ABSTRACT

This research describes a chrono-stratigraphic framework for alluvium within the lower Nueces River valley. The alluvial stratigraphy is then correlated to a eustatic curve, and the relationship between glacio-eustasy and valley-fill architecture is discussed.

Six informal allostratigraphic units were identified. Three older units incorporate the "Deweyville" terraces and the related alluvium (High, Middle, and Low Deweyville units). In addition there are at least two younger units. Deweyville units exhibit large amplitude and long wavelength meander scars (as compared to modern stream channel), and the underlying alluvium consists primarily of laterally accreted, coarse-grained sediments. The younger alluvium consists of vertically accreted, fine-grained sediments.

Thermoluminescence age-dates were obtained for the following strata: 1) Beaumont Fm- 91.7 ± 7.9 and 71.9 ± 6.1 ky (isotope Stages 5a and 5b); 2) High Deweyville unit- 52.6 ± 5.3 ky; 3) Middle Deweyville unit- 40.3 ± 3.2 and 41.3 ± 4.1 ky; and 4) Low Deweyville unit- 35.6 ± 2.1 and 31.4 ± 2.2 ky. Deposition of the Deweyville units occurred during isotope Stage 3. The younger alluvial units are pending age-dating, but are presumed Holocene in age based on examination of soil profiles.

Terrace gradient profiles trace to positions below modern sea level, at roughly - 30 m. Eustatic curves indicate that deposition of Deweyville units occurred when sea-level was 35 and 60 m lower than present, suggesting the lowered base level influenced the valley-fill architecture by controlling the paleofloodplain gradients. All three "Deweyville" units formed during a longer-term, complex glacio-eustatic fall, contrary to models that envision valley incision and sediment bypass during a eustatic fall and valley filling during eustatic rises and stillstands.

The differences in sediment grain-size, stratigraphic architecture, and the morphology exposed on the surface of the allostratigraphic units, when coupled with the timing of deposition suggest other factors may have influenced the stratigraphy as well.