NEW ARTHROPOD SITES IN THE MUD HILLS: MIOCENE BARSTOW FORMATION, CALIFORNIA

Three-dimensional silicified arthropod fossils are well known from lacustrine sediments of the Middle Member of the Miocene Barstow Formation. The fossils were originally studied in the Calico Mountains, but recent reports demonstrate the occurrence of Barstow Formation arthropods in the Mud Hills and in Black Canyon. These three outcrops of the Barstow Formation have been faulted into disjunct relationships that are separated by up to 25 kilometers. The Black Canyon arthropods are similar in faunal composition to the arthropods from the Calico Mountains (primarily midge fly pupa, brine shrimp, water beetle larva), but the previously reported Mud Hills arthropods are primarily ostracods, cladocerans and copepods. The reason for this faunal difference is unknown. The new fossil sites in the Mud Hills demonstrate a faunal composition (midge fly adult, brine shrimp, water beetle larva) that is similar to the typical Calico Mountains and Black Canyon fauna. This similarity is important, because it supports the idea that the paleoenvironment was similar across the three disjunct locations (Calico Mountains, Black Canyon, Mud Hills). It is likely that Miocene Lake Calico had a broad aerial extent, and that the unique paleoenvironmental conditions required for three-dimensional arthropod preservation occurred across the entire basin. The new Mud Hills arthropod sites are located in the south limb of the Barstow syncline in Rainbow Canyon and Owl Canyon. They occur near the type section of the Barstow Formation and between two volcanic tuff marker beds: 1) the Rak Tuff (16.3 Ma) and 2) the Oreodont Tuff (15.8 Ma).