Silicified stromatolite boundstone with oolites from the Precambrian of Minnesota, USA. (field of view ~6.4 centimeters across)

Northern Minnesota’s Mesabi Iron Range has numerous iron mines that exploit the Biwabik Iron-Formation (upper Paleoproterozoic, ~1.878 billion years). The Biwabik contains numerous iron-rich lithologies. The most visually intriguing lithology is stromatolitic ferruginous chert (stromatolitic jasper) (a.k.a. “Mary Ellen Jasper”). Stromatolites also occur in other specific lithologies - this specimen is a silicified stromatolite boundstone. Oolites are common in the sediment infilling between the stromatolites. The light-colored, convex-upward, layered, columnal structures in the rock are the stromatolites, which are built up by mats of cyanobacteria living in (typically) very shallow marine settings. The layering of stromatolites is principally the result of tidally rhythmic deposition of sediments atop the cyanobacterial mats.

These stromatolites have been assigned to Collenia undosa.

What’s especially intriguing about “Mary Ellen Jasper” and other stromatolite rocks in the Biwabik Iron-Formation is the sinuosity of the stromatolite columns. This has been considered to represent tracking of the sun over many seasons. Cyanobacteria are photosynthetic and, like sunflowers, they “want to face the Sun”, wherever it is. Summer-winter seasonal changes in the sun's position in the sky will be expected to result in stromatolite sinuosity.

Precambrian sinuous stromatolites from elsewhere have been used to show that the number of days per year has changed through time. Earth used to be rotating much more quickly than at present. The duration of the year hasn’t changed through time, but the rotation rate has. The early Earth had more than 1000 days per year!

Stratigraphy: Biwabik Iron-Formation, Paleoproterozoic, ~1.878 Ga

Locality: near the Mary Ellen Mine, near Biwabik, central St. Louis County, northeastern Minnesota, USA

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