Pacific Section AAPG Convention

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Depositional Framework And Controls On Coeval Deep-Water Slope Channel And Slope Fan Systems In The Eocene Juncal Formation, Southern California


The Eocene Juncal Formation, well exposed in the Transverse and Coast Ranges, of southern California, is comprised of deep-water, middle and upper active continental slope deposits that shallow to shelf depths. On the Pine Mountain fault block, a 19-km-wide stratigraphic exposure is interpreted to exhibit two coeval slope systems as much as 2,700 meters thick in outcrop. Sandstones and conglomerates within both systems are encased in mudstone tens to hundreds of meters thick, punctuated by mass-transport deposits tens of meters thick. The Western Slope System contains sandstone sheets and lobes ranging in thickness from a few to as much as 100 meters. Some are partially confined within 5-15m-deep scours, and exhibit high-energy bypass features such as pebble lags, scour surfaces, truncated bed tops, and basal injections. In contrast, the Eastern Slope System exhibits conglomerate-filled channels with 50-100m of confinement and change upward to fine- to coarse-grained sandstones with conglomeratic interbeds and increased content of terrestrial organic particles. Here, these two slope systems, and their contrasting grain sizes, sedimentary body geometries, and architectural elements, are described, discussed and interpreted with respect to depositional processes, various slope environments, and allogenic vs. autogenic controls. Distinguishing coarse-grained deposits of the Western Slope System are interpreted as line- to multi-sourced, sandy intra-slope aprons fed by longshore drift from a nearby delta. Those in the Eastern Slope System appear to be point sourced, where a confined slope channel system and associated overbank deposits at the base grade upward to shallow-water shelf edge beds sourced directly from the delta. Mass-failure scars created accommodation whereas the mass-transport deposit created an obstruction to trap sediments from subsequent down-slope flows. Sandstones are common overlying mass-transport deposits in both systems. Sea-level fluctuations are interpreted to control initial channel cutting, whereas tectonics and sediment supply control available grain-sized populations. Slope accommodation and rugged topography created by mass-transport deposits controlled the different architectures between the two systems. Results from this outcrop study of two coeval slope systems help bridge the data gap between large-scale seismic interpretations and smaller-scale detailed core analysis.