Ooidal ironstones from Ordovician Gondwana: a review
Abstract Ooidal ironstones occur widely in the Ordovician shelf sequences of SW Europe (the Western European Platform), the Avalonian Terranes and North Africa. They form the most important group of deposits of the earlier of the two major periods of Phanerozoic ooidal ironstone generation (Ordovician-Devonian and Jurassic-Paleogene). Individual ooidal ironstone horizons are generally thin (less than 2 m), but may have a very wide lateral extent. The ooidal ironstones represent sediments formed under extremely low sedimentation rates and were most commonly developed as the initial deposit above a disconformity. Ooidal ironstone-producing events occurred almost synchronously over wide areas of the Gondwanan shelf, reflecting the uniform sedimentary conditions across large areas of the shallow, low gradient shelf. Clastic sediments deposited during periods of “normal” sedimentation show similar remarkable uniformity over large areas. This geometry, together with the abundant fauna (both micro- and macro-) in the ironstone facies, means that these thin ironstone beds form important marker horizons for correlation within the region. The stratigraphic distribution of the ironstones shows strong correlation with periods of high relative sea level. Sedimentological study of examples in SW Europe indicate that they separate transgressive systems tracts (with parasequences bounded by phosphorites) from the early part of the following highstand systems tracts (without significant parasequences) and are therefore interpreted to lie on maximum flooding surfaces. Such maximum flooding surfaces occur in the early Llavirn, the late Llanvirn, the early Caradoc and the early Ashgill of SW Europe. Outside SW Europe the ooidal ironstones may have a different relationship to the sequence stratigraphy, possibly reflecting the influence of absolute water-depth.