Sand Shoal Development on Muddy Mississippi River Delta Shelf: ABSTRACT

Abstract

Trinity and Ship Shoals are transgressive sand bodies on the Louisiana inner continental shelf, and they represent the reworked sands of the abandoned Holocene Teche and Maringouin deltas. The development of these shoals is initiated by an episode of delta abandonment followed by subsidence-enhanced sea level rise. Through the process of shoreface retreat, the abandoned delta lobe evolves from an erosional headland with flanking barrier islands to a barrier-island arc and finally into a submerged inner-shelf shoal system. Trinity and Ship Shoals represent the final stage in the Mississippi River delta barrier shoreline cycle and provide a possible modern analogue for some Cretaceous shelf sandstones of the Western Interior. More than 1,000 km (620 mi) of high-resolution s ismic profiles correlated with cores provide the data base for interpretation of the depositional history of sand-body development on the muddy Louisiana shelf. Ship Shoal is the oldest inner-shelf shoal associated with the abandonment and subsequent reworking of the Maringouin delta 6,150 y.B.P. Located 25 km (16 mi) south of the Isles Dernieres, the Ship Shoal transgressive sands lie disconformably over the Maringouin deltaic muds. The Ship Shoal sand body is shore parallel, 32 km (20 mi) long and 2-4 km (1-2 mi) wide. The inner-shelf relief ranges (east to west) from 2-6 m (7-20 ft) with a corresponding decrease (east to west) in the water depth over the shoal crest from -6 to -3 m (-20 to -10 ft). The shoal profile is asymmetric landward. The Ship Shoal sand body is composed of an upward-coarsening sequence of well-sorted fine-grained sand with a median size of 125µ. Trinity Shoal is associated with the Teche delta, abandoned 3,500 y.B.P., and it is located 20 km (12 mi) south of Marsh Island. The base of Trinity shoal lies unconformably over the Teche deltaic sediments. Trinity shoal is a shore-parallel lunate sand body, 36 km (22 mi) long and 5-10 km (3-6 mi) wide. The inner-shelf relief ranges (east to west) 2-3 m (7-10 ft), with a corresponding decrease (east to west) in the water depth over the shoal crest 5-2 m (16-7 ft). The Trinity Shoal sand body is 5-7 m (16-23 ft) thick. End_of_Article - Last_Page 515------------