UNUSUAL MARGINAL MARINE RECORD OF OCEANIC ANOXIC EVENT 2

We studied a Cenomanian–Turonian succession of siliciclastic mudstones—the Pecínov Member—deposited during oceanic anoxic event 2 in the Bohemian Cretaceous Basin, Czech Republic. The Pecínov Member is usually interpreted as the product of offshore sedimentation. The succession is characterized by a number of erosional surfaces, visible in the field, that have been interpreted as a product of base level changes. In addition to these erosional surfaces, our textural, mineralogical, and geochemical data indicate the presence of multiple smaller scale erosional surfaces that suggests a dynamic sedimentary environment. The studied mudstones are carbonate-free and dominated by agglutinated foraminifera with taxa diagnostic for estuarine environments. The palynological assemblages are dominated by terrestrial taxa and become more marine upward. We interpret the environment in which the Pecínov Member was deposited as a proximal subtidal environment to tide-dominated estuary. We attribute erosional features within the Pecínov Member to autogenic processes, such as tidal channel and mud bedform migration. Two erosional surfaces coincide with a fundamental change in clay mineralogy: one from kaolinite-dominated to mixed kaolinite–smectite assemblages, which subdivides the Pecínov Member in two shallowing-upward units or parasequences, and the other, to smectite-dominated assemblages, which coincides with flooding of the basin and the onset of chalk sedimentation; the vertical clay mineral distribution pattern probably reflects particle size control on the distribution of clay minerals in response to sea level rise, rather than simply climate. Inflection points in the carbon isotope record that are a by-product of erosion complicate chemostratigraphic interpretation and correlation.