Magadi-type chert recorded in East Greenland: evidence for a Mid Triassic marine ingression

Magadi-type chert has been discovered in the predominantly continental Mid to Late Triassic succession of the Jameson Land Basin in East Greenland. Chert nodules, 1 to 10 cm in diameter, and more laterally continuous beds, up to 2 cm thick, are recognised which occur in a single interval within the regionally correlatable Gråklint Beds. The nodules commonly display cracked margins and a calcite rind, and are composed of micro-crystalline quartz which displays a patchy rectilinear extinction pattern. Such features are characteristic of Magadi-type chert which forms after Magadiite (NaSi₇(OH)₃ . 3H₂O). The chert occurs in the south east of the basin where the Gråklint Beds consist of up to 10 m of organic-rich calcareous mudstones which are divided by a distinctive coarse-grained bed 5-30 cm thick. The chert directly overlies this coarse grained unit which contains rounded quartz grains, bioclastic material, microbialite fragments and limestone clasts and can be correlated over 13 km. In thin section a tripartite, seasonal, lamination can be identified within the mudstones which is characteristic of lacustrine deposition. A low diversity macro-palaeontological assemblage also favours a largely lacustrine setting.

Two possible mechanisms are recognised for the formation of Magadiite; intensive evaporation of alkaline brines with high silica contents, and the mixing of such brines with more dilute acid waters, both of which can lead to supersaturation and therefore precipitation of silica. The identification of a single foram within the chert nodules suggests a marine influence and the underlying coarse grained bed is therefore interpreted as reflecting a high energy marine flooding event which resulted in the introduction of evolved brines and consequently the precipitation of Magadiite. An intra-basinal high is recognised to the NW, across which the Gråklint Beds thicken and contain an intermittently developed restricted marine macro-fauna (abundant modiolid bivalves and locally common spirorbid worms) suggesting that the flooding event occurred from the north.

The Gråklint Beds provide the southernmost record of marine influence from the Boreal Ocean during the Mid to Late Triassic.