Measured Stratigraphic Sections of Uranium-Bearing Upper Triassic Rocks of the Dockum Basin, Eastern New Mexico, West Texas, and the Oklahoma Panhandle with Brief Discussion of Stratigraphic Problems - 1983

Uranium (Nuclear Minerals) / Exploration, Mining, Marketing, & Acquisitions

Finch and Wright reported in 1983 that the stratigraphic sections contained herein were measured as part of the study of the uranium potential of Triassic rocks of the Southern High Plains of eastern New Mexico, west Texas, and the Oklahoma Panhandle [Recent information (here) and (here)]. The study was started by the late James C. Wright in 1967 and continued, after Dr. Wright's death in 1968 by Warren I. Finch from 1968 through 1972.

Most of these sections were made available in draft form to the Bureau of Economic Geology, University of Texas, in 1974 as part of a grant from the U.S. Geological Survey to study the Triassic rocks of west Texas. That unedited draft was referenced as "Open- File Report 1975" in McGowen and others (1979), a product of the grant study. Because of that reference these sections have been requested by the public. Many of these sections were used to construct two geologic cross sections, one north-south in west Texas (Finch and others, 1976a) and the second east-west extending from near Tucumcari, New Mexico to Palo Duro Canyon, Texas (Finch and others, 1976b).

There are 29 measured sections and their general locations are shown on plate 1. Their lines of measurement are shown on portions of either 7.5-minute or 15-minute topographic maps (figs. 1-29). Because many of the section localities are on private land, they are difficult to access. Color names and their numerical designations were determined from a rock-color chart (Goddard and others, 1948, 1970). All sections were measured using a Jacob's staff marked in the English system of units; a calculated equivalent in the metric system is given for each measured unit.

The Dockum basin of eastern New Mexico, west Texas, and the Oklahoma Panhandle is a distinct basin (McKee and others, 1959, pi. 4) and is separated from the Colorado Plateau Chinle basin to the west by absence of outcrops in a belt 25 to 50 miles wide marked by the Rio Grande trough. Correlation of rock units across this gap has not been done in any detail. For this reason extension of the name Chinle across into the Dockum basin is only provisional.

Other Triassic stratigraphic nomenclature in this report is also provisional. Local state and regional names are used with minimum attempts at correlations. The Triassic Dockum basin is in need of extensive research to unravel stratigraphy and depositional history relative to the Colorado Plateau Chinle basin and relative to plate tectonics. This reconnaissance study revealed several areas that unusual changes in stratigraphy take place. Many of these areas seem to correlate with major tectonic structures.

On the west side of the Dockum basin one notes a marked thinning of the Triassic section against the Rocky Mountain front from Las Vegas, NM northward. The Triassic thins about 10 miles (16 km) north of the New Mexico- Colorado border (Johnson and Baltz, 1960), and the stratigraphic sequence recognized at Santa Rosa, NM is not apparent along the mountain front. Another observation along the western edge of the basin is that near the Rio Grande Trough south of Santa Fe thick very coarse conglomerate lies at the base of the Triassic section. In the northern part of the Dockum basin a very dramatic change occurs in the stratigraphy between Triassic rocks that cropout in the area north of Tucumcari and southeast of Las Vegas and those that cropout to the north in the Cimmaron River Valley. The covered area between the two outcrop areas marks the boundary between the Tucumcari basin on the south and the Raton basin to the north.

Along the Canadian River east of Tucumcari at the New Mexico-Texas state line a marked change in the Triassic stratigraphy is noted (Finch and others, 1976b), and this is a very real "state-line fault boundary", which is the cause for problems in matching geologic mapping between the two states, particularly within the Tucumcari 2° quadrangle. Extension of the northeast-trending Bonito fault system (Dobrovolny and others, 1946) crosses the river at the state line.

In addition to changes within the Dockum, one notes regional but distinct changes in Late Jurassic and Cretaceous rocks. The Morrison Formation and Summervilie-like beds extend southward just to the north of the fault and the Entrada Formation barely extends across the fault. The Purgatorie Formation of Early Cretaceous age extends southward only in part across the line and the Edwards Limestone extends from the south northward barely to the line. There appears to have been a long-standing tectonic influence on sedimentation in this part of the Dockum basin.

In West Texas, the broad Matador arch apparently influenced Triassic sedimentation because shale of the Tecovas Formation does not extend over the arch area and there is a thick buildup of sandstone in the lower Dockum just north of the arch (Finch and others, 1976a). These changes may be important to uranium exploration in this area. In the literature the name "Santa Rosa Sandstone" is used extensively for basal Dockum sandstones, particularly in the subsurface. Yet, in the outcrop it appears that the Santa Rosa Sandstone of the type area does not extend to the east much beyond the "state-line fault boundary" mentioned above and to the south beyond Fort Sumner along the Pecos River. Work by McGowen and others (1979) shows a large number of source areas entirely around the periphery of the basin, each generated a local sandstone in the lower part of the Dockum.

Finally, the isolated Triassic conglomerate in the Glass Mountains in Brewster County south and outside the Dockum basin requires new study to describe the tectonic setting of southern edge of the basin. Each of these problems suggests a relation between structural growth and sedimentation. It is important to note that the problem areas are located around the edges of Dockum basin where the thickness of the Dockum is around 500 feet (150 m) or less whereas in the buried central part of the basin the sectionattains thicknesses of as much as 2,000 feet (600 m) (McKee and others, 1959, plate 5). Any research to solve the stratigraphic problems of this basin must include the subsurface well records at the outset.

[For more recent information in the Dockum Basin see (here)]