Age, Stratigraphy and Depositional Environment of the Pliocene Copper Canyon Formation, Death Valley, California.

摘要

The Copper Canyon Formation, a fluvial-lacustrine deposit exposed within the Black Mountains of Death Valley National Park, has only previously been broadly included in regional tectonic and depositional analyses. The formation is significant because it preserves numerous shoreline-playa features including highly abundant and diverse mammal and bird tracks. The formation represents a basin fill sequence consisting of approximately 1800 m of conglomerates, basalt flows, and lacustrine deposits. A measured type section along with x-ray diffraction, isotopic analysis, and thin section petrography shows an evolution of the ancient Copper Canyon lake from an hypersaline evaporative lake to a perennial saline lake fed by springs. The Copper Canyon Formation is divided into three new members; the Greenwater Conglomerate, Coffin Canyon, and Barnyard and three new basalt flows; Coffin Peak, Gyp Hill, and Carnivore Ridge based upon stratigraphic and lithologic characteristics. In addition, the Copper Canyon Formation is divided into nine lithologies and five lithofacies. The formation is further divided into the depositional environments: alluvial fan, sandflat, mudflat, ephermeral saline lake, and perennial fresh to saline lake. Tufa mounds reflecting active spring deposition are numerous within the upper two-thirds of the formation. Stable isotope data (18O ranges from -10.15 to -10.96 / PDB) from the tufa mounds indicate the lake was fed by relatively fresh groundwater. Lateral measurements of beds from lake margin to center demonstrates that the tufa mounds formed around the margin of the ancient Copper Canyon lake. Vertebrate track diversity and abundance is tied to the appearance and distribution of the tufa mounds. The spring water allowed a more abundant and diverse fauna/flora population to thrive. Radiometric age dating of three basalt flows within the CCF and magnetostratigraphic analysis of interbedded sedimentary rocks constrain deposition between 5 and 3 Ma. Further stratigraphic works limits the upper Copper Canyon Formation lacustrine deposition in Copper Canyon to between 4.73 and about 3.2 Ma, which is significant because it constrains the timing of the most prevalent animal activity. Age, measurement of a type section, sedimentary petrology along with review of shoreline features and fauna/flora, establishes timing and interpretation of the depositional environment of the ancient Copper Canyon Lake. In addition, a better understanding of the distribution of the animal tracks within the formation is understood. Temporal placement of the Copper Canyon Formation within the other Cenozoic basin fill deposits of Death Valley is also now possible.