ABSTRACT: An Ultramafic Inclusion from the Weeks Island Salt Dome, South Louisiana
DUEX, TIMOTHY W., and BRIAN E. LOCK, Department of Geology, University of Southwestern Louisiana, Lafayette, LA
Mines in Gulf Coast salt domes commonly encounter entrained clastic sedimentary material. The clastic sediments include red, brown, green, and yellow sands, silts, and clays in brecciated seams, which may represent shear zones within or between salt stocks. In January 1992, miners encountered a hard, dark-gray block of distinctive lithology within a clastic inclusion zone at the 1000 ft level of the Morton Salt Company's mine at Weeks Island, Iberia Parish, Louisiana. The square block measured approximately 18 in. on a side and had a depth into the freshly exposed salt face of a few inches. Thin-section petrography established that the dark gray rock is an altered ultramafic porphyry. Phenocrysts of serpentinitized olivine, pyroxene, and amphibole occur in a groundmass dominated by se pentine and opaque materials.
This new occurrence is significant because it extends the known range of mafic and ultramafic igneous activity within the Gulf area. There is no published record of foraminifera or other paleontological materials to enable the sediments to be dated, although Late Triassic or possibly Jurassic sporomorphs from the salt have been reported. It is normally concluded that the entrained clastic sediments are younger than the salt and were picked up during the salt's upward diapirism, although salt domes in the Arabian Gulf contain blocks of material that are clearly from the older underlying basement rocks, apparently "sucked" from below the rising salt mass.
AAPG Search and Discovery Article #91014©1992 AAPG GCAGS and GC-SEPM Meeting, Jackson, Mississippi, October 21-23, 1992 (2009)