Facies development along the tide-influenced shelf of the Burdigalian Seaway: An example from the Ottnangian stratotype (Early Miocene, middle Burdigalian)

Abstract

Herein, we report quantitative micropaleontological (benthic foraminifers, dinoflagellate cysts, calcareous nannoplankton), sedimentological (grain-size analysis) and geophysical (background gamma radiation) analyses from Ottnang–Schanze, the stratotype for the regional Ottnangian stage (Central Paratethys; Lower Miocene, middle Burdigalian). The revealed trends in bathymetry, primary productivity, bottom-water oxygenation and water energy allow exemplary insights into the paleoenvironment of the terminal Burdigalian Seaway. Several facies of a eutrophic environment are distinguished that reflect a transition from a suboxic outer neritic to upper bathyal towards a better oxygenated middle neritic setting under the influence of storm events and currents. A comparison with available data from Upper Austria and Bavaria consistently shows the regressive trend during the late early Ottnangian. In Upper Austria, the deep-water facies from the lower part of the stratotype represents the most distal sediments. The upper part together with localities closer to the northern coast records inner to middle neritic environments that are heavily affected by tidal currents. The facies distribution results from the progradation of a tide-influenced environment along the northern shelf of the North Alpine Foreland Basin, heralding the closure of the Burdigalian Seaway. The available age estimate for the stratotype constrains the onset of the regressive phase to 18 Ma. ► We present a detailed facies analysis of the Ottnangian stratotype. ► Microfossil and lithological data indicate a eutrophic environment. ► A shallowing water depth resulted in increasing water-energy and oxygenation. ► The revealed trends reflect the final regressive phase of the Burdigalian Seaway. ► The beginning of the regression can be dated to c. 18 Ma.