title: The Provenance and thermal histories of the Carboniferous Midland Valley of Scotland
Creators: name: McKenna, Eamon
Publisher: name: University of Glasgow
Year: 2022

description: Sedimentary basins are an important geological archive as they contain a wealth of information about the regions palaeogeographical setting, thermal evolution, and post depositional history, which are critical to many geological applications. However, establishing the source(s) of the sedimentary material stored within a basin, can be complicated by the refractory nature of some minerals often used as proxies for sediment provenance, meaning that a recycled component may be present and difficult to recognise. Thermal and burial history reconstructions based on the maturation of organic matter (vitrinite reflectance, VR) has been applied widely by the hydrocarbon industry to great success. However, due to the techniques inability to supply a quantification on time, its application in regions that lack post depositional constraints can lead to differing interpretations. In this thesis two geochronometers (zircon and apatite U-Pb geochronology) and one thermochronometer (apatite fission track, AFT) have been applied to the Carboniferous sedimentary strata of the central and eastern regions of the Midland Valley of Scotland. The aim of which is to provide a better understanding of the evolution of post-Caledonian sediment source(s) and pathways and to better constrain the post-Carboniferous thermal histories of the Midland Valley. This will reduce the geological uncertainties associated with the region, which is vital for Scotland to meet its future low carbon geo-energy commitments. A multi-proxy approach, applying U-Pb geochronology to detrital zircon and apatite has been utilised to investigate and constrain the provenance of the clastic material in Viséan to Westphalian B, Carboniferous sandstones of the Midland Valley of Scotland. The results of this provenance data clearly show three distinct sources for the sandstones: 1) Laurentian associated rocks (~900-2000 and ~2500-3100 Ma), 2) Caledonian orogeny associated rocks (395-475 Ma), and 3) Carboniferous volcanic associated rocks (~330-350 Ma). Applying U-Pb geochronology to both zircon and apatite from the same sandstone has allowed for the identification of recycled components, indicating that not all U-Pb age populations have been derived directly from their original source. Detrital zircon UPb age peaks that align with the Grenvillian (900-1200 Ma), Pinwarian (1450-1510 Ma), Labradorian (1600-1700 Ma), and Lewisian (2500-3100 Ma) orogenic events, lack equivalent apatite U-Pb counterparts. This suggests that Laurentian associated populations have been derived through the recycling of existing sedimentary and metasedimentary lithologies from the Devonian Old Red Sandstone, Grampian terrane, and Southern Uplands. Samples with complementary prominent Caledonian orogeny associated zircon and apatite U-Pb age peaks, are likely to be derived from first cycle sources from north of the Midland Valley, while Carboniferous ages are most likely derived from volcanic sources within the region. Spatial and temporal variation in the sample detrital spectra, indicates the provenance source did not remain constant over time. Early-mid Viséan (ArundianAsbian) U-Pb zircon samples are dominated by Proterozoic and Archean, Laurentian associated ages and lack a significant Caledonian peak, indicating recycling of Devonian Old Red Sandstone, Grampian terrane, and Southern Uplands dominated the sediment supply. U Pb apatite samples from the same time are dominated by a Caledonian peak suggesting either a very minor Caledonian associated source was actively supplying a limited amount of sediment or recycling of apatite through existing sedimentary lithologies is possible, as previous investigations have shown late Devonian Old Red Sandstone to be lacking in Caledonian aged U-Pb zircon. Late Viséan (Brigantian) samples in the eastern Midland Valley of Scotland have a marked increase in Palaeozoic Caledonian associated zircon ages, indicating the source region now included first cycle Caledonian associate rocks, although Laurentian associated components still dominate the spectra. A Brigantian sample collected to the west of the Bathgate Hills Volcanic Formation, located towards the centre of the study area, still shows a paucity in Caledonian related zircon ages, suggesting the volcanic high was a barrier or at least hindered sediment from the northeast being deposited across the region. Carboniferous peaks present in Brigantian zircon and apatite samples indicate the volcanic highs were also local sediment sources. A major change in sediment provenance occurs during the early Namurian (Pendleian) evidenced by the reversal in the dominant source signal, with Laurentian associated ages now relatively insignificant compared to Palaeozoic peaks, indicating first cycle Caledonian associated rocks now dominated the source area. A similar signal is recorded in Carboniferous strata in the Millstone Grit Formation in the Pennine Basin, attributed to a regional source to the north supplied via the Pennine delta. This distal source was also likely supplying material into the Midland Valley as well as proximal sources from Scottish Caledonian rocks, which combined to drown out the recycled component. Caledonian ages continue to dominate the Midland Valley samples in the Westphalian, contrary to Carboniferous basins to the south, which experience a reduction in the influence from the northern source during the Westphalian. Sources that dominated basins to the south were most likely prevented from entering the Midland Valley of Scotland by the Southern Upland high, which allowed the Caledonian sources to prevail. Undertaking detrital thermochronology is challenging due to the low yield of accessory minerals in some sedimentary rocks. However, results show AFT ages from the Midland Valley of Scotland reflect the geological structure of the region during the Carboniferous, with oldest ages located within the syncline cores and on structural highs. A complex series of forward and inverse modelling of individual and multi-sample profiles, combining outcrop and borehole samples, suggests the thermal histories can be sub-divided into three episodes: 1) Carboniferous Permian heating, 2) PermianMesozoic Cooling, and 3) Cenozoic cooling. . Inverse models also suggest initial heating was under the influence of an elevated geothermal gradient, supported by Carboniferous volcanic activity. These episodes are similar to those reported in other thermochronology investigations for onshore Scotland and show a contrast to those reported offshore, suggesting the onshore region was cooling as a coherent block from post Carboniferous times. Though the general trend observed in profile models is one of cooling through the Mesozoic, indicating an absence of significant burial under continued successions, some individual samples display an element of late Mesozoic reheating, which may be attributable to the Late Cretaceous transgression. The final Cenozoic cooling pulse, evident across the region, has been recognised in numerous investigations and has been attributed to processes associated with the emplacement of the proto-Icelandic plume or far field stresses related to compressional forces associated with the opening of the North Atlantic Ocean and Alpine collision. However, the temperature sensitivity of AFT is not enough to resolve this part of the thermal history. The results reported in this thesis reveal that the provenance of the Carboniferous sedimentary strata of the Midland Valley of Scotland did not remain constant through time and that post burial thermal maximum was attained following Carboniferous burial under an elevated geothermal gradient which was followed by episodes of cooling. These findings have important implications for assessing the future geo-energy potential within the Midland Valley of Scotland.