Abstract
In general, limited chronostratigraphic control in the rock record precludes us from determining how alluvial stratigraphy is assembled over short to intermediate time intervals (<1-10 kyr). Consequently, our resolution of the avulsion history and aggradation rate in natural systems is of the order of paleomagnetic, bio-stratigraphic, volcanic, or other coarse time data. Theoretical models have attempted to address this, and we have created an experimental basin-scale alluvial deposit in the Experimental Earthscape Facility ("Jurassic Tank") at the Saint Anthony Falls Laboratory, University of Minnesota. Using a unique subsidence mechanism, we designed an experiment to investigate the role of various subsidence rates and patterns, as well as climate, on alluvial architecture. The deposit was vertically sliced at 2-centimetre intervals perpendicular to mean flow direction to analyze the stratigraphy. Precise knowledge of the fluvial surface topography and the "basement" elevation every four hours of the 218 hour run allowed us to create maps of aggradation over short intervals. Sequential overhead photographs of the fluvial system allowed us to determine statistically which areas of the basin the flow occupied over the same time intervals, as well as map avulsion nodes and scour migration patterns. We compared these surface data with aggradation maps and the deposited stratigraphy in order to investigate the assembly of the stratigraphy. The aggradation maps over short time intervals (<10 hrs) do not necessarily reflect the subsidence. Rather, the basin is filled through time by a shifting patchwork of depocenters that opportunistically fill low-lying spots on the fluvial surface. These short-term depositional maxima do not correspond to the zones of flow occupation. The time needed before the rate of deposition begins to reflect the subsidence pattern is of the order of that required for aggradation of a sediment thickness a few times the mean channel depth. The same time scale governs the spreading of water over the sediment surface by channel switching and lateral migration.