A portion of the upper Miocene Modelo Formation exposed
in the vicinity of Canton and Devil Canyons, Los Angeles
County, California, represents lobes of a fan-delta and
intervening finer grained interlobe deposits. Eleven lithosomes
and three different, recurring combinations or associations
of these lithosomes are recognized in the Modelo in
this study area.
A fossiliferous, yellowish-gray breccia, conglomerate,
and sandstone association occurs at the base of the section
as a series of obliquely stacked clastic wedges that pinch
out progressively to the southeast upsection. The association is composed of six lithosomes which were deposited on
the locally steep, distal, marine portion of the fan-delta
mostly within the "transition zone" and upper slope environments
of the fan-delta. Each wedge of the association
represents a cycle of establishment, then abandonment, of an
individual lobe of the marine portion of the fan-delta. An
unfossiliferous, orange conglomerate and sandstone association
occurs near the top of the Modelo Formation as a single
clastic wedge that pinches out to the southeast. The association
is composed of three lithosomes which were deposited
on the proximal, subaerial portion of the fan-delta as multiple,
superposed fan lobes in an arid climate. An interbedded
mudstone and sandstone association, which interfingers
with the two conglomerate and sandstone associations,
is composed of three lithosomes which represent mostly deposition
on the prodelta and some deposition on the shelf.
The conglomerate clasts composed of Mendenhall Gneiss,
quartz monzonite, anorthosite, and other anorthosite-related
rocks come from local northeastern sources and from the San
Gabriel Mountains to the southeast. Sandstone petrography
indicates a source terrain of rugged relief and overall
semi-arid climate. Measurement of imbricated conglomerate
clasts indicates dispersal from a northeastern direction.
The disparity between direction of dispersal and probable
source terrain for clasts of anorthosite and anorthositerelated
rocks substantiates 60 km (37 mi) of right-lateral
movement along the San Gabriel fault during the Neogene.
Influx of these clasts of anorthosite to form the fan-delta
in the study area marks the inception of movement along the
San Gabriel fault.
The shoreline, which was to the northeast of the study
area prior to deposition of the Modelo Formation, shifted to
the southwest as sediment input temporarily filled the basin
and the fan-delta became emergent, then transgressed rapidly
back to northeast during deposition of the uppermost parts
of the Modelo.
Description:
Includes bibliographical references (pages 120-126)
California State University, Northridge. Department of Geology.