A Novel Approach to Constraining Carboniferous Tidal Simulations using Textural and Structural Sediment Properties

Abstract

A novel methodology shows that sedimentary textures and structures in tidal deposits can be used as a proxy for palaeotidal current velocity. Numerical tidal model simulations and paleogeographic reconstructions for the Carboniferous (358.9 – 298.9 Ma) are poorly constrained because of a scarcity of readily available proxy data in the literature. This investigation aims to rectify this problem by utilising relationships between sediment grain size, current ripple dimensions, and tidal flow conditions to estimate palaeotidal current speed from sedimentary data. Dimensional ripple data collected from a previously unstudied succession of 318 Ma tidal rhythmites in Wisemans Bridge, South Wales, UK suggests that the ripples formed at tidal flow velocities of 0.25 – 0.33 m s-1. At this site, there is somewhat poor agreement between these values and the simulated tidal current speed for the M2 constituent only (~0.16 m s-1). However, a simulation using the M2, S2, and K1 tidal constituents and a modified reconstruction to more accurately represent the Wisemans Bridge palaeogeography predicted current speeds of ~0.28 m s-1, agreeing excellently with the bedform-based current velocity estimates. Grain size data was collated from the literature for fifteen tidal deposits across the United Kingdom and United States of America to enable the validation of global tidal model simulations across six Carboniferous timeslices (330 – 305 Ma). The deposits were first used as palaeoenvironmental proxies to highlight inaccuracies in the palaeogeographical reconstructions. The reconstructions for all six timeslices were deemed fairly accurate because almost all proxies plotted into, or very close to, marine environments when their palaeolocations were mapped onto the reconstructions. The grain size data was then used to estimate palaeotidal current speeds. Upon comparison to simulated M2 tidal current speeds, the model underestimated current speed by 32 – 96%. This investigation demonstrates how tidal deposits can be used as proxies to simultaneously validate palaeogeographical reconstructions and palaeotidal models, but also that ripples may be a strong and widely applicable tool for palaeotidal current speed reconstruction.

Date of Award	6 Nov 2024
Original language	English
Awarding Institution	Bangor University
Sponsors	HEFCW
Supervisor	Mattias Green (Supervisor), Jaco Baas (Supervisor) & Sophie Ward (Supervisor)