Igneous activity and organic maturation in the Midland Valley of Scotland

Abstract

The general relationship of Erich Stach's research work to the effects of igneous activity on organic maturation is briefly considered. The geological history of the Midland Valley of Scotland is then reviewed. As a graben which developed in Devonian (Old Red Sandstone) times, and continued to do so as lithospheric tensional stretching was maintained throughout the Carboniferous, a significant factor in relation to the development of organic maturation was the widespread igneous activity that persisted throughout much of Upper Palaeozoic times. Calc-alkaline volcanic rocks are a major feature within the molasse-type deposits of the Old Red Sandstone. After a period of quiescence in later Old Red Sandstone times, the continuing tensional stretching initiated or rejuvenated major crustal fractures. Extensive volcanic, sill and dyke activity exercised an important influence on the predominantly shallow-water deltaic deposits of the Carboniferous. Alkaline-olivine basalts were erupted in the Early Carboniferous, while volcanic activity in the later Carboniferous was essentially phreatic. Two major intrusive episodes occurred; the first of alkaline sills in Early to mid-Carboniferous times which were closely related to volcanic plugs and eruptive centres, the second in post-Westphalian times when tholeiitic sills and dykes with no known relationships with any extrusive rocks were injected. The effects of this igneous activity on organic maturation can be considered under four headings. 1. (1) Although the general development of maturation of Carboniferous organic matter, which includes many coal seams and oil shales, was controlled to some degree by burial history, the regional pattern of igneous activity was also influential. Thus, there is a gradual increase of coalification from east to west in the Midland Valley which corresponds with an increasing thickness of volcanic pile in the central and westerly parts of the graben, suggesting that there may have been a greater regional "heat sink" in the west during the Carboniferous. The level of coalification is nowhere particularly high, except where affected by intrusions, and regional reflectance gradients generally vary between %Roil (vitrinite) = 0.2/km to 0.4/km, without displaying any consistent pattern. 2. (2) The post-Westphalian tholeiitic (quartz-dolerite) Midland Valley Sill, which underlies most of the central and eastern parts of the graben, has produced extensive reflectance (thermal) aureoles in the organic matter above and below the intrusion. The olivine-dolerite and teschenite sills of Namurian-early Westphalian times, many of similar thickness to the quartz-dolerite intrusions, fail to produce significant reflectance (thermal) aureoles. This major difference in organic matter response to the intrusions can only be explained in terms of the initial rank level of the organic matter, differences in degree of compaction of the sediments and particularly in the amount of water the sediments contained at the time of intrusion. 3. (3) Maturation of organic matter closely associated with extensive Surtseyan-type volcanic activity in the eastern part of the graben displays no relationship to the regionally developed coalification in associated sediments. Although some vitrinite clasts in the pyroclastic deposits have had their reflectances raised to meta-anthracitic rank, reflectance levels of clasts in tuffs and agglomerates are generally not much raised above the regional reflectance level, suggesting incorporation of organic matter in ash streams in which water was an important cooling and protective component. 4. (4) The close relationship of organic matter of all kinds with volcanic and intrusive igneous rocks has led to a marked increase of parent polycyclic aromatic hydrocarbons (PAH) in the aromatic fractions of organic extracts. The source of these hydrocarbons is the combustion and/ or pyrolysis of organic matter caused by the high temperatures of the igneous materials, whether volcanic or intrusive. These PAH may be directly generated or absorbed from the surrounding environment. © 1989.