Development of the Lower and Middle Old Red Sandstone Sedimentary Basin in Eastern Sutherland, North-West Scotland

Dec, Tomasz (1988) Development of the Lower and Middle Old Red Sandstone Sedimentary Basin in Eastern Sutherland, North-West Scotland. PhD thesis, University of Glasgow.

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The Golspie basin is a NE-SW trending, marginal component of the post- or late- orogenic Old Red Sandstone Orcadian basin. Structural evidence indicate the Late Caledonian strike-slip as a logical framework for initiation of the Old Red Sandstone sedimentation in the area. Deposits of the Golspie basin are represented by two sequences of the Lower and the Middle Old Red Sandstone age separated by an angular unconformity. Up to 400 m thick basal conglomerates of the Beinn Lunndaidh Formation (BLF), most extensively developed along the southern and the western margin of the basin, represent deposits of alluvial fans whose sedimentation was dominated by deposition from debris flows (Facies B and B1), often accompanied by sheetflood sedimentation (Facies C). The NE edge of the basin is fringed by sheetflood fanglomerates of Facies C1, whose maximum thickness is 180 m. Although the fault-controlled basin margins provided a physiographic framework for the alluvial fan development, sediment supply appears to have been a significant factor controlling the distribution and the diversity in style of the sedimentation. While the texturally homogeneous, sheetflood arkoses of Facies C1 represent a classical example of first-cycle sedimentation, the textural heterogeneity of the debris flow deposits of Facies B and B1 (i. e. presence in the same beds of chaotically distributed bimodal and polymodal textures, common bimodal-only beds, abundance of sandstone matrix (locally of distinctly different provenance than the gravel fraction - Facies B1), presence of outsize semi-lithified sandy intraclasts) strongly suggests that the debris flow-dominated alluvial fans were supplied by pre-existing, sand-enriched gravels, which had undergone a significant size segregation prior to resedimentation, remoulding with sand, and the deposition on the alluvial fans as debris flows. Although, clast composition of the BLF conglomerates mimics lithology of the adjacent Caledonian basement, diverse degree of clast roundness additionally indicates presence of the resedimented (second-cycle) deposits as well as a possible first-cycle component. The textural signatures of Facies B and B1 also suggest a cohesive nature of the debris flows. The cohesion factor might have been imparted not merely by electrostatic/electromagnetic interactions between clay particles but also by chemical bonds of early cements, present in the source sediments prior to the mobilisation. Deposits transitional between the debris flow and the sheetflood conglomerates have been recognized as subordinate. In the southern part of the Golspie basin, the development of gravelly alluvial fans was dramatically succeeded by sandy stream flow/sheetflood sedimentation (Facies G2) which resulted in the deposition of the over 600 m thick Beinn a' Bhragaidh Member (Glen Loth Formation). As a result of poor exposures it is impossible to infer detailed palaeodispersal and to correlate these deposits with distinctly different sediments of the Beinn Dhorain Member and the Ben Uarie Member (BDM and BUM), appearing above the BLF fanglomerates in the northern sector of the basin. In response to subsidence in the area of Glen Loth the sheetflood-dominated alluvial fans were brought into fan-delta-related interplay with a lacustrine environment, which was abundantly, externally supplied with exotic mud. This transgressive scenario is reflected in a dramatic, fining-upward transition from the BLF into the BDM. The subaqueous setting of the resulting fan-delta was dominated by cohesive mudflow sedimentation, and en mass mixing of the fan-borne, first-cycle gravels with the second-cycle lacustrine sediments. The resulted Facies E1 and E2 are characterised by shear-layering reflecting predominant laminar flow conditions (layer-parallel extension and shearing) during the mass flow emplacement. The main part of the BDM consists in the majority of the subaqueous mudflow deposits of Facies E2 derived from the north. An analogous fan-delta environment is represented by the lowest part of Ousdale Mudstones in Badbea basin, although here it was succeeded by stream flow and by lacustrine sedimentation (Facies G1). In the continuously subsiding northern sector of the Golspie basin the mudflow sedimentation of the BDM was followed by a northward emplacement of cohesive sandy debris flows of Facies F1 and sheetfloods of Facies F2. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Geology, Sedimentary geology
Date of Award: 1988
Depositing User: Enlighten Team
Unique ID: glathesis:1988-77746
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 11:53
Last Modified: 14 Jan 2020 11:53

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