Geologie der Iberger Klippen und ihrer Flysch-Unterlage

Zusammenfassung.

Die Deckenreste bei Iberg (Kt. Schwyz) umfassen sieben Einheiten, von unten nach oben: 1) Iberg-Wildflysch, ein tektonisches Mélange, in welches aber auch olisthostromale Mélanges inkorporiert sind; 2) eocaener Schlieren- Flysch; 3) kretazischer Schlieren-Flysch; 4) Klippen-Decke; 5) Arosa-Zone mit Ophiolithen; 5) Ober-Ostalpine Einheiten (Nördliche Kalkalpen), bestehend aus einer unteren Schuppe (Ober Roggen) und einer oberen Haupteinheit, mit Raibler Schichten und Hauptdolomit. In der Diskussion werden Argumente angeführt, welche eine nordpenninische Herkunft des Schlieren-Flyschs wahrscheinlich machen. Der von M. Caron verfasste Annex beschreibt die Mikrofauna der Couches Rouges, zwei begleitende Artikel von Dietrich (2006, dieser Band), sowie Hochuli & Frank (2006, dieser Band) behandeln spezielle Aspekte der Iberger Klippen.

Extended abstract.

Near Oberiberg, to the NE of Lake Lucerne (about N 47° 01′/E 8° 47′), three small peaks (1577–1778 m) are formed by klippen (outliers) of Penninic and Austroalpine nappes on top of the Helvetic Drusberg nappe (Figs. 1 & 2, Plate 1). Their allochthonous character was first recognized by Edmund C. Quereau (1892, 1893), from Aurora (Ill.). They provide a glimpse at the tectonic units once overlying the Helvetic nappes. These sheets are severely thinned out. Over a vertical distance of some 300 m, units derived from the European margin, from the Valais basin, from the Briançonnais rise, from the Piemont- Liguria ocean and from the Apulian (more or less African) margin can be found. Their emplacement occurred during various events, from mid-Cretaceous to Late Miocene time.

In most places, the youngest formations of the underlying Drusberg nappe are Upper Cretaceous shales. Most of the still younger sediments, of terminal Cretaceous to early Late Eocene age, have been stripped off and are now found in the Einsiedeln slices, located at the front of the Alps. These youngest sediments are only preserved on top of the Drusberg nappe where they have been downthrown by Late Eocene faults, before the décollement of the Einsiedeln slices and the associated arrival of the flysch nappes, which probably occurred near the Eocene-Oligocene boundary.

The Iberg Wildflysch (also referred to as Iberg Mélange), found at the base of the flysch nappes, is a remarkable feature. For its description and interpretation, I have heavily relied on the unpublished Ph. D. thesis of Andreas Bayer (1982). Its origin is complex. It may be considered as a tectonic mélange, incorporating Upper Cretaceous variegated shales, with boulders and finer detritus from a source consisting mainly of basement rocks (Isentobel assemblage), and a Middle Eocene shale-dominated flysch (Surbrunnen flysch of Bayer), with boulders, up to house size, and conglomerates derived from Triassic to Paleocene sediments. These exotic blocks can be reconstructed into one stratigraphic sequence (Roggenegg assemblage, presented in fig. 6). They have probably tumbled and slid from the advancing front edge of the Middle Penninic Klippen nappe (see cartoon of fig. 8). Slices of Schlieren flysch from the hanging wall also occur within the Iberg Wildflysch.

The overlying Schlieren flysch consists of two tectonic slices. The lower one (Eocene Schlieren flysch) is formed by Lower and Middle Eocene shales and coarse sandstones with nummulites. It is tectonically overlain by an Upper Cretaceous (Campanian – Maastrichtian) flysch (Cretaceous Schlieren flysch), with micaceous sandstones, micritic Alberese limestones and polygenic conglomerates. The Schlieren flysch slices of the Iberg region represent the easternmost outliers of a much thicker flysch nappe further to the W.

The paleogeographical origin of the Schlieren flysch is open to discussion. In western Switzerland, the closely related Gurnigel flysch lies at the external margin of the Prealps; it dips southwards underneath the Klippen nappe. Ch. Caron and other Fribourg geologists have found an Upper Cretaceous to Paleocene flysch on top of the Klippen nappe and have correlated it with the ageequivalent formations of the Gurnigel flysch. Therefore, most Swiss geologists (excluding Ken J. Hsü) have accepted the idea of a south-Penninic (Piemont- Liguria) origin of the Gurnigel-Schlieren flysch. In our area, however, a north- Penninic (Valais) origin of this nappe seems to be more probable. Arguments for this hypothesis include the close association of the Schlieren flysch with the Iberg Wildflysch, the absence of south-Penninic flysch between the middle Penninic Klippen nappe and the Arosa nappe as well as analogies between the Schlieren flysch and the (certainly north-Penninic) Rheno-Danubic flysch of the Eastern Alps. The solution of this dilemma has consequences for the reconstruction of Alpine sedimentary realms in Late Cretaceous and Early Paleogene times, and also for the kinematics of the Alpine orogeny.

The lowest unit of the Iberg klippen proper is the Middle Penninic Klippen nappe (or Nappe of the Préalpes Médianes). This tectonic unit is still well developed a few km to the W of our area, in the Mythen mountains, often regarded as the archetype of klippen. In the Iberg area it is strongly sliced and sheared, and not more than 120 m thick. This change of tectonic style may be due to the westward thinning of the Austroalpine overburden. The stratigraphic column of the Iberg Klippen (Upper Triassic to Eocene) is shown in fig. 10. Most widespread formations are the massive Upper Jurassic limestones and the marly limestones of the Couches Rouges (mainly Upper Cretaceous). The Albian to Eocene microfauna of the Klippen Nappe is described in Appendix 1 by Michèle Caron.

The South-Penninic Arosa zone is characterized by ophiolites (mainly metabasalts, often with pillow structures and associated with volcanic breccias). There is one outcrop of gabbro and one of badly sheared serpentinites. The petrology and geochemistry of the MORB-type Iberg ophiolites are described in a companion paper (Dietrich 2006, this volume). The sediments range from (presumably late Middle Jurassic) radiolarian cherts into a mid(?)-Cretaceous flysch (see fig. 15). The sequence of ophiolites and post-ophiolitic, partly oceanic sediments (Arosa zone s. str.) is imbricated within itself and with slices of Triassic shales and dolomites from the overlying Austroalpine nappes, thus forming a sort of tectonic mélange (Arosa nappe s. l.).

The two highest tectonic units preserved in the Iberg klippen are the westernmost occurrences of the Northern Calcareous Alps (Upper Austroalpine nappes). A tiny lower slice contains Rhetian and Early Jurassic limestones. The upper and main sheet is formed by Carnian Raibl Beds and Norian Hauptdolomit (see fig. 18). The fairly rich palynoflora from the Raibl Beds is described in a companion paper by Hochuli & Frank (2006, this volume). Facies relations exist with the internal parts of the western Northern Calcareous Alps, and also with the «Central Austroalpine» cover of the Sesvenna and Silvretta basement nappes. No rocks comparable to those of the Lower Austroalpine nappes have been found at Iberg.

The Iberg klippen lie about 65 km to the W of the westernmost tip, located in the Principally of Liechtenstein, of the Northern Calcareous Alps. This implies that the Upper Austroalpine décollement nappes extended as contiguous thrust sheets westwards to the meridian of Lake Lucerne, though probably not much further. On the other hand, the South-Penninic flysch nappes of the Western Alps (Simme nappe in the broadest sense) did not reach into our area, nor into areas further E.

We suspect, in accordance with Schmid et al. (2004), that the DentblancheSesia (-Margna?) Nappe, which for example forms the summit of Matterhorn, represents an isolated fragment (or «terrane»), detached from the Apulian margin, but without direct geometrical or paleogeographical links to the «true» Austroalpine nappes of the Eastern Alps.