Geochemical Search for Impact Signatures in Possible Impact-generated Units Associated with the Jurassic-Cretaceous Boundary in southern England and northern France

Abstract

Understanding of the extinction event at the Jurassic-Cretaceous (J/K) boundary is hampered by problems of faunal isolation and lack of comparable magneto- and biostratigraphy between the Boreal and Tethyan faunal realms. Three impact craters (Gosses Bluff, Mjolnir, and Morokweng) are known to have formed in the late Jurassic or early Cretaceous. The question of whether these impacts damaged the environment at the time of the J/K boundary sufficiently to affect extinction patterns remains unresolved but the impacts may also provide a tool to help resolve J/K stratigraphy. If an ejecta deposit from at least one of the craters can be located in both faunal realms, this would provide an independent time marker with which to compare the different Boreal and Tethyan biostratigraphies. This study reports on the search for geochemical markers of impact, notably the platinum-group elements (PGE), in two limestone-mudstone sections at Pointe aux Oises, near Boulogne sur Mer in France, and at Durlston Bay near Swanage in England that have been suggested to include possible impact layers. Both sections show zones with PGE enrichment, where individual PGE behave remarkably coherently, and which are invariably coincident with increased concentrations of clay. The highest concentrations of Ir and Ru (up to 0.08 ppb) are not particularly elevated when compared with available data for early Cretaceous clays in Southeast England not linked with any impact event, and the chondrite-normalized PGE patterns of these samples are consistently fractionated relative to chondrite. The PGE in these rocks are probably terrestrial and do not reveal any evidence for impact associated with the two layers. Nevertheless, the data provide one of the most comprehensive sets of baseline data for the PGE in such rocks and may be of value in helping to resolve impact signatures from natural PGE background in future studies.