Another maar lake, another story: Palynological study of Eocene lacustrine sediments at Groß Zimmern (Hesse, Southwest Germany)

The lacustrine succession at Groß Zimmern, 10 km east of Darmstadt (Hesse, SW-Germany), represents one of half a dozen of isolated Paleogene deposits scattered across the Sprendlinger Horst, the northern extension of the Hercynian Odenwald basement in Southwest Germany flanking the Upper Rhine Graben to the northeast. Scientific drilling in the center of the structure recently revealed a reference core, including 80 m of a volcanoclastic breccia overlain by 30 m of a lacustrine succession of clastic sediments and finely laminated bituminous shale. However, the whole lacustrine succession shows signs of slumping, redeposition or in-situ deformation. The discovery of massive volcanoclastic deposits proved that the overlying lake sediments have been deposited within a small maar structure with a diameter of 250 to 400 m. A first palynological analysis now suggests that the lake sediments were of Middle Eocene age. This age is consistent with the age of other Paleogene deposits on the Sprendlinger Horst, such as, e.g., the nearby maar lake of Messel. Our quantitative 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, dominated indicating a relatively high lake level causing flooded habitats in the crater area. In the upper part of the record these elements decreased significantly or disappeared completely. This can be interpreted by a change to less humid conditions accompanied by falling lake level. Changes in the pollen assemblages are mainly restricted to plants living at the lake shore. However, the dominating elements of the surrounding (para) tropical forest, such as Plicatopollis spp. (Juglandaceae) and Tricolporopollenites cingulum (Fagaceae) are characterized by cyclic frequency fluctuations. Since the lacustrine sediments are completely disturbed, probably due to seismic activity or rapid subsidence within the crater, the interpretation of palynological assemblages in terms of paleoenvironmental trends has to be treated with caution. However, the general trend obviously follows a pattern common to the lacustrine filling of other nearby maar structures which indicates that the succession was not completely mixed up by disturbance