Shelf Morphodynamics of Drowned Barriers on Louisiana Shelf: ABSTRACT

Abstract

Since the 1950s, Mississippi-delta researchers have hypothesized that Ship and Trinity Shoals, on the Louisiana continental shelf, were formed during the transgressive phase of the delta cycle following the abandonment of the Maringouin delta lobe during the Holocene. Historic, stratigraphic, and sedimentologic data strongly support this theory and suggest that the shoals originated through the drowning of delta-flank barrier islands. Maps published prior to 1840 show islands in the approximate location of the present shoals. Following submergence, the barrier/shoal system was subject to extensive modification by shelf currents. A comparison of U.S. Coast and Geodetic hydrographic surveys (1853, 1889-90, 1936) and a 1983 survey conducted by the authors illustrate that the shoals have migrated landward up to 2 km (1.2 mi) and been reduced in subaqueous elevation up to 3 m (10 ft). The overall trend is of erosion at the shoal crest and deposition seaward below 10-m (33-ft) water depth. Large scale ridge and swale features are also present in water depths below 6 m (20 ft). The shoals are composed of clean, well-sorted, fine-grained sands essentially devoid of bedforms detectable by side-scan sonar. Three days after Hurricane Alicia's passage within 160 km (100 mi) south of Ship Shoal, there were still no noticeable bedforms. This suggests that hurricanes may be of too short a duration to initiate shelf-wide flow of a magnitude great enough to significantly affect shelf-shoal sediment dispersal, at least in the shoals' present state. Ship and Trinity Shoals are still being modified today, and as long as they remain highs on the sea floor they will continue to be modified by shelf currents. It is believed that the original barrier morphology played an important part in the development of the present morphology. It is inferred that the presence of the original barrier chain, its composition, and its orientation relative to dominant wave and current conditions controlled the development of the present morphologic features. The only hope of preserving the present shoals would be if they were covered by prodelta deposits of another progradational cycle. Otherwise Ship and Trinity Shoals will continue to be modified until they reach a stable configuration for the Louisiana continental shelf. End_of_Article - Last_Page 468------------