TUFA MOUND DEPOSITS IN THE FLUVIAL-LACUSTRINE COPPER CANYON UNIT, DEATH VALLEY NATIONAL PARK, CALIFORNIA

The Copper Canyon Unit (CCU) is a 5-3Ma fluvial-lacustrine deposit exposed within Copper and Coffin Canyons, Black Mountains, Death Valley National Park. The sequence includes over 3000 meters of basin sediments deposited in a tectonic setting involving large magnitude extension, normal faulting, basin formation, deposition, and subsequent uplift. The fluvial-lacustrine deposits can be further divided into: evaporite facies; bioclastic carbonate facies; conglomerate/sandstone facies; and carbonate mudstone facies. Tufa mounds ranging from 5-30cm in diameter and height occur on several bedding planes within the bioclastic carbonate facies and associated with the carbonate mudstone facies. The tufa mounds consist of a central micritic pipe surrounded by porous calcium carbonate precipitate. The original internal structure in cross section consists of porous and crystalline (thinolite) fabric often associated with aquatic vegetation. Up to 25% of the original internal fabric has been silicified during diagenesis and infilled with chalcedony. Gastropod, ostracod and plant fragment coquinas occur in the same bedding plane with the tufa mounds. Fringe cements consisting of several layers of finely laminated micrite often cover the plant material. Additionally many of the gastropods and ostracods often serve as the nucleus of peloids. The tufa mounds accumulated in response to the precipitation of calcite when fresh calcium-rich spring waters mixed with bicarbonate-rich lake waters. The abundance of the tufa mounds and associated fauna suggest a standing body of freshwater for a sufficiently long period of time to allow tufa mounds to form and a fauna/flora population to be established. The tufa mounds occur stratigraphically above the carbonate mudstone facies that contains shoreline features including mammal and bird tracks suggesting a playa lake. Several cycles of tufa mounds followed by carbonate mudstone exist within the CCU further suggesting the ancient lake went through several perennial lake cycles, each followed by ephemeral, evaporate conditions.