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Ground-Penetrating Radar Study of Gulf of Mexico Holocene Beach Ridges as Sea-Level Indicators

Abstract

Holocene sea-level change in the Gulf of Mexico (GoM) is controversial. One view interprets basal peats from the Mississippi Delta to indicate continual sea-level rise for the GoM as a whole. An alternate view proposes that data from the subsiding delta represent a local signal only, and that sandy non-deltaic shorelines indicate that current sea-level was reached by the middle Holocene, ca. 6000 yrs BP, with minor oscillations since then. The use of relict sandy shorelines, commonly called “beach ridges”, has been criticized for its lack of precise sea-level indicators, relative to basal peats, and therefore discounted. This research focuses on further developing Holocene progradational beach ridges along the GoM as possible sea-level indicators. Ground-penetrating radar (GPR) imaging of well-dated beach-ridge successions is used to examine changes through time in the elevation of the topset-foreset break (TFB), the change between flat-lying foreshore and seaward-dipping shoreface facies, which corresponds to the intertidal zone. 2D lines of beach-ridge sets were obtained from Galveston, TX and Gulf Shores, AL, which are located sufficiently far from the Mississippi deltaic depocenter that they avoid the effects of region's subsidence. Beach-ridge successions that date from ca. 5500 yrs BP, display TFBs at current mean sea-level elevation, which supports the view that current sea-level was reached by the middle Holocene. Furthermore, GPR data processing is underway to constrain relative sea-level curves for both field areas for the late Holocene. In addition to contributing to our understanding of Holocene sea-level change, this research will contribute to the recognition and understanding of ancient shoreline trajectories.