The Dalradian Rocks of the Northern Grampian Highlands Of
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Revised Manuscript 8/7/12 Click here to view linked References 1 2 3 4 5 The Dalradian rocks of the northern Grampian 6 7 Highlands of Scotland 8 9 10 A.G. Leslie, S. Robertson, M. Smith, C.J. Banks, J.R. Mendum and 11 D. Stephenson 12 13 A. Graham Leslie British Geological Survey, Murchison House, West 14 Mains Road, Edinburgh EH9 3LA. 15 Steven Robertson (deceased) formerly British Geological Survey, 16 Edinburgh. 17 Martin Smith British Geological Survey, Murchison House, West 18 Mains Road, Edinburgh EH9 3LA. 19 20 Christopher J. Banks Slumberger Information Solutions, Ashley 21 House, Pitmeddden Road, Dyce, Aberdeen AB21 0DP; formerly 22 Department of Earth Sciences and Geography, Keele University. 23 John R. Mendum British Geological Survey, Murchison House, West 24 Mains Road, Edinburgh EH9 3LA. 25 * David Stephenson British Geological Survey, Murchison House, 26 West Mains Road, Edinburgh EH9 3LA. 27 [email protected] 28 0131 650 0323 29 30 * Corresponding author 31 32 Keywords: 33 Geological Conservation Review 34 Northern Grampian Highlands 35 Dalradian Supergroup 36 Lithostratigraphy 37 38 Structural geology 39 Metamorphism 40 41 42 ABSTRACT 43 44 The Northern Grampian Highlands are dominated by the outcrop of the 45 Grampian Group, together with infolds and structural outliers of 46 Appin Group strata and inliers of pre-Dalradian ‗basement‘, 47 consisting of Badenoch Group metasedimentary rocks. The south- 48 eastern limit of this mountainous region corresponds with the 49 regionally continuous Grampian Group-Appin Group boundary, which in 50 the south is marked by a high-strain zone corresponding to the 51 Boundary Slide of some authors. The more arbitrary southern 52 boundary runs north-west from Blair Atholl along the A9 road and 53 then westwards to Fort William. 54 The Neoproterozoic-age Grampian Group siliciclastic succession 55 accumulated during several transgressive and regressive cycles in 56 multiphase ensialic rift basins. The Badenoch Group constitutes 57 the crystalline floor to those basins and had experienced 58 amphibolite-facies metamorphism, migmatization, gneissification and 59 deformation between c. 840–800 Ma, prior to deposition of the 60 Dalradian strata. In contrast, evidence for only 470–450 Ma 61 62 63 64 65 1 2 3 4 Caledonian orogenic events is found at higher structural levels in 5 the Grampian and Appin group successions. Locating and 6 understanding the nature of the contact between the basement 7 gneisses and the Dalradian cover sequence has long been a major 8 challenge of Highland geology. Recent research has argued that not 9 only is a rift-basin architecture evident from the patterns of 10 Neoproterozoic stratigraphy, but also that it played a significant 11 role in influencing the geometry of the superimposed Caledonian 12 deformation, with the basin infill buttressed against its margins 13 or intrabasinal ‗highs‘. 14 The GCR sites in this region preserve important evidence of cover- 15 basement relationships, patterns of punctuated deposition, and 16 onlapping sequences. The effects of both pre-Caledonian and 17 Caledonian deformation and metamorphic events are also well 18 represented. Despite the deformation and metamorphism, spectacular 19 sedimentary structures are visible at several of the GCR sites and 20 there is evidence of the earliest recorded glacigenic sediments in 21 the Neoproterozoic rocks of the British Isles. 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 5 1 INTRODUCTION 6 7 A.G. Leslie 8 9 The Northern Grampian Highlands are dominated by a widespread and 10 thick succession of Neoproterozoic siliciclastic deposits referred 11 12 to as the Grampian Group (Figure 1). Most interpretations of the 13 regional geological relationships have suggested that the strata 14 were deposited upon an orogenic unconformity, now largely obscured 15 by a zone of ductile shearing at or near the base of the group 16 (Piasecki and van Breemen, 1979b, 1983; Piasecki, 1980; Piasecki 17 and Temperley, 1988a and references therein). These 18 interpretations were based upon structural and metamorphic 19 contrasts recognized between rocks referred to an older ‘Moine-like’ 20 crystalline basement of probable Grenvillian age and termed the 21 ‘Central Highland Division’, and a cover sequence referred to as the 22 ‘Grampian Division’ or Grampian Group. The basement rocks apparently 23 underwent amphibolite-facies migmatization, gneissification and 24 deformation prior to deposition of the cover sequence. While 25 lithologically similar to the Moine Supergroup of the Northern 26 Highlands, and formerly termed the ‘Younger Moine’, the Grampian 27 Group was included within the Dalradian Supergroup by Harris et al. 28 (1978) on the basis of the apparent stratigraphical, structural and 29 30 metamorphic continuity south-east of the Great Glen Fault in the 31 Grampian Terrane (Harris et al., 1994; Stephenson and Gould, 1995; 32 Strachan et al., 2002). Locating and understanding the nature of 33 the contact between the rocks of the Northern Highlands and 34 Grampian terranes has long been a major challenge of Highland 35 geology. 36 An alternative model viewed the rocks of the Northern Grampian 37 Highlands as part of a single stratigraphical succession in which a 38 regional metamorphic front separates the supposed basement and 39 cover sequences (Lindsay et al., 1989). Such a model was not 40 however supported by more-recent radiometric studies that confirmed 41 the existence of Neoproterozoic tectonothermal events (c. 840–800 42 Ma) in parts of the Northern Grampian Highlands (Noble et al., 43 1996; Highton et al., 1999), even though only Caledonian orogenic 44 events (470–450 Ma) are known at higher levels in the Dalradian 45 succession. Such a paradox, whereby comparable studies recognized 46 discrete tectonothermal events in different parts of an apparently 47 48 continuous stratigraphical succession, but were unable to separate 49 or define the limits of these events, continues to be one of the 50 key problems in Highland geology. 51 A lithostratigraphical framework has been erected for the Grampian 52 Group in the western and south-western parts of the Northern 53 Grampian Highlands, despite the problems of correlation across 54 major structures, polymetamorphism and the absence of 55 biostratigraphical control (Glover and Winchester, 1989; Glover et 56 al., 1995; Key et al., 1997). Those authors described an evolving 57 depositional basin in which marine and locally terrestrial 58 deposition occurred within multiphase ensialic rift basins, during 59 several transgressive and regressive cycles (Glover et al., 1995; 60 Glover and McKie, 1996). Smith et al. (1999) extended the 61 62 63 64 65 1 2 3 4 lithostratigraphical approach and integrated detailed mapping with 5 geophysical modelling to define a series of basin-bounding 6 structures in the Northern Grampian Highlands (Figure 2). Current 7 research continues to refine and improve the understanding of this 8 depositional framework (Banks and Winchester, 2004; Banks, 2005; 9 Banks et al., 2007). 10 The Grampian Group sediments were deposited in NE- to SW-trending 11 12 marine basins formed during a major phase of Neoproterozoic 13 rifting. These basins extended rapidly and accumulated up to 5 km 14 of turbiditic deposits, possibly within 20–30 Ma (Ryan and Soper, 15 2001). Later thermal subsidence is suggested by the regional 16 development of shallow marine-shelf environments and could have 17 occupied a similar length of time. The margins to these basins are 18 characterized by lateral facies and thickness changes, 19 stratigraphical omission and onlap relationships of both Grampian 20 and Appin group strata onto a basement of predominantly gneissose 21 rocks that records the older, pre-Caledonian tectonothermal 22 history. Stratigraphical relationships are summarized in Figure 3, 23 which is based largely upon Smith et al. (1999). 24 Smith et al. (1999) and Robertson and Smith (1999) argued that the 25 basin architecture thus determined is not only reflected in the 26 patterns of Neoproterozoic sedimentation, as would be expected, but 27 also played a significant role in predetermining the geometry of 28 the superimposed orogenic deformation in the Northern Grampian 29 30 Highlands. For example, the Geal-charn–Ossian Steep Belt has been 31 re-interpreted by those authors to reflect buttressing of basin 32 infill against the architecture of the basin margins or any 33 intrabasinal ‗highs‘. Such analysis of preserved ‗cover-basement‘ 34 relationships led Smith et al. (1999) to propose that a significant 35 stratigraphical and sedimentological break does indeed exist at the 36 base of Grampian Group, much in the manner originally suggested by 37 Piasecki (1980). Although there is presently insufficient 38 structural or metamorphic evidence to prove an orogenic 39 unconformity beyond all reasonable doubt, geochronological data do 40 confirm the presence of Precambrian events in the basement rocks 41 that have not been recognized in the cover. The basement rocks were 42 referred to informally as the Dava and Glen Banchor successions in 43 publications and on Geological Survey maps of the period 1999 to 44 2010, but have now been united formally as the Badenoch Group, with 45 Dava and Glen Banchor subgroups. 46 The GCR sites described within this paper (Figure 1) preserve 47 48 important evidence from locations where ‗cover-basement‘ 49 relationships, and the pattern of punctuated deposition and 50 onlapping relationships within the Appin Group, are preserved. 51 There are examples of the lithostratigraphical sequence in the 52 Grampian Group and evidence of the earliest recorded glacigenic 53 sediments in the Dalradian succession. Caledonian and pre- 54 Caledonian deformation and metamorphic events are similarly well 55 represented.