Neogene sturzstrom deposits, Split Mountain area, Anza-Borrego Desert State Park, California
Neogene sturzstrom deposits, Split Mountain area, Anza-Borrego Desert State Park, California (in Catastrophic landslides; effects, occurrence, and mechanisms, Stephen G. Evans (editor) and Jerome V. DeGraff (editor))
Reviews in Engineering Geology (2002) 15: 379-400
The Neogene stratigraphic section in the Split Mountain area exposes megabreccia deposits up to 12 km long with volumes up to 3 x 10 (super 8) m (super 3) . Shattered-rock domains still portray the bedrock distribution of lithologies. Jigsaw-puzzle fabric occurs on a variety of scales from microscopic to outcrop. Broken and stretched pegmatites tend to rise upward as step-ups in the inferred down-flow directions. Upper Miocene subaerial megabreccias about 65 m thick disturbed the underlying strata to depths less than a meter during their emplacement. This includes producing grooved and decapitated stones both in the substrate and below shear surfaces especially within the basal few meters of the megabreccia deposits. The lower portions of a megabreccia are rich in step-ups, ramps, and crushed-rock streamers that rise upward in the down-flow direction. After flooding of the basin by the ancestral Gulf of California, a lower Pliocene megabreccia moved across the sea floor deforming underlying sedimentary layers by injections and sunken megabreccia lobes that locally caused tightly folded bottom-sediment packages >35 m thick to rise as diapirs. Near the leading edge, on the southwest corner of the deposit, there is a small volume of more traditional sandy conglomerate deposited as the mass rapidly slowed and stopped. Both subaerial and subaqueous megabreccias contain lithologic domains that preserve the distribution of bedrock lithologies, jigsaw-puzzle fabric, step-ups and crushed-rock streamers; these features all require non-turbulent flow. These huge volumes of shattered bedrock moved 10-12 km distance in late Miocene as dry sub-aerial masses, and again across the floor of an early Pliocene inland sea. All the observed features strongly indicate flow as sturzstroms.