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

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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

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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.

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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.

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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.

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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

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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.

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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.

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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)

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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.

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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

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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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Arnaud, F., Boullier, A. M., & Burg, J. P. 2004. Shear structures and microstructures in micaschists: the Variscan Cévennes duplex (French Massif Central). Journal of Structural Geology, 26, 855-868.

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