mountain erosion in the Variscan belt
DEPOSITIONAL ENVIRONMENT
A marine sequence
The occurrence of marine fauna (brachiopods, crinoids) implies that the sandstones were deposited in a marine domain. The Sainte-Anne sandstones are immature, poorly-sorted and rich in plant debris, implying an important continental (detrital) contribution. A large amount of mud chips involve high energy currents reworking upstream shaly layers. These arguments
are in favour of a river drift, attributed to episodic floods, to the adjacent marine domain. The sandstones present the characteristics of gravity deposits, and no evidence for swell or tide influence has been observed. Moreover, neither mass-flow processes nor channeling forms have been observed, suggesting that the sandstones were deposited downslope, probably on the lobes. No indication allows us to precisely estimate either the bathymetry or the distance to the coast, as the density currents are able to transport sediments over large distances.
The palynomorphic assemblage is poorly diversified, generally implying proximal and shallow water depositional conditions [Butterfield & Chandler, 1992; Huysken et al, 1992].
As a consequence, it might have been reworked and deposited in a deeper and more distal environment by density currents. This could also explain why only isolated brachiopod valves and crinoid columnals are found in the sandstones.
Source of the terrigenous fraction
The Sainte-Anne sandstones are characterised by their compositional immaturity (i.e. a high proportion of feldspars and of lithic fragments) and by the large amount of angular fragments.
This implies both moderate conditions of weathering and proximal sources. Moreover, the nature of the detrital elements indicates sedimentary and volcanic sources rather than metamorphic and plutonic sources, thus suggesting an erosion of predominantly upper crustal material. The metamorphic and/or plutonic fragments probably arise from the erosion of a Proterozoic basement,or the erosion of Variscan materials.
Sedimentary elements can arise from various source areas. Well-rounded quartz grains are described in Ordovician formations of the Châteaupanne Unit (Fresnes Fm [Blaise et al., 1986; Wyns et al., 1998]) and the Tombeau Leclerc Unit ("Pélites à fragments" [Blaise et al., 1986; Piçarra et al., 2002]). Such quartz grains are also present at the base of the Chalonnes Fm [Cavet et al., 1970; 1971; Blaise et al., 1986]. Siliceous microcrystalline rocks are known in the Proterozoic basement of the Mauges Unit [Wyns et al., 1998] and in Silurian of the Tombeau Leclerc Unit [Philippot, 1950; Piçarra et al., 2002].
Several sources can be suggested for the origin of the volcanic components. Firstly, acidic and basic volcanics of Silurian to Devonian age [Carpenter et al., 1982] have been described north of the Sainte-Anne Fm, in the Saint-Georges-sur-Loire Unit (fig. IVa-1) [Arnaud, 1966; Pivette, 1978; Cartier & Faure, 2004]. However, a northern source for the volcanic components is highly unlikely, because palaeocurrents indicate a southern
Cambrian age, is located in the Choletais area, south of the Sainte-Anne Fm [Santallier, 1964;
Thiéblemont et al., 1987; 2001]. The volcanic fragments could derive from the erosion of these volcanics, in agreement with the palaeocurrent directions in the Sainte-Anne Fm (fig.
IVa-9). Thirdly, a last possibility could be volcanic rocks that were erupted shortly before or during the deposition of the Sainte-Anne Fm. This hypothesis would be consistent with the interpretation of the Saint-Georges-sur-Loire Unit as a back-arc (i.e. marginal) basin due to a northward subduction [Arnaud, 1966; Pivette, 1978; Cartier & Faure, 2004]. However, one has to contend that such Lower Devonian magmas are not known, either as plutons or lavas in a nearby domain, or as distal fall-out interbedded within the Devonian sequences. This casts doubt on both the back-arc model for the Saint-Georges-sur-Loire basin and for these assumed volcanics as a potential source for the detrital material.
Although we favour the second hypothesis, further studies are needed to resolve the issue, especially on the age of the detrital minerals (muscovite, zircon) within the Sainte-Anne Sandstone.
DISCUSSION
The transition from the reefal carbonates of the Chalonnes Fm to the gravity flow deposits of the Sainte-Anne Fm has two implications at a regional scale.
The Chalonnes Fm was deposited at shallow depths, as indicated by the reefal build-ups. The sedimentation of the Sainte-Anne Fm at the front of a prograding delta, but at much greater depths, abruptly follows the reefal sedimentation, which suggests a sudden and large increase of the bathymetry. The latter could be due to an eustatic rise of the sea-level, resulting in a transgression from the Pragian-Emsian transition until the Frasnian [Johnson et al., 1985; Haq & Schutter, 2008]. However, the sedimentation of the Sainte-Anne Fm implies that a large amount of detrital material of continental origin is made available, which is not easily reconcilable with a transgressive model. These arguments suggest that a relief has been created to the south of the study area coeval with an increase of the bathymetry. The increasing bathymetry may result from a locally- or regionally-controlled tectonic subsidence.
In the present state of knowledge, a clear separation of the tectonic contribution from the eustatic contribution is not possible.
The immaturity of Sainte-Anne sandstones and the large amount of plant debris suggest a brief transport and hence the close presence of an uplifted land. Moreover, the
sandstones of the Sainte-Anne Fm are at best coarse-grained sandstones and conglomerates had never been found. This implies a smooth relief, considering the brief transport suggested above. The reworked fragments arise from the erosion of superficial sources (sedimentary and volcanic) and then imply a gentle incision, which has not yet affected (or only incidentally) the Proterozoic basement and/or already exhumed Variscan materials. As a consequence, the Sainte-Anne Fm records the birth of a gentle relief during the late Lower Devonian (fig. IVa-12).
The geodynamic context of the Variscan belt during the Lower to Middle Devonian is still unclear.
1. High pressure (i.e. eclogite-facies) metamorphism is recorded in the Variscan belt from 415 Ma to 408 Ma (la Borie eclogite [Ducrot et al., 1983]; Marvejols tronhjemite [Pin &
Lancelot, 1982]; La Bessenoits eclogite [Paquette et al., 1995]). This time range (420-400 Ma) corresponds to the late Silurian to the late Lower Devonian [Gradstein et al., 2004].
FIG.IVa-12. – Reconstitution of the landscape during the late Lower Devonian (Emsian).
FIG.IVa-12. – Reconstitution du paysage au Dévonien inférieur (Emsien).
2. Following the high pressure event, partial melting of continentally-derived protoliths (paragneisses and orthogneisses) is recorded in both the Armorican Massif and the Massif Central. The age of this event is constrained by Rb-Sr whole-rock isochrons as well as U-Pb data on zircon and monazite at about 390 to 370 Ma [Duthou et al., 1994; Rolin et al., 2000; Cocherie et al., 2005; Faure et al., 2008], i.e. Givetian to Fammenian [Gradstein et al., 2004].
3. The erosional event recorded by the Sainte-Anne Fm, of Emsian age (407 - 397 Ma, [Gradstein et al., 2004]), is therefore coeval or slightly younger than the eclogite-facies event,
but older than the partial melting. The chronological data are consistent with the characteristics of the eroded material that implies mainly superficial sources. Because the relief is nascent, plutonic and metamorphic source rocks arise more likely from the Proterozoic basement than from exhumed Variscan metamorphic rocks.
Because the deposition of the Sainte-Anne sandstones took place simultaneously with relief formation and basin subsidence, one could be tempted to suggest a flexural basin model.
The flexural basin is expected to migrate either northward or southward simultaneously with the propagation of the thrust front. Occurrences of “flysch to molasse” deposits are not known either northward (Châteaulin, Laval [Morzadec et al., 1988]) or southward (Montagne noire, Mouthoumet [Feist et al., 1994]) during the Early Devonian times. Nevertheless, flexural basins are well described both in the northern and southern external zone of the Variscan belt.
This clastic sedimentation is recorded as soon as the Middle (South Cornwall [Floyd et al., 1987]) or Upper Devonian (Rhenohercynian domain [Engel & Franke, 1983]) in the northern external zone. In the southern external zone (Montagne noire, Mouthoumet, Pyrenees [Engel, 1984]; Moravo-Silesian Domain [Bábek et al., 2006]) flexural basins are not identified before the Visean. Further work on the Devonian deposits from the Chalonnes area is needed before any conclusive statement on the origin of the basin can be made.
CONCLUSION
In the southern Armorican Massif, the reefal carbonates of the Chalonnes Fm are overlain by the immature sandstones of the Sainte-Anne Fm. The main conclusions of the study of the Sainte-Anne Fm are as follows.
1. According to their petrological and sedimentological characteristics, the Sainte-Anne sandstones were formed by density currents in the marine domain, controlled by a high fluvial, continental contribution, providing large amounts of pieces of plants. As a consequence, these sandstones are interpreted as deposits at the front of a deltaic system, probably on the lobes of a submarine fan.
2. The immaturity of the sandstones and the large amount of terrestrial plant debris imply the proximity of an emerged land. Lithic fragments suggest the erosion of volcano-sedimentary sources during the building of the relief. The palaeocurrents are consistent with a southward derivation of the detrital material (possibly the Choletais volcanics), and therefore suggest that the relief was built south of the studied area.
3. A revision of the floristic content of the Sainte-Anne Fm suggests an Emsian age for this formation.
4. The transition from the Chalonnes Fm to the Sainte-Anne Fm implies an increase in the bathymetry during the Emsian times, at close proximity to an uplifting relief submitted to erosion.
5. The Sainte-Anne Fm is the oldest sediment testifying, in the internal zones of the Variscan belt, to the subaerial erosion of an uplifting relief that could be linked to the earliest stages of the convergence in the European Variscan belt.
Acknowledgments. - This work was realised as part of the PhD thesis for the first two authors.
We would like to thank D.G Strullu (Université d’Angers), A. Le Hérissé (Université de Bretagne Occidentale) and P. Steemans (Université de Liège) for their help in the plant, spore and acritarchs studies. Financial support was possible thanks to a MRT grant attributed to CD.
The authorisations for sampling in the Châteaupanne quarry were provided by its director (E.
Souchet) and his quarry chief (J. Binet). M. and Mrs. Porcher and M. and E. Mercier kindly allowed us to access the Sainte-Anne abandonned quarry and to the Orchère locality, respectively. Finally, M. Bohn (Microsonde Ouest, Brest), F. Redois (Université d’Angers), M. Giraldo (Université de Liège), and G. Beaulieu (Angers) are also thanked for their cooperation. P. Labaume, an anonymous referee and the editorial work of Y. Lagabrielle helped improve the content of the paper. Sara Mullin revised our English.