Ichnology of shallow sublittoral siliciclastics of the Burgschleinitz Formation (Lower Miocene, Eggenburgian) in the Alpine-Carpathian Foredeep (NE Austria)

The marine shallow water sediments of the upper Eggenburgian (Lower Miocene, Lower Burdigalian) in northeastern Austria illustrate a unique, complex palaeogeographic situation. The transgressing Paratethys Sea in the Alpine-Carpathian Foredeep encroached on a morphologically highly structured crystalline area of the Bohemian Massif, forming several small islands and bays. These morphological depressions were filled with mostly immature sediments derived from the first cycle of weathering. The Burgschleinitz Formation is characterized by an alternation of moderately to poorly sorted, coarse to fine sands with gravelly intercalations. Sedimentological and ichnological analyses allow the reconstruction of a wave- and storm-dominated littoral to shallow sublittoral environment in shallow bays sheltered by crystalline barrier islands. The marine sands of the outcrop "Burgschleinitz Kirchenbruch", which is the type locality of the Burgschleinitz Formation, were deposited in a small bay open to the west and close to the nearby crystalline elevations. In the Kirchenbruch quarry, at least 11 ichnotaxa were recognized, which is the highest diversity of trace fossils in the nearshore sediments of the Eggenburg Bay. In the middle part of the section the vertical trace fossil assemblage with Ophiomorpha, Skolithos, and Arenicolites suggests a colonization window; here, after a short regressive phase or stillstand, a stabilization of the sea floor related to new flooding can be assumed. This assemblage is indicative of the Skolithos ichnofacies, located in the upper shoreface. Upsection, the transition to the proximal Cruziana ichnofacies, which is typically positioned in the middle-lower shoreface, is indicated by Thalassinoides. In the muddy sands of the topmost Gauderndorf Formation, the lack of vertical trace fossils indicates the deepening during the proceeding Early Miocene (Eggenburgian) transgression.