Precambrian sediments of western Islay

Stewart, Alan; Hackman, B.D.

doi:10.1144/sjg09030185

Cited by 22 publications

(30 citation statements)

References 12 publications

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“…The Kilchoman Phyllite consists of grey metamudstones with calcareous bands; graded beds have been observed in this unit (Stewart and Hackman, 1973). Muir et al (1995) re-examined these rocks and concluded that the Crosprig Grit and the Kilchoman Phyllite are not separate formations but are simply the Rubha Gàidhealach Grit and the Kilchiaran Phyllite repeated by upright folding (Table 1).…”

Section: Descriptionmentioning

confidence: 97%

“…The Colonsay Group on Islay was mapped by Stewart and Hackman (1973), who identified ten formations that are essentially a series of gritty metasandstones, metamudstones and metagreywackes (Table 1).…”

Section: Descriptionmentioning

confidence: 99%

“…The Colonsay Group was first described by Wilkinson (1907), and more recently by Stewart (1969), Stewart and Hackman (1973), Fitches and Maltman (1984), Bentley (1988) and Muir et al (1995). The group is 5.5 to 6 km thick.…”

Section: Introductionmentioning

confidence: 98%

“…The upper part of the group is exposed only on Oronsay and Colonsay. Stewart and Hackman (1973) proposed a tentative correlation between the upper part of the Islay succession and the lower part on Oronsay, but Bentley (1988) thought it possible that up to a kilometre of intervening strata is covered by sea between Islay and Oronsay.…”

Section: Introductionmentioning

confidence: 99%

“…Stewart (1969) concluded that it could not be correlated reliably with any other stratigraphical unit, but Stewart and Hackman (1973) tentatively suggested a correlation with the Appin Group (Dalradian) of the Lochaber area. Fitches and Maltman (1984) did not rule out its correlation with the Dalradian, but they did have reservations on structural criteria.…”

Section: Introductionmentioning

confidence: 99%

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The Dalradian rocks of the south-west Grampian Highlands of Scotland

Tanner

Bendall

Pickett³

et al. 2013

Proceedings of the Geologists' Association

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The south-west Grampian Highlands, as defined here, include the Inner Hebridean islands of Islay and Jura, and extend north-east as far as Dalmally at the northern tip of Loch Awe. Due to a favourable combination of excellent coastal exposures and low tectonic strain, the late-Neoproterozoic rocks of the Dalradian Supergroup in this region are ideal for studying sedimentary structures. In addition, the diversity in protolith lithology from carbonate rocks to siliciclastic rocks of all grain sizes and volcanic rocks makes it possible to establish a very detailed lithostratigraphical succession and to recognize lateral facies changes. The stratigraphical range extends from the base of the Appin Group to the base of the Southern Highland Group and the area provides type localities for many regionally extensive formations of the Argyll Group. Rocks forming part of the basement to the Dalradian basins, the Rhinns Complex, are seen on Islay, where they are overlain by the Colonsay Group, a thick metasedimentary siliciclastic sequence of uncertain stratigraphical affinity. The structure of the Dalradian rocks in the south-west Grampian Highlands is controlled by early (D1) major folds (Islay Anticline, Loch Awe Syncline, and Ardrishaig Anticline), associated with a ubiquitous, penetrative, slaty or spaced cleavage. Most of the Dalradian rocks have been regionally metamorphosed under greenschist-facies conditions and amphibolite-facies (garnet zone) assemblages occur only in a narrow central zone, strongly affected by the D2 deformation. The area provides GCR sites of international importance for studying Neoproterozoic glacial deposits, splendidly preserved stromatolite bioherms and calcite pseudomorphs after gypsum. Deformed and undeformed sandstone dykes and interstratal dewatering structures are well displayed at several sites. Other features include thick sills of basic meta-igneous rock with unusual minerals such as stilpnomelane, and greenschist-facies rocks containing regional metamorphic kyanite. The area is of historical interest for the first recognition in Scotland, prior to 1910, of sedimentary way-up structures and pillow lavas in regionally deformed and metamorphosed rocksPeer reviewe

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Section: Descriptionmentioning

confidence: 97%

Section: Descriptionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Dalradian rocks of the south-west Grampian Highlands of Scotland

Tanner

Bendall

Pickett³

et al. 2013

Proceedings of the Geologists' Association

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The Colonsay Group

Bentley¹

1988

Later Proterozoic Stratigraphy of the Northern Atlantic Regions

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Some observations on brittle-ductile toggle

Saha

1995

Proc. Indian Acad. Sci. (Earth Planet Sci.)

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In low temperature deformation of polymineralic rocks the constituent minerals often show contrasting deformation mechanisms. In naturally deformed arkoses, feldspathic quartzites and grits under greenschist to aimandine-amphibolite facies condition, feldspar deforms by microbondinage (rigid-brittle behaviour), while quartz flows by a combination of dislocation creep, pressure solution and solution transfer. Boudin segments develop and separate in a phased sequential manner while quartz matrix flows in a ductile manner, indicating a brittle-ductile toggle during progressive deformation.Both the pressure solution and dislocation creep flows are volume-conservative. Therefore, a net volume increase during the above deformations is a necessity, unless compensated by a solution-transfer process. Hydrofracturing probably played a role in microboudinage formation as the ambient level of differential stress is estimated to be low around 45-75 MPa.To develop a synthetic flow law for the above type of deformation in arkoses, one needs to consider the significance of different rate-controlling mechanisms. As feldspar pull-aparts are syntectonically filled with quartz or metamorphic minerals crystallizing during progressive deformation, successive microboudin segmentation will depend on how fast/slow the matrix quartz moves to the open crack or the sealing takes place by transfer of appropriate solute components by pressure solution or solution transfer, the real rate-controlling process.

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Precambrian sediments of western Islay

Cited by 22 publications

References 12 publications

The Dalradian rocks of the south-west Grampian Highlands of Scotland

The Dalradian rocks of the south-west Grampian Highlands of Scotland

The Colonsay Group

Some observations on brittle-ductile toggle

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