Miocene Rock-Avalanche Deposits, Halloran/Silurian Hills Area, Southeastern California

Abstract

Sedimentary rock of the Miocene Shadow Valley Basin in the Halloran/Silurian Hills area, southeastern California consists of a terrestrial sequence that is divided into two units. The lower unit (up to 1500 m thick) consists primarily of red-bed fanglomerate. Interbedded with the fanglomerate are four mega-breccia sheets interpreted to be rock-avalanche deposits. Lithologic features of the sheets that indicate avalanche-type deposition include monolithologic composition, crackle and jigsaw breccia textures, large blocks up to tens of meters across, low (5–20 percent) matrix content, clastic dikes, and basal striations. The three stratigraphically lower sheets range between 5 and 10 m thick, whereas the highest sheet is approximately 75 m thick. Breccia clasts in the sheets consist predominantly of Paleozoic carbonate. The source area for the megabreccia is the Clark Mountains on the east. At the time 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 and for the thicker sheet, about 40 km. Volumes of the three thinner sheets is calculated to be on the order of 5 × 10 9 m 3 and the thicker sheet is about 4 × 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 emplacement of large-runout avalanches, including high pore water pressure, trapped air beneath the avalanche, intergranular impacts, and acoustic fluidization. Observations of the sheet bases in the Halloran/Silurian Hills suggest that the acoustic fluidization model fits best.