SILICIFIED MICROFOSSILS FROM BLACK CANYON: MIOCENE BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIFORNIA

Silicified three-dimensional microfossils have been reported from several disjunct outcrops of the Middle Member of the Barstow Formation. Most reports have described silicified arthropods and other microfossils occurring in limestone nodules from lacustrine mudstones in the Calico Mountains. Recently a similar microfauna has been described from the type section of the Barstow Formation in the Mud Hills (30 km north west of the Calico Mountains). This paper reports similar microfossils from similar limestone nodules that occur in the Middle Member of the Barstow Formation in the Black Canyon area (55 km northwest of the Calico Mountains).

SEM analysis of acid dissolution residues from the Black Canyon limestone nodules reveals exquisite three-dimensional preservation of brine shrimp tissues such as second antenna and reproductive cysts. Other soft tissues preserved at the site include three-dimensional midge pupae and ostracod appendages. Pennate diatom frustules from the nodules are referable to the following genera: Amphora, Nitzschia and Anomoeoneis. These pennate diatoms indicate a slightly alkaline lacustrine paleoenvironment.

Stratigraphic sections were measured in Black Canyon, in the Mud Hills and in the Calico Mountains. Analysis of the measured sections shows that unique marker beds can be used to confidently correlate the new fossil site (in Black Canyon) with the type section of the Barstow Formation in the Mud Hills and with the type location of the silicified microfossils in the Calico Mountains. Significant marker units include: 1) a thick microbial limestone near the base of the sections, 2) the silicified microfossil horizon and 3) the distinctive Oreodont Tuff zone.

This report is a significant extension of the known area of the Barstow Formation that contains silicified three-dimensional microfossils. This report illustrates the widespread nature of the unique geochemical conditions that caused the three-dimensional soft tissue mineralization of small aquatic organisms in the sediments of Miocene Lake Barstow. This report also emphasizes the use of this fossil horizon in correlating disjunct outcrops of the Middle Member of the Barstow Formation.