MARKER BED CORRELATIONS BETWEEN THE MUD HILLS, CALICO MTS., AND DAGGETT RIDGE, CENTRAL MOJAVE DESERT, CALIFORNIA

The early Miocene depositional sequence of volcanic and lacustrine rocks in the central Mojave Desert can be divided into lower, middle, and upper intervals. The middle interval contains distinctive lithologic markers that unite sections recognized in the Mud Hills, Calico Mountains, Lead Mountain, Harvard Hill and at Daggett Ridge. Their extent and depositional environment suggest an expanding lacustrine basin in Late Hemingfordian (late early Miocene) time, that, without reconstruction along late Tertiary strike slip faults, may have covered 777 Km2 (300 square miles). From the lowest, the marker units include a granitic-arkosic unit, green volcaniclastics, a stromatolitic limestone, brown platy limestones, and a horizon rich in strontium and borax minerals. The two former occur on the north margin of the basin while the latter three are recognized throughout the Miocene basin. Coarse clastic and volcanic lenses suggest proximity to highlands of specific lithology. Magnetic polarity data from the section in the Mud Hills that contains these markers suggests that lacustrine deposition in the broader Barstow Basin started around 17 mya and continued through 16.2 mya. Vertebrate faunas in sections to the south and east support this age range. The stratigraphic position of lacustrine siltstone suggests that the center of the broader basin migrated northwest through time. In the Mud Hills, lacustrine siltstone continued to be deposited until 13 mya. The portion of the basin that is now the Calico Mountains was deformed, eroded, and covered with gravity slides of volcanic debris. Throughout the eastern portion of the basin near Harvard, erosion has removed deposits, including the upper two marker units.