Geomorphological regional curves for prediction of drainage area and screening modern analogues for rivers in the rock record

A new approach is recommended for estimating the catchment size and paleohydrology of ancient rivers from analysis of their sedimentary record. Estimates of drainage area have significant predictive value across a broad spectrum of geological applications, though attempts to derive them are remarkably uncommon. Previous sedimentological studies have relied on discharge-area relationships created by amalgamating data from a wide range of hydro-physiographic catchment characteristics. This averaging can lead to large errors in drainage-area calculations. Geomorphologists and engineers have been developing an extensive database of relationships between drainage area and channel dimensions or discharge that account for the catchment response to specific climates, lithologies, and hydrological processes. These regional hydraulic geometry curves (“regional curves”) are independent of channel planform and are based on bankfull discharge, which is the effective channel-forming discharge and is more relevant to the construction of the rock record than the mean or peak discharge values in the global-average curves used by sedimentologists previously.

The value to sedimentologists of these region-specific relationships is demonstrated with a selection of examples that show how this approach is beneficial for assessing the uncertainty in catchment interpretations and for exploring the range of possibilities. It encourages the sedimentologist to think more carefully about the climatic and physiographic setting of the successions being interpreted. The approach assists with selection of modern analogues for rivers in the rock record, which should not be chosen merely by comparable channel dimensions, but also by comparable climatic regime, latitude, and catchment scale.

When applied to the Hawkesbury Sandstone (Middle Triassic) of the Sydney Basin, Australia, this approach indicates the modern Brahmaputra River is an inappropriate modern analogue, despite the similarity of channel dimensions. Applied to the Ferron Sandstone Member (Upper Cretaceous) of southwest Utah, the approach suggests that catchments may have been wider and deltas laterally more extensive. The method shows the Kayenta Formation (Lower Jurassic) of southwestern Colorado and eastern Utah cannot have been sourced only from the nearby Ancestral Rocky Mountains and suggests that it is the product of a much wetter setting than usually inferred. In effect, this approach forces sedimentological interpretations to be validated against geomorphological understanding of river behavior. The benefits and limitations are contrasted with existing sedimentological methods of determining drainage area, with the aim to better capture the range of uncertainty in interpretation for ancient rivers.