Silicification in the Belt Supergroup (Mesoproterozoic), Glacier National Park, Montana, USA



Nodular cherts can provide a window on the original sediment composition, diagenetic history and biota of their host rock because of their low susceptibility to further diagenetic alteration. The majority of Phanerozoic cherts formed by the intraformational redistribution of biogenic silica, particularly siliceous sponge spicules, radiolarian tests and diatom frustules. In the absence of a biogenic silica source, Precambrian cherts necessarily had to have had a different origin than Phanerozoic cherts. The Mesoproterozoic Belt Supergroup in Glacier National Park contains a variety of chert types, including silicified oolites and stromatolites, which have similar microtextures and paragenesis to Phanerozoic cherts, despite their different origins. Much of the silicification in the Belt Supergroup occurred after the onset of intergranular compaction, but before the main episode of dolomitization. The Belt Supergroup cherts probably had an opal-CT precursor, in the same manner as many Phanerozoic cherts. Although it is likely that Precambrian seas had higher silica concentrations than at present because of the absence of silica-secreting organisms, no evidence was observed that would suggest that high dissolved silica concentrations in the Belt sea had a significant widespread effect on silicification. The rarity of microfossils in Belt Supergroup cherts indicates that early silicification, if it occurred, was exceptional and restricted to localized environments. The similarity of microtextures in cherts of different ages is evidence that the silicification process is largely controlled by host carbonate composition and dissolved silica concentration during diagenesis, regardless of the source of silica.