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Stratigraphic Evidence from Miocene Megabreccia Deposits for a Regional-Scale Mesozoic Allochthon in the Western Soda and Avawatz Mountains, Eastern Mojave Desert, California

Kim M.Bishop
Dept of Geological Sciences, California State University, Los Angeles, 5151 State University Drive, Los Angeles, CA 90032

Basement rocks in the western Soda and Avawatz Mountains, southeastern California, consist of Paleozoic carbonate, Triassic-Middle Jurassic metavolcanic, and middle(?) Cretaceous granitic rocks that were involved in complex deformation during the Mesozoic. Grose (1959) interpreted that scattered Paleozoic carbonate outcrop blocks exposed in the western parts of the Soda Mountains are remnants of an extensive allochthon that was emplaced over metavolcanic rocks. However, because the outcrops of the possible allochthon are few and scattered, the evidence for a large-scale allochthon is underwhelming.

However, stratigraphic evidence from megabreccia deposits intercalated within the Miocene terrestrial Avawatz Formation supports Grose’s interpretation for a regional-scale allochthon. Important characteristics of the megabreccia, sheet-like geometry, preservation of source rock stratigraphy, matrix-poor fabric, and exposures that occur far from the basin margins. These characteristics support the interpretation that the megabreccias represent rock avalanche deposits analogous to the Pleistocene Blackhawk landslide in the southern Mojave Desert. In four of the rock avalanches, Paleozoic carbonate megabreccia is in contact with Mesozoic metavolcanic megabreccia. Significantly, at each of these locations, the carbonate megabreccia overlies the metavolcanic the metavolcanic megabreccia. Several lines of evidence indicate that the carbonate-metavolcanic contact is preserved from the source area, which could only have been the present-day metavolcanic basement outcrops south and west of the basin. The carbonate-over-metavolcanic geometry strongly supports Grose’s hypothesis for a major Paleozoic carbonate allochthon displaced tens of kilometers over the metavolcanic rocks. Regionally, the Soda-Avawatz allochthon may have counterparts nearby in the Silurian Hills and at Old Dad Mountain. If so, then the limited cumulative evidence from the three areas suggests that allochthon was west-vergent and may have been emplaced during tectonic extension along a low-angle normal fault. The tectonic environment for the Soda-Avawatz low-angle faulting is as yet unknown.


AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California