Ettrick Group, Silurian, Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
The Ettrick Group was first formally recognised in 2009 and utilised for the BGS 1:50k map sheets for Moffat and Ettrick (Scotland 16 W & E). In the Moffat–Ettrick area, the Ettrick Group incorporates strata lying to the south of the Moffat Valley Fault that had been formerly assigned to the Gala Group, and some strata lying to the south of the Laurieston Fault that had been previously regarded as lying within the Hawick Group. Farther south-west, it is likely that strata included within the Gala 8 tract, and the southern part of the Gala 7 tract, should now be regarded as forming part of the Ettrick Group. In the eastern part of the Southern Uplands, around Innerleithen, the Ettrick Group incorporates strata sometimes described in the older literature as either the ‘Garnetiferous Group’ or the ‘Buckholm Formation’.
The Ettrick Group is characterised by a turbidite lithofacies of well-bedded sandstone, locally coarse-grained, with mudstone interbeds up to about 40 cm thick. The individual sandstone beds are typically massive or poorly graded for most of the bed thickness, with grading to finer-grained sandstone only seen in the uppermost part, where cross-lamination may also be developed. Very thick sandstone beds occur sporadically, either singly or in multiples up to 20 m thick; thick massive siltstone units also occur locally. One division, the Grieston Formation, forms narrow outliers separate from the main outcrop and is largely composed of laminated to thin-bedded alternations of fine-grained sandstone, siltstone and mudstone. Sole marks are common on the base of the turbidite sandstone beds with linear grooves and flute casts establishing the flow direction of the depositing palaeocurrent. Once corrected for rotation during folding, the orientation of these structures suggests that the sediment transport direction was dominantly towards the south-west.
Petrographically, the sandstone is typically composed of poorly sorted angular grains set in a variable amount of fine matrix material. Quartz and feldspar (plagioclase and subordinate K-feldspar) dominate the grain population. Mica is generally a minor component but may be abundant in fine-grained sandstone. Garnet is common locally in coarse-grained sandstone; otherwise, tourmaline, zircon and epidote occur widely as minor accessory mineral grains. A variable proportion of lithic detritus includes grains of clastic sedimentary and metasedimentary rock, fine- and coarse-grained acidic igneous lithologies and conspicuous fine-grained, basic volcanic rocks that show a range of spilitic textures. Some carbonate (CaO up to 7 per cent) is usually present as replacement of matrix or framework grains. Whole rock geochemical analyses of the Ettrick Group sandstones show them to have a relatively homogeneous compositional character and allow them to be readily distinguished from both the Gala Group and Hawick Group sandstones (P912332).
Unusually for the Southern Uplands, the biostratigraphical age of the Ettrick Group is demonstrated by graptolite faunas from mudstone interbedded with the turbidite succession. It spans the upper Llandovery guerichi to spiralis biozones (P912326), and so overlaps with the range established for the youngest part of the Gala Group, and the oldest part of the Hawick Group.
In the central Southern Uplands, abundant outcrops of the Moffat Shale Group indicate relatively closely spaced tract-bounding faults and define numerous anastomosing tracts in parts of the Ettrick Group (P912331). Farther north-east, Moffat Shale outcrops become relatively rare and the tracts that can be recognised appear correspondingly broader, although a similar level of cryptic tectonic imbrication may well be present. Tract width also increases to several kilometres in the southern part of the Ettrick Group. However, the tract arrangement of the Ettrick Group is in some respects unusual, since the oldest strata in every formation is of guerichi or early turriculatus Biozone age, and the same base level is repeated across several adjacent tracts in the closely faulted areas; the sandstone succession in individual tracts may thence expand across several biozones. Uniquely, the Grieston Formation, also biostratigraphically wide ranging, forms outliers within the Gala Group, to the north of the main Ettrick Group outcrop.
Four formations are recognised in the main outcrop of the Ettrick Group in the central to northeastern Southern Uplands, distinguished primarily by their sedimentological and petrographical character. All of the top formation boundaries are taken at faults, but the basal transition from the Moffat Shale Group is clearly preserved in all but the Glendearg Formation. The Thornylee Formation spans two or three tracts, the Selcoth Formation is made up of about 9 or 10 tracts, the Kirkhope Formation bifurcates at its western end, whilst current information allows only a single tract to be assigned to the Glendearg Formation (P912331). To the north of the group’s main outcrop, the Grieston Formation occupies narrow, faulted outliers within the outcrop of the Gala Group.
The Thornylee Formation (at least 500 m) was distinguished originally as the ‘Garnetiferous Group’ since the coarse-grained massive sandstone contains significant garnet in fresh, angular and commonly large grains. The formation is well exposed at Thornylee Craigs (NT 402 370) where it is dominated by very thick-bedded or massive, coarse-grained sandstone, and in old railway cutting east of Thornielee House (NT 419 364) where tens of metres of medium- to thick-bedded, fine- to medium-grained sandstone and massive siltstone are interbedded with the massive coarse-grained sandstone. Brown mudstone is present locally and may be extensively bioturbated by the meandering feeding burrow Dictyodora. The outcrop of the formation narrows towards the south-west and is structurally terminated at the south side of Dob’s Linn. Near there, in the vicinity of St Mary’s Loch, the massive coarse-grained sandstone apparently forms lenticular bodies tens or hundreds of metres in thickness and 1–3 km in length within a background of medium- to thick-bedded sandstone. To the north-east, it is likely that the Thornylee Formation tracts broaden to occupy parts of the Lammermuir Hills and the Coldingham Moor inlier of Lower Palaeozoic strata (P912333). Throughout its outcrop, graptolite faunas from thin mudstone interbeds demonstrate the guerichi Biozone, perhaps extending up into the lower part of the succeeding turriculatus Biozone.
To the south-east of the Thornylee Formation, but extending farther to the south-west (P912325), the Selcoth Formation (500–2000 m) forms several narrow tracts within a demonstrably much imbricated outcrop (P912331) that pinches out between converging strike-parallel faults both north-eastward, near Clovenfords (NT 449 364), and south-westward, near Ae (NX 985 890). The transition from the underlying Moffat Shale Group is well exposed at a number of localities including those in the Selcoth Burn (NT 159 062) and Pot Burn (NT 180 090). At the transition, graptolitic mudstone of the Birkhill Shale Formation is overlain by about 8 m of greenish grey siltstone with thin interbeds of pale-coloured bentonite and, towards the top, dark mudstone laminae containing guerichi Biozone graptolites. Overlying the siltstone, the Selcoth Formation is consistently composed of medium- to thick-bedded sandstone with silty mudstone interbeds, interspersed with sequences of thinly interbedded sandstone and mudstone with sporadic thicker (1–3 m) sandstone beds. A typical section is exposed in the Selcoth Burn stream section (south-east from NT 137 072).
The Kirkhope Formation (at least 500 m) forms another south-westerly tapering outcrop, pinching-out between converging faults to the south-east of Moffat (P912325). In its southwestern extent, the Kirkhope Formation lies to the south-east of the Selcoth Formation, but farther north-east, where the latter is absent (from Clovenfords eastwards), the Kirkhope Formation abuts the Thornylee Formation. There is evidence from Moffat Shale Group outcrops for closely spaced faulting in the south-west of the outcrop, but subdivision cannot be carried north-eastward, where a single tract broadens to over 7 km in width in the Galashiels–Melrose area; large-scale folding may be responsible. In general, the Kirkhope Formation is composed of medium- to thick-bedded sandstone with thin mudstone interbeds, interspersed with sporadic, more thinly bedded units, 1–2 m thick. Massive, medium- to coarse-grained sandstone beds up to 2 m thick are common, locally forming sequences that amalgamate into laterally impersistent, coarse-grained units, tens, perhaps hundreds of metres thick. In common with the Thornylee Formation, the massive coarse-grained sandstone beds are commonly conspicuously garnetiferous. The typical lithology forming the lower part of the Kirkhope Formation is well exposed in the Ettrick Water around Ettrickbridge (NT 390 243), whilst characteristic higher parts of the formation are exposed to the north on Kirkhope Hill (NT 385 255). It is likely that the Kirkhope Formation continues north-eastward to form much of the Coldingham Moor Lower Palaeozoic inlier, where it is exposed in coastal sections from St Abbs Head towards Siccar Point (Figure 21). In this part of the outcrop, the distribution of numerous graptolite faunas from mudstone interbeds indicate that at least several hundred metres of strata in the turriculatus Biozone overlie a much thinner, condensed turbidite succession that here appears to span the sedgwickii to guerichi biozones, equivalent to the youngest part of the Moffat Shale Group as seen to underlie the Kirkhope Formation elsewhere. The youngest Kirkhope Formation strata, within the crispus Biozone, occupy a broad area in the north of the Coldingham Moor inlier that is characterised by large-scale folds, as clearly visible in the coastal section.
The Moffat Shale Group underlying the Kirkhope Formation crops out in narrow lenticular inliers in the Ettrick Water at Ettrickbridge and south of Melrose between Lindean Glen (NT 490 315) and Rhymers Glen (NT 527 327). At these localities the age of the transition up into the Kirkhope Formation lies within the guerichi Biozone, with graptolite faunas from inter- bedded mudstones higher in the Kirkhope succession ranging up to the crispus or griestoniensis biozones.
The Glendearg Formation (500–2000 m) forms the most south-easterly of the Ettrick Group tracts. It has a lenticular outcrop, attaining a maximum outcrop width of 4.5 km but pinching-out between converging faults near Glenkiln (NY 016 897) in the south-west and towards Selkirk in the north-east (P912325). The outcrop lies to the south of that of the Kirkhope Formation, but the Glendearg Formation extends a few kilometres farther to the south-west so that its northern margin abuts the Selcoth Formation. The Glendearg Formation is dominated by thickly bedded sandstone with generally thin mudstone interbeds, interspersed with sporadic thinly bedded sequences that only rarely exceed 2 m in thickness. Isolated, thicker sandstone beds (up to 2 m) are relatively common and may locally combine to form units up to 20 m thick. Red mudstone occurs sporadically, usually in beds a few centimetres thick that have gradational margins with grey mudstone. No Moffat Shale Group strata are preserved at the base of the Glendearg Formation, but graptolite faunas from the interbedded mudstones establish an age range from the turriculatus Biozone up to the spiralis Biozone. A characteristic section through the formation is exposed in the Glendearg Burn stream section (north from NT 230 062).
The boundary between the Glendearg Formation and the Hawick Group has been defined by a combination of geochemical data derived from sandstone samples (P912332) and from stream sediment. From the latter, of particular value is the distribution of CaO, which reflects an abrupt southward increase in carbonate detritus in the Hawick Group sandstones. The steep gradient in CaO abundance in the stream sediment is coincident with a strike-parallel fault zone exposed in Buck Cleuch (NT 334 145) that is now taken as the boundary between the Ettrick and Hawick groups.
North of the main outcrop of the Ettrick Group, the Grieston Formation (up to 500 m) occupies narrow, lenticular, fault-bounded outliers within the outcrop of the Queensberry Formation; these extend from the Megget reservoir (NT 209 223) north-east to Innerleithen and possibly as far as Lauder. The formation is composed of siltstone with a variable proportion of fine-grained sandstone in very thin to medium beds. Best known from the eponymous quarry at Grieston Hill (NT 313 361), these fine-grained deposits yield graptolite faunas ranging in age from the guerichi Biozone up to the griestoniensis Biozone, in contrast to the surrounding Queensberry Formation siltstone which is of convolutus to sedgwickii Biozone age. The ‘Grieston Shales’ have previously been regarded as part of the Gala Group, but seem more likely to have been deposited above Gala Group strata with a low angle unconformity intervening. The Grieston Formation can best be regarded as a distal component of the Ettrick Group, deposited in small basins that formed on top of the accretionary complex soon after the Gala Group tracts were incorporated therein.
As discussed above when considering the Gala 7 and 8 tracts, a case can be made for including within the Ettrick Group much of the strata previously assigned to those tracts in south-west Scotland. The evidence is drawn from whole-rock geochemistry, the presence of garnetiferous, coarse-grained sandstone beds, and the ages defined by graptolite faunas, all of which demonstrate considerable overlap. The graptolites from the Moffat Shale Group, seen locally at the base of the successions forming the Gala 7 and 8 and the Ettrick Group tracts, range up to the sedgwickii Biozone in all cases, whilst faunas from mudstone interbeds in the sandstones overlying the Moffat Shale in all of those tracts range from the guerichi Biozone up to the crispus Biozone. The principal sandstone-dominated facies association of the Gala 7 and 8 tracts is sedimentologically most similar to the Selcoth or Glendearg formations, but the thicker, mudstone-rich parts of the Gala 7 and 8 tracts are more akin to the Grieston Formation. However, the level of current information does not permit confident correlation at formational level.
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