Architecture of an Upper Slope Transient Fan System in the Eocene Juncal Formation, Ventura County, California

The Pine Mountain Fault Block in the Transverse Ranges near Ojai, California, contains exceptionally well exposed and relatively undeformed ancient continental slope deposits in an area otherwise plagued with complex faulting and folding, vertical axis rotation, and notoriously rugged terrain. Recent work in this field area demonstrated that three distinctly different but coeval slope systems are present; a coarse-grained canyon and channel system, a sandy intraslope fan system (depositionally lateral), and a finer-grained canyon and overbank system (depositionally down-dip). This study focuses on the sandy intraslope fan system, which is interpreted as a multi-storied transient-type fan system (Adeogba et al., 2005). Three fan bodies in the field area have been mapped, logged, and described in detail to construct high resolution architectural and depositional models. The fan bodies range from 3-5 kilometers wide and 10-20 meters thick, and are stratigraphically separated by no more than 20 meters of thin-bedded shale. Fan bodies are characterized by a relatively low relief incision (1-3 meters) at the axis into underlying strata, filled with thin-bedded, sheet-like, fine-grain sediment. Unconfined amalgamated coarse-grained sand overlies the basal channel fill and comprises the majority of the fan bodies. Bypass indicators are prevalent in both the fan bodies and the intervening thin-bedded intervals. Large bypass dominated channels are commonly incised into the tops of the fan bodies, sometimes incised through the entire thickness of the fan and into the underlying shale. Additionally, mass transport deposits are commonly observed underlying fan bodies, and fine-grained thin-bedded intervals often display both ductile and brittle small scale deformation. These observations lead to the interpretation of a system comprised of small weakly-confined elongate slope fans with a strong component of bypass, architecturally controlled by preexisting sea floor topography that creates local accommodation space.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California