CONTROLS ON STRATIGRAPHIC ARCHITECTURE OF CENOMANIAN-TURONIAN STRATA, WESTERN INTERIOR BASIN

Cretaceous strata of the Western Interior basin provide a unique opportunity to study controls on stratigraphic architecture in actively subsiding foreland settings, in part due to exceptional chronostratigraphy integrating biostratigraphy, lithostratigraphy, cyclostratigraphy, and radiometry. The development of this temporal framework has led to more accurate calculations of rates for various geologic processes, providing critical constraints for stratigraphic modeling studies, which are playing increasingly important roles in the hypothesis testing process. This study focuses on the detailed anatomy of a peak Late Cretaceous flooding event and sea level turnaround (Cenomanian-Turonian (C-T) Greenhorn cycle) in the Western Interior basin. A key attribute is correlation of an orbital time scale from the rhythmically bedded C-T strata of the basin center to the marginal marine section based on biostratigraphic and bentonite datums. Using this time scale, siliciclastic sediment accumulation (compacted) is estimated to have decreased from ~90 m/myr (late Cenomanian) to 50 m/myr (early Turonian). These high rates of accumulation characterize the unique accommodation and sediment supply attributes of the Sevier foredeep, which made possible the preservation of a complex but highly detailed record of relative sea level changes. This record captures three hierarchical orders of transgressive-regressive cycles defined as genetic sequences. The longest sequence is bounded by major flooding events, has a duration of ~800 kyr, and is penecontemporaneous with Oceanic Anoxic Event II. Medium-term and short-term genetic sequences are defined based on evidence of higher frequency transgressive-regressive events and they show quasi-periodicities of ~100 kyr and 20-40 kyr, respectively. Subaerial discontinuities that may represent episodes of transgressive-regressive and relative sea-level fall were identified at both medium- and short-term time scales. The timing and hierarchy of interpreted transgressive-regressive and relative sea-level changes are suggestive of Milankovitch-driven eustatic control. Using 2-D stratigraphic modeling, a preliminary test of the hypothesis of orbital control on sea level and its effect on stratigraphic architecture was performed.