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Lutetian to Priabonian dinocyst assemblages from the northwestern Tethyan margin (Adelholzen section, Eastern Alps, Germany)

Mohamed, Omar, Egger, Hans
Review of palaeobotany and palynology 2019 v.264 pp. 38-53
Eocene epoch, Miozoa, Retaria, limestone, marine environment, phosphates, sandstone, sea level, sediments, Germany
This paper is the first systematic documentation of Eocene organic-walled dinoflagellate cyst (dinocyst) assemblages from the northwestern shelf of the Tethys. A taxonomic list of the rich and well-preserved dinocyst assemblages found within 28 samples collected at the Adelholzen section in southeastern Germany is presented herein. The sediments accumulated during the Lutetian and Priabonian stages at a ramp-type margin. One-hundred organic-walled dinocyst species were found at the section, which is riddled with unconformities. The composition of the successive dinocyst assemblages reflects the sea-level changes within the depositional area. Areoligera coronata and Cordosphaeridium gracile dominate the assemblage within the glauconite-rich siliciclastic sandstone (lower Adelholzen beds) of the lower transgressive system tract, which spans the calcareous nannoplankton sub-Zones NP15a and NP15b. A hiatus comprising at least c. 1.5 million years truncates these deposits and indicates an erosional episode. Rapid sea-level rise in Biochron NP16 caused a new transgression and the deposition of calcareous marlstone and limestone rich in nummulitids. In this part of the section (middle Adelholzen beds), Homotryblium tenuispinosum is by far the most abundant dinocyst species, and rare but consistent occurrences of Impagidinium spp. suggest a more open marine environment. A lithological shift from limestone to marlstone, a change from nummulitids to discocyclinids, and finally the complete disappearance of larger benthic foraminifera indicates continuous deepening of the environment. An increase in Spiniferites spp., Operculodinium spp., and occurrences of Homotryblium floripes characterize the dinocyst assemblage of this marlstone (upper Adelholzen beds). A 0.5 m thick condensed layer, which consists essentially of iron-coated glauconite grains and scattered phosphate nodules, represents the maximum flooding surface of the transgressive sequence, for which a paleodepth of c. 300 m is assumed. The overlying marlstone (“Stockletten”) of the highstand system tract is punctuated by a hiatus of c. 4.3 million years spanning the entire Bartonian. The stratigraphically important species Rhombodinium longimanum, Rhombodinium perforatum, Distatodinium ellipticum, Nematosphaeropsis labyrinthus and Selenopemphix nephroides have their lowest occurrences in the Priabonian (Zones NP18 and NP19) of the section.