An extensive pro-foreland basin was formed during continent-continent collision between Europe and an indenting microplate, Adria, in the Tertiary. This basin developed around the perimeter of the arcuate Alpine orogen as a flexural response to thrust loading of the European continental lithosphere (Fig. 2A).
Regional facies distributions indicate that the basin
deepened eastwards towards the orogen (Cavalier 1984).
The basin-fill succession is superposed upon a Mesozoic
passive-margin wedge that developed at the western margin
of the Ligurian Tethys ocean (Lemoine et al. 1986;
Tricart 1984).
Colored lines delineate westward
pinchouts of successively younger chronostratigraphic
units of the Nummulitic Limestone Formation. The study
area, the Champsaur region, is located at southwest
margin of the Pelvoux basement massif.
The basal limestones of the Nummulitic Limestone
Formation were deposited on an east-facing carbonate ramp
at the western, distal margin of a NNW-SSE-oriented
Eocene seaway that was bounded to the east by the Alpine
orogenic wedge.
During Lutetian through Priabonian time successively
younger shallow marine strata were deposited above the
basal unconformity as the formation is traced westwards
onto the European foreland (Fig. 2B). This large-scale
stratigraphic onlap of the pre-Tertiary substratum
indicates that the distal-margin shoreline backstepped
progressively westwards during Eocene transgression of
the foreland.
The backstepping Nummulitic transgression and the
deepening-up basin-fill succession are interpreted as a
depositional response to a wave of flexural subsidence
migrating across the foreland plate during advance of the
Alpine thrust load (Gupta 1994).
The study area, the Champsaur region of Les Hautes Alpes,
is located at the southwest margin of the Pelvoux
external basement massif (Fig. 2B).
Here, crystalline basement rocks of the Pelvoux massif
are carried in the hanging wall of a set of reverse
faults, which are erosionally truncated by the Nummulitic
Limestone Formation at the basal unconformity (Fig. 3A).
Movement on the faults predates the Priabonian Nummulitic
transgression (Lory 1894; Boussac 1912; Chambers 1992;
Ford 1996).
The faults emplace basement rocks upon Triassic and
Jurassic strata that have been folded into large,
asymmetric footwall synclines (Fig. 3B; Chambers 1992;
Ford 1996; Gidon and Vernet 1952; Debelmas et al. 1989).
The overturned upright limbs of the synclines are
truncated by the faults, which clearly indicates that the
synclines were folded prior to the fault cutting through
the shared limb of an anticline-syncline pair.
FIG. 3A Map of the Champsaur area showing sub-Nummulitic
subcrop stratigraphy and structural elements, and outcrop
of Nummulitic Limestone Formation. Structural elements
are derived with modifications from Debelmas et al.
(1980) and Chambers (1992).
The basal foreland basin unconformity, in the Champsaur
area, is distinctly angular and downcuts stratigraphy in
a broadly south-to-north direction (Fig. 3A). The
overlying Nummulitic Limestone Formation oversteps
Jurassic and Triassic strata onto crystalline basement
rocks of the Pelvoux external basement massif.
The geometrical relationship of the Nummulitic Limestone
Formation to underlying structural elements demonstrates
the existence of complex structural paleotopography along
the basal unconformity. The unconformity shows a marked
increase in gradient where it oversteps across pre-
Nummulitic reverse faults onto basement. The Nummulitic
limestone succession thins and onlaps against basement
subcrop in the hanging wall to the pre-Nummulitic
basement reverse faults, which form prominent paleo-fault
escarpments (Figs. 3, 4).
The pattern of onlap indicates that the paleo-land
surface in the Champsaur area comprised a set of
structurally controlled basement paleotopographic highs
with an estimated relief of up to 200 m at the time of
the Nummulitic transgression.
FIG. 4. Overstep and onlap of the Nummulitic Limestone
Formation onto the basement paleo-fault scarp formed by
the hanging wall of the St. Bonnet fault on the western
side of the Tertiary syncline. The St. Bonnet fault shows
a tectonic transport direction out of the plane of figure
towards the reader.
FIG. 2A Geologic sketch map of the western Alps showing
location of study area relative to elements of the Alpine
orogen and Alpine foreland basin.
In southeastern France, Eocene-Oligocene deposits of the
Alpine foreland basin form an eastward-thickening wedge
up to 1.5 km in thickness, that is overthrust on its
eastern margin by Alpine internal zone units, and onlaps
to the west onto the European foreland (Figs. 2B, C).
Fig. 2B Map of southeastern France showing outcrops of
lower Tertiary foreland-basin strata and the external
basement massifs.
Stratigraphy
Above a well developed basal unconformity, the Eocene
basin-fill succession consists of a deepening-upward
succession that shows vertical transition from shallow
marine limestones through pelagic marls to turbiditic
sandstones, and is truncated by thrusts of the Alpine
deformation front (Fig. 2C).
Fig. 2C Chronostratigraphic diagram illustrating the
Eocene-Oligocene foreland basin stratigraphy along an
east-west cross section across the Alpine foreland basin
in the Hautes Alpes region. The section runs from the
Champsaur area to the Lus-la-Croix-Hautes syncline, which
is the westernmost preserved outcrop of foreland basin
strata in this sector of the basin
Setting of the Champsaur Study Area
The Basal Unconformity in the Champsaur Area
