Re-evaluation of the middle miocene eagle mountain formation and its significanceas a piercing point for the interpretation of extreme extension across the death valley region, California, USA

The Death Valley area of eastern California and southern Nevada has been highly influential in the development of ideas about extreme crustal extension. One of the tightest constraints on Death Valley extension is apparently provided by clasts in inferred alluvial-fan deposits of the Eagle Mountain Formation (similar to 15-11 Ma) and their source in the Hunter Mountain batholith, now located > 100 km from some of the deposits. Because alluvial fans are usually less than 10-20 km in radius, the remaining separation has been interpreted as tectonic. New research reported here suggests that the Eagle Mountain Formation at its type location was deposited in a fluvial-lacustrine setting, and provides no constraint on either the magnitude or the direction of tectonic transport and/or crustal extension. Confidence in palinspastic reconstruction thus depends on resolving ambiguities in the correlation of pre-extensional markers or on the recognition of demonstrably proximal facies tectonically distributed across the region. The succession at Eagle Mountain comprises (1) diffusely stratified monolithologic carbonate breccia and sandstone (similar to 140 m) onlapping Cambrian carbonate rocks at an unconformity with similar to 110-140 m of relief (fluvial or fluvially influenced); (2) similar to 10 In of tabular-bedded siltstone, diamictite, and sandstone (lacustrine); (3) cross-stratified and channelized sandstone and polymict conglomerate bearing Hunter Mountain clasts, with minor siltstone and carbonate (similar to 110 in; mostly fluvial); and (4) tabular-bedded sandstone, siltstone, and minor carbonate (similar to 140 m; mostly lacustrine). Eight prominent stratigraphic discontinuities mapped within the third interval are characterized by up to 15 m of local erosional relief, and by abrupt upward coarsening from siltstone or carbonate to conglomerate or sandstone. A fluvial interpretation for the same critical part of the succession is based upon the existence of the mapped surfaces; the ubiquitous development of channels, trough cross-stratification, and upward fining trends (particularly between the mapped surfaces); and the abundance of well rounded clasts in conglomerate. Paleocurrents are generally directed between southward and eastward, although with considerable dispersion, and they shift from approximately southward or southeastward in the mostly fluvial deposits to approximately eastward in the upper lacustrine interval. An unusual feature of the Eagle Mountain Formation at Eagle Mountain is the presence of five crosscutting conglomerate bodies, interpreted as vertically infilled fissures of tectonic origin. Numerous normal, reversed normal, and oblique-slip faults with up to 34 m of stratigraphic separation are thought to postdate sedimentation and tilting of the Eagle Mountain Formation after similar to 11 Ma.