RIVER TERRACES ALONG THE NUECES, FRIO, HONDO AND MEDINA RIVERS, SOUTH TEXAS: INCISION, TOPOGRAPHIC INVERSION AND LANDSCAPE EVOLUTION; A CLASS RESEARCH PROJECT

Up to six, unpaired fluvial strath terraces, ranging in elevation from ~320m amsl to ~250m amsl just downstream of the Balcones fault zone, are well developed along the south Texas Coastal Plain. Terraces range in age from Early Middle Miocene to Holocene. These terraces are used in five, field class exercises to constrain fluvial incision rates, stream capture, topographic inversion, landscape evolution, and possible flexure of continental lithosphere. Initially, students make a surficial geologic map of a small portion of the Medina River valley using air photos and detailed field mapping. A detailed DGPS cross-section is constructed across the valley and used to develop ideas regarding strath terrace formation, the concept of a graded river system and topographic inversion. Outcrops reveal the temporal relationship between terrace formation, stream capture, and development of the Middle-Late Miocene Balcones fault zone. The highest terrace, the Uvalde Gravel, is correlated downstream over ~200km from another DGPS survey. Using outcrop distribution, clast size and composition, the Uvalde Gravel is correlated to the Early-Middle-Miocene Oakville Sandstone, a locally conglomeratic, fluvial deltaic system. Students use an equation that describes contributions to the modern elevation of a terrace: E_TL1 (X) = E_TL0 (X) + DSL + I_S (X) + T(X), where E_TL1 is modern terrace elevation, E_TL0 is initial elevation at time of formation, DSL is change in eustatic sea level from time 0 to 1, I_S is an isostatic and flexural term from sediment redistribution, and T is a tectonic term. Several possible river profiles graded to the mid-Miocene sea level for the ancestral Nueces system are constructed assuming no divide migration and a graded profile thought time. Incision rates for the Nueces system are ~10^-5 m/yr. Assuming no tectonic deformation because the Uvalde Gravels are mostly downstream of the Balcones fault zone, students can estimate values for the amount of isostatic rebound along this part of the Texas Coastal Plain.