STRATIGRAPHIC EVIDENCE FOR NON-UNIFORM HYDRODYNAMICS IN THE TULLIG SANDSTONE, WESTERN IRISH NAMURIAN BASIN, CO. CLARE, IRELAND

Fluvial-deltaic systems are typically characterized by non-uniform flow condition. This creates a critical issue for analyzing the paleohydraulics of ancient fluvial-deltaic systems, as traditional reconstruction relations are based on steady and uniform flow conditions. Indeed, non-uniform flow conditions impact the stratigraphic record, as has been recently demonstrated in a number of numerical model and physical experimental studies. From this work, a series of methods and metrics have been proposed to facilitate identifying spatially varying channel hydrodynamics in ancient systems; however, they have yet to be rigorously tested using examples from the rock record. The present study investigates paleohydraulics in an ancient fluvial-deltaic system, preserved in the Tullig Sandstone of the Western Irish Namurian Basin (WINB). The spatial variability in sandstone facies were measured, including grain size and stratal architecture, and used to infer channel hydrodynamics. Paleohydraulic reconstructions, based on data from the Tullig Sandstone, indicate that this ancient channel system was located within the reach of non-uniform river flow. For example, the coarsest fraction of sediment decreases down-dip, and cross-set and bed thicknesses increase, and then decrease down dip, a pattern which is consistent with theoretical model predictions. Furthermore, statistics of the cross-set and bed thicknesses suggest enhanced channel bed aggradation, which is consistent with non-uniform flow. These observations and analyses agree with simulated hydraulic conditions, using a stratigraphic inversion scheme with a backwater morphodynamic model, for the channels of Tullig Sandstone. This study therefore provides a test for the theory used to assess trends in cross-set strata arising due to systematic spatial variability in channel hydrodynamics. Moreover, the distinct signatures of these trends provide a means to improve paleohydraulic reconstruction of ancient fluvial-deltaic systems.