AN EXPERIMENTAL-STUDY OF THE EFFECTS OF BASE-LEVEL CHANGE ON FLUVIAL,COASTAL-PLAIN AND SHELF SYSTEMS

Several of the concepts of sequence-stratigraphic models were tested b y experiments in a large flume. Using different gradients for successi ve runs, a physical model of a fluvial drainage basin, coastal plain, continental shelf, and continental slope was constructed. In all exper iments, initial gradients of the coastal plain and shelf were identica l. Systematic raising and lowering of the water surface simulated the changes in relative sea level (i.e., base level) that are similar to m ost sequence-stratigraphic models. Base-level changes significantly af fected the shelf area but they had little effect on the fluvial draina ge basin. During base-level falls, fluvio-deltaic progradation occurre d. Fluvial aggradation occurred only during periods of base-level stil lstand or rise. Because of the slow rate of headward erosion of incise d valleys, a significant time gap existed from the time base level fir st fell below the shelfbreak and the development of a cross-shelf bypa ss valley. During this lag time, deposition occurred on the exposed sh elf to form a fluvial braid plain. Concurrently, channels developed at the shelf-break and grew by headward erosion at various rates. Contin uous incision and widening of these channels formed valleys. These val leys were not linked directly to the main drainage basin until ultimat ely one captured the flow from that drainage. At that point, other val leys were abandoned and ceased to grow. During the subsequent base-lev el rise, the single, large, cross-shelf valley filled with coarser sed iments while the other valleys partially filled with finer suspended-l oad sediments. The main cross-shelf valley also supplied coarser sedim ents to the slope.