TUFA MOUNDS, TRACKS AND THE TECTONIC/CLIMATIC EVOLUTION OF THE LACUSTRINE MIOCENE-PLIOCENE COPPER CANYON “FORMATION,” DEATH VALLEY, CALIFORNIA

The Copper Canyon “Formation” (CCF) represents a diverse Cenozoic basin fill sequence controlled by tectonic (uplift rate, accommodation space, and spatial distribution) and climate (precipitation/evaporation P/E ratios and sediment supply) driven deposition within a hydrologically-closed lake. The CCF represents approximately 1800 meters of fanglomerate and fluvial-lacustrine sediments contained and well exposed within a five square mile rectangular fault bounded basin exposed within Copper and Coffin Canyons in the Black Mountains of Death Valley National Park, California. The CCF is significant because it preserves numerous shoreline-playa features including highly abundant and diverse mammal and bird tracks. Three basalt flows, along with a tuff bed stratigraphically above the CCF, constrain the age between ~5 and 3Ma. Initial CCF deposits represent active uplift recorded by numerous fanglomerates deposited as debris flows fining upward into mud-drapes. The fanglomerate provenance suggests a local source and rapid episodic deposition. Fanglomerates become less dominant up section and interfinger with trangressive-regressive playa-lake deposits. Cyclicity of lacustrine sequences is interpreted as humid-wet and arid-dry climate cycles consisting of: evaporite facies (reflecting a hypersaline lake); alternating beds of calcimicrite and dolomicrite (representing alternating fresh and saline conditions); and bioclastic carbonate and limestone beds containing tufa mounds (reflecting active spring deposition). Track distribution and abundance is tied into the appearance of the tufa mounds and associated bioclastic carbonates. Lower in the lacustrine section tracks and bioclastic carbonates rich in gastropods, ostracods and plant fragments are scarce; however higher in the section, where tufa mound deposits appear, tracks and bioclastic carbonates are very abundant. The evolution of the ancient Copper Canyon Lake represents a shift from an under-filled (evaporitic facies) to a balanced-filled (fluctuating profundal) lake. The CCF deposits end abruptly ~3Ma reflecting basin in-filling (loss of accommodation space), probably due to a decline in tectonic activity in Death Valley.