FORMATION OF ‘ABIOGENIC’ ANIMIKIE BASIN STROMATOLITES

Stromatolites are typically thought of as laminated, lithified structures formed through sediment trapping and binding or in situ biogenic precipitation by cyanobacteria- dominated microbial ecosystems. In many stromatolites from Mesoproterozoic and older successions non-biogenic sea floor precipitation is the predominant factor in the accretion of laminations. The abundance of sea floor precipitates has been linked with numerous factors, especially high carbonate concentrations in the early pre-Cambrian oceans. There are two distinct types of stromatolites in the Animikie basin sequence of Minnesota and Ontario: those interpreted as having been formed mainly by biogenic processes (Gunflint Formation, Schreiber locality) and those interpreted as abiogenic sinter deposits (Gunflint Formation, Mink Mountain. and Biwabik Iron Formation., Mesabi Range). However, the morphology of the ‘abiogenic' stromatolites, as well as their broad distribution and stratigraphic relationships make a hot spring depositional setting highly unlikely. The preserved stromatolites succession in the Cliffs-Erie mine (Aurora, MN) shows a progression of stromatolite macrostructures that are interpreted as having formed in laterally migrating ocean. These ‘abiotic' stromatolites are characterized by discrete, alternating thin dark laminations (mean 8 µm), and thicker lighter bands (mean 28 µm). The dark laminations consist primarily of iron oxides and silica cement and often are thinner than the diameter of cyanobacterial Gunflint microfossils. The lighter bands are composed of replacement chalcedony with ghost botryoidal fans interpreted as remnants of an original calcite or aragonite precipitate structure. We interpret the laminations as having formed when microbial mats of iron bacteria lowered local iron concentrations. Because iron is a strong calcite inhibitor, a decrease in iron concentration could have induced formation of the calcite precipitates. The microbial mats are also inferred to have played a role the formation of the stromatolites by colonizing sedimentation surfaces, providing a stabilizing effect. Our reinterpretation suggests that stromatolites formed under a strong biogenic influence and in a shallow marine, pertidal environment.