ABSTRACT

Sedimentological and petrographic data from two Lower Liassic ooidal ironstones (the Frodingham and Pecten Ironstones) and associated mudstones from eastern England have been collected. Within these ironstones early diagenetic berthierine, siderite, and minor glauconite have a limited lateral extent, associated with an intrabasinal high (the Market Weighton Swell). This mineral assemblage passes laterally and vertically into mudstones and silty mudstones within which pyrite is the dominant early diagenetic iron mineral. The ironstones are stratigraphically condensed, having experienced low net sediment accumulation rates and shallow, high-energy conditions.

Thermodynamic considerations indicate that berthierine and siderite should be stable in anoxic environments where sulfide activity is negligible. Pyrite is rare within the ironstones, suggesting that sulfate reduction was a negligible process during ironstone formation. Where pyrite is present it postdates berthierine and siderite, indicating that sulfate reduction may have been a later process in some beds. We propose that berthierine and siderite were precipitated during suboxic diagenesis, where sulfate reduction was inhibited and iron reduction was the dominant process of organic-matter oxidation. Suboxic diagenesis resulted from the extended periods of physical and biological sediment reworking during ironstone deposition, which favored the oxidation of iron minerals and aerobic egradation and loss of organic matter. Berthierine formed during early suboxic diagenesis at the expense of detrital clays and iron oxides. During later suboxic diagenesis, pore-water bicarbonate activities increased, leading to siderite precipitation. Early glauconite also formed in places, and it seems likely that it formed close to the oxic/suboxic interface. The predominance of pyrite in the mudstones indicates that sulfate reduction was the major process of organic-matter oxidation during early diagenesis.