Fossils and tectonics along the Highland Boundary Fault in Scotland

G. B. CURRY Department of Geology, The University, Glasgow, G128QQ, UK

Abstract: The tectonic significance of fossil discoveries fromthe Highland Border Complex of Scotland is described. These sparse but diverse macro- and microfossils have been recovered from a wide range of lithologies across Scotland, and range in age from early to late Ordovician, thereby refuting any attempt to incorporate the HBCwithin the Dalradian Supergroup. The Lower Cambrian trilobite fauna from the Leny Limestone could represent an isolated remnant of an early development of the Ordovician HBC basin(s), or alternatively may providean upper age limit for Dalradian sedimentation. Further palaeontological prospecting in the zone, especially in relatively undeformed lateDalradian sediments,could resolve this andother problems which complicate theregional tectonic interpretations.

About 30% of the land area of Scotland is composed of discoveries. The unravelling of the complex overstep fossiliferous rocks,and further extensive fossil-yielding Ordovicianstratigraphy in theSouthern Uplands (e.g., depositshave been well documented from the offshore Lapworth, see discussion in Williams 1962), the early continental margin. As a result palaeontological research in recognition of the Palaeozoic fauna1 continuity between Scotland spans a wide range of topics from micropalaeontol- Scotland and North America (e.g., Salter in Murchison ogy and palynology, through extensive taxonomic investiga- l859), and the role played by palaeontologists in the tions of a wide range of fossil marine organisms, to studies recognition of the Moine Thrust (McIntyre 1954), are all of freshwater and terrestrial vertebrates.Indeed Scottish good examples of such an application. It would clearly be Carboniferous successions are currently the focus of intense impossible toattempt even a synoptic review of these palaeontological interest having yielded, in recent years, the examples in this paper, especially as there are valid grounds first conodont animal (Briggs et al. 1983)>a remarkable for suggesting that all fossils are potentially of direct tectonic shark, fish, and crustacean fauna (Wood 1982),as well as significance because of the informationthey can provide the earliestamphibian fossil (Wood et al. 1985). Such on age, correlation and environment. Instead the intention ‘Konservat-Lagerstatte’deposits (Briggs & Clarkson 1983) here is to describe the considerabletectonic implications in the Carboniferous successions of the Midland Valley wil! of relatively recent,and hence possibly not widely probably continue to yield fossils of tremendous significance disseminated, palaeontological discoveries from one small for many years, and these localities are clearly destined to area of Scotland, which provide an ideal microcosmic become of worldwide renown. Similarly the Cambro- demonstration of the many facets of tectonic reconstruction Ordovician DurnessGroup of the NWHighlands, the to which palaeontology is animportant, if not essential, extensive Ordovician and Silurian deposits in the Southern adjunct. Uplands(e.g., at Girvan), and strata of Devonian age In comparison with-the examples mentioned above, the (which have yielded many non-marine fish and crustaceans), rocks of the Highland Border Complex (HBC), are poorly are widely known and intensely studied palaeontologically. known. The exposures are very small (a few square metres Scottish Mesozoic and Cenozoicrocks, although relatively in some cases) and are generally considerably altered and rare on land, nevertheless make a useful contribution to our intensely faulted. Establishing a valid stratigraphical understanding of biogeography duringthese eras, in correlation for such exposures across thebreadth of addition to their offshore economic importance. Indeed the Scotland has always proved a daunting task full of potential fundamental need for accurate dating and correlation in the pitfalls. Yetreferences to theserocks, as the ‘Highland Scottish offshore oil industryhas prompted considerable Border Series’ or the ‘Margie Series’, are common in interestin the micropalaeontologyand palynology of treatments of British stratigraphy, although it is clear that post-Palaeozoic sediments, which has been of considerable few geologists have much knowledge or appreciation of the benefit to the wider palaeontological community because it rocks themselves. This lack of understanding is reflected in has resulted in a greatly increased knowledge of such faunas the widespread usage of the term ‘Series’, because the and floras and of their stratigraphical importance. Highland Border rocks certainly do notconstitute a valid The intention of this paper, however, is not to dwell on chronostratigraphical ‘Series’ such as Arenig or Llandovery the broad subject of Scottish palaeontology, but to focus on (while the term ‘Complex’ is ideally suited-Henderson & the importance of Scottish fossil discoveries for interpreta- Robertson 1982; see discussion in Curry et al. 1984). Yet, it tions of tectonichistory. Even thisrestricted field of is the poorness and intractibility of the exposures which, referenceencompasses a vast range of possible examples, arguably,makes the fossil discoveries discussed below all representing the work of a great many palaeontologists past thebetter as demonstration a of the potential of and present, who influenced, readically altered, or created palaeontology for tectonics. Recent re-investigations of the new tectonic interpretations as a direct result of their fossil Highland Border Complex hadresulted in a novel 193

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interpretationfor their geological history which proposed 1963). By the early 1960s the clear consensus was that these incorporating them within the largely PrecambrianDal- fossils were not sufficiently diagnostic to date the host rocks radian Supergroup. However the discovery of Ordovician as anything other than Lower Palaeozoic (i.e. they could be fossils showed that this interpretation could not be justified Cambrian, Ordovician, or Silurian). (Curry et al. 1982, 1984), and also provided the first reliable In modern times, therefore, the Lower Cambrian age for method of correlating the disparate outcrops as well as the the Leny Limestone was the only firm palaeontological age first patchwork view of the possible tectonic relationships of datum, although many workers persisted with the the diverse lithologies represented. uncorroborated assumption that Ordovician rocks were also present.Palaeontological studies had effectively ceased following the discrediting of the earlier taxonomic The Highland Border Complex identifications although successful attemptswere made to Although the history of research on the rocks of the HBC recover microfossils fromsome lithologies, but no has been described elsewhere (Curry in Curry et al. 1984), unequivocal agedeterminations resulted (Downie et al. an historical perspective is as crucial as spatial a 1971). The macrofossils were stored in museums, or lost, appreciation of theoutcrops for an understanding of the and remained an enigmatic factor, especially as they had tectonic implications of the fossil discoveries, and will never been thoroughly monographed and indeed many have therefore be briefly outlined. neverbeen photographed. This was happeningat a time Highland Border limestoneswere extensively quarried when the combination of rapid advances in radiometric during theeighteenth century, butsustained academic dating and structural analysis were providing many very research was only initiatedduring the mid-nineteenth precise age determinationsand interpretations of tectonic century. Of particular significance was the recognition by history for theDalradian Supergroupand the Old Red Nicol (1850) of lithological similarities between the clastic Sandstone in Scotland. The HBC, relatively minute slivers components of theHBC and the fossiliferous Ordovician sandwiched betweenthese two major units (Fig. l), greywackes, cherts and shales of the Southern Uplands over remainedas a largely unexplained component of the 100 km to the south. This led Nicol (1850) to suggest that Scottish geological patchwork. The Highland Border rocks theHBC and Southern Uplands successions werelateral crop out in a narrow belt, between 150 m and 2000 m wide, equivalents, andthat the intermediate strata across the along the line of the Highland BoundaryFault (Fig. 1). Midland Valley were hidden beneath the Upper Palaeozoic Therefore, apart from explaining the geology of the rocks sediments of the Strathmore syncline (Fig. 1). Viewed in a themselves, research in the zone was further stimulated by moderncontext, the most significant aspect of this the hope that the results would throw considerable light on interpretation was the establishment of a link between this major fracture zone which has always been difficult to Ordovician fossiliferous successions in the Southern Uplands study in detail because of the lack of good exposure. andthe HBC which atthat time was not known tobe The debate as to the affinities of the HBC centres on fossiliferous. The implication thatthe HBC contained contrasting viewpoints as to their relationship with the Lower Ordovician rocks became widely disseminated during Dalradian Supergroup, which forms the northern margin of thelatter half of thenineteenth century.Barrow (1901) all HBC outcrops across Scotland (Fig. 1). Both sets of certainly proposed an early Ordovician age for the ‘Jasper rocks are steeply dipping or vertical, the Dalradian rocks and Green RockSeries’ which, together with the ‘Margie forming one limb of the Aberfoyle anticline. This latter Series’, constituted the first attempt to erecta stratigraphical structure, in fact a downward facing synform (Shackleton framework for the HBC. It is these two ‘Highland Border see Fig. l), represents the southermost culmination of the Series’,notionally assigned an Ordovicianage on criteria TayNappe, a major regional structureformed during an which would not now be accepted, which have permeated earlyphase of the Grampian Orogeny which deforms the discussions of Scottish geology for much of this century. Dalradian (Harte et al. 1984). Mapping of Highland Border The widespread acceptance of such age determinations rocks hadrevealed a downward facing bedding-cleavage was, however, apparently put on a much firmer footing by a relationshipin one locality whichwas similar tothat series of fossil discoveries which post-dates Barrow’s recordedfrom southern Dalradian rocks involved in the stratigraphicalsummary. The fossils, interpreted aspoorly Aberfoyle anticline (Shackleton1958). Such data did not preservedbrachiopods, graptolites, crustaceans andother necessarily imply that the HBC had experienced a similar organisms,were recovered from various localities across structural history as the Dalradian rocks, because this would Scotland(Campbell 1911, 1913; Jehu 1912) and were have required a clear demonstration that cleavage sets in the tentatively assigned an Arenig age. The distribution of the Highland Border Complex could be correlated with those in fossils was also considered to have provided good the adjoining Dalradian. In fact the Highland Border confirmative evidence for the existence of two divisions (still cleavage planes cannot be directly correlated across into called Series) separated by an unconformity and recog- adjacent Dalradian rocks, and indeed considerable rotation nizable both in E and W Scotland (Jehu & Campbell 1917). is required to bring the two sets of fabrics into geometric Thesubsequent discovery of undoubtedLower Cambrian alignment in key sections such as in the River North Esk trilobites and brachiopodsin the Leny Limestone (Pringle (Johnson & Harris 1967). 1939),a fault bounded shaley limestone which cropsout Subsequent remapping of the Highland Border Complex near Callender in much the same structural position as the was, however,thought to have turnedup sufficient HBC, clearly added a further dimension tothe Highland structural and other evidence to justify further elaboration Border story. The LowerCambrian age determination for of the tectonicand regional implications of shareda the Leny Limestone trilobites stands firm to the present day structural history for theHBC and the Dalradian (Cowie et al. 1972), but the ‘Ordovician’ fossils subsequently Supergroup (Henderson & Robertson 1982). In this proved to havebeen wrongly identified (see Anderson interpretationthe HBC was incorporated within the

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Fig. 1. Block diagram illustrating the tectonic setting of the HBC. Thediagram is a composite, incorporating information from a number of outcrops across Scotland. The orientationof the faulted contact between the Dalradian Supergroup and the HBC, hereshown as a thrust (as can be seen at Stonehavenfor example), is actually variable across Scotland, and other local variations in HBC geology have not been incorporated in this stylized summary of HBC relationships and internal structure.

Dalradian Supergroup, and an overriding nappe, related to the adjacent, but quite distinct, Midland Valley block to the butstructurally above theTay Nappe, was invoked to south (Bluck 1983, 1984). Previously the situation had been explain the southwards emplacement of the HBC from their entirely unsatisfactory with the unsubstantiated inference of postulated origin within the main Dalradian massif to the Ordovician rocks and the possibility of Dalradian affinities north (Henderson & Robertson 1982). In proposing an age existing in uneasy compromise at a time when the build up forthe HBC consistent with such aninterpretation, the of radiometric data was making it clear thatthe two authors wereconsiderably constrained by theplethora of suggestions were entirely incompatible. absolute(radiometric) age determinations forDalradian The recent fossil discoveries (Curry et al. 1982, 1984; metamorphism andfor igneousbodies which cross-cut Burton et al. 1984; Ingham et al. in preparation) have Dalradian fabrics. Such data indicate that structures such as proved that the Highland Border Complex contains a range the Tay Nappe were emplaced pre-520 Ma (see summary in of faunas which is undoubtedly of Ordovician age, which Harte et al. 1984). For these reasons the HBC had, by this rules out any possibility of incorporatingthem within the interpretation, to be assigned aCambrian or Precambrian Dalradian Supergroup as presently defined. The situation is age, and the reconstructedstructural scenario showed the clearly much too complicated to be explained away by two HBC moving as atectonic package during the Cambrian main structuralunits, and the new data points to (Henderson & Robertson 1982). This was not inconsistent considerable lateral movements in the Highland Boundary with the only valid palaeontological age determination Fault zone, to the possible presence of allochthonous rock (Lower Cambrian for the Leny Limestone) given that the assemblages, and to considerations of the HBC as the highly Ordovician ages were discredited. The incorporation of the tectonized remnants of one or more Ordovician basins. HBC in theDalradian Supergroup had the considerable The HBC fossils have beenobtained from limestones, advantage of explaining theseanomalous rocks within a sandstones, and black shales interbedded with spilites, and regional framework composed of the Dalradian massif and range in age from Arenig to Caradoc or Ashgill. Portions of

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these faunas have been illustrated and discussed elsewhere (460-440 Ma)and the dating of the initiation of the (Curry et al. 1982, 1984; Burton et al. 1984) but description Grampian Orogeny (pre-520Ma) is much too great to be of the biggest fauna, that from the Dounans Limestone at explained away by inherenterrors, and (b) because the Aberfoyle, has proved a major task. The fauna is silicified fossiliferous HBC are in some cases clearly interbedded with and extremely sparse; over 7.5 tonnes of rock was dissolved the ophiolitic rocks discussed by Ikin & Harmon.At the over a 3 yr period to yield insolubleresidues which very least it mustbe considered unrealistic to extrapolate contained minuteproportions of organicremains. Yet geochemical evidencefrom meta-igneous rocks to provide because of the amount of rock processed, a diverse fauna age determinationsfor theentire HBC; but the fact that has been recovered which is of considerable palaeontologi- some of these rocks are interbedded with Ordovician cal importance as well as being of major significance for the fossiliferous strata (which from Ikin & Harmon’s interpreta- geological history of the HBC. A full taxonomic treatment tion of the geochemistry would have beeninterpreted as of this fauna has been prepared (Ingham et al. 1985), but in Cambrian or older) makes it clear thatthere are serious the context of this paper it was considered worthwhile to doubts aboutthe validity of using geochemical data as provide illustrations demonstrating the extensive taxonomic grounds for correlating between rock units which have been spread of the recovered fossils (Fig. 2). This is in part subjected to various phases of chemical alteration. Certainly intended to dispel any doubts about the validity of the new geochemical studies are of great importance in unravelling age determination age for the Dounas Limestone. The fauna the geological history of theHBC, but it is totally contains over thirty genera. It includes several new species, unacceptable to considerstable isotope evidence to bea hascomplete growth series of sometrilobite species and more ‘direct’ source of information on age as compared with makes important contributions to the understanding of the ‘indirect’ palaeontological andradiometric data (Ikin & evolution of atleast one trilobite family. The assemblage Harmon 1984). incorporates taxa known from N America, Spitzbergen, and Similarily cleavage planes within tectonized complexes Ireland (and hence is of considerable importance for Arenig are unreliable indicators of correlation. Having been caught biogeographical syntheses) and has yielded large numbers of up in a polyphase fault zone, the HBC fabrics have certainly undoubted organicremains which are somewhatprob- experiencedvariable, and effectively undetectable, ge- lematical and may throwconsiderable light on enigmatic ometricreorientation related tothe several episodes of groups.Additionally, it is very precisely correlatable with post-Ordovician movement along the Highland Boundary one zone within the Arenig Series. Whatever uncertainties Fault. With the likelihood of considerable transcurrent and remain over the HBC, the age of the Dounans Limestone other faulting extending up to Carboniferous times, the has now been precisely determined,and the existence of original cleavage orientations have clearly beenlost, and Ordovician rocks across the breadth of the HBC in Scotland attempts to correlate on thebasis of co-alignment of fabrics has been unequivocally demonstrated. between the HBC and Dalradian are at best tentative. The importance of establishing the age of Highland Border rocks is amply illustrated by the fact thatthe implications of theirincorporation in the Dalradian Supergroup have beentaken upand furtherelaborated, Future developments especially the suggestion that overriding nappes, which have Current work on the HBC is focused on the possibility that beenextrapolated as farsouth as Girvan,to explain the it represents the remnants of an Ordovician basin or basins occurrence of exotic blueschists (Yardley et al. 1982). With which became intensely disrupted in a major fault zone. The an overriding nappe now discredited as a mechanism of unconformity between some Highland Border rocks and the emplacement for the HBC, this interpretation must also be ORS lithologies of theStrathmore syncline suggests that thrown into serious doubt. An attempt has also been made considerable thicknesses of HBC rocks could well be buried to reconcile apparently contradictory geochemical evidence beneath the Upper Palaeozoic cover of the Midland Valley. with the fossil evidence. Ikin & Harmon (1984) reported on Certainty unfolding theStrathmore syncline indicates that geochemical analyses of Highland Border andDalradian during the deposition of the ORS, the Dounans Limestone rocks, the results of which they interpret as indicating a was dipping gently to the south,and there are other clues to common origin. These data were considered to demonstrate the presence of Ordovician carbonate successions at depth thatthe Highland Border Complex was ‘pre-Grampian’, beneath the Midland Valley (Ingham et al. 1985). Other despite the fact that several of the early phases of Grampian units within the HBC appear tohave experienced a different deformation are accurately dated as pre-Ordovician (Harte geological history, testifying either tothe presence of a et al. 1984). Ikin & Harmon (1984) contendedthat the variety of unrelated rock sequences orto the differential apparently incompatible fossil and geochemical evidence can imprinting of earlydeformational events within a single be reconciled to comply with their pre-Grampian interpreta- basin. tion for the HBC on the grounds that (a) the absolute age The work onthe Highland Border Complex has difference between pre-Grampian andArenig (i.e. the emphasized the importance of determining the age at which Dounans Limestone fossils) is not significant because of the Dalradian sedimentation ceased. The uppermost units of the inherenterrors in radiometric and palaeontological age Southern Highlandsgroup are generally assumed to have determinations, and (b) thefossiliferous HBC rocks are only beendeposited during early Cambrian times (Anderton loosely associated with the meta-igneous rocks, and as such 1982), but the only palaeontological age determination in their conclusions from the latterneed not necessarily conflict Scotland comes from the Leny Limestone which may or may with the palaeontological ages for the former. Such points not be genuinea Dalradian rock. Radiometric age cannot be accepted because, (a) the fossils recovered to date constraints do not rule out the possibility of early Cambrian range up to the Upper Ordovician (Curry et al. 1984) and as a valid upper age limit for Dalradian sedimentation (see the difference in absolute terms between Caradoc to Ashgill Harte et al. 1984). If the Leny Limestone fossils do provide

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/1/193/4893460/gsjgs.143.1.0193.pdf by guest on 26 September 2021 Fig. 2. Silicified brachiopods (l-5), gastropods (6-15), orthocone (16), fncertae sedis (17), trepostome bryozoan (18), and trilobites (19-30) from the Dounans Limestone of the Highland Border Complex, Aberfoyle, Scotland.1,2, Nothorthis, HM L14761, X 5; 3-5, Archueorthis, HM L14735, X 5; 6, cf. Subulites, HM S22951, X 5; 7, cf. Murchisoniu, HM S22954, X 5; 8-10, cf. Strupurollina, HM S22946, HM S22945& HM S22944 respectively, all X 5; 11, cf.Cyrtodiscus, HM S22941, X 6; 12-15 cf. Maclurites, HM S22950, HM S22948, HM S22947 & HM S22949 respectively, all X 5;16, orthocone, HM S26898, X 8; 17, fncertae sedis, possibly conulariid plate, HM C11862, X 6; 18, trepostome bryozoan, PD 6346,X 4; 19,20, Kawinu, HM A21293 & HM A21294 respectively, both X 8; 21-24, Ectenonotus, HM A21348, HM A21340, HM A21226 & HM A21336 respectively, all X 8; 25-28, Sycqhnntia, HM A21224, HM A21234, HM A21230& HM A21225 respectively, 25,26 & 28 X 8,27 X 18; 29, Strotactinus, HM A21329, X 8; 30 Cerutocephala, HM A21351, X 8. All specimens are lodged in the HunterianMuseum, University of Glasgow (prefix HM), with the exception of the bryozoan which has been deposited in the Departmentof Palaeontology, British Museum (Natural History), London (prefix PD).

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a valid age constraint for uppermost Dalradian sedimenta- 463-6. tion,however, then the success in extractingmacro- and -, BLUCK,B. J., BURTON,C. J., INGHAM,J. K., SIVETER,D. V. & microfossils from the HBC suggests that a similar WILLIAMS, A.1984. Age, evolution, and tectonic history of the Highland BorderComplex, Scotland. Transactions of theRoyal Society of programme of palaeontologicalprospecting in adjacent Edinburgh, 75, 113-33. Dalradian rocks, which are often nomore deformed or DOWNIE, C. 1984. Acritarchsin British Stratigraphy. Special Report of the altered, shouldreveal confirmatory evidence. Preliminary Geological Society, London, 17, 26 pp. investigations have indeed suggested that such microfaunas -, LISTER,T. R., HARRIS, A. L. & FETTES,D. J. 1971. A palynological investigation of the Dalradian rocks of Scotland. Reportof the Znstitute of and floras do exist (Downie et al. 1971; Downie 1984) and Geological Sciences, London. 71 19. may eventuallyprovide workablea biostratigraphy for HARTE,B., BOOTH,J. E., DEMPSTER,T. J., FETTES,D. J., MENDRUM, J. &R. upper Dalradian rocks. WATTS,D. 1984. Aspects of the post-depositional evolution of Dalradian andHighland Border Complex rocks in theSouthern Highlands of I am grateful to all my colleagues in the University of Glasgow for Scotland. Transactions of the Royal Society of Edinburgh, 75, 151-63. theirguidance, encouragement,and companionship in the field HENDERSEN,W. G. & ROBERTSON,A. H. F. 1982. TheHighland Border rocks and their relation to marginal basin development in the Scottish duringthe last 5 years. In particular I would like tothank B. J. Caledonides. Journal of theGeologica1 Society, London, 139, 433-50. Bluck, C. J. Burton, B. L. Cohen, Sir Alwyn Williams and J. K. IKIN,N. P. & HARMON, R.S. 1984. Tectonic history of the ophiolotic rocks of Ingham (who kindly provided the trilobite photographs for Fig. 2). theHighland Border Fracture Zone, Scotland: stable isotope evidence The study of Highland Border geology and palaeontology was from rock-fluid interactionsduring obduction. Tectonophysics, 106, funded by NERCGrant GR3/3934 and an extension, whichis 31-48. gratefully acknowledged. INGHAM,J. K.,CURRY, G. B. & WILLIAMS,A. 1985. Lower Ordovician DounansLimestone fauna, Highland BorderComplex, Scotland. Transactions of the Royal Society of Edinburgh, 76, 4 (in press). References JEHU,T. J. 1912. Discovery of fossils inthe Boundary Fault Series,near Aberfoyle. Geological Magazine, 49, 469-70. ANDERSON,F. W. 1963. Palaeontology;Section 111, 1, In Summary of the - & CAMPBELL,R. 1917. The HighlandBorder rocks of theAberfoyle Progress of the Geological Survey of Great Britain (1962). district. Transactions of the Royal Society of Edinburgh, 52, 175-212. ANDERTON,R. 1982. 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K., BLUCK,B. J. & WILLIAMS,A. 1982. The YARDLEY,B. W. D., VINE,F. J. & BALDWIN, C. T.1982. The plate tectonic significance of a reliable Ordovician age for some Highland Border rocks setting of NW Britain and Ireland in late Cambrian and early Ordovician in CentralScotland. Journal of the Geological Society,London, 139, times. Journal of the Geological Society, London, 139, 455-63.

Received 7 March 1985; revised typescript accepted 11 June 1985

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