The Bolsa Quartzite

(Uploaded 12/27/08)

Early to lower middle Cambrian in age, the Bolsa Quartzite is the Southern Arizona equivalent to the Tapeats Sandstone in Northern Arizona and the Grand Canyon. The 550 million year old sediments can be found South of Tucson down to the southern border of Arizona, primarily in the Southeastern portion of the state. The Bolsa was named in 1904 by Ransome for exposures in the Mule Mountains in Cochise county. It is easily recognized as a dark brownish banded silicaceous quartzite at the base of the Cambrian sequence overlying the Precambrian crystalline basement rocks in the area. It is conformably overlain by the shales and limestones of the lower and middle Abrigo formation.

The Bolsa Quartzite varies in thickness from about 50 feet to over 500 feet. It is thickest in the Huachuca Mountains, and laps out to zero thickness to the north against the Precambrian basement rocks. Sedimentary structures include parallel and cross laminations, cross bedding, color banding, and biogenic trace fossils such as Scolithos and Correphioides. We have not observed any trilobite traces as of yet.

The silicaceous cement in the sandstone is very hard and so tightly bonded that it is difficult to see individual grains. Fossils are very difficult to remove, and usually results in the rock fragmenting into splinters. In a few localities, polygonal mud cracks have been reported, which we were able to confirm in outcrops near Sierra Vista. These images from French Joe Canyon.


Typical outcrop of Bolsa Quartzite from the Benson Area in Southern Arizona. It is normally a light brown with bands up to 12 inches thick of a lighter coloration. This is a Tidal flat and near shore deposit in shallow water since we found abundant Correphioides trace fossils at this locality.

Another view of the Bolsa in the bottom of a creek in a deep canyon. This view near Benson shows a flat surface in the foreground that on closer examination was full of Skolithos trace fossils, those of a bottom dwelling worm like animal.

Because the Bolsa is such a hard material, it fractures along random planes to the original bedding, and huge talus piles of angular blocks form. This is at the bottom of a canyon where a landslide piled the blocks up along the creek. Near Benson.

The Author stands on a tilted slope of Bolsa in the canyon bottom mentioned above, holding large blocks of Skolithos trace fossils. The Mississippian Escabrosa Limestone looms as a white cliff in the background.

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