ABSTRACT

The Colorado River of Texas drains a relatively low-relief basin of diverse geology and climate. Detailed geomorphic mapping of Colorado River flood plain and paleochannel morphology on the Texas Coastal Plain shows channel metamorphosis through the late Quaternary. The coarser sediment of the low-sinuosity river phases is relatively poorly sorted, lacking in silt and clay, and showing a ratio of C to M ranging from 12-80. Sedimentary properties are consistent with the fluvial regimen of a relatively arid climate. The high-sinuosity phases transported mostly silt, clay, and fine sand during relatively humid phases when interfluves were stabilized by thick soils and ample plant cover. The altering hydrologic regimen of the Colorado River during the Quaternary has resulted in long perio s of storage for alluvium in flanking terraces. With increasing time for weathering the alluvium is first enriched in plutonic rock fragments plus silica, and subsequently in siliceous clasts alone. During arid climatic phases these deeply weathered suites were introduced into the active channel alluvium. This "contamination" may result in anomalous roundness-versus-size relationships. The low-sinuosity, arid alluvial phases all have extremely well-rounded quartz associated in any given size class with poorly rounded limestone and chert and even with poorly rounded caliche fragments. The quartz rounding derived from multicyclic storage, weathering, and transport, while the softer components were more recently introduced from outcrops. Arid-phase alluvium also tends to have coarser partic es of a given lithology not necessarily better rounded than finer ones. Humid alluvial phases of the Colorado River show more normal relationships between roundness, lithology, and grain size.