A Novel Approach to Constraining Carboniferous Tidal Currents using Bedforms in Tidal Rhythmites

A novel methodology shows that the dimensions of current ripples within tidal rhythmites can be used as a proxy for tidal current velocity, allowing us to contribute to the validation of numerical tidal model simulations. Our understanding of changing tides through geological history is facilitated by tidal simulations, which are generally poorly constrained due to the limited availability of proxy data. We aim to rectify this by developing a new type of geological proxy for tides based on sedimentary textures and structures, as bedforms are widely reported but uncommonly measured in the literature. The Carboniferous is a particularly data-rich time period with globally abundant tidal lithofacies including tidal rhythmites; successions of rhythmically alternating coarser and finer layers which can be used to describe tidal cyclicity, changes in the Earth – Moon system, and palaeoenvironmental conditions. Using data collected from a previously unstudied succession of Late Carboniferous (318 Ma) tidal rhythmites in Pembrokeshire, South Wales, UK, and empirical relationships identified through a series of flume studies in the literature, we deducted that the current ripples in our studied outcrop were formed at tidal flow velocities ranging between 0.28 and 0.34 m s^-1. The latest palaeogeographical reconstructions depict South Wales as entirely continental, however the studied section revealed evidence of deposition in a shallow-marine palaeoenvironment. Identifying these palaeoenvironmental inaccuracies such as these allows us to rectify the palaeogeographical reconstructions; once tuned, the numerical tidal model simulation matched well with our proxy results. These promising findings demonstrate proof-of-concept of utilising bedforms as a proxy for palaeotides as well as its feasibility to validate tidal model simulations of other geological time periods and areas.