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Discussion |
1 1Division of Earth Sciences, Centre for Geosciences University of Glasgow, Glasgow G12 8QQ, UK (e-mail: g.tanner@earthsci.gla.ac.uk)
2 2Crustal Geodynamics Group, School of Geography & Geosciences, University of St. Andrews, St Andrews, Fife KY16 9AL, UK (e-mail gia@ st-andrews.ac.uk)
3 3School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT UK.
Geoff Tanner writes: Firstly, I would like to congratulate the authors for presenting a very carefully documented, and splendidly illustrated, account of the origin of the Stralinchy conglomerates. The aim of this contribution is to focus on one important question: is their controversial model for the Donegal Dalradian supported by studies of the stratigraphically equivalent Dalradian rocks of the SW Highlands of Scotland? If an orogenic unconformity is present in the Dalradian Supergroup, evidence for this should be found at the base of, or within, the Easdale Subgroup in this region. The field relationships seen at this stratigraphical level in Scotland are reviewed briefly, and an attempt made to reconcile the results with the Donegal model.
Jura Quartzite–Easdale Subgroup relationship in SW Highlands of Scotland. The contact between the Jura Quartzite and the Scarba Conglomerate Formation (which represents the base of the Easdale Subgroup) is best seen in excellent coastal exposures between Jura and Loch Creran. It is exposed on the SE side of the island of Jura; is excised by faulting on the islands of Scarba and Lunga; and re-appears on the SE side of the Rubha Aird Peninsula, 7 km north of Oban.
At Lussagiven on Jura [NR 6387 8666], the top of the Jura Quartzite is marked by cross-bedded quartzite followed by 23 m of thinly bedded, commonly gritty, sandstones with conglomerate lenses. These beds, which display cross-lamination, graded bedding and channel structures, pass upwards with perfect conformity into a 36 m thick unit of laminated slate with sandstone ribs, the Jura Slate member of the Scarba Conglomerate Formation. Gritty sandstone and conglomerate beds up to 1 m thick, that characterize the Scarba Conglomerate, appear 280m farther up sequence They contain pebbles to 3 cm; rip-up clasts of mudrock; and less common rafts of sandy sediment up to 7 m long. The entire sequence youngs consistently SE.
At Port Ban Mhic-a-phi [NR 6952 9536], 10 km to the NNE, it is clear, despite faulting, that the current-bedded Jura Quartzite becomes thinly-bedded before passing conformably upwards into gritty sandstones and conglomerates (to 1 m thick) of the Scarba Conglomerate Formation. The beds commonly have erosive bases, with some graded bedding and chanelling. Abundant cross-lamination and graded bedding found across the entire section show a consistent SE-younging.
The base of the Easdale Subgroup is next seen on the east side of the Garbh Aird peninsula [NM 8772 3724], where the Jura Quartzite is conformable with the Selma Black Slate, the local equivalent of the Jura Slate member. The Quartzite is highly fractured in outcrop, but bedding is preserved, and both cross-lamination and slump folding can be seen within 2–6 m of the contact.
At all three of the above localities, bedding maintains a constant orientation for tens of metres across each Quartzite–Scarba Conglomerate contact, and the pelitic rocks show a steep to vertical penetrative D1 cleavage, cut by a crenulation cleavage that dips at a moderate angle to the SW. There is no evidence of high strain at the contact, and no evidence for a structural discordance or orogenic unconformity between the Jura Quartzite and the Easdale Subgroup.
Relationship between the Scarba Conglomerate and the Selma Breccia. On Scarba and Lunga, a synsedimentary slump scarp has brought the Jura Quartzite into contact with the Scarba Conglomerate (Anderton 1977, 1979). The conglomerate reaches its maximum development on these islands, occurring as a series of slump and slide deposits, with intervening turbidites. On Lunga, mass flow deposits contain blocks of laminated siltstone and sandstone to 10 m long (Baldwin & Johnson 1976). Similar deposits on Scarba contain intrabasinal, randomly orientated, rafts of sandstone to 6m long (Anderton 1979).
The Selma Breccia, equated with the Scarba Conglomerate by Litherland (1980), occurs within basinal Easdale sediments at [NM 9024 3803], near Benderloch. It was interpreted by Litherland (1975) as a sedimentary slide tilloid, but work in progress by the author shows that the Breccia is a 29 m thick composite debris flow deposit (debrite) with clasts that range from granules to boulders in size, and are accompanied by occasional rafts of sediment to 3 m across. The original clasts were of undeformed sediment (later slightly deformed during D1), mainly of finely laminated siltstone and sandstone that preserve sedimentary structures, and mudrock. Intrabasinal dolomite clasts dominate the matrix-supported top 6 m of the debrite, which is capped by 2 m thick breccia containing many exotic, microfossil-bearing limestone clasts (Litherland 1975). Apart from the latter, the clasts are all of intrabasinal origin and can be matched with those reported from the slump and slide deposits of Scarba and Lunga (Anderton 1977; Baldwin & Johnson 1976).
The underlying Selma Black Slate (=Jura Slate) contains thick units of pebbly mudstone; horizons containing metre-scale rafts of sediment; isolated boulder-, pebble- and granule-sized clasts in a grey silty matrix; and numerous slump folds. These features indicate that a large part of the calculated >72 m thick sequence (exposed on the SE limb of a D1 anticline) was deposited from debris flows.
Discussion. Two conclusions follow from the observations above: (1) field data from the SW Highlands of Scotland do not indicate the presence of an orogenic unconformity in the Easdale Subgroup and (2) during the period in which the Scarba Conglomerate was deposited, basin margin slump and slide deposits represented by the sequences described from Jura, Scarba, and Lunga (Anderton 1977, 1979; Baldwin & Johnson 1976) gave rise to sporadic large-scale debris flows (Selma Breccia) which transported material from these deposits many km out into the sedimentary basin. From field and laboratory studies of debris flows and debrites (see Dasgupta 2003, and references therein) it is clear that certain types of flow are capable of carrying large blocks of mechanically weak material for long distances, and depositing them intact.
It is difficult to reconcile (1) with the model proposed by the authors. Viewed from a Scottish perspective it is perhaps more relevant to look again at the origin of the Stralinchy conglomerates, as they display most of the characteristics of a stacked debrite, and have many features in common with the Selma Breccia. These features are seen best in the middle member (as admirably illustrated in fig. 7), which consists of a wide variety of matrix-supported, unsorted, angular to rounded clasts, some of fragile materials, that range in size from granules to boulders. The conglomerates could therefore be interpreted not as a proximal deposit, as initially considered by the authors, but as the products of a dynamic system capable of carrying large blocks of rock for long runout distances. This raises the possibility that, were the schist fragments and foliated quartzite blocks to be of extrabasinal origin, a Selma-type debris flow(s) would be implicated, and the need for an unconformity removed. The main evidence that runs directly counter to this suggestion is the truncation, at the base of the conglomerate, of a pre-existing tectonic fabric in the underlying Slieve Tooey Quartzite (fig. 6, e & f), together with the correlation of the foliated quartzite clasts in the conglomerate with the same footwall Quartzite. The question is, are the authors certain that the contact labelled in figure 6 is the breccia–footwall contact, and not a breccia–megaclast contact?
As with the Selma Breccia, the single occurrence of the Stralinchy conglomerates would be more understandable if it were interpreted as resulting from a rapid, possibly multiple, depositional event, rather than representing a fundamental and widespread orogenic unconformity.
13 August 2004
Ian Alsop & Donny Hutton reply: We thank Tanner for his detailed comments on the Dalradian stratigraphy of the SW Highlands of Scotland and welcome this opportunity to clarify further and expand upon the important boundary relationships of the Islay–Easdale sub-groups. The two major concerns expressed by Tanner are (a) the apparent lack of similar structural and statigraphic relationships in SW Scotland and (b) the depositional nature and sedimentary characteristics of the Stralinchy conglomerates. We shall now address each of these points in turn.
The correlation of any erosive unconformity surface along strike may be problematical owing to variable footwall and hanging-wall geology, coupled with the added complication of incomplete exposure. In Donegal, the complete excision of the Islay Quartzite in the footwall of the unconformity clearly illustrates the problems of along-strike correlation when major marker horizons which underpin the stratigraphic framework are themselves missing. The preservation of geology in the footwall of the unconformity is thus partially dependent on any earlier deformation history which these rocks have suffered, whilst the variable hanging-wall geology may reflect depositional patterns controlled by the irregular unconformity surface (as noted in our original paper; Hutton & Alsop 2004). Indeed, the presence of major lateral facies variations coupled with syndepositional pinch-outs of the Argyll Group onto the underlying Appin Group has long been recognized in both Donegal and Argyll (Pitcher & Berger 1972; Litherland 1980). In the case of the unconformity in NW Ireland, the relationships are further complicated by subsequent regional deformation which affects both the footwall and hanging-wall sequences, and which in addition has reactivated the general level of the unconformity as a tectonic slide (Hutton & Alsop 1995, 1996, 1997; Alsop et al. 2001).
In terms of the early structural histories preserved within the Appin Group rocks, it is interesting to note that in the Loch Creran area of Argyll, Litherland (1982, p.220) identified a simple S1 cleavage in the Easdale subgroup which becomes a composite S1–S2 fabric when traced across the Benderloch Slide into the immediately underlying Appin Group. Importantly, this composite cleavage is axial planar to folds which affect both groups (Litherland pers. comm. 2004). Thus, S2 in the Appin Group may correlate with S1 in the Easdale subgroup, possibly supporting the presence of an early cleavage (i.e. pre-unconformity) as we have suggested in NW Ireland. The precise relationships of these early tectonic fabrics, together with the nature, extent and age of late Neoproterozoic tectonism in the British and Irish Caledonides is at present poorly constrained. However, a U–Pb titanite age of 669 ± 31 Ma has been obtained recently from a syn-kinematic titanite within contractional mylonites affecting parts of the Northern Highland Moine (Storey et al. 2004). The lower parts of the Dalradian succession, including the Appin Group, are generally considered to be older than this date (see Strachan et al. 2002 and Oliver 2002 for a discussion), and could therefore have been affected by deformation of this age. These recent developments and uncertainties clearly highlight the necessity for further geochronological and structural studies in these rocks.
The depositional nature and sedimentary characteristics of the conglomerates and breccias developed throughout the Dalradian succession are clearly an important area of study as they provide a record and insight into the tectonostratigraphic evolution of the entire sequence. Tanner has kindly provided details of his work in progress concerning the local Islay and Easdale successions in Argyll. Our observation suggest that some of these boundaries, e.g. the base of the Easdale subgroup at Garbh Aird, display strong fabrics and are tectonized. This deformation renders interpretations of the original nature of such contacts equivocal. However, most significant in his descriptions is the absence of recorded cleaved clasts within the conglomeratic units of the SW Highlands. As this is one of the single most distinguishing and important characteristics of the Stralinchy deposits, it may suggest that the conglomerates and breccias in the two sequences of NW Ireland and SW Scotland are sedimentologically unrelated, and may in fact be quite different to one another in terms of genesis (see below).
A major part of this discussion relates to the suggestion that debris flows may be capable of transporting large blocks of mechanically weak material for considerable distances. In such a scenario, the Stralinchy conglomerates would be interpreted not as proximal deposits, but as distal units containing extra-basinal clasts which have been carried large distances in to the basin. However, the Stralinchy deposit exhibits none of the characteristic sedimentary features necessary to infer that clasts may have been transported such large distances by this process (A.R. Prave pers. com. 2004). Such criteria typically include an abundant fine-grained matrix, a relatively low proportion of clasts to matrix, the isolation of clasts and separation from other large clasts, and a flow alignment of clasts with the larger clasts typically occurring near the tops of beds (Rodine & Johnson 1976; Takahashi 1981; Brunsden & Prior 1984).
In addition there is a close correspondence between quartzite, carbonate and pelite clast lithologies with the directly underlying sequence in the footwall of the unconformity. This leads us to suggest that the most likely source for many of these clasts is indeed the underlying Appin Group and Islay subgroup. That some of the clasts may be extra-basinal and exotic is not disputed as quartzite clasts clearly display a range of sedimentary characteristics including size, roundness and sphericity suggesting the possible presence of more than one population. Indeed, the extra-basinal origin of clasts within this unit together with conglomerates contained within the overlying Southern Highlands Group of the Dalradian has been suggested previously (Alsop et al. 2000). Thus, as we have indicated in our original paper, the presence of some extra-basinal clasts is to be expected along the unconformity, but this does not preclude the sourcing of other clasts from the footwall of the unconformity.
Tanner further suggests that the Cranford Limestone Fm exposed at Stralinchy may be similar in terms of its sedimentology and depositional system with the Scarba Conglomerates and Selma Breccia of the SW Highlands (Anderton 1977, 1979; Litherland 1980). It is important to note that whilst the Cranford Limestone Fm marks the base of the Easdale subgroup in Donegal, the Selma Breccia actually occurs within the Easdale subgroup of Argyll. The descriptions presented by Tanner of these rocks, together with our own observations of the Selma Breccia, seem to support their origin in terms of sediment gravity flow deposits. Larger and more angular blocks may be locally sourced, while the carbonate clasts may be more far travelled (Litherland 1975). However, in terms of sedimentology, the mixed matrix- to clast-supported conglomerates at Stralinchy comprise poorly sorted, polylithic angular blocks, many of which contain pre-existing tectonic fabrics (Hutton & Alsop 2004). Large (1 m) angular pelite clasts containing pre-existing tectonic fabrics and displaying extremely delicate margins (Fig. 1a) are found amongst clast-supported angular quartzite boulders as described in our paper (e.g. fig. 7e, f). These relationships are quite distinct from the conglomerates and breccias in the SW Highlands (Fig. 1b) and extremely difficult to reconcile with Tanner's suggestion of the Stralinchy conglomerates being the ‘product of a dynamic system capable of carrying large blocks of rock for long runout distances’. We therefore reiterate that the Stralinchy conglomerates and breccias display all of the characteristics, and are most readily explained in terms of being proximal deposits. We suggest that when the regional structural and stratigraphic patterns are also taken into account, then the most likely source for much of the conglomerate is the directly underlying sequence in the footwall of the unconformity.
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10 October 2004
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References |
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