Several cross-stratified ooid and bioclastic grainstone layers are present in the upper part of the cyclically deposited Mississippian Lodgepole Formation in central

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Montana. Two of those exposed in the Big Snowy Mountains were selected for a detailed study of diagenesis in ancient carbonate sandstones.

Evidence suggests that these sediments were deposited in a shallow marine-shelf environment. Diagenesis of the carbonate sands began with cementation in the intertidal or submarine environment. Partial cementation was followed by partial silicification of bioclastic debris and ooids. No definite order could be determined for later diagenetic events; they may have proceeded more or less simultaneously. Dolomitization appears to have postdated silicification and shows preferential replacement of ooid coatings and mud. The sandstones are compacted and stylolitized, and pressure solution may have served as a source for the coarse, blocky calcite which fills the remaining pore space. The distinctive red color of the upper grainstone layer may be due to recent weathering.

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