The Hackberry shale section of the middle part of the Frio Formation of southwestern Louisiana is one of the four deeper-water shale wedges in the post-Vicksburg Tertiary Gulf Coast section. The Hackberry section can be divided into two parts. The upper section ranges in thickness from zero to more than 3,000 ft., and consists predominantly of shale containing an outer-neritic (deep-water) assemblage; several thin, erratically distributed sandstone bodies are present. The lower zone ranges from zero to 700 ft. and consists mainly of sandstone.

To understand better the geological history of the Hackberry shale wedge, a discussion of the stratigraphy and structure of the Frio section has been included.

The complicated Frio stratigraphy of northern Jefferson Davis and Calcasieu Parishes is caused partly by a complex early (Frio) tectonic history and partly by variations in regional deposition of the Hackberry section. The geological history of the area is summarized as a sequence of eight steps. It should be emphasized that these eight stages are a general sequence of events, that they probably overlap one another, and that they may have occurred at slightly different times in different areas.

The steps are: (1) deposition of Vicksburg and Textularia seligi Zone of lower Frio; (2) development of lower unconformity (this may be a local unconformity); (3) deposition of lower Frio and Hartburg sequence; (4) uplift, folding, erosion, and development of pre-Hackberry unconformity; (5) tilting of unconformity surface and renewed erosion which formed channels; (6) deposition of basal Hackberry channel sandstone bodies; filling of channels which resulted in the development of a flat-surface sequence; (7) deposition of Hackberry with "arenaceous" fauna at the base; and (8) deposition of remainder of Frio.

The earlier structural movements formed folds and faults which then were truncated by regional erosion. On this eroded surface, large channels (600 ft. deep) were cut and later filled. The mechanism which cut and filled these channels is uncertain, but may be turbidity flows. The structural movements, history of erosion, and the complex stratigraphy of the Hackberry make exploration for Hackberry sandstone reservoirs a high-risk economic decision, but one which may pay high dividends.

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