HOLOCENE STRATIGRAPHY AND GEOMORPHOLOGY OF THE LOWER NUECES RIVER VALLEY

Three informally-defined allostratigraphic units (PD1, PD2, PD3-older to younger) were deposited in the Nueces River valley near Corpus Christi, Texas sometime after older Deweyville allostratigraphic units (Durbin, 1997; 1999). This research reports radiocarbon age-dates for the Post-Deweyville alluvial units, and relates geomorphologic, sedimentologic and stratigraphic features to probable sea-level and climatic controls on fluvial deposition. Recognition and comparison of paleosols and soil profiles developed in the vertically-accreted fine grained sediments allowed correlation of units in the valley, from which 17 samples were collected and submitted for radiocarbon age-dating. Six samples were collected from the PD1-unit, seven from the PD2-unit, and four from the PD3-unit. Samples consisting of bulk sediments, snail shells and wood from A, B, and C-horizons in paleosols were processed using both AMS and standard radiocarbon methods. The PD1 unit produced an age range of 9010 ±70 YrBP (snails) to 1870 ±60 YrBP (bulk sediment). The PD2 unit produced a max-min age range of 780 ±70 to 2140 ±70 YrBP (bulk sediment). The PD3 unit produced historic age-dates, 60 to 160 ± 50 YrBP (wood). The age-date ranges in PD1 and PD2 likely represent carbon contamination from modern deep root systems for all samples excluding snails. Thus, all ages must be considered as minimum dates. Correlation of units to previously published sea-level curves (Frazier, 1974) and climate conditions (Blum, 1994) suggest PD1, 2, and 3 were deposited during the rapid glacio-eustatic rise of Oxygen Isotope Stage 1. During the Holocene, the climate along the Texas Gulf Coast came under the influence of the larger and warmer Gulf of Mexico, and more modern atmospheric circulation patterns. The combination of rising sea level and more severe but less frequent rainfall events common under the Holocene climates favored overbank style flooding that allowed deposition via vertical accretion. Average flow conditions declined, and the higher sea level decreased the streams gradient allowing development of the smaller, highly convoluted meanders observed in the younger floodplain deposits.