Are Some Tillites Impact-Related Debris-Flow Deposits?

A number of ancient glacial deposits, or tillites, are currently interpreted as originating by subaqueous debris-flow processes at glacial termini. Problems in identifying the glacial origin of such deposits can arise because some criteria commonly used for establishing glaciation, especially outsize clasts interpreted as dropstones from icebergs, can apparently be produced by nonglacial debris flows and debris falls. Could some of these debris-flow diamictite deposits be related to large-body impacts, especially those that occurred at times other than established glacial periods? Debris-flow ejecta of known impacts seem to be marked by mildly shocked clasts showing evidence of plastic behavior with brittle failure, commonly resulting in multiple, partially displaced fractures, grading into crushed and brecciated clasts. These deformation features entail brief periods of high confining pressures, in accord with a hypervelocity-impact origin. I report here several debris-flow diamictites, currently interpreted as tillites, that exhibit these characteristic deformed stones.