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Entrada Sandstone
The Entrada Sandstone is a formation in the San Rafael Group that is found in the U.S. states of Wyoming, Colorado, northwest New Mexico, northeast Arizona and southeast Utah. Part of the Colorado Plateau, this formation was deposited during the Jurassic period sometime between 180 and 140 million years ago in various environments, including tidal mudflats, beaches and sand dunes. The Middle Jurassic San Rafael Group was dominantly deposited as ergs (sand seas) in a desert environment around the shallow Sundance Sea.〔(The formation of the Entrada Sandstone: Tectonics, Accommodation space, Wind and lots of Sand ). Written in Stone, 2011〕
==Age and history of investigation==
This formation has been dated to the early to middle Callovian stage of the latest Middle Jurassic.〔USGS GEOLEX database〕 The type locality and place for which the unit is named is Entrada Point, located in the northern part the San Rafael Swell in Emery County, Utah.〔Gilluly and Reeside, 1928〕
The Entrada Sandstone was named as one of the four formations of the San Rafael Group by James Gilluly and Reeside in 1928. In the 1928 description, the Entrada is overlain by the Curtis Formation, and overlies the Carmel Formation. In the Curtis Mountains region of northeastern Arizona, the Entrada is overlain by the Wanakah Formation.〔O'Sullivan, 2003〕 Gregory and Moore worked out the geographic extent of the formation and gave an overview of it in 1931. The extent was revised several times afterwards (most recently in 1992 by Condon). It was divided into the Gunsight Butte, Cannonville and Escalante members by Thompson and Stokes in 1970. The principal reference for the formation was written in 1988 by Peterson.
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