Chert is a hard, dense, cryptocrystalline sedimentary rock, essentially composed of tiny quartz crystals, that is, less than about 30 [micro]m in diameter, and may occur as nodular concretions and less commonly as layered formations (bedded chert
Trace fossils of a Middle Ordovician pelagic deep-ocean bedded chert
in southeastern Australia.
Outcrops of the latter formation on land contain nodular and bedded chert
mined by Archaic Indians.
It is characterized by thick basaltic rocks with bedded chert
, micritic limestone and hemi- pelagic mudstone.
At present, only 26 taxonomic occurrences of microfossils older than [approximately]2200 Ma meet these criteria: eight from the [approximately]2550-Ma-old Transvaal Supergroup of South Africa (15, 16, 17); two from the [approximately]2750-Ma-old Fortescuse Group of Western Australia (14); three from the [approximately]3400-Ma-old Onverwacht Group of the eastern Transvaal, South Africa (18, 19); and 13 from [approximately]3465-Ma-old units of the Warrawoona Group of Western Australia-two from the Towers Formation (20) and 11 from bedded cherts
of the Apex Basalt (4, 5).
Hydrogen and oxygen isotope ratios in nodular and bedded cherts
. Geochimica et Cosmochimica Acta, 40, pp.