Onlap of Upper Cretaceous synorogenic wedge-top deposits (Eastern Alps): Fragmentites, event beds, and coseismic deformation.

Abstract

Abstract In synorogenic wedge-top basins of the Alps, the basal strata are rarely exposed. Herein, a late Turonian to early Coniacian succession above an older rock substrate is described that comprises: (a) proto-/cataclasites, (b) slope breccias, (c) fluvial conglomerates, and (d) low-energy lagoonal limestones with event beds. The proto-/cataclasites are a vestige of a fault damage zone. The distribution of the other facies records a hilly vegetated terrain drained by a stream and that, upon sea-level rise, became encroached by a lagoon. A section of 'lagoonal' limestones contains a thick event bed of angular dolostone pebbles to cobbles derived from the local substrate, and embedded in hybrid arenite. Bioclasts (rudists, corals, red algae) are from open-marine environments. Angular shape and a corrugated surface of lithoclasts suggest that they stem from slope mantles of subaerially exposed areas. The bed records an event of exceptional coastal flooding and is tentatively interpreted as a tsunami deposit. In the slope breccias, the fluvial conglomerates, and in the event beds, many lithoclasts became fractured in situinto crackle, jigsaw, and mosaic subclast fabrics; fractures are filled with the same material (lime mud to arenite) as that comprising the primary matrix. In-situ clast fracture was produced by coseismic shaking, i.e., these fabrics record syndepositional earthquakes. In the Meso-Cenozoic wedge-top succession of the Eastern Alps, features related to seismic activity and coastal inundation expectably are widespread, but to date were overgone or not identified as such.