View source for Depositional environments, Ordovician and Silurian, Southern Uplands

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== Depositional environment ==
[[File:P008471.jpg|thumbnail|Typical sequence of turbidite beds. This example of Silurian (Llandovery) strata is exposed on the coast near Port Logan, Rhins of Galloway. P008471.]]
[[File:P008425.jpg|thumbnail|Flute casts, two examples from the Ordovician Kirkcolm Formation showing different styles and current directions: a) Finnarts Bay. P008425.]]
[[File:P008463.jpg|thumbnail|Flute casts, two examples from the Ordovician Kirkcolm Formation showing different styles and current directions. P008463.]]
[[File:P912328.jpg|thumbnail|Regional geochemical images showing the distribution of (a) strontium (Sr) and (b) chromium (Cr). P912328.]]
The thin successions of oceanic, pelagic mudstone and chert that form the Crawford Group and Moffat Shale Group accumulated very slowly across the broad expanse of the Iapetus Ocean, laid down on the volcanic rocks of the oceanic crust. Vastly more extensive volumetrically are the thick sandstone successions deposited in submarine fan systems adjacent to the Laurentian continental margin, with turbidity currents as the principal depositional agents as described in Chapter 1. There are three dominant lithofacies: channel-fill sequences composed of massive sandstone, coarse grained, thickly bedded and locally associated with intraclast (sandstone) breccia and conglomerate; lobe or unconfined sheet-flow deposits composed of well-bedded, fine- to medium-grained sandstone with subordinate mudstone; and interchannel or interlobe deposits composed of siltstone and silty mudstone. These lithofacies are interbedded in vertical alternations that range from a few tens of metres to several hundred metres in thickness, but they also pass laterally from one to another on a scale of hundreds of metres to several kilometres. Most of the sandstones contain a high proportion of fine-grained matrix enclosing ill-sorted grains and so can be classified as wackes.

Individual beds may display a complete or partial array of the ideal turbidite features. If substantial turbidity flows closely follow each other, a thick sequence may build up in which graded sandstone beds become amalgamated, with only subtle variations in grain size determining the margins of each bed. In more distal parts of the fan, or as a result of small, low-density flows, thin and fine-grained sandstone beds may be pervasively laminated and separated by thicknesses of silty mudstone, the finest-grained part of the submarine fan succession deposited in areas that were spatially or temporally removed from most of the depositional activity. Where the base of a turbidite sandstone bed overlies the fine-grained top of its predecessor, it is usually sharp and clearly defined ([[Media:P008471.jpg|P008471]]) though groove and flute casts are common on many of the bed bases ([[Media:P008425.jpg|P008425]] and [[Media:P008463.jpg|P008463]]). If the bed is imagined as being rotated back to the horizontal, the orientations of the groove and flute casts indicate the flow direction of the original turbidity current (the palaeocurrent), which also influenced the geometry of the ripples that show up in section as cross-lamination. This becomes an important interpretational tool when linked with the distinctive compositions of the sandstones forming some tracts, particularly those of the Leadhills Supergroup, which indicate derivation from a range of different provenance regions. Additionally, within the Leadhills Supergroup, the compositional differences are not only on a tract scale, but also appear within tracts as the interbedding of differently sourced sandstone units. This complexity has led to an extra level in the Ordovician stratigraphical hierarchy, relative to that applied to the Silurian rocks to the south, but that should not be taken to imply any fundamental change in tectonic and sedimentary processes at the Ordovician–Silurian boundary. However, to ensure clarity, the different levels of lithological variation do require slightly different structures to be adopted for the descriptive sections in the following account.

On a regional scale, compositional contrasts between the different tracts are sufficiently strong to be reflected in the geochemistry of the stream sediment eroded from their sandstones. Contoured maps of element concentration in stream sediment commonly show steep gradients coincident with or parallel to the tract boundary faults: two examples are shown in [[Media:P912328.jpg|P912328]]. The dominant feature in the distribution of strontium ([[Media:P912328.jpg|P912328]]a) is a marked decrease in its abundance across a narrow zone coincident with the Laurieston–Moffat Valley Fault. The change probably reflects variation in the plagioclase to K-feldspar ratio in the detrital grain population: more detrital plagioclase in the sandstones to the north-west of the fault, more detrital K-feldspar in the sandstones to the south-east. The distribution of chromium ([[Media:P912328.jpg|P912328]]b) is dominated firstly by the effects of the mafic and ultramafic igneous rocks forming the Ballantrae Complex, and secondly by a broad zone of very high values running parallel to, but north-west of, the Laurieston–Moffat Valley Fault. Detrital chrome-spinel in the sandstones underlying this part of the Southern Uplands is the likely cause of this second anomalous zone. In detail, and from multi-element integrations, the regional, stream sediment geochemistry suggests an intensity of cryptic imbrication that has not been fully resolved by field studies. Another useful property that has been exploited in the differentiation of the Leadhills Supergroup tracts is the magnetic susceptibility of the different sandstones, which varies with the proportion of iron-rich minerals that they contain.


== Bibliography ==
Clarkson, E N K, Harper, D A T, Owen, A W, and Taylor, C M. 1992. Ordovician faunas in mass-flow deposits, Southern Scotland. Terra Nova, Vol. 4, 245–253.

Duller, P R, and Floyd, J D. 1995. Turbidite geochemistry and provenance studies in the Southern Uplands of Scotland. Geological Magazine, Vol. 132, 557–569.

Elders, C F. 1987. The provenance of granite boulders in conglomerates of the Northern and Central Belts of the Southern Uplands of Scotland. Journal of the Geological Society of London, Vol. 144, 853–863.

Floyd, J D. 2001. The Southern Uplands Terrane: a stratigraphical review. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 91, 349–362.

Floyd, J D, and Kimbell, G S. 1995. Magnetic and tectonostratigraphic correlation at a terrane boundary: the Tappins Group of the Southern Uplands. Geological Magazine, Vol. 132, 515–521.

Floyd, J D, and Rushton, A W A. 1993. Ashgill greywackes in the Southern Uplands of Scotland: an extension of the Ordovician succession in the Northern Belt. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 84, 79–85.

Floyd, J D, and Trench, A. 1989. Magnetic susceptibility contrasts in Ordovician greywackes of the Southern Uplands of Scotland. Journal of the Geological Society of London, Vol. 146, 77–83.

Kemp, A E S. 1986. Tectonostratigraphy of the Southern Belt of the Southern Uplands. Scottish Journal of Geology, Vol. 22, 241–256.

Leggett, J K. 1987. The Southern Uplands as an accretionary prism: the importance of analogues in reconstructing palaeogeography. Journal of the Geological Society of London, Vol. 144, 737–752.

Merriman, R J, and Roberts, B. 1990. Metabentonites in the Moffat Shale Group, Southern Uplands of Scotland: Geochemical evidence of ensialic marginal basin volcanism. Geological Magazine, Vol. 127, 259–271.

Morris, J H. 1987. The Northern Belt of the Longford–Down Inlier, Ireland and Southern Uplands, Scotland: an Ordovician backarc basin. Journal of the Geological Society of London, Vol. 144, 773–786.

Phillips, E R, Smith, R A, and Floyd, J D. 1999. The Bail Hill Volcanic Group: alkaline withinplate volcanism during Ordovician sedimentation in the Southern Uplands, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 89, 233–247.

Phillips, E R, Evans, J A, Stone, P, Horstwood, M S A, Floyd, J D, Smith, R A, Akhurst, M C,and Barron, H F. 2003. Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland. Geology, Vol. 31, 625–628.

Stone, P, and Evans, J A. 2001. Silurian provenance variation in the Southern Uplands terrane, Scotland, assessed using neodymium isotopes and linked with regional tectonic development. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 91, 447–455.

Stone, P, Breward, N, Merriman, R J, and Plant, J A. 2004. Regional geochemistry of cryptic geology: variations in trace element distribution across the Southern Uplands terrane, Scotland. Applied Earth Science (Transactions of the Institution of Mining and Metallurgy B), Vol. 113, B43–B57.

Stone, P, Breward, N, Merriman, R J, and Barnes, R P. 2006. The interpretation and application of regional geochemistry: lessons from the Paratectonic Caledonides. Scottish Journal of Geology, Vol. 42, 65–76.

Tucker, R D, Krogh, T E, Ross, R J, and Williams, S H. 1990. Time-scale calibration by high-precision U-Pb zircon dating of interstratified volcanic ashes in the Ordovician and Lower Silurian stratotypes of Britain. Earth and Planetary Science Letters, Vol. 100, 51–58.

Waldron, J W F, Floyd, J D, Simonetti, A, and Heaman, L M. 2008. Ancient Laurentian detrital zircon in the closing Iapetus Ocean, Southern Uplands Terrane, Scotland. Geology, Vol. 36, 527–530.

Warren, P T. 1964. The stratigraphy and structure of the Silurian rocks south-east of Hawick, Roxburghshire. Quarterly Journal of the Geological Society of London, Vol. 120, 193–218.

[[Category:South of Scotland]]