DECIPHERING FLUVIAL RESPONSE TO CLIMATIC AND BASE LEVEL CONTROLS: A CASE STUDY OF TWO INCISED VALLEYS ALONG THE EAST TEXAS COAST

Climate and eustacy are the main forcing mechanisms driving stratal architecture along the passive continental margin of the East Texas coast over individual glacial-eustatic cycles. If published sea level curves for the last glacial-eustatic cycle for the Gulf of Mexico are accepted as valid, climate, and its effect on sediment supply, is left as the principle unknown. Yet other factors can mask the effects of climate. This study emphasizes the importance of accounting for both extrabasinal (allocyclic) and intrabasinal (autocyclic) controls on the incised valley fill of the last transgression and early high stand for two adjacent East Texas fluvial systems, the Brazos and the Trinity.

Numerous datasets, including 25 cores, 500 drillers’ water well logs, 17 cone penetrometer tests, 10 US Army Corp of Engineer core descriptions, 50 radiocarbon dates, seismic and ground penetrating radar data, and digital elevation models were integrated using GIS software (ESRI’s Arcmap and Rockworks Inc.) to analyze the lithologic and temporal variations of valley fill within and between these two fluvial systems. Lithologic variations of valley fill that might be attributed to climate change are shown to be dominantly controlled by changes in valley morphology as it is being filled for both the Brazos and Trinity. Temporal variance in the longitudinal profile of the lower Brazos valley floodplain shows a strong correlation to changes in the rate of sea level rise.

These results highlight the difficulty of interpreting the individual contributions of base level and climate on the preserved record of incised valley fill if autocyclic factors are not accounted for. Drainage basin size, antecedent topography of the lower drainage basin, and the surface geology of the upper drainage basin explain much of the variability observed in valley fill for the Brazos and Trinty. It is currently not possible to extract an overprint of climate change throughout the transgressive and early highstand aggradational fill of these two valleys in light of the effects that antecedent topography and changing rates of sea level rise have contributed.