The sedimentological evolution of a Lower Palaeozoic accretionary fore-arc in the Southern Uplands of Scotland


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    Department of Geology, Royal School of Mines, Imperial College, London SW7 2BP.


Thick turbidites accumulated along the northern margin of the Iapetus Ocean in Britain from mid-Ordovician to late Silurian times. Recent plate tectonic reconstructions hold that, during subduction, packets of these sediments, together with the underlying pelagic facies and thin portions of the uppermost ocean crust, were stripped from the descending plate and accreted to the inner trench wall on the Laurentian (North American) continental margin. The resulting accretionary prism is represented today by the Ordovician and Silurian rocks of the Southern Uplands of Scotland and the Longford-Down massif of Ireland. In these areas major reverse faults separate tracts of steeply dipping greywackes and mudstones with minor amounts of cherts and basalts. These tracts are up to several kilometres wide; their constituent beds face predominantly to the northwest, away from the site of the ancient ocean, while becoming progressively younger in each major fault slice towards the Iapetus suture in the southeast. From the stratigraphic sequences in these fault slices the sedimentary history of a portion of the Iapetus Ocean, and the British sector of its northern margin, can be reconstructed. In the Southern Uplands the earliest turbidites (mid- and late-Ordovician) are preserved in the northernmost fault slices. Regional facies trends, and vertical facies analysis, suggest that they accumulated in a trench dominated by a series of relatively small lower trench slope-derived fans. Pelagic sediments of the same age are found in the fault slices to the south, suggesting that the Ordovician turbidites were confined to the trench. During the lower and middle Llandovery, volcaniclastic trench turbidites were separated from quartz-rich ocean-floor turbidites (represented in the southern fault slices) by an elongate rise, on which pelagic deposits accumulated. This is interpreted as the outer trench high. In late Llandovery times the rise was overwhelmed, and thick laterally derived quartzose turbidites blanketed both the trench and the ocean floor. Sedimentation was strongly influenced by the evolution of the accretionary prism. By Llandovery times a trench slope break had emerged, supplying sediment both south to the trench and north to an upper slope basin in the Midland Valley of Scotland. In this basin early Silurian turbidites were followed by shallow-water and terrestrial sediments. Most of the sediment was derived from the emergent trench slope break: the volcanic arc and the Grampian orogenic belt to the north provided little or no detritus. Throughout the Ordovician and Silurian, sediment in the trench and on the ocean floor was derived from the volcanic arc, from the lower trench slope/trench slope break, from a degrading plutonic/metamorphic terrain (the Grampian Orogen), and locally by a minor amount of submarine sliding from carbonate-capped volcanic seamounts. Progressive elevation of the trench slope break in Silurian (and perhaps late Ordovician) times indicates that sediment from the arc-orogen hinterland must have bypassed the upper slope in the unexposed section of the margin to the northeast of the Southern Uplands, and travelled into the area axially along the trench floor.