Red Rock Park, New Mexico — © Dave Spier
East of Gallup, New Mexico (USA), Interstate 40 passes along the south edge of the reddish Wingate Cliffs composed of Entrada Sandstone. About 2/3rds of the way from Gallup to Wingate, at exit 31, you can access Red Rock Park (originally a state park and now part of the Navajo Nation) nestled into the dissected edge of the cliffs on the west side of Rt. 566. The Entrada Formation is part of the San Rafael Group on the Colorado Plateau. It formed during the Late-Middle Jurassic from cross-bedded sand dunes in a Sahara-like environment around 160 (+/- 20) million years ago. The unit is named after Entrada Point in Emery County, Utah.
In the vicinity of Red Rock Park, the lowest cliff layer was named the Iyanbito member which sits unconformably on top of the soft mudstones of the Triassic-age Owl Rock Member/Petrified Forest Member of the Chinle Formation underlying the valley eroded by the Puerco River and its tributaries. For a technical discussion with a map and rock columns, refer to USGS publication 1395d (1974).
It is unclear whether the Iyanbito has been renamed, but based on information on the New Mexico Bureau of Geology and Mineral Resources (NMBGMR) website, “The Entrada Sandstone [at Red Rock Park] is divided into two members… The Dewey Bridge Member consists of 40-60 ft of … reddish-orange silty sandstone and siltstone that form the base of the massive cliffs. The Slick Rock Member forms the spectacular cliffs and consists of 100-400 ft of reddish-orange well-cemented, thick-bedded, well-rounded sandstones, typical of ancient sand dunes. High-angle crossbeds or layers are seen in the sandstone. The sand dunes were cemented by silica and calcite from ground water and compacted to form the massive rock cliffs seen today.”
Again from the NMBGMR, “The overlying Recapture Member consists of 100 ft of reddish-brown to brick-red siltstone interbedded with white to green to yellow sandstone … well exposed at the base of Navajo Church [Church Rock]… The Recapture Member was deposited in both [river] and [wind] sand-dune environments. The overlying Acoma Tongue of the Zuni Sandstone is the prominent [wind-deposited] sandstone with east-dipping crossbeds at the base of Church Rock (Anderson, 1993). The Jurassic Morrison Formation overlies the Acoma Tongue… The Salt Wash Member…is visible on some of the mesas north of the park and at the top of Navajo Church. This unit consists of 130-230 ft of red to orange sandstone with thin lenses of siltstone and shale… It was deposited in a [river] environment and is host to most of the uranium resources in the Gallup-Grants area.”
The Red Rock Park’s Jurassic deposits are overlain with younger Cretaceous rocks which form high ridges north of the park.
Roadside Geology of New Mexico, by Halka Chronic (© 1987/reprinted 2005), published by Mountain Press
The Iyanbito Member (A New Stratigraphic Unit) of the Jurassic Entrada Sandstone, Gallup-Grants Area, New Mexico, by Morris Green, 1974, USGS publication 1395d at http://pubs.usgs.gov/bul/1395d/report.pdf
New Mexico Bureau of Geology and Mineral Resources Geologic Tour: http://geoinfo.nmt.edu/tour/state/red_rock/home.html based on the following:
Lucas, S.G., Heckert, A.B., Berglof, W.R., Kues, B.S., Crumpler, L.S., Aubele, J.C., McLemore, V.T., Owen, D.E., and Semken, S.C., 2003, Second-day road log from Gallup to Fort Wingate, Sixmile Canyon, Ciniza, Red Rock Park, Church Rock, White Mesa, Thoreau, and Grants, New Mexico Geological Society Guidebook 54, p. 35-68.
McLemore, V.T., 1989, Red Rock State Park: New Mexico Geology, v. 11, no. 2, p. 34-37
If you’re a geologist or geology buff familiar with the park, please comment or contact us. Corrections, comments and questions are always welcome at firstname.lastname@example.org or connect through my Facebook photo page or my personal page, Dave Spier (northeast naturalist). Other outdoor topics can be found on the parallel blogs http://northeastnaturalist.blogspot.com and http://adirondacknaturalist.blogspot.com/