Petrography and geochemistry of a limestone-shale sequence with early and late lithification: the Middle Purbeck of Dorset, England

Abstract

The Middle Purbeck Group (Upper Jurassic-Lower Cretaceous) consists predominantly of limestones and shales of shallow, brackish-water origins. The type-section, Durlston Bay, was investigated and correlations made with other exposures. Samples were studiedwerepetrographically, examined by X.R.D., and 200 of these analysed for 28 elements mainly by X.R.S. Porosities and heavy minerals were studied.The limestones, the "Purbeck Stone", are mostly bivalve biosparrudites. Bivalve biomicrudites, ostracod hiomicrites and charophyte biomierites also occur. The micrite was calcite mud, often argillaceous: most bivalves were aragonite and this is unchanged in thin shell beds within shales. Partial dissolution has led to precipitation -of fibrous calcite or 'beef. Porosities of biosparrudites increase with increase in aragonite, organic carbon and clay. Calcitized minor evaporites were discovered; calciostrontianite is reported.Chemistry of the limestones is mostly controlled by clay content and by extent of diagenesis. Sparry calcite contains less Mg than micrite. Some migration of Ni, Fe, Zn,S and organic matter from shales into aragonitic beds has occurred. Manganese contents, however, have apparently undergone little change during aragonite-calcite inversion.The shales consist of illite with montmorillonite. Kaolinite occurs above the Cinder Beds as do the heavy minerals kyanite and staurolite. The shales have chemical composition similar to the world average. Rubidium, Ba, Sr and Pb substitute for potassium. Zinc, Fe, Ni, Mn, P and organic carbon are relatively enriched in the calcareous shales. Sodium occurs in calcite. The clays have been derived from older marine sediments and there seemsu9W4rdsto have been a progressive increase in sedimentation rate probably due to increase in rainfall. Lithification of the biosparrudites, the most common limestones, took place in two major stages. Some were lithified early in vadose conditions. These are uncompacted. and occur as thick beds, occasionally with dinosaur footprints. Most biosparrudites, however, were compacted first and then lithified at a late stage. These, which retain more Sr, and organic matter, occur as thinner beds intercalated in shale sequences. Many characteristics, both chemical and petrographic, of these limestone types are detailed here and criteria given for their recognition.

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