ABSTRACT

Meandering stream channel sandstones and associated conglomerates of the Frontier Formation (Upper Cretaceous) in the Wyoming-Idaho-Utah thrust belt provide an excellent example of provenance (compositional) influence on diagenetic history. Fluvial sandstones in the southern thrust belt (northeastern Utah and southwestern Wyoming) are litharenites and sublitharenites dominated by chert, carbonate rock fragments, and monocrystalline quartz derived from erosion of Paleozoic strata exposed in thrust plates to the west.

Early diagenetic phenomena include compaction, as evidenced by deformed mudstone clasts and micas, and quartz overgrowth development. Subsequent diagenesis is variable and may include precipitation of sparry calcite, clay minerals (chlorite or kaolinite), or a combination of calcite and clay. Porosity values typically range from 3 to 15%. Porosity is dominantly secondary in origin and formed by dissolution of carbonate rock fragments and cement.

Frontier fluvial sandstones in the northern thrust belt (northwestern Wyoming and adjacent southeastern Idaho) are feldspathic litharenites, lithic arkoses, and arkoses derived from erosion of both sedimentary and volcanic terranes. Sedimentary detritus includes chert and carbonate rock fragments; volcanic detritus includes tuffaceous volcanic rock fragments and plagioclase. Diagenesis is characterized by extensive albite, chert, and kaolinite cementation and almost total concomitant porosity occlusion. Rarely, sandstones may possess several percent porosity due to secondary dissolution of detrital plagioclase.

Albite cement occurs as pore-filling aggregates of twinned euhedral crystals rather than as overgrowths, and it is interpreted to have been derived through dehydration reactions involving zeolite precursors such as heulandite or analcime.

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