Lower Proterozoic Stromatolites from Michigan

Massive Girvanella Stromatolites

Geologic Time: Lower Proterozoic (2.2 to 2.4 billion years old)

Fossil Site: Chocolay Group, Chocolay Hills, Northern Michigan

Stromatolite is the oldest of all fossils, and with much labor (cutting and polishing), it is most beautiful. The banding that commonly appears in stromatolite is a record of the growth patterns of colonies of microorganisms, principally photosynthetic prokaryotes (e.g, cyanobacteria). The colors that are often expressed are the result of the interaction of biological and sedimentary processes, together with subsequent chemistry and mineral exchange. Note the contrast in the polished and unpolished sides in the first two pictures.

Here are shown a number of stromatolites from the Chocolay Group in an area know as the Chocolay Hills located southeast of Ishpeming, Michigan. They all date to the lower Proterozoic some 2.2 to 2.4 billion years old. These stromatolitic carbonates have been hand-polished to a mirror finish on one side to bring out the vivid colors and layer pattering.

Also see: Stromatolites Across Geologic Time

The specimen below exhibits a rare structure for stromatolite, the so-called "Collenia" form genus comprising many small dome structures that has a pattern rather resembling a topological maps.

Collenia form genus close-up:

The unsual, rare and aesthetic characteristic of this example is a significant content of Rhodochrosite (the red areas). Rhodochrosite (MnCO3, Manganese Carbonate - whose name means rose-colored) is a very attractive mineral with an absolutely one-of-a-kind, beautiful color that is a gemstone in crystal form, and otherwise used in various jewelry applications. Although it can be an ore of manganese, it is its ornamental and display specimen qualities that make it a very popular mineral.

Stromatolites with Rhodochrosite

Another specimen with wavy lamination.

Also see: Stromatolites Across Geologic Time