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

Two major end-members of point-sourced submarine fans can be recognized: co arse-grained and fine-grained. The coarse-grained member is known from acti ve margins where the sediment source is rather close to the coast. Once rea ching 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 wi ll gradually carve out a canyon. On the basin floor, a submarine fan is con structed that gradually progrades into the basin. Its thickness and sand/sh ale ratio decrease downdip. The fine-grained member is common for passive margins. It is a bypass syste m. The sediment originates far from the coast, the fluvial system is long a nd a major delta commonly results. Exposure of the wide shelf requires a re lative lowering of sea level. Rapid aggradation of sediment near the shelfb reak results in high pore pressure that causes failure, followed by slumpin g and density flow transport. Deposition may commence at the base-of-slope. Leveed-channel transport is common on the mid-fan, while sheet-sand deposi tion takes place on the outer fan. Progradation of individual fan systems i s rapid, followed by lateral switching to minimize bottom topography. Global and regional climatic changes cause variation in the growth/decay ra tio 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 ex ist upon entering salt water. Although presently unknown, fine-grained subm arine fans may provide data required to help analyze paleoclimate. (C) 2001 Elsevier Science B.V. All rights reserved.