QUATERNARY GEOLOGY OF BIG BEND NATIONAL PARK, TEXAS

Surficial deposits in Big Bend National Park record the long and complex Quaternary geologic and climatic history of this border region. Since the Rio Grande became established, probably in latest Pliocene to earliest Pleistocene time, the fluvial system has been dominated by long-term incision punctuated by episodes of aggradation likely related to climate change. Large volumes of rock and sediment have been eroded from the landscape. For example, probable Pliocene-Pleistocene graben fill (i.e., upper part of Estufa bolson fill, the upper few meters of which has been interpreted as Quaternary) has been eroded into high relief, inverting its topographic position. Old gravel deposits that post-date erosion of the Pliocene-Pleistocene sediments from the upthrown block have strong calcic soils (cemented stage IV horizons) that likely took several 100 kyr to form, indicating that much of the erosion that produced the inverted topography of the Estufa bolson probably is pre-middle Pleistocene. Multiple levels of gravel-capped pediments are prominent throughout Big Bend National Park. Based on relative-age indicators (soil development and surface morphology), the oldest of these surfaces probably are middle (or possibly early) Pleistocene, the intermediate ones middle and late Pleistocene, and the youngest ones latest Pleistocene and Holocene. Along the Rio Grande, calcic soils suggest that axial terrace remnants range in age from middle Pleistocene for the highest preserved surfaces, about 55 m above the floodplain, to Holocene for those within several meters of the floodplain.

Eolian landforms in this arid region are relatively minor in extent. Alluvial valley floors have been deflated locally and in places are covered by small coppice dunes, but large areas of eolian sand deposition are lacking. In the northernmost part of the park, areas of patterned ground that resemble sand dunes on aerial photographs are not due to sand accumulation, but rather reflect variations in vegetation distribution and disruption of desert pavement and surface crusts by surface runoff.