Formation of hematite in the euxinic Orca Basin, northern Gulf of Mexico

Abstract The Orca Basin is a depression in the northern continental slope of the Gulf of Mexico, which is partially filled with hypersaline, anoxic brine pool. Sediment cores taken from this basin show millimeters to centimeters thick red layers within typical dark anoxic muds. Chemical and X-ray mineralogical analyses reveal that the red laminae are invariably enriched in iron in the form of hematite. They are formed primarily by interaction of ferrous iron in the brine with dissolved oxygen from overlying seawater during isolated incidences of enhanced mixing a cross the normally stable oxic—anoxic boundary. The presence of hematite red layers within generally anoxic sediment cores would complicate interpretation in the sedimentary record, and limit the usefulness of hematite as an indicator for environment of deposition. It also demonstrates that the close association of iron-formation with interbedded sulphic shales observed in many Proterozoic iron-formations is practically feasible. The processes associated with the formation of hematite in red layers might affect the extent and nature of organic matter eventually deposited in the basin as well, a fact of considerable significance for petroleum source bed formation.