Modern depositional environments and recent alluvial history of the Lower Colorado River, Gulf Coastal Plain of Texas

The Colorado River is the trunk stream of a large polyzonal fluvial system that drains the Edwards Plateau and Gulf Coastal Plain of Texas. This dissertation summarizes previous work on the late Pleistocene and Holocene alluvial history of major valley axes in the upper Colorado drainage, on the Edwards Plateau, then presents a new spatially- and temporally-controlled allostratigraphic framework for late Pleistocene and Holocene alluvial deposits of the lower Colorado River on the Gulf Coastal Plain. The late Pleistocene and Holocene alluvial history of major streams in the upper Colorado drainage on the Edwards Plateau consists of a series of informal allostratigraphic units and bounding disconformities that were defined on the basis of areally persistent soil-geomorphic and stratigraphic relations. Chronological control for this allostratigraphic framework is afforded by numerous radiocarbon ages. Large valleys in the upper Colorado drainage contain late Pleistocene terraces and underlying fills that record an extensive period of deposition centered on the full-glacial time period, ca. 20-14,000 yrs BP. After abandonment of late Pleistocene floodplains, major streams incised bedrock valleys to present depths by ca. 11,000 yrs BP. Since that time there have been two episodes of net deposition, from ca. 11,000-5000 yrs BP, and ca. 4600-1000 yrs BP, whereas the modem incised, and in many cases underfit, channels and associated depositional environments represent the last millennium of fluvial activity. A very similar sequence of allostratigraphic units and bounding disconformities has been identified in the lower Colorado valley, on the Gulf Coastal Plain. The oldest unit of interest has been defined as the Eagle Lake Alloformation, after localities near the town of Eagle Lake that display characteristic soil-geomorphic and stratigraphic relationships with both older and younger deposits. Radiocarbon ages indicate the Eagle Lake unit was deposited during the late Pleistocene from ca. 20-14,000 yrs BP, roughly contemporaneous with full-glacial conditions. The main valley fill of the lower Colorado River has been defined as the Columbus Bend Alloformation, named after localities near the town of Columbus, and subdivided into three members on the basis of soil-geomorphic and stratigraphic relations. Radiocarbon ages indicate that Columbus Bend Member 1 was deposited during the latest Pleistocene through early to middle Holocene, from ca. 13-5000 yrs BP, whereas Member 2 represents the period ca. 5-1000 yrs BP. Columbus Bend Member 3 constitutes the modem depositional system of the lower Colorado River and represents the last millennium. Late Pleistocene and Holocene allostratigraphic units and bounding disconformities in the upper Colorado drainage and through the lower Colorado valley correlate with independently-identified changes in climate, vegetation cover, and the characteristics of upland soil mantles, and are interpreted to reflect a series of morphological and sedimentary responses to changes in the relationship between the discharge regime and the concentration of sediments along valley axes. In the far downstream portion of the lower Colorado valley, allostratigraphic units and bounding disconformities persist but their geometry, or stratigraphic architecture, changes substantially in response to the last glacio-eustatic cycle. In this part of the drainage, genesis of the alluvial sequence is interpreted to reflect adjustments to changes in climate, but sequence architecture reflects the details of the last glacio-eustatic cycle