TECTONOSTRATIGRAPHY OF AN ICEHOUSE SHELF UNDERGOING LITHOSPHERIC FLEXURE: A CASE STUDY FROM THE ANTLER-SONOMA FORELAND BASIN, WESTERN UNITED STATES

Pennsylvanian and Early Permian sedimentation in western Utah was controlled by uplift of the West Central Utah Highland (WCUH) coupled with high-amplitude, high-frequency, icehouse sea-level fluctuations. Our study affirms the development of a regional unconformity of Late Pennsylvanian to Early Permian vintage, but indicates that the southern portion of the Ely shelf in the study area was not continuously exposed from late Desmoinesian through late Asselian time as asserted by previous workers. After a prolonged period of Morrowan to middle Desmoinesian carbonate sedimentation, the WCUH (re-activated Antler forebulge) began to exert its influence. From late Desmoinesian to Bursumian time, the shelf was subaerially exposed, except during relatively short-lived flooding events that deposited eight regionally limited, high-frequency sequences (MG 1- MG 8). Renewed marine flooding (Lower Absaroka II event) lasted through most of Asselian time. Subsequent sea-level fall, corresponding to the Lower Absaroka II-Lower Absaroka III transition stranded the Ely shelf during most of Sakmarian time. The Artinskian, however, marked a return to widespread flooding that continued through at least Middle Permian time. This interplay of tectonic warping and eustacy resulted in deposition of 11 carbonate-dominated sequences in western Utah. Sequences formed during maximum upwarping of the WCUH (sequences MG 1-MG 8, late Desmoinesian - Bursumian) are comprised of largely dolomitized, meter-scale cycles that lap out to the north over a distance of 10 to 20 miles. Bounded by abnormal subaerial exposure surfaces, these high-frequency sequences reflect flooding of the WCUH only during times of major third-order sea level rise. Insights from this study can be used to predict lithofacies and geometries of strata deposited on other updip, carbonate-dominated icehouse shelves. It also contributes to the emerging tectonostratigraphic framework of the region bounded to the east by the Ancestral Rockies.