The Mississippian Escabrosa Limestone

In Southeastern Arizona

(Uploaded 12/27/08)

The Escabrosa Limestone is the Redwall equivalent (lower Mississippian sedimentary rocks in southeastern Arizona). Outcrops primarily in the mountainous regions of Pinal and Pima counties, and also rests unconformably on the Devonian Martin Formation. The Escabrosa Limestone was named by Ransome in 1904, from Escabrosa Ridge near Bisbee. It is a highly resistant cliff forming limestone, occurring prominently throughout the region. Late Kinderhook to late Meramecian in age, it lies within the mid time frame of the Redwall limestone of northern Arizona during the period that the Thunder Springs, Mooney Falls, and Horseshoe Mesa members were being deposited.

The paleo fauna includes the same types as the Redwall, however, the fauna is dominated by brachiopods and rugose corals. Preservation, compared to the Redwall, is very poor and most fossils are badly worn and fragmented, almost inseparable from the host rock. And, as we discovered in the field, the Escabrosa is much less fossiliferous.

The Escabrosa Group is divided into two formations, the Keating Formation, and the Hatchita Formation. The Keating is subdivided into two members as well, the stratigraphically lower "A" member, and the upper "B" member. The "A" member is characterized by consisting of almost pure encrinite. Also, in the upper part of the "A" member, is a highly fossiliferous coral zone, with both silicified and calcified rugose and tabulate corals. Finally, the "A" member is lacking almost entirely in cherts.

The "B" member of the Keating Formation is thin bedded and highly cherty. We have found that many of the cherts have impressions of the encrinite matrix including crinoids and trilobites.

The Upper Hatchita formation is massively bedded, and contains large amounts of ground up crinoid hash and brachiopods. It is a darker gray color, and in some areas consists entirely of crinoidal remains at the exclusion of all other fossils.

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Exploring the Escabrosa in southeastern Arizona. The Highways 80/90 intersection is visible off to the right, and the peak of the ridge is up ahead. We are climbing up to to top, to find silicified fossils in both the cherts and limestones. My wife Dawn is in the foreground.

Viewed from the Highway 80/90 road cut, the beautiful tilted ridge we are climbing in the above image can be seen here. The top of the ridge is Escabrosa Limestone, and about half way down, the color changes to a more ruddy limestone indicates the Mississippian-Devonian Boundary. (The Devonian is the Portal Formation)

On the very top of the ridge at the above locality, the view is stupendous. Numerous reddish cherts in the Escabrosa were very fossiliferous, yielding a host of tiny rhynconellid brachiopods. We collected hundreds of them, each preserved in a limestone matrix and loose on the hilltop.

Spectacular outcrops of the Escabrosa Keating "A" member in the Mescal Mountains south of Globe. The ridge is actually made of countless crinoid fragments, with occasional brachiopods and other invertebrates. The ruddy layers in the bottom half of the ridge, are the Devonian Martin limestone.

After climbing to the top of an adjacent ridge on the opposite side of the canyon, the Escabrosa Keating "A" member was clearly seen. This is the same outcrop as the above image, in the Mescal Mountains south of Globe. We found extensive crinoidal material at the top of the ridge, with several complete crinoids, with holdfasts, stem, cirri, and full calyx and arms in the limestones at the summit!

Another view of the spectacular vista in the Mescal mountains south of Globe. Here, the large multi-armed Saguaro cactuses are growing on a plain of Martin formation. In the distance, the Escabrosa Limestone rises prominently. It is farther than it looks, the ridge is over a mile distant.

Chert nodules in the Escabrosa Keating "A" formation in the Mescal Mountains south of Globe. This characteristic feature was very prominent in beds and in isolated boulders. Large amounts of crinoids are in the limestone matrix.

Example of a complete crinoid, with holdfast, stem, and full calyx in the Mescal Mountains. We found many such "unremovable" specimens in addition to blastoids, urchin material, trilobite pygidia, and many brachiopods here.

Gigantic stromatoporoid found in the Escabrosa Keating "A" formation in the Mescal Mountains south of Globe. It was fully chertized, and weighed in over a hundred pounds. Such invertebrates were fairly rare, and finding one this size was quite a surprise.

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