arkose - medium-grained poorly sorted arkosic sandstone from the Lower Eocene Wasatch Formation, Sweetwater County, Wyoming - hand/display specimen
Wasatch Formation main body Sweetwater County, Wyoming
An arkosic sandstone is immature, meaning that the grains are not only quartz, but feldspar, granitic rock fragments and often mica. As sediment is transported, the feldspar and micas weather to form clay, with the more resistant quartz becoming the dominant remaining constituent in a pure sandstone.
A sandstone is generally described as an arkose when feldspar makes up 25% or more of the sediment grains, though many geologists call any sandstone where feldspar is present an arkose. The brightly reflecting grains in this arkose are mica. The clasts (sediment grains) are are poorly sorted and not well rounded. Small dust grains form a matrix filling the space between the larger grains, and the orange color is typical of many arkoses.
A fluvial basin-edge depositional environment of this arkose, and weathering of iron bearing minerals to hematite, account for the crossbedding and orange color of the Wasatch at this locality. The field photo shows the crossbedded nature of the outcrop on Highway 191 south of Rock Springs, Wyoming, where this was collected.
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