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The birth of Jurassic: from the sea or from the sky? Evidences from the Northern Apennines (Italy)

Fazzuoli M., Orti L.

Volumina Jurassica

2006

Vol. 4, no. 1

279-279

At the Triassic/Jurassic (T/J) boundary one of the most important mass extinctions occurred. The inferred causes for this extinction are numerous: eustatic variations; climatic changes; extraterrestrial impact events; strong volcanic episodes etc. One of the diagnostic evidences of a bolide impact in the stratigraphic records is the occurrence of PDF structures in quartz grains ("shocked quartz"). According to Bice et al. (1992), shocked metamorphosed quartz grains has been found in the Il Fiume Gorge, near Corfino (Tuscany) within three closely spaced shaly beds, at the boundary between the Rhaetic (Upper Norian) Calcare a Rhaetavicula and the inferred Lower Jurassic (Hettangian) Calcare Massiccio, suggesting "that multiple impacts occurred in latest Triassic, one of which coincided with a locally, and perhaps globally, significant extinction at the T/J boundary". According to our observations at the same stratigraphic section cited by Bice et al. (1992) the following results can be summarized: - The upper part of the Calcare a Rhaetavicula consists of dark grey bioclastic wackestone beds alternating with bioclastic packstone beds (storm layers) and with marly shales. - The top interval of this formation, 2 m thick, consists of 8-30 cm thick beds of dark grey oolitic grainstone, interlayered with three cm thick levels of marls; another 30 cm thick marly level tops this transition interval. In these marly levels the shocked quartz grains were found. - Above a sharp boundary, the overlying lithologies belong to the Calcare Massiccio (massive limestone). The lower interval, 90 cm thick, is slightly bedded and it is represented by barren mudstone; upwards the rock is massive, cliff forming, and also consists of barren mudstone, more or less dolomitized, up to about 30 m, where bioclastic and intraclastic grainstone/packstones occur. In this level abundant Rhaetic benthic foraminifers are present, such as Gandinella apenninica and Aulotortus sinuosus. - At the regional scale, a Rhaetic fossil assemblage (with Triasina hantkeni etc.) has been found also in other stratigraphic sections of the Calcare Massiccio in the same area (Fazzuoli et al. 1988). In conclusion, in the upper part of the Calcare a Rhaetavicula a lowering of the sea level occurred and the depositional environment evolved from a mid ramp to a high energy inner ramp and then to a lagoon, typical of the Calcare Massiccio. During this lowstand, according to the occurrence of shocked quartz, one or more impact events took place. However, considering the occurrence of Triassic fossils well above the levels with the shocked quartz, it is possible to affirm that the acclaimed extra-terrestrial event occurred within the Rhaetic, not at the Triassic/Jurassic boundary.