REEVALUATION OF EAGLE MOUNTAIN FORMATION AND IMPLICATIONS FOR EXTREME EXTENSION ACROSS DEATH VALLEY REGION, CA

The Death Valley region has been highly influential in the development of ideas about extreme crustal extension. Clasts shed from the Hunter Mountain batholith of the Cottonwood Mountains have been identified at locations more than 100 km away, in inferred alluvial fan deposits of the middle Miocene Eagle Mountain Formation (Niemi et al., 2001). Since alluvial fans are less than 20 km in radius, Niemi et al. conclude that tectonic transport was responsible for the separation of clasts and source beyond that distance. This constraint appears to be especially compelling because it circumvents the uncertainties in reconstructions based on pre-extensional markers.

The argument hinges on the Hunter Mountain clasts having been deposited at a fan. Since alluvial fans are prograding features with unconfined and supercritical flow, their deposits tend to show sheetlike geometry, parallel stratification, and coarsening-upward units that become bounded distally by lacustrine flooding surfaces. The present study reexamines the Eagle Mountain Formation at Eagle Mountain with a different approach: closely-spaced measured stratigraphic sections linked by traced surfaces. The results indicate that the interpreted fan deposits are not fundamentally characterized by those typical features and instead appear to be fluvial.

All of the Hunter Mountain clasts are found in the upper part of a roughly 90 km-thick section comprising what Niemi et al. infer to be debris flow and sheetflood deposits. The section is fundamentally channelized. Lower-flow-regime cross-beds and ripples are abundant. Parallel stratified and disorganized deposits, where present, are commonly interspersed with the cross-strata or found within channels. The section is punctuated by erosional surfaces bounding fining-upward units, many of which are capped by siltstones. At least one of the surfaces is a sequence boundary with incised valleys. At the base of the formation, the deposits are interpreted by Niemi et al. to result from rock avalanching. Yet most of these are at least diffusely stratified, and in places there is cross-stratification, wavy bedding, and lenticularity. If rivers instead of fans indeed deposited the Hunter Mountain clasts, the transport distances due to sedimentary versus tectonic processes cannot be discriminated.