Reconstruction of Holocene Relative Sea-Level Positions for the Morgan Peninsula and Gulf Shores Area of Alabama Using Ground-Penetrating Radar

Abstract

Holocene sea-level rise along the northern Gulf of Mexico coast has been debated. One hypothesis interprets basal peats from the Mississippi Delta to indicate continual sea-level rise for the Gulf of Mexico. An alternate hypothesis proposes that data from the subsiding delta is primarily a subsidence signal, and that sandy non-deltaic shorelines indicate that regional sea level reached present elevations by the mid-to-late Holocene, with minor oscillations since then. This research focuses on utilizing Holocene progradational sandy shorelines of the Morgan Peninsula in the eastern Gulf of Mexico as sea-level indicators. Sandy shorelines in this area are ideal to examine sea level change because they are well preserved, sufficiently distant from the subsiding delta, well mapped, and numerically dated by previous studies. To document Holocene sea level change, two-dimensional ground-penetrating radar imaging of well-dated beach-ridge successions is used to identify changes in the elevation of the shoreface clinoform topset-foreset-break through time. The topset- foreset break is observed in GPR imaging of the modern Morgan Peninsula shoreline and represents the transition between flat-lying foreshore and seaward-dipping shoreface facies. Because the topset-foreset-break occurs in the modern intertidal zone, relict topset-foreset-breaks observed in beach ridge successions are reliable sea-level indicators. Beach-ridge successions with optical luminescence (OSL) ages of 5.5 ka display topset-foreset breaks at -0.545 m below mean sea level. Topset-foreset-breaks occur at 0.075 m above mean sea level in beach-ridge successions from 3.5 ka and reach a maximum elevation of 0.205 m above mean sea level at 2.4 ka. These data support the view that current sea-level in the Morgan Peninsula was reached by the late Holocene before falling to current sea level positions. The sea-level curve developed in this study challenges the interpretations of continual Holocene sea level rise in the Gulf of Mexico (GoM) from basal peat data in the Mississippi delta. The beach ridge data in this study does not exhibit the deep-seated subsidence that occurs in the basal peat data and are therefore a more reliable sea level indicator. However, even after correcting for the deep-seated subsidence component in the Mississippi delta, the difference in sea level elevation between two curves cannot be resolved. Therefore, it is appears that both curves are local in nature, and neither is likely representative of Holocene sea-level change for the broader northern GoM. In addition to contributing to our understanding of Holocene sea-level change for the eastern GoM, results of this research provide context for sea-level conditions during which the Mississippi delta was constructed and can provide insight into future shoreline response to rising sea levels. Improved understanding of Holocene sea-level rise in the northern GoM requires further sea level curve constructions on progradation sandy shorelines along the Mississippi Delta.