Entrada Sandstone

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File:Carmel Formation below Entrada Sandstone along Park Avenue in Arches NP.jpeg

Entrada Sandstone conformably overlies the Carmel Formation, Park Avenue, Arches National Park

The Entrada Sandstone is a formation in the San Rafael Group 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.<ref name=Peterson1988>Template:Cite journal</ref> The Middle Jurassic San Rafael Group was dominantly deposited as ergs (sand seas) in a desert environment around the shallow Sundance Sea.<ref name="LucasAnderson1997">Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>Template:Self-published inline

DescriptionEdit

At its type section at Entrada Point, located in the northern part the San Rafael Swell in Emery County, Utah,<ref name=GillulyReeside1928/> the Entrada consists of red silty sandstone and lesser interbedded mudstone and is a slope-forming formation. This part of the Entrada is sometimes described as the "earthy facies".Template:Sfn Here the Entrada is overlain by the Curtis Formation, and overlies the Carmel Formation.<ref name=GillulyReeside1928/> To the south and east, the Entrada transitions to cliff-forming red or white crossbedded sandstone, sometimes called the "slickrock facies". This is actually more typical of the Entrada as a whole, and a principal reference section including both facies was designated by Peterson in 1988 at Pine Creek, Template:Convert north of Escalante, Utah, in the Kaiparowits Basin.Template:Sfn

At the reference section in the Kaiparowitz Basin, the Entrada is Template:Convert thick and is divided into three informal members. The lower member is Template:Convert of orange-red silty sandstone, with occasional beds of red mudstone, corresponding to the earthy facies. The middle member is Template:Convert of red to green mudstone interbedded with red to white sandstone. The upper member is Template:Convert of crossbedded white sandstone.Template:Sfn The white color is attributed to bleaching by organic-rich fluids from overlying beds. The formation rests on the Carmel Formation and is overlain by the Morrison Formation.Template:Sfn

In the Curtis Mountains region of northeastern Arizona, the Entrada is overlain by the Wanakah Formation.<ref name=OSullivan2003>Template:Cite journal</ref>

In the San Juan Basin of northwestern New Mexico, the Entrada consists of upper and lower sandy members and a medial silty member, the Rehoboth Member. The Curtis Formation is sometimes absent and the Entrada then overlies Chinle Formation. It is overlain by the Todilto Formation. Southeast of Fort Defiance, Arizona, the lower sandy beds are assigned to the Iyanbito Member. Total thickness is up to Template:Convert.<ref name=OSullivan2003/>

In the Slick Rock, Colorado area, the Entrada is divided into a "middle sandstone", the Rehoboth Member, and the Slick Rock Member, in ascending order.Template:Sfn

The Entrada has been dated to the early to middle Callovian stage of the latest Middle Jurassic.<ref name="CondonEtal1988">Template:Cite journal</ref>

Entrada Sandstone capped by Curtis Formation in Cathedral Valley.jpg

Entrada Sandstone overlain by Curtis Formation in Capitol Reef National Park's Cathedral Valley
Entrada at Curtis Pt.jpg

Entrada Formation at Curtis Point, Emery County, Utah.

History of investigationEdit

The Entrada Sandstone was named as one of the four formations of the San Rafael Group by James Gilluly and Reeside in 1928.<ref name=GillulyReeside1928>Template:Cite journal</ref> Gregory and Moore worked out the geographic extent of the formation and gave an overview of it in 1931.<ref name=GregoryMoore1931>Template:Cite journal</ref> The extent was revised several times afterwards (most recently in 1992 by Condon).<ref name=Condon1992>Template:Cite journal</ref> It was divided into the Gunsight Butte, Cannonville, and Escalante members by Thompson and Stokes in 1970.<ref name=ThompsonStokes1970>Template:Cite journal</ref> The principal reference for the formation was written in 1988 by Peterson.Template:Sfn

MembersEdit

Entrada members are (in alphabetical order):

Cannonville Member (UT)<ref name=ThompsonStokes1970/>
Cow Springs Member (AZ); equivalent to the upper member at the reference section in southern UtahTemplate:Sfn
Dewey Bridge Member (CO, UT) - named after the type locality at Dewey Bridge. This brick-red layer has a blocky look to it.<ref name="WrightEtal1962">Template:Cite journal</ref> Now assigned to the Carmel Formation.<ref name=OSullivan2003/>
Escalante Member (UT)<ref name=ThompsonStokes1970/>
Henrieville Member (UT);<ref name=ThompsonStokes1970/> abandoned by Peterson in 1988.Template:Sfn
Exeter Member (NM)<ref name=LucasEtal1985>Template:Cite journal</ref>
Gunsight Butte Member (UT)<ref name=ThompsonStokes1970/>
Iyanbito Member (NM)<ref name="Imlay1980">Template:Cite journal</ref>
Moab Member (CO, UT) or Moab Tongue (CO, UT) - named after the type locality of Moab, Utah. The whitish sands from inland dunes make up this "cap rock" layer, as seen atop Delicate Arch and Broken Arch in Arches National Park.<ref name="WrightEtal1962"/>
Red Mesa Member (AZ, NM, UT)Template:Sfn
Slick Rock Member (CO, UT) - named for the type locality at Slick Rock, Colorado; rounded beach sands were cemented together to create this uniform layer.<ref name="WrightEtal1962"/>