Bulletin 125—Petrography and depositional environments of the Lower Ordovician El Paso Formation

By R. E. Clemons, 1991, 68 pp., 34 tables, 49 figs., 2 appendices.

The Lower Ordovician (Canadian) El Paso Formation conformably overlies Bliss Sandstone across most of southern New Mexico and west Texas. Locally, lower El Paso beds overlie plutonic and volcanic rocks. Field examination, sampling, and petrographic study of 38 stratigraphic sections from southeastern Arizona, across southern New Mexico, to Van Horn, Texas, indicate the El Paso was deposited in shallow waters near a shoreline to the north. The southwestern edge of the North American craton had subsided gradually throughout most of Canadian time, receiving the up-to 420-m-thick El Paso carbonate blanket.

The El Paso Formation is subdivided by lithology into four members that are recognized throughout the region. In ascending order, the members are the Hitt Canyon (33-136 m), the Jose (3-33 m), the McKelligon (45-190 m), and the Padre (35-117 m). The gradually decreasing sand content upward through the Hitt Canyon Member indicates the Hitt Canyon was deposited farther from shore in waters that became deeper as the seas transgressed northward. Girvanella oncolites are locally abundant. Stromatolite mounds near the top of the Hitt Canyon and the presence of sand, ooids, and rounded bioclasts in the Jose Member record a shoaling environment. The McKelligon Member contains little or no sand; at several locales sponge-Calathium mounds and stromatolite mounds are prominent. The Padre contains a more restricted fauna that includes traces of ostracods. Beds in the lower part of the Padre Member are silty to sandy and locally contain thinly laminated zones.

There is no correlation between stratigraphic members of the El Paso Formation and occurrences of dolostone. Beds in the lower part of the Hitt Canyon Member are typically dolostone, but are limestone at 25% of the sections studied. The El Paso Formation is mostly to completely dolostone at Beach Mountain near Van Horn, Texas, at Bishop Cap, and in the Sacramento, San Andres, Oscura, and Pedregosa Mountains. The El Paso is predominantly limestone at San Diego Mountain and in the Caballo and Mud Springs Mountains, 60 km west of the Bishop Cap-San Andres-Oscura Mountains trend. At other locales, the limestone/dolostone ratios vary with no apparent geographic relationship. Locally, single beds of dolostone are interbedded with limestone sections. More typically, dolostone or limestone prevails for tens of meters of section. Evidence of supratidal dolostone is lacking except in some beds in the Jose and Padre Members. Hydrothermal dolostone is common in several sections.

Pervasive bioturbation of El Paso Formation beds and a varied biota of echinoderms, sponges, gastropods, trilobites, Nuia, Calathium, cephalopods, and algae (plus minor brachiopods, Pulchrilamina, and ostracods) indicate a predominantly shallow-subtidal environment. Intertidal conditions prevailed intermittently, and possibly supratidal deposition took place during parts of Jose and early Padre times. Low-energy platform environments, in which a large volume of micritic muds accumulated, were often disturbed by storms that produced abundant thin, poorly washed biosparite, intrasparite, and intrasparrudite (bioclastic and intraclastic grainstone) lenses.