Brackish and marine mollusks of the Hell Creek Formation of North Dakota: Evidence for a persisting Cretaceous seaway

Abstract

Molluscan species associations reflecting varying salinities in the final North American interior seaway occur in Cretaceous to Paleocene strata. Deposits of the last few million years of the Cretaceous sea record an open marine environment characterized by species-rich faunal associations documenting lowering levels of the Western Interior Sea. Relatively diverse molluscan associations at the beginning of the deposition of the Fox Hills Formation are dramatically reduced in both bivalves and gastropods in the younger brackish-water lithofacies of the formation, indicating continued loss of habitat. Marginal marine to marine conditions returned to the area of central North Dakota and South Dakota, as recorded in the transition deposits of the Fox Hills into the lower Hell Creek Formation. More fully marine (mesohaline to brachyhaline) conditions are documented by a relatively diverse molluscan fauna in south-central North Dakota, as evidenced in the Breien Member of the Hell Creek Formation. The short-lived species richness recorded in the Breien strata is overlain by evidence of isolated rare molluscan and arthropod fossils in the upper, but not uppermost, part of the Hell Creek. These molluscan faunules contain individual ammonite, scaphopod, and bivalve taxa that indicate a resurgence of westward shoreline advance or fluctuation in shoreline configuration and continued influence of sublit-toral marine conditions. Thus, as reflected in the deltaic depositional history of the Hell Creek Formation through much of its thickness, the molluscan record of the last 1-2 m.y. of the Cretaceous indicates environmental settings characteristic of the presence of an adjacent interior sea.