The Middle Eocene maar lake “Groß Zimmern” (Hesse, Southwest Germany): Vegetation dynamics in a disturbed lacustrine record

The maar lake of Groß Zimmern, 10 km east of Darmstadt (Hesse, SW-Germany), is one of about half a dozen isolated Paleogene deposits on and around the Sprendlinger Horst, which is the northern extension of the Hercynian Odenwald basement immediately east of the northern Upper Rhine Graben. A scientific core from 1997 which was drilled in the center of the lake basin revealed 80 m of volcanoclastic breccia and basaltic tuffs in the lower part followed by 30 m of a lacustrine succession of clastic sediments and finely laminated bituminous shale. The bituminous shales start abruptly at the top of the volcanic succession indicating meromictic conditions for the initial part of the deposits. A holomictic lake phase with clastic sediments, which fell as talus or were transported by mass flows from a collapsing crater wall into the basin as typical for the initial lake phase of maar craters, is not preserved. Furthermore, the disturbance of the complete lacustrine succession by slumping, redeposition or in-situ deformation is remarkable. The lamination of the clastic layers is discontinuous and sometimes up to 90° tilted. Possible explanations for the strong disturbance of the sediments are regional tectonic activity, a rapidly subsiding diatreme fill or a CO2 burst. A first palynostratigraphic analysis suggests that the lake sediments are of Middle Eocene age. This age is consistent with the age of other Paleogene deposits on the Sprendlinger Horst, such as the nearby maar lakes of Messel and Offenthal. Despite the disturbance of the lacustrine succession, our quantitative and qualitative palynological study revealed different pollen and spore assemblages. In the lower part of the succession aquatic plants and swamp elements, such as Hydrocharitaceae and Taxodiaceae, as well as marsh elements, such as Restionaceae, dominate. This indicates marginal swampy areas during times of a relatively high lake level. In the upper part of the record these elements decrease significantly or disappear completely and are replaced by elements of a paratropical rainforest with dominance of pollen from the juglandaceous and fagaceous alliances. This can be interpreted by a change to less humid conditions accompanied by a falling lake level. Furthermore, our study shows that the thermal alteration index of pollen and spores surprisingly increases continuously from the lower to the upper part of the core indicating that the maturity of the organic matter increases upward