Abstract

The Eagle Sandstone of central Montana was deposited during general eastward progradation of the shoreline in a narrow, north-south trending epicontinental seaway that extended from the Gulf of Mexico to the Arctic Ocean. Cordilleran highlands along the western margin were episodically uplifted and were the main source for terrigenous clastic sediments. Along the fluctuating western shoreline, the Eagle consists of a series of thick, blanketlike sandstones that were deposited in wave-dominated deltaic and interdeltaic environments. Farther seaward on the inner shelf, the Eagle consists of elongated sand ridges reworked by storm waves and sandstone units that resulted from rapid influx of sediment by seaward-directed, storm-generated rip currents.

Coastal sandstone of the Eagle grades eastward into offshore-marine siltstone and shale which are included in the Gammon Shale in eastern Montana. This fine-grained offshore sequence is considerably thicker than the time-equivalent, coarser-grained, coastal deposits due to increased rates of subsidence. In the subsurface in the vicinity of the Black Hills, coarsening-upward sandstone sequences, assigned to the Shannon Sandstone Member, are present in the upper part of the Gammon. The units form elongate bodies that trend northwest-southeast and are interpreted as sand ridges deposited on the outer shelf by south flowing, geostrophic currents induced by wind forcing.

East of the Black Hills, the lower part of the Gammon grades into the upper part of Niobrara Formation which contains widespread chalk tongues. The chalk tongues were probably deposited on a northwest sloping carbonate ramp which was essentially free of western-derived detritus. In central South Dakota, an unconformity, developed at the stratigraphic position of Shannon and younger Gammon rocks, occurs along a northeast-southwest trend and represents submarine erosion along the Transcontinental Arch. In southeastern South Dakota and southwestern Minnesota, the unconformity is present on a flat surface cut into the Precambrian basement on the Sioux Ridge which was an island near the eastern shoreline.