Proterozoic Impact Spherule/Ejecta Layers Younger Than 2.4 Ga

Abstract

The Precambrian has been more difficult to subdivide stratigraphically than the Phanerozoic. It extends from the beginning of the Solar System at 4566 Ma (Allègre et al. 1995) to the start of the Cambrian Period around 542 Ma (Grotzinger et al. 1995). This represents ~88 % of Earth’s history so Precambrian subdivisions generally cover vast stretches of time. Subdivision of the Phanerozoic Eon was also facilitated by the relatively rapid evolution of many different groups of organisms that produced skeletal remains (shells, teeth, etc.). Even though life appeared surprisingly early in Earth’s history (Wacey 2009), the Precambrian was, until almost the very end, a time of unicellular organisms that did not produce skeletal remains (Knoll 2003). Macroscopic body fossils appeared as early as 2 Ga (Han and Runnegar 1992) and microbial communities formed macroscopic structures known as stromatolites throughout most of the Precambrian, but neither one rivals skeletal fossils as a basis for detailed stratigraphic subdivision. Moreover, sedimentary rocks and the information they contain are increasingly likely to be affected by tectonic deformation or eroded away altogether as they get progressively older. Therefore distal ejecta layers should become progressively harder to recognize back through time.