Depositional Environments and Diagenetic Features of a Cretaceous Clastic Sequence, Fox Hills Sandstone of Northern Great Plains Province: ABSTRACT

Abstract

The Fox Hills Sandstone, youngest formation of the Montana Group, is a geographically extensive, thin unit that is composed primarily of silty sandstone and shale. This formation records the final withdrawal of the Upper Cretaceous epicontinental sea from the Western Interior of the US. It extends from central Canada southward through Montana, the Dakotas, Wyoming, and Colorado. This typical shoreline sequence is exposed along the flanks of the Cedar Creek anticline, which trends southeastward from eastern Montana into North and South Dakota. The upper Fox Hills consists of stacked, upward-fining, trough to planar cross-bedded sandstone bodies. These sequences grade laterally and vertically into interbedded clay and silt units containing root casts and are interpreted as deposits of a braided-stream system. A matrix mineral assemblage of chlorite, montmorillonite, calcite, and minor dolomite is characteristic of distributary channels and river mouth-bar facies. The lower Fox Hills consists of either irregularly stacked upward-coarsening, trough cross-bedded sand sequences or massive sand bodies separated by erosional surfaces. Outcrops locally contain the trace fossil Ophiomorpha or limestone concretions containing marine to brackish-water fossils, and have been interpreted as deposits of a rapidly advancing shoreline and strand plain. In the matrix, illite, kaolinite, and montmorillonite with minormore » calcite characterize beach, subtidal, and crevasse-splay environments. Diagenetic features include quartz overgrowths, clay alterations of potassium feldspar and rock fragments, and authigenic clay rims. Petrographic evidence indicates multiple source areas for these sediments and the primary influence of depositional environments on clay formation.« less