Deposition and diagenesis in an extensional basin: the Corallian Formation (Jurassic) near Oxford, England

Summary The mixed carbonate-siliclastic Corallian Formation was deposited on horsts and in grabens which were produced during Jurassic extensional tectonics. The high areas became sites for sandstone and carbonate deposition in shallow subtidal environments while clays and allochthonous debris accumulated in the depressions. Tectonic activity, perhaps confined to single fault blocks, produced depositional variation distinct to each area. The basin has been subdivided into four major east-west trending fault blocks. The most northerly of these is the Oxford Shallows, interpreted as a small accretionary-margin slope. The depositional facies and diagnesis of this area are discussed relative to the basin as a whole. The diagenesis of the Corallian sediments has been divided into three phases of cementation: pre-burial, shallow burial and deep burial. Phase 1 spars include marine precipitates and non-ferroan, neomorphic and drusy spars. Phase 2 cements are ferroan, precipitating during shallow burial, including an intermediate ferroan (mauve) spar which may represent development of a redox front. Phase 3 spar post-dates vuggy dissolution which probably took place during Barremian uplift. The cement precipitated during the ensuing burial phase. Geochemistry and stable isotope data show an overall fluid evolution path for the first two spars, although Sr 2+ values in replaced corals reveal variability in the degree of meteoric flushing. The chemistry of the late spars suggests low water/rock ratios, where local dissolution provided the cations for the cement. The oxygen isotope values suggest, however, that meteoric and connate waters were mixed at slightly elevated temperatures.