AN ALKALIC VOLCANO IN THE SOUTHEASTERN LOUISIANA SUBSURFACE: PRELIMINARY OBSERVATIONS OF SHELL OIL CO. CORE 3956 FROM DOOR POINT

A core drilled by Shell Oil Company in 1963 near Door Point, LA contains the only known in-situ volcanic rock in the entire offshore region of the Gulf of Mexico. Volcanic material occurs in the last 1300 ft of the 8538 ft deep core. The bottom 26 ft of the core was retrieved for preliminary study by optical and cathodoluminescent methods. Spontaneous potential and resistivity logs were available for the well from which the core was retrieved and a well ~2200 ft to the NE.

The core consists primarily of a grain supported framework of clasts, ~90% rounded to subangular lapilli of tuff and ~10% accidental sedimentary lithics cemented by a calcite and zeolite matrix. Lapilli are often cored with accidental lithic fragments of sedimentary country rock and volcanic clasts. Euhedral olivine is completely serpentinized and deformed phlogopite laths are partially to completely chloritized. Lath-like microcrysts are replaced by clays. Fresh phlogopite sometimes occurs as coronas around altered phenocrysts. None of phenocrysts were identified as plagioclase, they all appear to be altered olivine, pyroxene, or micas. This places the core firmly in the ultramafic alkalic suite of extrusive igneous rocks.

Optical cathodoluminescence shows multiple (at least 5) generations of carbonates with different responses ranging from a bright red response in late carbonate veins to no response at all in the carbonate matrix. Ocelli of carbonates in the juvenile clasts have no CL response at all and may be primary.

The top of the volcanics is marked by a zone of high resistivity at 7750 ft. The oldest formation that can be definitively placed above the volcanics is the heterostagina limestone and the Wilcox formation is missing in this well, but apparent in the one adjacent to it. This relation places the volcanic activity in the Paleocene-Eocene, not the Upper Cretaceous as has previously been reported.

Based on the occurrence of abundant olivine phenocrysts, cored lapilli, phlogopite, and multiple generations of carbonates this core likely samples an ultramafic tuffisitic carbonatite volcano, possibly a kimberlite. The Door Point carbonatite corresponds to a linear trend of alkalic volcanic rocks including lamproites and kimberlites that occur at a similar age in the Mississippi embayment, and in an approximately coeval trend across North America.