Late Oligocene to Early Miocene foraminifers and ostracods from Karben (Wetterau,Hesse State,Germany): stratigraphic occurrence and palaeoecological implications

Foraminiferal and ostracod assemblages from two upper Oligocene to lower Miocene sections located in the Wetterau region (NE Frankfurt a. M.) were analysed. The lithologie units investigated encompass Cyrenenmergel, Glimmersand, Vilbeler Kies, Cerithiensand, Cerithienkalk andCorbicula-Schichten. Hemicyprideis basiliensis, H. helvetica andCypridopsis? cyclocypriformae are characteristic of Cyrenenmergel deposits, representing the last marine influence around the Rupelian/Chattian boundary. The ostracod assemblages from the Cerithiensand are characterized by variousHemicyprideis species, of whichH. rhenana is the most frequent. The microfossil assemblages of the Cerithiensand indicate normal marine conditions for the basal and upper portions of the unit and slightly brackish to normal marine conditions for the middle portion. Very low salinities are indicated for some levels close to the base (around levels of emergence above sealevel) and for the clay layer around sample 30 in the lower part of the Cerithiensand. The Cerithienkalk represents a transitional facies from normal marine to brackish/freshwater conditions as indicated by successive higher contents of stress tolerant foraminifera and freshwater ostracods. TheCorbicula-Schichten are characterized by largeEucypris-moulds and various proportions ofCypridopsis spp. (freshwater indicators). When foraminifera (Bolivina spp.) are present, the fauna indicates slightly higher but still low salinities for a few levels of theCorbicula-Schichten. Oxygenation levels were generally high (presence of large ostracods and epifaunal foraminifera) and vary from suboxic to high oxic. As in the Mainz Basin, the last occurrence ofHemicyprideis rhenana marks the upper boundary of the Cerithienschichten.Protelphidium nonioninoides can be used to identify the Cyrenenmergel when it occurs in high numbers. Cerithiensand-samples show the highest diversities and contain high portions ofUvigerinella michelsi. Overall, highest diversities and normal marine salinities are reached during deposition of the basal and upper parts of the Cerithiensand, which may correspond to global sealevel rises at the end of the Chattian.