A contour map of the Precambrian surface for a part of northeastern New Mexico has been prepared from aeromagnetic, gravity, and drill-hole data. The area extends approximately from the Colorado border S. to 34 degrees N., and from the foothills of the Sangre de Cristo Mountains E. to 104 degrees W. Thirty-seven depths to Precambrian rocks were computed from aeromagnetic anomalies. Regional gravity anomalies were generally not suitable for quantitative analysis, but the gravity highs correlated with known areas of basement highs, providing a basis for contouring in areas of meager depth control. Drill-hole data provided 61 depths to basement in and near the survey area. The contouring along the E. edge of the Sangre de Cristo Mountains was guided by exposures of Precambrian rocks. A principal feature of the contour map is the Sierre Grande arch, a basement highland that trends SW. across the area to the NW. part of Guadalupe County. Major depressions are outlined W. of Vegas junction, NE. of Santa Rosa, and N. and NE. of Las Vegas. The largest of these, the Las Vegas basin, occupies more than 1000 sq. mi. and may be more than 10,000 ft. deep.

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