The Orcadian Basin is a
sedimentary basin
Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...
of
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 that formed mainly as a result of
extensional tectonics
Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere.
Deformation styles
The types of structure and the geometries formed depend on ...
in northeastern Scotland after the end of the
Caledonian orogeny
The Caledonian orogeny was a mountain-building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occ ...
. During part of its history, the basin was filled by a lake now known as ''Lake Orcadie''. In that lacustrine environment, a sequence of finely bedded sedimentary rocks was deposited, containing well-preserved fish fossils, with alternating layers of mudstone and coarse siltstone to very fine sandstone.
These
flagstone
Flagstone (flag) is a generic flat stone, sometimes cut in regular rectangular or square shape and usually used for paving slabs or walkways, patios, flooring, fences and roofing. It may be used for memorials, headstones, facades and other con ...
s split easily along the bedding and have been used as building material for thousands of years.
The deposits of the Orcadian Basin form part 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 ...
(ORS). The
lithostratigraphic
Lithostratigraphy is a sub-discipline of stratigraphy, the geology, geological science associated with the study of stratum, strata or rock layers. Major focuses include geochronology, comparative geology, and petrology.
In general, strata are ...
terms lower, middle and upper ORS, however, do not necessarily match exactly with sediments of lower, middle and upper Devonian age, as the base of the ORS is now known to be in the
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
and the top in 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 ...
.
Extent
The exact extent of the Orcadian Basin is uncertain due to later tectonic effects and burial beneath younger sediments, but it is known to have reached from the south coast of the
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 ...
to the
Shetland Islands
Shetland, also called the Shetland Islands and formerly Zetland, is a subarctic archipelago in Scotland lying between Orkney, the Faroe Islands and Norway. It is the northernmost region of the United Kingdom.
The islands lie about to the no ...
in the north and from
on the
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 ...
coast in the west, to the Outer Moray Firth and East Shetland Platform in the east, where it is proven by
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.
Continental sediments of the same age are also known from the
Clair oilfield
The Clair oilfield is an offshore oil field in Scottish territorial waters west of Shetland in water depths of up to . The field is the largest oilfield on the UK Continental Shelf with an estimated 8 billion barrels of oil-in-place, according ...
west of the Shetland Islands and have been tentatively identified in the West Orkney Basin. The connection of the Orcadian Basin to the Devonian basins of western Norway and eastern Greenland is not known in any detail. To the south the basin may continue almost as far south as the
Highland Boundary Fault
The Highland Boundary Fault is a major fault zone that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two different geological terranes which give rise to two distinct physiographic terr ...
, including the
half-graben
A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults.
Rift and fault structure
A rift is a region where the lithosphere ...
s at
Turriff
Turriff () is a town and civil parish in Aberdeenshire in Scotland. It lies on the River Deveron, about above sea level, and has a population of 5,708. In everyday speech it is often referred to by its Scots name ''Turra'', which is derived fr ...
and
Rhynie.
Tectonic setting
![Base of tuff layer in Lower Eday Sst](https://upload.wikimedia.org/wikipedia/commons/5/57/Base_of_tuff_layer_in_Lower_Eday_Sst.jpg)
In common with the Devonian basins of Norway and East Greenland, the Orcadian Basin lies entirely within the area affected by crustal thickening during the Caledonian orogeny. The recognition of extensional faulting at various scales in these areas at the same time as deposition led to the suggestion that these basins reflect the gravitational collapse of this thickened zone.
Other tectonic models have suggested that transtensional sinistral (left lateral) strike-slip movement on 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 ...
, which passes through the centre of the basin, was the main cause of basin formation.
The continuity of Middle Devonian
facies
In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...
belts across the trace of the fault zone, after accounting for subsequent reactivation of the Great Glen Fault in a dextral sense, has been used to argue against strike-slip activity during sedimentation.
However, the uplift of a block containing late
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 ...
veins within the basement
Moine complex in
Easter Ross
Easter Ross ( gd, Ros an Ear) is a loosely defined area in the east of Ross, Highland, Scotland.
The name is used in the constituency name Caithness, Sutherland and Easter Ross, which is the name of both a British House of Commons constituenc ...
, whose intrusion has been dated as earliest Eifelian, before deposition of the overlying Middle Devonian sediments, is evidence of some continuing activity along this structure into the middle Devonian period.
Development
Early Devonian
Evidence from onshore in
Easter Ross
Easter Ross ( gd, Ros an Ear) is a loosely defined area in the east of Ross, Highland, Scotland.
The name is used in the constituency name Caithness, Sutherland and Easter Ross, which is the name of both a British House of Commons constituenc ...
and offshore in the Inner Moray Firth shows that a series of isolated
half-grabens
A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults.
Rift and fault structure
A rift is a region where the lithosphe ...
were formed at this time filled with a mixture of coarse, often
conglomeratic sediments, combined with organic-rich lacustrine shales, such as those exposed at
Strathpeffer
Strathpeffer ( gd, Srath Pheofhair) is a village and spa town in Ross and Cromarty, Highland, Scotland, with a population of 1,469.
Geography
It lies in a strath west of Dingwall, with the elevation ranging from above sea level. Sheltered on ...
.
Middle Devonian
Sedimentation during the middle Devonian covered the full extent of the Orcadian Basin with only local exposed basement highs, such as 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 ...
in Orkney. The early part of this period, approximately equivalent to the whole of the
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 ...
and early Givetian, saw the development of a large lake, that covered most of the basin at times. Regular variations in its depth and extent, which have been linked to
Milankovitch cycles
Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term was coined and named after Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hypot ...
, led to pronounced cyclicity in the sequence. At the deep permanent lake stage in a typical cycle there is normally a laminite, consisting of fine-scale alternations of clastic, carbonate and organic laminae, thought to represent annual
varve
A varve is an annual layer of sediment or sedimentary rock.
The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (var ...
s. As the lake shallows and becomes ephemeral the sequence consists of alternations of silts and fine sandstones often with mudcracks, showing the repeated periods of drying out. The lake development culminated in a single very deep and particularly thick and extensive lake interval, the Achanarras Fish Bed Member. The lake at this time covered an area of at least 50,000 km
2 with a maximum estimated depth of at least 100 metres.
From the fossil fish fauna preserved at one level in this unit, it is known to have been particularly diverse and widespread in its occurrence.
This interval is also known as the Sandwick Fish Bed Member in Orkney and a series of other equivalent fish beds in Shetland and on the south side of the Moray Firth. A thick lake interval of similar age is also found in East Greenland. This deep lake interval is dated as late Eifelian and is correlated to the global
Kačák Event The Kačák Event (), also known as the Kačák-''otomari'' Event, is a widely recognised bioevent or series of events that occurred close to the end of the Eifelian Age (geology), Age of the Middle Devonian Epoch. It involved a global eustatic ris ...
of
marine anoxia, which was associated with significant extinctions. The increase in lake size is explained as the result of an intensification of the monsoon system.
This unit splits the flagstone sequence into two parts known regionally as the Lower and Upper Orcadian Flagstone formations.
In mid-Givetian times, the dominant lake environment gave way to mainly fluvial conditions, with the main sediment type changing from flagstones to sandstones. In parts of the basin, the sequence consists almost entirely of sandstones, mainly fluvial in origin, such as the Dunnet Head and Hoy Sandstones. Elsewhere, such as in Orkney in 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 ...
, the sequence shows more variety with major intercalations of marls and flagstones, marking the local return of lacustrine conditions. There is local evidence of marine conditions affecting the basin during one of these periods as shown by the presence of
scolecodont
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 billion ...
s (marine microfossils) in the Eday Marl Formation. This and other later marine incursions have been correlated with global sea level highstands previously recognised from the middle to upper Devonian sequences of
New York State
New York, officially the State of New York, is a state in the Northeastern United States. It is often called New York State to distinguish it from its largest city, New York City. With a total area of , New York is the 27th-largest U.S. stat ...
, including the Taghanic, Genundewa, Middlesex and Rhinestreet events.
On the Orkney Islands, volcanic rocks are found within the lower part of the Eday Group. These form isolated exposures of varying chemistry, indicating a change in magma source from one with a strong influence from earlier subduction to the alkali basalts more normally associated with
extensional tectonics
Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere.
Deformation styles
The types of structure and the geometries formed depend on ...
.
The Devonian of
West Mainland
The West Mainland of the Shetland Islands is the part of the Shetland Mainland lying west of Aith (1° 23′ W).
Geography
Points of interest include:
*Aith
* Twatt
*Sandness
*Walls
*Easter Skeld
* Reawick
*Wester Skeld
Wester can refer to:
...
in the Shetland Islands contains more volcanic rocks and the upper middle Devonian sequence there is intruded by the Late Devonian Sandsting Granite.
Late Devonian to Early Carboniferous
The later history of the Orcadian Basin is only known from offshore well data, as the youngest exposed Devonian known is Late Givetian in age. Well data shows that the depositional style remained similar throughout the Late Devonian and into the Early Carboniferous.
Later effects
The original form of the Orcadian Basin has been modified by a series of later tectonic events. In the late Carboniferous, the basin was partly
inverted during dextral reactivation of the Great Glen Fault system. This caused widespread folding and local small-scale
thrust fault
A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.
Thrust geometry and nomenclature
Reverse faults
A thrust fault is a type of reverse fault that has a dip of 45 degrees or less.
If ...
ing.
This was followed in the Permo-Triassic and Jurassic by a series of rift events, during which the Inner and Outer Moray Firth basins were formed. During the Early Cretaceous and again in the Cenozoic, the area was uplifted and eroded.
The Great Glen Fault was reactivated after the early Cretaceous, probably in sinistral sense by analogy with the connected Walls Boundary Fault,
although the exact timing is unknown.
Palaeogeography
The landscape consisted of rounded hills formed of older
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 ...
and
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
rock. The lakes varied in depth and extent from time to time, sometimes lapping against the side of the hills and sometimes retreating so that river flood plains were able to form.
[Stephen Jay Gould. ''The Book of Life''. Ebury Hutchinson, 1993] Stromatolites
Stromatolites () or stromatoliths () are layered sedimentary formations (microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). The ...
can be found at Stromness, indicating that the lake was sometimes saline. Locally
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 ...
(marine microfossils) have been found in the Eday Marl suggesting at least a temporary connection to the sea.
The hills were bare of vegetation.
The land was not colonised by large plants as it is today, so erosion would therefore be rather rapid and probably seasonal, as would be lake productivity. This is reflected in the deep lake carbonates, which show very fine laminations, interpreted as
varves
A varve is an annual layer of sediment or sedimentary rock.
The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (var ...
.
Fossils
Fauna
Fish living of the edge of the lake would float out into the centre, then sink and be preserved in the
anoxic
The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:
* Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...
conditions prevailing at depth. The
Achanarras quarry near
Thurso
Thurso (pronounced ; sco, Thursa, gd, Inbhir Theòrsa ) is a town and former burgh on the north coast of the Highland council area of Scotland. Situated in the historical County of Caithness, it is the northernmost town on the island of Great ...
has yielded the most extensive fish fauna and at least one example of all the following groups have been found there.
*
placoderms
Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally 'Plate (animal anatomy), plate-skinned') is a Class (biology), class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devoni ...
*
acanthodians
Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, which ...
*
actinopterygians
Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species.
The ray-finned fishes are so called because their fins are webs of skin supported by bony or hor ...
*
porolepiforms
*
osteolepiforms
*
dipnoans
Flora
Very few plant remains have been found and none in situ. There is evidence of algal and bacterial activity.
[R. O. Muir and J. M. Ridgway, Mineralium Deposita, Volume 10 Number 3, pp 205-215]
Economic importance
![Achscrabster quarry - geograph](https://upload.wikimedia.org/wikipedia/commons/4/4d/Achscrabster_quarry_-_geograph.org.uk_-_823145.jpg)
The flagstone facies of the Middle Devonian lacustrine sequence has provided local building material since at least the
neolithic period
The Neolithic period, or New Stone Age, is an Old World archaeological period and the final division of the Stone Age. It saw the Neolithic Revolution, a wide-ranging set of developments that appear to have arisen independently in several parts ...
. The houses at
Skara Brae
Skara Brae is a stone-built Neolithic settlement, located on the Bay of Skaill on the west coast of Mainland, the largest island in the Orkney archipelago of Scotland. Consisting of ten clustered houses, made of flagstones, in earthen dams t ...
, the tomb at
Maes Howe
Maeshowe (or Maes Howe; non, Orkhaugr) is a Neolithic chambered cairn and passage grave situated on Mainland Orkney, Scotland. It was probably built around . In the archaeology of Scotland, it gives its name to the Maeshowe type of chambered c ...
, the
Ring of Brodgar
The Ring of Brodgar (or Brogar, or Ring o' Brodgar) is a Neolithic henge and stone circle about 6 miles north-east of Stromness on Mainland, the largest island in Orkney, Scotland. It is part of the UNESCO World Heritage Site known as the Heart ...
and
Standing Stones of Stenness
The Standing Stones of Stenness is a Neolithic monument five miles northeast of Stromness on the mainland of Orkney, Scotland. This may be the oldest henge site in the British Isles. Various traditions associated with the stones survived into ...
, were all built with flagstone. The quarrying of flagstone became an important industry in the 18th century, particularly in Caithness. Flagstones from Caithness were exported round the world and are still being produced, although in more limited quantities.
The lacustrine sequence has also acted as an
oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
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 ...
, being responsible, at least in part, for the oil accumulation in the Beatrice field in the Inner Moray Firth.
See also
*
List of fossil sites
This list of fossil sites is a worldwide list of localities known well for the presence of fossils. Some entries in this list are notable for a single, unique find, while others are notable for the large number of fossils found there. Many of t ...
''(with link directory)''
*
Geology of Scotland
The geology of Scotland is unusually varied for a country of its size, with a large number of differing geological features.Keay & Keay (1994) page 415. There are three main geographical sub-divisions: the Highlands and Islands is a diverse area w ...
*
Geology of Orkney
The geology of the Orkney islands 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.
Metamor ...
References
*Gould, Stephen Jay, general editor (1993) ''The Book of Life''. London: Ebury Hutchinson
{{coord, 59.0, -1.5, display=title
Former lakes of the United Kingdom
Geology of Scotland
Paleontological sites of Europe
Devonian paleontological sites
Anoxic waters