Abstract: Formatlon of an Intraclast Megabreccia by Sliding on a Sand Bed, Miocene Modelo Formation, Sherman Oaks, California

DENISON, FRANK E., J. MANUEL SAENZ, and EUGENE A. FRITSCHE, California State University, Northridge, CA 91330-8266

An outcrop of the Miocene Modelo Formation in the headscarp of a landslide in Sherman Oaks, California reveals a sequence of laminated, diatomaceous shale beds overlain by a sandstone bed that contains in its upper part a layer of large, densely packed, disrupted and folded, diatomaceous shale intraclasts. The sandstone bed is vaguely graded from coarse to medium grained at the base upward to medium to fine grained, but otherwise is structureless. The basal contact of the sandstone is sharp, undulates only slightly, and is everywhere parallel with the underlying shale lamination. Sandstone dikes, ranging from 0.2 to 20 cm in thickness, penetrate downward from the sandstone bed into the shale for distances of from 5 to 60 cm; some branch into sills that intruded both along and across the shale laminations. The contact surface reveals that the dikes are relatively straight features that vary only slightly in trend, averaging N46W ± 5 degrees. Some intraclasts occur in the lower portions of the sandstone bed, but the zone of densely packed intraclasts begins ~45 cm above the base of the bed. The largest is 150 cm long and 30 cm thick. The intraclasts, all of the same diatomaceous shale composition, are tilted and folded in various directions and are surrounded by randomly oriented dikes of sandstone that range in thickness from 0.5 to 20 cm. Normal diatomaceous shale deposition resumes at a thickness of 250 cm above the base of the sandstone. We propose that a thick, fluid- saturated, turbidite sandstone bed was deposited on top of the underlying diatomaceous shale, then covered by more diatomaceous shale. Downslope removal of support allowed the sandstone bed to move down slope as a fluidized grain flow. The overlying shale broke into the intraclast blocks, which folded and foundered in the slurry, but generally remained floating on top. The underlying shale also moved a short distance down slope, failing along extension fractures as it slid, making room for sandstone dike and sill intrusion.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California