Geology of Orkney
Encyclopedia
The geology of the Orkney archipelago in northern Scotland
is dominated by the Devonian
age Old Red Sandstone
(ORS). In the southwestern part of Mainland, this sequence can be seen to rest unconformably
on a Moinian
type metamorphic
basement
.
and Yesnaby
on Mainland and on Graemsay
. Most of the basement consists of granitic
gneiss
es and later minor intrusion
s. These rocks are similar to those seen in northern Caithness
and have been correlated with the Loch Eil Group of the late Proterozoic
Moine sequence. The unconformity at the base of the overlying Devonian is highly irregular, with the exposed basement forming hills in this buried land surface.
control. These divisions are now known to approximate to the Lower, Middle and Upper divisions of the Devonian.
s, conglomerates
and sandstone
s deposited in an alluvial fan
setting, from the Yesnaby Sandstone which was deposited in an aeolian setting. The contact with the overlying Middle Devonian lacustrine
Lower Stromness Flagstone
s is an angular unconformity
.
. 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 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
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. Locally pseudomorphs of halite
have been found and rarely marine microfossils (scolecodonts
) 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. 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.
in age. However, there is no biostratigraphic data available to confirm this suggestion. Hydrocarbon exploration wells drilled in the Inner and Outer Moray Firth
basins show similar sequences continuing without break into the Lower Carboniferous.
system that lies just to the east of the islands, during the Carboniferous
. 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 thickess variations in the lower Eday Sandstone indicate that this structure was active during the middle Devonian.
of lamprophyre
and more rarely dolerite are exposed in many parts of Orkney. They trend mainly NE-SW to N-S and are dated as Late Permian, associated with a period of crustal extension.
s and are currently marginally mature at the surface as a result of burial after the Devonian. 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.
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.
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
is dominated by the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
age Old Red Sandstone
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
(ORS). In the southwestern part of Mainland, this sequence can be seen to rest unconformably
Unconformity
An unconformity is a buried erosion 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, but the term is used to describe...
on a Moinian
Moine Supergroup
The Moine Supergroup is a sequence of Neoproterozoic metamorphic rocks that form the dominant outcrop of the Scottish Highlands between the Moine Thrust Belt to the northwest and the Great Glen Fault to the southeast. The sequence is metasedimentary in nature and was metamorphosed and deformed in a...
type metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
basement
Basement Rock
Basement or Basement Rock music was a sub-genre coined in 2006 in an article by music magazine TGR. This was first in relation to the existence of underground record label Criminal Records but more for the independent bands they represent. The roots of the sub-genre are noted to be as far back as...
.
Metamorphic basement
Metamorphic rocks are exposed near StromnessStromness
Stromness is the second-biggest town in Orkney, Scotland. It is in the south-west of Mainland Orkney. It is also a parish, with the town of Stromness as its capital.-Etymology:...
and Yesnaby
Yesnaby
Yesnaby is an area in Sandwick, on the west coast of Orkney Mainland, Scotland, south of Skara Brae.Yesnaby is renowned for its Devonian geology, its crumbly rocks, sea stacks, blowholes, geos, towering cliffs and boiling seas....
on Mainland and on Graemsay
Graemsay
Graemsay is an island in the western approaches to Scapa Flow, in the Orkney Islands of Scotland. The island has two lighthouses.-Geography and geology:...
. Most of the basement consists of granitic
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es and later minor intrusion
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
s. These rocks are similar to those seen in northern Caithness
Caithness
Caithness is a registration county, lieutenancy area and historic local government area of Scotland. The name was used also for the earldom of Caithness and the Caithness constituency of the Parliament of the United Kingdom . Boundaries are not identical in all contexts, but the Caithness area is...
and have been correlated with the Loch Eil Group of the late Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
Moine sequence. The unconformity at the base of the overlying Devonian 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 biostratigraphicBiostratigraphy
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. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...
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 Hara Ebb Formation, a sequence of brecciaBreccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s, conglomerates
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
and sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s deposited in an alluvial fan
Alluvial fan
An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...
setting, from the Yesnaby Sandstone which was deposited in an aeolian setting. The contact with the overlying Middle Devonian lacustrine
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
Lower Stromness Flagstone
Flagstone
Flagstone, is a generic flat stone, usually used for paving slabs or walkways, patios, fences and roofing. It may be used for memorials, headstones, facades and other constructions. The name derives from Middle English flagge meaning turf, perhaps from Old Norse flaga meaning slab.Flagstone is a...
s is an angular unconformity
Unconformity
An unconformity is a buried erosion 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, but the term is used to describe...
.
Middle Devonian
Most of the Old Red Sandstone of Orkney is of Middle Devonian age. The lower part of the sequence, mostly Eifelian in age, is dominated by lacustrine beds of the lower and upper Stromness Flagstones that were deposited in Lake OrcadieOrcadian Lakes
The Orcadian Lakes are a series of lakes which existed during the Devonian period in the region which is now northern Scotland, Orkney and Shetland. The sedimentary rocks they left behind have been studied since the 1830's...
. 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 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
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
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. Locally pseudomorphs of halite
Halite
Halite , commonly known as rock salt, is the mineral form of sodium chloride . Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on the amount and type of impurities...
have been found and rarely marine microfossils (scolecodonts
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 to the present...
) 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. 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 into the Lower FrasnianFrasnian
The Frasnian is one of two faunal stages in the Late Devonian epoch. It lasted from 385.3 ± 2.6 million years ago to 374.5 ± 2.6 million years ago. It was preceded by the Givetian stage and followed by the Famennian stage...
in age. However, there is no biostratigraphic data available to confirm this suggestion. Hydrocarbon exploration wells drilled in the Inner and Outer Moray Firth
Moray Firth
The Moray Firth is a roughly triangular inlet of the North Sea, north and east of Inverness, which is in the Highland council area of north of Scotland...
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 FaultGreat Glen Fault
The Great Glen Fault is a long strike-slip fault that runs through its namesake the Great Glen in Scotland. However, the fault is actually much longer and over 400 million years old.-Location:...
system that lies just to the east of the islands, during the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
. 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 thickess variations in the lower Eday Sandstone indicate that this structure was active during the middle Devonian.
Intrusions
DykesDike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
of lamprophyre
Lamprophyre
Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions...
and more rarely dolerite are exposed in many parts of Orkney. They trend mainly NE-SW to N-S and are dated as Late Permian, associated with a period of crustal extension.
Petroleum geology
The presence of bitumens 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 rockSource rock
In petroleum geology, source rock refers to rocks from which hydrocarbons have been generated or are capable of being generated. They form one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been deposited in a variety of environments including...
s and are currently marginally mature at the surface as a result of burial after the Devonian. 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
GlacialGlacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
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.