Deep-water sandstones of the Hackberry unit of the Frio Formation form one of the most prospective exploration targets in southeast Texas and southwest Louisiana. The Hackberry is a wedge of sandstone and shale containing bathyal fauna that separates upper Frio barrier-bar - strand-plain sandstones from lower Frio neritic shales and sands. Major Hackberry sandstones lie above a channeled unconformity that forms the base of the unit. The deep-water "embayment" lies on the flank of the Houston delta system to the west and a delta system in south-central Louisiana, and downdip from a barrier-bar sequence.

Sandstones in a typical sand-rich channel at Port Arthur field grade upward from a basal, confined channel-fill sandstone to a more widespread, broad fan channel deposit, proximal to medial fan deposits and overbank turbidite deposits. The sequence suggests that Hackberry sandstones were laid down by an onlapping submarine canyon-fan complex. Fans with shallow channels formed southeast of 8004 deep canyons that eroded headward into the contemporaneous Frio barrier system.

Time-depth plots of water depth and sediment thickness indicate that most of the Hackberry Embayment in Texas could have been formed by normal subsidence during the late Oligocene, if the embayment was cut off from its supply of muddy sediment. Thick, sandy, lower Hackberry deposits filled deep canyons eroded into the retreating shelf margin, and formed small fan deposits seaward of the deep canyons.

Deep submarine canyons are found in delta-fringe settings in the Wilcox (Lavaca County area), the Frio (South Texas, Hackberry) and the Miocene of the Texas Gulf Coast, as well as in the Niger Delta and other Quaternary deltas. Many but not all, of these are associated with net transgression following major progradation - conditions that may favor sediment bypass and propagation of shelf-edge slumping. However, only the Hackberry has abundant clean, channel filling sandstones. This may be due to ponding of the fan facies in intraslope basins between salt diapirs, which raised base level and backed up sand deposition into the channels. If so, other delta-fringe environments within the salt basins may contain Hackberry-like, probably geopressured channel sandstones. Areas to examine include the Wilcox in Liberty and Hardin Counties, the Yegua in the same area, and the Pleistocene in High Island South and Galveston South areas.

Figure 1. Stratigraphic diagram of Frio and related strata, Jefferson County area, and diagnostic foraminifers, sand-body distribution (shaded), and marker horizons used for this study (A1 through A5).

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