A late Vendian age for the Kinlochlaggan Boulder Bed (Dalradian)?

Abstract

The Kinlochlaggan Boulder Bed occurs in Neoproterozoic Dalradian rocks of the Grampian Mountains of Scotland. It is currently thought to represent a glacial event ( c. 800 Ma) unique in the North Atlantic region, and be part of a sequence correlated by different workers with either the Grampian or Appin groups. We correlate the Boulder Bed with the Port Askaig Tillite of Vendian ( c. 650 Ma) age, and deduce that the sequence in which it occurs belongs to the younger Argyll Group. The latter is preserved within the D2 Kinlochlaggan Synform, and a previously unrecognized discontinuity of regional importance may separate inverted Argyll Group rocks from the underlying Grampian Group.

The Dalradian rocks of Scotland: an introduction

“…At the top of this succession is the Kinlochlaggan Boulder Bed, containing clasts of a variety of intra-and extrabasinal rock types including granite (Treagus, 1969(Treagus, , 1981Evans and Tanner, 1996).…”

The Dalradian rocks of the northern Grampian Highlands of Scotland

“…To the north-east, around Loch Laggan, comparable sediments in the Geal-charn-Ossian Steep Belt underlie the glacigenic Kinlochlaggan Boulder Bed (Treagus, 1969(Treagus, , 1981Evans and Tanner, 1996).…”

“…They are in mappable continuity with strata farther west, which include the Kinlochlaggan Boulder Bed and Quartzite and are of undoubted Appin Group affinities (see the Allt Mhainisteir and Kinloch Laggan Road GCR site reports) (Evans and Tanner 1996;Robertson and Smith 1999). The Aonach Beag Semipelite Formation is dominated by interbanded rusty-weathering schistose semipelite and micaceous psammite but also includes blocky white quartzite and quartzose psammites, some of which are pebbly.…”

“…The original orientation of the structures in the steep belt has previously been envisaged as either upright (Anderson, 1947b(Anderson, , 1956Thomas, 1979Thomas, , 1980 or recumbent (Temperley, 1990;Evans and Tanner, 1996).…”

“…At the opposite extreme, Evans and Tanner (1996) suggested that the boulder bed is equivalent to the Port Askaig Tillite at the base of the Islay Subgroup of the Argyll Group . 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 …”