CARTER, AMY E. and MICHAEL D. BLUM , Department of Geosciences, University of Nebraska-Lincoln, Lincoln, NE 68588
New data from the central Texas Coast indicate that the middle Holocene sea level was higher than today. The most important site consist of shore-parallel ridges in the mouths of tributaries to Copano Bay (and other bays), which attain elevations of +2 to +3m, and are underlain by ~2m of bioturbated sand and silt with shell hash. Abundant calcareous forams within these deposits indicate subtidal to intertidal depositional environments, whereas 14C ages from foram test indicate middle Holocene ages. These ridges are interpreted as shoals and spits that formed in the mouths of the then-flooded bay tributaries, and they are now emergent due to more recent sea-level fall. Accordingly, they indicate that middle Holocene sea level was at +2m or more.
A number of mainland landforms are now thought to record this highstand. Most important among these is an extensive beach ridge plain. Although long considered to be part of the "Ingleside" shoreline from the last interglacial period, this Holocene beach ridge is underlain by >4m of unweathered, massive to stratified sand that rests unconformably on weathered km's along the mainland shore (in between Matagorda, San Antonio, Copano, and Corpus Christi Bays), and can be 1-3 km's in width. Beach ridge plains are interpreted to represent progradation after maximum highstand, and during sea-level fall to present elevations or lower.
The middle Holocene highstand correlates to similar events at many low latitude and/or southern hemisphere sites, but may be the first highstand identified along a non-rebounding, non-uplifting coast in North America. Interpretation of what is sure to be a controversial highstand is bolstered by recognition of its imprint on landscape evolution, especially the extensive mainland-attached Holocene beach ridge plains.
AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas