PALEOENVIRONMENTS OF MIOCENE LAKE BARSTOW: BLACK CANYON, CALIFORNIA

Silicified microcrustaceans (fairy shrimp, copepods) were recently reported from Black Canyon in the northwest corner of Miocene Lake Barstow. This study compares the paleoenvironment associated with these new fossils with the paleoenvironment of a similar slicified arthropod fauna that occurs in the Calico Mountains (about 55 km southeast of Black Canyon). At both study sites, the silicified fossils occur within early diagenetic carbonate concretions.

In order to evaluate the Black Canyon paleoenvironment, petrographic and XRD analysis was performed on individual rock units. Stable isotope analysis (oxygen and carbon) was conducted on primary lake carbonates (micrites) and on the early diagenetic concretions. Calcite was the only carbonate mineral phase observed. Zeolites (clinoptilolite, phillipsite) and gypsum occur in the sediments associated with the concretions. Oxygen stable isotope values varied between -8.6 and -0.8‰ (VPDB). No covariance between oxygen and carbon stable isotope values was observed.

In general, the sedimentologic and geochemical results from Black Canyon mirror the sedimentologic characteristics previously reported from the Calico Mountains. Lacustrine sediments at both sites were deposited contemporaneously and under similar paleoenvironmental conditions in a hydrologically closed, saline/alkaline lake that periodically experienced evaporative conditions.