The stratigraphy and origin of the portaskaig boulder bed series (Dalradian)

Abstract

The late Pre-Cambrian Portaskaig Boulder Bed Series shows a very uniform sequence for some 600 km along the Caledonian strike, from Connemara on the west coast of Ireland to Aberdeenshire in Scotland. The Series, which is divided into four groups of beds, comprises numerous individual boulder beds separated by stratified sediments. The lower boulder beds are carbonate-bearing and contain abundant intrabasinal fragments of dolomite and few extrabasinal crystalline rocks while the higher beds are psammitic and contain mainly extrabasinal crystalline rocks. Varve-like banding, involutions and sandstone polygons suggest periglacial conditions. It is concluded that the boulder beds represent subaqueous tills deposited on unconsolidated sedimcnts. Contemporaneous deformation of some of these sediments is attributed to the movement of ice, although the possibility that sliding of the unconsolidated sediments occurred locally is considered.

Subglacial and proglacial glacitectonic deformation in the Neoproterozoic Port Askaig Formation, Scotland

“…The origin of the Great Breccia and the Disrupted Beds has been the subject of much debate. Kilburn et al (1965) and Spencer (1971Spencer ( , 1985 interpreted the Great Breccia as a btillQ deposited by grounded glacier ice. These workers also considered a glacial origin for the Disrupted Beds, but concluded that they probably record subaqueous slope failures.…”

“…The Disrupted Beds are a very heterogeneous group of rocks which exhibit large vertical and lateral variability (Kilburn et al, 1965;Spencer, 1971). They have a distinct banded appearance, consisting of alternating layers of cream-coloured dolostone, laminated dark blue siltstones, and diamictite (Fig.…”

The Dalradian rocks of the central Grampian Highlands of Scotland

“…Kilburn et al (1965), Spencer (1971Spencer ( , 1981Spencer ( , 1985, Eyles and Eyles (1983), Eyles (1988). These authors considered that the diamictites represent a major glacial event of global extent and that they may be correlated with the Varanger tillites of Scandanavia and East Greenland (but see Stephenson et al, 2013a for more-recently suggested correlations with other global glacial events).…”

“…Early workers identified the Selma Breccia and interpreted it as a ‗crush conglomerate' (Flett in Kynaston and Hill, 1908), but subsequent workers are agreed that it is of sedimentary origin (Kilburn et al, 1965). Litherland (1970Litherland ( , 1975 referred to it as a ‗sedimentary slide tilloid' but did not discuss the reasons for this interpretation.…”

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

“…Morgan (1966) carried out more-detailed mapping over a wide part of the Muckle Fergie-Inchrory area and provided detailed descriptions of the stratigraphy and structure. Then, following the recognition of the glaciomarine origin of the Port Askaig boulder bed (Kilburn et al, 1965), Spencer and Pitcher (1968) extended the correlation and interpretation to the Muckle Fergie Burn section and published a stratigraphical log, as well as noting metadiamictite occurrences farther north-east near Fordyce.…”