Abstract

An isopach map from the top to the base of chalk section, generally thought of by subsurface explorationists, as the Top of the Cretaceous to the Base of the Austin, more properly, the base of the Ector Chalk Member of the Austin Group, shows two areas of thinning south of the strongly thinned area of the Monroe Uplift and the Jackson Dome. Isopach thinning in both areas is due to missing upper portion of the Chalk section. The first thin area is a northeast to southwest dip-oriented linear trend normal to the Upper Cretaceous Shelf and parallel to the modern Mississippi River in Warren, Hinds, Claiborne, Jefferson, Adams, Franklin, Wilkinson, and Amite counties of western Mississippi and St. Landry, Avoyelles, Concordia, and Tensas parishes of eastern Louisiana. The second thin area is seen in Pearl River, Hancock, and Harrison counties of southern Mississippi and St. Tammany and St. Bernard parishes of southeastern Louisiana. We have named the second area the Pearl Basin Embayment. The Pearl Basin Embayment is oriented parallel to the east-west oriented Upper Cretaceous shelf trend and looks as if a bite has been taken from the Upper Cretaceous shelf.

Foraminiferal and nannofossil analysis of the chalk section in the HELIS #1 Poitevent well in T7S–R12E–34, St. Tammany Parish, Louisiana, which drilled 420 feet of chalk section, previously thought to be Cretaceous in age, showed that that the upper 320 feet was lowermost Danian in age. The only Cretaceous section was the lowermost 100 feet of section that is Coniacian age-equivalent to the Ector Chalk Member of the Austin Group. Seventeen plus million years of Upper Cretaceous section had been removed from the upper part of the Upper Cretaceous in the Pearl Basin Embayment. In addition, the Danian samples appear as if they were a jumbled mess, having been deposited quickly. The Cretaceous-Paleogene (K–Pg) section in the HELIS #1 Poitevent well in St. Tammany Parish is similar to that described by Kinsland (2018, 2019, 2021) in the LaSalle Parish core taken in the JUSTISS #2 LA Central IPNH well, Olla Field, 2–T9N–R2E. The HELIS well sits in a more downdip position, but still on the stable Upper Cretaceous Shelf. Our analysis supports the Kinsland’s interpretation of the K–Pg chalk section being outwash deposition from the Chicxulub impact.

Figures 1–9 provide illustrative support of this work.