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.