The Point Fermin submarine fan : a small late middle Miocene Age fan within the Monterey formation

Abstract

A distinctive sandstone and breccia lithofacies is channelized and interfingers with typical deep basin Monterey Formation deposits at Point Fermin, on the Palos Verdes Peninsula, California. Sea-cliff exposures display a thinning-and fining-upward sequence of channel-fill deposits consisting of sandstone, breccia and intraformational breccia, whose base is scoured into thin-bedded sandstone and shale. Thick-bedded facies A and B sandstone and breccia in the basal portion of the channel-fill sequence were deposited by high-density sediment gravity flows whereas facies F intraformational breccia beds in upper portions are representative of debris flow deposition, transitional to slumping. Thin-bedded sandstone and shale adjacent to the channel-fill sequence are representative of low-density turbidity currents separated by intervals of hemipelagic sedimentation. Sponge spicules and cold-water radiolarians and diatoms within shale beds imply that the entire sequence was deposited at bathyal depths, probably on the inner portion of a submarine fan, herein termed the Point Fermin fan. The glaucophane-rich sandstone and breccia of the fan contrast sharply with the arkosic composition of most coeval deep-sea fans of the Los Angeles basin, implying that the Point Fermin fan and its source were isolated from the rest of the basin. Paleocurrent data suggest that the source of the Point Fermin fan detritus was probably the Palos Verdes uplift to the north. Uplift of the Palos Verdes anticlinorium during the middle Miocene, and a small concomitant fall in sea level may have initiated the development of the fan.