The subsurface Hackberry wedge of the middle part of the Oligocene(?) Frio Formation of southwestern Louisiana is one of the four major deeper water shale wedges in the post-Vicksburg Gulf Coast Tertiary section. The Hackberry 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) microfaunal assemblage. Some thin, erratically distributed sandstone bodies are present. The lower section ranges in thickness from zero to 700 ft and consists essentially of sandstone.

In order to help the reader understand the geologic history of the Hackberry wedge, a discussion of the stratigraphy and structure of the entire Frio section is included.

The complicated Frio stratigraphy of northern Jefferson Davis and Calcasieu Parishes is caused in part by a complex tectonic history during the early part of Frio deposition and in part by depositional variation within the Hackberry section. The geologic history of the area can be summarized in a sequence of eight stages. It should be emphasized that these eight stages are a sequence of events that probably overlap one another and may have occurred at slightly different times in different areas.

1. Deposition of Vicksburg and lower Frio Textularia seligi zone.
2. Development of lower unconformity (this may be a local unconformity).
3. Deposition of the lower Frio and Hartburg sequences.
4. Uplift, folding, erosion, and development of "pre-Hackberry unconformity."
5. Tilting of the unconformity surface and renewed erosion which formed channels.
6. Filling of the channels with basal Hackberry sandstones to form a flat upper surface.
7. Deposition of Hackberry shale sequence with "arenaceous" fauna at its base.
8. Deposition of the remainder of the Frio Formation and the lower part of the Anahuac Formation.

The earlier structural movements were accompanied by folds and faults. The structures thus formed were truncated by regional erosion. Large channels (600 ft or deeper) were cut into the pre-Hackberry erosional surface and later filled. The manner in which these channels were cut and filled is uncertain, but turbidity flows may have been the cause. The structural movements, history of erosion, and the complex stratigraphy of the Hackberry make exploration for Hackberry sandstone reservoirs a high-risk economic venture, but one which may pay high dividends.