Sedimentology and diagenesis of the Simonstone limestone (carboniferous) of the Yoredale Group over the Askrigg Block, north Yorkshire

Izzidien, Saad Yousif (1984) Sedimentology and diagenesis of the Simonstone limestone (carboniferous) of the Yoredale Group over the Askrigg Block, north Yorkshire. University of Southampton, Doctoral Thesis.

Abstract

Sedimentological investigation of 196 outcrops, which constitute almost all accessible sections of the Simonstone Limestone over the Askrigg Block has been carried out in detail. Outcrops were lithologically described and correlated throughout Wharfedale, Littondale, Ribblesdale, Dentdale, Wensleydale, Garsdale, Swaledale and the nearby regions. Over 600 samples were collected on a bed-to-bed basis from the well exposed outcrops and more than 300 thin-sections were prepared and stained for petrographic examination. Many well preserved fossils have been collected and identified. For the limestones and the intercalated shales, several facies maps were constructed and palaeoenvironmental conditions were interpreted. 62 samples were selected for chemical analysis from seven more or less evenly distributed outcrops, in an attempt to explain some environmental and diagenetic problems. The samples were analyzed for SiO2, Al,o2, MgO, CaO,Na2O, K20 using the B-probe technique, CO2 and organic matter using the loss-on-ignition technique, Ti, Sr, Ma, and Ba using X-ray fluorescence analysis. X-ray diffraction analysis was used to determine the mineral content of specific samples. A scanning electron microscope has also been used to examine the mineral-content and microfossils in a few samples. The ideal section of the Simonstone Limestone as revealed from field investigations, comprises a thin shale bed at the base (Shale I); a lower division (0.50m-2.50m), of coarse grained calcarenites containing abundant crinoids and bioclasts with a distinctive Lithostrotion junceum biostromeat the top; a middle division (1.15m-5.74m), of several beds of calcilutite and calcarenite containing sparse crinoids and much finely comminuted shell debris, with a well defined mottled limestone bed and three thin shale beds (Shales II, III, IV); and an upper division (1.85m -6.1m) of coarse grained calcarenite containing abundant crinoid debris with oncolites and zaphrentoid corals, particularly near the top. Petrographic examination shows that the Simonstone Limestone consists mainly of packed biomicrites, packed biomicrudites and sparse biomicrites; biosparites are only local. All the in-place fossils and the derived bioclasts are exclusively marine. Deposition of the Simonstone Limestone took place in a shallow marine environment of normal salinity. Both the lower and upper divisions were deposited under semi-open water conditions throughout which crinoid thickets lived, died and disintegrated. The most favourable growth site was the southern edge of the Askrigg Block where their debris now dominates the rocks. Deposition of the middle division occurred under semi-restricted water circulation, unsuitable for significant crinoid growth, with signs of emergence at the level of the mottled limestone. Tectonic movements are interpreted to have altered the environmental conditions and caused such restriction. Deposition was followed by complicated diagenetic processes including compaction, cementation, neomorphism, micritization, dolomitization, veindevelopment and silicification. Such processes caused local or widespread changes in the rock textures and their mineral and chemical components. The latter is more apparent at levels close to compacted shales, from which migrating solutions brought excess of ions, particularly Mg2+ and Fe2+. Other elements, such as Mn, are more resistant to diagenetic alteration, andsupport field and petrographic evidence of palaeoenvironmental conditions.

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