The North Helvetic Flysch between Pte. Des Martinets and
Croix de Javerne, Canton Waadt, Western
Switzerland1
Abstract
The research area comprises three of the main tectonic
units of the Swiss Alps on the eastern flank of the
Rhône-valley between Martigny and Monthey; 1. The
Aiguilles-Rouges-Gneisses and their autochthonous sedimentary
cover as part of
the Autochthone Massives in the adjacent. 2. The overturned
limb of the recumbently isoclinally folded Morcles Nappe as
part of the Helvetic Nappes in the hanging wall.
3.
Intercalated between these two units North Helvetic
Flysch crops out.
The rocks of the Aiguilles-Rouges-Massiv (Triassic to
Cretaceous) and the Morcles Nappe (Cretaceous and Tertiary)
were mapped and described according to their lithological
characteristics.
(map and legend)
The Tertiary North Helvetic Flysch was investigated using
sedimentological, tectonical and petrographical techniques. It
could be divided in two parts on sedimentological grounds:
In the lower area, the Flysch is composed of medium-bedded to
thick-bedded turbidites, which mainly consist of medium-grained
to very coarse-grained sands. These turbidites are
interpreted as lower-fan deposits according to Walker (1978).
Because of their petrographical and sedimentological
characteristics, they are probably part of the Grès du
Val d' Illiez Member.
The upper part of the Flysch consists of alternating laminated
to at most middle-bedded marls and fine-grained sandstones.
These sediments are interpreted as turbidites deposited at the
boundary of the lower fan / basin plain according to Walker
(1978) or as lobe-fringe facies according to Mutti (1977). In
a few areas, this part of the Flysch is interpreted as
contourites because of its characteristic sedimentology.
The upper part of the Flysch lies conformably on the Morcles
limestones and is therefore assigned to the Morcles Nappe
("Morcles-Flysch"). Consequently, the contourites also
originated in the same depositional environment as the rocks
of the Morcles Nappe ("Morcles-Basin"). The original position
of the depocenter has been found not very far to the South
between the Aiguilles-Rouges-Massiv and the
Mont-Blanc-Massiv.
Two models for the emplacement of the Flysch over the
Aiguilles-Rouges-Massiv are proposed:
1. The Grès du Val d'Illiez is interpreted as
autochthonous Flysch, which has been deposited in a
northwestern direction at an early stage of the Molasse Basin.
The middle part of the Flysch represents the thrustzone of the
Morcles Nappe. The upper part of the Flysch ("Morcles-Flysch")
was placed on top of the Grès du Val d'Illiez during
the overthrusting.
2. The Morcles-Flysch is firstly deposited in the Morcles-Basin
followed by the deposition of the Grès du Val
d'Illiez. Later, both are overthrust on top of the
Aiguilles-Rouges-Massiv together. In this case, the whole Flysch
sequence corresponds to the Grès du Val d'Illiez
Formation (lying inverse); the autochthonous Flysch of the
Aiguilles-Rouges-Massiv is missing.
In both models, the cause and the direction of the bottom
currents, which formed the contourites in the relatively
restricted Morcles-Basin stay unclarified.
References:
MUTTI, E., 1977. Distinctive thin-bedded turbidite facies
and related depositional
environments in the Eocene Hecho Group (South-central
Pyrenees, Spain).
Sedimentology, 24: 107-131.
WALKER, R.G., 1978. Deep-water sandstone facies and ancient
submarine fans:
Models for exploration for stratigraphic traps. AAPG Bull.,
62/6: 932-966.
1PREUSCHE, Ch., 1994. Der Nordhelvetische Flysch
zwischen Pte. des Martinets und Croix de Javerne,
Kanton Waadt, Westschweiz. Unveröff. Dipl.-Arb., Geol.
Inst. Univ. Freiburg,
94 S.
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