MARINE DEPOSITIONAL EVENTS CONTROLLED BY SEDIMENT SUPPLY AND SEA-LEVEL CHANGES

Under conditions of high fluvial or coastal sediment supply, individua l sandy storm layers (tempestites) and turbidites form after significa ntly shorter time intervals than Milankovitch cycles. To provide the m aterial for mass flows of very high volume, large, mountain-borne rive rs require about ten thousand years. With little sediment supply, howe ver, all event deposits become rare or are missing. Third-order sequen ces (about 1 - 4 Ma) vary considerably in thickness depending on the s edimentation rates in the basins. Thick and widely extended mud flows and megabreccias are preferentially triggered by sea level fall below the shelf edge. Mixed siliciclastics - carbonate systems along the foo t of carbonate shelves generally become coarser grained and poorer in carbonates during lowstands. Turbidite successions also frequently occ ur in lowstand deposits, but may be present in the other systems tract s as well if a delta is prograding continually. The position of tempes tites in third or higher order shallow water sequences is less clear. In studies on the migration of coastal sands in relation to falling an d rising sea level, one should distinguish, apart from sediment supply , between settings with and without substantial wave scour. The genera tion of sandy tempestites appears to be favoured if coastal progradati on and wave scour operate simultaneously for example during late highs tand and early lowstand.