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Autocyclic Modulation of Allocyclic Signals in Submarine Fans: An Experimental Study


The interplay between allocyclic and autocyclic controls dictates the timing and distribution of sediment supply on the slope and basin floor. Allocyclic controls are external to the sedimentary system and include sea-level, climate, and tectonics whilst autocyclic controls are system-internal, including processes such as channel avulsion, levee growth and compensational lobe stacking. The degree to which autocyclic processes mask allocyclic controls, and the relative timescales at which they operate, is still poorly-understood. Here, successive experimental sediment-laden turbidity currents with incrementally increasing then decreasing sediment supply rates have been used to emulate the effect of a waxing-to-waning sediment supply cycle on submarine fan evolution. Associated with sediment supply rate increase, channel incision and overbank deposition (levee growth) was observed on the slope, with concurrent lobe progradation on the basin floor. Conversely, when supply rate was lowered, the opposite was recorded: a reduction of channel incision and overbank deposition, and progressively back-stepping lobe deposits that onlapped the slope. These observations suggest a direct correlation between sediment supply (allocyclicity) and submarine fan architecture and that predictions about sediment distribution can be made on this basis. However, topography created by deposits from earlier turbidity currents caused subsequent flows to divert around them, favouring topographic lows, promoting lateral spreading and compensational lobe stacking. Continued lateral spreading resulted in a decrease in accommodation space on the basin floor, exacerbating the back-stepping trend of lobe deposits documented during the waning phase of the sediment supply cycle. This morphodynamic evolution indicates that allocyclic factors may drive initial conditions, but progressively, autocyclic processes and products modulate that signal and play a relatively larger role in lobe development, promoting back-stepping and possible avulsion of the system. The progradation, then rapid lobe switching recorded in these experiments may account for bed stacking patterns observed in outcrop and core where lobe packages often coarsen and thicken upwards, with abrupt tops, and more generally for sediment distribution in lobe systems in subsurface datasets.