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Memoir 8—Stratigraphy and Paleontology of the Mississippian Sytem in southwestern New Mexico and adjacent southeastern Arizona

By A. K. Armstrong, 1962, 99 pp., 41 figs., 12 plates, 1 index.

The Mississippian system was studied in western Cochise County, Arizona, and in Luna, Hidalgo, and Grant Counties, New Mexico. The primary concern was with the Osage through Meramec Escabrosa Limestone, its stratigraphy, paleoecology, and biologic contents. The Escabrosa Limestone is regarded as a group and has been divided into two formations. These are, in ascending order, the Keating Formation, and the Hachita Formation. The Escabrosa Group has a minimum thickness of 650 ft in the Peloncillo Mountains and a maximum thickness of 1,000 ft in the Big Hatchet Mountains of New Mexico. In the area of this report, it is primarily an encrinite with minor amounts of microcrystalline limestone. These sediments represent almost continuos deposition through all of Osage and Meramec time. The strata were deposited over a slowly sinking shelf area in shallow normal-marine waters.

The corals, brachiopods, blastoids, and endothyrids collected from the Escabrosa Group are described and illustrated. The following new species were found: Chonetes klondikia, Unispirifer balki, Amplexizaphrentis sonoraensis, A. northropi, Vesiculophyllum sutherlandi, Lithostrotionella lochmanae, and Michelinia leptosphragma.

The late Meramec to middle Chester Paradise Formation was studied and part of its brachiopod fauna described. The Paradise Formation is an alternating series of medium-bedded limestones and shales, with a maximum thickness of 220 ft in the Big Hatchet Mountains. It thins rapidly to the north and to the west, and is only about 80 ft thick in the eastern Chiricahua Mountains of Arizona. The Helms Formation of Chester age was studied in the Franklin Mountains and its coral fauna, including the new coral species Koninickphyllum elpasoensis, described.

The Mississippian system in southwestern New Mexico and southeastern Arizona is represented by the Escabrosa Limestone and by the locally present Paradise Formation. Previous published material on the Escabrosa Limestone was lithostratigraphic studies done primarily in central Cochise County, Arizona. The extent and thickness of the Escabrosa Limestone in New Mexico was an enigma. The Escabrosa Limestone is part of the extensive Cordilleran Osage-Meramec sequence. The fossil contents of these strata are little known. In the past these rocks have been correlated in broad generalities with the Mississippian type sections of the Mid-continent region. This generalized attitude was expressed for the sections in central Cochise County, Arizona.

The biostratigrapher is confronted with two problems when he attempts to zone the Escabrosa Limestone and related formations in the Cordilleran region. The first problem is the poor preservation and fragmentary nature of the fossils within these rocks. Fossils are not abundant and are generally difficult to extract from the outcrop. The majority of the specimens are so poorly preserved that when they are studied in the laboratory, generic-level identification is difficult.

The second problem is that the fauna consists of two elements apparently derived from two biologic provinces. The brachiopod fauna consists of a large number of forms which are conspecific with described species from the Mid-continent region. In the Cordilleran region there is no assurance yet that these species have exactly the same stratigraphic range as they have in the Mississippi Valley. Furthermore, in many areas in the Cordilleran region there was continuous deposition and a stable environment throughout Osage and Meramec time. Within these sections, the generic lineages are vertically a series of intergrading species.

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