Bulletin 79—Paleozoic and Mesozoic strata of southwestern and south-central New Mexico

By F. E. Kottlowski, 1963, 100 pp., 2 tables, 18 figs., 1 index.

Paleozoic strata, totaling 8,000–11,000 ft in thickness for relatively complete sections, consist of the Cambrian and Ordovician Bliss Sandstone which thins depositionally northward; the Ordovician El Paso Limestone and Montoya Dolomite; the Silurian Fusselman Dolomite; argillaceous Devonian rocks; crinoidal Mississippian limestones; a thin to thick Pennsylvanian sequence; and thick, largely marine Permian strata with basal beds marked by a southward transition from red beds into limestones. Pre-Pennsylvanian units thin northward chiefly because of various erosional episodes but are thick in southern NM. More than 3,000 ft of Pennsylvanian beds were deposited west of the Pedernal landmass in north-south-trending basins such as the Orogrande and Pedregosa. Reefoidal masses locally characterize the Ordovician, Mississippian, Pennsylvanian, and Permian limestones.

The early Mesozoic was a time of uplift and erosion which accelerated during the Early Cretaceous when all Paleozoic rocks were stripped from the Burro uplift in western Grant County and all the upper Permian beds removed throughout large areas of southwestern NM. Early Cretaceous rocks are 7,000–20,000 ft thick in the southwestern part of NM and consist of red beds, shoreline sandstones, nearshore conglomerates, fossiliferous biohermal limestones, and volcanic detritus. The northernmost shoreline of this Early Cretaceous sea may have paralleled an east-southeast-trending line from Silver City toward the Cornudas Mountains. Late Cretaceous sediments, the Dakota Sandstone, Mancos Shale, and Mesaverde Formation, are about 2,000 ft thick and rest on middle Permian beds or on a northward-tapering edge of Early Cretaceous strata. The dark Mancos Shales of south-central NM grade southward into the shaly Eagle Ford sandstones near El Paso. Thick sections of latest Cretaceous rocks near Steeple Rock and near Elephant Butte Dam are mainly conglomeratic volcanic sediments. Erosion during Tertiary time locally removed the Cretaceous strata and cut down to lower Permian beds.

There may be more unknown than known facts on the Paleozoic and Mesozoic stratigraphy of southwestern and south-central NM but enough detailed new work has been done in the past 10–15 years to allow this study to be reasonably accurate, and to provide a springboard for future attacks on the more pressing problems. A summary of the conclusions from this study was presented before the Southwestern Regional Meeting of the American Association of Petroleum Geologists at El Paso, TX, November 3, 1961, as part of a symposium on the sedimentary and tectonic framework of northern Mexico and southwestern US. Requests for more detailed information than could be given in a talk led to the compilation of this report.

The basin-and-range country of southwestern and south-central NM consists of isolated fault-block ranges scattered like islands amid a sea of sandy, semiarid plains. On the northwest is the Datil-Mogollon plateau where thick flows and huge pyroclastic masses of volcanic rocks have buried the pre-Tertiary strata. On the east is the Sacramento section of the Basin and Range province where long, narrow mountain ranges are separated by long, wide basins. In the central part of the area, the Rio Grande structural depression appears to die out southward against the syncline on the west flank of Sierra de Las Uvas. South and west of Silver City, Deming, and Las Cruces complex structure is the rule: thrust faults, klippe, gravity faults, and many volcanic features are seen everywhere in the bedrock that is exposed above the Bolson plains. One can merely hope that the pre-Tertiary strata buried in the Bolsons have not been deformed greatly and have not been subjected to severe erosion during earlier Cenozoic time.