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South Coast Geological Society

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Meeting Date: Monday, February 2nd
    - Social Hour Begins at 6:00PM

Location (Map Below): DoubleTree Club

Speaker: Dr. Kim Bishop

Topic: Insights on the Mechanics of Long-Runout Rock Avalanches (Sturzstroms) from the Anatomy of a Rock Avalanche Deposit at Old Dad Mountain, eastern Mojave Desert


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.

Speaker Information:

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.

Address: DoubleTree Club by Hilton 7 Hutton Drive, Santa Ana, CA

Meal: Regular or Vegetarian meals Complimentary tea and coffee will also be available

Cost: Members $30; Non-members $35; Students and Professors $15, $5 late fee for reservations made after 5pm the Friday prior to the meeting.

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