The Cretaceous of the Breccia nappe (French and Swiss Prealps). New data and an attempt towards a stratigraphic and palaeogeographic synthesis

Authors
Citation
S. Dall'Agnolo, The Cretaceous of the Breccia nappe (French and Swiss Prealps). New data and an attempt towards a stratigraphic and palaeogeographic synthesis, ECLOG GEOL, 93(2), 2000, pp. 157-174
Citations number
47
Language
FRANCESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
ECLOGAE GEOLOGICAE HELVETIAE
ISSN journal
0012-9402 → ACNP
Volume
93
Issue
2
Year of publication
2000
Pages
157 - 174
Database
ISI
SICI code
0012-9402(2000)93:2<157:TCOTBN>2.0.ZU;2-8
Abstract
Two new formations are defined in the lower and middle Cretaceous sediments of the Breccia nappe within the Swiss and French Prealps, the Bonave Forma tion and the Joux Verte Formation. The Bonave Formation replaces the historical and informal "Calcaires a Sile xites" which were considered to constitute a member within the Upper Brecci a Formation. The definition of the Bonave Formation is based on the argumen t that sufficient lithologic change exists within the Upper Breccia Formati on to warrant the definition of a new formation. According to calpionellids , the Bonave Formation ranges from the Late Tithonian to the Barremian and comprises a fine-grained lower part with abundant nodular chert and a detri tal upper part with less abundant chert dominated by calciturbidite deposit s topped by breccias. The Bonave Formation exists in its entirety only in t he Chablais region; in the Romandes region only small remnants may be found . The study of the spatial extension and facies variations in the Chablais region suggests a ESE-WNW oriented basin which was fed from the north-west. The Joux Verte Formation replaces the "Serie a Quartzites" (also known as " Gault" or "Mesocretace"). The definition of the Joux Verte Formation is par tly based on the argument that in the formerly accepted "Serie a Quartzites ", contrary to what its name suggests, detrital quartz represents only a mi nor part of the series, most of the SiO2 being of diagenetic origin. The Jo ux Verte Formation exists only in the Chablais region and is absent in the Romandes region. It ranges from the Late Barremian to the Middle Turonian. The lithology consists mostly of black shales with intercalations of spongi al, radiolarian and foraminiferal cherts. Three major successions can be de fined, each beginning with thin laminated black shales followed by relative ly thick cherty limestones. Areal extension and facies variability suggest an ESE-WNW oriented basin with locally differential subsidence. The black s hale levels can be correlated within the basin and compared to coeval world -wide oceanic anoxic events.