Field Trip 4 Deep Water Sedimentation on Top of a Growing
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ZOBODAT - www.zobodat.at Zoologisch-Botanische Datenbank/Zoological-Botanical Database Digitale Literatur/Digital Literature Zeitschrift/Journal: Geo.Alp Jahr/Year: 2016 Band/Volume: 013 Autor(en)/Author(s): Ortner Hugo Artikel/Article: Field trip 4 Deep water sedimentation on top of a growing orogenic wedge - interaction of thrusting, erosion and deposition in the Cretaceous Northern Calcareous Alps 141-182 Geo.Alp, Vol. 13 2013 141 - 182 Field trip 4 Deep water sedimentation on top of a growing orogenic wedge - interaction of thrusting, erosion and deposition in the Cretaceous Northern Calcareous Alps Hugo Ortner Institute of Geology, University of Innsbruck, Innrain 52, 6020 Innsbruck 1 Goals and highlights of excursion The distribution of Cretaceous upper-footwall- and thrust-sheet-top deposits in the western This field trip will show deformation and related Northern Calcareous Alps allows to separate ini- deposition during stacking of thrust sheets in tial Cretaceous nappe-stacking from later thrust the Cretaceous Northern Calcareous Alps. The sheet movements, and in-sequence from out-of itinerary of the field trip is chosen to illustrate sequence thrusting. The analysis of the distribu- different types of synorogenic sediments follow- tion of different types of synorogenic deposits ing the definition of Ortner (2003a) and Ortner & shows that the Inntal thrust is out-of-sequence Gaupp (2007): The first part of the field trip will (Ortner, 2003a), and splays from the Lechtal thrust. visit the spectacular outcrops at Krabachjoch in In the Karwendel mountains, the Inntal thrust the Arlberg region, where a klippe of the Inntal looses offset, the continuity of Inntal- and Lech- thrust sheet overlies the synorogenic deposits tal thrust sheet is partly preserved (e.g., Figs. 48 of the Lech Formation. The latter is an example and 96 in Ampferer, 1942; Ortner & Kilian, 2013). of upper-footwall deposits in the foreland of a thrust sheet. The second part of the field trip visits the Gosau Group of Muttekopf in the vicinity of Hanauer Hütte. There, coarse clastic conti- nental to deep marine synorogenic sediments 2 Geological setting and introduction of overlie the deeply eroded Inntal thrust sheet. basic concepts This type of synorogenic succession has been ter- med thrust-sheet-top deposits. Up to 1000 m The Northern Calcareous Alps formed in a two- of mostly coarse clastic, deep marine deposits are stage process (Froitzheim et al., 1996; Neubauer preserved in a km-scale syncline. Several angular et al., 2000; Schmid et al., 2004). In the western unconformities document fold growth during part of the Northern Calcareous Alps, initial inver- deposition; the most prominent angular uncon- sion of the former passive continental margin formity in the area is the Schlenkerkar uncon- occurred during the late Early to Late Cretaceous formity that formed as a consequence of folding in the foreland of the closure of the Meliata part of and concurrent tear faulting, and was recently the Neotethys ocean (Fig. 1a; Frank, 1987; Ortner, defined as the type example of a fault-related pro- 2003a). At the same time, subduction of the Alpine gressive unconformity (Ortner et al., 2015). The Tethys, also referred to as Penninic ocean, had vicinity of the Hanauer Hütte offers seismic scale already started. Thus, the Adriatic microplate and outcrops of synorogenic sediments and their rela- the Northern Calcareous Alps at its northwestern tionship to preorogenic deposits comparable to edge either occupied an intermediate position the Southern Pyrenees (e.g., Puigdefabegras et between two subductions in the Upper Creta- al., 1992). ceous (e.g., Schmid et al., 2004; Wagreich, 2001), or 141 the position of a single subduction shifted in time directions: Cretaceous nappe stacking and fold- (Stüwe & Schuster, 2010). At the time of stacking ing was NW-directed, while Paleogene shorten- of thrust sheets, the Northern Calcareous Alps ing was N to NNE-directed (Eisbacher & Brandner, were a typical foreland fold-and-thrust belt. In a 1996). WNW-striking fold axes are well developed second stage, the Northern Calcareous Alps were in the thrust sheets of the Rhenodanubian Flysch transported piggy-back as a part of the upper (Rothmund, 2015) and the Arosa Zone (Biehler, plate onto the European lower plate (Fig. 1b). This 1990). However, post-collisional Paleogene and process culminated in continental collision dur- Neogene shortening repeated the structural his- ing the Eocene that extincted the Penninic ocean. tory with Late Paleogene NW-transport, followed From a structural point of view, Cretaceous nappe by Neogene NNE-directed shortening (Ortner, stacking and subsequent transport onto Penninic 2003b; Peresson & Decker, 1997). units are separate events in terms of transport NW SE Central Alps not preserved in subduction transpressive Northern Calcareous basement Eastern Alps of Penninic northern margin of Alps foreland fold-and- involved suturesuture of ocean Adriatic microplate thrust belt thrusting MeliataMeliata ocean future future ATS LTS ITS Arosa CRS Zone ? Penn + . inic o ce an ic c ru st not to scale! a) Albian/Cenomanian NSfault Inntal Tauern + + shear zone . Periadriatic Munich . Window Dolomites Valsugana + . thrust 0 20 km -50 km Penninic units Molasse sediments Adriatic Mesozoic European Mesozoic b) Adriatic basement European basement Recent Fig. 1: The two-stage evolution of the Northern Calcareous Alps. (a) Stacking of thrust sheets at the boundary from the Early to the Late Cretaceous. The foreland fold-and-thrust belt of the Northern Calcareous Alps developed in the foreland of the closure of the Meliata ocean. Penninic subduction had, however, already started (adapted from Wagreich, 2001). CRS = Cenomanrand- schuppe, ATS = Allgäu thrust sheet, LTS = Lechtal thrust sheet, ITS = Inntal thrust sheet. (b) Present day cross section of the Eastern Alps east of Innsbruck. The Northern Calcareous Alps are transported piggy-back on the Helvetic and Penninic thrust sheets, that form the fold-and-thrust belt related to Cenozoic orogeny, onto the European plate (adapted from Lammerer & Weger, 1998). Legend denotes paleogeographic affinity of units. 142 Geo.Alp,Vol.13 2016 The thrust sheets of the Northern Calcarous Alps Frisch & Gawlick, 2003; Tollmann, 1976), still in use. were essentially defined at the start of the 20th In the western Northern Calcareous Alps, three century by Ampferer & Hammer (1911) in the west, main thrust sheets are present. These are, from and, for the central part in Salzburg and Upper base to top, the Allgäu thrust sheet, the Lechtal Austria, by Hahn (1912, 1913). The two tectonic thrust sheet and the Inntal thrust sheet (Fig. 2c). classifications are, with some modifications (e.g., At the northern margin of the Allgäu thrust Late stage - internal shortening thrust-sheet-top deposits erosion preorogenic deposits sea level biogenic buildups Early stage - structure growing olistolithes upper-footwall deposits Gosau-Gr. LTS Branderfleck-Fm. Branderfleck-Fm. south ITS Branderfleck-Fm. ATS FSK LTS north Lech-Fm. Tannheim-Fm. Losenstein-Fm. Tannheim-Fm. Losenstein-Fm. Raibl marker FSK Inntal thrust sheet Allgäu thrust sheet Lechtal thrust sheet “Cenomanrandschuppe” (CRS) Fig. 2: Relationship of thrusting to synorogenic sedimentation. (a) Ramp-flat model illustrating the effects of thrusting in a deep marine environment during an early stage (bottom) and a late stage (top). (b) Distri- bution and timing of synorogenic sedimentation in the Northern Calcareous Alps roughly along the trace of the cross section below. (c) Cross section of the Northern Calcareous Alps (adapted from Eisbacher et al., 1990). Trace of section given in Figure 3. CRS = Cenomanrandschuppe, FSK = Falkenstein klippe, ATS = Allgäu thrust sheet, LTS = Lechtal thrust sheet, ITS = Inntal thrust sheet. 142 Geo.Alp,Vol.13 2016 Geo.Alp,Vol.13 2016 143 sheet, a tectonically deeper unit, the Cenoman- rock may break off the allochthon creating olisto- randschuppe (a marginal slice characterized by liths that are overthrust subsequently. While off- Cenomanian sediments) is present; the Inntal set across the thrust increases and the thrust thrust sheet carries the klippen of the Krabach- sheet climbs up the ramp, relative sea level falls joch nappe in a few places. above the growing hanging-wall anticline, until conditions are suitable for the growth of small, The thrust distance of the nappes is large. Eis- shortlived carbonate platforms or biogenic build- bacher et al. (1990) reconstructed some 20 km for ups. If eroded, this material is redeposited into the Inntal thrust, which is the thrust plane at the the surrounding basins. Upon further growth, the base of the Inntal thrust sheet. The definition of hanging-wall anticline is uplifted above sea level. the large thrust sheets is based on the observation It is eroded and the thickness of the thrust sheet of old rocks thrust onto much younger, usually reduced. As a consequence, the thrust sheet is Cretaceous rocks (Fig. 3). However, in map view, mechanically weaker and gets deformed inter- most of the large thrusts of the Northern Calcar- nally. This is expressed by folding during further eous Alps end laterally, where the stratigraphic shortening, while continental sedimentation offset disappears. The Lechtal thrust, for example, starts. ends near Lech in the Arlberg region (Fig. 3, north of label 1; e.g., Spengler, 1953), and is replaced by In respect to a thrust sheet two types of synoro- the Mohnenfluh thrust to the