Sedimentary environments and basin evolution of the upper Dalradian: Tayvallich subgroup and Southern Highland group

The Dalradian Supergroup is divided into four groups, the uppermost of which is the Southern Highland Group (SHG). To date there are few sedimentological data that pertain to the SHG. In this thesis new field, petrographic and geochemical data are provided for the SHG and a model for its evolution outlined. The Killin and Aberfoyle areas together provide one of the most complete sections through the SHG. New sedimentological data from these sections are used here as a basis on which to build a correlative stratigraphy for the entire SHG basin which extents from Banff in NE Scotland southwestward into N. Ireland. The sediments of the SHG and the underlying Tayvallich subgroup of the Argyll Group change are dominantly turbiditic in character. They change stratigraphically upward from predominantly carbonate turbidites to siliciclastic turbidites. Volcaniclastic units are present, especially in the lower parts of the sequence. The volcaniclastic sediments document distal fall out of ash derived from plume-related volcanoes. The sedimentary sequences record the history of an evolving basin developed on the southern margin of Laurentia during the opening of the Iapetus Ocean. This history spans sedimentation during initial active rift faulting with rapid subsidence rates through to a drift stage with sedimentation along a passiv.e margin marked by thermal subsidence. Those parts of the sequence deposited on oceanic crust have been removed by later tectonic activity. Using present day geographic co-ordinates, the long axis of the basin was orientated NE-SW with the majority of the sediments derived from the N and NW and deposited by sediment gravity flows in a slope apron or a submarine ramp environment. The source terrain comprised mainly granitic basement-type rocks with abundant quartz veins. However parts of the sequence show an igneous component with the geochemical signature of within-plate, plume-type basalts. Geochemical data show that the major change in provenance was temporal rather than geographic. Geochronological data derived from detrital minerals suggest that erosional systems in the hinterland were exposing increasingly old, probably Archaean basement, rocks as SHG deposition continued