Mesozoic Hydrothermal Impact on Rotliegende and Bunter Immature Sandstones of the High Rhine Trough and Its Adjacent Eastern Area (Southern Black Forest, Germany)
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Sedimentary Geology 234 (2011) 76–88 Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo Mesozoic hydrothermal impact on Rotliegende and Bunter immature sandstones of the High Rhine trough and its adjacent eastern area (southern Black Forest, Germany) Olaf Brockamp a, Andreas Schlegel a,⁎, Norbert Clauer b a Fachbereich Geowissenschaften, Universität Bremen, Postfach 330 440, 28334 Bremen, Germany b Laboratoire d'Hydrologie et de Géochimie de Strasbourg (CNRS-UdS), Université de Strasbourg, 1, rue Blessig, 67084 Strasbourg, France article info abstract Article history: The Mesozoic hydrothermal and geodynamic evolution of a complex region of the European crust is studied here Received 5 May 2010 on immature and highly reactive Rotliegende and Bunter sandstones of the intramontane High Rhine trough Received in revised form 9 December 2010 (southern Black Forest) and its adjacent eastern area bounded by the Upper Rhine Graben and the Alpine foreland. Accepted 12 December 2010 Microscopic observations outline varied degrees of illitization of detrital feldspar grains, muscovite flakes and Available online 17 December 2010 illite particles. XRD analysis shows that the clay fractions are dominated by illite with minor illite/smectite mixed- Editor: B. Jones layered minerals of R3 and R1 ordering. The chemical composition of these minerals is controlled by precursor minerals, since the illitic material inherited its chemical components from detrital muscovite and illite. Towards Keywords: the centre of the trough, the sandstones were more strongly affected by the hydrothermal impact, showing High Rhine trough extensive replacement of feldspar grains by Al-rich illite. Upper Rhine Graben K–Ar dating of the authigenic illite-type material points to multiple hydrothermal alteration episodes. A Late Hydrothermal fluid flow Jurassic event at 147 Ma seems to have affected extensively the study area, especially towards the centre of the Chemical composition of illite trough. At the periphery of the trough, relics of an Early Jurassic activity are likely preserved. An additional Early – K Ar dating Cretaceous hydrothermal event seems to have occurred locally near Schopfheim and next to the Rhine Graben boundary fault long before its main rifting phase. The 147 Ma event was obtained also nearby in the previously investigated Breisgau–Schramberg, Offenburg–Teinach and Baden–Baden troughs and is characteristic for the Black Forest and some other areas outside this region. It concludes obviously a period of regional hydrothermal activities between 160 and 145 Ma, whereas the Early Jurassic and Cretaceous signatures are also known throughout Central and Western Europe, suggesting repeated fluid pulses. The hydrothermal flows were controlled by reactivated basement faults due to periods of extensive reorientation of the tectonic stress during the break-up of Pangaea. Illitization was triggered by hot and low saline solutions of about 180 °C and b10 wt.% NaCleq,whereasalater fluid pulse of about 80 °C and N20 wt.% NaCleq caused quartz overgrowth. Fluid flows delivered by the long lasting orogeny of the nearby Alps were probably limited towards the Black Forest. Becoming part of the European margin of the Tethys in the Mesozoic, the pre-Alp area evidently developed a separate fluid-flow system. © 2010 Elsevier B.V. All rights reserved. 1. Introduction states. The known changes are the selective loss of heavy-mineral species and the increase of the clay matrix at the expense of the feldspar Sandstones of low compositional maturity contain large amounts of component. The resulting clay products that are suitable for isotopic incompletely altered coarse-grained minerals and unstable rock dating are also geologically meaningful as time markers for both the fragments associated mostly with a clay-rich matrix. Such sandstones diagenetic history and the regional geodynamic evolution. The have a high chemical reactivity and are very susceptible to post- mineralogical properties of sandstones clearly provide a significant sedimentary processes like increasing pressure and temperature, and/or archive and are, therefore, expected to yield considerable genetic varied pore water compositions. They can, therefore, induce significant information about post-depositional evolution. exchange reactions and authigenesis of minerals in the sediments after However, Wetzel et al. (2003) argued that the reactivation of deposition, leading to more equilibrated mineralogical and chemical basement structures in favourable stress fields did not necessarily occur at the same time everywhere throughout Europe because the basement consisted of blocks interacting mutually, which does not ⁎ Corresponding author. allow large-scale synchronous fluid migration. Thus, the events in E-mail address: [email protected] (A. Schlegel). each region have individual characteristics and just their synopsis 0037-0738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2010.12.001 O. Brockamp et al. / Sedimentary Geology 234 (2011) 76–88 77 may reveal local activities. For instance, Clauer et al. (2008) identified four hydrothermal episodes at 210–185, 175–155, 110–95 and 65– 60 Ma by K–Ar dating of illite from Bunter sandstones of the Vosges Mountains, along the fault systems of the western side, and the centre of the Upper Rhine Graben. These episodes coincide with periods of hydrothermal alteration in Central Europe and episodes of vein mineralization in the Black Forest (Wetzel et al., 2003; Pfaff et al., 2009). On the other hand, K–Ar studies of illitic material from Rotliegende and Bunter sediments of the northern and central Black Forest troughs only identified a Late Jurassic hydrothermal event at about 145 Ma, together with a vague Cretaceous signature of a local overprint (Zuther and Brockamp, 1988; Schlegel et al., 2007), while illite K–Ar data from trough sediments and zircon fission track analysis of the crystalline basement rocks from the adjacent northern Switzerland did not record this Late Jurassic thermal episode (Schaltegger et al., 1995; Timar-Geng et al., 2006). To understand better the interplay of rock alteration, hydrothermal fluid flows and tectonic processes in the Black Forest, immature and highly reactive arkoses and sandstones of Rotliegende and Bunter formations from the High Rhine trough (southern Black Forest) and its directly adjacent eastern area were investigated. The reason was to complete the regional aspect of our previous investigations about alteration processes of cover rocks in the Black Forest with the southern area being the missing link. Some of the unsolved questions to be addressed here were whether this area has been altered hydrothermally at all during the Mesozoic and if so either by a single or multiple fluid pulses, and which tectonic stress Fig. 1. Simplified map of the Black Forest region showing the position of the Permo- fields might have prevailed during the time of reactions. Because of its Carboniferous intramontane troughs (after Franzke & Werner, 1994) and the study area fi speci c tectonic position between the Upper Rhine Graben to the west (rectangle). The Variscan suture zones (black lines with triangles) between the and the Alpine foreland to the south, the study area appeared also well- Saxothuringian (SAX) and Moldanubian (MOL) crustal blocks and along the Badenwei- suited for studying mineral authigenesis and the related fluid flows that ler-Lenzkirch zone (BLZ) are also included. (1)=Baden–Baden zone, (2)=Central might have been supplied from these surrounding areas during tectonic Schwarzwald Gneiss Complex, (3)=Southern Schwarzwald Gneiss Complex. movements to the Black Forest at different periods. The history of the expected post-sedimentary processes will be chemical and marine sediments (Geyer et al., 2003). The area of the questioned in this study from point of view of the mineral content and High Rhine trough is also called the Dinkelberg block (Laubscher, the chemical composition of the samples from the trough and its 2003). In contrast, the SSGC adjacent to the east is directly overlain by adjacent eastern area. The study is therefore based on microscopic Upper Bunter sandstones up to 15 m thick and by Muschelkalk, observations of thin sections and microthermometric analysis of fluid Keuper and Jurassic sediments further to the east. inclusions in thick sections, with special emphasis on X-ray diffraction The tectonic deformation of the cover rocks was mainly caused by (XRD) and chemical analysis of clay minerals from the b0.2 and b2 μm reactivation of NW–SE and NNE–SSW-trending basement faults as a fractions, as well as on K–Ar dating of the illitic material. The results result of changing paleostress fields during the post-Variscan break-up are expected to provide comprehensive insights into hydrothermal of Pangaea, during the Alpine orogeny (e.g. Ziegler et al., 2004)and episodes and alteration processes within the High Rhine trough in the probably also during Late Cretaceous Africa–Iberia–Europe convergence southern Black Forest, and the geochronological response to these (Kley and Voigt, 2008). An example of the tectonic impact is the repeated features at a regional geological scale. reactivation of the faults to the north and east of the High Rhine trough since Permian time. The result was the independent evolution of the 2. Geological background High Rhine trough, which was not uplifted as much as the other parts of the Black Forest (Geyer and Gwinner, 1991). Consequently, the SSGC is The basement of the Black Forest is part of the Central European now about 750 m above the trough sediments in the eastern part of the Variscides and consists of Saxothuringian and Moldanubian crystal- study area. The major faults (Fig. 2)aretheE–W-trending Kandern fault, line units that are separated by major shear zones (Fig. 1, Kalt et al., the N–S-trending Wehr fault zone and the NNE–SSW-trending Rhine 2000). After collapse of the Variscan orogenic belt, a system of narrow Graben boundary fault respectively to the north, east and west of the NE-trending Permo-Carboniferous intramontane troughs developed High Rhine trough.