Facies and implications of a coarse-grained lacustrine onshore paleo-tsunamiite: An integrated study of an upper Miocene bouldery cobble gravel

摘要

Gravelly tsunami event beds are well known from some contemporary shores, but they are far less known from the pre-Quaternary depositional record. Lacustrine examples are particularly exceptional. Here we present multidisciplinary evidence for a lacustrine cobble to boulder gravel deposited as an onshore paleo-tsunamiite and discuss features that help to identify the depositional setting. The deposit was formed on the rocky shore of a large peninsula within the late Miocene Lake Pannon (Hungary). The lake was several hundred meters deep, and near to the study area it was about 50-80 km wide. The inferred paleo-tsunami deposit includes subangular clasts that are exclusively derived from the underlying Cretaceous sandstone. Imbrication of the bouldery gravel points to landward transport of the clasts, however, a 15-cm-thick lens of cross-bedded sand records an opposing flow direction. The clayey sand matrix contains fossils, i.e. fragmented and articulated mollusc shells, as well as ostracods, that lived on the nearby sandy shoreface and offshore. The facies of the gravel indicates a short-distance transport of clasts typical for an extreme wave event. None of the depositional features e.g. various degree of clast roundness, seaward dipping imbrication, bidirectional transport indicators, clast-supported fabric, large thickness of beds, and lack of grading is either unique to tsunami deposits or excludes storm wave origin. An extreme storm wave origin, however, is discarded in this case, when paleogeography and climate are considered. The combination of matrix-supported fabric, presence of a mixed clayey-sandy matrix in the clast-supported parts, and preservation of articulated mollusc shells, as well as mixture of clasts and fossils from different zones of the shore, identify these beds as paleo-tsunami deposits. As coeval volcanism is not known, a hypothetic tsunami could have been related either to an earthquake along nearby normal faults or to large-scale sliding events on the closely located shelf slope due to rapid deposition or seismic shocks. The recognition of these event beds reinforces structural observations on the syn-depositional character of nearby faults, active after the middle Miocene climax of synrift tectonics in the Pannonian Basin. Thick, boulder-sized onshore paleo-tsunamiites are useful indicators of a paleogeographic situation in which tsunami waves were amplified to produce noticeable beds in the rock record.