A MIOCENE LONG-RUNOUT ROCK AVALANCHE DEPOSIT, SOUTHEASTERN CALIFORNIA, CONTAINING A LARGE "CLOT" OF SEMI-COHERENT SOURCE ROCK: AN ANALOG FOR SOME RECENT LANDSLIDE MASSES?

The Old Dad Mountain area in the eastern Mojave Desert exposes Miocene terrestrial basin strata containing intercalated megabreccia deposits derived from Paleozoic carbonate. The internal structure of the megabreccia is revealed from erosion. Several characteristics of the deposits indicate they formed from long-runout rock avalanches (aka sturzstroms). These characteristics include monolithology, jigsaw texture, high clast-to-matrix ratio, sheet-like geometry, and long-runout distance.

The best exposed sheet occurs near the base of the Miocene sequence and displays an unusual morphology. Across most of its 4 km width, the deposit is relatively thin with an average thickness of about 15 m. However, in its central part, the deposit contains a "clot" about 200 m thick and 1 km wide. The thin parts of the sheet consist of uniformly brecciated carbonate with clasts generally 5 to 20 cm in diameter. In contrast, the thick "clot" mostly consists of coherent source rock, except at its base, where there is a 2 to 4 m thick brecciated zone similar to the breccia in the thinner parts of the sheet. The geometry of the deposit is interpreted to indicate that much of the rock avalanche material became brecciated and spread laterally during emplacement and that a large block of the source rock remained intact during emplacement and rode atop the flowing breccia.

Two recent landslide masses in southeastern California may have a similar origin. One of these masses is informally known as Elephant Hill and is located in Deep Springs Valley near Bishop, California. The other is known as Baker Hill and is located west of Baker in the eastern Mojave Desert. Both of these landslide masses are exposed as a large hill surrounded by alluvium. I propose that both masses may be "clots" resting atop extensive, thin breccia deposits that have been buried beneath alluvium.