MIOCENE ROCK-AVALANCHE DEPOSITS, HALLORAN SILURIAN HILLS AREA, SOUTHEASTERN CALIFORNIA/

Sedimentary rock of the Miocene Shadow Valley Basin in the Halloran/Si lurian Hills area, southeastern California consists of a terrestrial s equence that is divided into two units. The lower unit (up to 1500 m t hick) consists primarily of red-bed fanglomerate. Interbedded with the fanglomerate are four megabreccia sheets interpreted to be rock-avala nche deposits, Lithologic features of the sheets that indicate avalanc he-type deposition include monolithologic composition, crackle and jig saw breccia textures, large blocks up to tens of meters across, low (5 -20 percent) matrix content, elastic dikes, and basal striations, The three stratigraphically lower sheets range between 5 and 10 m thick, w hereas the highest sheet is approximately 75 m thick. Breccia clasts i n the sheets consist predominantly of Paleozoic carbonate. The source area for the megabreccia is the Clark Mountains on the east. At the ti me of deposition, the avalanche deposits covered an area of 300 to 400 km(2), Runout distances for the three thinner sheets is about 30 km a nd for the thicker sheet, about 40 km, Volumes of the three thinner sh eets is calculated to be on the order of 5 x 10(9) m(3) and the thicke r sheet is about 4 x 10(10) m(3). Thickness to runout ratios vary from 0.002 for the thicker sheet to 0.0004 for the thinner sheets. Several mechanisms have been proposed to decrease friction that allows for em placement of large-runout avalanches, including high pore water pressu re, trapped air beneath the avalanche, intergranular impacts, and acou stic fluidization. Observations of the sheet bases in the Halloran/Sil urisn Hills suggest that the acoustic fluidization model fits best.