SEVIER OROGENESIS AND NONMARINE BASIN FILLING: IMPLICATIONS OF NEW STRATIGRAPHIC CORRELATIONS OF LOWER CRETACEOUS STRATA THROUGHOUT WYOMING

Early Cretaceous strata in Wyoming (WY) record two periods of tectonic uplift and basin deposition separated by a period of reduction in mountain-belt topography and development of a basin-wide unconformity. New correlations reveal a basin-fill morphology that exhibits a foredeep, forebulge, and backbulge depozone. The correlations are based on distinctive hyperconcentrated flow deposits (diamictites & wackes), a change in the compositions of extraformational clasts, and new palynological age dates, all integrated with previous biostratigraphic and fission-track dates. Upsection, the new correlations are as follows. The upper Ephraim Formation (Fm) to the west correlates with the Cloverly Fm A interval in central WY and the Lakota Fm L1 interval to the east (the latter two intervals are dominated by dark chert). These strata are Neocomian and record subsidence & mountain-belt uplift. The Aptian Peterson Limestone to the west correlates with an unconformity throughout the rest of WY, and both reflect a subdued mountain belt and a decrease/cessation in subsidence. The Bechler Fm to the west correlates with the Cloverly B/C and Lakota L2/L3 intervals. Cloverly and Lakota strata record a change in provenance (occurrence of clasts such as Ephraim, silicified limestones, red chert, quartzites, vein quartz) indicative of progressive unroofing of the mountain belt. Such clasts within diamictites & wackes of the Cloverly C interval previously have been misinterpreted as gastroliths. The Bechler and correlatives are Albian and reflect renewed orogenesis & subsidence.

Strata in western WY are an order of magnitude thicker than elsewhere and are foredeep deposits. East of the foredeep, strata progressively thin into central WY and were deposited adjacent to (west of) and on a forebulge. The unconformity represents basin-wide erosion, which was greatest on the forebulge. Strata thicken from central to eastern WY (the Black Hills). Hence, Lakota strata to the east were deposited in a backbulge depozone. The styles of syndepositional structures (compressional in the foredeep; extensional on the forebulge; compressional & transpressional in the backbulge depozone) are consistent with these settings. The basin-fill morphology reflects subsidence driven not only by orogenic loading, but by dynamic loading as well.