A sequence-stratigraphic interpretation of a mudstone-dominated succession: the Lower Jurassic Cleveland Ironstone Formation, UK

Abstract

Microfacies analysis (using optical and backscattered electron imagery) has been undertaken on 37 samples from the Cleveland Ironstone Formation, Staithes, Yorkshire, UK. Four lithofacies were encountered: clay-rich mudstones, silt-rich mudstones, sand-rich mudstones and concretionary carbonates (two types: berthierine/siderite-rich ironstones and apatite-rich concretionary horizons). Successive units were found to stack systematically into upward-coarsening units on a small (0.1–1.0 m) scale. Upward-coarsening and upward fining packages were also found on a larger (1.0–3.0 m) scale. The small-scale upward-coarsening units are interpreted to be parasequences; while the large scale upward-fining and upward-coarsening packages are interpreted to be retrogradational and progradational parasequence sets respectively. The ironstones occur at the level where the stacking patterns change from being overall progradational to retrogradational, while the apatite-rich concretionary units occur at the level where the stacking patterns change from being retrogradational to progradational. The concretionary carbonates are interpreted to have formed close to/at major stratal surfaces. The ironstones are interpreted to be either the correlative equivalent of sequence boundaries or part/all, of the forced regressive system tract; while the apatite-rich units are interpreted to have precipitated close to/at maximum flooding surfaces (condensed sections) and/or major transgressive surfaces.