The diagenesis of the Great Estuarine Group, Middle Jurassic, Inner Hebrides, Scotland

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Abstract
Summary: The Bathonian Great Estuarine Group consists of sandstones, silty shales and mainly shelly limestones, deposited in micro-tidal brackish lagoons in a warm, seasonal climate. The rocks show, in general, a lack of intense early diagenesis or deep-burial diagenesis, and are thus suitable for the study of shallow-burial diagenetic processes. Early diagenetic changes can be directly linked to depositional environment. They were only important on the lagoon margins, where cyanobacteria flourished in schizohaline ‘algal marsh’ settings and dolomite formed in response to evaporation of low-salinity lagoonal waters. During burial to a few hundred metres, and after initial compaction, cementation by ferroan calcite occurred. This was pervasive in limestones and formed large concretions in sandstones. Quartz and feldspar grains in sandstones were marginally corroded by the porewaters that precipitated calcite, and some feldspars had previously or concurrently suffered partial solution. Aragonitic mollusc shells were replaced by calcite in permeable rocks but remained unaltered in shales. Widespread volcanism in the Palaeocene buried the area beneath basaltic lavas and dykes were intruded. These events produced little metamorphic or burial-diagenetic change, except in close proximity to minor intrusive contacts or in the vicinity of plutons. The evolution of porewaters can be monitored in the carbon and oxygen isotopic composition of diagenetic calcites. After initial burial, they were dominantly of meteoric derivation. Vitrinite reflectance measurements confirm the mild thermal history of most of the sediments, and show the local effects of heating by igneous intrusions.