TILLITES, DIAMICTITES, AND BALLISTIC EJECTA OF LARGE IMPACTS

The primary evidence for the temporal and spatial distribution of glac iation prior to the Cenozoic is the occurrence of tillites. These are diamictites (poorly sorted mixtures of very coarse-grained to fine-gra ined sediments), and associated deposits, which have features consider ed diagnostic of glacial action. The discovery that many diamictite-be aring sequences were the products of debris flows and related mass flo ws, with contemporaneous rainout of coarse sediment, has led to the re interpretation of many tillites as glaciomarine debris-flow deposits. Fallout of ballistic ejecta from the impact of large asteroids or come ts can also create large-scale debris flows with simultaneous rainout of debris. Recent calculations of the predicted volume and distributio n of impact-generated diamictites suggest that they should be common i n the geologic record. Moreover, ballistic debris-flow deposits can ap parently exhibit a number of features considered characteristic of gla cial deposits (e.g., striated clasts, eroded and striated pavements). The possible confusion between glacial and impact diamictites-a few di amictite deposits formerly interpreted as glacial or questionably glac ial in origin are already known to be impact ejecta-suggests that a re view of the properties of diamictites of various origins is required. Evidence of shock deformation in diamictites is the most important cri terion for determining an impact origin. It is possible that some diam ictites identified as glaciogenic may actually be ejecta of large impa cts, which may help to explain climatic anomalies, such as Lower Prote rozoic tillites at a time of predicted global warmth, and the low-lati tude distribution of some Upper Proterozoic glacial deposits.