Modern-day stromatolites primarily form in carbonate-dominated environments through binding particulate matter to the sticky extracellular polysaccharides secreted by the cyanobacteria. Evidence of the operation of this process in constructing stromatolites extends back into the Phanerozoic. However, Precambrian stromatolites do not contain abundant silt- to sand-sized particles characteristic of this process. Instead they are primarily composed of material that has precipitated from seawater. This is commonly a carbonate phase, though chert stromatolites are common in some Proterozoic and Archean sequences. In recent years controversy has developed in the literature as to whether the chert stromatolites in the Gunflint Formation and similar rock units represent silicification of carbonate or are primary silica precipitates (see: Knoll and Simonson, 1981; Simonson, 1985; Knoll, 1985; Siever,1992; Maliva et al., 2005). Some of this work utilized outcroppings of the Gunflint Formation that are highly diagenically altered, and thus ambiguous concerning primary mineralogy. Roadwork on Highway 590 near Kakabeka Falls exposed an excellent example of chert microbialites and stromatolites entombed in ankeritic grainstone with a sharp boundary between the two, providing an opportunity to add to the debate on the possible primary origin of the silica in chert stromatolites.
The 1.878+-1 Ga Gunflint Formation is a chemical sediment dominated shelf succession deposited on Archean basement. It records a number of transgressive-regressive cycles. Hofmann (1969) and Franklin et al. (1982) described stromatolites occurring at two different stratigraphic levels; at the base of the Gunflint growing on Archean units and the basal conglomerate and secondly, on a hard-ground in the middle of the Formation developed during regression. The basal biogenic sediment at Kakabeka Falls directly overlies Archean granodiorite and consists of a lower one meter thick, continuous chert microbialite. The millimeter-scale, crinkly layering is sub-horizontal, with organic carbon only giving the unit a dark colouration in limited places. A large stromatolitic head develops off the upper microbialite layers. It is the size of a beach ball, with a slightly narrower base. There appears to be a sharp contact between the microbialite-stromatolite and carbonate grainstone, which overlies the microbialite and abuts against the stromatolite. The layering in the stromatolite does not build on top of the lower grainstone laminae, indicating that the stromatolite was fully grown before the grainstone entered the environment.
Samples of all lithologies were investigated with a petrographic microscope, a field emission scanning electron microscope with an energy dispersive spectrometer, inductively coupled plasma atomic emission spectrometry and mass spectrometry.
The diagenetic history is more complex than it appears to the naked eye. The stromatolite contains abundant microquartz, megaquartz, chalcedony, and organic matter. The silica is rather clean and well developed with little to no carbonate inclusions or ghosts. Carbonate present in the stromatolite is seen replacing chalcedony fans. Distinct growth zones can be seen in the outer portions of the carbonate. Rarely, quartz crystals present in the stromatolite show carbonate overprinting, replacing them to varying degrees. Carbonate is visible replacing megaquartz grains and overprinting microquartz as very fine grained specular carbonate. Growth zoning is present in most of the carbonate grains. The carbonate grainstone unit overlying the stromatolite has been highly silicified. The quartz is texturally similar to the quartz in the stromatolite; microquartz and megaquartz are abundant with minor amounts of chalcedony. The megaquartz is seen in the intergranular spaces as space-filling cement. Higher magnification shows extinct carbonate grains that have been silicified with small amounts of carbonate remaining behind.
However, further from the contact with the stromatolite the carbonate grainstone unit is silicified to a much lower degree. Some of the carbonate grains have experienced some silicification, but the rock is mainly composed of carbonate with some organic material. Silica in the form of microquartz is forming within the carbonate grains. A closer observation reveals that the quartz developing in the carbonate grains is itself being over-printed by carbonate growth. The carbonate grains have growth zoning in their outer portions. Rare earth element plots for the ankerite and chert are generally similar, though this probably reflects both inheriting the seawater pattern. The most striking difference between the two lithologies is the Ba/Sr ratio. The stromatolitic cherts have one consistent ratio, whereas the carbonates and known replacement cherts from the area have a different consistent ratio. All of the above is consistent with the microbial and stromatolitic cherts being a primary precipitate, but it is not conclusive. If the silica is primary new models will need to be developed to explain how Precambrian cherty stromatolites form.