The Mosby Sandstone Member of the Greenhorn Formation is composed of thin (less than 6 ft or 1.8 m), very fine-grained to fine-grained sandstone cycles separated by shale. The sandstones occur either as individual beds, generally less than 1 ft (9.3 m) thick and separated by interbeds of shale, or as amalgamated beds. The base of each cycle is a planar to undulating erosional

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surface, and the top is gradational with the overlying shale. The dominant sedimentary structures are parallel lamination and low-angle cross-lamination filling broad, shallow troughs. Wave-ripple cross-lamination and wave ripples are commonly developed in the upper parts of the cycles. Trace fossils are generally restricted to the rippled surfaces and consist of horizontal Ophiomorpha and Thalassinoides, Diplocraterion, and Chondrites. Shells, including gastropods, bivalves, and ammonites, occur as lenses near the base of the cycles and as concretions laterally where the sandstones are not developed.

The thin sandstone cycles occur as elongate bodies that are a few tens of miles across and several tens of miles long. The bodies collectively occur in a southward-projecting lobe that covers an area of 40,000 sq mi (64,372 sq km) in central Montana. The sediments were transported as much as 700 mi (1,127 km) in a south-eastward direction from the Dunvegan delta in north-western Alberta. The sandstone cycles are interpreted to have been deposited by storm events on a broad shallow shelf. The sand was probably transported by intense wave action and storm-generated currents and deposited after erosion during the waning stages of the storm.

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