Abstract

King et al (1945) used the term Magdalena to include all strata of Pennsylvanian age in the Hueco Mountains and subdivided it into three lithologic subdivisions designated as lower, middle and upper. The Magdalena Group unconformably overlies the Mississippian Helms Formation and is unconformably overlain by the Permian Hueco Group. Recent fusulinid data (Williams, 1963) indicates that the upper 200 feet of the Magdalena Group north of Hueco Canyon is Lower Permian in age.

The lower division of the Magdalena Group of Morrowan and Atokan age is 500 feet thick and consists predominantly of light to dark gray, massive bedded, cherty, crinoidal and coralline biomicrites.

The middle division of the Magdalena Group is Atokan and Desmoinesian in age (King, 1945) and consists of 300 feet of fossiliferous marl, shale and thin limestones.

The upper division of the Magdalena, of Desmoinesian, Missourian, and Virgilian age, is 500 feet thick on the north side of Powwow Canyon where it consists of distinct cycles of deposition. In general, the upper division reflects a change back to a dominantly carbonate environment that was periodically interrupted by thin conglomerates and fine clastics from erosion of local highlands that began to develop at this time.

The base of the Permian Hueco Group in the vicinity of Powwow Canyon is marked by a major unconformity that increase in magnitude southward where the group rests on underlying rocks ranging from Pennsylvanian age downward to Cambro-Ordovician age. Williams (1963) renamed the Hueco Limestone the Hueco Group and assigned the lower, middle, and upper divisions of King et al, (1945) as formations. The lower division was named the Hueco Canyon Formation, the middle unit became the Cerro Alto Limestone, and the upper division was named the Alacran Mountain Formation respectively. The Hueco Canyon Formation and the Cerro Alto Limestone are both Wolfcamp in age. Williams (1963) places the Wolfcamp-Leonard boundary within the upper part of the Alacran Mountain Formation.

The Hueco Canyon Formation consists of a basal conglomerate and red bed sequence (Powwow Member) followed by 600 feet of olive-gray, medium and thick-bedded limestones, containing abundant stylolites and chert nodules at certain levels. The Cerro Alto Limestone consists of 460 feet of light to medium gray, thin and medium bedded limestone that weathers to gently rounded hills. The Alacran Mountain Formation is very similar, both lithologically and faunally, to the Hueco Canyon Formation. It has a thickness of 622 feet and, except for the 122 foot thick Deer Mountain Red Shale Member, consists of light olive-gray and olive-gray, medium to thick-bedded limestone.