Geology and petrology of the Lava Mountains, San Bernardino County, California

Abstract

The Lava Mountains are a range of low mountains along the northern edge of the Mojave Desert, California. The pre-Tertiary rocks consist of a few small pendants of metamorphic rocks in Atolia quartz monzonite. Overlying these are small patches of volcanic and sedimentary rocks, probably Tertiary, which project into the later formations. The major sedimentary unit is the middle Pliocene Bedrock Spring formation; it consists chiefly of arkosic sandstone and conglomerate with lesser amounts of siltstone and brecciated volcanic rocks. Overlying this formation are two late Pliocene formations, the Almond Mountain volcanics and Klinker Mountain volcanics in the eastern and western parts of the area, respectively. Small areas of other late Pliocene(?) volcanic rocks are locally present. Overlying these are flows of the Lava Mountains andesite of very late Pliocene age. The Pleistocene(?) Christmas Canyon formation is restricted to the eastern half of the area. A few small dikes of basalt cut this formation. Quaternary gravels, alluvium, and travertine are the youngest deposits. Within the area, three fault systems converge. The Garlock fault trends N. 75° E. along the north side of the area; it is a left lateral fault. The Blackwater fault trends N. 45° W. in the southeastern part of the region; it has right-lateral and normal displacements. The Brown's Ranch fault zone and its associated faults trend about N. 55° E. in the western part of the area; both left-lateral and dip-slip displacements are found. The Dome Mountain anticline trends parallel to and south of the Garlock fault. Remnants of late Cenozoic pediments indicate that the drainage at that time was toward the north; later, the eastern part of the region warped, thus blocking the previous drainage direction. No commercial mines are in the area although extensive prospects are present in the southwestern portion. All of the volcanic rocks are plagioclase andesite porphyries. Plagioclase (3-23 percent), biotite (0-2 percent), orthopyroxene and clinopyroxene (0-2 percent), quartz (0-3 percent), and opaques (1-11 percent) are found as crystals in a partly-crystalline to glassy groundmass. As the Pliocene volcanic sequence progressed, the lavas became more homogeneous, more effusive, and more frequently erupted. The mineral and chemical compositions did not change systematically. The melt originated by the fusion of quartz monzonite. There is no evidence of fractionation, crystal settling, or continued assimilation during the volcanic sequence.