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Transcript of Florac Report
James Sean Dickson, CID No: 00692568
A Geological Compendium and History of the Florac
Area of the Cévennes National Park, France
Submission for the Academic Year 2013
A report submitted in partial fulfilment of the requirements for the degree of BSc Geology at Imperial College London and the Associateship
of the Royal School of Mines.
It is substantially the result of my own work except
where explicitly indicated in the text.
The report may be freely copied and distributed provided the source is explicitly acknowledged.
A Geological Compendium and History of the Florac
Area of the Cévennes National Park, France
James Sean Dickson
Page � of �2 43
Table of Contents
List of Figures 4 Abstract 5 1. Introduction 6
1.1.Location 6.................................................................................1.2.Area Profile 6............................................................................1.3.Hazards 8.................................................................................1.4.Previous Research On The Area 9...........................................
2. Regional geological setting 10 3. Summary of the geology of the mapped area 11 4. Stratigraphy 12
4.1.Introduction 12..........................................................................4.2.Abridged Stratigraphic Sequence 12........................................4.3.Basal Sediments (590-680m) 12..............................................4.4.Upper Dolomite (680-740m) 14................................................4.5. Interbedded Marl and Limestone (740-840m) 16....................4.6.Cherty Limestone (840-930m) 19.............................................4.7.Karst Limestone (930-1025m) 20.............................................4.8.Ripple Limestone (1025-?m) 22...............................................
5. Structure 24 5.1.Variscan Basement Schists 24.................................................5.2.Jurassic Sediment Bedding and Faulting 27 5.3.Variscan Orogeny and Associated Regional Metamorphism295.4.Contact Metamorphism of the Mont Lozère Granitoid 32.........5.5.Contact Metamorphism of the Dykes 34..................................
6. Igneous intrusions 35 6.1.Mont Lozère Granitoid 35.........................................................6.2.Lamprophyre 36.......................................................................6.3.Aplite 37...................................................................................
7. Geological history 38 7.1.Variscan Compression (550 - 330 Ma) 38................................7.2.Variscan Extension (330 - ~300 Ma) 38...................................7.3.Jurassic Sediments (201.3 - 166.1 Ma) 39...............................
8. Acknowledgements 41 9. References 42
Page � of �3 43
List of FiguresFigure 1: Map of the Cévennes National Park.
Figure 2: A topographic map of the area covered by the project.
Figure 3: Regional geological map.
Figure 4: BRGM regional geological map.
Figure 5: An exposure of the palæoweathered basal schists.
Figure 6: Cross bedding structures at the base of the Upper Dolomite unit.
Figure 7: The bipedal carniverous dinosaur trace fossil Grallator minisculus.
Figure 8: A Diplocrateron trace fossil burrow.
Figure 9: Harpoceras falciferum ammonoid fossil.
Figure 10: Amaltheus margaritatus ammonoid fossil.
Figure 11: Sketch of a pyritised gastropod shell.
Figure 12: Sketches of Harpoceras falciferum and Amaltheus margaritatus.
Figure 13: The discontinuous beds of the Interbedded Marl and Limestone unit.
Figure 14: Bivalve shell within the Interbedded Marl and Limestone unit.
Figure 15: A vast Karst Limestone block on the edge of the Causse Méjean.
Figure 16: Karstic exposures visible from 3 kilometres away.
Figure 17: Symmetrical ripple marks in the Ripple Limestone.
Figure 18: Stereonet projection of poles to S1 planes.
Figure 19: An F1 fold expressed within a quartz lens.
Figure 20: Stereonet projection of poles to F1 planes.
Figure 21: Stereonet projection of poles to S2 planes.
Figure 22: Stereonet projection of poles to F2 FAP planes.
Figure 23: Stereonet projection of poles to bedding.
Figure 24: Map showing post-Jurassic faulting.
Figure 25: A diagram showing the main detachments of the Massif Central.
Figure 27: A sheared quartz lens.
Figure 27: Garnet crystals in the Graphite Schist.
Figure 28: AFK diagram that describes the Graphite Schist as a metapelite.
Figure 29: Contrasting exposures of the Graphite and Hornfels Schist units.
Figure 30: A table giving the elemental composition of the Mont Lozère Granitoid.
Figure 31: Lamprophyre exposure.
Figure 32: The Variscan Orogeny during the Late Carboniferous
Figure 33: Map of the Causse Méjean sea.
Page � of �4 43
Abstract
A series of Lower Proterozoic to Early Palæozoic metapelitic, heavily graphitic
schists form the basement to the Florac and Cévennes area. Having undergone
north-south compression during the Variscan Orogeny, the schists have sub-
horizontal slaty cleavage and folding features; which are expressed within their
graphite, muscovite, and quartz mineral phases. The metamorphism of the
pelitic rocks fits within the amphibolite facies. ~306 Ma, following the orogenic
collapse of the Variscan mountain belt, the Mont Lozère Granitoid body intruded
into the graphitic schists, along with aplite and lamprophyre dykes of the same
magmatic source. The broadly monzogranitic unit is laccolithic in shape, with its
feeding root to the west - it therefore thins to the east. The heat of the intrusive
body has created a half-kilometre wide aureole of Hornfels Schist, featuring
slightly different exposure patterns and mineralogy to the standard Graphite
Schist. After a break of ~100 million years, a series of Jurassic dolomites and
limestones were deposited over ~37.8 million years. The basal units are
relatively complex, with a convolute fluvial, shallow sea and, lagoonal history.
The unit can contain clasts of the Mont Lozère Granitoid and schist units. These
basal sediments give way to less complex limestone units, which are more
homogenised, with much smaller changes in the palæoenvironment. These
sediments have a homogenous gentle south west dip, and feature 3 major
normal faults of an unknown absolute age. Activities relating to the Karst
Limestone are a major economic feature of the regional geology, with many
outdoor pursuits companies making use of its landscape features.
Page � of �5 43
1. Introduction
1.1.Location
Florac is a small medieval
village of ~2000 people,
which serves as the seat of
the Parc national des
Cévennes, and as such, is
located towards the north
west of its boundaries. The
Cévennes National Park
covers 913 km2 in the south of the Massif Central, a mountainous area in south-
central France. Florac served as the base for the project, with the mapped area
extending mostly eastwards from the village itself.
1.2.Area Profile
1.2.1.Map
The mapped area is roughly bounded by Salièges, Florac, Rochefort, La
Salle Prunet, La Chaumette, les Craix, Cocurès and Bédouès, all of which are
mountains or settlements.
Page � of �6 43
Figure 1. Map of the Cévennes National Park. Parcs nationaux de France site officiel.
1.2.2.Elevation and Topography
The mapping area is relatively mountainous, with an elevation range of just
over 500m. To the west of Florac (which lies at ~590m), the elevation rises
rapidly to an extensive plateau at ~1050m; and to the east, Mont de
Lempézou and La Chaumette reach similar heights. The elevation presented
a problem in that a car could not be procured for the project, increasing the
time required in the field, and making it more physically enduring.
1.2.3.Climate
Florac’s climate is typically Mediterranean in the Summer (i.e. warm and dry)
- although many days consist of intense heat of up to 45ºc during the day,
giving way to thunderstorms in the evening and afternoon. The terrain
Page � of �7 43
1km
Figure 2. A topographic map of the area covered by the project. An enhanced copy of this map was also used for the fieldslip. Total of 17.7 km2. The red line is the line of section.
IGN (2006)
encouraged rapid change in the weather.
1.2.4.Exposure
Exposure in the area is relatively poor - the high annual rainfall of the area
means that much of it is heavily forested with a mixture of deciduous trees
and shrubbery, and coniferous woodland; as a result nearly the entirety of the
project is based upon road and path cutting exposures. The D16 has recently
been rebuilt (and crucially, widened), resulting in fresh, clear cut exposures
along much of its length. The Karst Limestone crops out magnificently, but is
very difficult to reach, owing to the steep slopes that surround it. Other than
the aforementioned, the only rocks to naturally crop out were the Mont Lozére
Granitoid and Hornfels Schist units.
1.2.5.Access and Agriculture
Access is almost a non-problem because being a national park, most of the
land is open to the public for hiking etc. Otherwise, the two times access had
to be requested for access onto private agricultural land, access was granted
with good will. Agriculture is the biggest private land use outside of the built
up residential areas, and consists of livestock rearing on the steep slopes,
and some arable farming on the western plateau.
1.3.Hazards
The biggest hazard in Florac, as is the case in most other mapping projects is
the roads. This is particularly true of the busy and fast N106 bypass, often used
by freight lorries. Second to this is the local weather, which is often extremely Page � of �8 43
hot and can rapidly change to thunderstorms in the afternoon. Beyond this,
standard geological hazards like rivers and cliffs apply.
1.4.Previous Research On The Area
Mapping of the area has not previously been conducted by students of
Imperial College London. The most recent known geological map was
created in the 1930s by the Bureau de recherches géologiques et
minières (BRGM), the French equivalent of the BGS, and was created at
1:50000, 20% of the resolution of this map. In addition, the cited papers
in the accompanying booklets are mostly pre-war, in a language of
which the author is not fully fluent - and they are mostly inaccessible.
However, due to the academic importance of understanding orogenies,
there are more recent regional scale papers on the Mont Lozère
Granitoid and Graphite Schist units.
Page � of �9 43
2. Regional geological setting
Fully contained within the mountainous Massif Central area of south central France,
the regional geology can be split into three main lithological groups, all of which
feature within the mapping area. The Grands Causses limestone plateaus lie to the
west and south of the mapping area. To the north and east, lies the Velay igneous
complex of intrusive granitoid bodies. More locally, the basement of the region is
exposed by the Tarn and Tarnon rivers, and is comprised of a series of graphitic
Page � of �10 43
Figure 3. Talbot et al. 2004 produced this regional map of the area. The mapping area is roughly outlined in red. The Grand Causses limestone plateaus are to the west of Florac, and are not patterned or shaded.
amphibolite facies schists. The Cévennes mountain range of the Massif Central is
bounded by the Cévennes Fault to the south east.
3. Summary of the geology of the mapped area
The exposed basement of the area studied consists of Variscan-
metamorphosed schists, originally laid down as late Proterozoic early
Palæozoic flysch ~ 570 - ~400 Ma ago. Heavily graphitic, the schists might
easily be confused for phyllites, but for certain areas containing well formed
garnet crystals, confirming its amphibolite facies metamorphic profile. The Mont
Lozère Granitoid intrusion lies to the north east of the Florac area, just beyond
Cocurès. A Jurassic sedimentary sequence comprises the western Causse
Mejéan plateau and the hills of Mont de Lempézou and La Chaumette.
Page � of �11 43
Figure 4. Geze (2006) and previous researchers of the BRGM have produced this map of the local area. Much of the information is pre-war, but remains correct. The limestone plateaus are blue, the granites are red and purple, and the schist is green.
4. Stratigraphy
4.1.Introduction
Literature descriptions of the rocks in the area are unsatisfactory, with unclear
descriptions of the units (though this may be due to the greater resolution of this
project), and over-interpreted subdivisions. For this reason, a new unit subdivision
is proposed, summarising the sequence of ~450m. The BRGM descriptions and
map together (Beze 2006), are however sufficient to gleam the ages of each newly
created unit. The sequence begins in the Hettangian - the base of the Jurassic
Period, and ends in the Callovian, of the late Middle Jurassic (Beze 2006). The
sequence therefore represents ~37.8 million years of sedimentary deposition.
4.2.Abridged Stratigraphic Sequence
The Basal Sediment unit is a chaotic sequence of conglomerates and dolomitic
sublitharenites. This grades into the Upper Dolomite, a series that begins with
sandy limestones, but is mostly comprised of silty, honey coloured dolomitised
beds. The self explanatory Interbedded Marl and Limestone unit follows, which
covers 100m. After this, the Cherty Limestone is notable for its siliceous nodules.
The sandy and non-stratified Karst Limestone follows, which dominates the
landscape due to its strong exposure pattern and classic karstic exposures. Finally,
the Ripple Limestone tops the sequence, stretching into the plateau beyond the
mapping area boundary.
4.3.Basal Sediments (590-680m)
Page � of �12 43
4.3.1.Lithological Description
The Basal Sediment unit covers the entire
Hettangian stage (Geze 2006), from 199.3
± 1 to 201.3 ± 0.6 Ma. At the base, a
matrix of honey coloured, dolomitised silt
to medium sand sized sediments supports
a variety of clasts. The clasts vary from
very coarse sand to very large pebble in
size (depending on the outcrop), and are
mostly sub-rounded to sub-angular. The material making up the clasts
depends on the location, with the western clasts only featuring quartzite,
schist and mica flakes; with the schist sometimes appearing green due to
the presence of chlorite. In the east, smaller clasts are more common:
partially due to the presence of feldspar and quartz grains, rather than
quartzite clasts. Many outcrops have well developed fining up sequences,
and palæochannels 0.5m wide were observed at . Towards the top of the
unit, glauconitic mudstone layers exist between layers of more competent
dolomite. Bed thickness ranges from 2-45 cm.
4.3.2.Fossils
Sea urchin spines and fractured bivalve shells were found at L:138. Species
identification could not be conducted. Beze (2006) notes the existence of
Liostrea gastropod casts.
Page � of �13 43
Figure 5. When exposed, the unconformity reveals a palæoweathering texture of the schist.
4.3.3.Lithological Interpretation
Chaotic and highly variable throughout its exposures, the unit represents
deposition of eroded sediments, derived from the Graphite Schist and Mont
Lozère Granitoid mountains which existed into the Hettangian. The transport
distance cannot have been large because mica flakes are preserved. The
dolomitisation of the sediments and minor fossil presence indicates that
there must be a marine element to the palæoenvironment, although the
presence of channels and fluvial style conglomerate clasts conflicts with
this. It is suggested therefore that the unit represents a palæoriver network
flowing into a lagoon. The glauconitic mudstone beds represent a
condensed sequence of slow sedimentation.
4.4.Upper Dolomite (680-740m)
4.4.1.Lithological Description
The Upper Dolomite stretches
from the Sinemurian to the Lower
Pliensbachian (Geze 2006),
186.5 ± 8.1 to 199.3 ± 1 Ma. The
unit begins with a fossiliferous
calc-arenite, with a coarse silt-
fine sand grain size. Lithic
fragments are still present, but are
less abundant than in the Basal Sediments. Planar cross beds were found
in one outcrop. The remainder of the unit is homogenous - 5-100cm beds
Page � of �14 43
Figure 6. An enhanced image of the cross bedding structures found at the base of the unit.
(mostly 15-25cm) of honey coloured silty dolomites with calcite veining.
4.4.2. Fossils
The site of fossilised footprints of a bipedal theropod dinosaur, identified as
the trace fossil Grallator minisculus by Parcs nationaux de France (2011), is
a popular tourist destination for Cévennes hikers. The fossil site is outside of
the mapping area in the village of Saint Lauren de Trèves (L:28), but was
included in the study for its importance.
L:20 is a fossil rich locality, featuring
brachiopod and bivalve shells,
echinoids and belemnite guards.
Species identification could not be
conducted, though Beze (2006)
notes the existence of the Lytoceras
fimbriatum ammonoid and the
Cycloceras acteon nautiloid.
4.4.3.Lithological Interpretation
The change at the unit transition from a
fossil-barren dolomite to a fossiliferous calc-arenite is probably caused by a
marine transgression (towards the East). Whilst lagoons can support flora
and fauna, they are commonly highly saline - especially so in arid regions
with high evaporation rates (Nichols 2009). Given that the area would have
been ~30ºN (Blakey, 2013) in the Early Jurassic, high lagoonal salinity is
likely if Saharan latitude evaporation and precipitations conditions are
Page � of �15 43
Figure 7. The bipedal carniverous dinosaur trace fossil Grallator minisculus.
extrapolated from the current day. This would reduce the abundance and
diversity of life - hence, the fossils and limestones could represent an
opening of the lagoon to a range of both marine nutrients and life during a
transgression.
A regression would have then occurred to recreate the original
palæoenvironment found at the top of the Basal Sediments. It is certain that
a lagoon existed at one point, for this explains the mode of formation of the
dinosaur footprint suggested by Parcs nationaux de France (2011):
•A lagoon filled with lime muds at its base dries out (possibly due to an
exceptionally low tide) and becomes plastic
•A dinosaur then walks across the weak surface, leaving its footprints in the
mud
•The mud dries further, creating a strong cast of the footprint
•The lagoon is refilled with water and fresh muds, filling in the cast and
preserving it
The footprints confirm that the Florac area was at a terrestrial-marine
boundary in the Sinemurian to the Lower Pliensbachian, as the footprint is
of a terrestrial carnivore. It also confirms a lagoonal palæoenvironment, as
this explains the creation and preservation of the footprints.
4.5. Interbedded Marl and Limestone (740-840m)
4.5.1.Lithological Description
The Interbedded Marl and Limestone unit covers the most time of any Page � of �16 43
division, stretching from the Upper Pliensbachian to the Aalenian (Geze
2006), 170.3 ± 2 to 186.5 ± 8.1. Micritic throughout, the main difference is
that of lithification textures - i.e., marl beds and limestone beds. Most beds
are 0.25-0.5 m thick. Dewatering structures and discontinuous beds are
present. There is a question as to whether or not it is
laterally homogenous given the locality L:284, but as
it is a single locality, it is discounted for this study.
Because of its chemical and physical
homogeneousness, little more can be stated.
Biologically speaking however, this unit is very
diverse.
4.5.2.Fossils
The feeding pattern trace fossil Zoophycos was
found at several localities, L:24 being this report’s ‘type locality’.
Diplocraterion, a burrowing trace fossil was also found throughout the unit,
which was best preserved at L:21 with a 15 cm deep structure. Belemnite
guards are common.
Les Bondons (L:27), like Saint Laurent de Trèves, is a locality found outside
of the mapping area that is included for its important fossil evidence which
allows for further interpretation. Gastropods and the ammonoids Harpoceras
falciferum and Amaltheus margatitatus are spectacularly pyritised here.
The ammonoids Paltopleuroceras spinatum, Ludwigia murchisonae,
Litoceras jurense and Hildoceras bifrons are noted by Beze (2006), as well
as the bivalves Gryphæa, Ostrea, Pecten, Lima lima, and the brachiopods
Page � of �17 43
Figure 8. A Diplocrateron trace fossil burrow. Finer details include meniscus-like patterns within the burrow.
Rhynochonella epiliasina and Terebratula perovalis.
4.5.2.1.Lithological Interpretation
A standard tropical shallow sea set of
micrite mud depositions summarises this unit. Heavily fossil rich, the unit
would have had a strong nutrient input, possibly from nearby rivers or
upwelling currents. The change in lithology throughout the unit is
cyclical, and most likely relates to small changes in diagenetic
conditions. I.e. the unit before diagenesis of each layer would have been
relatively homogenous, grey, calcareous muds. This is supported by the
(poorly formed) dewatering structures and the non-continuous limestone
Page � of �18 43
Figure 11. A sketch of a pyritised gastropod shell (possibly Nerinea). Note that the fossil is mainly comprised of calcite - only the outside is pyrite.
Figure 12. Field sketches of figures 9 and 10. Sutre lines are drawn in detail to the right the sketch of figure.
Figures 9 and 10. The ammonoids Harpoceras falciferum (left, 5 cm wide) and Amaltheus margaritatus (right, 3.5 cm wide, with clearly visible sutre lines) preserved via pyritisation.
beds which can form lenses. That said, diagenetic processes cannot
have been especially extensive, for fine Thalassanoides burrow traces
exist even within the more lithified limestone beds.
4.6.Cherty Limestone (840-930m)
4.6.1.Lithological Description
The Cherty Limestone is Bajocian in age (Geze 2006), 168.3 ± 3.5 to 170.3
± 3 Ma. It is named for its prominent nodules of silica, which begin to appear
at 840m along with a transition to a sparite limestone form, defining its base.
At the base, the nodules are white in colour, and are heavily porous,
absorbing any liquid dripped onto them. In the centre of the unit, they are
much less porous, grey-black in colour, and they fracture conchoidally.
Between 850-870m exists a transitional zone where the outer parts of the
nodules are still white, but the centre is grey-black.The nodules phase
towards the top of the unit again becoming more porous and white,
disappearing altogether in the last 20 metres of the unit. The unit is notable
for having a clean, white fresh surface. The nodules are surrounded by a
sparite cement.
Page � of �19 43
Figure 13. The discontinuous beds of the Interbedded Marl and Limestone unit.
Figure 14. a bivalve found within the Interbedded unit. Most probably Lima lima.
4.6.2.Fossils
The only fossil found within the unit was a 1cm bivalve shell of an
unidentifiable species at L:31, and Geze (2006) has no fossils recorded for
this unit at all.
4.6.3.Lithological Interpretation
Whilst a thin section to confirm the presence of spicules replaced by calcite
could not be procured, it is likely that the silica in the chert nodules has a
biogenic origin, as suggested by Townson (1975) for the Jurassic Portland
Formation in Dorset. In this model, siliceous spicules which comprise the
‘skeletons’ of sponges and other marine invertebrates are replaced by
calcite in a diagenetic processes; which results in nodular silica precipitating
in burrow voids (Clayton 1987). Gorman et al. (1993) suggest that the white-
grey ‘halo’ surrounding the chert proper is a gradational reduction in silica
content, giving way to calcite. The white fresh surface is suggested to be
caused by a high calcite chemical purity.
4.7.Karst Limestone (930-1025m)
4.7.1.Lithological Description
The Karst Limestone unit was deposited in the Bathonian (Geze 2006),
Page � of �20 43
166.1 ± 4 to 168.3 ± 3.5 Ma. It is by
far the best exposed rock in the
area, with its vast karstic blocks still
visible several kilometres away. The
karstification appears to have
happened in the present, and is
therefore not a palæotexture. It has
a beige-yellow (sometimes yellow-
pink) fresh surface, and consists of
a sparite matrix supporting medium
to coarse quartz sand grains, which
make up < 10% by volume of the
rock. At the base, it is completely
non-stratified; only in the top 10
metres does it begin to show signs of graded stratification. Sub-mm wide
calcite veins are present in some outcrops. No dolomites were observed,
though this unit is
regularly referred to as
a non-stratified
dolomite in the
available literature
(Beze 2006; Lagrave
1990). Beze does
however note that
there are localised
areas with no
dolomitisation - it may
be that the study only
Page � of �21 43
Figure 16. Shows the Karst Limestone cropping out strongly along the horizon as vast grey blocks. Photograph taken from ~3 km away. The photograph also clearly shows the D16 road used for the stratigraphic log.
Figure 15. One of the vast Karst Limestone blocks visible on the edge of the Causse Méjean plateau. ~40 m of the stratigraphy is visible. Note poor stratification beginning towards the top.
included these.
4.7.2.Fossils
No fossils were found in this study for the Karst Limestone unit, but Geze
(2006) notes that casts of the gastropod Nerinea exist within the unit, albeit
rarely.
4.7.3.Lithological Interpretation
Sandy epicontinental limestones ‘can be very thick… because deposition…
can continue uninterrupted for tens of millions of years’, states Nichols
(2009). It is suggested that this is the reason for the non-stratified structure
of the deposit. Bousquet & Vianey-Liaud (2002) confirm the existence of a
Jurassic epicontinental sea in the area, and it is already known that the
Florac area is often on the edge of this sea. Sediments being washed off the
land explain the detrital content of the limestone, whilst the shallowness of
epicontinental seas allows for the growth of a limestone. Significant
diagenetic processes involving groundwater are assumed to have taken
place, in order to recrystallise the original lime muds into a sparite cement.
One rock sample is noted to have a calcite nodule with crystals over a
centimetre in length. This strong diagenetic process also explains the
possible absent dolomitisation for the unit elsewhere.
4.8.Ripple Limestone (1025-?m)
4.8.1.Lithological Description
Page � of �22 43
The beginning of this unit is not known as it is the erosional surface, but it is
at least known that its oldest beds lie in the Callovian (Geze 2006), which
ended 166.1 ± 4 Ma. The unit was so named for obvious asymmetrical
ripple marks found at L:35. With beige-grey fresh surfaces, the unit is
comprised of 3-10cm thick beds (most being ~5cm), with a fine micrite mud
matrix. Calcite veins were pervasive throughout the unit.
4.8.2.Fossils
The ammonoid Perisphinctes was found at L:164. Reineckeia anceps and
Hecticoceras, also ammonoids, are noted to exist in this unit by Geze
(2006).
4.8.3.Lithological Interpretation
It is likely that the Ripple Limestone was deposited in a similar
palæoenvironment to the limestones of the interbedded unit - i.e. lime muds
being lithified via diagenetic fluid flush. Little is written about the unit as the
study covered ~40m of broadly homogenous rock.
Page � of �23 43Figure 17. The symmetrical ripples for which the unit was named. The ripples suggest a palæocoastline strike of 200-20º.
5. Structure
5.1.Variscan Basement Schists
The Variscan Orogeny is discussed further under 6.1. In this section, its only
relevance lies in its compression direction - with σ1 being north-south, with an
east-west sutre. The main structure of the Cévennes schist is a sub-horizontal
S1, which is sub-parallel to S0 (Arnaud 2004; Talbot et al. 2004). This foliation is
a direct result of the compression shortening. Measurements of the D1 features
of slaty cleavage and isoclinal F1 fold FAPs can be taken throughout the schist
unit because they are so well formed and displayed. D2 structures such as F2
FAPs and crenulation planes were taken where possible, although they were
much rarer.
5.1.1.D1
D1 structures are well expressed - the best exposed structure being the
slaty cleavage; this is defined by the planes produced by the graphite and
muscovite mineral phases within the Graphite Schist. They are so strongly
expressed that they are often preserved within the baked Hornfels Schist
unit. F1 folds however are only observed in the Graphite Schist unit -
expressed in the folding of quartz lenses. The visibility of the folds is highly
variable, and is heavily dependent on the quantity of quartz lenses at the
outcrop, something that changed with each location. It is considered that the
quartz lenses just make the folds more easy to see - and that they still exist
within graphite layers, just without the visibility.
5.1.1.1.S1
Page � of �24 43
S1 is well expressed throughout the Graphite Schist unit and much of
the Hornfels Schist unit, allowing for 146 separate measurements.
The poles plotted to bedding show a very clear WNW-ESE trend, and
infer a SSW-NNE σ1, which lines up well with the north-south
compression known to exist in the Variscan Orogeny.
5.1.1.2.F1 Fold Axial Planes
With a smaller dataset of 40, the F1 FAP trend provides almost identical
trend and σ1 inferences. The folds themselves were all isoclinal and
Page � of �25 43
Figure 18. Poles plotted to the slaty cleavage planes. All broadly sub-horizontal, their north-south spread indicates north south compression, with a fold axial plane that runs WNW-ESE. The plane drawn on the stereonet represents the average compression trend for all of the poles.
recumbent, with centimetre-decimetre scales.
5.1.2.D2
5.1.2.1.F2 Fold Axial Planes
20 F2 FAP measurements were taken at various places in the Graphite
Schist unit, with the best F2 folding expressed at the spectacular 181
river locality. Again, the same trend and σ1 can be inferred, which this
report concludes is due to a continuation of the compression regime that
formed the F1 folds and S1 slaty cleavage. Interlimb angles are ~110º.
5.1.2.2.S2
18 S2 measurements were taken, and like with the F2 FAPs, the same
Page � of �26 43
Figure 19. Folds are generally best expressed within quartz rich outcrops, as the contrast between the black graphite and off-white quartz is stark.
Figure 20. The poles to the F1 FAPs give a similar indication as to the σ1 and trend of the orogeny.
compressional regime and trend is present.
5.2.Jurassic Sediment Bedding and Faulting
5.2.1.Bedding
Bedding is generally found to dip gently to the south west, regardless of the
unit it is recorded under. A stereonet was utilised to get an averaged figure
for the Florac area sedimentary bedding, and this yields 07/241, very much
in line with expectations.
Page � of �27 43
Figure 21. Left. S2 poles. Figure 22. Right. F2 FAP poles. The same σ1 is inferred, possibly with some clockwise rotation of σ1.
Figure 23. Stereonet projection of poles to bedding. The average bedding reading resulted in the shallow plane displayed here, has a dip and dip direction of 07/241.
5.2.2.Faulting
Three faults are known to cut through the sediments of the mapping area.
Whilst two have a general north-south trend, one has a firmly east-west
trend. It is known that all are post deposition of the Jurassic sediments that
they cross cut, but further inferences cannot be made. It is unlikely that
these relate to the orogenic collapse and extension of the Variscan
mountain belt, as this was already ending in the Late Carboniferous, ~100
million years before the sediments were even being deposited.
Page � of �28 43
Figure 24. The final copy of the map, displaying the two roughly north-south trending faults, and the east-west trending faults. The fault to the west must be post-Callovian, but the age of the other two faults is less certain, although probably the same.
5.3.Variscan Orogeny and Associated Regional
Metamorphism
5.3.1.Tectonic Context
The geological development of the Massif Central is a result of the
subduction of of the northern margin of the Palæo-Tethys Ocean (Ledru et
al. 2001), and the subsequent collision of the continents Gondwana and
Euramerica - also known as Laurussia (Matte 1986). Nappe stacking to
accommodate shortening began in the north, travelling south via a series of
southern-propogating thrust faults (Ledru et al. 2001; Brichau et al. 2008).
5.3.2.Graphite Schist Unit
The Graphite Schist unit is the rock that the 5.1 discussed metamorphism
resulted in. So named for its high graphite content, the rock is black-grey in
colour, apart occasional bands of quartz. With a estimated average graphite
percentage of 40%, the rock often has a greasy lustre, and is somewhat
friable. The percentage of quartz is highly variable by outcrop. This is
especially dependent on the presence of semi-pure quartz layers - of which Page � of �29 43
Figure 25. Ledru et al. (2001). A schematic diagram showing the main crustal detachments of the Massif Central.
L:85 is a good example, because of its pervasive 1-15cm quartz beds. The
majority of the quartz in the unit however (making up ~bv% 30), is
somewhat granular, and tends to exist in layers, mostly associating with
muscovite. Muscovite is the final major mineral phase, again comprising
roughly bv% 30 of the rock. If its presence is greater than this, it begins to
lighten the colour of the rock, and decrease the greasiness of its lustre.
Garnet is perhaps the most important mineral in terms of diagnostics found
within the unit - comprising up to 3 % of the rock by volume even in the few
locations that it appears (e.g. L:65; L:181). Its presence confirmed in the
field that the rock had entered the amphibolite metamorphic facies, and
therefore was indeed a schist, not a phyllite, as was mooted in the first
couple of days as a mistake in the literature, or a local lithology of the same
unit. Minor hæmatite staining occurs at some outcrops, which is thought to
be a weathering feature of pyrite, which could have crystallised in the anoxic
black muds that form
the unit’s protolith.
Finally, crenulation
cleavage was
common and
measured, but not
included due to the
homogeneity of the
more useful planar
results.
Page � of �30 43
Figure 26. Garnet crystals within the graphite schist unit, confirming that the schist underwent ampibolite facies metamorphism.
5.3.3.Regional Metamorphism in the Literature
Caron (1994) dated the end of the primary metamorphism of the Cévennes
area using 40Ar/39Ar geochronology studies, yielding 340-330 Ma - notably
younger than that of the northern Massif Central - which had already ended
its compressional phase by 360-350 Ma (Brichau et al. 2008). Arnaud
(1997) undertook geothermobarometric studies, which gave metamorphic
temperatures of 500ºC ± 12 and pressures of 520 ± 80 MPa, specified to
440 ± 30 MPa via primary fluid studies of apatite crystals, again by Arnaud
(1997). These peak metamorphic conditions would have occurred 343.1 ±
4.4 Ma (Caron 1994), and have resulted in amphibolite facies
metamorphism.
The rocks deformed by this metamorphism were Proterozoic and Early
Palaeozoic flysch deposits - thick sequences of carbon rich muds. These
sediments would have been deposited on the northern margins of the
former supercontinent of Gondwana. Geze (2006) asserts that the
deposition of the protolith sediments
extended from the Pre-Cambrian into the
Lower Silurian, noting that it may even
stretch into the Dinantian of the
Carboniferous.
The main structure that has resulted from
the Variscan metamorphism is a sub-
horizontal S1 (Arnaud 2004; Talbot et al.
2004), as detailed in 5. This foliation is a
direct result of the crustal shortening that
Page � of �31 43
Figure 27. Shearing is suggested as the primary method of formation of the quartz lenses by Arnaud et al. (2004).
the area underwent during the orogeny. It was considered most likely in the
field that the ‘quartzite beds’ within the Graphite Schist were preserved
sedimentary features, but work by Arnaud et al (2004) refutes this; noting
that they exist only within shear zones of the rock unit. They therefore
suggest formation via a dissolution-crystallisation process relating to a
change in P-T conditions. A second stage of metamorphism was instigated
at 325 ± 3 Ma, featuring a peak HT-LP regime of 4-5kb, and temperatures of
less than 680ºC (Najoui et al. 2000). This has been attributed to the
emplacement of the large Velay igneous complex (Rakib 1996), which has
the same magma source as the Mont Lozère Granitoid. The intensity of the
second metamorphic stage in Florac cannot be estimated, though it should
be assumed that the metamorphic effects were significantly less severe
than would be found to the North East.
5.4.Contact Metamorphism of the Mont Lozère
Granitoid
A roughly half kilometre thick zone of thermal metamorphism encapsulates the
Mont Lozère Granitoid unit. Though it is defined by a thermal gradient, a cut off
point is necessary for effective mapping of the extent of the aureole. One of the
most striking features of this ‘Hornfels Schist’ unit is its exposure type - which is
far more ‘blocky’ and resistant than the standard Graphite Schist unit. Whilst it is
arbitrary and based upon human judgement of fine differences, the point at
which this texture is no longer dominant was defined as the boundary of the
aureole. At any rate, the vast majority of the metamorphic effects of the
granitoid unit are already minor or dissipated completely by the time this
happens. The unit has grey fresh-surface, often weathering to a brown-red.
Page � of �32 43
Quartz and muscovite have been recrystallised to some extent, reducing the
anisotropy of the unit. Graphite is far less present than in this unit than in the
Graphite Schist, although the reason for this remains unclear - the
metamorphism may have introduced oxidising fluids, or it may have simply
been an area with less original carbon upon deposition. The unit does allow us
to further confirm that the protolith of the Graphite Schist (and Hornfels Schist)
was a metapelite, as in some locations (e.g. L:51), the minerals sillimanite and
cordierite exist with random orientation. This combined with the other regular
minerals allows for the use of an AKF diagram - and the area defined by these
minerals is metapelitic (Figure).
Page � of �33 43
Figure 28. After Nelson (2011). The blue region represents the general area in which metapelites lie. The purple overlay joins the data points on the ternary plot.
Figure 29. The contrasting patterns of exposure of the Hornfels Schist unit (right), and the less blocky, more erodible Graphite Schist unit (left).
Sillimanite
Cordierite
Muscovite
Garnet
A
K F
5.5.Contact Metamorphism of the Dykes
Highly localised metamorphism on the scale of < 3 m occurs on the edges of
the Aplite dykes discussed in 7.3 that are pervasive throughout the area. The
main effect is the production of a zone slight hardening surrounding the dykes,
followed by the introduction of mostly randomly orientated amphibole crystals.
These will sometimes align with the pre-existing foliation. The lamprophyres
meanwhile have introduced a cordierite phase to the rocks that they have
intruded into, with weathering pits clearly visible for up to a metre around the
dykes. This section was included as an artefact of interest, with no geological
significance being inferred.
Page � of �34 43
6. Igneous intrusions
6.1.Mont Lozère Granitoid
The Mont Lozère Granitoid is so named for its variable composition: broadly
speaking, it is a monzogranite, but in several localities this character changes,
with an increased presence of alkali feldspar. This variability is seen in certain
localities where the rock has an alkali feldspar granite, or, more commonly, a
syenogranite composition. Universally however, it has a grey-pinkish-beige
coloured leucogranitic texture, with never more than ~2% amphibole and ~3%
biotite. Plagioclase and quartz form phenocrysts in this porphyritic unit, the
former of which exhibits well developed simple twinning, and exhibits lengths of
up to 1cm. The groundmass is a mixture of the two former, with alkali feldspar,
the source of the pink-beige tint.
The crystallisation of the intrusion has been dated at ~306 Ma, after a series of
conflicting papers which had a range of 425 ± 30 to 279 ± 15 Ma (Brichau et al.
2008). The literature was also conflicted on the relative emplacement age. It is
considered now that the intrusion is post tectonic, i.e. it was emplaced during
the orogenic collapse of the Variscan mountains, which were undergoing east-
west extension from ~320-300 Ma (Brichau et al. 2008; Faure, 1995).
Gravimetry combined with field observations and magnetic lineation data (AMS
data) indicate that the granites have a laccolithic shape, with a root lying to the
west, and thicknesses ranging from 5-6 to 3-4 km (Talbot et al. 2004; Faure
1995; Faure et al. 2001). Najoui et al. (2000) found that the emplacement
occurred at 1–2 kb of pressure, at 690–700°C, in ‘relatively cold host rocks’.
Page � of �35 43
Mialhe (1980) gave the following elemental composition of the unit 8 km at le
Pont de Montvert from where this report’s observations were made.
Roberts (1993) suggests that calc alkaline, high-K, I-type granitoid magmas are
solely derived from partial melting of hydrous, calc-alkaline to high-K calc-
alkaline, mafic to intermediate metamorphic rocks in the crust; and that there is
no implication that subduction processes are necessary for the generation of
these magmas.
6.2.Lamprophyre
The two lamprophyre dykes observed exist within the Hornfels Schist unit in
only in one location - the hill ‘les Craix’. The mineral group of amphiboles and
plagioclase dominate, defining the dykes as spessartites under the Streckeisen
(1979) classification scheme. A porphyritic texture is present, with mm wide
biotite crystals, and mm long amphibole phenocrysts being easily identifiable
within the aphanitic plagioclase and amphibole groundmass. It is likely that the
pyroxene group is also present, with some dark crystals weathering to red-
brown clays. An estimation of the mineral percentages in the field was
conducted: 70% amphibole, 15% plagioclase and 10% biotite/phlogopite, the
remainder being attributed to pyroxene and/or olivine.
It has been proposed that lamprophyres of the calc-alkaline magma series are
parental magmas to granitoid plutons (Leat et al. 1987; Rock, 1987), lining up
Page � of �36 43
Figure 30. Mialhe (1980) confirms that the granite is a high-K calc-alkaline I type granitoid.
wt % SiO2Al2O3
Fe2O3
MnO
MgO CaO Na2
O K2O TiO2P2O
5LOI Total
Pont de Montvert 69.74 14.41 2.86 0.06 1.76 1.77 2.84 5.04 0.36 - 0.66 99.30
with the work done by Roberts (1993); and suggesting that the lamprophyres
were intruded in a similar time frame and are heavily related.
6.3.Aplite
The aplite series of dykes are related to the final crystallisation stage of the
Mont Lozère Granitoid melt. Originally mostly felspathic (> 90%), most of the
exposed dykes are heavily weathered, with the majority of the alkali feldspar
content having been altered via hydrolysis to clays, which was interpreted as
primarily kaolinite. The dykes are generally a dull beige-grey, and have
thicknesses ranging from 0.5-10 m. Because of these minor thicknesses,
despite being a pervasive and important rock unit within the Graphite Schist, the
unit was not mappable. Former amphibole phenocrysts with hexagonal basal
sections are visible in some localities (e.g. L:67), but by far the most common
phenocrysts are quartz, which tend to form 10-20 bv % of the rock. 1-2 mm
wide, they are regularly green or beige.
Page � of �37 43
Figure 31. The lamprophyre dykes were the only unit mapped via remote sensing. Because of their mineral content, they are easily weathered and form good soils- hence the luscious vegetation in their planes of intrusion. Satellite and aerial photography both clearly show the two lines of vegetation, and at an acceptable scale.
7. Geological history
7.1.Variscan Compression (550 - 330 Ma)
On the northern continental margins of the former continent of Gondwana, a
thick series of carbon rich muds were deposited. These underwent north-south
compression in the Variscan orogeny and were metamorphosed to the
amphibolite facies grade without significant retrograde metamorphism. The
folding of these metapelites happened in two stages - the first being the
concurrent formation of the F1 folds and the slaty cleavage; and the second
being the crenulation cleavage, which was followed by open F2 folds. Caron
(1994) dates the end of this metamorphism event to 340-330 Ma.
7.2.Variscan Extension (330 - ~300 Ma)Page � of �38 43
Figure 32. Blakey (2011) produced this map of the orogen during the Late Carboniferous. The west-east trend is clearly visible.
Following extension relating to the Variscan orogenic belt’s collapse, a granitic
body was intruded into the schists. ~306 Ma is the date given by Brichau
(2008) for the emplacement of the magma. Broadly monzogranitic in
composition, the leucogranitic intrusion lies on the high-k calc alkaline magma
series. The intrusion propagated eastwards from its western root and has a
laccolithic geometry (Talbot et al. 2004; Faure 1995; Faure et al. 2001).
7.3.Jurassic Sediments (201.3 - 166.1 Ma)
A break in the geological record of ~100 million years is ended by the
deposition of the Basal Sediment unit. The unit is a relatively complex sequence
which is first deposited on palæoweathered schists, and consists of
conglomerates and dolomitic sublitharenites. This grades into the similar but
less lithologically variable Upper Dolomite, a series that begins with sandy
limestones, but is mostly comprised of silty, honey coloured dolomitised beds. it
is considered most likely that the palæoenvironment varied between fluvial
systems flowing into muddy lagoons, and very shallow seas.
A transgression follows, with a series of interbedded marls and limestones. It is
understood that these beds were laid down identically, with diagenetic changes
explaining the cyclical bedding patterns. After the Interbedded Marl and
Limestone unit the Cherty Limestone appears, and is notable for its siliceous
nodules. A shallow sandy tropical sea is interpreted here, with strong diagenetic
processes inferred to create the sparitic texture of the limestone. The sandy,
non-stratified Karst Limestone exists after this, also interpreted to have strong
diagenetic processes, and very little variation in deposition conditions. Finally,
the Ripple Limestone tops the Causse Méjean sequence, and is interpreted as Page � of �39 43
a slightly shallower sea than was seen in the last two units - allowing for the
preservation of ripple marks.
Page � of �40 43
Figure 33. Figure Bousquet & Vianey-Liaud (2002). A map of the sea in which the sedimentary sequence was deposited. Florac is at the very edge, resulting in the shallow dolomites which were deposited in lagoons.
8. Acknowledgements
I wish to give thanks to my mapping partner Chloe Kirkpatrick, our fellow Florac
mappers Ritwika Sengupta, Conor O’Sullivan, Arkadyuti Sarkar and Rachael Fletcher,
and our supervisor, Mike Streule; all for their role in ensuring the realisation and
success of this research.
Page � of �41 43
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