Vitrinite Reflectivity and the Structure and Burial History of the Old Red Sandstone of the Midland Valley of Scotland

Journal of the Geological Society, London, Vol. 151, 1994, pp. 425-438, 12 figs. Printed in Northern Ireland Vitrinite reflectivity and the structure and burial history of the Old Red Sandstone of the Midland Valley of Scotland J.E.A.MARSHALL', P.D.W. HAUGHTONZT3& S.J. HILLIER'.4 'Department of Geology, University of Southampton, Southampton S09 SNH, UK 2Department of Geology, University of Glasgow, Glasgow G12 8QQ, UK 3Present address: Badley Ashton & Associates Ltd, Winceby House, Winceby, Horncastle, Lincolnshire LN9 6BP, UK 4Present address: Geologisches Institut, Universitat Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland Abstract: Vitrinite reflectivities from Lower Old Red Sandstone (ORS) sediments of the Strathmore region in the Midland Valley of Scotland give values of 1.2% for the Stonehaven Group (Kincar- dineshire), 0.9% for the Arbuthnott Group (Angus) and 1.3% forthe Strathmore Group (Perth- shire).These values are corroborated byclay mineral diagenesis studies. The StonehavenGroup reflectivityvalues are considerablylower than expected underthe 9 kmthickness of sediments aggregated for the Lower Old Red Sandstone of Kincardineshire and demonstrate a maximum burial of some 3 to 5 km. These results together with detailed vitrinite reflectivity measurements in the Dundee Formation, consideration of post-Devonian burial history and kinetic burial modelling, show that the present level of Lower ORS thermal maturation results from Late Carboniferous maximum burial.This is confirmed by comparison with a vitrinite reflectivity gradient from the Strathclyde Group of Fife. To reconcile the vitrinite reflectivity data with the stratigraphy and structureof the Lower Old Red Sandstone,burial models invoking structural repetition, depocentre migration and structurally as- sembledstratigraphy are considered.It is concluded that the Lower Old Red Sandstone is a structurally assembled series of basins with the Cowie Formation of the Stonehaven Group being part of a separate depositional succession and structurally juxtaposed into its present position as partof an apparently contiguous sequence. The Lower OldRed Sandstone (ORS) of the Midland vitrinite reflectivity normally increases with depth, mapping Valley of Scotland (Figs 1 & 2) comprises a thick succession its distribution at differentstratigraphical levels can give dominated by sandstoneand conglomerate. These largely clues on how thestratigraphy has been stacked within fluviatile depositswere traditionally linked to uplift and basins, with implicationsfor their tectonic history. This is erosion of Dalradian metamorphic rocks lying to the north, particularly appropriate for the Midland Valley Lower ORS with the thick succession accommodated by syndepositional with its variableand apparently very thick stratigraphic faultmovements on the intervening Highland Boundary succession. Fault.Bluck (1984) has shown that ORS deposition post-dated uplift of the Dalradian, and that the Highland Boundary fault represents a probable terrane boundary on Geological setting which there has been a long prior history of displacement. Generalaccounts of the Lower ORS geology of theNE The Lower ORS rocks flanking the Highland Boundary are Midland Valley are given by Armstrong & Paterson (1970), important for they straddle part of the interval between the Bluck(1978) andMykura (1991). Armstrong & Paterson Ordovician,when the Midland Valley cannot have been (1970, reiterated by Mykura 1991)in theirstratigraphic adjacent to the Dalradian (Bluck 1984) and the Upper ORS, revision of the ORS of the Midland Valley proposed a thick when there is aclear provenance link with the Dalradian sequence totalling some 9 km in SE Kincardineshire. This (Bluck 1984). The structure of the Lower ORS basins, and thickness has been determined from a measured, internally theprovenance of thesediment they contain, can thus correlatable and apparently continuous coastal section south provideinformation about the Late Silurian to Early from Stonehaven. This contrasts with a thinner succession in Devonian tectonic history of the Highland Boundary. While Angus and Perthshire thinning again to the SW where the progresshas been made with theprovenance of coarse Lower ORS is just over 1 km thick on Arran, the Kintyre conglomerates (Haughton 1988; Haughton et al. 1990) poor peninsula, and the Balmaha area. inland exposure and the lack of an adequate biostratigraphy Recent research suggests that it is incorrect to view all have hampered a more detailed analysis of the basin(s). the Strathmore Lower ORS as the fill to a single basin, as Thiscontribution emphasizes the value of thermal previously attempted in accounting forthe thickness maturationstudies (e.g. Whitham & Marshall1988), variation. Instead a series of separate basins, each with its specifically vitrinitereflectance, in testingstratigraphic own history, is suggested by acombination of sedimen- models for basins like those of the ORS. Non-marine basins tologicalwork, palaeocurrent analysis andprovenance characterized by coarse clastic sediments and rapid lateral studies(Haughton 1988; Haughton & Bluck1988). Thus facies changes,have few stratigraphical markers and poor where the succession is thickest, in Kincardineshire, at least palaeontologicalzonation rendering analysis difficult. As two separate basins are considered to havebeen 425 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/3/425/4889897/gsjgs.151.3.0425.pdf by guest on 23 September 2021 426 J. E.MARSHALL, A. P. D. W. HAUGHTON & S. J. HILLIER Fig. 1. The Strathmore Old Red Sand- stone (ORS) in the Midland Valley of Scotland. The location of the Strathclyde Group section in east Fife is indicated. For clarity no geological boundaries are shown outside the Midland Valley. From Cameron & Stephenson (1985). superimposed, with the older Crawton basin forming a small amajor stratigraphic gap with the youngestpreserved sub-basin beneathmorea extensive Strathmore basin. Lower ORS being of Emsian age. However the Stonehaven and Dunnottar Groups, the lowest Littleprevious information is available onthermal units in the succession, occur as a single section dipping to maturationin the Lower ORS inthe MidlandValley of the south and with a thickness of some 3 km. Scotland,however Kennedy & Leith(1987, pers. comm. Correlation of strata has proved difficult in the ORS of 1990) report vitrinite reflectivities of 0.82% (Strathmore Gp, the Midland Valley with its paucity of both lithological and Callander) and 0.92 and 1% (Arbuthnott Gp, Forfar). They palaeontological markers. Fossils are limited to fish, plants concluded that maximumformation temperatures of and spores. Fish are particularly common in the lacustrine between120-150°C were attained under a total burial of Dundee Fm (Formation) of theArbuthnott Gp(Group) some 3.5-4.5 km of overburdenand under Devonian with furthervertebrate and arthropod faunas in the sedimentationand burial. Organicgeochemical dataare Stonehaven GP. Plant spores occur that date the Strathmore availablefrom a single sample in theDundee Fmof the Gp as Emsian (Richardson 1967) and the Arbuthnott Gp as Arbuthnott Gp (Robinson et al. 1989) wherebiomarker early Gedinnian in age (Richardson et al. 1984). The Cowie ratios indicate low maturation levels. Fm of theStonehaven Gp is of late Wenlock to early Ludlowage (Marshall 1991), significantly olderthan the Arbuthnott GP. Spores are restricted to these three groups Burial history of the Lower ORS and do not permit detailed correlation or sub-division of the Although no post-Upper ORS rocks are now exposed in the sequence.As regards lithostratigraphy, marker beds are Strathmore region, the probable burial history of the Lower generally absent. An exception is the Lintrathen Porphyry ORS can be inferred from comparisons with other parts of andits correlatives in Glenbervie (Fig. 3), aseries of the MidlandValley andfrom scattered outcrops across distinctive porphyritic ignimbrites that mark the top of the Scotlandand in Northern Ireland where a more complete Crawton Gp (the Crawton Volcanic Fm). sequence has been preserved under Tertiary lavas. Many of A major unconformity separates the Lower ORS from the arguments for this burialhistory are detailed by both theUpper ORS, which is of LateDevonian to Early Hall (1991) and Hillier & Marshall (1992) which, although Carboniferousage. The Upper ORS, hasa thickness of dealing specificallywith the ScottishHighlands and about 1 km in the eastern Midland Valley increasing to over Orcadian Basin respectively, have many parallels with the 2 km in the Firth of Clyde area (Paterson & Hall 1986). It is Midland Valley. in stratigraphic continuity with the overlying Dinantian with It is likely thata fairly completesequence of which it has a conformable transitional contact. The age of Carboniferous rocks was widespread in the Midland Valley, theUpper ORS is poorly constrained, with the first asrocks of this age are stillwidely distributed in the stratigraphicallyuseful fossils beingspores of lateTourn- Midland Valley (Figs 1 & 3) and its extension in the North aisian age(Paterson & Hall 1986) from the overlying Sea (Andrews et al. 1990). In the Midland Valley the Upper Carboniferous.This demonstrates that at least theupper ORS is the basalunit toa major depositional sequence, parts of the Upper ORS are of Carboniferous age. Thus its largely of Carboniferous age, resting with a marked angular affinities are very much with Carboniferous rocks and above unconformity on Lower ORS. Thus the presence

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