Influence of water and sediment supply on the long-term evolution of alluvial fans and deltas: Statistical characterization of basin-filling sedimentation patterns

Abstract

The temporal and spatial variability of sedimentation, resulting from sediment storage and release and the lateral mobility of sediment transporting flows, imparts fundamental patterns into the stratigraphic record. Recent studies show that paleoenvironmental (allogenic) signals preserved in stratigraphy may be contaminated by internally generated (autogenic) sedimentation patterns; however, it is unclear how the magnitude of autogenic dynamics is related to allogenic forcings. Utilizing statistical methods, we quantify basin-filling trends in three laboratory experiments where input water and sediment flux were varied. We use the compensation index and compensation time scale to estimate the strength of compensation, defined here as the tendency to fill topographic lows faster than would result from random deposit stacking, and to estimate the time scales over which autogenic processes operate. In the experiments, topography of channelized deltas formed by noncohesive sediment was monitored in a basin experiencing temporally and spatially uniform relative subsidence. Each experiment resulted in construction of a stratigraphic package in excess of 25 channel depths thick. We find that compensation strength in the experiments is not influenced by absolute magnitudes of water and sediment flux but does increase as a function of the ratio of water to sediment flux. A compensation time scale, defined as the maximum depth of a system's channels divided by the long-term deposition rate, accurately defines the maximum time scale at which autogenic dynamics occur in all experiments. When applied to field-scale systems, we predict that autogenic dynamics occur out to time scales between 5 and 150 kyr. ©2013. American Geophysical Union. All Rights Reserved.