Molecular fossil constraints on the water column structure of the Cenomanian–Turonian Western Interior Seaway, USA

Abstract

Bulk-geochemical and biomarker (molecular fossil) data were collected for four Cenomanian–Turonian sections in the Western Interior Basin, USA. The four sections represent a 900km wide transect across the basin from Kansas to New Mexico. Rock–Eval and biomarker analyses indicate an east–west thermal maturity trend due to progressive deeper burial associated with the tectonic flexure of the foreland basin since the Cenonamian–Turonian. At each site the source of extractable organic matter is dominantly marine with minor terrestrial contribution. The presence of isorenieratene derivatives in samples from Bunker Hill (KS), Pueblo (CO), and Red Wash (NM) indicates the presence of the green sulfur bacterium Chlorobium in the water column, and shows that the southern part of the Western Interior Seaway (WIS) experienced events of photic zone anoxia. At maximum transgression, sedimentary features indicative of water column oxygenation occur within single beds with geochemical evidence of photic zone anoxia. This is consistent with a scenario in which bottom waters experienced alternating oxic/dysoxic and anoxic conditions. These so-called ‘intermittent anoxic events’ underline the high variability in redox conditions in the WIS. The distribution of isorenieratene derivatives shows a dynamic water column structure of the WIS, and indicates that anoxia was a permanent feature of the transgressive and regressive intervals of the Greenhorn Cyclothem, rather than at maximum transgression. Hence, the oceanographic circulation is dominated by near field effects inherent to the seaway, rather than by direct proto-Atlantic elements such as the oceanic anoxic event (OAE).