Sequence Biostratigraphy of Carboniferous–Permian Boundary Strata in Western Utah: Deciphering Eustatic and Tectonic Controls on Sedimentation in the Antler–Sonoma Distal Foreland Basin

Abstract

The stratal architecture of the upper Ely Limestone and Mormon Gap Formation (Pennsylvanian–Lower Permian) in west-central Utah reflects the interaction of icehouse sea-level change and tectonic activity in the distal Antler–Sonoma foreland basin. Nineteen stratigraphic sections correlated by physical and biostratigraphic means provide a basis for tracing Carboniferous–Permian boundary strata over a north–south distance of 60 km. These formations can be subdivided into 14 unconformity-bounded, third-order depositional sequences of differing internal architecture and regional extent. Conodonts and fusulinids provide ages for selected sequences and parasequences, permitting correlation with tectonostratigraphic units in the proximal foreland in north-central Nevada and with selected Midcontinent cyclothems. The 14 third-order sequences stack into three second-order supersequences characterized by distinctive differences in facies and facies stacking patterns, regional continuity of cycles, relative abundance of dolomite and limestone, calculated rock accumulation rates, and the frequency and inferred duration of sequence-bounding hiatuses. These reflect the effect of high-frequency sea-level change on an intermittently subsiding distal foreland shelf. Sediment accommodation was relatively high during the Bashkirian through middle Moscovian (upper part of Lower Absaroka I supersequence) and again during the late Sakmarian and Artinskian (lower part of Lower Absaroka III supersequence) as a function of continuous subsidence and high-amplitude sea-level change. During the late Moscovian through upper Sakmarian (Lower Absaroka II supersequence), however, subsidence slowed or ceased in response to tectonic activity in north-central Nevada, with concomitant development of the West-Central Utah Highlands (forebulge). During this episode of reduced subsidence, intermittent sedimentation was driven by second- and third-order eustatic fluctuations in sea level. Constituent strata form a wedge of onlapping, northward-thinning sequences and parasequences deposited during selected third-order highstands of the Lower Absaroka II second-order sea-level event. Depositional sequences in the distal foreland are bounded by low-relief disconformities of variable duration, in contrast to the angular unconformities and intensely deformed tectonostratigraphic domains that characterize the proximal foreland basin in north-central Nevada.