Late Quaternary/Holocene Evolution of the Nueces Incised Valley, Central Texas
Alexander Ray Simms, Rice University, Houston, Texas
Many sea-level records from the last transgression starting around 20 ka are preserved within the incised valleys of the northwestern Gulf of Mexico. The sea-level records preserved within these valleys are up to 30 m different than time-equivalent records in equatorial locations far removed from the last great ice sheets. This difference in sea-level histories is attributed to the effects of glacio-hydro-isostasy. Glacio-hydro-isostasy refers to the gravitational and flexural deformation of the earth in response to the melting of the last great ice sheets and a redistribution of mass from the poles to the oceans. The ice sheet reconstructions that best reconcile modeled sea levels within the Gulf of Mexico with observations suggest a Laurentide source for meltwater pulse 1-A.
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 experienced an aggradation of the same environments in nearly the same location over this time period. Its aggradational character is attributed to a delicate balance between sea-level rise and sediment supply, provided by the reworked deposits of the transgressive Colorado delta to the north and relict Pleistocene Headlands, and the pining of the island to a wall of a tributary of the Nueces Incised Valley. Corpus Christi Bay experienced no less than four periods of abrupt reorganization marked by the backstepping of estuarine environments around 9.5, 8.0, 4.8, and 2.5 ka. These backstepping events are marked by up to 15 km of landward displacement of environments within a couple hundred years. A possible rapid increase in the rate of sea-level rise 8.0 ka, climatic changes around 5 and 2.5 ka, and the flooding of relict fluvial terraces contributed to these backstepping events.
The fill within the Nueces Incised Valley represents one endmember in a continuum of incised-valley types. The Nueces Incised Valley is largely filled with a thin layer of fluvial deposits at its base, estuarine deposits in the middle, and marine deposits capping the sequence. 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. The Nueces Incised Valley represents an under-filled incised valley. This contrasts with other incised valleys such as the Colorado and Brazos valleys which are almost completely filled with fluvial deposits with a thin cap of transgressive deltaic deposits at their tops. These valleys generally lack fluvial terraces. Instead, their rivers cut multiple highstand valleys and transgressive channels during the time of terrace deposition within smaller rivers. The Brazos and Colorado rivers did not incise in the same location through multiple glacio-eustatic cycles. The Brazos and Colorado Rivers represent the other endmember type of incised valleys, over-filled incised valleys.