EVENT DEPOSITS - THE ROLE OF SEDIMENT SUPPLY AND RELATIVE SEA-LEVEL CHANGES - OVERVIEW

This overview on event deposits is based on (1) a brief summary on den udation rates in regions of various relief and climate as derived from the suspended and bed loads of rivers, (2) the fractions of sand and mud present in the fills of various basins, and (3) the mechanisms con trolling sediment remobilization. In continental settings, size and fr equency of event deposits (debris flows on alluvial fans, avalanching on fan deltas and overbanking on floodplains) directly reflect erosion al processes in the source areas and the ratio of denudation area/accu mulation area. In marine environments, both sediment supply and the ch ange in near-shore accommodation space largely control the nature of s tratigraphic sequences. Under conditions of high sediment supply and l ow-frequency sea-level changes, the thick systems tracts tend to show only minor differences in the presence of event deposits, including te mpestites. With decreasing sediment supply, event deposits are increas ingly concentrated in the lowstand systems tract. As shown by a number of models (with differential subsidence or uplift of the basin margin ), rapid relative sea-level fall accentuates both coastal and submarin e sediment remobilization (rich in sand), particularly during the earl y lowstand phase, as well as delayed valley incision. The resulting su bmarine fans tend to be sand-dominated, whereas large fans fed by majo r rivers are dominated by turbidite muds. In regions of coastal uplift , valley incision persists longer than the lowstand period, and sea-le vel changes may cause 'pulses of uplift' and phases of punctuated clif f erosion. Along carbonate buildups, lowstands of third-order or highe r frequency sea-level changes are often recorded by coarse skeletal de bris and megabreccias and/or, in the case of mixed systems, by silicic lastic turbidites. In rapidly closing foreland basins, high-frequency sea-level cycles only tend to affect both the proximal and distal basi n margins, whereas third-order sea-level changes have a limited potent ial to directly control depositional sequences and event deposits clos e to the overthrust front. With high sediment supply, individual event deposits (such as debris flows, sandy and calcareous tempestites and turbidites) mostly form at intervals of tens to hundreds up to some th ousands of years. Longer recurrence intervals occur in settings with l ow sediment supply or characterize very large mass flows and megaturbi dites.