FACIES CONTROL ON FOSSIL PRESERVATION IN THE BARSTOW FORMATION, MOJAVE DESERT, CALIFORNIA

The Barstow Formation (middle Miocene) crops out in the central Mojave Desert, notably in the Mud Hills near Barstow, California. The Barstow Formation preserves mammals that define the Barstovian North American Land Mammal Age and contributes substantially to our understanding of middle Miocene mammalian faunal change. The majority of fossil localities in the Barstow Formation occur in the (unnamed) upper and middle members, and few occur in the basal Owl Conglomerate Member, although vertebrate trackways are preserved low in the section. These three members of the Barstow Formation are distinguished by major lithologic differences and a general upward-fining trend, and they can be further divided into several major facies whose distribution changes throughout the section. Based on lateral correlations of 31 sections measured in the field and compiled from the literature, we recognize six major facies in the Barstow Formation of the Mud Hills: (1) amalgamated coarse sandstone and conglomerate, (2) alternating interbeds of fine sandstone, mudstone, and marl, (3) sandstone, siltstone, and nodular marl, (4) stacked, multi-story sandstone, (5) bentonitic and weakly pedogenic mudstone, and (6) siltstone and marl with numerous root casts. These facies represent deposits of alluvial fans, shallow or seasonal lakes, and floodplains in an internally-drained asymmetrical basin.

Fossil concentrations are common in two of the six facies. Vertebrate body fossils are most abundant in facies 5 and 6, which are prevalent in the upper part of the Barstow Formation. We interpret these facies as having formed in spring-fed wetlands and adjacent better-drained floodplain. Body fossils are absent or rare in facies 1, 2, 3, and 4, which make up the majority of deposits in the lower and middle Barstow Formation, but vertebrate trackways occur in facies 2. These facies represent active alluvial environments and the margins of shallow lakes. The transition from coarse-grained, fan-dominated landscapes to fine-grained floodplain environments reflects basin-wide changes in depositional settings during the filling of the Barstow Basin. As the basin filled, the developing floodplain environments attracted mammals and may have provided settings more conducive to fossil preservation than had existed earlier in the basin’s history.