The El Paso Limestone

In Southeastern Arizona

(Uploaded 12/27/08)

The El Paso Limestone lies unconformably on the Coronado Quartzite in Greenlee county, and on the Abrigo and Bolsa Quartzite in the Dos Cabezas Mountains. It can attain thicknesses of several hundred feet, and is upper Cambrian to lower Ordovician in age. Of great interest to us was the Ordovician component at Dos Cabezas, where we found fossils of gastropods, and orthocone cephalopods replaced with red chert in a limestone matrix. Most of the Ordovician rocks were removed from Arizona by erosion, and what little remains is in isolated outcrops in remote localities.


Gordon Nelson points out the ridge like outcrop in the distance in the foothills of the Dos Cabezas Mountains that is the El Paso Limestone. The Abrigo is off to the right down the hill, which is the Cambrian. The outcrop is only a few hundred feet thick here, and the exposure is small. Rick Hill, a "paleo-astronomer" such as myself, is on the right.

Another view at the base of the Dos Cabezas of the Abrigo Limestone straight ahead, and the Ordovician outcrop to the left.

My wife Dawn picks through the outcrop on the ridge to find good specimens of Ordovician flat spiral shaped gastropods, which filled the rocks in this area. The rock in outcrop is dark and weathered, with a coating of desert varnish.

The red specks in this rock are parts and whole gastropods. They occurred in specific layers on the ridge, and were largely ground up pieces of thin shell sections.

A close up of an orthocone cephalopod (Orthoceras??) found in situ. We were able to remove it in one piece, but found out that this limestone is like pounding on concrete to free specimens!

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