IMPACT SPHERULE LAYERS IN EARLY PRECAMBRIAN SUCCESSIONS

Eleven or more layers rich in sand-size spherules of former silicate melt are known from early Precambrian successions. The spherules have distinctive textures and can be reliably distinguished from "normal" spheroidal grains such as ooids and accretionary lapilli using a hand lens. Based on geochemical anomalies and similarities with Phanerozoic impact spherule layers, they are clearly ejecta from impacts by large extraterrestrial bodies. All but one of the known layers occur in South Africa or Western Australia and are clustered either in the early Archean or in the late Archean to earliest Paleoproterozoic. The layers in the younger cluster all occur in fine-grained, deeper water successions as coarse-grained "event" beds deposited under anomalously high-energy conditions. Each layer is preserved over tens of thousands of square kilometers and serves as an excellent time-stratigraphic marker. Impact ejecta layers differ from fallout tuffs in that they can blanket the entire surface of the planet; in fact, two late Archean spherule layers in South Africa and Western Australia may be products of a single impact formed within hours of one another. Confirmation of such intercontinental correlations by detailed mineralogical and geochemical studies would permit global time-stratigraphic correlation among Precambrian successions with unprecedented precision. In both early Precambrian clusters, one spherule layer was deposited roughly every 70 million years. This suggests over two dozen impact spherule layers have yet to be identified in the Proterozoic. While the use of impact spherule layers for stratigraphic correlation among Precambrian successions has great potential, wide application will be hindered by the fact that they are unlikely to survive intact in higher energy environments. They are also likely to be less useful in Phanerozoic successions because of the wide distribution of burrowers and perhaps faster dissolution in silica-poor seawater.