Precambrian Spherule Layers: Unique Archives of Both Asteroid Impact and Early Earth History

Bruce M. Simonson

Precambrian spherule layers are a largely untapped source of information about impacts and early Earth history. They form where ejecta fall and get buried in low energy environments, thereby outlasting the craters that spawned them. Spherule layers always occur in stratigraphic context, so they provide good baselines to assess environmental change (or lack thereof) before and after major impacts. Although they have been replaced wholesale during diagenesis, many spherules retain textures that offer clues to their original composition, which is largely inherited from target rocks. Precambrian spherules appear to be more basaltic on average than Phanerozoic ones, perhaps reflecting a secular increase in the area of granitic continental crust through time. As the only surviving record of terrestrial impacts prior to ca. 2 Ga, spherule layers may shed light on the flux and composition of early Earth impactors, but only if more can be identified. Ultimately, they might serve as time planes with a precision on the order of days for correlating stratigraphic successions that are billions of years old on different continents.

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