The petrology and diagenesis of Middle Jurassic clastic sediments, Ravenscar Group, Yorkshire



    1. Geology Department, University of Hull, Hull HU6 7RX, U.K.
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      Koninklijke Shell Exploratie en Produktie Laboratorium, Volmerlaan 6, Rijswijk ZH, The Netherlands.


The petrology of non-marine clastic sediments from the Middle Jurassic Ravenscar Group was investigated to identify and to assess the significance of possible controls on diagenesis. Diagenetic modifications took place in three broad regimes: within the depositional groundwater (eogenesis), during burial (mesogenesis), and during uplift and erosion (telogenesis).

Eogenesis involved the initial interaction of the original sedimentary assemblage with its depositional pore waters. Eogenetic processes were influenced strongly by bacterial degradation of organic matter present in the finer grained sediments. This bacterial activity caused a lowering of pore water pH, and subsequently reduced Eh. Feldspar dissolution and alteration of muscovite to kaolinite occurred. Quartz overgrowths and vermiform kaolinite precipitated in sands with oxygenated and mildly acidic pore waters, whereas chlorite and quartz overgrowths formed in sands with anoxic and neutral pore waters. This caused complete reduction of porosity in both finer-grained and texturally less-mature floodplain-facies sandstones. Channel-facies sandstones were cemented only with a thin veneer of quartz overgrowths, creating a rigid but still porous quartzose framework. Bacterial ferric-iron reduction throughout the water table subsequently established more ubiquitous conditions, raised pH, and caused rhombic siderite cementation within the remaining porosity.

Mesogenetic modifications, of the mineralogy formed at the surface, occurred when pore water composition changed during burial. Bicarbonate saturated solutions migrated through the surviving porosity, precipitating replacive, poikilotopic ferroan calcite. Carbonate dissolution occurred subsequently. Carbonate dissolution was possibly caused by the same solutions which then precipitated dickite and further authigenic quartz.

Telogenetic modifications took place during uplift and exposure. Groundwater reactions and subaerial exposure probably caused further carbonate and feldspar dissolution, as well as alteration of chlorite to interstratified chlorite vermiculite.

It is concluded that the diagenesis of these non-marine clastic sediments reflects various factors, the complex interrelationship of which precludes the identification of any single factor as wholly controlling their diagenesis. Eogenetic modifications involved the interaction of the sediments and their pore waters. As a result of the metastability of the mineral assemblage established at this time, these modifications can still be recognized. despite the sediments' subsequent history.