The palaeoecology of the Lower and Middle Eocene at Helmstedt - a study in contrasts

Abstract

The lignite-bearing Eocene sediments at Helmstedt are divided into a Lower Eocene and a Middle Eocene unit separated by a partially marine bed of about 40 m in thickness. Lignites occur in both units in several seams varying somewhat in number, composition and thickness due to lateral and vertical facies variation. In each unit the lower seams are separated by more or less marine deposits while the upper seams are associated mainly with fluvial sediments. The general environmental setting in either unit is rather similar, characterized by an alternation of peat forming and marginal marine to fluvial clastic environments located on a coastal plain. The seams, however, are totally different in petrographic constitution as well as palynological and palaeobotanical content. The seams of the Lower Eocene have a high content of fusain and woody tissue including marked tree stump horizons. The 10 m thick main seam (“Hauptflöz”) and seam 1 in the Lower Eocene in particular are characterized by rapidly changing palynofloras with pronounced peaks of Sphagnaceae, fern spores, Taxodiaceae and palms. In contrast, the seams in the Middle Eocene are almost devoid of fusain layers and finely dispersed fusinite and recognizable woody tissue is rare while the Fagaceae and Juglandaceae form the dominant element in the palynoflora. In the Lower Eocene, environments of peat formation appear to have been frequently disrupted by a fluctuating water table allowing for periodic or episodic fires and floods, while peats in the Middle Eocene grew under the influence of a rather persistent intermediate water table and a very equitable perhumid climate. In contrast, climatic and environmental conditions during the Lower Eocene appear to have been significantly less stable and slightly cooler.