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 Scotlandgive 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. Theseresults 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 theDalradian, 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 theDalradian 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

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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 particularlycommon 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 NorthernIreland 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 thearguments 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 OrcadianBasin 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 of Upper

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CALLANDER BRECHIN STONEHAVEN letres FORFA R STRATHFINELLAFORFAR ,000

I000

‘000

j000

iooo

,000 LR. O.R.S. Crawton

3000

2000 F STRATAFAULTED OUT Fig. 3. Vitrinite reflectivity sample localities, Arbuthnottand U EXPOSED NOT Stonehaven Groups. Box ‘a’ indicates the areaof Fig. 4; box ‘b’ that of Fig. 12. From Armstrong & Paterson (1970). 1000

recommencedduring theLate Permian with thina 0 Permo-Triassic (hundreds rather than thousands of metres) succession, culminating in a general Late Triassic cessation of sedimentation. Fii. 2. Simplified ORS stratigraphyof the Strathmore Old Red Jurassic sediments deposited during successive transgres- Sandstone. Approximate locationof vitrinite reflectivity samples sions are widely distributed in andaround Scotland, such shown. The Cowie Fm is as defined by Armstrong & Paterson that it is reasonable to assume that similar sediments were (1970). From Mykura (1991) after Armstrong & Paterson (1970). present in a sedimentary basin such as the Midland Valley. Similarlyduring theEarly Cretaceous, evidencefrom the Moray Firth indicates a thin marginal facies of ‘greensand’ ORS as outliers in areas without Carboniferous rocks such was probably also present. There is more direct evidence for as atMontrose implies that Carboniferous sedimentation the presence of Chalk, as derived Late Cretaceous flints are has occurred here with the ORS representing the erosional foundat high levelsinTertiary age gravels on the remnants of its base. Further evidence for the presence of Grampians. Its presence at these elevations, on what is now Carboniferousrocks is provided by the lack of extensive apositive area,indicates its likely presencealso in the alteration in the Lower ORS. Such alteration is a marked basinal setting of the Midland Valley. phenomenen in the ScottishCarboniferous (Francis 1991) andoccurred during the interval of Permian uplift and erosion. The lack of asimilar level of alteration in the Material and methods Lower ORS indicates a degree of protection from overlying Sites sampled are shown on Figs 1,3 & 4, and locality Carboniferous strata whilst it was being removed by erosion details together with the reflectivity results are given in during latest Carboniferous and Early Permian times. The Table 1. Most samples were collected from the Dundee Fm extent of the missingCarboniferous succession is more of the Arbuthnott Gp of Angus and Perthshire, some 50 km difficult to determine, as demonstrated by the considerable from theStonehaven Group samples atStonehaven. An variationin stratigraphy (Francis 1991) of thepreserved exception is an Arbuthnott Gp locality at Dellavaird (loc. Carboniferous rocks sequences. However arealistic estimate 26) whichis only 8 km fromthe Stonehaven Gp sample for thickness would be some 2-3.5 km. This Carboniferous localities. The Strathmore Gp localities (Fig. 1) are all from burial marks the culmination of a phase of (deep) burial for the Callander area some 100 km west of the main group of the Lower ORS sinceduring much of the succeeding Arbuthnott localities. Permian there wasan episode of extensiveuplift, erosion Coalswere also sampled (Figs 1 & 5) from the andplanation. Following this inversion,sedimentation Strathclyde and Lower Limestone Gps of east Fife, which is

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Fig. 4. Vitrinite reflectivity sample sites from the Dundee Formation, Arbuthnott Group. These stratigraphically well-constrained samples were selected to test burial models. Note the lack of systematic variation both along strike and down dip. The general location of these samples is shown in Fig. 4. Simplified from Geological Survey of Great Britain (Scotland) Sheet 49, 1 :50 OOO solid.

the first sectionyounger thanthe Lower ORS tocontain dominated by semifusinite with onlyminora vitrinite vitrinite. The section,described by Forsyth & Chisholm component. Figure 7 is a cross plot of vitrinite and exinite (1977), is sufficiently removed from the Midland Valley Sill reflectivities which shows a close relationship to that derived and has coals throughout the succession. (Smith & Cook1980) from a suite of Carboniferousto As Lower ORS mudrocks that contain kerogen generally Tertiary coals. It is auseful test of consistency in these onlypossess it in low amounts,the best results were Silurian and Early Devonian kerogens. achievedusing demineralized kerogen concentratesand Thepresence of Silurian vitrinite in theStonehaven polishedthin preparations(Hillier & Marshall1988). samples does notimply thepresence of woodytissue, as Reflectivity determinations were madeas described by vitrinite as defined has an unspecified origin from a variety Hillier & Marshall(1992). The claymineralogy of the of plant tissues and is characterized by its morphological, <2 pm fraction was studied by X-ray diffraction using the chemical and physical properties rather than its precursors. methods described by Hillier & Clayton (1989). An explanation for the paucity of the Silurian and Devonian vitrinite can be found in the nature and occurrence of early landplants. These plants were small, lacked significant Vitrinite quantities of structured tissue and occurred in low numbers. Vitrinite is the remains of structured plant tissue which has This gave them a low preservation potential in the terrestrial 'gelled' duringdiagenesis with the loss of itsoriginal environment as the volumeof accumulating plant debris was structure. Its importance as a thermal maturation index was less ableto promote an anoxic environment unlike the largely established on studies of Carboniferous coals (e.g. Carboniferouscoal swamps. These early accumulations of Stach et al. 1982), therefore its application to rocks of plant debris were thus less likely to 'gel' to vitrinite; instead Silurian and Early Devonian age requires some justification. oxidation to semifusinite and 'inertinization' of spores being Particularly important is theidentity of any 'vitrinite' the more common diagenetic processes. formedfrom early land plants. These are small and lack Vitrinite reflectivities, once acquired, can be interpreted substantiveplant tissue unliketheir coal-forming Car- by a variety of techniques. For example, reflectivity values boniferous successors. Examination of kerogen isolates from alone can be used to compare relative rank levels. They can theStonehaven samples as polishedstrew slides in both alsobeused to model the burial in sedimentarya reflectedand transmitted light, permitsmorphological succession; through conversion to temperatures using either description in bothcoal petrographic and palynological relationshipsbetween vitrinite reflectivity andburial terminology. temperature compiled from large data sets (e.g. Barker & Intransmitted light thespores are morphologically Pawlewicz 1986), or application of kineticbased models obvious, and similarly so in reflected light (Fig. 6 a, c, d and (e.g.Sweeney & Burnham1990) such as BusinMod"" e). They also formthe maceral group with the lowest (PlatteRiver Associates Inc.,Denver, Colorado, USA). reflectivity (Fig. 6c) although it is noticeablethat a small The relative importance of time to these proportion (Fig. 6 a & d) are converted to an inertinitic or vitrinite/temperaturerelationships has been the subject of vitrinite/serni-fusinite material.Inertinite witha tube much debate,however, consensusa is emerging(e.g. morphology (Fig. 6i) is rare.This leaves the bulkof the Barker 1989; Robert 1988) which seesmaximum tempera- kerogenas structured plant debris showing arange of ture as the dominant factor. A further approach tried in the reflectivity. Semifusinite can usually be recognized (Fig. 6e) MidlandValley of Scotland is the use of local vitrinite by itsangular appearance, obvious phytoclast origin and reflectivity gradients. If these can be defined for the basin, wide range of reflectivity. The vitrinite (Fig. 6 b,c,f,g & h) theycan then be used to predict depths of burial for the has a lower andmore consistent reflectivity range, is successions which have a comparable thermal history. Lack generallysmaller, less well-formed,and has an irregular of subsurfacematerial precluded the establishment of an morphologywith ragged edgesand no fluorescencewhen ORS vitrinite gradientbut a satisfactory alternative was within theappropriate maturity range. Mostsamples are measured from the Carboniferous of Fife.

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Locality No. Grid.RV ref. n Re s.d. n s.d. I I/S C CR K Sm

Stonehaven Group Cowie Formation Carron Wood 1 NO 8078481.14 0.13 58 0.82 40 0.17 - Carron Wood 1.19 0.14 83 0.88 36 550.13 CarronWater 2 NO 8258531.16 0.14 35 0.80 16 50 0.11 Carron Water Carron 1.16 0.19 52 0.83 39 55 0.11

Arbuthnott Group Dundee Formation Slatefield 3 NO 454491 2.12 0.05 6 2.03 36 30 0.16 Slatefield 2.12 0.22 26 2.10 25 30 0.16 Slatefield 2.04 0.14 27 2.04 35 0.16 - Westcraig 4 NO 455485 0.96 0.11 34 0.66 12 30 0.13 Westcraig 0.91 23 0.08 0.61 29 0.13 - Craiksfold 5 NO 528537 0.90 0.10 37 0.58 23 0.08 - Craiksfold 0.85 0.11 42 0.45 24 0.11 - Balmashanner 6 NO 458492 0.95 0.11 36 - - - 30 Balgavies 7 NO 539519 0.99 0.12 40 0.65 26 30 0.11 Balgavies 0.950.11 35 0.47 21 30 0.11 Friockheim 8 NO 597484 0.99 0.09 32 - - - 30 Friockheim 1.OO 0.10 33 - - - 25 Pitairlie 9 NO 500372 0.910.10 28 0.53 29 0.08 30 Pitairlie 0.860.10 38 0.40 36 20 0.09 Pitairlie 0.930.10 42 0.47 31 0.06 - Pitairlie 0.870.11 42 0.44 5 0.09 - Cunmont 10 NO 490368 2.64 0.50 33 - - - 30 Arbirlot 11 NO 605403 0.830.07 21 - - - - Panlathy Mill 12 NO 563376 0.77 0.09 16 - - - - Crombie Mill 13 NO 530402 0.74 0.10 33 0.30 38 0.05 - Crombie Burn 14 NO 535401 0.98 0.09 27 0.61 13 0.07 - Clearie Woods 15 NO 538394 0.68 0.12 13 0.26 36 30 0.05 Clearie Woods 0.77 0.12 20 0.33 33 0.06 - Panmure 16 NO 545375 0.76 35 0.08 0.35 39 0.06 - Panmure 0.730.11 31 0.32 33 0.06 - Denhead 17 NO 491379 0.87 0.10 43 0.43 35 0.07 - Denhead 0.84 0.12 39 0.45 33 0.06 - Kellyfield 18 NO 588406 0.73 7 0.06 0.31 26 0.08 - Wellbank 19 NO 469371 0.79 0.09 29 - - - - Wellbank 0.71 0.11 38 0.38 38 0.07 - Quarry Cottages 20 NO 474378 0.69 0.10 34 - - - - Westwellbank 21 NO 459377 0.780.12 41 0.36 38 0.08 30 Westwellbank 0.76 0.12 37 0.44 33 0.06 30 Westhall 22 NO 446367 1.040.12 37 0.68 34 0.08 - Westhall 1.050.11 32 0.73 32 0.07 - Duntrune 23 NO 441370 1.090.11 33 0.68 36 0.08 - Duntrune 1.04 0.14 39 0.66 40 0.09 - Powrie 24 NO 418349 1.010.15 47 0.68 44 0.08 30 Powrie 1.01 0.09 38 0.76 40 0.08 30 Powrie 1.11 0.10 29 0.77 36 0.10 - Wormit 25 NO 385258 0.91 0.11 38 0.65 20 50 0.05 Wormit 1.04 0.11 25 0.72 10 0.08 - Wormit 1.020.12 29 0.61 12 40 0.09 Dellavaird 26 NO 730820 0.96 0.12 40 0.60 42 0.14 - Dellavaird 0.930.09 31 0.52 45 0.11 - Dellavaird 0.94 0.13 40 0.58 40 0.11 - Den of Morphie 27 NO 722656 0.88 0.12 37 - - - -

Strathmore Group Teith Formation Brae of Boquhapple 28 NN 656019 1.310.11 29 1.36 39 350.14 Brae of Boquhapple 1.31 0.12 33 1.25 80 350.14 Brae of Boquhapple 1.280.10 32 1.23 35 0.12 - Cassafuir 29 NN 615023 1.340.16 41 1.17 25 350.14 Cassafuir 1.29 31 0.08 1.07 34 30 0.11 Ballanucater Farm 30 NN 630022 1.17 0.13 76 0.96 46 30 0.11 Ballanucater Farm 31 NN 630023 1.35 0.12 26 1.37 34 0.08 - Ballanucater Farm 1.400.10 28 1.21 32 0.08 30 Gellyburn 32 NO 094389 2.29 0.25 37 2.37 36 0.31 -

Sample localities are shown on Figs 3 and 4. I, illite; SM, smectite; I/S, illite/smectite; C, chlorite; CR, corrensite; K, kaolinite

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Vitrinite reflectivities from coals in the Strathclyde and \ Lower Limestone Gp are shown in Fig. 5 and range from \ 0.5 to 0.7% across 2 km of continuous coastal section. \ \ \ Interpretation and discussion \ These results fromtheStonehaven, Arbuthnott and Strathmore Gps (Table 1) demonstrate two clear problems \ when correlated with implied stratigraphical position. \ First, the values for Stonehaven are far too low to have \ been generated under the thick sequence of ORS envisaged in thenortheastern Midland Valley. This is shown by application of the empirical temperature-reflectivity conversion of Barker & Pawlewicz (1986): log, R = 0.0096T - 1.4 where R is thevitrinite reflectivity andT the maximum temperature in "C. Assuming the maximum vertical thickness of 9km and an average geothermal gradient of 30°Ckm-' (conservative viewin of the significant contemporaneousvolcanism in large areas of the Midland Valley), a reflectivity of over 3% is obtained for burial by Arbuthnott GP+ the end of the Early Devonian. This is twice that actually (RV S 0-89 measured at Stonehaven. n 39) Second,the mean values from the Stonehaven and \ Arbuthnott Gps are 1.2 and 0.9% respectively, not greatly \ differentconsidering the standard deviation of 0.1. These \ samples are supposedly separated by some4 km of Stonehaven GP*+ intervening strata in thestratigraphical succession of \ Armstrong & Paterson (1970). It can be argued that since the Arbuthnott Gp samples are from quite separate geographi- cal localities (Fig. 3), they need not be expected to conform

I 1 1 1 to adepth versusburial relationship from acomposite 2 0.3 0.5 1.0 2.0 % section some 50 km to the northeast. However a key group VlTRlNlTE REFLECTIVITY of Arbuthnott Gp samples is from Dellavaird, some 8 km from the Stonehaven Gp localities and with a mean vitrinite Fig. 5. Vitrinite reflectivity profile, Strathclyde and Lower reflectivity of 0.94%, clearlya comparable value. Strat- Limestone Groups, Fife. The histogram shows reflectivity values igraphic separation by the 4 km implied by Armstrong & from the Arbuthnott Gp, the regression line passes through the Paterson (1970) would give a predicted vitrinite reflectivity sample mean of these data. Only the mean value is shown for the difference of some 2%,under a geothermal gradient of Stonehaven GP. The regression line shows some 2 km of strata to 30 "C km-' and taking the Dellavaird vitrinite reflectivity of have occurred above this Carboniferous succession. Extrapolation 0.94% as the base value. of this gradientto Arbuthnott and Stonehaven Group vitrinite The Strathmore Gp samples have the highestvitrinite reflectivity values shows some5 km and 6 km of total burial for reflectivities seen in the Midland Valley ORS although the these sequences. Thicknessof Carboniferous section from Forsyth youngest rocks in the succession. They are, however, some & Chisholm (1977). 100km distantfrom the main groups of Stonehavenand Arbuthnott Gp samples and not directly comparable. These initial observationsreveal difficulties in the Vitrinite reflectivity results reconciliation of these vitrinite reflectivity data with existing These are given in Table 1 which summarizes the result of geologicalmodels. This prompted a more thorough over 7000 measurements on 60 samples from 32 localities. interpretation of the observed vitrinite reflectivity data and Reflectivities from Stonehaven give consistent values which its ability to constrain thepost-ORS burial history of the range from 1.14 to 1.19%. Those from the Arbuthnott Gp Midland Valley. rangefrom 0.69% to 1.11%(mean 0.9%) with a further group between 2 to 2.6%. which generally correlate with the presence of intrusions(e.g. Cunmount, loc. 10). The Prediction of burial from measured vitrinite Arbuthnott Gp samples nearest to Stonehaven (Dellavaird, reflectivities loc. 26) havea mean reflectivity of 0.95%, close to those Using the time-independent temperature versus reflectivity from the Stonehaven GP. conversion of Barker & Pawlewicz(1986), temperatures Samplesfrom the Strathmore Gp have reflectivities in were calculated for the Midland Valley ORS (Table 2). therange 1.17 to1.4%. They are all fromthe Callander All vitrinite reflectivity values aremeans for each Gp areasome 100 km from the main Arbuthnott Gp of excluding anomalies caused by local igneous heating. These localities. The onlyPerthshire locality is atGellyburn results suggesttotal burial depths of about4 km forthe (2.3%, loc. 32) and associated with an igneous intrusion. Arbuthnott Gp andabout 5 km forthe Stonehaven GP.

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Fq. 6. Photomicrographs of kerogen isolatedin reflected light. All illustrations X550 and from the Stonehaven Gp (loc. 2) except for (a) which is loc. 26, Arbuthnott GP.(a) Micrograph illustrating spores preserved as different macerals; Se, sporeas exinite; Si, spore as inertinite; Sf, spore as semi-fusinite. These spores are morphologically obviousbut clearly show reflectivity differences.Most of the remaining phytoclasts are semi-fusinite.(b) Vitrinite phytoclast, largeand well formed. (c) Phytoclasts of typical Stonehaven Gp vitrinite, note the lower reflectivity exinite (Se). (a) A cryptospore tetrad Tetrahedraleres medinemis preserved as vitrinitic material. (e) Semi-fusinite phytoclast. (f) Vitrinite phytoclast. (g) Vitrinite phytoclast with spore (Se) preserved as exinite.(h) Vitrinite phytoclast. (i) Inertinite with a tube morphology.

These estimates must include post-Lower ORS burial. The stratigraphicalrelationships between localities couldbe values are bestregarded guide,asa asimportant estimatedwith confidence andthe effects of differing assumptions include a geothermal gradient of 30 "C km-'. It geothermal gradient believed negligible. Examination of the requiresonly shorta interval of elevatedgeothermal results (Fig. 4) shows there to be no correlationbetween gradient,quite likely in suchanextensional tectonic reflectivity andstratigraphical level, with valuesoften situation with significant igneous activity, toreduce these increasing into younger rocks (compare localities 16 with 12 depths of burial by over 1km. It is alsoclear that the and 18 with 11). Inthis area suchsmall scale reflectivity difference in totalburial between theStonehaven and variations probably reflect as much local variation in igneous ArbuthnottGps is about 1 km assuming the 30 "C km-' heating as minor differences in burial. geothermal gradient. The imposition of a similar range of reflectivity values In order to test further the relationship between depth across theStrathmore ORS outcroparea (Stonehaven to and vitrinite reflectivity, additional samples were collected Dundee) 'and agerange (late Wenlock/early Ludlow to overa small area in theDundee Fm (Fig. 4) where Emsian) is characteristic of apost-Lower ORS thermal maturation maximum that overprinted the maturity values attained at the end of the Early Devonian. This is because Table 2. Temperature for fhe Midland Valley the measured thermal maturities do not relate to the Lower ORS ORS stratigraphybut instead all showsimilar values ~ stemmingfrom a common post-Lower ORS depositional GroupRV "C Depth(km) history. This is entirelycompatible with the geological history of the MidlandValley, with its change from Strathmore Gp 1.3 173 5.7 deposition to uplift and erosion during the Mid Devonian Arbuthnott Gp 4.50.9 135 Stonehaven Gp 1.16 161 5.4 inversionaccompanied by erosion prior tothe renewed sedimentation of the Upper ORS.

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Arbuthnott Group 0 Stonehaven 18 1- 0 Strathmore 11 ,/Re RV relationship

DEPTH

l \ I \ \ " / Arbuthnott GP- / /

I I I 1 300 200 1bo I I TIME (Ma) 1.0 2.0 RV o/o (vitrinitereflectivity) Fig. 8. Burial history (BasinMod'") for the Arbuthnott Group of Strathmore, specifically based on the Forfar area. The strata are Fig. 7. Exinite and vitrinite cross-plot for Lower Old Red backstripped using the Lower ORS sequence togetherwith an Sandstone samples from the Strathmore Group. The Re versusRV inferred stratigraphyof strata now removed by erosion. relationship is for a group of Carboniferous to Tertiary coals (Smith & Cook 1980) and shows the same relationship as determined for the ORS samples.

TheStrathmore Gp samples have similarbut slightly highervalues, although forming the youngest part of the Westphalian succession.However, as theproductive Strathmore Gp sampleswere found at some distance from those of the Arbuthnottand Stonehaven Gps, any stratigraphic com- I parisons are of little validity. nNarnurian In an attempt to elucidate the nature of this post-Lower ORS burial,usewas made of thekinetic model U.ORS/ BusinModTM. Figure 8 graphically representsthe burial Dinantian including majorepisodes of inversionduring the Mid- Devonianand Early Permian. The model is based on the StrathmoreGp vitrinite reflectivities andstratigraphy of theArbuthnott Group from Forfar.However to achievemodelled reflectivity valueswhich match those measured in the ArbuthnottGp, the stratigraphic thicknessesused forthe Arbuthnott,Garvock and Strathmore Gps have all been Arbuthnott Gp reduced. The validity of thisreduction is acceptedas stratigraphic thicknesses have been largely derived from dip measurementsand trigonometric estimates rather than directlogging. Experience has shown thatdirect measure- Fig. 9. Vitrinite reflectivity versus time relationship determined mentgives thinner sequences as stratigraphic duplication using a kinetic model (BasinMod'") for the Arbuthnott Group, acrosssmall faults is removed.The stratigraphy younger specifically based on the Forfar area and using the burial history thanthe Strathmore Gp is based onthe stratigraphical shown in Fig. 8. The mean vitrinite reflectivities from the argumentsfor the post-Lower ORS burialhistory of the Arbuthnott Group of Forfar (0.9%) matches that produced by Midland Valley as outlined previously. Figure 9 shows that a kinetically calculated values shownby the line. maximumvitrinite reflectivity of 1% wasachieved in the Late Carboniferous, and that post-Carboniferous burial has not been sufficient to overprint the maturity level achieved was selected as it is the closest area with rocks most likely to by that time. These results demonstrate that the succession besimilar to thosewhich overprinted the Lower ORS canbesuccessfully modelled using the burialhistory maturity values. The reflectivity gradient obtained (Fig. 5) is outlined previously. basedon coal samples rich in vitrinite,rejecting those To establish furtherthe validity of this concept of containing algal materialand amorphous organic matter maximum burial in Late Carboniferous times, attention was with theirsuppressed vitrinite reflectivities. Application of focused on the Carboniferous sequence from Fife. This area thevitrinite reflectivity totemperature relationship of

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Barker & Pawlewicz (1986) to the Fife vitrinite reflectivity I values, gives a geothermal gradient of 23 "C km-' for this sequence, which is aboutcrustal average and acceptable given theassumptions involved in itsdetermination. This gradient is somewhatlower than those measured in Carboniferousrocks tothe east (Raymond & Murchison 1988), andthe difference is attributedto the thermal influence of the MidlandValley Sill. The Fifegradient measured here runs through some 2 km of measured section andthe fact thata gradient can be established is itself significant, as it shows that the burial of this succession after its Permo-Carboniferous uplift, tilting and erosion has been of insufficient thickness to overprintthe Carboniferous reflectivity gradient.This is evidencefor maximum burial beingachieved in Fifeduring the Carboniferous and by analogyalso in Strathmore.The low reflectivities values (0.5-0.7%) measured in this Fife succession also substanti- atethere onlyhaving been a moderate cover of post-Carboniferous rocks. The thickness of the cover can be estimated by extrapolating the reflectivity gradient (Fig. 5) to the minimum value (between 0.2 and 0.3%) characteristic of unconsolidatedsediments. This indicates amaximum thickness of some 2-3 km whichis geologically realistic considering its likely burial history. Application of this gradient to the ORS should be made with caution,as assumptions include a similar geothermal gradient persisting in both areas throughout the Devonian to Carboniferousinterval, and a comparable post-Devonian depositionalhistory in an area known to showstructural control on deposition.Accepting these assumptions, total burial depths for the Stonehaven and Arbuthnott Gps can be estimated using this gradient. Figure 5 shows the vitrinite reflectivities for the Arbuthnott and Stonehaven Gps plotted on an extrapolation of the Fife gradient. This gives a total burial estimate of 5-6 km for the Arbuthnott Gp with some 1 km of interveningstrata between it andthe overlying Strathclyde Gp. The latter is similar to the known thickness Upper ORS in Fife. The same extrapolation applied to of I 1 I I I l the Stonehaven Gp give a depth of burial of some 6-7 km of 30 25 20 15 10 5 totaloverburden. Both these sets of values are clearly comparable to thosederived from thevitrinite and O28 CuKa palaeotemperatureconversion. As a time interval with an Fig. 10. X-ray diffraction patterns (A, Air dried; G, glycolated; H, elevatedgeothermal gradient is likely in theextensional heated to 375 "C for one hour) for three samples to illustrate the setting of the Lower ORS, these values should be regarded types of clay minerals present. LOC.1, Stonehaven Gp; loc.9, as a maximum. Arbuthnott Gp and loc. 28 from the Strathmore Cp. I, illite; I/S, illite/smectite; C, chlorite; CR, corrensite;SM, smectite. Only selected peaks for each mineral are labelled. Clay mineralogy Asthese vitrinite reflectivity resultswere based on pre-Carboniferous vitrinite and produced results contrary to mixed with small amounts of smectite. A relatively greater thoseexpected from the then current understanding of abundance of illite/smectite is a distinctive feature of Midland Valley ORS stratigraphy, it was felt necessary to samples from the Strathmore Gp (Fig. 10). These samples undertakecorroborative a study using clay mineral arecharacterized by abroad but distinctive illite/smectite diagenesis. These results are based on the same sample set superlattice in the low angleregion indicating thatthe used for the vitrinite reflectivity samples. illite/smectite is R = 1 ordered.The proportion of With few exceptions all clay mineral assemblages contain expandable layers is commonly about 25%. Estimates of the abundantand relativelyconstant amounts of illite and proportion of expandablelayers for samples from the chlorite,together with morevariable amounts of Arbuthnott and the Stonehaven groups were not generally illite/smectite andcorrensite (Table 1, Fig.10). Forthe possible, because illite/smectite is normally only present in most part the illite present has relatively sharp diffraction small quantities. The few samples from both groups which peaksindicating that it wellis crystallized(Fig. 10). contain more significant amounts, show an obvious but weak However, the base of the lOA illite peak is almost always superlattice reflection and expandabilities of about 20%. broad and asymmetrical and changes shape with glycolation Chlorite is present in all butone of the samples and heat treatments, indicating that well crystallized illite is examined and the relative intensities of its basal reflections

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indicatesa tendency towards a magnesium-rich variety. In burial diagenetic sequence. It is considered significant that this respect itis notablethat corrensite, whichis usually the diagenetic grade is the same as that determined from the Mg-rich, is also present in every sample containing chlorite, vitrinite reflectivities. althoughoften in only tracequantities. The abundance of corrensite in samples from loc. 9 (Fig. 10) makes it probable The ORS of the Midland Valley: a new burial thatthe smectite identified atthis locality is aMg-rich model trioctahedralvariety, i.e. a saponite, but no attempt was made to confirm this. Kaolinite is abundant in onlytwo The reflectivity data and the burial model discussed above samples (loc. 25), which are also notable for the absence of with its thin ORS sequence and Late Carboniferous burial chlorite. maximum, needsbeto reconciled with existing depositional/structuralmodels for the Lower ORS. This Interpretation phenomenon of aburial history at odds with measured The clay mineralogy of the ORS of the Midland Valley was stratigraphicthickness, has been recognized in other studied by Wilson (1971), and the results here are generally sedimentary basins and has been explained in one of two in agreement.One major difference is attributedto the ways. Eitherthe measured thicknesses are nota true lithology of thecompared to samples with the largely reflection of the actual thickness, because, for example, of sandstonesamples studied by Wilson (1971) containing unrecognizedstructural repetitions; orthe discrepancy is abundant kaolinite in contrast to the mudrocks investigated real and a function of the particular basin geometry, as for here. example in theDevonian Hornelen Basin of western Clay minerals can be useful indicators of diagenetic or Norwaywhere depocentre migrationhas produced a metamorphic grade (e.g. Kisch 1983), and such an estimate stratigraphic thickness of some 25 km, but the true vertical of grade was the aim of the present investigation. However, thickness is probably <8 km (Steel & Gloppen1980). In the clay minerals in a rock are normally of several origins attempting to reconcile theanomaly of thickstratigraphic and are not therefore necessarily indicative of the level of sections with low levels of thermal maturation, a number of diagenesis. One of the most frequently used grade indicators mechanisms (possibly in combination) will be considered. is the smectite-to-illite reaction (Hower et al., 1976) and the subsequentincrease in illite crystallinity; thepresence of Structural repetition? (Fig. lla) mixed-layered illite/smectite minerals being characteristic of The9 km thicksuccession in Kincardineshirehas been thediagenetic zone (Kisch 1983). At least somesmall determined (Fig. 12) from the almost continuously exposed amount of illite/smectite is present in all samples from the coastal sections of the Stonehaven, Dunnottar and Crawton MidlandValley, which, where it canbe furthercharac- Gps,together with the Arbuthnott, Garvock and Strath- terized (Strathmore GP), is the R = 1 ordered variety, and more Gps cropping out in less well-exposed inland sections this mineralproperty is typical of moderatediagenetic along the Bervie Water northwestwards to Strathfinella Hill. conditions(Hoffman & Hower 1979).Because In the coastalsections, a number of strikefaults cut the illite/smectite is present, even in only small quantities, the moreabundant, well-crystallized illite is interpretedas a successionalong thenear vertical northwestlimb of the detrital mica ratherthan product a of deep burial Strathmore Syncline. In at least one instance, displacements metamorphism. Where they could be made, estimates of the are considered to havebeen both early and transcurrent (Robertson 1987), with pervasive soft sediment deformation proportion of expandable layers in illite/smectite of 20-25% indicate broadly similar degrees of diagenesis for all three and block rotations across a diffuse displacement zone. In this andanother case, these ENE-WSWstrike faults stratigraphic groups. Hoffman & Hower (1979) indicate that juxtaposeconglomerates with differentdispersal directions R = 1 illite/smectite occurs at temperatures of c. 100-170 "C in Mesozoic andTertiary rocks, but they warn against (by up to as much as 180") and/or differing clast assemblages translatingthese temperatures to Palaeozoicrocks for (Haughton1989). As these conglomerates are sedimen- reasons of kinetics that will causethem to be lower. tologicallydistinct, they are unlikely to bepart of a Therefore,the upper limit of about170°C for R = 1 structurallyduplicated section of conglomeratesoriginally illite/smectitecan be considered as a rough estimate of depositedatsimilara stratigraphic level. Additionally, maximum temperaturefor the Devonian samples of the megacycles and sections without cyclicity within the faulted present study. conglomeratessequences cannot be matched across the faults, and it is thus unlikely that these faults involve large The origin of the abundant chlorite is more difficult to discover.Nevertheless, its ubiquitousassociation with stratigraphic repetitions. corrensite,and the indication that it is amagnesium-rich Theupper part of theStrathmore succession is less variety, indicates that it is probably authigenic rather than well-exposedand consequently there is somescope for detrital.Regardless of the origin of theseMg-rich clay concealingrepetitions here. However, structural dips are minerals, the presence of corrensite suggests temperatures low and there are a number of distinctive lithologies (e.g. volcanicrocks in theArbuthnott and Garvock Gps, the between 100 "C and 250 "C (Kubler 1973), and the isolated Edzell Mudstones in the Strathmore GP) which suggest that occurrence of saponite,that the lowerregion of this temperature bracket is more probable. repetition is nota problem. Consequently the apparently thicksuccession cannot have been caused by structural In summary,theclay mineral assemblages from mudrocks of the Midland Valley ORS, although not giving repetition. very precise indications of diagenetic grade, are consistent with thelater stages of claymineral diagenesis. Samples Depocentre migration? (Fig. llb) from all three stratigraphic groups have similar assemblages Depocentre migration was originally proposed for a number andhence thereare no obvious indications of anoriginal of ORS basins,including theStrathmore succession, by

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a Strathmore region, recognizing that there was a southwesterly overstep of the older lithostratigraphic groups. Bluck (1978) expanded this model, suggesting that sedimentation tookplace in abasin flanking a fault system that actively propagated in a southwesterly direction. More recent work (Bluck 1984; Haughton 1988; Haughton & Bluck 1989) has demonstrated that models for Strathmore deposition, which involve expansion of a single basin in this way, are difficult to reconcile with evidence for early Lower ORS basins of a style quite different from the large elongate basin in which the upper lithostratigraphic groups accumulated. Thus in the NE Midland Valley, new data (Haughton 1988; Haughton & Bluck 1989) fromconglomerates in theCrawton Gp (beneaththe Crawton Volcanic Fm) reveal asoutherly provenance for much of the succession preserved on the SE limb of the Strathmore syncline (Fig. 12). This implies that at this timeLower ORS deposition was in a relatively narrow,largely conglomerate-filled basin, with alluvial fans/braidplainsprograding from faulted margins lying to both thenorth and south. Incontrast tothe upper stratigraphic groups, axial sediment dispersal (from the NE) was suppressed in favour of transverse sources. As a dacite correlated with the Lintrathen Porphyry rests unconformably on the Highland Border Complex in the NorthEsk section (Fig. 3, Edzell),and given thatthe Lintrathen Porphyry has also been correlated with a similar porphyry close to the top of the Crawton Gp at Glenbervie (on the Bervie Water, Fig. 12), the thick succession beneath thetop of theCrawton Gp appearsto havebeen overstepped in theshort distance (20km) between Glenbervie and the North Esk. The Crawton and Dunnottar Gpsare thus envisaged to haveaccumulated in anarrow sub-basin which was later draped by a more extensive basin, with theanomalously thick succession in Kincardineshire reflecting amalgamation of thesetwo basins. It is thus unnecessary to invoke depocentre migration to account for theoverall geometry. In addition, ‘strata1 shingling’ (Crowell 1982), a feature of many strike-slip basins whose depocentre has migrated in a direction opposite to that of the strike-slip movement, is absent. Depocentre migration normal to the strike of the basin marginmust also be considered. Thereis, however, no evidence for backfaulting of the northwestern margin of the basin(s). Migration of the depocentre towards the basin is recorded in somebasins, and should be manifest from internal unconformities within the basin fill. These have not been identified in the Lower ORS, although the nature of Fig. 11. Schematic cartoon showing possible explanations for the the exposure means that low angle unconformitiesmay have discrepancy between stratigraphic thickness and vitrinite reflectivity beenoverlooked. On balancehowever there is no results. (a) Structural repetition leading to overestimated thickness convincing evidence for progressive depocentre migration. of succession overlying the Cowie Formation. (b) Depocentre migration, such that the vertical thickness at any time was less than the stratigraphic thickness. The numbers 1, 2 & 3 referring to A structurally assembled stratigraphic section? (Fig. successively younger units. (c) A structurally assembled section 1 lc) involving juxtaposition of parts of three separate basins. 1 is the Given that deposition overlapped with a period of a regional Stonehaven Gp, 2 the Dunnottar and Crawton Gps and 3 the lateCaledonian (Acadian) strike-slip deformation(Hutton Arbuthnott, Garvock and Strathmore Gps. 1987; Soper et al. 1987), serious consideration must begiven to the possibility of syn-and post-depositional fault Bryhni(1964). It wassuggested thatthe depocentre displacementsbringing together parts of the succession migratedtowards thesouthwest, parallel tothe basin which were originally not in the position we now see them. margin, and in consequence explaining the restriction to the The similar and low values of vitrinite reflectivity for both NE Midland Valley of the lower lithostratigraphic groups of the Stonehaven and Arbuthnott Gps imply they suffered a Campbell(1913). Armstrong & Paterson (1970) modified similar history of burial, despite their wide separation on the andextended Campbell’s stratigraphy to all of the stratigraphic column (Fig. 2). This could have been achieved

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Fig. U. Map of Kincardineshire showing basis of Lower Old Red Sandstone stratigraphy as describedby Armstrong & Paterson (1970). The NE Midland Valley section (Fig. 2) is measured from the base of the Stonehaven Gp at Stonehaven. Correlation between the coastal section to that in the Bervie Water is achieved using the lavasof the Crawton Volcanic Fm.S/D bdry Campbell (1913) is the Stonehaven/DunnottarGp boundary as defined by Campbell (1913) and possibly the line along which the sequence has been structurally assembled.S/D bdry Armstrong & Paterson (1970) is the Stonehaven/Dunnottar Gp boundaryof Armstrong & Paterson (1970), the intervening unfilled area being the Carron Fm. From Haughton (1988) after Armstrong & Paterson (1970) and Campbell (1913)

only if the two groups (where sampled) originally occupied a sandstones, which containabundant rounded quartzite similar(and shallow) structural position within the basin. pebbles, are analogous to sandstoneintercalations in the Significantly loc. 26(Fig. 3) fromthe Arbuthnott Gpat Dunnottar Gp, with the real break in terms of process and Dellavaird is close to the Stonehaven sample sites, yet has a sedimentcomposition occurring at theboundary between reflectivity similar tothe more remote Arbuthnott Gp theCowie and overlyingCarron Fms asdefined by samples from Angus. The problem is thus not merely one of Campbell (1913). Unfortunately this contact has yet to be geography.Could the stratigraphic columnbe partly an directly observed as it lies concealed beneath the superficial artifact of laterdisplacements, and where might such deposits of Stonehaven Bay. The vitrinite reflectivities of the displacements have occurred? Cowie Fm can be explained if these rocks never underlay Thetop of theStonehaven Group (Fig. 12) was the thickoverlying succession, which commences with the originally defined (Campbell 1913) at the top of the present Carron and contiguous formations and for which there is no Cowie Fm., with the overlying unit (now the Carron Fm.) directevidence forstratigraphic continuity. Attention has being attributedto the Dunnottar GP. But Armstrong & beendrawn above tothe probability that grossthickness Paterson (1970) redefined the Stonehaven Gp (Figs 2 & 12) variation in the Lower ORS may reflect the superimposition to include the overlying sandstones and pebbly sandstones of a regional basin containing the upper stratigraphic groups of the Carron Fm, on the grounds that the contact between (e.g. Strathmore GP) on one or more small sub-basins (e.g. theseand the coarse conglomerates at the base of the the Crawton GP). Where the sub-basins are absent beneath Dunnottar Gp waswell-defined andmappable. However, the upper lithostratigraphic groups, the overall succession is the disparate nature of the Cowie Fm is a striking feature of considerably thinner. We suggest that the Stonehaven Gp (1 the Stonehaven coastal section,with its distinctive structure, in Fig. llc) originallyunderlay thinnera succession pattern of dispersal,petrography (Armstrong et al. 1978) comprisingjust theupper stratigraphic groups (3 inFig. andpalaeomagnetism (Sallomey & Piper1973; Trench, llc), hence the similar vitrinite reflectivity values. Ithas pers.comm.),which distinguishes it fromthe rest of the subsequently been laterally displaced into juxtaposition with succession. The recent demonstration of a late Wenlock to one of the sub-basins. As the Stonehaven succession rapidly early Ludlow age (Marshall 1991) for the Cowie Fm, rather thins to the SW (see above), the amount of displacement thanthe previous Pfidoli assignment, confirms this dis- does not have to be large (a minimum of some 10 km). The tinction,as a discernible age gap is nowshown to occur structure on which suchdisplacements occurred may have before the next securely dated succession which is the early been the unseen junction separating the Cowie and Carron GedinnianArbuthnott GP. In contrast,the Carron Fm Fms.

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ORS sedimentation in the NE Midland Valley may thus - 1985. The Scottish paratectonicCalcdonides. Scottish Journal of comprise at least three basins. (1) Alate Wenlock-early Geology, 21,437-464. LudlowSilurian basin, resting unconformably the BRYHNI,1. 1964. Migrating basins on the Old Red Continent. Nature, 202, on 384-385. Highland BorderComplex, of which theStonehaven Gp CAMERON,1.6. & STEPHENSON,D. 1985. The Midland Valley of Scotland. (Cowie Fm of Armstrong & Paterson 1970) is adisplaced British Regional Geology, British Geological Survey. fragment.The low-energy fluvial depositshave aclose CAMPBELL,R. 1913. The geology of south-easternKincardineshire. analogy with a similar style of Wenlock sequence occurring Transactions of the Royal Society of Edinburgh. 48, 923-960. CROWELL, J.C.1982. The tectonics of the Ridge Basin, southern California. in anidentical structural position and of comparableage In: CROWELL, J.C.& LINK, M.H. (cds)Geologic history of Ridge Barin, close to Louisburgh in the west of Ireland(Palmer et al. southern California. SEPM Pacific Section, Los Angeles, California, US, 1989). (2) A narrowconglomerate filled basin,probably 25-42. developedalong a majorterrane boundary, in which the FoRsvlli, I.H. & ClilSIIOLM, J.I. 1977. The geology of East Fife. Memoirs of the Geological Survey of Great Britain. Carron Fm, Dunnottar and Crawton Gps accumulated. (3) FRANCIS,E.H. 1991. Carboniferous. In: CRAIG,G.Y. (ed.) Geology of Anelongate, largelyaxially filled trough which extended Scotland 3rd edition. The Geological Society, London, 347-392. across the northern Midland Valley, overstepping both these HALL,A.M. 1991. Pre-Quaternarylandscape evolution in the Scottish twoearlier basins and in which theupper Lower ORS Highlands. Transactions of the Royal Society of Edinburgh: Earth Sciences, 82, 1-26. groups are preserved. HAUGIITON, P.D.W.1988. A cryptic Caledonian Flyseh Terranc in Scotland. Reflectivity values taken for the Strathmore Gp of the Journal of the Geological Society, London, 145, 685-703. centralMidland Valley (Teith Fm) are geographically - 1989. Structure of someLower Old Red Sandstone conglomerates, remotefrom the Stonehaven and Arbuthnott Gpdata Kincardineshire,Scotland: deposition from late-orogenic antecedent discussedabove. Nonetheless, thefact that vitrinite streams? Journal of the Geological Society, London, 146, 509-525. - & BLUCK,B.J. 1988. Diverse alluvial sequencesfrom the Lower Old reflectivities are higher than any for the lower stratigraphic Red Sandstone of the Strathmore region, Scotland and implications for intervals (excepting those close to intrusions) indicates that therelationship between late Caledonian teetonics and sedimentation. significant along-strike changes in the burial history of the In: MCMILLAN, N.J., EMBRY, A.F.& GLASS, D.J. (eds) Devonian of the Strathmore succession have probably occurred. World. Canadian Society of PetroleumGeologists Memoirs, 14, II, 269-293. The application of vitrinite reflectivity data to the Lower -, ROGERS,G. & HALLIDAY, A.N.1990. Provenance of Lower Old Red ORS of the MidlandValley demonstrates how thermal Sandstone conglomerates. SE Kincardineshire: evidence for thetiming of maturitiescan be used to highlightambiguities in the Caledonianterrane accretion in centralScotland. Journal of the stratigraphicsuccession and unravel the sequence and Geological Society, London, 147, 105-120. HILLIER,S.J. & CLAYTON,T. 1989. Illite/smeetitediagenesis in Devonian stratigraphicsuccession in sedimentary a basin. The lacustrinemudstones from northern Scotland and its relationship to conclusion thatthe Stonehaven Gp is aseparate basin-fill organic maturity indicators. Clay Minerals, 24, 181-196. with an earlier history and not stratigraphically contiguous - & MARSHALLJ.E.A. 1988. Arapid technique to make polished thin with youngersediments, continues the disassembly of the sections of sedimentaryorganic matter concentrates. Journal of Strathmore ORS as started by Haughton (1988) with the Sedimentary Petrology, 58, 754-755. - & - 1992. Organicmaturation, thermal history andhydrocarbon recognition of separate Crawton and Strathmore Basins. generation in theOrcadian Basin, Scotland. Journal of the Geological Society, London, 149, 491-502. J.E.A.M. acknowledgesfinancial support from theUniversity of HOFFMAN,J. & HOWER,J. 1979. Clay mineralassemblages as low grade metamorphic geothermometers: application to the thrust fault disturbed Southampton (84/M,Volcanics of Scotland).P.D. W.H. acknow- belt of Montana, U.S.A. In: SCHOLLE. P.A.& ScliuLcERA, P.R.(eds) ledges support from the Royal Society of Edinburgh and BP. S.J. Aspects of Diagenesis SEPM Special Publications. 26, 55-79. H. thanksNERC and BP Exploration for CASEa research HOWERJ., ESLINGER E.V., HOWER M.E.& PERRY E.A.1976. Mechanism of studentship. C. Cornford (IGI Ltd) is thanked for arranging access burial metamorphism of argillaceoussediment: 1. mineralogical and to BasinModl” chemical evidence. Geological Society of America Bulletin, 87, 725-737. HUITON, D.H.W. 1987. Strike-slip terrane and model for the evolution of the British and Irish Caledonides. Geological Magazine, W, 405-425. KENNEDY,D.A & LEITH, L. 1987. Constraintson the diagenetic history of Lower Old Rcd Sandstone sediments in the Midland Valley, Scotland. References In: BSRGAbstracts. Publications of theDepartment of Geologyand Mineralogy, University of Aberdeen, 6, 75. ANDREWS,I.J., LONG,D.. RICIIARDS, P.C., THOMSON, A.R., BROWN, S., KISCIIH.J. 1983. Mineralogy andpetrology of burial diagencsis (burial CIIESIIER, J.A.& M~ORMAC,M. 1990. The Geology of the Moray Firth, metamorphism)and incipient metamorphism in clastie rocks. 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Rcccivcd 6 Junc 1993; revised typcscript acccptcd 15 July 1993

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