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Masters Thesis

Eocene marine to nonmarine deltaic deposits Lower Piru Creek, Los Angeles and Ventura Counties, Southern California

A siltstone and sandstone section, up to 1600 m thick, north of Canton Canyon and Agua Blanca Creek in the vicinity of lower Piru Creek, 16 km north of the town of Piru, California, previously was unnamed and considered as "undifferentiated Eocene." The siltstone and sandstone are similar in age, stratigraphic position, and lithology to the Juncal Formation and the Matilija Sandstone, respectively. The 900-m-thick siltstone unit (Juncal Formation) is in fault contact with the Whitaker Peak granodiorite basement complex north and west of the study area, but it rests unconformably on the basement contact in the study area. Where it is in depositional contact, it is characterized by having a veneer of transgressive lag at the base of the section. The transgressive lag, which consists of medium to coarse sandstone and some conglomerate, is overlain by a few meters of shoreface carbonaceous sandstone that grade vertically upward into transition-zone siltstone and fossil-bearing, storm deposits of sandstone. Calcareous nannofossils, planktic and benthic foraminifera, and molluscan assemblages in the siltstone (Juncal Formation) indicate an early through early medial Eocene age. The 760-m-thick sandstone unit (Matilija Sandstone) unconformably and locally conformably overlies transition zone siltstone of the Juncal Formation and records three subdivisions of a tide-dominated delta (subaqueous delta plain, lower delta plain, and upper delta plain) which are characterized by tidal deposits. The base flat, and braided stream/river of the Matilija Sandstone is characterized by an upward-coarsening sequence that is fine at the base to medium at the top and comprised of laminated and/or bioturbated sandstone. Shallow-marine molluscs diagnostic of a late early through early medial Eocene age are present within lenses of the medium sandstone. These rocks were deposited on a subaqueous delta plain. Above the subaqueous delta-plain deposits is structureless and bioturbated medium to coarse sandstone with common herringbone cross-bedding, planar crossbedding, planar laminations, and scoured-and-filled structures. These structures are interpreted to have been formed in a tidal-flat environment. Coal lenses higher in the section are interpreted to be marsh deposits. Overlying these rocks is an interval of interfingering tidal-flat and braided-river deposits. The remaining section is unfossiliferous and is comprised of a sequence of upward-fining cycles overlain by an interval of structureless sandstone. This uppermost section was deposited by a braided-river system. Sediments were derived from uplifted continental basement rocks composed mainly of felsic plutonic and high-rank metamorphic terranes. The continental basement complex was present to the north and northeast of the study area during the early through early medial Eocene and provided detritus for a southwest prograding tide-dominated deltaic system. Although sedimentation is coincident with a global sea-level rise and fall, there is evidence that the study area was influenced by regional and local tectonics and that the transgressive and regressive sequence was due to changes in subsidence and sedimentation rate.

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