Strategies for finding the record of Early Precambrian impact events (in The Precambrian Earth; tempos and events )

Developments in Precambrian Geology( 2004), 12:45-62

Index Terms/Descriptors: Africa; Archean; basalts; Canada; chemical composition; chemostratigraphy; Chesapeake Bay impact structure; Chicxulub Crater; craters; depositional environment; diamond; Eastern Canada; exploration; geochemical anomalies; geochemistry; geomorphology; geophysical methods; igneous rocks; impact craters; impact features; lithostratigraphy; magnetic susceptibility; mantle; mantle plumes; marine environment; metamorphic rocks; meteor craters; mid-ocean ridge basalts; Morokweng; mylonites; native elements; Ontario; Paleoarchean; paleoenvironment; paleomagnetism; Precambrian; pseudotachylite; research; sedimentary rocks; sedimentary structures; sedimentation; SEM data; South Africa; Southern Africa; spherules; spherulites; stratigraphic units; submarine environment; Sudbury Structure; surveys; tektites; terrestrial environment; textures; volcanic rocks; volcanism

Latitude & LongitudeS35°00'00'' - S22°00'00'' and E16°00'00'' - E33°00'00''N46°25'00'' - N46°50'00'' and W81°40'00'' - W80°45'00''

Abstract

Impact debris layers should be common in Archean sediments. We estimate that there should be at least 350 global layers that range in age from 2.5 to 3.8 Ga. Most of these layers would be from impacts into the Archean abyssal ocean. We have studied Phanerozoic abyssal impact ejecta in deep-sea sediment layers in order to identify their distinguishing characteristics. These layers usually contain: 1) platy magnetite, 2) microtektites, and 3) microkrystites. The most common high-pressure mineral from abyssal impacts would be impact diamond rather than shocked quartz or shocked zircon. Distal abyssal impact layers often have sharp contacts, internal laminations, dark colors, and high magnetic susceptibility. The top ten centimeters of unusual sedimentary layers with high magnetic susceptibility values should be examined for impact spherules, exotic minerals, and other debris that characterize Phanerozoic sediment layers resulting from abyssal impacts.