CLAY MINERAL EVIDENCE OF POSSIBLE PROVENANCE CHANGE FOR CAT ISLAND, OFFSHORE MISSISSIPPI

Cat Island, located offshore of Gulfport, MS, is the westernmost barrier island in the Gulf Islands National Seashore. The island marks the northeastern extent of the St. Bernard subdelta of the Mississippi River. It is assumed that the unusual “T” shape of Cat Island is due to reworking of the original shore-parallel island by westward longshore currents following the abandonment of the St. Bernard delta. Standard petrographic and XRD analyses were performed on surface and subsurface (0 to ~200 cm) samples collected from and around Cat Island. High-energy sands are composed of quartz and 0-5% potassium feldspar and plagioclase feldspar regardless of sample depth or location on the main shore-parallel island body or on the reworked “T”. Clay minerals from surface samples are rare, but where present (on the purportedly reworked “T”) are composed of subequal amounts (~10-20% each) of kaolinite, chlorite, mixed-layer illite/smectite (I/S), and ~50% illite. Offshore muds recovered from beneath the reworked “T” consist of lesser amounts of kaolinite and chlorite (~10% each), and subequal amounts of I/S and illite (~40% each). The explanation for the differences in the clay mineral suite of surface and subsurface samples at Cat Island is unclear. Variable amounts of illite, I/S, chlorite, and kaolinite are present in modern Mississippi River sediments. The mineralogic differences observed in this study may represent a change in sediment provenance from the Mississippi River during St. Bernard delta formation, to an I/S-poor, eastern source after delta switching. Clay mineral analyses of sediment from east of Cat Island are uncommon, but suggest that this material, derived predominately from weathering of the crystalline southern Appalachians, is I/S-poor and kaolinite-rich. Another possible explanation for the observed differences is preservation of dissimilar clay mineral suites in originally different depositional environments (high-energy shoreface vs. low-energy offshore).