidized weathering profile; (2) each unit consists of river terraces that merge downdip with delta plains; (3) the surfaces of older units occur at higher elevations in updip

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margins of the coastal plain, and exhibit greater degrees of stream dissection than younger units; and (4) older surfaces have steeper slopes and are onlapped by younger surfaces farther downdip. Early genetic interpretations for large-scale coastal plain units were developed when the Pleistocene was divided into 4 long glacials with sea-level lowstands separated by long interglacials and sea-level highstand. Following Fisk's (1944) model for the Mississippi valley and Louisiana coast, Bernard and LeBlanc (1965), among others, inferred valley entrenchment and sediment bypass for glacial periods, and major depositional units were interpreted to represent floodplains and delta plains constructed during transgressions and interglacial highstands. The Beaumont Formation was traditionally assigned to the last long-lived “Sangamon” interglacial. Detailed mapping of the Texas Coastal Plain during the 1960s and 1970s improved understanding of Beaumont and younger depositional environments (e.g. Fisher et al., 1972; McGowen et al., 1975, 1976; Brown et al., 1976). This mapping showed that large rivers like the Brazos and Colorado had constructed extensive fan-shaped alluvialdeltaic headlands, whereas smaller rivers like the Trinity flowed into the Gulf at interdeltaic bights and had constructed small alluvial–deltaic plains fronted by estuarine and barrier island/strand plain environments (see Fig. 3). Bernard et al. (1970) and Winker (1982) showed the Beaumont surface consists of multiple cross-cutting meanderbelts with intervening flood basins, and interpreted alluvial plain surfaces to have been constructed by a series of autogenic meander-belt avulsions during sealevel highstand. Winker (1982) attempted to define major coastal plain units in three dimensions, correlate coastal plain units with a newly emerging offshore record, and develop a chronological framework based on data other than correlations with Pleistocene glacial cycles. He suggested that contacts between Willis, Lissie, Beaumont, and post-Beaumont strata on the coastal plain project downdip to widespread seismic reflectors in the marine record (Fig. 5). Willis strata were assigned a Pliocene age based on normal magnetism thought by Kukla and Opdyke (1972) to represent the Gauss polarity epoch (3.4 to 2.48 Ma; see Harland et al., 1989), whereas Lissie strata were assigned an early Pleistocene age based on projections downdip to biostratigraphic markers in offshore wells, and reversed polarity characteristic of the Matayuma epoch (2.48 to 0.79 Ma). Beaumont strata were correlated to recently developed oxygen isotope curves that served as a proxy record of glacio-eustasy (see Fig. 4A), with widespread aggradation of alluvial plains and multiple avulsions assigned to the “Sangamon” interglacial, now recognized

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Fig. 5. Long profiles of major depositional surfaces of the Colorado River and inferred correlations with seismic reflectors in the marine record offshore (after Blum and Price, 1998; based on original in DuBar et al., 1991). The first downhole appearance of the foram Trimosina denticulata, “Trim A”, placed at approximately 600 kyr BP by Armentrout and Clements (1990), occurs just above the reflector correlated to the top of the Lissie unit in offshore wells elsewhere in the Gulf.

as oxygen isotope stage 5e (OIS 5e), and deltaic progradation on the shelf with sediment bypass of the coastal plain during the subsequent “Wisconsin” glacial cycle (now OIS 5d-2). 3.2. Post-Beaumont valley fills 3.2.1. Deweyville terraces In the early part of the 20th century, post-Beaumont deposits and landforms received little attention. Beginning with Barton (1930), however, workers noticed large relict channel scars on terraces of east Texas Rivers, noting they were distinct from those on the older Beaumont or younger modern floodplains. Bernard (1950) later formally recognized the “Deweyville beds” as underlying a terrace that is lower in elevation than Pleistocene Beaumont surfaces but higher than Holocene floodplains, and which exhibits relict channel dimensions much larger than the modern Sabine. He noted similar terraces along the smaller rivers of the Texas coast (Fig. 6A), and suggested that large arcuate scars along valley walls in the Brazos and Colorado valleys might be Deweyville correlatives, but they are buried by younger deposits. Early workers recognized that Deweyville terraces and deposits are younger than Beaumont strata, and older than modern floodplains. Bernard (1950) inferred a latest Pleistocene age, whereas Bernard and LeBlanc (1965), Gagliano and Thom (1967), and Saucier and Fleetwood (1972) cited 14C ages of ca. 30–17 ka from Deweyville deposits in Arkansas. More recent estimates ranged over an order of magnitude, with Alford and Holmes (1985) suggesting an early to middle Holocene age for Dewey-

ville terraces in east Texas, and Anderson et al. (1992) and Thomas and Anderson (1994) placing Deweyville terraces of the Trinity valley in OIS 5c and 5a, ca. 105 ka and 80 ka, respectively, based on correlations with the Trinity incised valley fill offshore, and ages for the offshore record that were inferred from oxygen isotope curves (Fig. 6B). Barton (1930) recognized the large channel scars on terraces were significantly larger than modern channels, and suggested rainfall must have been greater. By contrast, Bernard (1950) favored linking Deweyville deposition and terrace formation to rising then falling sea level during the latest Pleistocene. Gagliano and Thom (1967), Saucier and Fleetwood (1972), and Alford and Holmes (1985) again favored climatic controls, using hydraulic geometry relationships to suggest Deweyville meander scars represent mean annual discharges or mean annual floods that were significantly greater than modern, whereas Saucier (1994) suggested changes in precipitation seasonality and intensity, and changes in vegetation, were more important than changes in mean annual discharge. Anderson et al. (1992) and Thomas and Anderson (1994) revisited the sea-level control model, and linked deposition to rising sea level during isotope stage 5c and 5a. 3.2.2. Holocene floodplains The stratigraphic framework and geochronology of post-Deweyville floodplains on the coastal plain has received relatively little attention. The most notable study would be Bernard et al. (1970), who described a series of cores and geophysical logs across the lower Brazos floodplain. In doing so, they identified buried channelbelts, discussed the history of a recently-abandoned

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Fig. 6. (A) Air photo illustrating the scale of Deweyville paleochannel scars in the Trinity valley, as compared with the modern Trinity channel (modified from Blum and Tornqvist, 2000). (B) Line drawing and seismic line from the cross-shelf Trinity valley, illustrating early interpretations of Deweyville correlatives on the shelf, which were interpreted to represent OIS 5c and 5a (modified from Anderson et al., 1992). Numbers in (B) correlate to oxygen isotope stages. Symbols in (B) as follows: SB = sequence boundary, HST = highstand systems tract, TST = transgressive systems tract, DLS = downlap surface, LB = lower bay facies, UB = upper bay facies, BL = bayline.

channel course (the Oyster Creek meanderbelt), and examined facies typical of modern Brazos point bars. Other studies include Aten's (1983) identification of different phases of fluvial–deltaic activity for the lower Trinity River from archaeological data. By contrast, numerous workers have focused on latest Pleistocene and Holocene stratigraphic records in upstream reaches of the major coastal plain river systems (e.g. Blum and Valastro, 1989; Hall, 1990; Blum and Valastro, 1992; Blum et al., 1994; Waters and Nordt, 1995). From these studies, it is clear the upstream reaches of fluvial systems that eventually discharge to the Gulf of Mexico record a variety of morphological and sedimentary adjustments during the late Pleistocene and Holocene. In this largely tectonically-inactive setting, far removed from the effects of glaciation or glacio-eustasy, most authors have attributed changes in fluvial activity through time to climate change.

(c) fluvial systems were increasingly recognized to be more sensitive to upstream climatic controls, and (d) emerging new data from the shelf provided a unique opportunity to develop an understanding of fluvial system evolution from source-to-sink. Research efforts were therefore undertaken on the Colorado, Trinity, and Nueces Rivers. These studies relied on mapping from multispectral satellite imagery, collection of continuous cores, description and characterization of facies and facies successions from outcrops and cores, and development of geochronological frameworks using radiocarbon and thermoluminescence dating techniques. The following summarizes current thinking on Beaumont and postBeaumont landforms and deposits of the Texas coastal plain, updated from Blum (1994), Blum and Valastro (1994), Blum et al. (1995), Morton et al. (1996), Durbin et al. (1997), Blum and Price (1998), Aslan and Blum (1999), and Blum and Tornqvist (2000).

4. Current thinking on Texas coastal plain fluvial deposition

4.1. Beaumont alluvial plains

By the early to middle 1990s, it was clear that older concepts of Gulf Coastal Plain fluvial deposition needed reevaluation, since (a) the collective understanding of Quaternary sea-level change had changed substantially, (b) the development of sequence stratigraphic models provided new conceptual frameworks to test and refine,

A number of criteria have long suggested that Beaumont alluvial–deltaic plains are more complex than early interpretations inferred, and represent a longer and/or different time period. First, the areal extent of Beaumont alluvial plains is much greater than the same rivers have constructed during the present highstand. Second, several workers (e.g. Winker, 1982; Paine, 1991) noted mature

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paleosols within Beaumont deposits, indicating significant periods of non-deposition and soil development before burial by more Beaumont deposits. Finally, Winker (1982; see also DuBar et al., 1991) correlated the onlapping contact between Beaumont and Lissie onshore with a regional seismic reflector offshore that occurs below the first downhole appearance of Trimosina denticulata, placed at ca. 600 ka by Armentrout and Clements (1990; see Fig. 5). Winker (1982) retained the interpretation that Beaumont strata represented the last interglacial highstand (OIS 5), and therefore inferred a significant time gap between Lissie and Beaumont deposition. However, an alternative interpretation would be that the Beaumont Formation represents everything above the inferred Lissie contact in Fig. 5, and therefore much of the middle to late Pleistocene. Blum and Price (1998) and Blum and Aslan (unpublished data) subsequently developed a revised stratigraphic framework for Beaumont strata, focusing on the

alluvial–deltaic plain of Colorado River. This framework applies strictly to the Colorado River, but the same patterns may apply elsewhere. 4.1.1. Beaumont stratigraphic framework Previous mapping (e.g. Fisher et al., 1972; McGowen et al., 1975, 1976; Brown et al., 1976) differentiated Beaumont surfaces into channelbelt and floodplain depositional environments, in part on the basis of morphological and tonal characteristics identified in air photos. Blum and Price (1998) took this a step further, and noted distinct cross-cutting relationships between channelbelt axes in multi-spectral satellite imagery, systematic discontinuities in surface drainage patterns, and superimposed deeply-weathered paleosols in outcrops. They also presented a series of thermoluminescence (TL) ages from Beaumont channelbelt and crevasse splay sands of the Colorado alluvial plain, and provided the first

Fig. 7. (A) Satellite image of Colorado alluvial plain. (B) Simplified geologic map of Beaumont and younger strata of the Colorado alluvial plain, subdividing the Beaumont surface into multiple large-scale cross-cutting valley fills. From Blum and Price (1998).

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chronological framework for alluvial plain deposition along the Gulf of Mexico coast. From these data, the Beaumont alluvial plain of Colorado River was differentiated into at least three principal incised valley fills (Fig. 7), with key criteria as follows: 1. Each Beaumont valley fill has an areal extent similar to that of the post-Beaumont valley; 2. Each valley fill surface consists of well-defined channelbelt axes with clearly identifiable high-reflectivity (bright tones in satellite imagery, indicating welldrained) channelbelt sands that anabranch or become distributary in the downstream direction, and are flanked by low reflectivity (dark-toned, indicating poorlydrained) flood-basin muds; 3. Each valley fill is bounded by a paleovalley wall that can be identified on the basis of discontinuities in surface drainage patterns, with headward-eroding channels that extend from the paleovalley wall to the adjacent older alluvial plain surface (see Posamentier,

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2001 for a discussion of this relationship, as observed in seismic data); and 4. Each valley fill is capped by a deeply-weathered soil profile, and successively older valley fill surfaces show increased degrees of weathering and diagenetic alteration. In far down-dip reaches of the coastal plain, or in updip reaches proximal to paleovalley walls, deeplyweathered soils from older valley fills are buried and/or partially truncated by floodplain facies and deeplyweathered soils from younger valley fills. The oldest Beaumont valley fill occurs along the western flanks of the Colorado alluvial plain, where it is partially occupied by the present-day Navidad River, and was informally referred as the Lolita valley fill (Blum and Price, 1998). The Lolita surface is buried by younger flood-basin facies, but is defined by a deeply-weathered soil profile that has been penetrated in far updip locations in shallow cores, is exposed in bluffs along Navidad River (Fig. 8A), and is exposed farther downdip along the

Fig. 8. Stratigraphic relations between paleosols that define the top of the Lolita valley fill, and overlying floodplain muds of the El Campo valley fill, in which the surface soil has developed. (A) Exposure along Navidad River, illustrating Lolita channelbelt sands, and paleosol, buried by younger floodbasin muds and the surface soil. TL dates of 323 ± 51 and 307 ± 37 kyr BP were obtained from this locality. (B) Exposure farther downdip along Lavaca Bay, illustrating the top of Lolita valley fill is roughly parallel to the present coastal plain surface. Locations shown on Fig. 7A.

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margins of Lavaca Bay (Fig. 8B). Depths of burial for the Lolita surface remain relatively constant (2–4 m) through this 70 km distance, indicating this former depositional surface had a gradient comparable to the present coastal plain, and was graded to sea-level highstand positions that were similar to present. TL ages from Lolita highstand channelbelt sands (see Figs. 8A, 9E) suggest deposition during the OIS 9 interglacial period. A poorly-exposed valley fill of intermediate age is centered over, and informally named after, the town of El Campo. The deeply-weathered soil that defines the El Campo alluvial plain surface is the surface soil throughout most of its mapped extent, and is only buried by younger strata along its eastern and farthest downdip margins. The youngest and most clearly defined unit was referred to as the Bay City valley fill and has been reoccupied by the modern Colorado channel (Blum and Valastro, 1994; Aslan and Blum, 1999). Its western boundary is partly defined by Tres Palacios River, a small stream that has eroded headward from a drowned channel (Tres Palacios Bay) into lower topography along the contact between flood-basin depositional environments and the El Campo valley fill. The post-Beaumont valley fill defines the eas-

tern margins of the Bay City unit. TL ages constrain the Bay City unit to the OIS 6 to 5 glacial–interglacial cycle. The El Campo valley fill is undated but from stratigraphic relations it must be intermediate in age between the Lolita and Bay City valley fills, and was interpreted to represent the OIS 8 to 7 glacial–interglacial cycle. 4.1.2. Beaumont alluvial architecture and facies Because of reoccupation by the present Colorado River, the Beaumont Bay City valley fill is exposed in modern channel cut banks for a distance of 50–60 km (see Fig. 7). A series of outcrops along the river and a series of cores across the valley fill provide important data on valley fill architecture and facies. Exposures along the modern Colorado channel can be subdivided into 2 distinct types of facies successions. The first occurs in reaches dominated by channelbelt traces, as interpreted from satellite imagery and previous mapping efforts, and typically consists of a fining-upwards succession, 10–12 m or more in thickness, of coarse to fine sands that are capped by a well-developed surface soil (Fig. 9A, B). Basal parts of these sand bodies are typically trough cross-bedded, with cross-sets up to 75 cm in height

Fig. 9. Measured sections from Beaumont valley fills. (A) Measured section typical of Beaumont Bay City highstand channelbelt sandbodies, veneered by highstand floodbasin facies. TL age of 119 kyr BP obtained from this locality, just downstream from the town of Wharton (see Fig. 7B). (B) Short measured section of Beaumont Bay City highstand channelbelt near town of Bay City, where channelbelt sands are not veneered by younger floodbasin facies. (C) Measured section typical of Beaumont Bay City floodplain succession, where transgressive to highstand floodplain facies include crevasse–channel and crevasse–splay sands encased in floodbasin muds, and resting on paleosols developed in falling stage to lowstand channelbelt sands. TL age of 115 kyr BP obtained from this locality, as shown. (D). Measured section typical of channelbelt sands in Beaumont El Campo valley fill. (E.) Measured section of highstand channelbelt sands of the Beaumont Lolita valley fill, with Lolita paleosol buried by floodbasin facies of the El Campo and/or Bay City valley fills. Locations shown on Fig. 7A.

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Fig. 10. (A) Photomosaic of typical transgressive to highstand facies, illustrating crevasse channel tracing laterally to thin sands isolated within floodplain muds. Person (arrow) for scale. (B) Typical exposure in transgressive to highstand flooplain facies, overlying paleosol developed in falling stage to lowstand channelbelt sands. (C) Close-up view of the same outcrop, illustrating the top of this deeply weathered paleosol, and the sharp contact with overlying floodplain muds.

and decreasing in scale upwards, and commonly contain reddish mud rip-up clasts up to 3–5 cm in diameter. The upper part tends to be dominated by plane-bedded and ripple-laminated sands or sands that have been modified by deep weathering and soil development. These sandbodies are interpreted to represent point-bar and channelbar facies deposited in association with channelbelts that were graded to sea-level positions comparable to the present-day highstand (hereafter referred to as highstand channelbelts). Similar highstand channelbelt sandbodies can be seen in the older El Campo and Lolita valley fills as well (Fig. 9D, E). Exposures in highstand channelbelts contrast markedly with exposures common to reaches dominated at the surface by flood-basin environments, as interpreted from satellite imagery. Here, the upper 5–10 m of most

sections consist of lenticular to sheet-like sandbodies ranging from 0.5 to 2 m in thickness, and dominated by ripple-laminated to plane-bedded fine sand, that are interbedded with, or bounded by, laminated to massive calcareous reddish mud (Figs. 9C, 10A). The entire succession is capped by a well-developed and deeply leached black vertic paleosol (see also Stiles et al., 2003; Driese et al., 2005 for studies of surface soils). These facies are interpreted to represent interbedded crevasse– channel and crevasse–splay sands and flood-basin muds that were genetically-related to the highstand channelbelts described above (hereafter highstand floodplain facies). TL ages from interpreted highstand channelbelt and crevasse–splay sands range from ca. 102–119 kyr BP (Blum and Price, 1998), which corresponds to the OIS 5 highstand period.

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Highstand floodplain facies typically rest on leached and weathered paleosols that formed in a buried sand body that consists of trough cross-bedded to ripple-laminated medium to fine sand (Figs. 9C, 10B, C), or in interbedded fine sand and reddish mud with clearly-identifiable lateral accretion surfaces. Sandy facies are interpreted to represent upper point bar and channel bar deposits associated with well-developed channelbelts, whereas sandy and muddy facies are interpreted to represent abandoned channel fills. The thickness of overlying highstand floodplain facies, and the corresponding depth of burial for these paleosols, increases in the downstream direction when measured with respect to the Beaumont alluvial plain surface, and paleosols dip below present sea level some 20 km upstream from the present-day shoreline. Such geometric relationships indicate these buried channelbelts were graded to shorelines that were considerably lower in elevation, and farther basinward than average highstand positions, and therefore formed during a glacial period falling stage or lowstand (hereafter referred to as falling stage to lowstand channelbelts). TL ages of ca. 155 kyr BP (Blum and Price, 1998) suggest these channelbelts correspond to the OIS 6 glacial period falling stage and lowstand. Core data from the Bay City valley fill complex illustrates these same facies successions are representative, and valley fill facies architecture is spatially complex. From mapping data, the valley fill is interpreted to extend some 20–40 km along strike. As shown in the interpreted crosssection represented by Fig. 11, distinct paleosols and subjacent channelbelt sand bodies were encountered at depths of 10–20 m below the alluvial plain surface, and are interpreted to represent the falling stage to lowstand

channelbelts seen in outcrops (Fig. 10B, C). Basal scours beneath sandbodies were rarely penetrated, but assuming an average sand-body thickness of 10–15 m, overall valley-fill thickness is interpreted to range from 25–35 m. A number of valley-fill characteristics can be inferred from this cross-section, in conjunction with outcrop data. 1. Falling stage to lowstand sandbodies tend to be amalgamated and sand-dominated, and have few associated floodplain facies. Fine-grained facies are present, but tend to be limited in lateral extent, and concentrated within lenticular channel fills. 2. In some cores, laminated gray muds rest on paleosols developed in falling stage to lowstand channelbelt sand bodies. These are interpreted to represent estuarine muds that reflect sea-level rise and temporary flooding of the valley. These facies were not identified farther upstream where paleosols are encountered above present-day sea level. 3. Thick channelbelt sands with associated sandy crevasse–splay facies occur above falling stage and lowstand channelbelts, as well as interpreted estuarine facies, but are in turn buried by floodplain facies associated with highstand channelbelts. These are interpreted to represent channelbelts that were active during transgression and the early stages of valley filling. 4. At least 2 significant highstand channelbelt sand bodies can be identified, but ribbon-like crevasse channels, sheet-like crevasse–splay sands and reddish flood-basin muds represent the most volumetricallysignificant facies assemblages in the upper half of the valley fill as a whole.

Fig. 11. Cross-section of the Beaumont Bay City valley fill, as interpreted from a series of continuous cores. Location shown on Fig. 7A.

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5. Channel-in-channel stacking is common, such that transgressive and highstand channelbelts cut out and/or reoccupy channel courses that were active during falling stage and lowstand. This pattern continues to the present, since the Colorado channel avulsed away from the post-Beaumont valley fill and has reoccupied a Beaumont Bay City highstand channelbelt, which in turn occupies a falling stage to lowstand channel course. 4.2. Post-Beaumont valley fills Beaumont alluvial plain surfaces were abandoned as active depositional environments at the end of OIS 5, ca. 100–80 kyr BP, and river systems incised through the highstand prism and extended their courses across a newly subaerial shelf. The post-Beaumont record consists of the Deweyville units, and latest Pleistocene to Holocene strata, as summarized below. 4.2.1. Deweyville units 4.2.1.1. Deweyville stratigraphic framework. Studies by Blum et al. (1995), Morton et al. (1996), and Durbin

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et al. (1997) refined Bernard's (1950) original Deweyville model, and concluded that at least 3 suites of landforms and/or deposits that correspond to Bernard's original Deweyville concept are present in major river valleys. However, some of these have a similar age, origin, and genetic significance, but no longer have surface expression as a terrace, since they have been onlapped and buried by younger strata. Fig. 12 illustrates Deweyville stratigraphic relationships in extrabasinal vs. basin-fringe system, whereas Fig. 13 summarizes Deweyville stratigraphic relations in the Trinity valley, where they are best defined. Blum et al. (1995) summarized Deweyville stratigraphic relationships as follows: 1. All Deweyville units cross-cut, are inset, are lower in elevation, and are therefore younger than Beaumont alluvial plain surfaces; 2. The oldest Deweyville unit in any given valley occurs at the highest elevations, with successively younger units downstepping to successively lower elevations; 3. The multiple Deweyville units rest on a composite basal post-Beaumont valley-fill unconformity that

Fig. 12. Schematic valley cross-sections contrasting “Deweyville allostratigraphy” in unfilled vs. filled coastal plain incised valleys, at similar distances updip from modern highstand shorelines. (A) Unfilled valley such as the Trinity, where 2 of the oldest Deweyville surfaces remain as terraces, but lowest and youngest Deweyville surfaces are buried and onlapped by Holocene floodplain or delta plain strata. (B) Filled valley, such as the Colorado, where all “Deweyville” surfaces have been onlapped and buried by Holocene strata. Relative scale of valley fill sequences as indicated. “Deweyville” allostratigraphic units are shown occurring on one side of the valley for illustration purposes only.

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4.

5.

6.

7.

traces up and out of the valley to deeply-weathered soils developed on Beaumont alluvial plain or older surfaces; Each Deweyville unit is bounded at its base by unconformities that trace up, and laterally, to soils developed on older Deweyville or Beaumont units, and the upper boundary to each unit is defined by a deeply-weathered soil profile; Individual Deweyville units consist of well-developed channelbelts that record lateral migration at a specific elevation for an extended period of time (∼ 103–104 yr); Individual Deweyville surfaces represent former floodplains, and project seaward to shorelines that were considerably lower in elevation and farther basinward than today. When exposed at the surface or near surface, Deweyville units display the large abandoned meander loops identified by early workers (Figs. 6A, 13A); The updip limits of onlap and burial of Deweyville units by younger overbank strata, defined as the onlap distance (see Blum and Tornqvist, 2000), is correlated to sediment supply and inversely correlated to valley gradient. For this reason, Deweyville units are still exposed in the unfilled valleys of the basin-fringe systems like the Trinity River, but completely onlapped and buried in the filled valleys of the extrabasinal Colorado and Brazos systems to distances exceeding 100 km upstream from the highstand shoreline (see also Sylvia and Galloway, 2006-this volume).

Deweyville geochronology remains a topic of investigation. Stratigraphic relations indicate that Deweyville units postdate abandonment of Beaumont alluvial plain surfaces, which occurred during late OIS 5 as channels incised and extended across the shelf in response to sealevel lowering. At the other end of the time window, the youngest Deweyville units have produced a suite of minimum 14C ages from channel fills that rest on top of channelbelt sands. These fall within OIS 2, ca. 23–16 kyr BP (ages calibrated from original data reported in Blum and Valastro, 1994), and Deweyville units are onlapped and buried by late Pleistocene and Holocene strata (see also Sylvia and Galloway, 2006-this volume). From these data, it can be inferred that, in general, Deweyville units of

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the Colorado River represent the OIS 4–2 glacial period. Durbin et al. (1997) report TL ages from the Nueces River, which support inferences based on stratigraphic position plus bracketing 14C and TL ages from the Colorado system, namely that Deweyville units represent the OIS 4–2 glacial period, especially OIS 3 and 2. Similarly, Sylvia and Galloway (2006-this volume) report 14C ages that place interpreted buried Deweyville units of the Brazos River in OIS 3 and 2. Accordingly, Deweyville units are considered to represent falling stage to lowstand channelbelts that were deposited within distinct incised valleys, and by channels that were extended to sea-level positions on the mid to outer shelf or shelf edge. 4.2.1.2. Deweyville alluvial architecture and facies. Early workers recognized that Deweyville facies typically are coarser than Beaumont or Holocene deposits (e.g. Gagliano and Thom, 1967; Saucier and Fleetwood, 1972). Blum and Valastro (1994) subsequently noted that (a) gravely or sandy point- and channel-bar facies extend to the top of most sections in Deweyville correlatives of the Colorado valley, (b) fines are concentrated in lenticular channel fills, and (c) true laterally extensive vertical accretion facies are rare, which contrasts with Beaumont alluvial plain or Holocene deposits where vertical accretion facies are thick, widespread, and volumetrically significant (Fig. 14A, B; see also Blum and Straffin, 2001). Aslan and Blum (1999) later described a series of cores in the postBeaumont valley fill of Colorado River, 20 km updip from the modern shoreline, and identified 3 paleosols and subjacent sand bodies at depths of 5–15 m below the modern floodplain surface. They interpreted these units to represent Deweyville units (Fig. 17B). From these observations, one can generalize that Deweyville units consist of multiple cross-cutting and therefore laterally amalgamated sandy and gravelly channelbelts, with isolated muddy channel fills. Observations in the Trinity and other valleys support this view, especially that of limited to non-existent vertical accretion facies in Deweyville units (Fig. 14C, D). Indeed, the scroll topography commonly viewed in air photos from Deweyville terrace surfaces (see Figs. 6, 13A) would not be so visible if these surfaces were covered by appreciable thicknesses of vertical accretion facies. Where

Fig. 13. (A) Satellite image of the lower Trinity valley, illustrating large channel scars typical of Deweyville units. (B) Geologic map of the lower Trinity valley, illustrating distribution of Deweyville units within the post-Beaumont incised valley. Modified from Aslan and Blum (1999). (C) Cross-section of the lower Trinity valley at location A–A′ in (A), illustrating stratigraphic relations between Beaumont valley walls, Deweyville units and Holocene channelbelts. (D) Cross-section of the lower Trinity valley at location B–B′ in (A), illustrating stratigraphic relations between Beaumont valley walls, Deweyville units and Holocene channelbelts. Note increased depth of burial of Low Deweyville unit, relative to cross-section A–A′. Modified from Morton et al. (1996).

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Fig. 14. Photographs of Deweyville facies. (A) Photograph of Deweyville facies of Colorado River, upstream from the onlap point, illustrating deeply weathered paleosol developed in channel and point bar gravel and sand, with no fine-grained overbank strata. (B) Photograph of Deweyville facies of Colorado River downstream from the onlap point, where paleosols developed in channel and point bar gravel and sand is buried by younger Holocene floodbasin mud. (C) Photograph of Deweyville facies of Trinity River, illustrating deeply weathered paleosol developed in channel and point bar sand, with no fine-grained overbank strata. (D) Photograph of Deweyville facies of Trinity River, where youngest Deweyville unit is onlapped and buried, illustrating deeply weathered paleosol developed in point bar sand and channel fill mud, with no fine-grained overbank strata.

present in these valleys, fine-grained facies represent either lenticular channel fills inset into point and channel bar sand, or younger vertical accretion facies that rest unconformably on top of, and onlap, deeply-weathered paleosols developed in Deweyville strata. Sylvia and Galloway (2006-this volume) extended Aslan and Blum's (1999) cross-section across the Brazos valley, and interpreted a series of paleosols resting on deposits that were correlated to Deweyville units. Brazos deposits are, in general, finer than those of the Colorado and other rivers, but fine-grained vertical accretion facies were again considered rare. It seems clear, therefore, that Deweyville units represent a succession of sand-dominated channelbelts from the last glacial period falling stage and lowstand, but the

significance of the large Deweyville paleochannels remains a topic of discussion. Current workers favor climatic explanations of some type. However, Blum et al. (1995) suggested, in contrast to earlier views, that Deweyville units did not represent larger floods than modern, whereas Sylvia and Galloway (2006-this volume) argue for significantly larger channel-forming discharges than today. The view taken here is that is not yet possible to correlate specific Deweyville units with specific climatic conditions in hinterland source regions, due to lack of geochronological data for Deweyville units themselves, and the general paucity of independently-defined OIS 4–2 paleoclimate records in the southcentral United States. These different views should be tested in future research efforts.

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4.2.2. Latest Pleistocene and Holocene strata As noted above, few early studies addressed the nature of post-Deweyville deposits along the lower reaches of coastal plain rivers. Following earlier work in the upper Colorado drainage, Blum (1994) and Blum and Valastro (1994) reexamined and reinterpreted the latest Pleistocene and Holocene record of Colorado River within the mixed bedrock–alluvial valley of the Inner Coastal Plain, then traced deposits and surfaces downstream to the subsiding alluvial–deltaic plain so as to investigate the nature of interactions between upstream climatic and downstream glacio-eustatic controls.

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4.2.2.1. Latest Pleistocence and Holocene stratigraphic framework. In the mixed bedrock–alluvial valley of the Inner Coastal Plain, Blum and Valastro (1994) defined latest Pleistocene and Holocene deposits as the Columbus Bend Alloformation, with three allomembers (from oldest to youngest, CBA-1 to CBA-3). Fig. 15A summarizes stratigraphic relationships for the mixed bedrock–alluvial valley. In general, these relationships indicate 3 periods of channelbelt formation and lateral migration, punctuated by periods of floodplain stability and soil formation, and followed by renewed lateral migration and deep overbank flooding with burial of paleosols. CBA-1 and CBA-2

Fig. 15. (A) Schematic cross-section of the post-Beaumont valley fill within the mixed bedrock–alluvial valley of Colorado River near La Grange, Tx. (B) the post-Beaumont coastal plain incised valley fill near Wharton, illustrating downstream persistence of allostratigraphic units and paleosols (vertical lines) with downstream changes in stratigraphic architecture. (C) Longitudinal profiles for the youngest Deweyville unit, and different members of the CBA. Upstream from the town of Eagle Lake, longitudinal profiles for CBA-1 and CBA-2 are essentially the same, and plotted as such. Downstream from that point, longitudinal profiles for the youngest Deweyville and CBA-1 are essentially the same, and are plotted as such (from Blum and Valastro, 1994).

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deposits underlie a distinct composite terrace surface at ∼10–12 m above present low water channels, rest on preQuaternary bedrock at elevations very near that of the present low water channel, are stratigraphically inset into Deweyville equivalent deposits or older units, and range in thickness from 10 to 15 m. The two units are separated from each other by an erosional unconformity or a welldefined paleosol, and in some cases veneers of CBA-2 strata bury paleosols developed in CBA-1. CBA-3 represents the pre-dam channelbelt and genetically-related floodplain strata. The height of CBA-3 point bars and floodplain surfaces is 6–9 m or so above the low water channel, and therefore 2–3 m below the elevation of CBA-2 surfaces. However, veneers of CBA-3 strata cover and bury soils developed in both CBA-1 and CBA-2, especially in locations proximal to the CBA-3 channelbelt (Fig. 16). Downstream from the town of Eagle Lake and extending to the town of Wharton, the same basic components are present in the CBA, but stratigraphic relationships change significantly. This is the updip limit of the coastal plain incised valley, and the onlap point for Deweyville strata, which were deposited when the Colorado channel was incised below Beaumont alluvial plain surfaces, and extended to shorelines in a mid-shelf or shelf-edge position. Between the towns of Eagle Lake and Wharton, CBA-1 maintains a longitudinal profile similar to Deweyville surfaces, whereas CBA-2 and CBA-3 onlap and bury both units, and depth of burial of paleosols

developed in CBA-1 increases in the downstream direction to a maximum of 4–5 m at Wharton. Fig. 15B illustrates stratigraphic relationships for the updip reaches of the coastal plain incised valley, whereas Fig. 15C illustrates longitudinal profiles of the youngest Deweyville and CBA depositional surfaces as they go from the mixed bedrock–alluvial valley to the coastal plain incised valley. Downstream from Wharton, the Colorado River abandoned its late Pleistocene and Holocene incised valley some 2–300 yr ago, and now occupies a Beaumont channelbelt (Aslan and Blum, 1999). 14 C ages provide a robust chronological framework for the CBA succession, and indicate that deposition of CBA1 occurred during the latest Pleistocene through middle Holocene, from ca. 14–5 kyr BP, deposition of CBA-2 occurred during the late Holocene, from ca. 5–1 kyr BP, and CBA-3 represents the last 800 years or so. In contrast to the OIS 4–2 period, where correlation of specific Deweyville units to specific climatic conditions is speculative, periods of change in the CBA succession correspond to independently-identified periods of climate change (Toomey et al., 1993). Accordingly, they have been argued to reflect climatically-controlled changes in the relationship between discharge and sediment supply, and in the magnitude and frequency of extreme flood events (Blum et al., 1994; Blum and Valastro, 1994). 4.2.2.2. Latest Pleistocence and Holocene alluvial architecture and facies. In the mixed bedrock–alluvial

Fig. 16. (A) Exposure typical of the CBA-2, showing thick overbank strata resting on channel — and point bar sands, with a weak paleosol buried by veneers of overbank strata from CBA-3. (B) Exposure illustrating stacked paleosols typical of the CBA succession, with a paleosol developed in CBA-1 overlain by CBA-2 overbank strata, which in turn has a paleosol developed at the top, and it then buried by overbank strata from CBA-3.

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valley, the CBA succession contains a typical array of channelbelt facies, interpreted to represent deposition by coarse-grained point bars similar to those described for the modern Colorado river (see McGowen and Garner, 1970). However, in sharp contrast to older Deweyville units, vertical accretion floodplain facies are ubiquitous within the CBA succession, commonly exceeding 3–5 m in thickness. Primary morphological and sedimentological characteristics of floodplain environments and facies are wellpreserved as well, with ridge and swale topography easily

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visible in aerial photographs. Within CBA-1 and CBA-2, overbank facies are modified by post-depositional bioturbation, weathering, and pedogenesis, but most exposures consist of lenticular mud interbedded and inset within laterally-extensive tabular bodies of silt and fine sand (Fig. 16A). Veneers from CBA-2 commonly truncate and/or bury soils developed in CBA-1, and consist of fine sand and mud some 20–200 cm in thickness. Overbank facies from CBA-3 are still accumulating, and thin veneers occasionally bury soils developed in both older units (Fig. 16B).

Fig. 17. (A) Satellite image of the post-Beaumont valley fill of the lowermost Colorado River, which is now abandoned by avulsion (see Fig. 7). The most recent channel of Colorado River is now referred to as Caney Creek. Also shown are locations of the San Bernard River, and early to middle Holocene channel course of Colorado River, and the present-day Brazos River. (B) Cross-section of the post-Beaumont valley fill, as interpreted from a series of continuous cores (locations as shown in (A).

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The CBA is not exposed in far downstream reaches due to avulsion and abandonment of the lowermost part of the coastal plain incised valley, so precise stratigraphic relations are not known at the same level of detail possible farther upstream from outcrops, and it remains unclear as to the differentiation of this succession into specific units within the CBA, as defined farther upstream. However, Aslan and Blum (1999) examined alluvial architecture in the lowermost Colorado valley from a series of cores (Fig. 17), and in the lowermost Trinity valley from outcrops, with special reference to the role of avulsion during different phases of valley filling. In each system, the early phases of post-Deweyville deposition consists of sandy and muddy channelbelts that reoccupy abandoned falling stage and lowstand channels, with erosion and reworking of older channelbelt sands, followed by avulsion and reoccupation of another falling stage to lowstand channelbelt course. As valley filling progresses, avulsion was interpreted to occur by repeated diversion into floodplain depressions. This produces successions of massive to laminated floodbasin muds that encase thin (<5 m) ribbon-like crevasse channels and thin (<2–3 m) sheet-like splay sands, and comprise a large proportion of the total valley fill. Deposits of individual avulsions are separated by massive to slickensided muds or buried weakly-developed soil horizons that represent periods of slow sediment accumulation or floodplain stability between episodes of avulsive deposition. As valley filling nears completion, avulsion by channel reoccupation again becomes the dominant process, and again results in amalgamated channelbelts. Because of the differences in sediment supply, the Trinity River is in the early stages of valley filling and avulsion is still dominated by reoccupation of Deweyville channelbelts, whereas the larger Colorado system has filled its previously incised valley and completed this succession of stages. Valley filling in the Colorado progressed to such a point that some 2–300 yr ago the channel avulsed completely out of its incised valley from the last glacial cycle, and reoccupied an OIS 5 Beaumont channelbelt (Aslan and Blum, 1999).

5. Discussion 5.1. Correlations between onshore and offshore records Abdullah et al. (2004) provide an overview of the stratigraphic evolution of the east-central Texas shelf, which includes the cross-shelf incised valleys, and the shelf-phase and shelf-margin deltas of the Trinity, Brazos, and Colorado Rivers. Their interpretations are based on a network of seismic data, platform borings, and a limited number of cores and radiocarbon ages. Much of their chronostratigraphic framework is based on oxygen isotope analysis of microfauna from cores, and correlation with published oxygen isotope curves. Based on the framework presented therein, it is now possible to correlate fluvial activity within the youngest Beaumont valley fill, as well as the post-Beaumont record, with their downdip offshore equivalents. For the Colorado and Brazos systems, Abdullah et al.'s (2004) offshore record begins with incised valleys and shelf-margin deltas interpreted to represent isotope stage 6, the penultimate glacial period lowstand. These strata should correlate with the falling stage to lowstand channelbelts identified in the Beaumont Bay City valley fill (see Figs. 10, 11), which have produced TL ages of ca. 155 kyr BP. The location of Abdullah et al.'s (2004) interpreted isotope stage 6 incised valleys on the innermost shelf does not correspond to their mapped location on the coastal plain (Fig. 18A), as presented in Blum and Price (1998), but instead suggests the location of isotope stage 6 valleys was similar to post-Beaumont valley fills. However, Abdullah et al. (2004) note that stage 6 deposits are usually obscured below the seafloor multiple, except at the shelf margin, so their mapped location on the inner shelf appears to be inferential. By contrast, OIS 5 highstand strata of the Colorado River on the shelf are welldocumented, and correspond to the mapped occurrence of highstand systems tracts of the OIS 5 Beaumont Bay City valley fill (Fig. 18B). OIS 4 and 3 of the Colorado and Brazos systems are represented in the shelf record of Abdullah et al. (2004) by

Fig. 18. Paleogeographic maps synthesizing evolution of depositional systems and sequences, and correlation between units on the coastal plain and shelf. (A) OIS 6, illustrating mapped location of falling stage to lowstand channelbelts of the Beaumont Bay City complex on land, and their downdip correlatives, as mapped by Abdullah et al. (2004). Dashed lines illustrate their map units, where they do not agree with onshore data (see text discussion). (B) OIS 5e, illustrating the mapped distribution of the Beaumont Bay City highstand complex, plus their offshore equivalents. (C) OIS 4/ 3, illustrating mapped location of post-Beaumont coastal plain incised valleys, with Deweyville falling stage units, and their downdip correlatives, including mid-shelf deltas. (D) OIS 2, illustrating mapped location of post-Beaumont coastal plain incised valleys, with Deweyville lowstand unit, and its downdip correlatives, including shelf-margin deltas and slope systems. (E) OIS 1, illustrating mapped location of post-Beaumont coastal plain incised valleys, with transgressive to highstand units, and their downdip correlatives, including backstepping deltas interpreted to represent river mouth backstepping in response to transgression. (F) Simplified glacio-eustatic curve for the last 250,000 yr for reference. Letters above sea-level curve correspond to specific panels on this figure.

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what has been interpreted to be channelbelt sands within incised valleys that feed large deltas located in mid-shelf positions (Fig. 18C), whereas the isotope stage 2 lowstand of the Colorado, Brazos, and Trinity Rivers is represented by the same or similar valleys that feed shelf-margin deltas and slope systems (Fig. 18D). The OIS 4–2 part of the offshore record correlates to Deweyville units on the coastal plain, which occur inset below Beaumont highstand alluvial plain surfaces, and fully within the wellmapped post-Beaumont valley fills, suggesting Deweyville units are the updip equivalent. With the exception of their westernmost interpreted OIS 3 delta of Colorado River, Abdullah et al.'s (2004) mapped occurrence on the shelf coincides with their position within the postBeaumont valleys of these rivers as well. This anomalous western delta cannot be linked to the Colorado incised valley, which was much farther east, and more likely represents one of the smaller intrabasinal rivers (the Lavaca River) located farther west. OIS 1 strata on the shelf are interpreted to represent a succession of backstepping deltas within, or proximal to, the major incised valley axes (Abdullah et al., 2004). These backstepping deltas correlate to the early stages of valley filling on the coastal plain, and the progressive onlap and burial of Deweyville units by late Pleistocene and Holocene strata of the Columbus Bend Alloformation (Fig. 18E). 5.2. Depositional model and sequence stratigraphic implications The following outlines a general model for Texas coastal plain fluvial deposition, emphasizing interactions between climate and sea-level change over multiple time scales. The view taken here is that post-Beaumont strata represent a continuation of patterns of deposition that are also represented by Beaumont alluvial plains. Accordingly, the discussion below applies to Beaumont and postBeaumont deposits and landforms, as described above, and is framed within the context of sequence stratigraphic interpretations of incised-valley fills. 5.2.1. Partitioning of valley fills in response to 100-kyr glacio-eustatic cycles At the most general level of stratigraphic organization, Beaumont and younger strata can be subdivided into multiple cross-cutting and/or superimposed incisedvalley fills (Blum and Price, 1998; Blum and Tornqvist, 2000; Fig. 19); in a general way, Beaumont and postBeaumont alluvial plain successions correspond to what Holbrook (2001) defined as a multi-valley complex, based on studies of Cretaceous strata. Valley fills exposed

at the surface or near surface represent the last 400 kyr, and individual valley-fill successions are interpreted to represent the 100-kyr cycles of glacio-eustasy that have characterized this time period. Valley fills are interpreted to be stratigraphically partitioned when sea level falls below the interglacial highstand depositional shoreline break (see Fig. 2B). At this time, (a) channels incise through highstand aggradational and progradational wedges and extend across the newly subaerial shelf, (b) valley axes on the coastal plain become fixed in space, and (c) the river is no longer able to flood over valley walls so as to deposit sediments on the highstand surface. During the falling stage and lowstand, multiple episodes of lateral migration and channelbelt construction, degradation, and/or abandonment of floodplains with soil formation occur within incised and extended valleys. This creates a composite, strongly timetransgressive basal valley-fill unconformity (Blum and Price, 1998; see also Törnqvist et al., 2003), as well as multiple smaller-scale stratigraphic units within the valley fill itself. The composite nature of this basal valley-fill unconformity reflects the cross-cutting and laterallyamalgamated nature of falling stage to lowstand channelbelt complexes (Blum and Price, 1998). While sea level remains lower in elevation than highstand wedges, and shorelines remain in mid-shelf or farther basinward positions, older abandoned highstand alluvial plain surfaces are characterized by non-deposition, weathering, and soil development. With transgression and highstand, and related channel and valley shortening, incised valleys fill at paces set by upstream controls on sediment delivery: post-Beaumont coastal plain incised valleys of the smaller, lower sediment yield systems like the Trinity are presently unfilled, with significant remaining accumulation space, whereas coastal plain incised valleys of the larger extrabasinal systems like the Colorado are filled. As valley filling nears completion during highstand, veneers of floodplain facies spread laterally and bury deeply weathered soils that developed on subsiding downdip margins of the older alluvial plain, and along former valley walls. Accordingly, composite basal valley-fill unconformities trace up and out of the valleys to these deeply-weathered paleosols, and paleosols represent the time period during which the incised valley was initially partitioned, and subsequently evolved until filling. In this sense, these deeply weathered paleosols serve as sequence boundaries, as envisioned for ancient successions by Wright and Marriott (1993), McCarthy and Plint (1998), and Gibling and Bird (1994), among others. Available data suggest that total thickness of individual coastal plain incised-valley fills ranges from 15–20 m updip to 35 m or so at the highstand shoreline (Blum and

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Fig. 19. Cartoon model of Beaumont and post-Beaumont alluvial plains as a succession of 100-kyr valley fills (from Blum and Tornqvist, 2000).

Price, 1998), whereas individual valley fills may extend 10–40 km along strike. For the Texas coast, the crossvalley dimensions of incised valleys directly reflect the scale and number of falling stage to lowstand channelbelts, with channelbelts themselves likely scaled to drainage basin size, discharge, and sediment delivery to the coastal plain. Accordingly, valley fills of the extrabasinal Colorado and Brazos Rivers have a greater alongstrike spatial extent than those of the smaller basin-fringe systems. Valley-fill thickness, however, may correlate to discharge volumes and sediment loads, and/or geometric relationships between coastal plain and shelf gradients, and their effects on channel geometry and depths of incision. As a result, mud-dominated systems like the Brazos or Trinity, with narrow and deep channels, may have greater depths of incision and correspondingly greater valley fill thicknesses than sand-dominated systems like the Colorado, Guadalupe, or Nueces Rivers. These scaling relationships need to be explored in greater detail. 5.2.2. Stratigraphic organization and evolution of valley fills during a 100-kyr cycle The stratigraphic organization and evolution of individual valley fills is interpreted to reflect interactions between climatically-modulated changes in discharge regimes and sediment supply, and sea-level controls on channel extension, shortening, and changes in long profiles. The following outlines a proposed model for valley fill evolution.

5.2.2.1. Falling stage to lowstand systems tracts. Early work on the Texas Coastal Plain generally followed the views developed in Fisk's (1944) work on the Lower Mississippi Valley, where sea-level fall led to valley incision, and sea-level rise led to valley filling (e.g. Bernard and LeBlanc, 1965). The original sequence stratigraphic models of Posamentier and Vail (1988) followed the same process-based reasoning, and did not include systems tracts that developed during sea-level fall, but instead assumed fluvial incision, sediment bypass of the coastal plain, and production of an unconformity during that time. In more recent years, a number of workers have stressed the importance of “forced regressive” or “falling stage” systems tracts in the sequence stratigraphic literature (Posamentier et al., 1992; Hunt and Tucker, 1992; see Plint and Nummedal, 2000 for an overview), but have for the most part retained interpretations of fluvial sediment bypass of the coastal plain during the falling stage, with some authors arguing that incision updip is necessary to produce the volume of sediments needed for systems tracts farther downdip. Such arguments appear in the recent summaries of work on the Gulf of Mexico shelf as well (Abdullah et al., 2004; Anderson et al., 2004; Wellner et al., 2004). Fisk's original process model has not withstood scrutiny in the Mississippi Valley where it was first developed (Saucier, 1994; Blum et al., 2000). Moreover, there are few if any well-studied Quaternary examples where incision with complete sediment bypass during

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sea-level fall has actually been demonstrated rather than assumed (Blum and Tornqvist, 2000; see also Törnqvist et al., 2000, 2003; Amorosi and Colalongo, 2005). Moreover, Blum and Tornqvist (2000) showed how the volume of sediment produced by complete excavation of a valley is insignificant relative to the volume of sediments transported to the basin margin via the fluvial conveyor belt. In fact, correlations between onshore and offshore records, such as that presented above for the Texas coastal plain and shelf records, demonstrate that long-held concepts of incision and complete sediment bypass during falling sea-level are not realistic, and that incision updip is not necessary to produce the volume of sediments required to produce systems tracts farther downdip. Van Heijst et al. (2001) drew similar conclusions through experimental flume studies, and comparisons with the OIS 3 record of the Colorado River offshore. As noted above, however, sea-level fall below a clearly demarcated highstand depositional shoreline break does indeed result in partitioning of an incised valley within which subsequent fluvial activity will take place. Moreover, it seems clear that rates of net deposition and sediment storage on the coastal plain are significantly less during falling stage and lowstand because (a) channels and channelbelts are extended to mid-shelf or farther basinward shoreline positions, and (b) channels and channelbelts are confined within the boundaries of a distinct valley that is cut below elevations of the surrounding highstand alluvial plains. However, it is equally clear that complete sediment bypass is a myth, and instead, there is a recognizable and volumetrically significant stratigraphic record from the long and complex glacial period falling stages and lowstands. For the Texas coastal plain, the record of such deposition consists of the Deweyville units within postBeaumont valleys, and similar strata within Beaumont valley fills. These falling stage to lowstand fluvial deposits should be considered within the context of the following premises: 1. Large extrabasinal and basin-fringe fluvial systems of the Texas coastal plain are most likely graded, when measured in terms of their mixed bedrock–alluvial valley long profiles, to average late Quaternary shoreline positions. Over the last 400 kyr, the time period of Beaumont and post-Beaumont deposition, average shoreline positions have been in the mid-shelf or farther basinward, and some 60 m and more lower in elevation than present highstand positions (see Fig. 5). Full interglacial highstand shoreline positions, like those of today, are anomalous, as are the low gradients

of the lowermost reaches of these rivers, which are graded to these anomalous highstand shorelines. 2. Channels incise the low-gradient highstand alluvial plain surface following sea-level fall below the highstand depositional shoreline break, where the major break in slope exists (see Fig. 2B). But the lag time for upstream propagation of a wave of incision should be insignificant compared to the maximum rates and duration of sea-level fall itself. In short, glacio-eustatic sea-level fall can proceed at maximum rates of mm/yr, whereas large rivers like those of the Texas Coastal Plain can incise multiple meters, and deliver vast quantities of sediments, during individual flood events. As sea level falls below this elevation, channels cut down rapidly to their average long profiles, hence the time gap between highstand alluvial plain deposition and falling stage deposition within the incised valley may be less than a few thousands of years, i.e. geologically instantaneous. 3. The premise of incision with complete sediment bypass of the coastal plain was always based on faulty reasoning: sea-level lowering can force complete sediment bypass if, and only if, it results in downstream increases in sediment transport rates due to downstream increases in slope (see Blum and Tornqvist, 2000). To be sure, channel slopes increase locally within the incised valley as the channel cuts through the highstand prism and extends across the newlyemergent shelf. However, to produce downstream increases in sediment transport rates within the valley as a whole, and over any significant period of time, slopes in the newly incised valley would have to be greater than slopes in the mixed bedrock–alluvial valley upstream from the limits of any sea-level influence. A downstream increase in slope as a river enters a depositional basin seems unlikely, at best. Hence, changes in slope forced by sea-level lowering are insufficient to trigger complete sediment bypass of the coastal plain for a significant length of time, unless there is a corresponding decrease in sediment supply from the drainage basin. The above concepts suggests a different model for incised-valley evolution during falling stage and lowstand. Through most of a 100-kyr glacial period, when the shoreline is basinward of highstand positions and lower in elevation than highstand alluvial plain surfaces, channels are extended to shoreline positions on the shelf or at the shelf-edge. The coastal plain part of the fluvial system then becomes an extension of the mixed bedrock–alluvial valley farther upstream, with similar sediment transport and storage capabilities, and undergoes multiple episodes

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of channelbelt formation and sediment storage punctuated by renewed valley incision and/or sediment bypass, during an individual 100-kyr glacio-eustatic cycle. Periods of active channelbelt construction within the incised valley during the falling stage and lowstand represent high sediment concentrations delivered from upstream sources, relative to discharge and transport capacity. Unconformities that separate these units represent periods of net channel incision due to decreases in sediment supply relative to discharge and transport capacity. Well-drained soils develop on abandoned floodplain surfaces and represent the upper boundary to these allostratigraphic units. Falling stage to lowstand sand-bodies may be 10– 15 m in thickness and extend 2–10 km along strike, and, as noted above, it is the falling stage to lowstand sandbodies that determine the lateral extent of incised valleys themselves. Plint and Nummedal (2000) stressed that depositional offlap is the fundamental signature of falling stage systems tracts. This model applies most readily to prograding and downward-stepping shorelines, and when viewed in dip-oriented sections. Along slowly subsiding continental margins like the Texas Coastal Plain,

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genetically-linked strata within paleovalley fills, as documented herein, will consist of multiple downwardstepping channelbelt sandbodies when viewed in strikeoriented cross-sections, and should be connected to, and feed, the offlapping and prograding falling stage to lowstand deltas or shoreface sandbodies or (Fig. 20). Hypothetically, if rates of subsidence were to increase, and approach rates of eustatic sea-level fall, multiple falling stage to lowstand sand-bodies will no longer be downward-stepping, but rather would form a multilateral amalgamated sheet sand at the base of the valley fill. If rates of subsidence approach or exceeds rates of eustatic sea-level fall, such that the fluvial system actually experiences relative sea-level rise, sand bodies that correlate to global sea-level fall should actually form multistory aggradational packages, with increases in rates of storage of fines (Amorosi and Colalongo, 2005). A different set of problems emerges when attempting to correlate falling stage and lowstand fluvial deposits within coastal plain incised valleys to the record on the shelf: these problems mirror those in the literature, as they relate to sequence stratigraphic nomenclature and to placement of the sequence boundary relative to the

Fig. 20. Model for falling stage to lowstand deposition within coastal plain incised valleys, and links with incised valleys on the shelf, and downwardstepping and offlapping shorefaces. Falling stage to lowstand fluvial deposits in the coastal plain incised valley would link downdip genetically, and chronostratigraphically, with all deposits that reside above the composite basal regressive surface of erosion and downlap surface, and the overlying transgressive surface of erosion and onlap surface. The preference here would be to place the sequence boundary underneath falling stage and lowstand fluvial deposits, and below the downlap surface that demarcates progradation during falling sea level and maximum lowstand. Modified from Posamentier et al. (1992) to include falling stage to lowstand fluvial deposits, and concepts presented in this paper.

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falling stage or forced regressive systems tract. Abdullah et al. (2004), for example, use the original Exxon term, “late highstand systems tract”, for alluvial-deltaic deposits identified on the shelf, even though they are interpreted to represent deposition during marine isotope stages 4–3. Following the original Exxon approach, Abdullah et al. (2004) then place their sequence boundary above these deposits, and below strata interpreted to represent the maximum lowstand fluvial incision. The original Exxon terminology was always confusing, in some cases internally inconsistent in definition and use (see Blum and Tornqvist, 2000), and reflected confusion between geometric and genetic connotations of terms such as lowstand, transgressive, and highstand systems tracts. In the case of Abdullah et al. (2004) it seems unnecessary to use the term “late highstand systems tract”, as initially defined from stratal-geometry alone, for time periods over which sea level is known to have fallen by ∼ 100 m. Moreover, this approach subdivides strata into packages that are linked through a conceptual model with confusing terminology rather through changes in process: when the shelf record is correlated to the coastal plain, for example, the Abdullah et al. (2004) approach has the unfortunate consequence of placing one part of a distinct incised-valley fill within one depositional sequence, and another part within a second sequence. For example, Deweyville fluvial deposits correlate with the OIS 4 and 3 “late highstand systems tract”, and the OIS 2 “lowstand systems tract” of Abdullah et al. (2004), but reside within morphologically distinct coastal plain incised valleys that are incised below previous highstand alluvial plain surfaces. Under the Abdullah et al. (2004) approach, then, Deweyville units deposited during OIS 3 falling sea level would be placed within the same depositional sequence as the youngest Beaumont transgressive and highstand alluvial plain strata, whereas the Deweyville lowstand unit would be placed within the post-Beaumont depositional sequence. To be sure, placement of the sequence boundary has figured prominently in previous discussions of the falling stage or forced regressive systems tracts (see Plint and Nummedal, 2000). From data presented herein, and from other well-studied late Quaternary systems (see Törnqvist et al., 2000, 2003; Amorosi and Colalongo, 2005), it seems clear that the major change in process regimes occurred at the end of isotope stage 5, when ice volumes increased and global sea-level fell significantly, and valleys incised and became fixed in placed as channels extended across the newly subaerial shelf. From this reasoning, a process-based sequence boundary should

reside under the falling stage and lowstand systems tract (Fig. 20). 5.2.2.2. Transgressive to highstand systems tracts. It seems clear that along the Texas coast, sea-level rise and highstand results in an overall trend of channel shortening, and an overall flattening of longitudinal profiles due to valley filling and increases in rates of sediment storage in updip reaches. The dominant theme is one of overall valley filling of previously incised valleys through aggradation and progradation, with faciesscale architecture dominated by successions of isolated channelbelt, crevasse–channel, and crevasse–splay sands encased in flood-basin muds. Within this broader picture, three issues deserve special mention. 1. The different stages of valley filling observed along the present coast correlate to rates of sediment delivery from the hinterland (see Fig. 3), which has significant implications for correlation of inland and offshore records within a sequence stratigraphic framework. For example, the incised valleys of the high sediment yield Colorado and Brazos Rivers are filled, the majority of the valley fill corresponds to the period of sealevel rise, and correlates to transgressive systems tracts in the offshore record. By contrast, incised valleys of the basin-fringe lower sediment yield Trinity and Nueces Rivers are unfilled, and the bulk of the valley fills have yet to be deposited. They will ultimately correspond to sea-level highstand and what might be referred to as highstand systems tracts in the offshore record. 2. The latest Pleistocene to Holocene succession contains at least 2 moderately well-developed paleosols (see Fig. 16), which indicates the overall trend of valley filling during transgression and highstand was punctuated by episodes of floodplain stability and soil formation. Blum and Valastro (1994) interpreted periods of deposition, followed by periods of floodplain stability and soil formation within the Colorado incised valley fill to correlate to independently-identified time periods of climate change and fluvial response in hinterland source regions (see Toomey et al., 1993; Blum et al., 1994). 3. Avulsion, and changes in avulsion styles through time, play a critical role in the architecture of the transgressive to highstand valley fill (Aslan and Blum, 1999). During the early stages of valley filling, avulsion occurs by reoccupation of abandoned falling stage and lowstand channels, with erosion and reworking of older channelbelt sands. This produces channel-in-channel stacking patterns, or multilateral

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type may comprise >50% of the total valley fill. During the late stages of valley filling, rates of aggradation are low, avulsion by channel reoccupation again becomes the dominant process, and again results in amalgamated channelbelts (Fig. 21C). As noted in Aslan and Blum (1999), the Trinity incised valley is unfilled, in the early stages of filling as described above, and avulsion has so far taken place by reoccupation of Deweyville falling stage to lowstand channelbelts. By contrast, the Colorado valley is filled, and has progressed through the entire sequence described above. In fact, as noted above, during the most recent avulsion, the Colorado River completely abandoned its filled valley from the last glacial cycle, and reoccupied a Pleistocene Beaumont channelbelt from the previous interglacial highstand. Hence avulsion plays a pivotal role in incised valley fill architecture, and in the broader distributive pattern of multiple 100-kyr valley fills. If sea level were to fall today, and channels were to incise and extend across a newly subaerial shelf, the next incised valley would form in its present location, some 20–30 km distance alongstrike from the previous 100-kyr incised valley complex (see Fig. 19). 5.3. Glacial vs. interglacial contrasts in fluvial process regimes

Fig. 21. Model for changes in avulsion style during different stages of valley filling. (A) Avulsion by reoccupation of falling stage and lowstand channels during early stages of valley filling, when rates of floodplain aggradation are low. (B) Avulsion by frequent crevassing into floodplain depressions when rates of channel and floodplain aggradation are high. (C) Avulsion by reoccupation of older highstand channel belts, when the valley is filled or nearly filled (modified from Aslan and Blum, 1999).

and multistory channelbelts (Fig. 21A). As rates of valley filling increase, channelbelts aggrade rapidly and create raised alluvial ridges with significant cross-valley gradients, and avulsion occurs by repeated diversion into floodplain depressions. This creates ribbon-like channelbelts, ribbon-like crevasse channel sands, and thin (< 5 m) multilateral and multistory crevasse–splay sheet sands that are encased in thick successions of massive to laminated floodbasin muds (Fig. 21B). Avulsion deposits of this

River systems on the Texas Gulf Coast and elsewhere demonstrate a significant contrast in glacial vs. interglacial process regimes and their resultant stratigraphic and sedimentologic signatures (e.g. Blum and Straffin, 2001). Within the Texas coastal plain incised valleys described above, these contrasts coincide with changes in stacking patterns that reflect sea-level change, so it might be tempting to link these contrasts in a cause-effect manner to sea-level rather than other controls. However, these contrasts also can be found within the mixed bedrock– alluvial valleys farther upstream, much farther upstream than the influence of sea-level changes, so their fundamental characteristics most likely reflect climaticallymodulated changes in discharge regimes and sediment load. A number of workers, including Sylvia and Galloway (2006-this volume) suggest linkages between Deweyville units and climate change, a significant goal unto itself, and these models can be used to define multiple testable hypotheses that can guide future research. In fact, Deweyville units, and the contrast between Deweyville units and those of the Holocene, may represent a superb opportunity to quantitatively examine

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regional-scale fluvial responses to climate change. Here, however, fluvial responses to climate change along the Texas Coast are discussed at 2 more general scales. First, it seems clear there are fundamental contrasts in channel geometry, sediment load, and depositional style that correspond with glacial vs. interglacial climates. Glacialperiod systems are represented by the Deweyville units and correlative falling stage to lowstand sandbodies within Beaumont valley fills, and were dominated by coarse-grained large-wavelength meandering channels, with a general paucity of fine sandy and muddy floodplain strata deposited by deep overbank flooding. By contrast, Beaumont and Holocene transgressive and highstand units contain appreciable thicknesses of floodplain facies. Hence, extreme overbank flooding was not significant during the cooler glacial period, most flood events were contained within bankfull channel perimeters, and fine sediments were mostly bypassed through the system. By contrast, extreme overbank floods are important during the warm interglacials, especially evident in the late Holocene, and a significant volume of fine sediment is sequestered in floodplain settings. Second, glacial vs. interglacial periods resulted in different amplitudes and frequencies of fluvial adjustment to climate change. High-amplitude but low-frequency adjustments characterized the long isotope stage 4–2 glacial period, with 3 extended periods of lateral migration and sediment storage punctuated by episodes of valley incision, as recorded by the Deweyville alloformation. Low-amplitude but high-frequency adjustments have been more typical of the relatively short Holocene, with millennial-scale periods of high magnitude floods and deep overbank flooding, punctuated by millennial-scale periods of floodplain stability and soil formation. This high-frequency signal is absent in Deweyville landforms and deposits from the glacial period. This glacial vs. interglacial dichotomy is not unique to the Texas Coast, but instead appears to be common to many large unglaciated midlatitude fluvial systems. 70– 80% or more of any 100-kyr glacial cycle is characterized by substantial ice volume, cooler land surface temperatures, cooler and smaller ocean basins, and midshelf or farther basinward shoreline positions. Hence a glacial-period process regime represents the norm, and an interglacial regime is relatively unique and nonrepresentative. In short, when measured in terms of boundary conditions and process regimes, the present is the key to only the late Holocene, or perhaps isotope stage 5 and older full interglacial periods. Large unglaciated midlatitude fluvial systems may therefore be in long-term equilibrium with a “glacial period”

environment, so fluvial systems respond to major changes in climate, discharge regimes, and sediment loads, but they may be relatively insensitive to higherfrequency changes. Short interglacials like the Holocene are, by comparison, periods of relatively low-amplitude climate changes, but fluvial systems appear to exhibit a greatly increased sensitivity to subtle changes in discharge regimes that produce frequent periods of disequilibrium. 6. Conclusions 6.1. General conclusions This paper summarizes a body of research on rivers systems of the Texas Gulf Coastal Plain. Results of this research highlight a number of fundamental points that differ in scale and kind from previous generations of workers. 1. The Beaumont Formation was traditionally viewed as representing a series of autogenic channelbelt occupations and avulsions during the last interglacial highstand (OIS 5). However physical stratigraphic relations and TL ages from the Colorado alluvial plain now suggest that Beaumont strata consist of a series of distinct valley fills, with each valley fill representing a 100-kyr climatic and glacio-eustatic cycle. Each valley fill is bounded by a composite basal unconformity that traces up and out of the paleovalley to a deeplyweathered paleosol. 2. Post-Beaumont valleys represent the last glacial– interglacial cycle, but are in various stages of filling, which reflects variations in sediment supply from the respective drainage basins. Incised valleys of the lower sediment yield basin-fringe systems, like the Trinity River, are unfilled, and channels still discharge to bayhead deltas in the upper reaches of estuarine systems. By contrast, incised valleys of the higher sediment yield extrabasinal systems like the Colorado and Brazos are filled, and these river systems have constructed highstand alluvial–deltaic plains. 3. Beaumont and post-Beaumont valley fills contain a complex alluvial architecture that can be subdivided into distinct systems tracts. Falling stage to lowstand systems tracts occur within the basal parts of valleyfill successions. These consist of multiple laterally amalgamated sandy channelbelts that reflect deposition within a discrete paleovalley that was incised below highstand alluvial plain surfaces, and extended across a subaerially-exposed shelf. Well-drained and oxidized paleosols developed on channelbelt surfaces

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after they were abandoned by renewed valley incision. The lower boundary to falling stage and lowstand units comprises the composite valley fill unconformity, and is time-transgressive in both crossand down-valley directions, whereas upper boundaries are defined by the well-drained paleosols, or erosion surfaces that truncate channelbelt sandbodies below the depth of weathering and soil development. 4. Well-drained paleosols that define the top of falling stage to lowstand systems tracts are onlapped and buried by heterolithic sandy channelbelt, sandy and silty crevasse channel and splay, and muddy floodbasin strata deposited during transgression and highstand. The facies-scale architecture that develops during the overall period of valley filling reflects changes through time in dominant styles of avulsion, and follows a predictable succession through different stages of valley filling. 5. Beaumont and post-Beaumont valley fills are similar in scale. These 100-kyr valley fills can be 30–40 m in thickness at the highstand shoreline, and extend 20– 40 km along strike. The lateral extent of individual valley fills reflects the scale of falling stage to lowstand channelbelts, and therefore likely correlates to drainage basin size, discharge, and other factors that control channel and channelbelt dimensions. 6. Key suites of deposits identified within coastal plain incised valleys can be correlated with the record on the now-submerged shelf. These correlations suggest a continuity of process and stratigraphic organization between the linked updip vs. downdip components of these large river systems, and emphasize the important role of sediment supply from the continental interior. 6.2. Signatures of climate vs. sea-level change Beyond the specific issues discussed above, a number of suggestions can be made regarding the signatures of upstream climate change vs. downstream sealevel controls within incised valley successions such as those described herein. 6.2.1. Signatures of climate change Signatures of climate change are suggested to be as follows. First, there appears to be a continuity to stratigraphic units when traced from uplifting continental interiors to the coastal plain and shelf, as expressed by tracing of long profiles onshore, and correlations with the shelf record. This continuity indicates that major episodes of net incision or deposition persist through the valley system from the mixed bedrock–alluvial valley to the

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distal reaches, wherever that may be at the time. This continuity, in turn, suggests that, fundamentally, the development of major stratigraphic units records upstream controls on sediment supply, and the unsteady nature of sediment supply due to climate change, and is not directly forced by sea-level change. Second, there is a downstream persistence in facies architecture between uplifting continental interiors and the coastal plain, such that contrasts between glacial and interglacial facies successions are robust, and persist from the mixed bedrock–alluvial valley to the lowermost reaches of the coastal plain incised valley. Such relationships are not known from the shelf, where most data is seismic in nature. This downstream persistence in facies architecture again suggests that, fundamentally, depositional processes and facies are strongly coupled to prevailing climate conditions within continental interior source regions. 6.2.2. Signatures of sea-level change Sea-level change is an important control as well, but is manifested in different ways: the signatures of sea-level change can be stated as follows. First, sea-level fall below the elevation of highstand depositional shoreline breaks results in channel incision through the highstand prism, and extension across the newly emergent shelf. This in turn results in partitioning of the major 100-kyr incised valley complexes: the sequence boundary that forms from this point is a composite surface, in both strike and dip directions, that represents multiple episodes of channelbelt formation, lateral migration, and incision during the falling stage and lowstand. Second, even though deposits and key surfaces can be traced from uplifting continental interiors to the coastal plain, there are significant downstream changes in geometric relations that roughly correspond to the transition between the mixed bedrock–alluvial valley and the coastal plain incised valley. Upstream from this point, there is a long-term trend for channels to incise bedrock in response to uplift of continental interiors. However, long-term trends of net bedrock incision are punctuated by periods of channelbelt formation and lateral migration, which results in flights of terraces. Downstream from this point, rivers enter the updip margins of the subsiding basin, and sediment accumulation is the longterm trend. Downstream from this point, sea-level change, and resultant channel incision and extension during sea-level fall and lowstand, with channel shortening and delta backstepping during transgression, plays a fundamental role in controlling the stacking patterns, or stratigraphic architecture, within incised valley successions on the coastal plain and shelf.

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