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Local subsidence after Oligocene Nonion struma deposition created an elongate channel 15 mi long extending north-south from Indian Village field to South Manchester field; the channel ranged in width from 3 to 6 mi. This subaqueous channel was developed within the marine slope environment and subsequently was filled with Hackberry sediments supplied from an updip delta.
The area can be subdivided into updip and downdip areas based on structure of the pre-Hackberry section and the lithology of the Hackberry section. The updip section is characterized by slump block faulting in the pre-Hackberry section and a dominant shale lithology in the Hackberry interval. The downdip section consists of strongly folded and faulted pre-Hackberry sediments; the lithology of the Hackberry section is dominantly shale with massive sandstones in the lower part.
The Hackberry section, which was deposited unconformably on the slope surface, contains the highest percentage of sandstones in the channel axis. Association of the sandstones with deep water pelagic shales, the geometry of the sandstone bodies, and the sequence of primary sedimentary structures strongly suggest that the sands were deposited by turbidity currents in deep water.
As Hackberry mud and sand deposition continued the channel was filled, the depositional slope was reduced, and less sands were carried downslope into the basin. In "Cibicides hazardi time" gradation was achieved and a prograding shallow-marine sequence was deposited.
Hackberry sandstones are productive in the study area, producing a condensate-rich gas from stratigraphic and structural traps.
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