Sequence stratigraphic analysis of the Fox hills and Hell creek formations (maastrichtian), eastern Montana and its relationship to dinosaur paleontology

Abstract

The Upper Cretaceous Hell Creek Formation near the Fort Peck Reservoir, eastern Montana, has received significant investigation due to the large variety and exceptional preservation of included fossil material. Workers have focused mainly on taphonomic and paleontologic issues, but lack of a stratigraphic framework within which to place fossil finds in order to address questions of evolution, population diversity, and paleoecology. This study uses sequence stratigraphy to correlate within the Fox Hills and Hell Creek Formations such that paleontologic data can be interpreted in relation to a series of linked depositional environments and their relationship through time. Four key surfaces are present in the study area. First is a sequence boundary capping the Fox Hills marine shoreface strata. Incision occurs locally at this boundary and the resulting topography is filled by incised valley-fill strata of the Colgate Member of the Fox Hills Formation (lowstand and transgressive systems tracts). The second key surface is a flooding surface internal to the transgressive systems tract which is associated with extensive Skolithos burrows into the Colgate Member below (Glossifungites surface?). This flooding surface separates white, trough cross-stratified Colgate sandstone from thin carbonaceous, bioturbated and pedogenically altered units of the lower Hell Creek Formation. Capping this carbonaceous member is the maximum flooding surface which separates transgressive deposits below from highstand deposits above. Highstand deposition of the lower Hell Creek consists of the high accommodation, estuarine, inclined heterolithic sandstone which contains the most complete and associated Tyrannosaurus rex. The fourth key surface separates lower Hell Creek estuarine deposits from upper Hell Creek fluvial system deposits. These fluvial deposits consist of mixed sandstone, siltstone and mudstone which are interpreted to have formed on a flood plain through processes associated with meandering stream deposition and pedogenic alteration. These deposits reflect a return to low accommodation depositional patterns (lowstand systems tract). Changes in accommodation during Maastrichtian time, as interpreted from regional stacking patterns, facies changes and facies tract dislocations can be used in conjunction with taphonomic data from paleontologic sites in order to understand and predict the nature of preservation and distribution of fossil material