Quaternary morphology, acoustic characteristics, and fan growth of the conception fan, Santa Barbara Basin, California Continental Borderland

The Quaternary Conception fan forms the gentle northwestern slope of the Santa Barbara Basin. Acoustic facies and morphology of the Conception fan were defined using seismic profiles, and core and boring logs, in order to determine the fan growth. This fan was fed by two canyon systems. Each canyon developed a sediment wedge, which shows conspicuous differences in growth patterns. The older Cojo canyon fed the western fan sector and the younger two Gaviota canyons, Sacate (west) and Alegria (east), fed the eastern fan sector. The Cojo canyon-system is a reactivated Illinoian conduit, which fed a "typical" radial fan turbidite system characterized by: (1) a single persistent conduit, (2) extensive rhythmic interbedded Ievees in the upper to upper-mid fan, (3) a high frequency of episodic turbidity flows, (4) an apparent well-developed distributary system which buiIt an extensive overlapping sand lobe and (5) contained channel flows which deposited distal sands on the lower fan and basin plain. The Sacate and Alegria canyons, which are located 7 mi to the east, formed subsequently to the Cojo canyon-channel. These canyons incised along fault zones where headward erosion was enhanced by fluvial and slumping processes. During the Iate PIeistocene sediment suppIy to the Gaviota canyons was greatly increased by the capture of the Santa Ynez River near the Gaviota Creek. Capture also resulted in an increased sand bedload that was transported directly into the Gaviota canyon heads. As a result of channel aggradation and tectonism Sacate canyon sequentially developed three en-echelon channels: namely, Western, Pescado and Central channe Is. The Ionger-lived Central channel shows greater morphologic development than the rei ict Pescado and Western channels; Pescado shows varying degrees of burial and the Western system exhibits anomalous scour. The Eastern channel fed by the Alegria canyon is characterized by slumping and numerous coalescing slope gullies. These features resulted from oversteepening of the fan slope due to growth along the Hondo fold. The Central channel shows similar morphologic development. Due to the absence of distinct gradient breaks, channel morphoIogy was used to determine fan divisions: (1) upper fan gradients range from 1:25 to 1:40 and channels are broad with well-developed levees; (2) middle fan gradients range from 1:40 to 1:100, where levee height decrease and channels narrow in the lower half; (3) lower fan gradients are less than 1:100 with relatively smooth surface morphology and the major channels die out. These higher gradients on the Conception fan are unlike radial fan systems and have resulted in: (1) relatively straight channels, incised into the fan surface, (2) no distinct distributary system with evidence of channel braiding or avulsion on the fan, and (3) no suprafan bulge despite high sand supply. By mid-Holocene time, the fan was separated from its supply of coarse sediment and hemipeIagic deposition became the dominant process. These deposits form an acoustically transparent blanket over the relict fan surface. The central upper and upper-mid fan, which is being winnowed by currents, has hummocky mesotopography and no hemipelagic sediments.