Biostratinomy and Paleoecology of a Cretaceous Brackish Lagoon

Abstract

Shell beds in a Cretaceous lagoonal fill resulted from a combination of low sediment input, high organic productivity, and reworking by storm processes. The skeletal concentrations occur in a shelly shale unit 2-3 m thick in the upper sandstone member (Late Cenomanian) of the Dakota Formation of northeastern Arizona. The shell beds are parautochthonous relics of a former brackish-water community dominated by the infaunal suspension-feeding bivalve Caryocorbula ovisana. Analysis of 74 samples and more than 12,000 specimens allows subdivision of this association into four subsets characterized by the bivalves Flemingostrea prudentia/Anomia ponticulana, Brachidontes filisculptus, and corbulid sp. A, and the gastropod Voysa sp. Biostratinomic, sedimentologic, and paleoecologic data allow reconstruction of a stratified brackish-water lagoon system. Saline, low-oxygen waters below storm wave base occurred at least locally in deeper parts of the lagoon system and were characterized by a monospecific assemblage of lucinid bivalves. The low-oxygen water was overlain by brachyhaline waters that were below fairweather wave base, but were within the reach of storms. This habitat supported a moderately diverse fauna of benthic molluscs. The lagoon was capped by water of strongly reduced salinity (mesohaline regime) and contained a very low diversity fauna. Progradation of the low-energy lagoonal shore resulted in the establishment of nonmarine environments in some parts of the lagoon. This sequence is overlain by reworked barrier-bar sands deposited during the ensuing transgression