Early Burdigalian infill of the Puchkirchen Trough (North Alpine Foreland Basin, Central Paratethys): Facies development and sequence stratigraphy

Abstract

To improve the prediction of gas and oil in strata along the tectonically imbricated southern margin of the Puchkirchen Trough (North Alpine Foreland Basin, Central Paratethys), a better understanding of facies distribution and stratigraphic control of the basin is essential. The present study provides bio- and chemofacies analyses and a biostratigraphic evaluation for the pelitic Hall Formation from the borehole Hochburg 1 in the central part of the Puchkirchen Trough. A statistical evaluation of benthic foraminiferal assemblages together with geochemical proxy records (TOC, sulphur, hydrogen index, δ13CTOC, δ15NTN) reveals a succession of early Burdigalian depositional environments.Following a major subaqueous erosional hiatus, conglomeratic sands at the base of the section, which contain reworked Chattian and Aquitanian foraminiferal assemblages, record the reactivation of a basin-axial channel system. During the early Burdigalian eustatic sea-level rise, the channel was cut off from its sediment sources on the shelf and a deepening bathyal environment was established. Agglutinated foraminiferal assemblages with abundant Bathysiphon filiformis developed that were adapted to an unstable environment with frequent deposition of turbidites. The middle part of the Hall Formation is characterized by prograding high-relief clinoforms, which are fed by the Wachtberg Delta of the Paleo-Inn river. High sedimentation rates are interpreted and increased input of terrestrial-derived organic matter is documented with a strong correlation of TOC and TOC/S and low HI values, as well as frequent occurrences of Ammodiscus spp. and other opportunistic agglutinating foraminifers. A renewed transgression reestablished a eutrophic and suboxic bathyal environment, followed by the development of an oxygenated outer–middle neritic shelf environment heralding the ultimate infill of the deep-water basin.Based on a comparison of the revealed development to existing sequence stratigraphic models for the Puchkirchen Trough, three sequences and their corresponding systems tracts can be identified for the lower, middle and upper Hall Formation. Biostratigraphic evidence from benthic foraminifers and calcareous nannoplankton indicates that they correspond to the regional substages of the middle and upper Eggenburgian and lower Ottnangian, and to global 3rd-order sequences Bur 1–3. The observations suggest a primary control of eustatic sea-level on the Puchkirchen Trough rather than Alpine tectonics during the Burdigalian.