The sedimentology of the Marlstone Rock Bed and Dyrham Silt Formations (Pliensbachian, Lower Jurassic) of the Cotswold Hills
The formations were examined along the Cotswold scarp (160km), and subcrop data were also utilised. The spatial and temporal characteristics of the formations allow sedimentological patterns to be related to structures in the pre-Permian basement and in the overlying Middle Jurassic strata. Sedimentation was strongly controlled by an actively subsiding block faulted basement, which formed part of the North Atlantic Rift system. The generalised Pliensbachian-Bajocian model of Sellwood and Jenkyns (1975) is supported by evidence in the Cotswolds. Both formations show cyclic sedimentation characterised by upward changes in grain size, mineralogy, thickness, sedimentary structures and fauna. Spatial patterns reflect the N-S structures of the basement. Primary controls on the cyclicity are shown to be tectonic rather than eustatic. Five facies are recognised in the Marlstone Rock Bed Formation. The stratigraphic interpretation of the formations is refined. There was a break in sedimentation at the end of the Pliensbachian. The base of both formations is diachronous, and spread from the centre of the basin outwards to both E and W margins. Randomly-interstratified illite-smectite in these rocks is interpreted as a weathering product of illite, while smectite was produced by alteration of air-fall volcanic ash. Both were derived from adjacent land areas. Ferruginous ooids probably formed through mechanical and/ or. algal accretion in temporary reducing conditions on the sea bed. The iron-rich sediments were formed at the boundary between siliciclastic and carbonate regimes. True ironstones are virtually absent as a result of rapidly changing patterns of sedimentation within the rift. Widespread 'wavy' bedding is shown to be mostly diagenetic pseudo-bedding, although some appears to have been produced by wave rippling or by compaction alone.