A RECORD OF EOCENE-OLIGOCENE TECTONISM IN THE TITUS CANYON FORMATION, DEATH VALLEY, CA

Death Valley, CA, contains one of the most complete sedimentary records of Cenozoic extension in the Basin and Range continental rift. The oldest syn-extensional formation is the Eocene-Oligocene Titus Canyon Formation (TCF), a terrestrial deposit composed of conglomerates, sandstones, siltstones, and limestones that is exposed in the southern Grapevine and northern Funeral Mountains. The TCF contains a unique assemblage of fossil mammals and potentially records a Paleogene phase of extension not otherwise preserved in Death Valley. Efforts to precisely date the TCF and place its fossil localities in stratigraphic context have been hampered by the TCF’s lateral variability, lack of pristine tuffs for radiometric dating, and structural complexity imposed by multiple generations of normal faults. We present 10 detailed measured sections, comprising 2,600 meters of strata, a 1:24,000 scale geologic map, and seven detrital zircon U-Pb analyses for maximum depositional age (MDA) analysis. Our observations lead us to divide the TCF into two members: an older, finer-grained Redbed Member and a younger, coarser-grained Variegated Member. Robust MDA constraints are only available for samples from the Variegated Member (35.0^+0.5_-0.7 Ma to 30.0^+1.5_-2.5 Ma); samples from the Redbed Member reveal only a Mesozoic provenance signature. Our new MDA ages, when combined with biostratigraphy, indicate that the Redbed to Variegated Member transition coincides with the Eocene-Oligocene boundary. The detrital zircon age spectra indicate a change in provenance between the members, transitioning from a Sierra Nevada Arc source to a mixture of Sierran and recycled Neoproterozoic and Paleozoic miogeocline, possibly due to an increasing contribution from extensionally exhumed local bedrock. Relocated fossil localities are restricted to the Redbed Member, requiring them to be older than the oldest MDA of 35.0 Ma, consistent with the Duchesnean North American Land Mammal Age assignment of the fossils (Lander, 2019). Changes in sedimentary facies, volcanic input, provenance, and fossil content between the Redbed and Variegated Members at ~35-34 Ma may be a reflection of broader climate cooling and/or more rapid continental extension coincident with the Eocene-Oligocene transition.