Geology of Orkney
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The geology of the Orkney islands in northern
Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
is dominated by the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
age
Old Red Sandstone The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...
(ORS). In the southwestern part of
Mainland Mainland is defined as "relating to or forming the main part of a country or continent, not including the islands around it egardless of status under territorial jurisdiction by an entity" The term is often politically, economically and/or dem ...
, this sequence can be seen to rest
unconformably 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 o ...
on a Moinian type
metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...
.


Metamorphic basement

Metamorphic rocks are exposed near
Stromness Stromness (, non, Straumnes; nrn, Stromnes) is the second-most populous town in Orkney, Scotland. It is in the southwestern part of Mainland Orkney. It is a burgh with a parish around the outside with the town of Stromness as its capital. E ...
and Yesnaby on Mainland and on
Graemsay Graemsay () is an island in the western approaches to Scapa Flow, in the Orkney Islands of Scotland. The island has two lighthouses. Graemsay lies within the parish of Stromness. Geography and geology Graemsay lies between Hoy and Stromness ...
. Most of the basement consists of
granitic A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quart ...
gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...
es and later minor
intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
s. These rocks are similar to those seen in northern
Caithness Caithness ( gd, Gallaibh ; sco, Caitnes; non, Katanes) is a historic county, registration county and lieutenancy area of Scotland. Caithness has a land boundary with the historic county of Sutherland to the west and is otherwise bounded by ...
and have been correlated with the Loch Eil Group of the late
Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...
Moine sequence. The unconformity at the base of the overlying Devonian rocks is highly irregular, with the exposed basement forming hills in this buried land surface.


Devonian

The stratigraphic breakdown of the Devonian sequence has traditionally been into Lower, Middle and Upper ORS due to a general lack of
biostratigraphic Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...
control. These divisions are now known to approximate to the Lower, Middle and Upper divisions of the Devonian.


Lower Devonian

Rocks of probable lower Devonian age are exposed near Yesnaby, on western Mainland. A ridge of granitic basement divides the Harra Ebb Formation, a sequence of
breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
s, conglomerates and
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...
s deposited in an
alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...
setting, from the Yesnaby Sandstone which was deposited in an aeolian setting. The contact with the overlying Middle Devonian
lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...
Lower Stromness Flagstones is an angular
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 o ...
.


Middle Devonian

Most of the
Old Red Sandstone The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...
of Orkney is of Middle Devonian age.Marshall, J.E.A., & Hewett, A.J. 2003. Devonian. In: Evans, D., Graham C., Armour, A., & Bathurst, P. (eds), The Millennium Atlas: petroleum geology of the central and northern North Sea The lower part of the sequence, mostly
Eifelian The Eifelian is the first of two faunal stages in the Middle Devonian Epoch. It lasted from 393.3 ± 1.2 million years ago to 387.7 ± 0.8 million years ago. It was preceded by the Emsian Stage and followed by the Givetian Stage. North American s ...
in age, is dominated by lacustrine beds of the lower and upper Stromness Flagstones that were deposited in Lake Orcadie. The two flagstones sequences are divided by the Sandwick fish bed, equivalent to the Achanarras formation of Caithness, representing an unusually persistent, deep and widespread lake that filled much of the Orcadian Basin. The sequence continues with the Rousay flagstones deposited in a similar lacustrine environment. The flagstones show a marked cyclicity in their sedimentation, with 86 such cycles being counted for the whole sequence. These are interpreted as representing regular climatically driven changes in lake level caused by Milankovitch cyclicity. The
Eday Group The Eday Group is a Devonian lithostratigraphic group (a sequence of rock strata) in Orkney, northern Scotland. The name is derived from the island of Eday where the strata are exposed in coastal cliffs. Outcrops These rocks are exposed through ...
overlies the Rousay Flagstones and represents a change to dominantly fluvial/aeolian deposition with only occasional lacustrine intervals. The Eday Group consists of three sandstone units, the lower, middle and upper Eday Sandstones separated by the Eday Flagstones and Eday
Marl Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part o ...
respectively. The Eday Marl consists mainly of red and green mudstones and siltstones and is thought to have been deposited in a river floodplain environment.Upper Middle and Upper Devonian Sediments, Hall, Adrian and Brown, John, "Orkney Landscapes" (online) (Edinburgh, Scotland) September 2005, Last update 10-Nov-2008
/ref> Locally
pseudomorphs In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by ...
of
halite Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, p ...
have been found and rarely marine microfossils (
scolecodonts A scolecodont is the jaw of a polychaete annelid, a common type of fossil-producing segmented worm useful in invertebrate paleontology. Scolecodonts are common and diverse microfossils, which range from the Cambrian period (around half a billi ...
) indicating that the basin was affected by occasional marine incursions at this time. The age and stratigraphic equivalents of the Hoy sandstone sequence are a matter of continuing debate. Traditionally the presence of an unconformity at the base of the lavas beneath these sandstones on southwestern Hoy has been interpreted to mark a major time break.Mykura, W. (with contributions by Flinn, D, & May, F.) 1976. British Regional Geology: Orkney and Shetland, Institute of Geological Sciences, Natural Environment Council, 149pp. More recent studies have suggested that this unconformity is of only local significance as a hiatus of this age has not been recognised elsewhere in the Orcadian Basin sequence. It has been proposed that the Hoy sandstones are the lateral equivalents of the Eday Group, the thickness and associated facies changes being caused by active extensional faulting during deposition. The presence of lavas within the Eday Flagstone Formation has been used to lend support to this idea, but differences in the chemistry between these lavas and those on Hoy mean that such a correlation remains uncertain.


Upper Devonian

The Upper Eday Sandstone is considered to extend from the Upper
Givetian The Givetian is one of two faunal stages in the Middle Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Eifelian Stage and followed by the Frasnian Stage. It is named after the town of Givet in Fra ...
into the Lower
Frasnian The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage. Major reef-building was under way during th ...
in age. However, there is no biostratigraphic data available to confirm this suggestion.
Hydrocarbon exploration Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology. Exploration methods Vis ...
wells drilled in the Inner and Outer
Moray Firth The Moray Firth (; Scottish Gaelic: ''An Cuan Moireach'', ''Linne Mhoireibh'' or ''Caolas Mhoireibh'') is a roughly triangular inlet (or firth) of the North Sea, north and east of Inverness, which is in the Highland council area of north of Scotl ...
basins show similar sequences continuing without break into the Lower Carboniferous.


Structure

The Devonian and older rocks of Orkney are cut by a series of WSW-ENE to N-S trending faults, many of which were active during deposition of the Devonian sequences. The largest of these faults are the North Scapa Fault, East Scapa Fault and Bring-Risa Fault. Both the North Scapa and Brims-Risa faults show evidence of local reversal along part of their lengths, thought to be due to a period of transpressional deformation along the
Great Glen Fault The Great Glen Fault is a strike-slip fault that runs through the Great Glen in Scotland. The fault is mostly inactive today, but occasional moderate tremors have been recorded over the past 150 years. Location Aligned northeast to southwest, t ...
system that lies just to the east of the islands, during the
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
. Small-scale folding is common and one large-scale fold is developed, the north–south trending Eday syncline. This syncline is parallel to neighbouring normal faults and thickness variations in the lower Eday Sandstone indicate that this structure was active during the middle Devonian.


Intrusions

Dykes of
lamprophyre Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxid ...
and more rarely
dolerite Diabase (), also called dolerite () or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grained ...
are exposed in many parts of Orkney. They trend mainly NE-SW to N-S and are dated as
Late Permian Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...
, associated with a period of crustal extension.


Petroleum geology

The presence of
bitumen Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...
s throughout Orkney show that hydrocarbons have at least locally been produced and migrated. The permanent lake laminites from the flagstone sequences are moderate to good
source rock In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been depos ...
s and are currently marginally mature at the surface as a result of burial after the Devonian.Hydrocarbon Potential Orkney Devonian, Hall, Adrian and Brown, John, "Orkney Landscapes" (online) (Edinburgh, Scotland) September 2005, Last update 10-Nov-2008
/ref> Two large exhumed oil accumulations have been described from Orkney, the Yesnaby and Houton Head reservoir systems. At Yesnaby bitumens occur throughout the presumed trap in the aeolian Yesnaby Sandstone with good porosity and permeability. At Houton Head, bitumens are found within a large area of aeolian/fluvial lower Eday Sandstone of similarly good reservoir characteristics.


Quaternary deposits

Glacial A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
and periglacial sediments are found locally throughout Orkney. A full history of the glaciations is not recorded with the last glaciation being the only event for which there is much evidence.The Ice Age in Orkney, Hall, Adrian and Brown, John, "Orkney Landscapes" (online) (Edinburgh, Scotland) September 2005, Last update 10-Nov-2008
/ref>


References

{{Orkney Islands
Orkney Orkney (; sco, Orkney; on, Orkneyjar; nrn, Orknøjar), also known as the Orkney Islands, is an archipelago in the Northern Isles of Scotland, situated off the north coast of the island of Great Britain. Orkney is 10 miles (16 km) north ...
Orkney