Thrust tectonics in the Wetterstein and Mieming mountains, and a new tectonic subdivision of the Northern Calcareous Alps of Western Austria and Southern Germany

We investigate the tectonic evolution of the Wetterstein and Mieming mountains in the western Northern Calcareous Alps (NCA) of the European Eastern Alps. In-sequence NW-directed stacking of thrust sheets in this thin-skinned foreland thrust belt lasted from the Hauterivian to the Cenomanian. In the more internal NCA major E-striking intracontinental transform faults dissected the thrust belt at the Albian–Cenomanian boundary that facilitated ascent of mantle melts feeding basanitic dykes and sills. Afterwards, the NCA basement was subducted, and the NCA were transported piggy-back across the tectonically deeper Penninic units. This process was accompanied by renewed Late Cretaceous NW-directed thrusting, and folding of thrusts. During Paleogene collision, N(NE)-directed out-of-sequence thrusts developed that offset the in-sequence thrust. We use this latter observation to revise the existing tectonic subdivision of the western NCA, in which these out-of-sequence thrusts had been used to delimit nappes, locally with young-on-old contacts at the base. We define new units that represent thrust sheets having exclusively old-on-young contacts at their base. Two large thrust sheets build the western NCA: (1) the tectonically deeper Tannheim thrust sheet and (2) the tectonically higher Karwendel thrust sheet. West of the Wetterstein and Mieming mountains, the Imst part of the Karwendel thrust sheet is stacked by an out-of-sequence thrust onto the main body of the Karwendel thrust sheet, which is, in its southeastern part, in lateral contact with the latter across a tear fault.

Lithostratigraphic definition of the Anisian carbonate-ramp deposit of the Annaberg Formation (Middle Triassic, Northern Calcareous Alps, Austria)

Concerning the Middle Triassic stratigraphic succession of the Northern Calcareous Alps (NCA), a modern, litho- and biostratigraphic oriented evaluation of the early- and middle Anisian Annaberg Formation is presented. Due to the fact, that Middle Triassic formations are characterized by a wide distribution within the NCA, any lithostratigraphic definitions of these formations would be of great benefit for mapping geologists, engineers and hydrogeologists. The lithostratigraphic term Annaberg Formation may substitute former designations like “Alpiner Muschelkalk”, “Anisian Limestone and Dolomite” or, partly, “Gutenstein Limestone”. It is exclusively of Anisian age and earlier then the Steinalm and Reifling Formation. Mainly based on microfacies data and lithological data, we define the Annaberg Formation (former: Annaberg Limestone) as one of the most significant Middle Triassic lithostratigraphic units within the NCA. After a detailed description of the type area, findings gained in other areas of the NCA are incorporated to obtain the largest possible overview about the lithological variability and constituents of the Annaberg Formation. As a result, we can describe the Annaberg Formation as mainly organic-rich, medium bedded wackestone, containing remnants of crinoids, little bivalves and gastropods. Typically, fossil-rich layers with accumulations of bivalves and crinoids can often be observed within the Annaberg Formation. In contrast to the Gutenstein Formation no siliceous concretions or fossils (like radiolarians) appear and the fauna is in the main shallow marine. The rock-colour varies from dark- to medium-grey and the bench thicknesses are greater than within the Gutenstein Formation sensu stricto. The fossil content is also larger than in the essentially anaerobe Gutenstein Formation. With respect to the Virgloria Formation the Annaberg Formation is rather planar bedded, not so rich in bioturbation-structures and poor in silica and clay. Hence, the depositional environment of the Annaberg Formation can be described as a restricted carbonate ramp succession, with only minor water movement and separated from the open sea by a shoal with crinoid and brachiopod meadows. Breccias may be an indication for collapse-structures and slumping. In addition, knife-cavity structures (“Messerstichkalke”) indicate an occasional hypersaline environment with precipitation of evaporite-minerals like gypsum. Fossil-rich layers with accumulations of molluscs and crinoids may indicate short-term storm affected sedimentation.

Ophiolite derived material as parent rocks for Late Jurassic bauxite: evidence for Tithonian unroofing in the Northern Calcareous Alps (Eastern Alps, Austria)

In shallow-water limestones of the Plassen Formation in the Tirolic nappe of the Northern Calcareous Alps, bauxite was formed on karstified and tilted platform margin grainstones to boundstones around the ?Kimmeridgian/Tithonian boundary, or in the Early Tithonian as proven by Protopeneroplis striata Weynschenk, Labyrinthina mirabilis Weynschenk, and Salpingoporella pygmaea Gümbel. The platform established on top of the obducted ophiolite nappe stack. The onset of unroofing at the Kimmeridgian/Tithonian boundary exposed ophiolites to weathering, forming laterites, and bauxites. The weathered ophiolitic material was shed on the tilted, emerged, and karstified platform, where the bauxite accumulated. Continued subsidence led to flooding, and a Tithonian transgressive carbonate sequence sealed the bauxites. XRD analysis of the bauxite yields a composition of mainly boehmite with hematite and some berthierine, kaolinite, and chromite. SEM analysis verified magnetite, hematite, rutile, chromite, zircon, ferropseudobrookite, ilmenite, monazite, xenotime, and garnet distributed in pisoids and within the matrix. The pisoids reach a millimeter in size and partly show cores of older, larger pisoids. The composition of the chromites indicates an ophiolitic origin. Geochemical examination using major- and trace elements points to a mafic andesitic to basaltic parent material contaminated with highly fractionated rocks from an island arc. Formation of Early Tithonian bauxites in shallow-water limestones confirms Middle to Early Late Jurassic ophiolite obduction. This was followed by uplift and unroofing of the orogen from the Kimmeridgian/Tithonian boundary onwards after a period of relative tectonic quiescence with an onset of carbonate platforms during the Kimmeridgian on top of the nappe stack and the obducted Neo-Tethys ophiolites.

A multistratigraphic study of the Campanian Postalm section (Northern Calcareous Alps, Austria)

<p>The Postalm section in the Gosau Group (Northern Calcareous Alps) exposes pelagic deposits of northwestern Tethyan origin. We present a magneto-, bio- and chemostratigraphic assessment of this Santonian to uppermost Campanian record, as well as a cyclostratigraphic model for the Tethyan Campanian based on three independently assessed proxies; the δ<sup>13</sup>C signature, the elemental ratio of Fe and the thickness of limestone/marl couplets (Wolfgring et al., 2021).</p><p>The Santonian/Campanian transition is characterised by condensed greyish packstones, the Campanian strata exhibit a succession of limestone-marl couplets that represent orbital precession of an approximate duration of 20ka. A magneto- and biostratigraphic (based on planktonic foraminifera and calcareous nannofossils) framework is supported by carbon isotope and strontium stratigraphy.</p><p>The Sr isotope record matches the data for the Upper Cretaceous and suggests no major gaps in the Postalm succession. A robust cyclostratigraphic assessment of three independently assessed data series (L/M couplets, Fe and δ<sup>13</sup>C) resulted in the identification of eighteen 405 ka eccentricity cycles spanning the middle to upper Campanian (<em>Contusotruncana plummerae</em> to <em>Gansserina gansseri </em>Zones or CC17/UC15 to CC23/UC16 nannofossil zones).</p><p>Carbon isotope stratigraphy identifies the LCE (Late Campanian Event) and possibly the SCBE (Santonian Campanian Boundary Event). Magneto- and biostratigraphic data, in particular the position of the top of the <em>R. calcarata </em>planktonic foraminifera Zone, the position of the LCE and the top of Chron C32r.1r served as primary tie points and constraints to match the floating cyclostratigraphic model to the Laskar solution and to compare it to other cyclostratigraphic solutions and reference sections for the upper Campanian.</p><p>References: Wolfgring, E., Wagreich, M., Hohenegger, J., Böhm, K., Dinarès Turell, J., Gier, S., Sames, B., Spötl, C., Jin, S., 2021. An integrated multi-proxy study of cyclic pelagic deposits from the north-western Tethys: The Campanian of the Postalm section (Gosau Group, Austria), Cretaceous Research, 120, 104704, doi.org/10.1016/j.cretres.2020.104704.</p>