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Coeval Deepwater Slope Channel and Slope Fan Systems in the Eocene Juncal Formation, Coast and Transverse Ranges, Southern California

Turner, Olivia C. and Dykstra, Mason
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The Eocene Juncal Formation, well exposed in the Transverse and Coast Ranges of Southern California, consists of a deepwater (500 to >1500 m water depth) continental slope succession that shallows overall to inner shelf deposits (~150 m water depth).

On the Pine Mountain fault block, coeval slope successions preserved within the Juncal Formation are dominated by conglomeratic channel bodies in the east and sandy, sheet-like deposits in the west. The Eastern Slope Succession is characterized by conglomeratic channel bodies sitting within erosional confinement (50-150 m deep) at the base of the section, and fines to coarse grained massive sands with conglomeratic interbeds and increased organic content. This entire succession is interbedded with thick intervals (10s of meters to 100s of m) of fine-grained shale. Mass-transport deposits up to tens of meters thick are common. The Western Slope Succession is characterized by thin (few meters) to thick (~100 m) sandstone bodies that only locally sit within modest (5-15 m deep) confinement and are often laterally discontinuous. They are largely bypass dominated and also exhibit high-energy features such as erosive bases, pebble lags, and injections. These sandstone bodies are interbedded with fine-grained intervals (10s – 100s m thick). Mass-transport deposits are common throughout, especially below the sand intervals.

We interpret the Eastern Slope Succession as deposits of deeply confined slope channels and their associated overbank at the base of the section grading to outer shelf deposits at the top of the section. The Western Slope Succession is interpreted as the deposits of a sandy, slope fan system. Here we compare these two coeval continental slope systems, and discuss how their radically different grain-size populations, body geometries, and architectural features relate to their respective depositional environments.


AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013