HIGH-RESOLUTION SEQUENCE STRATIGRAPHY AND CHEMOSTRATIGRAPHY OF LATE MISSISSIPPIAN (CHESTERIAN) FORELAND BASIN RAMP CARBONATES-SILICICLASTICS IN EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA

A new sequence stratigraphic framework of Late Mississippian (Chesterian) mixed siliciclastics and carbonates in east-central Idaho and southwest Montana provides insights into foreland basin sedimentary response to changes in regional tectonics and global climate. Fifteen outcrops (each up to 600 m thick) comprise a dip profile across the foreland basin ramp. The resulting cross section records an eastwardly thinning wedge of marine carbonates and siliciclastics that document an overall 2^nd order rise and fall of sea level. The profile distinguishes unique depositional settings in the western, central and eastern portions of the ramp.  The western and central portions of the ramp record an abrupt deepening not evident in eastern sections that may indicate foredeep flexure and reactivation of Antler foreland tectonics.  Deep water carbonates on the western ramp shallow upward into a thick skeletal carbonate bank, overlain by prograding siliciclastics in the late Chesterian. The central ramp includes deep water facies throughout that likely formed in a back bulge or intrashelf basin, and which only shallow to sea level in the latest Chesterian.  The eastern portion of the ramp includes heterolithic marginal marine facies containing abundant unconformities and paleosols. Detailed conodont biostratigraphy and chemostratigraphy (⁸⁷Sr/⁸⁶Sr and d¹³C) provide the basis for sequence correlations.

Geochemical analyses of 700+ whole rock samples and >100 well preserved fossil allochems comprise a composite d¹³C curve for the Late Mississippian.  The basal part of the curve averages +1.5‰ (PDB) and has low amplitude isotopic fluctuations.  A mid-Chesterian negative excursion and moderate amplitude isotopic fluctuations between 0‰ to –1‰ are followed by high amplitude isotopic fluctuations between –2‰ and +2‰ in the upper Chesterian.  A positive excursion to +2‰ across the mid-Carboniferous boundary coincides with published data from the US mid-continent and Europe.  The timing of this excursion and the gradual transition from low to high amplitude oscillations throughout the curve may signal a change in primary productivity associated with modified oceanic circulation in the foreland basin and the initiation of global Gondwanan icehouse conditions in the latest Chesterian.