CHRONOSTRATIGRAPHY OF THE TRACK-BEARING COPPER CANYON UNIT, DEATH VALLEY NATIONAL PARK, CALIFORNIA

Extremely well preserved ichnofossils including tracks and trackways of mammals and birds occur throughout the fluviolacustrine deposits of the Copper Canyon Unit (CCU) exposed within Copper and Coffin Canyons in the Black Mountains of Death Valley National Park, California. No body fossils occur however and the stratigrahic position of this unit has not yet been established. This study addresses the age of the Copper Canyon Unit with radiometric dates tied to magnetostratigraphy. The CCU consists of claystones and siltstones with interbedded carbonates and evaporites that grade abruptly into marginal sandstones and conglomerates. The sequence covers over 3000 meters deposited in a basin formed by large magnitude extension and normal faulting. Uplift has deformed the basin into a large syncline with dips on the limbs up to 70 degrees.

The timing of deposition in the CCU is constrained by Ar40/Ar39 ages from the three laterally extensive basalt flows in the bottom third of the section and magnetostratigraphy on the basalts and sedimentary units sampled along the eastern side of the basin. Total fusion weighted plateau Ar/Ar analyses gave ages of 5.20 ± 0.07, 4.73 ± 0.05, and 4.33 ± 0.07 Ma for the three basalts. An Ar/Ar analysis on micas from a tuff near the top of the section did not reach a stable plateau and is not considered valid. Oriented slabs collected about every five meters were drilled in the lab and thermally demagnetized after an initial series of AF demagnetization steps. Steep dips allowed timing of remanence acquisition to be evaluated. Magnetic polarities from the basalts (C3n.4n to C3n.2n) agree with the current GPTS. Interbedded and overlying sedimentary rocks extend the magnetostratigraphy up into the lower part of chron 2. Tracks are most abundant in the top third of the unit, but are present throughout the fluviolacustrine deposits. They are present in both reversed and normal polarity units and their major stratigraphic control appears to be related to the depositional environment.