The Composition of Coarse Sediment in Basal Upper Old Red Sandstone Alluvial Fan Conglomerates of the Northwest Midland Valley, Scotland: A Provenance Study

Goold, David K (1986) The Composition of Coarse Sediment in Basal Upper Old Red Sandstone Alluvial Fan Conglomerates of the Northwest Midland Valley, Scotland: A Provenance Study. MSc(R) thesis, University of Glasgow.

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A provenance investigation of coarse detritus (sand to boulder size) associated with basal Upper Old Red Sandstone (Upper Devonian) alluvial fans has been carried out in the northwest Midland Valley of Scotland. Detailed Petrographic, XRD and SEM studies of sandstones (lithic arenites) interbedded with conglomerate units confirm previously suspected compositional differences for three localities, Portencross, Wemyss Bay and Roseneath along the Firth of Clyde Upper ORS dispersal system. Much of this detritus is interpreted as first cycle and reflects abrupt compositional changes in essentially metamorphic basement lithologies beneath the northwest Midland Valley. These changes may be summarised as increasing metamorphic grade towards the north/northwest ranging from low greenschist facies in the southeast to garnet grade in the northwest. This is combined with a trend from a significant basic plutonic/volcanic input to a more acid igneous component in a similar direction. A substantial amount of reworking of older sediment is also implied for the entire region; characteristic lithic fragments suggest sediment of at least Silurian to early Devonian age contributed detritus to the developing Upper ORS basin. Textural investigation of sand sized material indicates immaturity, and no significant decrease in grain size for these alluvial sediments in the direction of supposed palaeoflow (S/SW-N/NE). This has also been detected in the cobble-boulder size conglomerate material. This, together with the unique compositional assemblages, reinforces the view that the Upper ORS basin in the Firth of Clyde subsided diachronously with earlier deposits overlain by younger sediments as the basin margins receeded towards the south/southwest. Major oxide and trace element analysis of southerly derived intermediate and acid igneous boulders at Roseneath, the most northerly of the localities investigated, reveals that they have clear affinities with Lower Devonian (c410 Ma) granitoids intruded into the SW Highlands Dalradian metamorphic terrain some 30-40+km to the north. Many granodiorite clasts also display evidence of incipient Cu mineralization, a feature also recognised in the SW Highlands igneous suite. Preliminary geochemical and petrographical analysis of the metamorphic clasts at Roseneath also reveals many significant similarities with lithologies of similar metamorphic grade in the SW Highlands 30-40km to the north. In contrast, however, Rb-Sr isotopic data obtained from co-magmatic granodioritic boulders suggests an emplacement age of at least 516+/-5Ma for an intrusion whose initial 87Sr/86Sr ratio was of the order of 0.704. This significantly predates the SW Highlands granites although the initial strontium ratio together with trace element features implies that the 'Roseneath pluton' and many SW Highlands granites must have shared a common origin in the deep crust or mantle; the latter being preferred on account of a large (?) Grenville inherited zircon component as detected by U-Pb isotopic analysis. The paradoxical nature of much of the evidence obtained in this study makes conclusions regarding provenance of the basal Upper ORS difficult to draw. The simplest based model for the northwest Midland Valley may have parallels in westward extension of the Midland Valley in Ireland (Connemara) where Dalradian and largely undeformed Cambro-Ordovician plutonic lithologies are combined in a complex thrust related mixed terrain.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Keywords: Geology, Sedimentary geology
Date of Award: 1986
Depositing User: Enlighten Team
Unique ID: glathesis:1986-77515
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 09:06
Last Modified: 14 Jan 2020 09:06

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