A Radical Interpretation of Stratigraphy and Structure in the SW and Central Highlands of Scotland
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Journal of the Geological Society, London, Vol. 145, 1988, pp. 69-678, 10 figs. Printed in Northern Ireland Dalradian slides and basin development: a radical interpretation of stratigraphy and structure in the SW and Central Highlands of Scotland R. ANDERTON Department of Applied Geology, University of Strathclyde, 75 Montrose Street, Glasgow G1 lXJ, UK Abstract: Movements on syndepositional listric faults are responsible for many of the lateral varia- tions in Dalradian stratigraphy. Sedimentary facies and thickness variations show that the surface traces of these faults correspond, in many cases, to what have previously been interpreted as tectonic slides, although normal faults, thrusts and sedimentary contacts may also mark their position. Three main episodes of faulting are described in the Islay to Lochaberarea. Appin Group sedimentation was controlled by a fault now represented by the Loch Skerrols Thrust-Fort William Slide-Ossian steepbelt. In the Islay-Jura areaan anomalouslythick Islay Subgroup succession accumulated in a half-graben bounded by a listric normal fault to the NW and terminated by transfer faults at each end. The deposition of a thick Easdale Subgroup section was controlled by movements on what is now the Benderloch-BallachulishSlide. This listric fault terminates southwards in the Scarba Fault and probably extends north into the Ossian steep belt. The thinness of the Appin and lower Argyll Group succession between the Ericht-Laidon and Bridge of Balgie Faults is an original feature. The stratigraphic gap represented by the Boundary Slide here is reinterpreted as a disconformity. However, further east the Boundary Slide may well be a listric fault, movement on which accommodated the thick succession around Schiehallion. The development of the major structurewas influenced by the geometry of the sedimentary basins. The Schiehallion-Tummel area is reinterpreted as lying on the lower limb of the Ben Lui fold. This resolves the problem of the emplacement of the Tay nappe which is likely to be a gravity structure derived from'above the Ossian steep belt. The interpretation of structure in the Dalradian has always Dalradian sedimentary and tectonic evolution been dependent on a knowledge of stratigraphy. In ground which is, at best, only moderately exposed, where way-up The Dalradian is a thick sequence of metasediments with criteria are oftenabsent and where thereare no minor metavolcanics that extendsfrom the Shetland Isles stratigraphically useful fossils, a knowledge of the local across Scotland tothe west coast of Ireland. It was stratigraphic succession is essential for the mapping of folds depositedin the latePrecambrian to Cambrian and and faults. Structurally complex areas can be disentangled deformed by theGrampian phase of the Caledonian with the help of a stratigraphic sequence established in an Orogency in late Cambrian to early Ordovician times. The adjacent, less deformed area. However, this assumes that Appin, Argyll and Southern Highland Groups only are thereare no significant lateralchanges in stratigraphy included here in the Dalradian (Fig. 1). The underlying between the two areas. If this assumption is invalid, the Grampian Group, included in the Dalradian by Harris et al. structural interpretation may be quite wrong. The problem (1978) but still referred to as Moine by many workers (e.g. is that the identification of lateral changes in stratigraphy is, Thomas 1980), is sufficiently different to be considered as a in itself, dependentonthe structural interpretation. separate stratigraphic unit (Anderton 1985). Although it should be possible, in theory,to distinguish The sedimentary evolution of the Dalradian has recently betweenstructural and stratigraphicfeatures during been reviewed by Anderton (1985) and can broadly be mapping, in practice many complex field relationships can consideredin terms of deposition on the NW side of an be equally well interpreted as dueeither to lateral facies ensialic marine gulf which was ruptured along its axis, at variations or tectonic dislocations or a combination of both. around the Precambrian/Cambrian boundary, to form the Untilrecently, structural interpretations of the Dalradian site of the Iapetus Ocean. Thus, most of the Dalradian, up have assumed that the stratigraphic sequence is fairly well to about the topof the Argyll Group (Fig. l), was deposited known and can be assumed to extend across Scotland and during thelate Precambrian in an environment of Ireland without rapid lateral changes. It is shown here that progressive lithosphericstretching prior to continental this assumption is not justified and that major changes in the failure. It shows evidence of increasing tectonic instability stratigraphicsequence can beinferred over quite short with time,for example, in the change from dominantly distances. Soper & Anderton (1984) have argued that some shallow water sediments in the Appin Group to deep water of the features thathave been interpreted as slides may have turbidite basins and volcanics in the upper Argyll Group. originated as syndepositional listric faults. The stratigraphic The post-rifting Cambrian part of the succession, the analysis presented here goes further and suggests that listric Southern Highland Group, may then have been deposited faulting is the key to understanding both the lateral onthe newly formed, thermally-subsiding Laurentian variations in Dalradian stratigraphy and some of the major continental margin. features of the Dalradian's subsequent structural evolution. Anderton (1979) inferred thatmajor syndepositional 669 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/669/4889420/gsjgs.145.4.0669.pdf by guest on 28 September 2021 670 R. ANDERTON Group Subgroup Formation by Anderton (1985, Figs 4-7) were hypothetical and deduced only from facies and thickness variations. It is now shown that many of these hypothetical faults can be equated with mapped slides while some slides can be interpreted as stratigraphicdiscontinuities which may have beena consequence of listric fault-controlled extensional tectonics. This new insight comes from considering Dalradian faulting in terms of arcuate listric faults of the type shown in Fig. 2a. These large scoop-shaped fault surfaces can be divided into normal, transfer and sole fault sectors. The rotational fault TAYVALLICH block above the listric fault is normally, but not necessarily, 154 elongatedin the direction of the normalfault. Transfer faults may curve round gently into the normal and sole fault sectors (Fig. 2a) or may be more regional featuresthat truncateseveral fault blocks (Gibbs 1984). Not only will there be rapid facies and thickness variations as one goes across the faultsbut the half-graben geometry of the resulting sedimentary basins will result in an attenuated succession, broken by disconformities, being deposited in the updip parts of the grabens (Fig. 2b). Even in a folded succession it should still be possible to see the original geometric and stratigraphic relationships of the faults. For example, if the regional fold axes lie roughly parallel to the long axes of the half-grabens, down-plunge profiles should Jura Quartzite show the listric geometry (Fig. 2b) while down-dip view ISLAY perpendicular to this may show aseries of scoop-shaped discontinuities (Fig. 2c) Bonahaven Dot. Port Askalg Tilllto Islay-Jura archipelago Anderton (1985) argued thatthe along-strike facies and BLAIR ATHOLL Llsmore Lst. thickness variations seen in the Islay and Easdale Subgroup ----- rocks could be interpreted in terms of the activity of two BALLACHULISH transfer faults, one on Islay and one in the Scarba area. The ___-_ former was invoked to explain the increase in thickness of the Islay Subgroupfrom less than 2 km in SW Islay to Leven Schist8 6+ kminthe N Islay-Jura-Garvellachs area.The north-eastwards thickening is seen in all the formations in LOCHABER Quartrlte Okncoe the Subgroup, viz. the PortAskaig Tillite, Bonahaven Dolomite and Jura Quartzite, but does not appear to have Blnneln Quartzlte affected the overlying Easdale Subgroup. The thickening of theJura Quartzite is mainfested by a 30" divergence Ellde Quartzlte between the trend of the top and bottom contacts in south Fig. 1. Summary of Dalradian stratigraphy. Only selected central Islay (Fig. 3). Here, the 1:50000 Geological Survey formations are named. See Anderton (1985) for details. map (Sheet 19) shows a 5 km length of the lower contact truncating limestone horizons in the Appin Group. If this part of the contact is a fault it could be interpreted as the normal faults, with throws of several kilometres, controlled actual trace of what originally would have beena Easdale Subgroup sedimentation in Argyll. He later realized north-eastward-dipping transfer fault. This is an alternative that such faultsmust also have been responsiblefor the to Basahel's (1971) explanationfor the thickening of the rapid lateral thicknesschanges in Islay Subgroup rocks JuraQuartzite that the whole of its lower contact is a (Anderton 1982). Soper & Anderton (1984) drew attention tectonic slide. 2.5 km to the east of the inferredtransfer to the well-known tectonic slides in the Dalradian pointing fault is a roughly parallel fault, shown as such onthe out that, as they usually have an extensional geometry, they Geological Survey map, which runs NNE into the Sound of may have been initiatedas the low angle parts of Islay. It is responsible for further north-eastward thickening syndepositonal listric faults.