Depositional environments in the Petershill Formation, Bathgate, West Lothian

The name "Petershill Fm. is proposed for the 30-40 m thick sedimentary sequence of Visean (V3c) age, occurring within a thick pile of Carboniferous lavas which form the present day Bathgate Hills. These lavas were erupted in the middle of the Midland Valley graben, to form a mid-basin high. The Petershill Fm. (&bbrev. Ph. Pm.) marks an exceptional interval of quiescence and sedimentation along the southern flanks of this volcanic pile. Current exposure is confined to a northsouth strip, which provides a nearshore-off shore profile of the successive environments deposited across the platform. The Ph. Fm. is roughly divisible in half, into an upper, clastic, Silvermine Mbr. and a lower, carbonate, Reservoir Mr. The Reservoir Mbr. forms a transgressive-regressive sequence terminating in two major intervals of erosion and subaerial exposure. The first sediments to overstep the lava surface form a continuous level-bedded sequence, deposited in progressively more open-marine environments. Differences in the faunal composition of level-bedded facies may be related to substrate properties and lithological composition. Sequences of indistinct burrowing in argillaceous biomicrosparites indicate that they formed gel substrates (capable of thixotropic behaviour), which were likely to have been unstable. Trace fossils additionally reveal that some slightly argillaceous limestone horizons underwent an increase in consolidation from a gel to plastic state. Epifauna concentrate at these horizons, suggesting that the increase in consolidation may be attributed to penecontemporaneous lithification associated with intervals of non-deposition. The succeeding regressive facies are laterally differeEtiated into a mosaic of adjacent environments, consisting of a shoreward lagoon, a nearshore turbulent zone, and a biohermal build-up, passing into fine-grained biomicrosparites further offshore. In the nearshore zone, disorientated and overturned, massive cerioid corals, winnowed lag horizons of Gigantorroductus, and current cross-stratification establish that currents were strong and destructive. The laterally equivalent build-up, which possessed topographic relief, accumulated in a distal nearshore zone of maximum biological productivity and sediment preservation potential. It shows a faunal succession which may be attributed to regional shallowing over the platform as a whole. Offshore from the build-up, fine-grained biomicrosparites accumulated in a zone of quieter and slightly deeper water, as shown by a delicate, yet undisturbed fauna. Comparisons between these contemporaneous facies reveals that substrate consistency and mobility were key factors in controlling faunal distribution as well as the subsequent diagenetic history of a litholoy. Mobile, granular substrates (characteristic of turbulent zones) supported taxa with robust growth forms, in some cases (e.g. Chaetetes) especially adapted to high-energy conditions. Firm substrates (characteristic of the build-up and further offshore fades) provided ideal surfaces for epifaimal colonization and encrustation. The open gallery systems of Thalassinoides paradoxica forma minuta (forma nov) and other structures (e.g. intraclasts and fractures) provide evidence of firmness and limited early lithification. Firm substrates preserve much of their depositional porosity and were therefore susceptible to early meteoroic diagenesis which created breccias and cavity systems that, in some cases, resemble Stromatactis. Algae, particularly branched filamentous types (new), Girvanella, dasycladaceans and microproblematica, often regarded as algae, are widespread throughout the Reservoir Mbr. in some cases forming structures attributable to binding. Algae are widely distributed in several facies and thus not likely to have been the sole sediment stabilizing agents. A combination of factors, including rate and type of sediment influx, turbulence and the stabilizing effect of the fauna itself, probably ultimately determined the properties of the sediment surface. Regression culminated in subaerial exposure and the formation of sandstone, tuff and plant debris infilled fissures, which provide evidence of a complex history of events not preseed elsewhere in the sequence. The overlying Silvermine Mbr. consists of a basal sandstone facies followed by a coarsening-upward (Cu) facies. The basal sandstone shows a shoreline-offshore profile comparable to that in the carbonate facies underneath. Coastal bay sandstones pass via an inflection point into a system of nearshore sand bars. The CU sequence above accumulated during mulling of a coastal bay, progradation being finally terminated by renewed volcanic activity and lava outpouring. The Ph. Fm. is unique in the Midland Valley in the diversity of its fauna and facies. Relief over the surrounding basinal areas, provided by the Bathgate volcanics, located in the middle of the graben, and the shape of the underlying platform are most likely to account for the exceptional nature of the Ph. Fm. sedimentation. CONTENTS Page Frontispiece INTRODUCTION 0.1 The Bathgate Hills 1 0.2 Exposure 1 0.3 Structure 1 0.31 Overview 1 0.32 Correlation across the Bathgate Hills 3 0.4 The Bathgate Volcanics 5 0.41 Distribution and geometry 5 0.42 Petrology and interbedded sediments 6 0.43 Flow morphology and its significance 7 0.5 Tectonic setting 8 0.51 Overview 8 0.52 Basinal areas 8 0.53 Burntisland Anticline 10 0.6 Stratigraphy 10 0.61 Regional correlation 12 0.62 The Petershill Fm. : a history of correlation 12 0.7 Lithostratigraphical position of the Petershill Fm. 13 0.71 Facie soecificity of the Neilson Shell Bed 14 0.72 Sedimentological similarity of the limestones beneath the NSB 14 0.73 Lateral persistence of the Petershill FI:i. 15 0.8 Biostratigraphy 15 0.9 History and previous research 16 0.10 Summary of geological setting 19 0.11 Methods 21 0.11. 1 Mapping 21 0.11. 2 Sampling 24 0.11. 3 Sampling strategies 24 0.11. 4 Analytical procedure 25 0.11. 41 Notes on techniques 27 0.12 Statistical analysis 32 0.13 Curation 32 o. 14 Presentation 33 Page THE PETERSHILL FM. Ch 1 TYPE DESCRIPTION 0.