A Subsurface Structural Model for Natural Subsidence Patterns in South Louisiana

Bryan Stephens
[email protected]

Fundamental gaps remain in our understanding of the causes, rates and patterns of geologic subsidence and their implications for the sustainability of south Louisiana. The coastal landscape is the product of spatial and temporal variations in vertical movements, results of diverse processes which have interacted in an ordered structural framework throughout the geologic history of the Gulf Coast. Depth-migrated offshore 3-D seismic surveys are revealing unprecedented views to depths below 40,000' and providing analogs for understanding this subsurface framework. The basement fabric is the first-order control on coastal geomorphology. Ongoing movements of the basement have exerted a profound level of control on all superposed geological processes. Surface manifestations of ancient crustal boundaries are evident in the Holocene coastal landscape. Salt tectonic processes are the second-order control. Salt withdrawal 'minibasins' circumscribed by down-to-the-basin and counter-regional faults are arranged in similar fashion to the underlying rift architecture. These basins are areas of greater overall subsidence, providing accommodation space for the deposition of delta lobes as well as the means to their destruction. Sedimentation is considered to be a third-order driver of the coastal fabric. This model challenges the traditional river-dominated view of the Mississippi Delta in favor of a view in which the river is a slave to tectonically-controlled depositional topography. Generalizations of tectonic stability and dismissal of ongoing salt tectonism seem implausible to experienced practitioners of subsurface exploration. The subsurface geology must be taken into account if we are to understand and affect the future landscape of south Louisiana.

AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63^rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013