The Random Formation is a rock unit in

Newfoundland Newfoundland and Labrador (; french: Terre-Neuve-et-Labrador; frequently abbreviated as NL) is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region ...

dating to the

early Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...

period, dominated by tidal quartz

arenite Arenite (Latin: ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00246 in) and 2 mm (0.08 in) and contain less than 15% matrix. The related adjective is ''arenaceous''. The equivalent ...

s deposited in a near-shore environment, but also incorporating

intertidal The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of Marine habitat, habitats with var ...

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 minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

s), and red channel

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

s. It was deposited quickly and is approximately 175 m 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 their

herringbone 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 pꞒ, 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 the ...

, 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 Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...

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 Brachiopoda, are a phylum of trochozoan 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 end, w ...

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 language, 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 i ...

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 upper ...

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 approxim ...

) has been reported as being within the Random Formation, or possibly in the overlying Bonavista Formation. The overlying Smith Point Formation hosts the first

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

Palaeontology

The trilobite-like trace fossils ''

Rusophycus ''Rusophycus'' is a trace fossil ichnogenus allied to ''Cruziana''. ''Rusophycus'' is the resting trace, recording the outline of the tracemaker; ''Cruziana'' is made when the organism moved. The sculpture of ''Rusophycus'' may reveal the approx ...

'' and ''

Cruziana ''Cruziana'' is a trace fossil consisting of elongate, bilobed, approximately bilaterally symmetrical burrows, usually preserved along bedding planes, with a sculpture of repeated striations that are mostly oblique to the long dimension. It is fo ...

'' occur in the Random Formation, alongside a range of other

ichnofossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...

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 traces ...

Scolicia ''Scolicia'' de Quartrefages, 1849 is a parataxon of ichnofossils present in sedimentary rocks of marine, marginal lacustrine, alluvial, or fluvial facies. These fossil traces appear in a wide geological range, from the Cambrian The Cambri ...

, 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 mollusc that was assigned to the Gastropod stem group but may also belong to a paraphyletic "Monoplacophora Monoplacophora , meaning "bearing one plate", is a polyphyletic superclass of molluscs with a ...

attleborensis'' assemblage.

References

{{coord missing, Newfoundland and Labrador Rock formations of Canada