Fold–thrust styles in the Absaroka thrust sheet, Caribou National Forest area, Idaho–Wyoming thrust belt

Abstract

The Bear Creek, Big Elk and Black Mountain anticlines are macroscopic structures located in the hanging wall of the Absaroka thrust within the Idaho–Wyoming fold–thrust belt. Structural mapping of the area was conducted by draping existing geologic maps and digital orthophotos over digital elevation models (DEM) to obtain a three-dimensional perspective of the area. The map was further refined by reconnaissance field mapping of selected outcrops. Construction of balanced cross-sections suggests that these structures detach at three stratigraphic levels, within the Cambrian Gros Ventre Formation, Devonian Darby Formation, and Triassic Dinwoody Formation. The folds in the upper stratigraphic package consist of symmetric to asymmetric detachment folds, which show variable vergence along trend. The structures in the lower stratigraphic packages primarily evolved as low-amplitude detachment folds, which were subsequently displaced over fault ramps to form fault–bend folds or duplexes. The duplex geometries in the Cambro–Ordovician units under the Bear Creek and Poker Peak anticlines vary from independent anticlines in the south to completely overlapping stacks in the north. The macroscopic structural patterns of folds proposed in this study will be useful in interpreting subsurface structures in other parts of the Idaho–Wyoming fold–thrust belt.