The Stratigraphic Architecture of Small Incised Valleys Along an Active Margin: Examples from the Oceanside Littoral Cell of the Southern California Coast
Published:January 01, 2019
Baird L. King, Alexander R. Simms, Lauren M. Simkins, 2019. "The Stratigraphic Architecture of Small Incised Valleys Along an Active Margin: Examples from the Oceanside Littoral Cell of the Southern California Coast", From the Mountains to the Abyss: The California Borderland as an Archive of Southern California Geologic Evolution, Kathleen M. Marsaglia, Jon R. Schwalbach, Richard J. Behl
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Compared to their equivalents along passive margins, less is understood about the stratigraphic architecture of incised-valley fill along active margins. Using approximately 10 km of shallow-marine seismic data and five vibracores, we compare the coastal incised-valley fill of two small southern California mountainous streams within the slowly uplifting and semiarid Oceanside Littoral Cell to incised-valley fill models from other active and passive margins. Our seismic data images the upper 16 m of the valley fill and contains three seismic units. The top unit is composed of a discontinuous drape of high-amplitude discontinuous subparallel reflections between 0.2 to 2 m thick, assuming a seismic velocity of 1500 m/s. The second unit is a 5.5- to 8-m-thick unit of faint chaotic reflections discontinuously compartmentalized by higher-amplitude mounded reflections. The lowest unit recorded in the seismic data is composed of a series of faint horizontal continuous reflections. A prominent reflector at 10 m separates the middle chaotic seismic unit from the lowest seismic unit. The shallow vibracores sampled the upper two seismic units, revealing a moderately well-sorted fine to very fine sand overlain by two silty units. The upper silt unit is bioturbated while the lower silt unit is well-laminated with laminations of gypsum sand. We interpret the silty units to represent a well-flushed mudflat overlying an enclosed evaporative mudflat. The second of these two facies appears to be unique to southern California estuaries undergoing uplift as an equivalent facies is not found within the estuaries developed within subsiding basins of the southern California coast. The sandy unit is interpreted to represent sandy lagoon or sandflat deposits. No cores sampled the lowest seismic unit; however, based on previously published data from neighboring incised valleys, we interpret it to be an open-estuary/central basin deposit. The fill within these small incised valleys is similar to that found along passive margins. The architecture of the valley fill is not dominated by tectonics but by eustatic sea-level rise. It lacks the multiple progradational phases and large volumes of coarse clastics common to other incised-valley fills from active margins. We attribute this difference to the generally low rates of uplift along this portion of the coast.