Depositional and tectonic setting of Quadrant and Tensleep Sandstone, Montana and Wyoming

The Quadrant Sandstone in southwestern Montana and the equivalent Tensleep Sandstone in northwestern Wyoming reflect transition from marine to nonmarine depositional settings. Thickness and facies relationships were influenced by tectonic events that preceded and followed deposition. Three important tectonic events are indicated in the Carboniferous strata: (1) separation of the Big Snowy Group and older Mississippian rocks from the overlying Amsden Group, (2) separation of the Amsden from the overlying Quadrant-Tensleep, and (3) separation of the latter from overlying Permian rocks. Locally, the Quadrant and Tensleep were deposited above rocks of significantly different ages within relatively short distances, owing to differential uplift and erosion of older rocks during the second tectonic event. Initial Quadrant and Tensleep deposition occurred as coastal dunes and as littoral sands in a shallow epeiric sea. Marine deposition was succeeded by progradation of a sand sea, characterized by extensive eolian dune and minor interdune facies. The dune facies seems more extensive in areas of subsidence, but this may be due to erosion of the eolian facies from positive areas during the third (pre-Permian) tectonic event. Some thickness and facies differences of the Quadrant are coincident with linear structural trends such as the northeast-trending Greenhorn lineament inmore » southwestern Montana. Similar structural trends may also have affected deposition of the Tensleep Sandstone in the Bighorn basin.« less