Lateral and vertical variations in pedogenesis and other early diagenetic phenomena, Middle Jurassic Ravenscar Group, Yorkshire

SUMMARY The effects of early diagenetic processes are widespread in shallow marine and non-marine (deltaic) sediments of the Ravenscar Group. These can be related to both lateral variations in ground water chemistry within the original depositional environment on the lower coastal plain, and to the introduction of sea water into the non-marine sediments and of fresh water into the marine sediments. In the non-marine sediments lateral (catena) variations in soil profiles can be related to floodplain aggradation and abandonment. Near the major channel sands in the Saltwick Formation at Whitby West Cliff, a ganister and seatearth formed in elevated and well drained sediments that were frequently flushed with fresh water. In more distal floodplain areas, ground-waters were influenced by fresh water, but remained generally suboxic, as a result of which siderite cementation occurred. The influence of meteoric water is indicated by the isotopic composition of the cements that formed ( δ 13 C PDB −3.7 to −9.6 ‰, δ 18 O PDB −6.0 to −8.5 ‰). Rhombic siderite cemented the sands, and siderite nodules formed in both sands and shales. Siderite spheruliths formed within more distal soil horizons (such as the Long Nab Member at Crook Ness) where bacterial processes dominated the supply of bicarbonate into the ground water, and in non-marine sediments on abandoned and subsiding delta lobes. Bacterial influence is reflected in the more negative carbon isotopic composition relative to that of the nodules ( δ 13 C PDB −16.2 ‰, δ 18 O PDB −3.5 ‰). Sea water was introduced onto the floodplain locally, providing sulphate ions that were reduced and incorporated into the pyrite that precipitated in the siderite soil horizons. In the marine Millepore Bed sediments at Yons Nab early diagenetic, possibly aragonite, fringes formed on bioclasts and ooliths. These were subsequently deformed by the weight of overburden following renewed progradation. Grain-to-grain contact dissolution and kaolinite precipitation occurred as the fresh groundwater invaded these marine carbonates from the prograding delta above.