DETRITAL SHOCKED MINERALS: EVIDENCE OF ANCIENT IMPACTS IN THE SEDIMENTARY RECORD

Crater records from the inner solar system demonstrate that large meteorite impacts occurred for several hundred million years after accretion of planets (e.g., the Late Heavy Bombardment, LHB). One estimate indicates that on Earth ~900 structures >200 km in diameter and ~3000 structures >100 km formed by 3.9 Ga, however no early craters have been identified; erosion and/or burial have destroyed these structures. However, shocked minerals eroded from these craters may be preserved; Hadean detrital zircons that pre-date the LHB have been identified in younger sedimentary rocks. This raises the possibility that shocked minerals eroded from impact basins on the early Earth may reside in younger sediments.

We have documented detrital shocked minerals (zircon, quartz, and monazite) in diverse sedimentary environments from the two oldest and largest impact basins known: the 2.02 Ga Vredefort Dome (South Africa) and the 1.85 Ga Sudbury basin (Canada). At Vredefort shocked minerals occur in proximal and distal (>750 km) modern fluvial sediments, and also in ancient (Miocene) fluvial sediments (river terraces) at distances of >600 km. At Sudbury they occur in proximal modern sediments, and also in Holocene glacio-fluvial deposits, such as eskers and glacial outwash deltas.

Petrographic (TL, RL) and SEM methods (SE, BSE, CL, EBSD) are used to document shock microstructures on external surfaces and polished interiors. Shock microstructures in detrital zircons include planar fractures (PFs) in up to 8 orientations (Erickson et al., this volume), granular texture, and µm-scale deformation twins (detectable by EBSD). Detrital quartz preserves decorated planar microstructures visible in TL that are bright in CL images. Detrital monazite preserves PFs in up to 4 orientations, and also granular texture. At Vredefort, impact basin provenance of detrital zircons was confirmed using U-Th-Pb dating on grains with PFs to determine basement ages, and grains with granular texture to determine impact age. Analysis of cross-cutting relationships among microstructures allows additional constraints to be placed on the origin of grains from different locations within an impact basin. Shocked detritus from impact basins of any age large enough to form shock microstructures in these minerals will be preserved in the sedimentary record.