The 5.5 m thick Dynow Marlstone in well Oberschauersberg 1 in the Upper Austrian Molasse Basin was studied using mineral and maceral petrography, SEM, organic geochemistry, and C- and N-isotopy of the organic material. The well is located on the former upper slope of the northern basin margin. The depositional period of the Dynow Marlstone covers parts of the Early Oligocene nannoplankton Zone NP23, which corresponds to the culmination of the first Paratethys isolation. The Dynow Marlstone represents a carbonate-rich, organic-poor interval (0.5–2 % TOC) intercalated between organic-rich formations. The onset of the deposition of the Dynow Marlstone was characterized by an abrupt increase in primary carbonate productivity, but persisting photic zone anoxia. Both high organic carbon productivity and photic zone anoxia prevailed during deposition of the Dynow Marlstone. These constant conditions were overprinted by cyclic increases in the trophic level favouring blooms of calcareous nannoplankton. Limestones with low TOC contents were deposited during algal blooms, whereas organic-rich marls accumulated during periods with low production of calcareous nannoplankton. Sulphate reduction extended into the water column. The intensive consumption of labile organic material decreased the hydrogen index. Intensified photic zone anoxia and an increase in salinity worsened the ecological environment for calcareous nannoplankton and led to deposition of the organic-rich marls of the Eggerding Formation within a constantly eutrophic and normal-marine environment.