Size and scale of rivers versus valleys in ancient source to sink systems

Source to sink systems include 1) the catchment area, which comprises tributive river systems that feed, 2) a trunk river that in turn disperse sediments in a, 3) distributive system. Each component contains channels that may cover a range of sizes and scale. Depth, width, grain size, integrated with facies architectural analysis can be determined in ancient stratigraphic successions to estimate paleodischarge. Channel depths typically range over 3 orders of magnitude from 1 up to 100 meters, whereas paleodischarge can vary over 6 orders of magnitude up to 200,000 cumecs. Interpreting ancient rivers requires distinguishing single channel fills, confluence scours, and incised valleys. Valleys are recognized by a mismatch between the size of the incision versus the size of an individual channel fill and commonly contain trunk rivers. Confluence scours may be expressed as abnormally-thick channel fills exhibiting large scale foresets that fill the scour, but are rarely multi-story. Confluence scours occur at the downstream end of bars or downstream of tributary junctions, and can be distinguished by the scale (i.e., depth relative to other channel features). Bars and bedforms scale to flow depths such that larger flows produce larger bars and bedforms and these can be deciphered in ancient systems using facies architectural analysis. However, flood stage bars and bedforms may be reworked during falling or low stage flows. Ancient rivers systems can show highly variable preservation of bars and bedforms and this may increase the uncertainty in estimating paleodischarge. However, regional considerations, such as reconstructed paleogeography, can be used to place upper limits on the scale of an ancient source to sink system. This presentation will illustrate these points with examples from the Mesozoic of North America, including the Torivio Sandstone in New Mexico, the Ferron Sandstone in Utah, and the Permo–Triassic Ivishak Formation in Alaska.