Nested Submarine-Fan Channels in the Capistrano Formation, San Clemente, California

Abstract

At San Clemente State Beach, eight turbidite-filled channels are exposed in a 550-m-long, 30-m-high sea cliff. The channel margins show a progressive northwestward lateral shift in position and thus are nested one alongside another. Three adjacent channels trend between 270° and 300°, with west-northwest turbidity-current flow. Four other adjacent channels trend between 230° and 240°, with southwest flow. Seven of the eight channels received mud and silt deposits in the form of a drape over the channel walls before deposition of graded sand beds took place. In some channels, these graded beds belong to the “classical” turbidite facies, but in others, the sandstone deposits are pebbly, beds are thick, and interbeds of shale are thin to absent. In three of the channels, there is an overall upward fining and thinning of beds, implying progressive channel abandonment. By contrast, the beds filling one channel become coarser and thicker upward, implying a prograding turbidite lobe. The channels are assigned to the braided suprafan part of the submarine-fan model.