Towards a Quantitative Method for Estimating Paleohydrology from Clast Size and Comparison with Modern Rivers

Abstract

Abstract An outcrop-based methodology is presented to reconstruct bankfull depth of fluvial channels from clast size based on well established geomorphological procedures. Results obtained from a Lower Old Red Sandstone (Ludlovian to Emsian) example from the Midland Valley Basin, Scotland, are calibrated against existing methodologies for reconstruction of bankfull channel depth in fluvial sandstones which utilize preserved channel thickness and preserved dune foreset height. The proposed methodology can be applied to pebbly sandstones and conglomerates of fluvial origin. It does include significant uncertainties, but these are of a range similar to the existing approaches for reconstructing bankfull depth in sandy fluvial systems. Estimates of bankfull channel depth can be used to reconstruct paleo-drainage area using paleo-discharge data, derived from geomorphological “regional curves.” These regional hydraulic-geometry curves are constructed for modern rivers with specific hydro-physiographic catchment properties and constrain the relationship between bankfull depth and discharge, and bankfull depth and drainage area. By comparing the results of outcrop datasets with the regional curves from similar inferred climate regimes, an estimate of paleo-discharge and paleo-drainage area can be derived. Utilizing these paleohydrological approaches from outcrops of fluvial strata allow quantitative constraints to be placed on paleogeographic reconstructions, give a better understanding of paleoclimate and help to constrain provenance studies. In addition, comparison of the drainage areas and bankfull discharges obtained from this analysis for Lower Old Red Sandstone fluvial systems with global data for modern rivers reveals good agreement. This indicates that using region-specific hydraulic-geometry curves to reconstruct ancient fluvial systems can be applied to large rivers interpreted to have a dryland setting possibly sourced from another climatic region, as is commonly the case in modern dryland river systems. It is recommended that by integrating several techniques more accurate estimates of paleohydrology can be obtained to better reconstruct ancient fluvial environments.