Geo.Alp, Vol. 13 2016 215 - 228 Field trip 8 Innsbruck´s geology in a nutshell: from the Hafelekar to the Hötting Breccia Karl Krainer1 and Michael Meyer1 1 Institute of Geology, University of Innsbruck, Innrain 52, 6020 Innsbruck 1 Introduction Alpen, southwest of Innsbruck Stubaier Alpen and north of Innsbruck Karwendel as part of the Hafelekar Spitze provides an excellent panora- Northern Calcareous Alps. mic overview of the main geologic units around The excursion starts at the cable railway station Innsbruck. The city of Innsbruck is located on an Hungerburgbahn next to the Innsbruck Congress alluvial fan which was formed by the Sill river at Centre (Fig. 1). The Hungerburgbahn will bring us its confluence with the Inn river. The mountain onto the Hungerburg terrace (868 m), from there group southeast of Innsbruck is termed Tuxer we continue with a cablecar (Seegrubenbahn ) to Fig. 1: Map showing excursion route to Hafelekar Spitze (Wetterstein Reef Complex) and Hungerburg – Alpenzoo (Hötting Breccia). 215 Figure 1 Seegrube (1905 m) and ascend Hafelekar (2260 m) composed of the famous Höttinger Breccie (see . via a second cablecar. From the top station of the excursion route part 2 – the Hötting Breccia) Hafelekarbahn the excursion route follows the The Valley which extends from Innsbruck to the trail to Hafelekar Spitze and continues East where Brennerpass in the south (Silltal-Wipptal) repre- the trail meets the Goetheweg, one of the most sents a big west-dipping normal fault (Brenner spectacular panoramic trails around Innsbruck. Abschiebung) which formed as a result of N-S- Following the Goetheweg we come back to the compression and E-W-extension of the Eastern cablecar station at Hafelekar. Hafelekar Spitze Alps. Deformation along the fault was brittle in provides a good opportunity to introduce the the northern part and ductile in the south (Bren- regional geological setting and to summarize the nerpass). The fault was active during the Neogene. Quaternary dynamics of the Inn valley. Several At the Oligocene-Miocene boundary the rocks of outcrops along the hiking trail allow the Hafelekar the western part of the Tauern Window were still Reef Complex to be studied (Fig. 1). located at a depth of approximately 23 km. Since The second part of the excursion focuses on a 13 Ma rocks of the Tauern Window and Innsbruck famous Pleistocene clastic sediments succession Quartz Phyllite formed the footwall which were north of Innsbruck known as the Hötting Brec- exhumed along the Brenner normal fault. In the cia. The excursion route starts at Hungeburg and north the Brenner normal fault joins the sinistral follows the trail to Lepsiusstollen, a classic loca- fault running along the Inn Valley. tion of Quaternary Geology in the Alps, and from there to an abandoned quarry (Mayr´scher Stein- bruch), where the Höttinger Breccie is well expo- 2.2 Northern Calcareous Alps sed. From the quarry we will walk to the Alpenzoo Hafelekarspitze is one of the peaks of the Nord- (Innsbruck´s zoological garden) from where the kette which is part of the Karwendel Mountains. Hungerburgbahn carries us back to Innsbruck The Karwendel Mountains belong to the Nor- (Fig. 1). thern Calcareous Alps, a main geologic unit of the Eastern Alps extending from Liechtenstein over approximately 500 km to the East (Baden bei Wien). The Northern Calcareous Alps are a com- plex geologic unit formed of a number of stacked nappes. NCA are overthrusted on the underlying 2 Geologic Overview nappes of the Helvetic zone and Rhenodanubian Flysch. From Schwaz to the East rocks of the Gray- 2.1 Inn Valley and Sill Valley wacke Zone are exposed south of the NCA. The The Inn Valley marks one of the large, approxi- Graywacke Zone formed the basement of the mately ENE-WSW-trending sinistral faults sepa- thick sedimentary pile of the NCA. West of Schwaz rating the Northern Calcareous Alps north of the the Inntal Fault separates rocks of the Innsbruck fault from the geologic units (Innsbruck Quartz Quartz Phyllite and Ötztal-Stubai Metamorphic Phyllite Complex, Ötztal-Stubai Metamorphic Complex from the NCA. Complex) to the south (Fig. 2). The NCA are mainly composed of carbonate sedi- A core drilling near the Innsbruck airport showed mentary rocks, subordinately of siliziclasitc rocks that the Valley in the area of Innsbruck is filled and cherty sediments (radiolarite); volcanic rocks with more than 360 m thick Quaternary sedi- are very rare (see Fig. 2). Sedimentation started ments. The Inn Valley SW, SE and NE of Innsbruck is during the Lower Permian. During the Permian also characterized by distinct terraces („Mittelge- nonmarine red beds and evaporitic rocks („Hasel- birgsterrassen“) formed by Quaternary sediments gebirge“) were deposited. In the westernmost (lacustrine sediments – Bändertone – grading into part of the NCA acid volcanic rocks are interca- sandy and gravelly glacial outwash sediments – lated. During the Lower Triassic a transgression Vorstoßschotter – overlain by lodgment till of the from East to West occurred, causing deposition last glacial maximum). The terrace north of the of shallow marine siliciclastic and carbonate sedi- city of Innsbruck (Hungerburgterrasse) is mainly ments of the Werfen Formation in the Eastern 216 Geo.Alp,Vol.13 2016 Fig. 2: Stratigraphy of the Triassic successionFigure in the western 2 part of the Northern Calcareous Alps. part of the NCA which towards the West inter- Upper Triassic extremely thick platform carbona- finger with the nonmarine deposits of the Alpine tes accumulated (Wetterstein Formation: 800 m, Buntsandstein (Fig. 2). During the Middle and Hauptdolomite up to 1800 m. Fig. 2). During the Upper Triassic mainly shallow marine carbonate Jurassic dominantly pelagic sediments (limesto- sediments were deposited. Locally intraplatform nes, marls, cherty sediments) were deposited in basins developed which were filled with basinal a number of pull apart basins which developed sediments (limestone and mudstone). The sedi- as a result of the opening of the Central Atlan- mentary succession of the Triassic is more than tic and Penninic Oceans. During the Cretaceous 3000 m thick, particularly during the Middle and pelagic sedimentation was increasingly replaced 216 Geo.Alp,Vol.13 2016 Geo.Alp,Vol.13 2016 217 by siliciclastic deep sea sediments as a result of Mesozoic (Fig. 3). The succession can be divided plate tectonic processes (beginning of the sub- into a Permotriassic part composed of quartzite, duction of the Penninic Ocean). evaporitic rocks (Rauhwacke), limestone, dolo- Compressional tectonics („Pre-Gosauic move- mite, mudstone and sandstone, and a Jurassic ments“) resulted in overthrusting of nappes and part including sandstone, mudstone, breccia uplift above sea-level. During a short extensional (Tarntaler Breccie), cherty limestone, calcareous phase sedimentary basins (Gosau Becken) deve- schist, cherty sediments (Radiolarit), serpentinite loped which were filled with fluvial, lacustrine and ophicalcite. The rocks underwent polyphase and shallow- to deep marine sediments. Com- alpidic deformation. The Tarntal Mesozoic is part pressional tectonics caused deformation of the of the Lower Austroalpine Nappe System (Tarntal Gosau sediments and overthrusting (Tollmann and Hippold nappes). Reckner Nappe (calcareous 1976, Brandner 1984, Mandl 2000). schists, phyllitic rocks, marble, siliceous schists, serpentinite, ophicalcit) are assigned to the Pen- ninic nappe system of the Tauern Window (Enzen- 2.3 Innsbruck Quartz Phyllite Complex berg, 1966; Rockenschaub et al., 2003b). This geologic unit which is exposed southeast of Innsbruck (Fig. 3) is a thick succession of mainly phyllitic rocks. The succession is divided into 2.6 Tauern Window (Penninic and a lower unit composed of quartz phyllite with Subpenninic nappes) intercalations of greenschist and porphyroids, a South of the Innsbruck Quartz Phyllite and Tarntal middle unit composed of phyllitic rocks and inter- Mesozoic rocks of the Tauern Window are expo- calated marbles, and an upper unit consisting of sed and composed of various rocks that belong black phyllitic rocks and intercalated marbles. to the Penninic and Subpenninic nappes respec- An Ordovician to Devonian age is assigned to tively (Fig. 3). Orthogneiss („Zentralgneis“) and the succession, based on conodonts and litho- metamorphic rocks of continental crust („Altes logic correlations with similar geologic units in Dach“: micaschist, paragneiss, amphibolite) form the Eastern Alps. The rocks are characterized by a large part of the Subpenninic Nappes. Meta- a polyphase (pre-alpidic and alpidic) deformation morphically overprinted Permotriassic to Juras- and metamorphic history (Haditsch and Mostler, sic sedimentary rocks of the Helvetic shelf such 1982; Rockenschaub et al., 2003b). as quartzite, marble, various phyllites („Bünd- The Innsbruck Quartz Phyllite was long conside- nerschiefer“) are also part of the Subpenninic red part of the Lower Austroalpine Nappe system, Nappes. Penninic nappes include the Matreier but recently was assigned to the Upper Austro- Zone and Nappes forming the northern margin alpine Nappe System (Silvretta-Seckau Nappe of the Tauern Window). These nappes are mainly System) by Schmid et al. (2004). composed of metamorphic sedimentary rocks (various types of phyllitic rocks; originally deep marine marly and muddy sediments of the Pen- 2.4 Patscherkofel Metamorphic Complex ninic Ocean) locally containing olistoliths of Tri- Locally rocks of the Innsbruck Quartz Phyllite assic