Fine-grained submarine fans as possible recorders of long- and short-term climatic changes

Abstract

Two major end-members of point-sourced submarine fans can be recognized: coarse-grained and fine-grained. The coarse-grained member is known from active margins where the sediment source is rather close to the coast. Once reaching the coast, most of the sediment goes into longshore transport and may move down a shelf depression to the slope. The movement of the sediment will gradually carve out a canyon. On the basin floor, a submarine fan is constructed that gradually progrades into the basin. Its thickness and sand/shale ratio decrease downdip. The fine-grained member is common for passive margins. It is a bypass system. The sediment originates far from the coast, the fluvial system is long and a major delta commonly results. Exposure of the wide shelf requires a relative lowering of sea level. Rapid aggradation of sediment near the shelfbreak results in high pore pressure that causes failure, followed by slumping and density flow transport. Deposition may commence at the base-of-slope. Leveed-channel transport is common on the mid-fan, while sheet-sand deposition takes place on the outer fan. Progradation of individual fan systems is rapid, followed by lateral switching to minimize bottom topography. Global and regional climatic changes cause variation in the growth/decay ratio of continental glaciers and in the type of fluvial effluent (hypopycnal and hyperpycnal outflows). Major global climate changes can cause ice ages, while smaller climate changes often influence transport and depositional variations. Shelf bypassing may result if hyperpycnal outflow conditions exist upon entering salt water. Although presently unknown, fine-grained submarine fans may provide data required to help analyze paleoclimate.