| Skolithos Temporal range: | |
|---|---|
 | |
| Skolithos trace fossils. Scale bar is 10 mm. | |
Trace fossil classification  | |
| Ichnogenus: | Skolithos Haldeman, 1840 |
Skolithos (formerly spelled Scolithus or Skolithus [1] ) is a common trace fossil ichnogenus that is, or was originally, an approximately vertical cylindrical burrow with a distinct lining. It was produced globally by a variety of organisms, mostly in shallow marine environments, and appears as linear features in sedimentary rocks. [2]
Skolithos ranges in age from early Cambrian [2] to the present [3] [4] and is found throughout the world. They occur in sediments and sedimentary rocks, primarily sands and sandstones. They are typically marine in origin, [5] and are commonly associated with high-energy environments close to the shoreline. [2] They have also been reported from freshwater lacustrine settings, [6] but have rarely been described from carbonate rocks. [7] Vertical Skolithos can also occur in alluvial sediments such as braided river deposits, where the periodic fluctuation of water is an important factor in the formation of this structure. [8] This periodic water fluctuation corresponds to tidal activity in shallow marine environments, but also occurs over longer time intervals in alluvial deposits. [8]
The ichnogenus Skolithos was first described as a subgenus of the supposed seaweed Fucoides in 1840, by Samuel Stehman Haldeman, a renowned Pennsylvania naturalist in the early 19th century, who labeled the structure as the “oldest fossil in the state”. [9] He named the trace fossil Skolithos, meaning “worm-stone”, suggesting its morphologic similarity to a worm. [9] James Hall published the first illustrations of Haldeman’s discovery in his journal Paleontology of New York Volume I (1847), changing the name to Scolithus. [9] 1943 marked the revival of Haldeman’s research as Benjamin Howell reported the occurrence of the trace fossil in the Hardyston Formation in Pennsylvania. [9] Howell restored the name Skolithos in accordance with the International Code of Zoological Nomenclature. In the 1960s structural geologists discovered the use of the trace fossil as a strain marker by which it could record rotation and strain in highly deformed rocks. [9] This led to a series of experiments that extend to present-day analyses to determine the extent of the strain marking properties of Skolithos. Skolithos linearis, found in the Blue Ridge Mountain region, is the oldest known trace fossil in Virginia. [10] Trypanites is superficially similar in form but is a boring excavated in hard substrates, and lacks the diagnostic lining of Skolithos.
The structure of the trace fossil is cylindrical and elongated in shape, usually at a perpendicular angle to the surface where it has been deposited. They can reach lengths of up to about 35 cm (14 in) [6] and diameters of up to about 5 cm (2.0 in). [6] The vertical burrows are composed of the same mineralogy as its surrounding matrix which allow it to deform homogenously with the parent rock. Variations in observed Skolithos structures include burrow curvature, angle to the plane of deposition, and size of the fossil’s aperture. [11] Funnel-shaped apertures of Skolithos reflect the filter- and suspension-feeding habits of burrowing genera. The high intensity of bioturbation of these organisms indicate the shallow water paleoenvironment in which the Skolithos burrows formed shortly after the deposition of the bed. [11]
Unstrained Skolithos structures are normal to the plane of the bed. [11] In zones where tectonic deformation is intense, such as thrust zones, the deformed Skolithos burrow can be used to evaluate the local strain on the region. [11] This technique is performed by comparing the angle between the specimen and the bedding surface, with the original 90o geometric relationship. Since the trace fossil shares similar material properties to the surrounding matrix, they are inferred to deform by the same mechanism. [11] This technique can be applied in areas where other strain markers may have been destroyed by tectonic activity or cataclastic flow. [11]
Unit strain ɛ can also be defined using the elongation of the structure:
where
• ɛ is the unit strain due to elongation
• l is the deformed length of the structure
• lo is the initial length of the structure
The structure length and orientation may be influenced by the directional behaviour of the burrowing organism, therefore observing the widths of the burrow may provide a more precise strain estimation. [12]
The famous "Pipe Rock" of northwest Scotland is a well-known example of Skolithos. The 'pipes' that give the rock its name are closely packed straight Skolithos tubes that were presumably made by a worm-like organism. [13] The Pipe Rock can be found in the Stack of Glencoul region beneath the Moine Thrust Belt, Scotland. [14] This area which has a history of thrust faulting activity is a highly deformed mylonite zone with a quartzite protolith where many structural geologists have used microstructures such as the Skolithos borings in conjunction with other strain markers, such as quartz vein recrystallization, in order to approximate strain in the region. [14] Using three-dimensional analysis of the strain markers, geologists inferred flattening of the region parallel to the thrust direction, stretching along the vertical strain direction and shortening perpendicular to the foliation of the lithology. [14] The deformation history of the mylonite belt which is characterized by large translation of thrust faults, can be deduced from the apparent clockwise rotation of these structures. Assuming simple shear, the westward displacement of the 800 m thick Moine Thrust mylonites at Loch Eriboll where the average shear strain determined using the trace fossils is approximately 10, was calculated to be about 8 km. [11]
Assumptions regarding the undeformed burrow and its geometric relationship cannot directly be determined, and only estimated. [15] While it is common for Skolithos burrows to form normal to the deposition plane, this is not always true, in which case, the ideal, undeformed state can no longer be used as a reference orientation. [15] Shallow depositional sediments are also susceptible to damage by erosion and tectonic stress forces, which can influence average measurements and geometric orientations. [7] Since the rheological properties between the structure and the host rock are usually very similar, observations of the fossils are conducted with the assumption that they have deformed homogeneously, where the deformation forces are distributed evenly along the entire deformation zone. [12] This is directly contradicted by the presence of folding and varying elongation measurements of the fossil at different locations in the same deformation zone. Deformation mechanisms are difficult to distinguish using this strain marker, as the thinning and flattening of the highly deformed rocks where they are found, cannot necessarily be attributed to pure shear since the planes may have simply rotated near parallel to the shear plane. [15] It is therefore only possible to make accurate strain determinations of the host rock provided the correct assumption of the deformation mechanism and original measurements.
A trace fossil, also known as an ichnofossil, is a fossil record of biological activity by lifeforms but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or by mineralization. The study of such trace fossils is ichnology - the work of ichnologists.
In structural geology, a fold is a stack of originally planar surfaces, such as sedimentary strata, that are bent or curved ("folded") during permanent deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur as single isolated folds or in periodic sets. Synsedimentary folds are those formed during sedimentary deposition.
The Moine Thrust Belt or Moine Thrust Zone is a linear tectonic feature in the Scottish Highlands which runs from Loch Eriboll on the north coast 190 kilometres (120 mi) southwest to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of thrust faults that branch off the Moine Thrust itself. Topographically, the belt marks a change from rugged, terraced mountains with steep sides sculptured from weathered igneous, sedimentary and metamorphic rocks in the west to an extensive landscape of rolling hills over a metamorphic rock base to the east. Mountains within the belt display complexly folded and faulted layers and the width of the main part of the zone varies up to ten kilometres, although it is significantly wider on Skye.
In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear zone takes the form of a fracture called a fault. In the lower crust and mantle, the extreme conditions of pressure and temperature make the rock ductile. That is, the rock is capable of slowly deforming without fracture, like hot metal being worked by a blacksmith. Here the shear zone is a wider zone, in which the ductile rock has slowly flowed to accommodate the relative motion of the rock walls on either side.
The Caledonian orogeny was a mountain-building cycle recorded in the northern parts of the British Isles, the Scandinavian Caledonides, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly 490–390 million years ago (Ma). It was caused by the closure of the Iapetus Ocean when the Laurentia and Baltica continents and the Avalonia microcontinent collided.
Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a classification based on the textural appearance of the rock.
The Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6 billion years old. The rocks have experienced multiple metamorphic events at amphibolite or granulite conditions, resulting in often complete destruction of original igneous or sedimentary (protolith) textures. Importantly, it contains the oldest known samples of the Earth's crust: samples of zircon from the Jack Hills portion of the Narryer Gneiss have been radiometrically dated at 4.4 billion years old, although the majority of zircon crystals are about 3.6-3.8 billion years old.
In geology, shear is the response of a rock to deformation usually by compressive stress and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear. Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics.
Fortune Head is a headland located about 1.6 km (0.99 mi) from the town of Fortune on the Burin Peninsula, southeastern Newfoundland.
In geology, texture or rock microstructure refers to the relationship between the materials of which a rock is composed. The broadest textural classes are crystalline, fragmental, aphanitic, and glassy. The geometric aspects and relations amongst the component particles or crystals are referred to as the crystallographic texture or preferred orientation. Textures can be quantified in many ways. A common parameter is the crystal size distribution. This creates the physical appearance or character of a rock, such as grain size, shape, arrangement, and other properties, at both the visible and microscopic scale.
The Ecca Group is the second of the main subdivisions of the Karoo Supergroup of geological strata in southern Africa. It mainly follows conformably after the Dwyka Group in some sections, but in some localities overlying unconformably over much older basement rocks. It underlies the Beaufort Group in all known outcrops and exposures. Based on stratigraphic position, lithostratigraphic correlation, palynological analyses, and other means of geological dating, the Ecca Group ranges between Early to earliest Middle Permian in age.
The Narooma Accretionary Complex or Narooma Terrane is a geological structural region on the south coast of New South Wales, Australia that is the remains of a subduction zone or an oceanic terrane. It can be found on the surface around Narooma, Batemans Bay and down south into Victoria near Mallacoota. It has attached itself to the Lachlan Fold Belt and has been considered as either an exotic terrane or as a part of the fold belt. Rocks are turbidites, block in matrix mélange, chert, and volcanics. The accretionary complex itself could either be the toe of a subduction zone, or an accretionary prism. It was moved by the Pacific Plate westwards for about 2500 km until it encountered the east coast of Gondwana. It is part of the Mallacoota Zone according to Willman, which in turn is part of the Eastern Lachlan Fold Belt, which is part of the Benambra Terrane.
The Hebridean Terrane is one of the terranes that form part of the Caledonian orogenic belt in northwest Scotland. Its boundary with the neighbouring Northern Highland Terrane is formed by the Moine Thrust Belt. The basement is formed by Archaean and Paleoproterozoic gneisses of the Lewisian complex, unconformably overlain by the Neoproterozoic Torridonian sediments, which in turn are unconformably overlain by a sequence of Cambro–Ordovician sediments. It formed part of the Laurentian foreland during the Caledonian continental collision.
Chondrites is a trace fossil ichnogenus, preserved as small branching burrows of the same diameter that superficially resemble the roots of a plant. The origin of these structures is currently unknown. Chondrites is found in marine sediments from the Cambrian period of the Paleozoic onwards. It is especially common in sediments that were deposited in reduced-oxygen environments.
The geology of the Isle of Skye in Scotland is highly varied and the island's landscape reflects changes in the underlying nature of the rocks. A wide range of rock types are exposed on the island, sedimentary, metamorphic and igneous, ranging in age from the Archaean through to the Quaternary.
In structural geology, strain partitioning is the distribution of the total strain experienced on a rock, area, or region, in terms of different strain intensity and strain type. This process is observed on a range of scales spanning from the grain – crystal scale to the plate – lithospheric scale, and occurs in both the brittle and plastic deformation regimes. The manner and intensity by which strain is distributed are controlled by a number of factors listed below.
The Nordfjord—Sogn Detachment (NSD) is a major extensional shear zone in Norway up to 6 km in thickness, which extends about 120 km along strike from Nordfjord to Sognefjord, bringing Devonian continental coarse clastic sedimentary rocks into close contact with eclogite facies metamorphic rocks of the Western Gneiss Region. It formed towards the end of the Caledonian Orogeny and was mainly active during the Devonian. It has an estimated displacement of at least 70 km and possibly as much as 110 km. It was reactivated during the Mesozoic and may have influenced the development of fault structures in the North Sea rift basin.
Pre-collisional Himalaya is the arrangement of the Himalayan rock units before mountain-building processes resulted in the collision of Asia and India. The collision began in the Cenozoic and it is a type locality of a continental-continental collision. The reconstruction of the initial configuration of the rock units and the relationship between them is highly controversial, and major concerns relate to the arrangements of the different rock units in three dimensions. Several models have been advanced to explain the possible arrangements and petrogenesis of the rock units.
The Bokkeveld Group is the second of the three main subdivisions of the Cape Supergroup in South Africa. It overlies the Table Mountain Group and underlies the Witteberg Group. The Bokkeveld Group rocks are considered to range between Lower Devonian (Lochkovian) to Middle Devonian (Givetian) in age.
The geology of the Ellsworth Mountains, Antarctica, is a rock record of continuous deposition that occurred from the Cambrian to the Permian periods, with basic igneous volcanism and uplift occurring during the Middle to Late Cambrian epochs, deformation occurring in the Late Permian period or early Mesozoic era, and glacier formation occurring in the Cretaceous period and Cenozoic era. The Ellsworth Mountains are located within West Antarctica at 79°S, 85°W. In general, it is made up of mostly rugged and angular peaks such as the Vinson Massif, the highest mountain in Antarctica.
