Geology and palaeontology of the Telychian (Silurian), reservoir formation of the North Esk inlier, near Edinburgh, Scotland

Bull, Elizabeth Eleanor (1996). Geology and palaeontology of the Telychian (Silurian), reservoir formation of the North Esk inlier, near Edinburgh, Scotland. PhD thesis The Open University.



The North Esk inlier is one of several outcrops of Silurian sediments in the Midland Valley of Scotland. The lowermost sediments exposed are the Reservoir Formation, and these have been studied in detail. These sediments are interbedded shales and siltstones, deposited as the deepest part of a regressive sequence. The sediments are suggested to have been deposited predominantly by storm processes in an outer shelf or marginal basin environment, and not by turbidity currents into a deep ocean basin as previously suggested. Decreasing water depths can be traced by changes in sedimentary structures and in the animals and their bioturbation traces fossilised therein. A correspondence between the graptolites preserved in the North Esk group and models for distribution of graptolites in depth related zones is observed and is compared with other taxa thought to be indicative of water depth. Cycles of sedimentation are identified and also record fluctuations in the water depth.

The NEI sediments were deposited in the remnant lapetus ocean basin at the time of a three plate collision. The sediments are currently sub-vertically oriented. Emplacement at this attitude was probably associated with thrusting at depth, in turn related to strike-slip movement o f separate terranes along the southern margin of Laurentia. Minor small scale bedding-parallel thrusts are evidence of this at the surface. The North Esk inlier sediments were probably part of a tcrranc now of unknown extent. Minor folding, local thrusting and faulting, indicating one major phase of tectonic deformation are described. Compressional kink folding took place, with pressure applied obliquely to foliation (bedding). Areas of chevron folding and kink bands correspond to deformation along Caledonian trends. The northern boundary of the NEI, previously thought to be fault controlled, is reinterpreted as the regional unconformity at the base of the Upper Old Red Sandstone. Metabentonite horizons are identified. The volcanic source of these ash falls is suggested to have been of calc-alkaline character, and an increase in alkalinity of the volcanic source in Scotland during the Telychian is suggested.

A study of the fossils preserved has led to a review of late Llandovery graptolite biostratigraphy. A biozonal scheme based on that currently in use in other parts of the world, particularly in Bohemia, is introduced. Three Llandovery graptolite biozoncs arc recognised between the crenulata biozone and the base of the Wenlock. The Reservoir Formation sediments are correlated with the middle of the O. spiralis graptolite biozone, upper Telychian. Comparisons are made with the biozonal schemes for other fossil groups, and the P. celloni. to P. amorphognathoides conodont biozonc boundary is accurately identified within the Gutterford Burn sediments, facilitating accurate correlation between graptolite and conodont biozonal schemes.

Revised taxonomic descriptions and notes on a number of species of graptoloid and dendroid are presented. Further work on the dendroid Dictyonema pentlandica has identified normal and abnormal dendroid growth patterns. The response of dendroids to trauma, and disease are described and possible uses of dendroids as palaeoenvironment and palaeocurrent-direction indicators are discussed. A model for the secretion of dendroid holdfasts by mobile zooids distributed across the sediment surface is suggested, and possible dendroid reproductive processes are outlined.

A new technique of producing graptolite “Profile Plots” is introduced. These plots are used to identify, characterise and compare different graptoloid species, where a number of species with similar thecal structure are known. The rate of expansion of the stipe at different levels within the colony and the thecal size can be compared, and characteristic profiles plotted for each species. This technique has possible future uses in the computerised identification of graptolites.

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