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Review of the mineralogy of the Cretaceous-Tertiary boundary clay: evidence supporting a major extraterrestrial catastrophic event

¹ Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18002 Granada, Spain, ² Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, 18002 Granada, Spain, and ³ Sédimentologie et Géodynamique, SN5, Université de Lille I, 59655 Villeneuve d’Ascq cedex, France

^* E-mail: mortega{at}ugr.es

(Received 17 May 2002; revised 3 September 2002)

The proposed impact event at the end of the Cretaceous resulted in mass extinctions and subsequently significant variations in the geochemical and mineralogical composition of the sediments marking the K/T boundary. The impact-generated material derived from target rocks produced the ejecta layer deposits around Chicxulub crater, which were subsequently diagenetically altered to mainly smectite in marine sections and to kaolinite in continental sections. The fireball layer represents the cosmic dust dispersed and deposited globally and contains smectite derived from the alteration of microkrystites and the finest fraction. The lowermost Danian clay layer, recognized in marine sections, resulted from the sudden decrease in ocean productivity and represents a reduced sedimentation deposit. Its clay mineral associations depend on local environmental conditions and diagenetic processes. Overall, the diagenetic alteration of the boundary materials resulted in a significant modification of original signatures. The composition of the clay mineral phases can, however, still be evidence of the nature of the precursor materials providing evidence for an extraterrestrial impact event.

KEYWORDS: Chicxulub, K/T, clay minerals, ejecta layer, fireball layer, clay boundary layer, impact-glasses, microkrystites, diagenesis, DSDP, ODP

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