A Petrographic Study of the Star Point Sandstone and Blackhawk Formation (Upper Cretaceous), East Mountain, Emery County Utah

A total of 23,400 point counts were made on 75 thin sections of sandstones from the Star Point Sandstone and the Blackhawk Formation (Upper Cretaceous) to determine their petrographic characteristics. Both units are litharenites, fine to medium grained, and moderately well sorted. The major lithic components in the Star Point are dolomite and chert with minor amounts of metamorphic rock fragments. In the Blackhawk channel sands, dolomite and intrabasin slump material are the major lithic components. The sand was derived from the Sevier Orogenic Highlands composed of sedimentary terrane to the west and both sedimentary and metamorphic terranes to the north of the study area. Diagenesis was controlled mainly by organic decay and dissolution due to pressure solution. Authigenic minerals and their order of precipitation are the same for both formations. These are quartz and pyrite, calcite, dolomite, and vivianite. The bleach zone in the Star Point was created when acidic waters from the overlying swamp dissolved the ferroan dolomite from the sands. Sandstone dikes within the Hiawatha coal seam (basal Blackhawk) were created during a severe drought when sand was blown from dry stream beds into dessication cracks in the organic material. The Blackhawk channel sandstones are usually very competent and provide good roof stability for underground mining. Well-developed secondary porosity and slump material decreases the competence of the sandstone. Monitoring the occurrence and abundance of these features may indicate areas of potential roof collapse.