The Hackberry depositional system has been long recognized by Gulf Coast geologists by its anomalous deepwater fauna, rapid lateral variation in sandstone thicknesses, and prominent basal erosional unconformity. Hackberry sandstones also serve as major hydrocarbon reservoirs in many fields in southeast Texas and southwest Louisiana. A vertical succession through the Hackberry typically shows the basal erosional surface overlain by a variably sandy interval, informally termed the lower Hackberry sands, and capped by a thick deep-water interval, the Hackberry shale.

High-quality seismic data indicate that the Hackberry has a distinctive seismic signature. This seismic signature permits the Interpretation of Hackberry lithologic characteristics in a sequence stratigraphic framework. Key elements of the seismic signature include: (1) a series of half-graben-like slumps, marking the updip limit of the Hackberry depositional system, and representing the failure of an immediately pre-existing shelf edge, (2) a basal erosional surface, in some places channelized as deep as 1800 ft. and cutting as deeply as the Eocene, representing a prominent sequence boundary upon which the Hackberry was deposited, and (3) a pronounced downlap surface with well-developed suprajacent clinoform geometries, best developed in updip positions and representing a maximum flooding surface within the Hackberry shale.

Most Hackberry sandstone is confined to the lower Hackberry :,and interval immediately above the sequence boundary. The Hackberry sandstone isopach shows numerous linear to ovoid-shaped areas of thickly developed sandstone separated by areas of little or no sandstone. In some places, linear sandstone depocenters can be related to eroded and channelized slope paleotopography that is discernable seismically. Elsewhere, linear areas of thickly developed sandstone are not associated with a clear erosional expression on seismic records and may represent broadly linear, aggradational turbidite fills in paleolow positions. Other, more irregular to ovoid-shaped sand patterns represent ponded aggradational deposits in intraslope paleolow areas and basins. Paleontologic data strongly suggest that this system was deposited at the time of the large mid-Oligocene sea-level lowstand shown on the Haq et al. (1987) coastal onlap curve. Deposition of lower Hackberry sands commenced when fluvio-deltaic systems bypassed the foundered shelf edge and sediment-gravity flows ensued through a tortuous network of upper slope channels, gullies, and other paleolows. Collectively, these deposits represent the upper and middle portions of a sand-rich slope-fan complex.

REFERENCE

Haq, B. U., Hardenbol, J., and Vail, P. R., 1987, Chronology of fluctuating sea levels since the Triassic. Science, v. 235. p 1156-1167.

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