MASS-BALANCE EFFECTS IN DEPOSITIONAL SYSTEMS

Net deposition is necessarily accompanied by overall loss of sediment mass from the transport system; on long time scales the deposition compensates for subsidence and creates the stratigraphic record. Although its spatial pattern changes and it can be locally stopped or reversed, depositional mass loss is a fundamental driving influence on the morphology and behavior of depositional systems. Mass extraction to deposition leads directly to facies changes as sediment flux declines; indirectly it usually leads to down-channel sediment fining as deposition preferentially removes coarser particles. Laboratory experiments illustrate the effects of systematic mass extraction on fluvial channel stacking and deposit grain size, and show how a mass-balance framework allows consistent comparison of facies despite major shifts in depocenter location. Mass-balance analysis of experimental fluvial and turbidite systems, and a turbidite mini-basin from the Gulf of Mexico, show a change from channel-dominated to lobe-dominated deposits at about 80% total mass extraction. Experimental and field studies also show a close connection between rate of mass loss and rate of downstream grain-size fining, as predicted by a mechanistic theory based on mass balance. These are first steps towards a framework for quantitative basin analysis based on mass extraction. A mass-balance framework allows for consistent, quantitative comparison across basins of varying scale and shape. Mass-balance frameworks can account for overall and/or generic mass-balance effects, and help separate them from local effects. One way to use such generalized models effectively is to view the results as reference cases of down-transport change resulting from the basic interplay of spatial mass-extraction pattern and sediment supply. Where such reference cases do not provide detailed predictions of specific field cases, they can still provide a baseline to separate local, case-specific features from generic system behavior. The approach is analogous to the way the geoid serves as a reference case against which to measure gravity anomalies.