Spatial and temporal variations in early diagenetic organic matter oxidation pathways in Lower Jurassic mudstones of eastern England

The Lower Liassic succession of the East Midlands Platform, eastern England, comprises shallow marine mudstones and oolitic ironstones. In these sediments, temporal and spatial variations in the relative importance of early diagenetic anaerobic organic matter oxidation pathways are recorded. The ironstones are stratigraphically condensed and were deposited upon the Market Weighton Swell (MWS), a topographic high in the basin, in shallow water and high energy conditions at times of low relative sea-level. At times of higher sea-level stratigraphically condensed mudstones were deposited upon the MWS. Away from the MWS stratigraphically expanded mudstones were deposited throughout the Lower Liassic. The ironstones contain berthierine (ooids and pore-filling cement) and siderite (pore-filling cement), with rare pyrite replacing berthierine and siderite. Mudstones deposited away from the MWS contain pyrite as the sole early diagenetic iron mineral. Contrastingly, mudstones deposited upon the MWS contain both early diagenetic pyrite and Mn-rich siderite. The presence of berthierine and siderite in the ironstones indicates that sulphide was absent from early diagenetic pore-waters and that Fe(III) reduction dominated anaerobic organic matter oxidation. In contrast, in mudstones deposited away from the MWS the presence of pyrite indicates that sulphate reduction was a predominant pathway for anaerobic organic matter oxidation. In mudstones deposited upon the MWS the presence of both pyrite and Mn-rich siderite suggests that both sulphate reduction and Fe(III) reduction were important during early diagenesis. It is proposed that this spatial and temporal variation in relative importance of early diagenetic anaerobic organic matter oxidation reactions was a result of variation in physical sedimentary parameters (sediment accumulation rate and energy of environment). At times of low net sediment accumulation rates, in shallow water, high energy environments, suboxic diagenesis (Fe(III) and Mn(IV) reduction) dominated, giving rise to the characteristic berthierine-siderite ironstone assemblage. At times of higher sedimentation accumulation rates in deeper, lower energy environments, sulphate reduction dominated, leading to pyrite formation. At intermediate conditions, Fe(III), Mn(IV) and sulphate reduction were all important, leading to precipitation of both pyrite and Mn-rich siderite.