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The Bighorn Dolomite, which is 38-53 m (125-174 ft) thick, lies disconformably between the Cambrian Gallatin Limestone and Mississippian
Madison Limestone. It is approximately equivalent to the productive Red River Formation in the Williston basin. Above the basal Lander Sandstone, the unit is massive to slabby (toward top), fretted-weathering, locally vuggy to cavernous, low-insoluble, finely crystalline dolomite. An abundant marine fauna and pervasive, bedding-controlled Thalassinoides-type burrow patterns indicate a subtidal environment of normal salinity for the original carbonates prior to dolomitization.
The positive elements of the fretted-weathering pattern represent burrow fillings, and consist of xenotopic, decimicron-size dolomite; the recessed areas, or "matrix," are composed of xenotopic to idiotopic dolomite. Whereas the matrix is also chiefly decimicron size, the inclusion of more skeletal debris (particularly pelmatozoan fragments) than in the burrow fillings contributes to the coarses texture of the matrix. However, the significantly greater intercrystalline to mesovuggy (mesomoldic?) porosity in the matrix results in its lesser resistance. Where weathered tubes are even with or recessed below the matrix, pores in the matrix are filled with chalcedonic quartz. Well-preserved fossils, relatively fine crystallinity, conodont color-alteration index (1), and oxygen-isotope data (x = -4.7% PDB) strongly suggest early diagenetic dolomitization of both burrow fillings and matrix at relatively shallow burial depths. Isotope values and pseudomorphically replaced pelmatozoan fragments suggest mixing-zone effects. The widespread Bighorn has generally similar characteristics throughout much of its extent in the northern Rocky Mountain area.
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