Stromatolites as Facies Indicators in Iron-Formation (Paleoproterozoic, Minnesota)

Stromatolites have been shown to be useful facies indicators in a variety of marine and lacustrine settings. These models are largely based on modern examples dominated by trapping-and-binding cyanobacterial ecosystems. Focus has been placed on the macrostructure as gross environmental indicator. However, pre-Cambrian stromatolites display a variety of mesostructures and microstructures and it is likely that differences in stromatolite morphologies at these finer scales also yield facies information.

Interpretations of stromatolites of the Paleoproterozoic Biwabik Iron-Formation (Mesabi Range, Minnesota) have varied, hindering refinement of models of iron-formation deposition and basin evolution. The stromatolites have been interpreted as abiogenic sinter deposits, based on microstructure, and tidal deposits, based on comparison with modern marine analogues. Thus, the Biwabik provides a good example of a deposit where the macrostructure and sedimentology is at odds with microstructural interpretations.

Our measurements of in situ biostromes in open-pit taconite mine cliffs, coupled with petrographic and electron microscopic analyses clearly demonstrate that the columnar, domal, and stratiform Biwabik stromatolites are shallow subtidal deposits. The microstructure of all the stromatolites, irrespective of macrostructure, is composed of micrometer-scale laminae of high inheritance. Wall structure is common and adjacent centimeter-scale columns are joined with bridging laminae. The stromatolite morphologies indicate biological control over formation, either by iron oxidizing or photosynthetic benthic ecosystems.

Previous studies recognized that the stratigraphy of the Biwabik is best explained by transgressive-regressive-transgressive cycles. Based on our reinterpretation, the stromatolites occur in several shallowing-upward parasequences developed on flooding surfaces—typical of younger, peritidal carbonate facies.

The pre-Cambrian has long been known to hold challenges for extending models based on modern marine stromatolites. Our examination of Mesabi Range stromatolites demonstrates that an actualistic approach with a focus on multiple scales of stromatolite morphology can enable more refined facies analysis of both marine and lacustrine settings.