Isopollen maps as a tool for the reconstruction of a coastal swamp from the Middle Eocene at Helmstedt (Northern Germany)

Abstract

In the Middle Eocene lignite bearing succession at Helmstedt, isopollen maps have been constructed from a thin, highly carbonaceous horizon in the interbed between two seams of the Wulfersdorf member. During mining progress this carbonaceous band could be traced in the mine high wall over an area of 2 km by 0.5 km and is considered to represent an isochronous horizon. 28 samples were collected and analyzed. On the basis of the isopollen maps it is possible, to distinguish between allochthonous and autochthonous elements. Ultimately, by comparison of the maps individual plant associations and environmental conditions may be recognized. In case of the carbonaceous horizon of the Wulfersdorf seam group a coastal swamp succession can be traced from a mangrove fringe through a brackish marsh and a freshwater marsh to a mire forest. The distribution of mangrove elements likeRhizophora, Avicennia andNypa in this horizon clearly shows that the shoreline was to the northwest and advanced from there inland to the southeast. A fourth element,Psilodiporites iszkaszentgyoergyi, is restricted in distribution and closely associated with the other known mangrove types and therefore considered here to be a part of the mangrove fringe. Behind the mangrove zone, pollen of Restionaceae and Sparganiaceae as well as fern spores are more abundant than elsewhere in the Wulfersdorf seam section suggesting that a reed-like vegetation with fern and aquatic herbs developed here under rather wet conditions. A hammocky distribution of corresponding plants is indicated by the isopollen map of palms likeArecipites spp. and other elements. The main peat forming vegetation, however, was a mire forest dominated by Fagaceae alternating with a Betulaceae dominated forest including Myricaceae and Juglandaceae.