A middle Eocene treefall pit and its filling: a microenvironmental study from the onset of a forest mire in the Geiseltal (Germany)

The treefall pit of a large tree at the base of the middle Eocene Mittelkohle has been studied in the former open-cast mine Neumark Nord of the Geiseltal Mining District. Above a light clastic soil, the sedimentation in the pit started with backfall and downwashed material grading upwards into pond deposits that filled the pit. This represents a high-resolution section equivalent in time to the initial peat-forming environment. Palynology revealed a striking difference between a fern-dominated herbaceous plant community below the pit and a pulse of washed-in fagaceous pollen ( Tricolpopollenites liblarensis ) at the base of the pit. This is overlain by an assemblage of woody taxa typical of a peat swamp forest associated with a number of tropical elements. Comparison with a nearby but undisturbed transition from underlying sediments to the seam confirms the ecotonal character of the parent plant of T . liblarensis , and suggests that Cupressaceae s.l., Nyssaceae and Myricaceae especially benefited from moisture and light in the clearing of the treefall pit. The transition from the underlying sediment to the lignite represents a sharp break in the sedimentary regime from clastic to purely organic material with a bounding surface colonised by large trees, most of them preserved as individual stumps, but including some fallen logs.