Abstract: Structure, Stratigraphy, and Depositional Environment of the Heterostegina Limestone and Overlying Sands in the Lake Pontchartrain Area of Southeast Louisiana

Spencer B. Street, III

The Heterostegina zone of the Oligocene Anahuac Formation in southeastern Louisiana occurs in the subsurface as an extensive shelf reef complex. The Heterostegina limestone is overlain by strata associated with the Oligocene Discorbis and lower Miocene Robulus (43) biostratigraphic zones.

Examination of electric logs and drill cuttings from wells in the Lake Pontchartrain area of southeastern Louisiana reveal the importance of the Heterostegina reef as a paleoenvironmental punctuation marking a significant shift in regional depositional patterns that occurred between the generally transgressive Oligocene seas and the generally regressive Miocene seas. The Discorbis and Robulus (43) zone sediments in the study area are interpreted to represent a transition of environmental conditions following the transgressive reefal carbonate environment in place during Heterostegina reef deposition. Fauna identified in thin section from the Heterostegina reef interval suggest deposition in a warm, shallow marine environment relatively free of significant clastic influx. An eastward migration of late Oligocene-early Miocene stream systems introduced an influx of clastic sediments onto the ancient shelf of the Lake Pontchartrain area, which influenced the termination of favorable conditions for Heterostegina reef growth.

The Robulus (43) zone strata are characterized by a cyclic sequence of lime, shale, and sand. Within this interval, two general lithofacies are identified. Lithofacies I is characterized by thick, shore-parallel sand deposits, and is interpreted to have been deposited in association with a barrier-beach/tidal-inlet

channel environment. Lithofacies II is characterized by shale-prone sand intervals, which are immediately overlain by calcareous shales and limestones deposited in the offshore inner-middle neritic environment.

Five oil and gas fields in the study area have produced hydrocarbons from the interval of interest. The occurrence of hydrocarbons at these locations with respect to mechanisms of entrapment and areal extent of the reservoirs is characterized through detailed subsurface mapping.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994