Cleaved clasts in Dalradian conglomerates: possible evidence for Neoproterozoic compressional tectonism in Scotland and Ireland?

Abstract

Cleaved metasedimentary clasts are present in stratigraphically and geographically distinct conglomerates in the Argyll and Southern Highland Groups of the Neoproterozoic Dalradian succession in the SW Scottish Highlands and NW Ireland. The significance and relationships of these clasts are that: (1) they were unequivocally reworked and deposited by sedimentary processes; (2) their internal foliation is probably due to contractional deformation that pre‐dates regional Caledonian fabrics; and (3) most of the cleaved clasts are only moderately deformed psammites and pelites and thus cannot be construed as having been derived from extensional mylonites. These conclusions, coupled with the generally accepted inferences that the Dalradian succession post‐dates Grenville deformation (c. 1100–1000 Ma) and pre‐dates early Palaeozoic Caledonian deformation (c. 470 Ma) and that the lowermost exposed Dalradian rocks, the Grampian Group, are truncated by a c. 806 Ma shear zone, imply that the clasts must have been foliated during an episode of mid‐Neoproterozoic contractional deformation. The clasts may thus represent further evidence in support of the contentious c. 870–800 Ma Knoydartian orogeny and thereby further render as equivocal interpretations that the Neoproterozoic tectonostratigraphic evolution of the Scottish Highlands and NW Ireland is a record of long‐lived ‘episodic’ extensional tectonism. Copyright © 2000 John Wiley & Sons, Ltd.

The Dalradian rocks of Scotland: an introduction

“…The possibility of a major stratigraphical break at this horizon has been proposed by Prave (1999) and Alsop et al (2000) who pointed to the presence of cleaved clasts of metsedimentary rock with up to two deformational fabrics.…”

“…This boulder bed has also been correlated tentatively with a younger tillite that has been recognized in the Dalradian of Donegal, which could represent the Varanger glaciations at c. 620-590 Ma (Condon and Prave, 2000).…”

“…Anderton, 1985 , 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 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 1988) and detailed field observations are now revealing previously undetected intrabasinal unconformities and periods of nondeposition. The scale and significance of such discontinuities is currently a matter of much debate, in some cases leading to speculation that major stratigraphical and orogenic breaks occur within the Dalradian successions of both Scotland and Ireland (Prave, 1999;Alsop et al, 2000;Hutton and Alsop, 2004), though this has been challenged by Tanner (2005).…”

“…Correlations with various glacial periods, some as young as Ordovician, have been suggested, though a correlation with the glacigenic deposit in the Southern Highland Group of Donegal (Condon and Prave, 2000) and the Varanger tillites (accepting their revised age of 620-590 Ma) seems to be the most likely on present evidence (see below).…”

The Dalradian rocks of the north-east Grampian Highlands of Scotland

“…Hence, the glacial deposits more likely correlate with dropstone localities near the base of the Southern Highland Group in Donegal (Condon and Prave, 2000), which would imply that only the lowermost parts of the group are exposed in the North-east Grampian Highlands, albeit in a relatively thick sequence. Figure 26 shows the composite cross-section along the coast, based upon recent British Geological Survey mapping and the overall structure is illustrated in Figure 3a.…”