Mid-dinatian (chadian) limestones in Gower

Abstract

The Chadian rocks of Gower comprise the Caswell Bay Oolite beneath and the Caswell Bay Mudstone above. The Oolite is generally a massively bedded ‘pure’ light-grey limestone of shallow-water origin, marking nevertheless not ‘eustatic’ marine ‘ regression ’ but sustained subsidence of more than 40 m. In detail it shows much variation and includes macrofossil beds, some of crinoid-brachiopod-coral limestones, that indicate proximity to open-sea environments. It gives signs of rapid lithification, including contemporary channels cut into it. Algal wisps and fragments, and algal veneers, are not uncommon in the formation but are a very minor feature of it, so that its terminal member, the Heatherslade Bed, is in great contrast: as an algal sheet, stromatolitic and spongiostrome, the bed formed a carpet over perhaps the whole of Gower, and probably continued to east and to west for many kilometres. A non-sequence above the Heatherslade Bed is indicated by a breccia, haematitestained, with pseudomorphs after gypsum, interpreted as a desiccation breccia. The Caswell Bay Mudstone, the breccia its basal member, reaches a thickness of 13 m. It is a very mixed group of rocks, greatly contrasting with the Caswell Bay Oolite in an association of calcilutites, pellet rocks, algal layers, pisolites, ‘im pure’ oolites, and gypsiferous and sabkha-type sediments, in most of which silt-size detrital quartz grains, almost wholly absent from the Oolite, are common. A description of the rocks as ‘lagoonaP is more or less appropriate, but the very rapid lithological alternations in laminar sequence imply a fluctuating multiplicity of controls needing more refined analysis of the sediments than is offered by Tagoonal’. Fossils, although abundant in the formation, are in restricted facies; but recurrent thin layers and pockets, and more widely scattered fragments, of crinoids and brachiopods, among the laminae otherwise Tagoonal’, again point to proximate open-sea sources; and although the rocks are of very shallow-water intertidal or supertidal origin, their sustained accumulation points to an accordance of sedimentation with subsidence, and not to ‘eustatic’ marine ‘regression’. Pre-Arundian slumping is repeated at several horizons in the formation, some of the slumped masses incorporating exotic corals carried in from neritic sources. A palaeogeography of the stratigraphical changes is organized in terms of a fluctuating depth of sedimentation (within a narrow range) and of access to open sea in a bank or shelf environment undergoing slow subsidence. The great differences between the Oolite and the Mudstone cannot be explained simply by ‘internal’ differences in rock formation, but they may well reflect gentle tectonic movement hinted at by the Heatherslade Bed and the immediately succeeding non-sequence, and by the intraformational slumping in the Mudstone. The Chadian sequence is abruptly terminated by Arundian overstep, demonstrated by pre-Arundian erosion, a basal Arundian breccia, visible unconformity, great contrasts in lithology, and the fossils. A reconstruction of the form of the overstep suggests gentle uplift to the south of Gower, in anomalous tectonic relation with the generalized palaeogeography of Dinantian sedimentation in South Wales.