PENN-PERM STRATIGRAPHY AND KARST OF THE MOGOLLON RIM AND SLOPE, ARIZONA

Differences exist between the stratigraphy observed Pennsylvanian and Early Permian strata exposed at the surface along the Mogollon Rim and the equivalent section occurring in wells on the Mogollon Slope. To understand these differences, correlation sections were constructed using surface and subsurface data.

The cross-sections show that the Penn-Perm interval can be divided into two depositional sequences separated by a regional unconformity. In turn, the two sequences are composed of at least twenty-two cyclothems. The cyclothems show depositional environmental change from humid swamp to arid aeolian in a near shore setting.

Sequence 1 is Des Moinesian to Virgil age and is laterally equivalent to the Horquilla Formation of southern Arizona and correlative with the lower part of the Supai Group in the Grand Canyon. Sequence 2 is Wolfcampian to Leonardian age and include rocks of the Amos Wash, Big A Butte, Corduroy, Schnebly Hill and Coconino Formations and is equivalent to the Esplanade Sandstone and Hermit Shale of the Grand Canyon.

The sequences are affected by two dominant diagenetic processes, acid sulfate diagenesis and evaporite dissolution. Acid sulfate diagenesis changed the color, composition, texture and volume of rocks in both sequences with removal of evaporite units also occurs in Sequence 2.

In the subsurface, both sequences contain abundant pyrite created by bacteria in organic rich, water-saturated, sediments. Some oxidization of sulfides occurred during deposition of the individual cyclothems, and the diagenetic products were incorporated into overlying units. However, much of the diagenesis occurred after lithification.

Sequence 1 contains carbonates, carbonaceous clastics, and coals. These units may be the source of the hydrocarbons, helium, and carbon dioxide in the region’s wells

Sequence 2 is a carbonate / evaporate system. In the subsurface, the halite and intra-halite units are preserved and the salt bodies are surrounded by dolines and non-tectonic faults. In outcrop and near subsurface, erosion has removed the salt beds allowing shallower horizons collapse forming a founder breccia. Almost half of the depositional thickness of Sequence 2 has been removed at the Mogollon Rim. Mobilization of halite and sulfate salts from Sequence 2 has had a long-term effect on the region’s groundwater.