Provenance of Eocene river sediments from the central northern Sierra Nevada and implications for paleotopography
Geochronology of fluvial deposits can be used to characterize provenance, the paleotopography of sediment source regions, and the development of regional drainage systems. We present U-Pb and (U-Th)/He ages of detrital zircon grains from Eocene gravels preserved in several paleoriver systems along the western flank of the central and northern Sierra Nevada. These ages allow us to trace the sourcing of detritus in paleorivers and to constrain the evolution of the Sierra Nevada range front. U-Pb zircon age distributions are bimodal, with a dominant peak between 110 and 95 Ma and smaller but significant peaks in the Middle to Late Jurassic, matching the predominant ages of the Sierra Nevada batholith. A small fraction (<6%) of grains has pre-Mesozoic ages, which consistently match ages from prebatholithic assemblages within the northern part of the range. (U-Th)/He ages of a subset of double-dated zircons cluster between 114 and 74 Ma and are consistent with batholithic (U-Th)/He cooling ages in the northern Sierra. Our results indicate that the Eocene river systems in the central northern Sierra Nevada likely had proximal headwaters and had relatively steep axial gradients, draining smaller areas than was commonly thought. This also suggests that the northern Sierra Nevada would have had an established drainage divide and would have acted as a major topographic barrier during the early to mid-Cenozoic. The data presented here support a model of the Eocene northern Sierra Nevada characterized by a western slope with a gradient broadly similar to that of today.
Additional Information© 2010 American Geophysical Union. Received 2 April 2010; accepted 31 August 2010; published 14 December 2010. We thank Jim Wood for his assistance in the field and Victor Valencia for his help with U‐Pb zircon analysis. This work was supported by NSF‐EAR 0606967 to M. Ducea, Arizona LaserChron Center NSF‐EAR 0732426 to G. Gehrels, and graduate students research scholarships from Chevron and the Geological Society of America to M. R. Cecil. A.M. acknowledges support through the LOEWE funding program of Hesse's Ministry of Higher Education, Research, and the Arts. The authors thank J. Saleeby for a critical discussion of the paper and Greg Stock and an anonymous reviewer for thoughtful and thorough reviews, all of which significantly improved the manuscript.
Published - Cecil2010p12384Tectonics.pdf
Supplemental Material - 2010tc002717-ds01.txt
Supplemental Material - 2010tc002717-ds02.txt