Unreciprocated Sedimentation: How Differential Accumulation Decoupled the Shelf and Slope of Offshore Louisiana, Gulf of Mexico, USA

Abstract

According to generally accepted eustatic and bathymetric models, sedimentary cyclicity in the Gulf of Mexico is modulated by sea-level change, while smaller-scale intraslope accumulation is controlled by the filling of pre-existing bathymetric depressions. We have tested both hypotheses using 2D and 3D seismic reflection data, as well as biostratigraphy, from the early Pliocene to Holocene of offshore Louisiana, and reach different conclusions. We determine that differential accumulation and associated salt tectonism dislocated the shelf and slope, preventing sedimentary reciprocity at 10-100 kyr time scales. Current eustatic and bathymetric models therefore need to be re-evaluated, particularly for actively-deforming continental margins composed of fine-grained accumulations. From the early Pliocene to middle Pleistocene, a reduction in sediment flux resulted in basinward thinning shelf sediments and in relatively slow sedimentation on the continental slope. During the onset of North American glaciation in the late Pliocene (∼2.9 Ma), accumulation rates decreased, and the depocenter shifted from the shelf to the slope, owing to the healing of inboard and large-scale listric normal faults. Mass transport complexes (MTCs) are uncommon during this time. From the middle Pleistocene to present, an increase in available sediment led to basinward thickening shelf deposits, and to a marked increase in the rate of slope accumulation. The most notable interval is the glacially controlled middle Pleistocene to Holocene (∼0.45-0 Ma), when a four-fold increase in the rate of sediment accumulation led to salt-related oversteepening, and to associated MTC development on the slope. Salt tectonics at this time were not an important factor in the shallow water part of the system, where local structures were largely healed and little additional sediment accumulated. Our results suggest that sedimentation for the last ∼3.7 Myr was controlled at a regional scale by varying rates of differential siliciclastic accumulation and associated salt-related subsidence. Subsidence, in turn, controlled the timing and location of MTCs, which have been found largely to track maximum rates of deposition. Our data suggest that MTCs can be used to identify depocenter migration through time, with applications in oil and gas exploration at continental margins with mobile substrates.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016