Exposed at Old Dad Mountain 25 km southeast of Baker, California is an erosionally dissected Miocene terrestrial sedimentary basin sequence. Near the base of the sequence is a megabreccia deposit interpreted as a long-runout rock avalanche deposit analogous to the well-known Blackhawk landslide in the southern Mojave Desert. Although the Old Dad Mountain deposit shares many similarities to the Blackhawk deposit, it also has an important difference. Near the center of the 5 km long megabreccia outcrop is a hummock 1 km wide and 250 m thick that consists of mostly non-brecciated carbonate, except in the lowest 1 to 3 meters, where it is thoroughly brecciated. North and south of the hummock, the megabreccia consists of 10 to 30 m thick deposits that extend outward approximately1.5 km northwest and southeast from the hummock. Unlike the hummock, these “wings” are thoroughly brecciated and look much like the Blackhawk megabreccia. The geometry of the Old Dad Mountain deposit argues against the “air cushion” and “bulk fluidization” models to explain the low apparent friction of the landslide. Instead, mechanical processes acting in the basal zone appear to explain the long-runout. Based on the observations from this study, a model to explain variations in long-runout rock avalanche geomorphologies is proposed.
Attended UCLA as an undergraduate and then Cal State Los Angeles and USC for graduate work. Worked as an Engineering Geologist, first at GeoSoils, Inc. and then Pacific Soils Engineering, in the Los Angeles area between 1980 and 1988. Professor at Cal State Los Angeles for past 21 years.
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