The Argyll Group is a thick sequence of metamorphosed Neoproterozoic sedimentary rocks that outcrop across the Central Highlands of Scotland , east of the Great Glen , as well as appearing in the north of Ireland. It is a subdivision of the Dalradian Supergroup and is itself divided into four units; from oldest to youngest these are the Islay, Easdale, Crinan and Tayvallich subgroups.
21-678: The lower boundary of the Islay Subgroup and hence the Argyll Group as a whole is defined at the base of the Port Askaig Tillite Formation , a diamictite which displays limestone clasts overlain by quartzite and granite clasts. The tillite is thickest in its type area and on the Garvellachs . The Schiehallion Boulder Bed of the eastern Grampians correlates with this tillite. The tillite is overlain by
42-515: A Snowball Earth , where ice sheets extended to very low latitudes. The PATF has been correlated with both of these. A number of techniques have been used to reduce the uncertainty. Rhenium–osmium dating has been carried out on pyrite grains found within layered baryte deposits that are part of the Easdale Subgroup of the Argyll Group. This gives values of 604.0 ± 7.2 Ma and 612.1 ± 18.6 Ma, which can be compared to earlier dating of
63-492: A finer-grained matrix, are interpreted to be tillites of glacial origin. The sediments are organised into units interpreted to represent 28 periods of glaciation, 25 periods where glacial conditions were close (peri-glacial) and 23 non-glacial episodes. The onset of the earliest of the glacial cycles is recognised within the uppermost part of the Garbh Eileach Formation. The end of the overall glacial interval
84-663: A major depocentre for the Argyll Group, possibly as a result of active extensional faulting . Even in this area the unit thins markedly to the west and the base is marked by an unconformity with the underlying Lossit Limestone Formation , part of the older Appin Group . On Garbh Eileach , the largest island of the Garvallachs, the base of the PATF is conformable, following on from the Garbh Eileach Formation of
105-408: A period of erosion or non-deposition. Disconformities are marked by features of subaerial erosion. This type of erosion can leave channels and paleosols in the rock record. A nonconformity exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock. Namely, if the rock below
126-420: A region or were subsequently eroded before the next deposition. The local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus . It is a kind of relative dating . A disconformity is an unconformity between parallel layers of sedimentary rocks which represents
147-531: Is dominantly limestone though this is replaced in the east by psammite and quartzite . Lavas and pelites are seen around Tayvallich in Knapdale, after which area the subgroup is named, where it consists of the Tayvallich Slate and Limestone Formation and the overlying Tayvallich Volcanic Formation. The succession is between 100 and 250m thick but can reach up to 2.5 km in the east where
168-684: Is exposed along the Dalradian outcrop from Galway , Mayo and Donegal in Ireland in the west through Islay and the Garvellachs in the Inner Hebrides to Schiehallion , Braemar and Fordyce to the east on mainland Scotland. The formation records a time in Earth's history where there were repeated glaciations where ice sheets extended to low latitudes, sometimes referred to as a Snowball Earth . The PATF forms scattered outcrops along
189-518: Is presented by the Briançonnais realm (Swiss and French Prealps) during the Jurassic. Angular unconformities can occur in ash fall layers of pyroclastic rock deposited by volcanoes during explosive eruptions . In these cases, the hiatus in deposition represented by the unconformity may be geologically very short – hours, days or weeks. A paraconformity is a type of unconformity in which
210-597: Is unclear, although the uppermost member only has a few, thin diamictite beds. There is nothing within the overlying Bonahaven Formation that matches a typical Cap carbonate . During the Neoproterozoic cold period, the Cryogenian, there are two globally distributed glacial sequences, the older Sturtian glaciation (~717–660 million years ago} and the younger Marinoan glaciation (<654–632 million years ago), both of which are regarded as probable examples of
231-646: The Moray Firth coast, the unit can reach to 3 km thick. The Crinan Subgroup is formed by the Crinan Grits Formation, a submarine fan deposit. These rocks which are between 1.5 and 3 km thick, extend the width of Scotland from Kintyre to Fraserburgh . Their base is characterised by a coarse quartzite in Argyll , otherwise it is formed from pelites, semipelites and psammites. The lithology of this subgroup varies across Scotland but
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#1732783399837252-605: The Tayvallich Volcanic Formation within the overlying Tayvallich Subgroup, of 595 ± 4 Ma and 601 ± 4 Ma, using U-Pb and Sm-Nd methods. These results suggest that the PATF is best correlated with the Marinoan glaciation. Detrital zircon analysis has also been carried out on zircon grains from the sandstones interbedded within the PATF. The results support the sequence being Sturtian in age, with "youngest single grains" throughout being consistent with
273-564: The sedimentary geologic record . The significance of angular unconformity (see below) was shown by James Hutton , who found examples of Hutton's Unconformity at Jedburgh in 1787 and at Siccar Point in Berwickshire in 1788, both in Scotland. The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents time during which no sediments were preserved in
294-714: The Bonahaven Dolomite and then the Jura Quartzite which can reach up to 5 km thick but which is more typically between 500 and 1000m thick. Psammites and pelites occur within the sequence in the Ladder Hills area. The Easdale Subgroup consists of a basal Scarba Conglomerate overlain by the Craignish Phyllite . It also includes pelites, semipelites, psammites and some volcaniclastic rocks. Extending between Jura , Knapdale and
315-438: The break is igneous or has lost its bedding due to metamorphism, then the plane of juncture is a nonconformity. An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenic activity. A typical case history
336-403: The likely depositional ages. Unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer, but the term is used to describe any break in
357-441: The lowest (oldest) three of which are particularly well-exposed on the Garvallachs. The sequence contains 48 beds of diamictite interbedded with sandstones of non-glacial origin, forming 35–40% of the unit, typically deposited in deltaic to shallow water marine environments. The rest of the formation is made up of minor amounts of siltstones and dolomites . The diamictites, rocks consisting of large to very large clasts set in
378-675: The psammites and semipelites occur. The Loch Tay Limestone Formation extends from Campbeltown to Glen Isla . Port Askaig Tillite Formation The Port Askaig Tillite Formation (PATF) (also known as the Port Askaig Formation ) is a sequence of glacigenic sedimentary rocks deposited during the Cryogenian period of the Neoproterozoic era, forming part of the Dalradian Supergroup . It
399-400: The sedimentary layers above and below the unconformity are parallel, but there is no obvious erosional break between them. A break in sedimentation is indicated, for example, by fossil evidence. It is also called nondepositional unconformity or pseudoconformity. Short paraconformities are called diastems . A buttress unconformity also known as onlap unconformity, occurs when younger bedding
420-411: The uppermost Appin Group. On Islay the top of the PATF is marked by a transition to the mixed siliciclastic -carbonate Bonahaven Formation . In this central area the sequence is extremely well exposed and the deformation state is generally low, meaning that original sedimentary structures are well-preserved, allowing for confident environmental interpretation . The PATF is subdivided into five members
441-640: The whole ca. 600 km extent of the Dalradian Supergroup. It is always metamorphosed and variably deformed as a result of the Grampian Orogeny . The thickness of this unit varies greatly across the outcrop, with its thickest development, estimated at 1,100 m, in the Inner Hebrides on the Garvallachs and Islay. The type section is found on Islay, near Port Askaig , for which the formation is named. This area has been identified as
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