Random Formation | |
---|---|
Stratigraphic range: | |
Type | Formation |
Unit of | Young's Cove Group, [1] Musgravetown Group, [2] or none, [3] depending on authority (and location) |
Underlies | (Unconformably) Bonavista Fm and others [4] |
Overlies |
|
Thickness | From a few metres to 250 m [6] |
Lithology | |
Primary | Herringbone-crossstratified white arenites |
Other | Grey-green silts and shales |
Location | |
Region | Newfoundland |
Country | Canada |
Type section | |
Named for | Random Island |
Bedrock distribution of the Random Formation |
The Random Formation is a rock unit in Newfoundland dating to the early Cambrian period, dominated by tidal quartz arenites deposited in a near-shore environment, but also incorporating intertidal and open-shelf deposits, including glauconitic and mud-cracked mudstones (now shales), and red channel sandstones. [4] [7] It was deposited quickly and is approximately 175 m (574 ft) thick. [8] The Blue Pinion Formation was originally recognized as a separate formation, but is now interpreted as an expression of the Random Formation. [9] [10]
Its white arenitic sandstones are very distinctive, recognizable from their herringbone cross-stratification that denotes the influence of storm systems. Interbedded green-grey sands and silts represent an intertidal setting. [5]
It is widespread throughout Newfoundland unlike its underlying units. It was deposited in intertidal mud flats to subtidal setting, then (later) near the bottom of an open marine shelf. [11]
The top of the unit intergrades with the overlying strata. [10]
Some studies [1] assign the Random Formation as the lowest unit of the Adeyton Group, but strictly it is below that group. [5]
It overlies the Chapel Island formation, which contains the Precambrian/Cambrian boundary. [12]
The unit is often strongly folded. [5]
The formation is difficult to date, but is considered Precambrian, as it lies unconformably under the Bonavista Formation and conformably above Ediacaran strata. [2] It does sit above the Chapel Island Formation on the Burin peninsula meaning that, at that location, the lower boundary is in the Lower Cambrian. Inarticulate brachiopods have been found in its upper strata, dating these to the late early or even early mid Cambrian. [10]
However, it is likely that the unit is diachronous and may date into the Ediacaran further to the east. [10]
The Cambrian stage 2 - stage 3 boundary (Tommotian/Atdabanian) has been reported as being within the Random Formation, or possibly in the overlying Bonavista Formation. The overlying Smith Point Formation hosts the first trilobites. [13]
The trilobite-like trace fossils Rusophycus and Cruziana occur in the Random Formation, alongside a range of other ichnofossils including Diplocraterion, Paleodictyon, Scolicia, and Squamodictyon . [13] Body fossils include the small shelly fauna Aldanella attleborensis assemblage. [14]
The Ediacaran is a geological period of the Neoproterozoic Era that spans 96 million years from the end of the Cryogenian Period at 635 Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last period of the Proterozoic Eon as well as the last of the so-called "Precambrian supereon", before the beginning of the subsequent Cambrian Period marks the start of the Phanerozoic Eon, where recognizable fossil evidence of life becomes common.
The Precambrian is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic Eon, which is named after Cambria, the Latinized name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.
Charnia is an extinct genus of frond-like lifeforms belonging to the Ediacaran biota with segmented, leaf-like ridges branching alternately to the right and left from a zig-zag medial suture. The genus Charnia was named for Charnwood Forest in Leicestershire, England, where the first fossilised specimen was found. Charnia is significant because it was the first Precambrian fossil to be recognized as such.
Glauconite is an iron potassium phyllosilicate mineral of characteristic green color which is very friable and has very low weathering resistance.
Aspidella is an Ediacaran disk-shaped fossil of uncertain affinity. It is known from the single species A. terranovica.
The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The term "Ediacara biota" has received criticism from some scientists due to its alleged inconsistency, arbitrary exclusion of certain fossils, and inability to be precisely defined.
The Stratigraphy of the Cambrian period currently has several schemes used for ordering geologic formations from the period. The International Commission on Stratigraphy−ICS scheme has set a stratotype section for the base of the Cambrian, dated quite accurately to 538.8 ± 0.2 million years ago. Russian and Chinese scientists have developed a different scheme.
Syneresis cracks are a sedimentary structure developed by the shrinkage of sediment without desiccation – not to be confused with desiccation cracks. Syneresis is the expulsion of a liquid from a gel-like substance. Syneresis cracks are formed by the contraction of clay in response to changes in the salinity of a liquid surrounding a deposit. The cracks can occur, for example, in mudstones deposited between two beds of sandstone. The markings would have been formed subaqueously on the bedding surface and could resemble desiccation mudcracks, but are not continuous and vary in shape. They commonly occur in thin mudstones interbedded with sandstones, as positive relief on the bottom of the sandstone, or as negative relief on the top of the mudstone. Subaqueous shrinkage cracks can develop on and through a surface that has been continuously covered in water. Syneresis cracks in some shales and lime mudstones may initially be preserved as small cavities, which then usually fill with silt and sand from either the overlying or underlying beds and laminae. Usually there is no pattern to the cracks, and they do not connect to form geometric shapes. Rather they are discontinuous and shaped in one of the following categories:
The Chapel Island Formation is a sedimentary formation from the Burin Peninsula, Newfoundland, Canada. It is a succession of siliciclastic deposits, over 1,000 metres (3,300 ft) thick, that were deposited during the latest Ediacaran and earliest Cambrian.
The Attawapiskat Formation is a geologic formation in Ontario. It preserves fossils dating back to the Silurian period.
The Brigus Formation is a fossiliferous upper lower Cambrian geologic formation in Newfoundland and Labrador.
The Mistaken Point Formation is a geologic formation in Newfoundland and Labrador. It is recognized as a Lagerstätte preserving fossils dating back to the Ediacaran period. It contains a stratum dated to 565 ± 3 million years ago.
The Ingta Formation is a geological unit containing green sandstones and shales; it crops out in the Canadian Mackenzie Mountains. Its age is poorly constrained, though it straddles the Precambrian/Cambrian boundary. Below the boundary its ichnofauna comprises subhorizontal Planolites burrows; above it, Phycodes burrows immediately appear, with Nemakit-Daldyn SSFs appearing soon after.
One of the major depositional strata in the Himalaya is the Lesser Himalayan Strata from the Paleozoic to Mesozoic eras. It had a quite different marine succession during the Paleozoic, as most parts of it are sparsely fossiliferous or even devoid of any well-defined fossils. Moreover, it consists of many varied lithofacies, making correlation work more difficult. This article describes the major formations of the Paleozoic – Mesozoic Lesser Himalayan Strata, including the Tal Formation, Gondwana Strata, Singtali Formation and Subathu Formation.
The Rencontre Formation is a geological formation just below the Cambrian-Ediacaran boundary in Newfoundland, deposited in a fault-bounded enclosed basin. U-Pb dates obtained just below its base give a maximum age of 552 ± 3 million years ago.
The Smith Point Formation is an Early Cambrian, fossil-rich, pink to brick red limestone formation cropping out in Newfoundland.
The Mooring Cove Formation is a formation of volcanic rock exposed in outcrops in Newfoundland.
The Vampire Formation is a formation of delta-front siliciclastics deposited in the early Cambrian. The upper unit is trilobitic Cambrian. Some authors place the underlying Ingta formation at least partially in the Cambrian; in places it overlies the sub-Cambrian unconformity.
A "Palaeopascichnid" describes a multitude of elongate fossils made up of multiple sausage-shaped chambers. They appear only in Ediacaran sediments. Fossils of Palaeopascichnids consist of an occasionally branching series of globular or elongate chambers. These fossils started appearing in the Vendian about 580 million years ago. Fossils of Palaeopascichnids are found in East European platform, Siberia, South China (Lantian), Australia, India (Tethys), Avalonia
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