Middle Holocene Sea-Level and Evolution of The Gulf of Mexico Coast (USA)

Abstract

New data published in BLUM et al. (2001) suggest that middle Holocene sea level along the Texas Gulf of Mexico coast was at −9 m at ca. 7.8 ka, then rose rapidly to 2 m or more by ca. 6.8 ka. This view contrasts with the traditional, widely accepted interpretations of continual submergence until ca. 2–3 ka. or later. A middle Holocene sea level higher than present should have left a significant imprint on the coastal landscape, yet coastal landforms and deposits of middle Holocene age have not been identified in previous studies. Our recent research has now identified extensive Holocene beach-ridge plains on the mainland central Texas coast, landward of Holocene barriers, that may represent the geomorphic manifestation of this highstand. Long considered to be part of the isotope stage 5 interglacial period shoreline, these Holocene beach-ridge plains attain elevations of 2.5–3 m, extend for 10's of km along the mainland shore, and can be 1–3 km in width, roughly the same scale as the Holocene barriers. To further test the concept of a middle Holocene highstand, we have also investigated previously mapped Holocene shorelines along the Alabama coast. A series of optical luminescence ages suggest that some of the shorelines are middle Holocene in age, whereas others represent the earlier part of the late Holocene, prior to ca. 2–3 ka. In aggregate, these data suggest that relative sea level was at, or very close to, present elevations throughout the middle to late Holocene along the Gulf of Mexico shoreline, both to the west and east of the subsiding Mississippi depocenter, and the model of continual submergence until 3-2 ka. or later needs reevaluation.