Facies architecture and sedimentology of a meandering fluvial system: a Palaeogene example from the Weisselster Basin, Germany

Abstract

The Schleenhain open pit coal mine, located 30 km south of Leipzig, Germany, exposes Upper Eocene and Oligocene non‐marine strata representing fluvial deposition in the centre of the Weisselster Basin. Active mining and successive cuts provided the rare opportunity to obtain a three‐dimensional perspective of laterally extensive surface outcrops. These were used to construct a detailed fence diagram, which provided the basis for recognition of key architectural elements in the weakly consolidated meandering stream deposits. In addition to the eight basic architectural elements of Miall (1985), the element SL (shallow lake deposits) was newly defined and the element CH (channel) was subdivided into CHg (palaeo‐river system) and CHk (small channel). The profiles contain parts of two fining‐upward cycles, which are separated by an unconformity spanning the Early Oligocene. Deposits of the first cycle begin with transverse sand bars (downstream accretion deposits‐DA) and point bars (lateral accretion deposits‐LA). The upper part of the cycle is represented by overbank fines (OF) and the element SL, which consists of laterally discontinuous lenses of dark, plant‐bearing, kaolinite‐rich clays, that were deposited in shallow lakes adjacent to the active channel. Coal seams interlayered with palaeosols are the main constituents of element OF. Sheetlike bodies of medium to fine gravels (gravel bars and bedforms‐GB) on an erosive coal surface mark the beginning of the second cycle. Dissolution of underlying Permian salts and sulphates prior to, during, and after the deposition of the Palaeogene strata caused the development of two synclines within the outcrop. Coal seams and clay horizons which thicken and dip towards the centre of the synclines, provide evidence for their chronological development.