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

SAGEMAN, BRADLEY B., and ERLE G. KAUFFMAN,* University of Colorado, Boulder, CO

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. Resolution is greatest in late transgressive, early highstand, and late highstand systems tracts, when increasing stress gradients drive rapid evolution. High-resolution event chronostratigraphy provides regional correlation a 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 (e.g., high diversity levels characterize early TST, late TST, and late HST; low diversity assemblages are predominant during much of he TST and early HST coeval with water stratification and benthic oxygen reduction); (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 biofacies patterns for each systems tract.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)