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The Random Formation is a rock unit in
Some studies assign the Random Formation as the lowest unit of the Adeyton Group, but strictly it is below that group.
It overlies the Chapel Island formation, which contains the Precambrian/Cambrian boundary.
The unit is often strongly folded.
The formation is difficult to date, but is considered
Newfoundland
Newfoundland and Labrador is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region of Labrador, having a total size of . As of 2025 the population ...
dating to the early Cambrian
The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordovic ...
period, dominated by tidal quartz arenite
Arenite (from the Latin ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00245 in) and 2 mm (0.08 in) and containing less than 15% matrix. The related adjective is ''arenaceous''. The e ...
s deposited in a near-shore environment, but also incorporating intertidal
The intertidal zone or foreshore is the area above water level at low tide and underwater at high tide; in other words, it is the part of the littoral zone within the tidal range. This area can include several types of habitats with various sp ...
and open-shelf deposits, including glauconitic
Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color which is very friable and has very low weathering resistance.
It crystallizes with a monoclinic geometry. Its name is derived from the Greek () m ...
and mud-cracked mudstones (now shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
s), and red channel sandstone
Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
s. It was deposited quickly and is approximately thick. The Blue Pinion Formation was originally recognized as a separate formation, but is now interpreted as an expression of the Random Formation.
Sedimentology
Its white arenitic sandstones are very distinctive, recognizable from theirherringbone cross-stratification
Herringbone cross-stratification is a type of sedimentary structure formed in tidal areas, such as tidal flats, where the current periodically flows in the opposite direction.
Formation
During the conventional formation process of cross-stratifica ...
that denotes the influence of storm systems. Interbedded green-grey sands and silts represent an intertidal setting.
Spatial distribution
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. The top of the unit intergrades with the overlying strata.Stratigraphy
Some studies assign the Random Formation as the lowest unit of the Adeyton Group, but strictly it is below that group.
It overlies the Chapel Island formation, which contains the Precambrian/Cambrian boundary.
The unit is often strongly folded.
Age
The formation is difficult to date, but is considered Precambrian
The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) 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 t ...
, as it lies 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 ...
under the Bonavista Formation and conformably above Ediacaran
The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...
strata.
It does sit above the Chapel Island Formation
The Chapel Island Formation is a sedimentary formation from the Burin Peninsula, Newfoundland, Canada. It is a succession of siliciclastic deposits, over thick, that were deposited during the latest Ediacaran and earliest Cambrian.
Stratigraph ...
on the Burin peninsula meaning that, at that location, the lower boundary is in the Lower Cambrian. Inarticulate brachiopod
Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear e ...
s have been found in its upper strata, dating these to the late early or even early mid Cambrian.
However, it is likely that the unit is diachronous
In geology, a diachronism ( Greek ''dia'', "through" + ''chronos'', "time" + ''-ism''), or diachronous deposit, is a sedimentary rock formation in which the material, although of a similar nature, varies in age with the place where it was deposite ...
and may date into the Ediacaran further to the east.
The Cambrian stage 2 - stage 3 boundary (Tommotian
Stage 2 of the Cambrian is the unnamed upper stage of the Terreneuvian Series. It lies atop the Fortunian and below Stage 3 of the Cambrian. It is commonly referred to as the Tommotian, after the Cambrian stratigraphy of Siberia. Neither the uppe ...
/Atdabanian
Cambrian Stage 3 is the still unnamed third stage of the Cambrian. It succeeds Cambrian Stage 2 and precedes Cambrian Stage 4, although neither its base nor top have been formally defined. The plan is for its lower boundary to correspond approxi ...
) has been reported as being within the Random Formation, or possibly in the overlying Bonavista Formation. The overlying Smith Point Formation
The Smith Point Formation is an Early Cambrian (c. Tommotian), fossil-rich, pink to brick red limestone formation cropping out in Newfoundland.
Palaeontology
Its fossil assemblage includes trilobites, hyoliths, and microbialites (stromatolite ...
hosts the first trilobite
Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...
s.
Palaeontology
The trilobite-like trace fossils ''Rusophycus
''Rusophycus'' is an ichnogenus of trace fossil (fossil records of lifeforms' movement, rather than of the lifeforms themselves) allied to '' Cruziana''. ''Rusophycus'' is the resting trace, recording the outline of the tracemaker; ''Cruziana' ...
'' and ''Cruziana
''Cruziana'' is a trace fossil (fossil records of lifeforms' movement, rather than of the lifeforms themselves) consisting of elongate, bilobed, approximately bilaterally symmetrical burrows, usually preserved along bedding planes, with a sculp ...
'' occur in the Random Formation, alongside a range of other ichnofossil
A trace fossil, also called 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 part ...
s including ''Diplocraterion
''Diplocraterion'' is an ichnogenus describing vertical U-shaped burrows having a spreite (weblike construction) between the two limbs of the U. The spreite of an individual ''Diplocraterion'' trace can be either protrusive (between the paired ...
, Paleodictyon
''Paleodictyon'' is a trace fossil, usually interpreted to be a burrow, which appears in the geologic marine record beginning in the Precambrian/Early Cambrian and in modern ocean environments.Swinbanks, D. D., 1982: ''Paleodictyon'': the tra ...
,
Scolicia, and '' Squamodictyon''.
Body fossils include the small shelly fauna
The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambri ...
''Aldanella
''Aldanella'' is an extinct Paleozoic animal that was assigned to the Gastropod stem group but may also belong to a paraphyletic "Monoplacophora" or Annelida
The annelids (), also known as the segmented worms, are animals that comprise the ...
attleborensis'' assemblage.
See also
*Inlet Group
The Inlet Group was proposed as a geological unit for southeastern Newfoundland. It was made up of the Brigus Formation, Chamberlain's Brook Fm, Manuels River Fm, Chapel Island Formation
The Chapel Island Formation is a sedimentary formation ...
References
{{coord missing, Newfoundland and Labrador Rock formations of Canada