Prevenance, preservation, and transport of the detrital “coffee grounds” of the Mississippi Delta

Abstract

Detrital organic fragments or “coffee grounds”, common in the swash zone, and in the lee of barrier islands off the Mississippi Delta consists of up to 40% high-volatile bituminous coal particles of medium to coarse sand size (0.25–1 mm). These coal fragments produce both a well preserved Cretaceous-Paleocene flora and a less well preserved Pottsville (Early Pennsylvanian) flora. This Pottsville flora has close affinities with one described by Kosanke (1950) from the Early Pennsylvanian Illinois Basin coals. If the provenance assignment is correct, these coal particles were transported by the Mississippi River a minimum of 1000 miles from the Interior Coal Province to their present locality in the south Pass area of the Mississippi Delta. However, spillage contamination during a century of barge transport within the Mississippi drainage cannot be ruled out as a possible source for the “coffee grounds” coal. High-volatile coals crop out in the Black Warrior Basin in Alabama to the northeast of the delta. If this is a source of the coal, then an entirely different drainage-transport system would be involved. It would invoke drainage from the Appalachian Province southward, with subsequent longshore current transport, westward — along the Gulf Coast shoreline. Heavy-mineral differences exist between these two sedimentary provinces, with the Appalachian-derived heavy minerals dominated by kyanite, staurolite, and tourmaline; and the Mississippi River Canadian Shield source dominated by amphiboles and pyroxenes. The heavy-mineral assemblage recovered from the “coffee grounds” material is more like those of the Mississippi River Province, and dominated by green hornblende. Vitrinite particles of the coals show no evidence of oxidation rims although they have been in oxygenated waters for an extended period of time during transport. The spore preservation is adequate for specific identification, and is further evidence of minimal oxidation. Coal rank analysis and palynoflora identifications provide a dual approach in reconstructing provenance, transport history, and origin of these detrital organic deposits.