ARROYO SAN FERNANDO: A CRETACOUS SUBMARINE VALLEY-LEVEE COMPLEX BUILT WITHIN AND OUT OF A SUBMARINE CANYON

Arroyo San Fernando exposes a deep marine valley-levee complex in a submarine canyon. Preserved in the arroyo are three-dimensional exposures of the valley fill, the levees, and the bounding submarine canyon wall rocks. The valley fill consists of conglomerate bodies, sandstone bodies, and interbedded siltstone/sandstone intervals. The levee facies consists of thinly interbedded siltstones and sandstones, sometimes slumped, and the canyon wall facies consists primarily of structureless foram-rich siltstones. The sequence of events leading to deposition of the valley levee complex is interpreted as follows: A North-South trending submarine canyon was cut in slope deposits in the current location of Arroyo San Fernando. Aggradational turbidity currents flowed down the canyon, leaving deposits of coarse-grained channel facies, medium and fine-grained interchannel facies, and fine-grained levee facies, and building a valley-levee system with levees that possessed relief and onlapped the submarine canyon wall. Deposition continued in this fashion until the submarine canyon was filled, building conglomerate bodies up to ~50 m thick, and generally sheetlike in geometry, but relatively isolated laterally and vertically within the finer-grained interchannel deposits. We interpret these deposits as resulting from sedimentation focussed within the submarine canyon necessitating a greater degree of vertical aggradation because sediment could not be spread laterally as widely as in a less laterally confined system. As the valley-levee system aggraded up and out of the submarine canyon, however, the valley-levee system became laterally unconfined and could widen, essentially decreasing the sedimentation rate in any one location, and allowing thicker and more highly amalgamated conglomerate bodies to form. The uppermost portion of the Arroyo San Fernando valley levee complex contains a very thick (~200 m), and laterally extensive >5 km conglomerate body that was deposited within the valley formed by the levees after the entire system had aggraded up and out of the submarine canyon. The overall trend of the system stratigraphically is from a narrow, restricted valley-levee system with isolated conglomerate bodies to a wide system with thick, laterally extensive and amalgamated conglomerate bodies.