The Richfield Member of the Lucas Formation of the detroit river group (lower middle devonian), Michigan Basin, Michigan, USA

Abstract

The Richfield Member of the Lucas Formation is composed of numerous anhydrite and carbonate cyclothems which prograded across the Michigan Basin from the west to the east forming a wedge of sediments. The sulfates were precipitated as either anhydrite or gypsum (1) from the evaporation of pore fluids in the supratidal facies, (2) as lath crystals in the intertidal and subtidal facies, and (3) as subaqueous palmate crystals within the ponds on the supratidal flats and in the deeper basin. The low-relief platform was frequently inundated by basinal waters with only slight changes in water level. Thus, the sabkha was flooded with sheets of water which caused subaqueous palmate gypsum to precipitate so that a single anhydrite bed had both supratidal and subaqueous origins. The intertidal and supratidal zones were characterized by dolomitic algal mats which frequently were partially or totally dolomitized by replacement. The dolomite is typically euhedral to sub-euhedral with an idiotopic texture forming intercrystalline porosity. The subtidal facies was composed of peloids, ooids, and micrite with partial dolomitization. Basinal ionic concentrations regulated whether anhydrites or carbonates were deposited in this zone. Maximum thickness of the Richfield, up to 225 feet, occurred on the western edge where the maximum number of cyclothems were developed. To the east, the unit has fewer carbonate-anhydrite cyclothems and greater amounts of unaltered limestone which thinned to a feather edge. 9