SEDIMENTOLOGY AND PETROGRAPHY OF 2.2 GA RANDVILLE DOLOMITE STROMATOLITES, UPPER PENINSULA OF MICHIGAN

The Lomagundi-Jatuli Event (LJE) is ~2.2 billion years old (Ga) and is a time of mass change in the global carbon cycle, as most carbonates from this time have enriched carbon isotopes (+5-10 ‰). The exact causes of the LJE remain uncertain, with various hypotheses involving shifts in global oxidation, or changes in local environments. The 2.2 Ga Randville Dolomite in Michigan’s Upper Peninsula was deposited during the LJE, but contains less enriched carbon isotopes (0-3‰) compared to more well-studied neighboring deposits such as the Kona Dolomite. One possible explanation for local differences in LJE isotopes are different depositional environments. Heavier isotopes are often concentrated in restricted, evaporitic basins such as the Kona Dolomite, while less-restricted shorelines typically record carbon isotopes in global seawater. To address this question, samples of Randville Dolomite were collected from Iron Mountain, Michigan. In the field, we examined sedimentology to interpret ancient environments. In the lab, we examined microscopic textures and performed isotopic analyses. In addition, chemistry data comparing layers within stromatolites can track environmental changes over short periods of time. Despite a slightly higher metamorphic grade, Randville Dolomite preservation is comparable to the Kona Dolomite, and any differences in chemistry between the two are likely primary. Within the Randville Dolomite, there are traces of cross-beds, conglomerates, and a variety of stromatolites including conical, domal, nodular, and inclined. With one possible exception, there are no instances of evaporative minerals, unlike the Kona Dolomite. These results indicate a shallow marine environment with slight variations over time, but little or no restriction as observed in many other LJE environments. The Randville Dolomite provides a unique look into the LJE, potentially showing that heavier carbon isotopes were focused in more restricted basins.