Lithology and Petrology of the Stanley and Jackfork Formations

The sandstones of the Stanley and Jackfork formations represent the two arenite types indigenous to geosynclines. Bedding characteristics, intra-formational stratigraphy, and depositional structures are described. The Stanley formation consists of shale, graywacke, silts tone, siliceous shale, and tuff, interbedded in more or less rhythmic fashion. The graywackes do not show clear evidence of grading, although they present other features characteristic of sequences of graded arenites. The Jackfork formation consists principally of sandstone, with minor amounts of shale. Ripple marking and other bedding-plane structures, and cross-bedding, rare in the sandstones of the Stanley, are not uncommon in those of the Jackfork. The quartz grains of the sandstones were assigned to either an igneous or a metamorphic source on the basis of shape. Percentages of rock fragments, feldspar, and heavy minerals were also determined, and the roundness of grains estimated. In addition, grain size distributions were plotted. All lines of evidence suggest that the Stanley was derived from a source composed primarily of igneous and other crystalline rocks and the Jackfork principally from metasediments. The Stanley graywackes are interpreted as having been deposited in relatively deep water, possibly by turbidity flows, and the clean Jackfork sands in shallow water.