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Estimating the Size of Ancient River Systems and Linked Depositional Systems


The source to sink concept is focused on quantification of the various components of siliciclastic sedimentary systems, from initial source areas through the dispersal system and deposition within a number of potential ultimate sedimentary sinks. The maximum size of depositional systems, such as rivers, deltas, and submarine fans, is significantly controlled by the area, relief, and climate regime of the source area, which in turn may linked to the plate tectonic and paleogeographic setting. There has also been significant recent debate regarding estimates of the size of ancient river systems and the identification of big rivers. Examination of ancient river systems in the rock record, and especially the largest trunk rivers, which are typically within incised valleys, can be used to estimate paleodischarge, which in turn can be linked to the drainage basin to make estimates about the size and sale of the source area. The best estimates can be made in basins with well-constrained data that allow details of cross-sectional or plan-view channel-architecture to be determined, such as extensive outcrops, or abundant subsurface data, and especially where higher resolution 3D seismic data are available. Paleodischarge estimates of lowstand Quaternary-age continental-scale ancient rivers from passive continental margins, using seismic data, are orders-of-magnitude higher (1000's of cumecs) than smaller-scale Cretaceous lowstand systems that drained into the Western-Interior Seaway of North America (100s of cumecs). Paleodischarge of rivers can also be estimated independently by integrating estimates of drainage basin area and paleoclimate. These can be compared with paleodischarge estimates based on the river deposits themselves. A more quantitative approach to estimating the scale of sedimentary systems, and especially in the context of source areas, also puts constraints on the size and scale of potential hydrocarbon reservoirs and thus has economic value.