SEDIMENT MASS EXTRACTION AS A PREDICTIVE TOOL IN SOURCE TO SINK STUDIES

The systematic down-transport loss of sediment mass to deposition leads to systematic downstream changes in transport regime, grain size, and sedimentary facies. To the extent that the spatial distribution of this mass loss can be deduced from large-scale stratigraphic geometry (e.g. by mapping volumes within approximate chronostratigraphic surfaces), it can be used to make approximate predictions of sedimentary attributes. A straightforward means of doing this is to replace downstream distance with a dimensionless variable chi representing the fraction of supplied sediment lost to deposition to that point, or an equivalent dimensionless measure of relative bypass. We illustrate this using a series of examples from channelized systems. Theoretical modeling suggests that for a given sediment input, the rate of bulk downstream fining should be directly related to the relative rate of mass extraction (deposition), and recent field observations support this idea. Systematic variation in channel stacking density and predominance of channels versus lobe deposits is also predictably related to the chi profile in fluvial and turbidite systems, both field and experimental. In particular, the transition to lobe-dominated sedimentation appears consistently to occur at an extraction (chi) value of about 0.7 – 0.8. Mass extraction analysis provides a potential unifying theme across environments and length and time scales. Although it is clear that not all variability in sedimentary systems can be accounted for using mass balance, it is also clear that at this point the methods have not been developed or applied to the point where we can estimate how much stratigraphic variation it can account for.