Late Quaternary/Holocene evolution of the Nueces incised valley, central Texas
Simms, Alexander Ray
Anderson, John B.
Doctor of Philosophy
A detailed investigation of the Nueces incised valley of central Texas focused on three problems: (1) the sea-level history of the region over the last 18 ka, (2) how coastal systems responded to sea level and other forcing mechanisms, and (3) the facies architecture of the valley as compared to other Gulf of Mexico valleys and existing incised-valley facies models. A difference in sea-level histories between the Gulf of Mexico and equatorial regions of up to 30 m over the last 18 ka is attributed to the effects of glacio-hydro-isostasy, the gravitational and flexural deformation of the earth in response to the melting of the last great ice sheets. Numerical modeling of glacio-hydro-isostasy suggests that ice sheet reconstructions with a Laurentide source for meltwater pulse 1-A best fit sea-level observations with predictions within the Gulf of Mexico. The coastal systems preserved within the Nueces incised valley record two fundamentally different responses to essentially the same sea-level and climatic changes over the last 10 ka. Mustang Island aggraded over the last 9.5 ky. Corpus Christi Bay experienced no less than four periods of abrupt reorganization marked by the backstepping of estuarine environments around 9.6, 8.0, 4.8, and 2.5 ka. A possible rapid increase in the rate of sea-level rise, climatic changes, and the flooding of relict fluvial terraces contributed to these backstepping events within Corpus Christi Bay. The Nueces incised valley represents one end member in a continuum of incised-valley types. The Nueces valley contains a thin layer of fluvial deposits at its base, estuarine deposits in the middle, and marine deposits at the top. The valley is marked by a terraced morphology and is located in the same location as other valleys cut by the Nueces River during older glacio-eustatic cycles. This contrasts with other incised valleys such as the Brazos River valley which is almost completely filled with fluvial deposits with a thin cap of transgressive deltaic deposits. Instead of forming fluvial terraces, the Brazos cut multiple highstand valleys and transgressive channels and did not incise in the same location through multiple glacio-eustatic cycles.