The Stratigraphic Architecture of Incised Valleys Created by Two Small Southern California Mountainous Streams

Small mountain streams terminating in coastal areas are thought to deliver disproportionately large amounts of sediment to the world’s oceans. However, compared to their equivalents along passive-margins, less is understood about the stratigraphic architecture of their incised-valley fill. Using approximately 10 km of shallow-marine seismic data and five vibracores, we document the valley fill of two incised valleys of small mountainous streams of the Southern California coast. 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 low-amplitude discontinuous parallel reflections between 0.25-2 m thick, assuming a seismic velocity of 1500 m/s. The second unit is a 5.5-8 m thick package of faint chaotic reflections discontinuously compartmentalized by a higher-amplitude dipping reflection. The lowest unit recorded in the seismic data is composed of a series of high amplitude, horizontal, and continuous reflections below a relatively constant depth of approximately 10 m. The shallow vibracores sampled the upper two seismic units revealing a thick medium grained sand deposit overlain by an organic rich mud. We interpret these units to be, respectively, 1.) an anthropogenic unit composed of dredge material and closed lagoon muds, 2.) a middle thick sandy unit representing coalesced tidal channels and/or sand flats, and 3.) an underlying 6 m thick central basin or fine grained marine deposit. Despite their small size, these incised valleys may provide important archives of past environmental changes as well as provide excellent aquifers and reservoirs.

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