Renik, Byrdie1, Nicholas Christie-Blick1
(1) Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY
ABSTRACT: Re-evaluation of Miocene Evidence for Extreme Crustal Extension Across the Death Valley Region, California
The geology of the Death Valley region has been highly influential in the development of ideas about extreme crustal extension. Distinctive clasts as large as boulders derived from the Hunter Mountain batholith of the Cottonwood Mountains have been identified by Niemi et al. (2001) in purported alluvial fan deposits of the middle Miocene Eagle Mountain Formation at locations more than 100 km from the batholith. These data have been used to infer uniquely precise constraints on the direction, timing and magnitude of tectonic transport (equivalent to >500% extension), apparently circumventing uncertainties in earlier reconstructions based upon the correlation of pre-extensional markers. Re-evaluation of the Eagle Mountain Formation at Eagle Mountain casts doubt on the published interpretation, and suggests an alternative hypothesis: that the rocks represent a series of unconformity-bounded fluvial-lacustrine sequences. The overall stratigraphic motif between erosion surfaces is upward fining, the opposite of what would be expected of an alluvial fan building laterally into a lake. The abundance of channelization and cross-stratification in conglomerate and sandstone is not consistent with sheetflooding, the most important fluid gravity process responsible for the construction of alluvial fans, but is typical of braided fluvial deposits. Appreciable extension has undoubtedly taken place across this region. However, if rivers rather than fans were responsible for the deposits in which the Hunter Mountain clasts are found, the roles of sedimentary and tectonic transport cannot be discriminated, and both the amount of extension and the manner in which the crust was extended remain unresolved.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.