Lower Miocene olistostromes and giant-olistoliths: A new interpretation of the Eocene Waschberg Limestone occurrences and consequences for the structural composition of the southern Waschberg–Ždánice Unit in Lower Austria

Abstract

The Waschberg–Ždánice Unit links the Alpine and Carpathian orogens. Its complex structural and sedimentary structures lack a modern interpretation, particularly in the Austrian part. In recent years, the southern end of the Waschberg–Ždánice Unit has been geologically mapped in detail. Nine large occurrences (km-size) of the Waschberg Limestone, particularly at Waschberg, Michelberg, Praunsberg, and at some unnamed places continue into and strike in line with the widespread olistostromes. They are consequently interpreted as giant-olistoliths and represent products of submarine mass transport processes contemporaneous with the adjacent olistostromes. Signs for large-scale imbricate structures (repetitive sequences) or interpretation as tectonic klippen were not found. Based on the detailed geological mapping, some previously unknown structural elements are introduced, such as Haselbach Wedge and ”crunch-zone”. The Waschberg Limestone itself is an allochthonous mixed sediment (high density debrites and turbidites) that contains shallow water benthic (e.g., Nummulites) and deep-water planktic foraminifera of different age. Formation and final deposition of the Waschberg Limestone included sedimentation of Ypresian larger foraminifera and other biogenic grains in an Ypresian/basal Lutetian basin, detachment and transport towards the north-west, mixture with crystalline basement fragments and Flysch components in an Egerian or basal Eggenburgian foredeep, exposure on unstable slopes of the thrust front, and finally mobilization and basinward transport of olistostromes and Waschberg Limestone giant olistoliths during the Eggenburgian. The formation of olistostromes and giant-olistoliths may be indicative for the increased velocity or higher intensity of the thrusting processes during the early Miocene.