The Port Askaig Tillite Formation (PATF) (also known as the Port Askaig Formation) is a sequence of glacigenic

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

s deposited during the

Cryogenian The Cryogenian (from , meaning "cold" and , romanized: , meaning "birth") is a geologic period that lasted from . It is the second of the three periods of the Neoproterozoic era, preceded by the Tonian and followed by the Ediacaran. The Cryoge ...

period of the

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

era, forming part of the Dalradian Supergroup. It is exposed along the Dalradian outcrop from

Galway Galway ( ; , ) is a City status in Ireland, city in (and the county town of) County Galway. It lies on the River Corrib between Lough Corrib and Galway Bay. It is the most populous settlement in the province of Connacht, the List of settleme ...

, Mayo and Donegal in Ireland in the west through

Islay Islay ( ; , ) is the southernmost island of the Inner Hebrides of Scotland. Known as "The Queen of the Hebrides", it lies in Argyll and Bute just south west of Jura, Scotland, Jura and around north of the Northern Irish coast. The island's cap ...

and the Garvellachs in the

Inner Hebrides The Inner Hebrides ( ; ) is an archipelago off the west coast of mainland Scotland, to the south east of the Outer Hebrides. Together these two island chains form the Hebrides, which experience a mild oceanic climate. The Inner Hebrides compri ...

Schiehallion Schiehallion (; , ) is a prominent cone-shaped mountain in the Breadalbane, Scotland, Breadalbane region of the Scottish Highlands, in the county of Perthshire. It rises to and is classed as a Munro. Schiehallion has a rich flora, interesting ...

Braemar Braemar is a village in Aberdeenshire, Scotland, around west of Aberdeen in the Highlands. It is the closest significantly-sized settlement to the upper course of the River Dee, sitting at an elevation of . The Gaelic ''Bràigh Mhàrr'' p ...

and Fordyce to the east on mainland Scotland. The formation records a time in Earth's history where there were repeated glaciations where

ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacier, glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are the Antarctic ice sheet and the Greenland ice sheet. Ice s ...

s extended to low latitudes, sometimes referred to as a

Snowball Earth The Snowball Earth is a historical geology, geohistorical hypothesis that proposes that during one or more of Earth's greenhouse and icehouse Earth, icehouse climates, the planet's planetary surface, surface became nearly entirely freezing, fr ...

Occurrence

The PATF forms scattered outcrops along 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 In geology, a stratotype or type section is the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. If the stratigraphic unit is layered, it is called a stratotype, whereas the ...

is found on Islay, near Port Askaig, for which the formation is named. This area has been identified as 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 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 ...

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 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.

Interpretation

The PATF is subdivided into five members the lowest (oldest) three of which are particularly well-exposed on the Garvallachs. The sequence contains 48 beds of

diamictite Diamictite (; from Ancient Greek (): 'through' and (): 'mixed') is a type of lithified sedimentary rock that consists of nonsorted to poorly sorted terrigenous sediment containing particles that range in size from clay to boulders, suspended ...

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 Dolomites ( ), also known as the Dolomite Mountains, Dolomite Alps or Dolomitic Alps, are a mountain range in northeastern Italy. They form part of the Southern Limestone Alps and extend from the River Adige in the west to the Piave Va ...

. The diamictites, rocks consisting of large to very large clasts set in a finer-grained matrix, are interpreted to be

tillite image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

s 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 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.

Correlation

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 a

, where

s 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 The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

grains found within layered

baryte Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate (Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...

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 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 the likely depositional ages.

References

{{Reflist Geology of Scotland Geology of Ireland Islay