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Bulletin-31—Geology of the Thoreau Quadrangle, McKinley and Valencia (now Cibola) Counties, New Mexico

By C. T. Smith, 1954, 36 pp., 1 fig., 1 plate, 1 index.

The Thoreau 15-min quadrangle is on the north flank of the Zuni Mountains, McKinley and Valencia Counties, NM. The Zuni Mountains represent a northwest-southeast-trending domal uplift forming the central southern margin of the San Juan Basin, a physiographic division of the Colorado Plateau. Rocks exposed in the Thoreau quadrangle are correlated with formations which are defined either farther north and west on the Colorado Plateau or considerably south and east of the area on the edges of the Permian Basin of NM and west TX. The Yeso Formation of Permian age, the oldest unit exposed in the Thoreau quadrangle, is overlain by the Permian Glorieta and San Andres Formations. These beds are unconformably overlain by strata assigned to the Triassic Chinle Formation which in turn underlies a series of sandstones and siltstones variously correlated with the Glen Canyon Group, the San Rafael Group, and the Morrison Formation of Jurassic age. The marine upper Cretaceous beds of the Dakota and Mancos Formations truncate the underlying beds and form the upper part of the geologic column. Two small patches of basalt are presumed to be of Tertiary or Quaternary age. Recent alluvium and detritus mask the relationship between the older rocks in many places.

Beds in the Thoreau quadrangle dip from 3° to 5° northward or northeastward under the younger rocks of the central part of the San Juan Basin. The gentle homoclinal dip is interrupted by several persistent fault zones of small throw. Structures within the quadrangle appear to be closely related to and controlled by the uplift of the Precambrian core of the Zuni Mountains south of the mapped area.

Extensive deposits of uranium ores are being exploited to the east of the Thoreau quadrangle, and some mining has been done within the quadrangle. The uranium mineral occurrences are confined principally to the Jurassic and Cretaceous rocks. The beds which were particularly favorable to mineralization are shown in red on the geologic map. The ground-water resources of the area have not been developed to their maximum capacity, although conditions favorable to an extensive artesian circulation are present.

Resent discoveries of uranium deposits along the north and east flanks of the Zuni Mountains in McKinley and Valencia Counties, NM, have focused attention upon the need for detailed geologic information in this region. National interest in uranium as well as sporadic production of coal, fluorite, and copper from the Zuni Mountains has prompted the NM Bureau of Mines and Mineral Resources to publish pertinent information as rapidly as it can be assembled.

Gradational contacts between many map units allow considerable latitude in placing geologic boundaries as well as allowing variation in interpreting field relations defined by map symbols. Thus, some individuals may have interpreted a vaguely defined contact as concealed, while others may have interpreted the same contact as approximately located; such differences are particularly common along alluvium contacts. Rock descriptions follow standard terminology and classification. Mudstone is used to describe a poorly-sorted, non-laminated sediment composed principally of clay-size particles. Such a rock differs from shale because of a lack of fissility or laminae and differs from claystone because of the poor sorting.

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