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Quaternary Conception Fan, Santa Barbara, California: Two Distinct Canyon(s)-Fan Systems

Susanne M. C. Kraemer

The Quaternary Conception fan forms the gentle northwestern slope of the Santa Barbara basin, California. Two distinct canyon(s)-fan systems which show conspicuous differences in growth pattern coalesced to form this sediment wedge.

The older Cojo canyon-fan system fed a typical radial fan characterized by (1) a single persistent conduit, (2) rhythmic interbedded levees in the upper and mid-fan built by frequent episodic turbidity flows, (3) an apparently well-developed distributary system, and (4) distal sand deposition on the lower fan and basin plain.

Subsequently, the Gaviota canyons formed a radial fan/slope apron system. Two subparallel canyons were incised along fault zones where headward erosion was enhanced by slumping and increased fluvial sediment supply during the Pleistocene due to the capture of the Santa Ynez River near Gaviota Creek.

Tectonism produced channel aggradation, which resulted in a series of en echelon channels. The eastern channel is associated with slumping and numerous coalescing slope gullies. These features resulted from over-steepening of the fan slope.

Due to the absence of distinct gradient breaks, recent channel morphology was used to define fan divisions: (1) upper fan, broad channel with levees, gradient range from 1:25 to 1:40, (2) middle fan, levee heights decrease and channels narrow, gradients range from 1:40 to 1:100, and (3) lower fan, relatively smooth surface morphology, gradients less than 1:100.

These higher gradients on the present Conception fan are unlike radial fan systems and have resulted in (1) relatively straight incised channels, (2) no distinct distributary system with evidence of channel braiding or avulsion, and (3) no suprafan bulge despite high sand supply.

Morphology of the Cojo canyon-fan system suggests gradients were lower during deposition than presently observed on the fan.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.