Random Formation

Random Formation Stratigraphic range: Early Cambrian PreЄ Є O S D C P T J K Pg N
Some facies of the Random Formation: Synaresis cracks in mudstones, with overlying white cross-bedded tidal sandstones
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	Chapel Island Fm (conformably, Burin Peninsula) Crown Hill Formation (conformably, Random Island)[5]
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 near-shore rock unit dating to the early Cambrian period, dominated by tidal quartz arenites, 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 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]

Sedimentology

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 speak to an intertidal setting.^[5]

Spatial distribution

It is widespread throughout Avalonia - Canada, and Boston, unlike its underlying units. It was deposited in intertidal mud flats / subtidal setting, then (later) near the bottom of an open marine shelf.^[11] The top of the unit intergrades with the overlying strata^[10]

Stratigraphy

Synaresis cracks in the Random Formation, Trinity Bay, Newfoundland

Some maps^[1] refer it to the Adeyton group, but strictly it's below this.^[5]

It overlies the Chapel Island formation, which contains the Precambrian/Cambrian boundary.^[12]

The unit is often strongly folded.^[5]

Age

Megaripples in the quartz arenites of the Random Formation

Difficult to date; Some authors slip it in before the Cambrian, by virtue of its lying unconformably underbneath tbe Bonavista formation and conformably above Ediacaran strata.^[2]

It does sit (conformably?) above the Chapel Island formation on the Burin peninsula, meaning that at least there its lower boundary is in the Lower Cambrian; and places inarticulate brachiopods have been found in its upper strata, dating these to the late early or even early mid Cambrian.^[10]

That said, it is likely that the unit is diachronous, so it may go back into the Ediacaran further east.^[10]

On the other hand, the Stage 2 - Stage 3 boundary (Tommotian/Atdabanian) has been supposed to lie within the Random Formation, or possibly in the overlying Bonavista Formation; the Smith Point Formation above that hosts the first trilobites.^[13]

Palaeontology

The triolobite-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]

References