Macro- and micro-structure comparison of ancient and modern iron-rich stromatolites

The Gunflint and Biwabik Formations comprise the middle units of the Paleoproterozoic Animikie Group that crop out near the northwestern shore of Lake Superior. Stromatolitic units were investigated and sampled at 9 outcrop sites, 4 mine sites and in 12 drill-cores. Controversy exists as to whether the stromatolitic units that are now composed of chert were deposited as a silica gel or represent carbonate replaced by chert. There is also debate in the literature as to whether the iron oxides that coat many microfossils are primary or deposited during later diagenesis. Younger examples of siliceous stromatolites are very rare and iron hydroxides coating bacteria are only known from limited modern environments. Thus, understanding the primary composition of the Gunflint microbialites will provide information on ocean chemistry during this interval.
\nThree stromatolitic members have previously been described within these formations: the Lower Stromatolitic Member containing stromatolites that grow directly on the peneplained Archean basement or on the conglomerate which forms the base of the Animikie Group, a Middle Member 45 to 50 meters above the base, and the Stromatolitic Limestone Member at the top of the formation directly beneath the Sudbury impact layer. Based on this work, a new Upper Stromatolitic Member 50 meters below the Limestone Member in the Gunflint Formation was recognized. This member contains abundant iron-silica-carbonate-manganese oncolites, and is correlative to stromatolitic units described from the Biwabik. The oncolites represent a primary-to-very-early authigenic, precipitated mineral assemblage. The accretion of lacustrine ferromanganese nodules at the sediment-water redox boundary provides a modern analogue for the development of the oncolites described in this work.
\nThe presence of exceptionally well-preserved, hematite coated microfossils, encased in chert in the relatively unmetamorphosed Middle Stromatolitic Member together with positive cerium and europium anomalies indicates that these sediments were deposited at a redox boundary in ocean waters near saturation with respect to silica and iron. Conversely, precipitates containing carbonaceous filaments in the Lower Stromatolitic Member contain a distinct negative cerium anomaly, indicating deposition in an oxidized environment.
\nAlternating manganese- and iron-rich laminae within the Upper Stromatolitic Member indicate a fluctuating redoxcline and oxygen levels in the shallow subtidal to peritidal environments within the Animikie Basin. The fluctuation reduces the Fe:Mn ratio, allowing for the less readily oxidized manganese to precipitate. The presence of dehydration cracks which cross-cut original ooid and stromatolitic laminae indicate that deposition of the silica and iron most probably occurred as amorphous gels of opal-A and iron hydroxides.
\nThis study strongly indicates the Gunflint and Biwabik stromatolites were originally siliceous and formed by a different precipitation mechanism than that of Proterozoic carbonate stromatolites or modern agglutinated forms.

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