MIDDLE HOLOCENE SEA-LEVEL AND EVOLUTION OF THE US GULF OF MEXICO COAST

Study of Holocene sea-level change along the US Gulf of Mexico coast has been dominated by two contrasting schools-of-thought. One widely-accepted interpretation would argue for continual submergence, with present sea-level elevations not reached until 1-2 ka. This view finds support in recent work from the Mississippi delta plain, and from now submerged estuarine and near offshore settings. However, it is difficult to reconcile such views with the many subaerial shorezone landforms along the Gulf Coast, which have been studied for close to 50 years, and which almost certainly formed prior to ca. 3-2 ka. A second alternative view would then have sea level reach present elevations by the middle Holocene, with one or more Holocene highstands since that time. This view also finds support in recent work, especially that which documents a variety of coastal landforms in Texas occurring at elevations 1-3 m higher than their modern analogs.

Ongoing research is focused on testing the middle Holocene high sea level model through analysis of coastal landforms and deposits that may have formed during that key time period. First among these would be beach-ridge plains on the mainland central Texas coast, landward of Holocene barriers. Long considered to be part of the last interglacial shoreline, these Holocene beach-ridge plains attain elevations of 2.5-3 m, extend for 10’s of km’s along the mainland shore, and can be 1-3 km’s in width. These mainland beach-ridge plains remain undated, but must have formed prior to development of the barrier chain, which severely limits fetch, and are therefore interpreted to represent maximum transgression during the middle Holocene, followed by sea-level fall to present elevations. We have also investigated Holocene shorelines along the Morgan Peninsula of the Alabama coast, to the east of Mississippi delta. Optical luminescence ages show that some beach-ridge sets are middle Holocene in age, ca. 6.7-4 ka, whereas others are late Holocene in age, ca. 3.5-2.5 ka. In aggregate, these data suggest that relative sea level was at, or very close to, present elevations since the middle Holocene along the Gulf of Mexico shoreline, both to the west and east of the subsiding Mississippi delta.