Thursday, September 27, 2012

To the Edge of the Entrada Erg

The eponymous stone sculptures of Arches National Park in southeast Utah were once sand dunes -- part of the vast Entrada erg of middle Jurassic time (Delicate Arch, National Park Service photo).
The Entrada erg (dune field), now preserved as the Entrada Formation, was huge.  It covered much of Colorado, eastern Utah, northern New Mexico and northeast Arizona.  After millions of years of deposition, followed by burial, lithification, uplift and erosion, the ancient erg now is part of the spectacular sandstone scenery of the Colorado Plateau.
In the McInnis Canyons National Conservation Area near Grand Junction, Colorado, trails wind among domes and alcoves of the Entrada Formation -- remains of ca 175-million-year-old sand dunes.
Entrada sandstone in the McInnis Canyons area is especially amenable to tafoni development (cavities).
Weathering and erosion have brought out the beautiful cross-bedding, as well as tafoni, in this exposure of Entrada sandstone; Twenty-mile Wash, Grand Staircase - Escalante National Monument, southern Utah.
One of the many cross-bedded hoodoo inhabitants of Devils Garden, carved out of Entrada rocks; off the Hole-in-the-Rock Road east of Escalante, Utah.
Even the most extensive ergs do not go on forever, of course.  The Entrada erg was bordered on the north and west by ocean, and was periodically flooded and exposed with changes in sea level.  Instead of massive deposits of sand, there were sometimes low dunes, sometimes tidal flats.  The result is a very different Entrada, like the exposures below at Capitol Reef National Park, located about a half mile west of the east Park entrance.  Cross-bedded sandstone strata -- old dunes -- are present, but are only about a foot thick, and alternate with layers of siltstone, the remains of tidal flats.
Thin veins of gypsum run parallel to and cut across strata.
The Entrada (left center), capped by the greenish-brown Curtis Formation -- deposited during a later marine advance from the north.

Sources and Additional Information

Doelling, H. H. et al.  2000.  Geology of Grand Staircase-Escalante National Monument, Utah.  Utah Geological Association Publication 28.

Fillmore, Robert. 2010. Geological evolution of the Colorado Plateau of eastern Utah and western Colorado, including the San Juan River, Natural Bridges, Canyonlands, Arches, and the Book Cliffs.  Salt Lake City: University of Utah Press.

Orndorff, R.L., Wieder, R.W. and Futey, D.G.  2006.  Geology underfoot in southern Utah.  Missoula, MT:  Mountain Press Publ. Co.

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