Abstract

I wish to thank W.J. Stewart and Texaco, Inc., Midland, Texas, for providing me with fusulinid thin sections and logs on the Alsabrook No. 3 Riley, Gaines County, Texas. I thank A.E. Kauffman, with Exxon Company, U.S.A., Midland, Texas, for logs and fusulinid data for a number of wells in the Midland and Delaware basins which I used as background data. R.G. Todd, with Exxon Production Research Company, Houston, Texas, provided me with much of the data for the cross section in the northern Delaware basin and a copy of his soon-to-be published manuscript on progradational deposition of the San Andres in the Midland basin. To these persons and others too numerous to mention who have helped in formulating some of the ideas suggested herein, I offer sincere thanks.

Sedimentary and other aspects of geologic history in the greater Permian basin are amply documented by the numerous excellent surface exposures and thousands of well sections. Fusulinids have been used extensively for correlation, zonation, and detection of stage boundaries both in the surface and subsurface because of their common occurrence, small size, complexity, and demonstrated biostratigraphic value. Thus, vastly more detailed data are available for these microfossils than for most other Permian fossils. Sufficient biostratigraphic information is available on several fossil groups, however, to suggest that differences in placement of boundaries reflect more personal preference than incomplete knowledge.

In the Midland basin, the base of the Permian commonly is placed arbitrarily because similarly appearing clastic shale above and below this boundary yields fusulinids only locally. In shelf areas (e.g., Central Basin platform), earliest Wolfcampian beds generally are lacking so that middle Wolfcampian carbonates overlie Late Pennsylvanian carbonates. Here, conglomerates, red beds, and glauconite provide physical indication of the boundary, and fusulinid evidence also is abundant. For continuously deposited carbonate environments (e.g., Big Hatchet Mountains of sourthwestern New Mexico) the evidence is so overwhelming that the systemic boundary can be picked precisely.

The Wolfcampian-Leonardian boundary is commonly recognized as the top of the Monodiexodina linearis Assemblage Zone of the Wolfcampian and the base of the Schwagerina crassitectoria Assemblage Zone of the Leonardian. This horizon is easily recognized on the basis of fusulinids and also is a regional tectonic boundary. Attempts to use the base of the Dean sand of the Midland basin as the Wolfcampian-Leonardian boundary because of convenience or possible agreement with brachiopod zonal boundaries recently proposed for the Glass Mountains only serves to distort local sedimentary history. The Dean sand is close to this boundary in the northern part of the Midland basin, but lies a few hundred feet above it to the south. Also, in low areas to the north (e.g., Hobbs channel), the Dean equivalent again lies a few hundred feet above the base of the Leonardian.

The Leonardian-Guadalupian boundary presently is not uniformly defined by many workers. The Road Canyon Formation of the Glass Mountains is considered as Leonardian by brachiopod and ammonoid specialists whereas this unit has been generally regarded by fusulinid workers as an early Guadalupian equivalent of the Cutoff Formation. On the basis of its sedimentary history, the Road Canyon and equivalents might be separated from both the Leonardian and Guadalupian as the Roadian Stage (Furnish, 1966). This, then, would eliminate the need for the three-fold subdivision of the Guadalupian favored by many, but might result in other difficulties. For example, Perrinites is considered by ammonoid workers as a marker for the Leonardian and Roadian stages. The type locality of the type species (P. hilli), however, is on the Eastern shelf in the Dog Creek Formation, the uppermost unit of the San Andres Group. If the Dog Creek is as old as Road Canyon, as ammonoids suggest, then a considerable thickness of San Andres (conventionally middle Guadalupian) of the Midland basin is lacking on the Eastern shelf.

The Brushy Canyon, Cherry Canyon, and Bell Canyon Formations of the Delaware basin yield distinctive fusulinid faunas. At least part of the Brushy Canyon is coeval with the Road Canyon, the Cherry Canyon is a Word equivalent, and the Bell Canyon is Capitan. A profound break in both fusulinid faunas and sedimentation occurs at the top of the Cherry Canyon. Tracing of bentonite horizons here supports fusulinid correlations from shelf to basin.

Identical fusulinid faunas in the Lamar Limestone of the Delaware Basin edge and Tansill Formation on the shelf permit some of the most precise fusulinid correlations for the entire Permian. Evidence from surface sections in Seven Heart Gap of the Apache Mountains suggests that the Lamar-post Lamar beds of the Bel Canyon grade upward into the Castile Formation. Thus, it seems reasonable to suggest that Castile deposition began with water depths of at least a few hundred feet rather than in a shallow, sabkha-type pan.

Extinction of fusulinids near the end of the Permian corresponds with extinctions among other Paleozoic organisms. The tendency is to consider these extinctions as rather abrupt, which carries a connotation of catastrophic biological revolution. Amongst the fusulinids, however, tendency toward extinction of the group began rather early and was accompanied by 1) massive migrations away from former centers of proliferation and 2) experimentation both in shape, size, coiling, tunnel number and position, and in shell composition. Absence in latest Guadalupian rocks of large genera, such as Polydiexodina, and development of minute, complicated forms with possibly different shell compositions suggest gradual but extreme environmental change.

Fusulinologists generally regard Polydiexodina faunas of North America as coeval with those of the Middle East and Asia. These Asiatic so-called Polydiexodinas, however, lack well-developed central tunnels and commonly have discontinuous secondary apertues as in Skinnerina. Skinnerina is early Guadalupian, Eopolydiexodina (Asiatic Polydiexodina) is middle Guadalupian, and Polydiexodina is late Guadalupian. The last corresponds to the lower Yabeina zone elsewhere. The Yabeina globosa zone equivalent in the Permian of the southwestern United States is represented only by Paradoxiella Paraboultonia - Codonofusiella - Reichelina faunas; the minute Yabeina texana of the Lamar Limestone, is probably a dwarfed late representative of the genus. The Lepidolina zone equivalent in North America is probably post-Guadalupian (early Ochoan-early Dzhulfian). Palaeofusulina, not found in North America, occurs elsewhere with Codonofusiella and Reichelina, probably in the late Ochoan (late Dzhulfian) Stage.