Sedimentation Styles and Accumulation Rates on Modern Louisiana Shelf and Slope

Harry H. Roberts, James M. Coleman

Data generated from drop cores, soil borings, and high-resolution geophysical profiles associated with foundation studies have defined various styles and rates of sedimentation associated with the last sea level cycle. Approximately 500 borings have been analyzed along with numerous drop cores and thousands of kilometers of high-resolution seismic profiles. A synthesis of the last sea level cycle indicates that during falling to low sea level, channel migration and aggradation caused abundant deposition of coarse clastics on the exposed shelf. During rising sea level, sedimentation was first focused on filling the deeply cut alluvial valley (average accumulation rates above the oxygen isotope stage 1-2 boundary, 25-40 cm/100 years), leaving the shelf and slope covered wit a thin, time-transgressive unit of carbonates and carbonate-rich hemipelagics with a distinctive clay mineral suite as compared to the Modern Mississippi River clays. Carbon-14 dating suggests these carbonate-rich deposits accumulated at a rate of 0.5 to 2.0 cm/100 years. Once the valley-filling process neared completion, deltas again started building on the shelf. Delta switching caused the locus of deposition to shift in time. Individual delta lobe average sediment accumulation rates reach magnitudes as high as 11 m/100 years in the coarse distributary mouth facies to 30 m/100 years in fine-grained mudflow deposits. Distal shelf and slope sediments opposite active delta lobes are characterized by thin alternating units of hemipelagics and fine-grained turbidity flow deposits, average ccumulation rates from 4 to 6 cm/100 years. In contrast, distal shelf/slope areas that have received primarily hemipelagic deposition over the last sea level cycle (especially during the rising phase) accumulate sediments at average rates of 1 to 2 cm/100 years. Older sea level cycles suggest similar sedimentation styles and accumulation rates.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.