Description
This report is of a previously unmapped 30 square mile area in a northern portion of the &ierra Juarez, Baja California, Mexico. The oldest rocks are a metamorphic series that includes quart-mica-schist, muscovite-quartz-phyllite, gneiss, quartzite and marble. They probably correspond to the Triassic{?) age Julian schist series in San Diego County, California. Tonalite was emplaced during Cretaceous (?) time, probably as at least three separate plutons. Two 0£ these plutons assimilated part of the country rock, making a mixed rock unit which consists of two types of tonalite, metamorphic rocks, and an abundance of inclusions. Tonalite with a different composition than either of the tonalites in the mixed rocks has been mapped as a separate unit. Three sedimentary formations were deposited on the irregular, eroded surface of the metamorphic and plutonic rocks. Tertiary (?) age, Table Mountain Formation conglomerate is capping many of the metamorphic and plutonic rock hills, and extends down into the central valley of the mapped area. Late Cenozoic (?) siltstone and sandstone lie unco11formably on the Table Mountain Formation in places in the valley. A lacustrine formation that probably formed within a depression in the sandstone facies of the siltstone and sandstone fonnation, is found in a limited area at the eastern edge of the sedimentary rocks. Pegmatites are abundant in the mixed rocks and metamorphic rocks, and less common in the tonalite. Several quartz veins, each several feet thick and more than onô€€¨ hundred feet long are located in the metamorphic rocks. The foliation and schistosity of the metamorphic rocks has a dominate northeast trend which suggests a northwest southeast stress is responsible £or some of the metamorphism. Contact metamorphism also probably influenced the character of these rocks. Marble is presently being mined to make cement, and it is possible economically profitable gold, tungsten, and quartz deposits will be developed or discovered and mined in the future.
