Int J Earth Sci (Geol Rundsch) (2003) 92:338–347 DOI 10.1007/s00531-003-0321-3
ORIGINAL PAPER
O. Bruguier · J. F. Becq-Giraudon · N. Clauer · H. Maluski
From late Visean to Stephanian: pinpointing a two-stage basinal evolution in the Variscan belt. A case study from the Bosmoreau basin (French Massif Central) and its geodynamic implications Received: 3 April 2002 / Accepted: 9 February 2003 / Published online: 17 April 2003 Springer-Verlag 2003
Abstract Post-convergence evolution of the Variscan belt is characterized by the development of intramontane coal-bearing basins containing volcano-sedimentary successions. In the French Massif Central, K–Ar ages on clay particles from fine-grained sediments of the Bosmoreau basin (Limousin area), help pinpoint the evolution of the basin. In the lower part of the sedimentary pile, illite in a siltstone underlying a volcanic layer previously dated at 332€4 Ma by the U–Pb method on zircon, yields a consistent K–Ar age of ca. 340 Ma. Upward in the sedimentary succession, illite yields Stephanian K–Ar ages, which can be combined to provide a mean deposition age of 296.5€3.5 Ma. The Bosmoreau basin, albeit mainly filled with Stephanian deposits, was initiated during the late Visean, i.e. ca. 30 Ma earlier than inferred from biostratigraphical constraints. During the Stephanian, the same structure was reactivated and late Visean deposits were eroded and subsequently blanketed by thick clastic sediments. These results emphasise a twostage evolution for the Bosmoreau basin, which is closely related to extensional tectonics identified on basement country rocks, and they are used to propose a geodynamic evolution of the studied area.
O. Bruguier ()) Service ICP-MS, cc 056, ISTEEM, Universit de Montpellier II, Place E. Bataillon, 34095 Montpellier, France e-mail:
[email protected] J. F. Becq-Giraudon BRGM, 3 Avenue C. Guillemin, BP 6009, 45060 Orlans, France N. Clauer CGS-EOST, Universit Louis Pasteur-CNRS, 1 rue Blessig, 67084 Strasbourg, France H. Maluski Laboratoire de Gophysique, Tectonique et Sdimentologie, CNRS-UMR 5573, ISTEEM, Universit de Montpellier II, Place E. Bataillon, 34095 Montpellier, France
Keywords Delamination · French Massif Central · Intramontane basins · Stephanian · Visean
Introduction Extensional tectonics is preferentially located along orogenic belts with a thickened crust and collapse of mountain belts represents an important feature of postcollisional orogenic stages (e.g. Ratschbacher et al. 1989). Implicit to this is the creation of a pervasive series of continental basins accompanying extension in the upper crust. Whereas most studies addressing evolution of orogenic belts usually deal with basement and deep crustal processes (Faure and Pons 1991), an alternative approach, whenever possible, is to focus on sedimentary records and to use sedimentary basins as tectonic markers, considering that their formation mirrors deeper processes (Zoback et al. 1993). Basin opening and sedimentary infilling are often tectonically controlled (e.g. Bruguier et al. 1997). A key issue, therefore, is to determine precisely both the age of basin formation and the main stages of infilling, and the relationships between basin initiation and tectonic structures such as major fault systems. These parameters potentially carry important information that have implications for understanding the tectonic control on the sedimentary record, and to punctuate the different stages of extensional tectonics. The implied requirements for such an approach to be fully operative are tightly related to the possibility of selecting suitable sedimentary records and appropriate chronological methods. The French Massif Central is one of the most important exposures of the Internal Zone of the Variscan Belt, which extends along ca. 3,000 km from the Iberian Massif in the West to the Bohemian Massif in the East (Fig. 1). In the whole belt, the Late Carboniferous–Early Permian time interval is characterized by numerous coalbearing intramontane basins corresponding to isolated troughs closely associated with fault-zones and filled with coarse, clastic, non-marine sediments deposited unconformably on the metamorphic and igneous basement. The
339 Fig. 1 Position of the French Massif Central within the European Variscides (after Matte 1986)
structural control of the opening and further infilling of these basins is obvious from their association with faults, at least along one of their borders, and it has been widely documented (e.g. Faure 1995, and references therein). In this study, we have focused on the Bosmoreau basin located in the northern part of the French Massif Central (Fig. 2). The sedimentary successions are well preserved in the basin, and they are known in detail. In addition, a biostratigraphic support provided by floral records allows age constraints to be placed (Becq-Giraudon 1985). This basin is located in the hanging-wall blocks of the Argentat fault, a major fault system of this part of the Variscan orogen (Fig. 2). Following our previous work on this basin (Bruguier et al. 1998), our goal is herein to date the sedimentary succession, by selecting illite particles for K– Ar analyses from fine-grained sedimentary rocks sampled at different levels in the sedimentary pile. This approach was thought to help to pinpoint basin infilling and relationships with nearby structuring faults, which may be viewed as a basin response to more global, crustal scale, processes.
Geological setting The Bosmoreau coalfield is a small graben filled in by continental Carboniferous deposits and located in the north-western part of the French Massif Central (Limousin area). It is entirely bounded by normal brittle faults and developed in the hanging-wall block of the northern end of the Argentat fault (Fig. 2). Sedimentary rocks unconformably overlie the Late Devonian Guret granitic massif dated at 356€10 Ma by the Rb–Sr method on whole rocks (Berthier et al. 1979). Basin fill is estimated to be about 600 m thick and consists mainly of siliciclastic fluvio-lacustrine to palustrine sediments, including coal seams. The lithostratigraphy of the basin can be divided into four third-order sequences (Fig. 3). Sequence 0 is only exposed in the south-western part of the basin and consists of about 30 m of fine-grained sediments, which include a 60-cm-thick volcanic ash layer. U–Pb zircon results from this volcanic ash layer yielded a Late Visean upper intercept age of 332€4 Ma, interpreted as the age of eruption of the magma and,
340
Fig. 2.a Location of the main Stephanian–Autunian basins of the French Massif Central. b Simplified geologic sketch map of the Bosmoreau basin showing the main features (after Becq-Giraudon 1985)
therefore, of deposition of the airborne ash in the Bosmoreau basin (Bruguier et al. 1998). Sequence 0 is separated from sequence 1 by an erosional unconformity and the ages of sequence 1 to 3 are upper Stephanian according to their palaeobotanical and palynological records (Becq-Giraudon 1985).
Analytical techniques K–Ar isotopic determinations were made following a procedure close to that reported by Bonhomme et al. (1975). K was measured by flame spectrophotometry with a global accuracy of €1.5%, based on systematic controls of international standards. For Ar analysis, the samples were pre-heated under high vacuum at 100 C for at least 12 h to reduce the amount of atmospheric Ar adsorbed on the mineral surface during sample preparation and handling. The Ar isotopic results were periodically controlled by analysis of the international GL-O standard, which averaged 24.40€0.15x10-6 cm3/g STP (2s) of
radiogenic 40Ar for five determinations. The blank of the extraction lines and the mass spectrometers was also determined repetitively. During the course of the study, the amount of residual 40Ar was always below 110-7 cm3 and the 40Ar/36Ar ratio of the atmospheric Ar averaged 293€6 (2s). The usual decay constants were used for the age calculations (Steiger and Jger 1977), and the overall error of the K–Ar determinations was evaluated to be systematically better than €2% (2s).
Results Three fine-grained sediments were sampled at different levels in the sedimentary pile for K–Ar analyses (see Fig. 3). Sample ST1 belongs to sequence 0 and was taken from a micaceous siltstone underlying the volcanic ash CI1. Sample ST4 is from the base of sequence 1 and was taken from a 0.5-m-thick micaceous siltstone located 3 m above sequence 0. Sample ST7 is a brownish micaceous siltstone located at the base of sequence 3. The clay
341 Fig. 3 Lithostratigraphic column of the sedimentary sequence accumulated in the Bosmoreau basin. A symbol denotes sample locations
342 Table 1 K–Ar isotopic results and mineralogical composition of the investigated clay fractions. Constants used: l40Kb=4.96210-10 an-1, l40Ke=0.58110-10 an-1, 40K/Ktot=1.16710-4. Ar* radiogenic
Ar; the amounts in 40Ar* are given in the STP system. I/S Illite/ smectite mixed layers; chl chlorite; sm smectite; fsp feldspar
Sample
Grain size (m)
Mineralogy by XRDa
I/S (%)
Illite crystallinity
K2O (%)
Ar* (%)
40
Ar* (10 6 cm3/g)
Age (Ma€2s)
ST1
