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.