Sedimentary Geology 186 (2006) 187–211 www.elsevier.com/locate/sedgeo Lower Triassic sequence stratigraphy of the western part of the Germanic Basin (west of Black Forest): Fluvial system evolution through time and space Sylvie Bourquin a,*, Samuel Peron a, Marc Durand b a Ge´osciences Rennes, UMR 6118, Univ. Rennes 1, 35042 Rennes cedex, France b 47 rue de Lavaux, 54520 Laxou, France Received 30 September 2004; received in revised form 21 October 2005; accepted 10 November 2005 Abstract The aim of this paper is to analyse the fluvial evolution of the Lower Triassic in the western part of the Germanic Basin through time and space, as well as the impact of the geodynamic and climatic setting on the preservation of fluvial deposits. The Lower Triassic crops out only in the Vosges Massif and the Black Forest, so well-log studies are required to realise sequence stratigraphy correlations and establish comparisons with others parts of the Germanic Basin. In a first step, we use well-log data analyses to characterise the electrofacies associations in the Triassic and then define the well-log signatures of each formation. In a second step, the characterisation and recognition of genetic sequences and their stacking pattern allow us to define seven minor cycles integrated into two major cycles. Finally, the quantification of the lithologies at different stages of basin evolution leads to the reconstruction of paleoenvironmental maps to illustrate facies evolution through space and time. A comparison with cycles defined in the Germanic Basin allows us to propose correlations of the Lower Triassic on either side of the Rhine Graben and leads to a discussion of the evolution of fluvial systems through time and space. During the Scythian, the fluvial style is characterised by braided fluvial systems evolving laterally into lake deposits towards the central part of the Germanic Basin. During this stage, the basin was a huge depression with very few marine connections in its extreme eastern part. The stratigraphic cycles represent rhythmic fluctuations in relative lake level that could be attributed to sediment supply and/or lake level variations in an arid setting. Four minor stratigraphic cycles are observed that are integrated within a single major stratigraphic cycle. During the period of the stratigraphic base-level rise of the major cycle, a maximum of 233 m of sediment would represent a duration of sedimentation in the Paris Basin of at least 1.8 m.y. During the period of the stratigraphic base-level fall of the major cycle, a maximum of 65 m of sediment would have accumulated over 2.5 m.y. On the western edge of the Germanic Basin, the top of the Scythian is marked by a major sedimentary break characterised by a planation surface, with preservation of the first paleosols, followed by the Hardegsen unconformity. This unconformity is tectonically deformed, leading to the development of a new sedimentation area in the west of the basin. The fluvial sedimentation above this discontinuity shows a trend towards enhanced development of floodplain or lacustrine-type environments at its western margin, with the formation of paleosols. The fluvial systems are linked with sabkhas, and then with a shallow sea connected to the Tethys Ocean. In this context, the * Corresponding author. Fax: +33 2 23 23 61 06. E-mail address: [email protected] (S. Bourquin). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.11.018 188 S. Bourquin et al. / Sedimentary Geology 186 (2006) 187–211 stratigraphic cycles are caused by variations in relative sea level and/or sediment supply. The fluvial deposits are preserved in an exoreic basin. D 2005 Elsevier B.V. All rights reserved. Keywords: Lower Triassic; Fluvial sedimentation; Sequence stratigraphy; Well-log analysis; Paris and Germanic basins; Paleoenvironmental maps 1. Introduction 2. Geological setting The Triassic is a period of transition associated with During the Early Triassic, the Paris and Bresse-Jura the beginning of the break-up of the Pangean supercon- basins formed the western end of the Germanic Basin. tinent and the development of the Mesozoic basins, in a The Paris Basin only existed as an independent basin globally warm and dry climate (Ziegler, 1990; Frakes et from the Middle Carnian onwards (Bourquin and Guil- al., 1992; Dercourt et al., 1993; Lucas, 1998; Golonka locheau, 1993, 1996). In the Vosges (Fig. 1B), the and Ford, 2000; Reinhardt and Ricken, 2000; Boucot Lower Buntsandstein units (Senones Sandstones or and Gray, 2001). Anweiller Sandstones) can be attributed to the Upper- Within the basins around the western part of Tethys, most Permian, i.e. Zechstein equivalents (Durand et al., the Triassic succession is characterised by (Dubois and 1994). Whereas in the major part of the Germanic basin, Umbach, 1974; Courel et al., 1980): (i) fluvial and the bBuntsandstein GroupQ is separated from the Rotlie- playa deposits during the Early Triassic, i.e. Buntsand- gends by the typical-Zechstein carbonate-evaporite fa- stein facies, (ii) evaporite and marine deposits during cies (Uppermost Permian), in France the latter are the Middle Triassic, i.e. Muschelkalk facies and (iii) completely lacking. This is why the French geologists mainly evaporite and fluvial deposits during the Late place the base of their bBuntsandsteinQ at the level of a Triassic, i.e. Keuper facies. In western Europe, the major unconformity between fanglomerate prone depos- Buntsandstein is mainly represented by two fluvial its, localised in relatively restricted basins, and wide- styles: (i) large bed-load sand sheets associated with spread fluvial deposits (Courel et al., 1980). Such a lake deposits characterising the Lower and Middle concept of bBuntsandsteinQ prevailed in South Germany Buntsandstein (i.e. Scythian, Ro¨hling, 1991; Aigner until the adoption of a unified lithostratigraphic scale and Bachmann, 1992), which pass vertically up into (Richter-Bernburg, 1974). Thus, in the French sedimen- (ii) fluvial systems bordering an evaporite sabkha or a tary basins, deposits referred to as bBuntsandsteinQ can shallow sea, typical of the Upper Buntsandstein (i.e. be attributed either to Permian or to Triassic (Durand, in Upper Scythian to Middle Anisian, Durand, 1978; press). Actually, the bLower BuntsandsteinQ of the Courel et al., 1980; Durand et al., 1994). Vosges (Senones Sandstone and Anweiller Sandstone) At the scale of the western part of the Germanic can be attributed to the Upper Permian, i.e. Zechstein Basin, this study aims to analyse (i) the evolution of equivalents (Durand et al., 1994). the fluvial systems through time and space and (ii) Therefore, the Middle and Upper Buntsandstein units the mechanism of preservation of fluvial deposits in are attributed mainly to the Lower Triassic. These facies the geodynamic and climatic context of the Early are characterised by fluvial deposits that make up the Triassic. following formations (Fig. 1C), from base to top (Courel To investigate the relationship between the fluvial et al., 1980): bConglome´rat basalQ, bGre`s vosgiensQ, environments and the stratigraphic context, we pro- bConglome´rat principalQ, bCouches interme´diairesQ, pose (1) sequence stratigraphy correlations based on and bGre`s a` VoltziaQ. The bCouches intermediaiesQ For- well-logs (Fig. 1) located in the western part of the mation is commonly separated from the bConglome´rat Germanic Basin (Paris Basin, including Rhine Gra- principalQ by the bZone limite violetteQ Formation that ben, and Bresse-Jura Basin), (2) to reconstruct characterises by the first occurrence of Triassic soils in paleoenvironmental maps from a quantification of this area. The bed-load fluvial systems of bConglome´rat the lithologies based on well-log and outcrops data, basalQ, bGre`s vosgiensQ and bConglome´rat principalQ are and (3) to compare and correlate our results with data attributed to braided type networks developed in an arid from other parts of the Germanic Basin (Richter-Bern- climatic environment, as indicated by the occurrence of burg, 1974; Ro¨hling, 1991; Aigner and Bachmann, reworked and in situ aeolian sand dunes and wind worn 1992; Van der Zwan and Spaak, 1992; Bachmann and pebbles (Durand, 1972, 1978; Durand et al., 1994). The Lerche, 1998). bed-load fluvial deposits of the bCouches inter- S. Bourquin et al. / Sedimentary Geology 186 (2006) 187–211 189 Fig. 1. (A) Location of the studied area and the Bockenem well (Bo). (B) Location of studied wells and transect. B: Bertray1; G: Granville 109; M: Montplone1; F: Francheville1; L: Lorettes1, S: Saulcy; J: Johansweiller; SSF: Soultz-sous-Foreˆts, K: Kraichgau; E: Emberme´nil. (C) Lithostrati- graphic column, sedimentary environment variations and biostratigraphic data for the Lower Triassic succession in the eastern part of the Paris Basin based on Francheville well (after Bourquin et al., 1995). (a): Durand and Jurain (1969), Gall (1971), (b): Kozur (1972), Adloff et al. (1982), Khatib-Nguyen and Thi (1977), (c): Kozur (1972), Khatib-Nguyen and Thi (1977). me´diairesQ correspond to low sinuosity rivers with trans- al., 1994). The only biostratigraphic evidence in this verse bars (Durand, 1978), and are associated with Buntsandstein series concerns the bGre`s a` VoltziaQ, hydromorphic paleosols (Durand, 1978; Durand and where macrofauna and palynoflora allow the attribution Meyer, 1982). The bGre`s a` VoltziaQ shows an evolution of a Lower to Middle Anisian age according to location from low sinuosity fluvial systems in the bGre`s a` (Durand and Jurain, 1969; Gall, 1971). The bGre`s a` meulesQ, with weak marine influence, to the fluvio- VoltziaQ evolves upwards to deposits with increasingly marine environment of the bGre`s argileuxQ (Gall, marine influence (bGre`s coquillierQ, bComplexe de 1971; Durand, 1978; Courel et al., 1980; Durand et VolmunsterQ, bDolomie a` Myophora orbicularisQ), then 190 S. Bourquin et al. / Sedimentary Geology 186 (2006) 187–211 into the evaporitic facies (bCouches rougesQ...) of the Graben (Soultz-sous-Foreˆts well, Fig.