Hardbottom Characterization and Relationship to the Geologic Framework in Long Bay, South Carolina

Abstract

Hardbottom seafloor is a common element among sediment-starved portions of the inner continental shelf along the U.S. Atlantic margin. These areas are characterized by indurated sediment surfaces that are heavily altered by biological and physical processes. Long Bay, in northeastern South Carolina, offers ideal environmental conditions for hardbottom exposure with only patchy Holocene sand deposits, interspersed with extensive hardbottom areas. Here we use high-resolution multibeam bathymetry, CHIRP subbottom profiling and electrical resistivity data, along with surficial sediment samples, hardbottom thin sections, and water column radioisotope (radon-222) analysis to investigate the origin and geologic framework of a region of hardbottom seafloor in central Long Bay. Based on petrographic analyses, Long Bay seafloor hardbottom is characterized as phosphatic glauconite sandstone, while loose beach hardbottom samples are characterized as quartz sandstone or fossiliferous limestone. The presence of glauconite and older foraminiferal species comprising the seafloor hardbottom samples suggest that the hardbottom within the study area likely formed during the Cretaceous and Tertiary. Correlation of bathymetry and CHIRP data suggests that the hardbottom is outcropping, truncated and tilted sedimentary rock strata that outcrops at the seafloor as a result of the location of the Mid-Carolina Platform High. As such, it appears that the underlying geologic framework does provide spatial control on the distribution of hardbottom, where hardbottom is often associated with ancient outcropping sedimentary strata. Mineralogical differences between seafloor hardbottom and loose beach hardbottom samples suggest that there may be other types of hardbottom within Long Bay that were not sampled. Lastly, electrical resistivity and radon-222 data show that there are indicators of groundwater discharge associated with regions of hardbottom, though no potential pathway could be identified at this time

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oai:digitalcommons.coastal.edu:etd-1051Last time updated on 5/7/2019View original full text link

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