Biostratigraphic and Paleoecologic Characterization of Cretaceous Sequences of the Western Interior Basin of North America

Abstract

Five second-order and numerous third-order sedimentary sequences have been recognized in Cretaceous strata of the Western Interior basin, North America. Regional definition of the predominantly transgressive (TST) and highstand systems tracts (HST), from nearshore coarse clastics to basinal fine-grained facies, depends on a system of detailed biostratigraphy and chronostratigraphic correlation. A predominantly molluscan-based biostratigraphy utilizing composite assemblage zonation provides correlations across the basin at 0.1-0.3 Ma/biozone levels of resolution. High-resolution event chronostratigraphy provides regional correlation at finer (40-100 Ka) intervals, based on integrated bioevent, chemoevent, physical, and composite event units tied through graphic correlation to assemblage biostratigraphy and geochronology. Further, paleoecologic data that reflect changes in relative sea level can be used to better define systems tracts, especially in fine-grained basinal facies where sequence stratigraphic boundaries may be poorly expressed. These data include (1) taphonomic and community data reflecting condensation and offshore starvation during TST and TST-HST boundaries (e.g., shell lags and marine oyster beds); (2) systems tract-specific communities; (3) macrofaunal diversity trends; (4) temporal biofacies patterns where predominantly epifaunal communities characterize most of the basinal TST and early HST, whereas mixed epi- and infaunal communities characterize earliest TST and later HST and lowstand systems tracts; and (5) lateral biofaciesmore » patterns for each systems tract.« less