Abstract

In the western U.S. Cordillera Late Devonian-Early Mississippian overthrust loading resulted in (1) development of the Antler foreland basin and (2) a transition of depositional framework from shelf carbonate sedimentation on a passive continental margin to sedimentation in an active, convergent tectonic setting. On the deformed cratonward foreland the transition is marked by a regional unconformity on which the Lower Mississippian Lodgepole/lower Madison/Henderson Canyon formations were deposited as the initial episode of renewed carbonate sedimentation. This carbonate succession is characterized by 25 microfacies and 13 lithofacies, which are grouped into seven facies associations: foreslope, offshore skeletal-peloidal bank, deep ramp, middle ramp, foreshoal subtidal, shoal and peritidal-lagoonal.

The facies associations and their related lithofacies and microfacies are interpreted to have been deposited on a carbonate platform characterized by a carbonate ramp and an offshore skeletal-peloidal bank. This differs from the “classic” ramp model in that a carbonate bank developed along the ramp-to-foredeep basin transition in eastern Idaho and northeastern Utah. The platform facies model could be described by three broad facies zones: shallow ramp, middle/deep ramp and bank/foreslope, which correspond to three distinct tectono-paleotopographic settings: stable craton in central Wyoming, secondary foreland basin in western Wyoming and forebulge in eastern Idaho and northeastern Utah respectively.

With conodont zonations providing time-stratigraphic control, the Lower Mississippian succession records a shallowing upward deposition of the carbonate ramp and offshore bank due to aggradation and progradation from the early Siphonodella isosticha-Upper S. crenulata Zone to the Gnathodus typicus Zone.