STABILITY AND FACIES ASSOCIATION OF EARLY DIAGENETIC MINERAL ASSEMBLAGES - AN EXAMPLE FROM A JURASSIC IRONSTONE-MUDSTONE SUCCESSION, UK

Sedimentological and petrographic data from two Lower Liassic ooidal i ronstones (the Frodingham and Pecten Ironstones) and associated mudsto nes from eastern England have been collected. Within these ironstones early diagenetic berthierine, siderite, and minor glauconite have a li mited lateral extent, associated with an intrabasinal high (the Market Weighton Swell). This mineral assemblage passes laterally and vertica lly into mudstones and silty mudstones within which pyrite is the domi nant early diagenetic iron mineral. The ironstones are stratigraphical ly condensed, having experienced low net sediment accumulation rates a nd shallow, high-energy conditions. Thermodynamic considerations indic ate that berthierine and siderite should be stable in anoxic environme nts where sulfide activity is negligible. Pyrite is rare within the ir onstones, suggesting that sulfate reduction was a negligible process d uring ironstone formation. Where pyrite is present it postdates berthi erine and siderite, indicating that sulfate reduction may have been a later process in some beds. We propose that berthierine and siderite w ere precipitated during suboxic diagenesis, where sulfate reduction wa s inhibited and iron reduction was the dominant process of organic-mat ter oxidation. Suboxic diagenesis resulted from the extended periods o f physical and biological sediment reworking during ironstone depositi on, which favored the oxidation of iron minerals and aerobic degradati on and loss of organic matter. Berthierine formed during early suboxic diagenesis at the expense of detrital clays and iron oxides. During l ater suboxic diagenesis, pore-water bicarbonate activities increased, leading to siderite precipitation. Early glauconite also formed in pla ces, and it seems likely that it formed close to the oxic/suboxic inte rface. The predominance of pyrite in the mudstones indicates that sulf ate reduction was the major process of organic-matter oxidation during early diagenesis.