DOOR POINT: VOLCANO IN THE GULF

The Door Point structure penetrated by the Shell Oil S.L. 3956 well no. 1, located offshore in St. Bernard Parish Louisiana represents the singular observed subsurface occurrence of igneous rock in the entire offshore region of the GoM that is Cretaceous in age. Identified in the literature as a volcano, sill or other primary intrusive/extrusive feature, with an age of 82 ± 8 Mya, the Door Point “Volcano,” has received very little study since its discovery in the 1970’s.

Preliminary study of the core shows that the unit consists of volcaniclastic rocks. Initial review of the geophysical logs provided by Shell Oil and Drilling Info show a drastic and apparent offset in the resistivity, with a maligned or absent deflection of the Spontaneous Potential curve. The sequence of volcanics extend vertically a minimum of 396m with no base identified, and the singular cored interval taken of the final 8.5m. Initial petrographic examination has revealed there to be both massive un-bedded polymictic agglomerates as well as bedded volcanic ash. The basalt cobble agglomerates sequentially alternate with the packages of finer grained volcanic ash. Grain size for the igneous cobbles within the agglomerates range in diametric extremes from a maximum of 5 cm to a minimum of 1 – 2 cm. Sedimentary structures are absent in the coarser sequences, but are exhibited in the volcanic ash beds, in the form of cross bedding. The clast populations vary in composition from fragments of sedimentary rock, to the igneous rocks of interest. Possessing a porphyritic texture and angular in appearance the clasts contain visible clinopyroxene phenocrysts, in an aphanitic groundmass; CI > 90.

These results suggest the cobbles are sourced from multiple volcanic centers, and have undergone a multistage cooling history. Coupled with the presence of the calcite cement, the cross bedding is indicative of a marine or fluvial depositional system. We are able to reconstruct a regional story of episodic volcanism in the nascent Gulf of Mexico Rift during the late Cretaceous.