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