Bulletin-77—Geology of the San Pedro Mountains, Santa Fe County, New Mexico

By W. W. Atkinson, Jr., 1961, 50 pp, 2 figs., 10 plates, 1 index.

The San Pedro Mountains are one of a chain of four groups of hills in north-central NM. The chain consists of intrusions of monzonite and related porphyry into sedimentary rocks ranging from Pennsylvanian to Eocene. Igneous rocks in the northern two mountain ranges erupted and were reworked to deposit the Espinaso volcanics, which contain definite Oligocene vertebrate fossils.

The San Pedro Mountains are part of the horst in the Tijeras fault system. The Dooley fault crosses the east end of the mountains. Many faults in the mountains are either nearly parallel to the Tijeras fault system or perpendicular to it, other faults radiate from intrusive centers. Five stocks, two laccoliths, and many dikes and sills intruded the sedimentary rocks. The main rock types are monzonite, monzonite porphyry, latite porphyry, and rhyolite porphyry.

Sedimentary rocks have been extensively metamorphosed by the igneous intrusions, yielding tactite, hornfels, and marble. Important mineral deposits occur in veins and contact metasomatic deposits. The San Pedro mine has been the most important and has produced more than 13 million pounds of copper since 1900, from a contact-metasomatic deposit in limestone. Lead, zinc, and gold have been produced from mesothermal vein deposits. Gold placers have been among the richest in the state. Nonmetallic deposits include quartz sand, garnet, and limestone. The quartz sand is being developed at present. Water is scarce but available from wells at an average depth of 500 ft.

The San Pedro Mountains are about 4 mi long in an easterly direction and as much as 2 mi wide. They are dominated by two eminences, Oro Quay Peak and San Pedro Mountain. These peaks reach altitudes of about 8,200 ft, rising 1,300 ft above the surrounding pediment surfaces. The slopes surrounding the mountains are similar in nature and origin to those around the Ortiz Mountains, a few miles to the north, where they were termeda conoplain. The San Pedro Mountains are part of a chain consisting of four groups of mountains which are aligned in a northerly direction. From north to south they are Los Cerillos, Ortiz Mountains, San Pedro Mountains, and South Mountain.