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Abstract

A regional study, based on detailed descriptions of 17 outcrops across east-central Idaho and southwestern Montana, provides a dip-oriented cross section in which to better understand the distribution of Upper Mississippian (Chesterian) stratigraphy on the distal margin of the Antler foreland basin. Chesterian strata constitute an eastward-thinning wedge of mixed carbonate and siliciclastic rocks that formed on a west-facing ramp. Foreland-basin tectonism subdivided the ramp into three distinct depositional settings: the western, central, and eastern ramp. The western ramp records nearly continuous Chesterian deposition, whereas the central and eastern ramps have significant unconformities. Mud-rich subtidal carbonate predominates on the western ramp, but this interfingered during the late Chesterian with tidally influenced siliciclastics. The central ramp contains an intraramp basin with deep subtidal siliciclastics and carbonate that formed adjacent to shallower-water facies to the west and east. The eastern ramp has mostly peritidal carbonate and shallow marine to fluvial siliciclastics, but a transgression from the north during the late Chesterian inundated this portion of the ramp with open marine carbonate.

New conodont biostratigraphic constraints indicate that these Chesterian strata are a second-order megasequence (10-12 My duration) composed of more than seven third-order depositional sequences (S0-S7), each having a duration of 1-5 My. The sequences are grouped into three composite sequences (I, II, and III) that define long-term changes in accommodation controlled by syndepositional tectonism. Composite sequence I was deposited during a period of tectonic loading that partitioned the ramp via subsidence loading and extension. Composite sequence II records a period of tectonic stabilization and deposition during the most extensive eustatic flooding, whereas composite sequence III is dominated by a localized subsidence event in the Big Snowy Trough. Higher-frequency (fourth- and fifth- order) parasequences are common throughout the study interval, but they are only locally correlative. A change from thick-bedded carbonate- dominated parasequences in the early and middle Chesterian to thinner-bedded mixed carbonate and siliciclastic parasequences in the late Chesterian likely reflects the onset of moderate- to high-amplitude, high-frequency eustatic fluctuations caused by the initiation of Gondwanan glaciation.

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