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

Bulk-geochemical and biomarker (molecular fossil) data were collected for f our Cenomanian-Turonian sections in the Western Interior Basin, USA. The fo ur sections represent a 900 km wide transect across the basin from Kansas t o New Mexico. Rock-Eval and biomarker analyses indicate an east-west therma l maturity trend due to progressive deeper burial associated with the tecto nic flexure of the foreland basin since the Cenonamian-Turonian. At each si te the source of extractable organic matter is dominantly marine with minor terrestrial contribution. The presence of isorenieratene derivatives in sa mples from Bunker Hill (KS), Pueblo (CO), and Red Wash (NM) indicates the p resence of the green sulfur bacterium Chlorobium in the water column, and s hows that the southern part of the Western Interior Seaway (WIS) experience d events of photic zone anoxia. At maximum transgression, sedimentary featu res indicative of water column oxygenation occur within single beds with ge ochemical evidence of photic zone anoxia. This is consistent with a scenari o in which bottom waters experienced alternating oxic/dysoxic and anoxic co nditions. These so-called 'intermittent anoxic events' underline the high v ariability in redox conditions in the WIS. The distribution of isorenierate ne derivatives shows a dynamic water column structure of the WIS, and indic ates that anoxia was a permanent feature of the transgressive and regressiv e intervals of the Greenhorn Cyclothem, rather than at maximum transgressio n. 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). (C) 2001 Elsevier Science B.V. All right s reserved.