Lacustrine Paper Shales in the Permocarboniferous Saar-Nahe Basin (West Germany) — Depositional Environment and Chemical Characterization

Abstract

Lacustrine paper shales in Carboniferous and Permian rocks of the Saar-Nahe Basin, West Germany, are described in terms of their sedimentology, petrography and geochemistry.

The lakes formed mostly on flood plains associated with fluvial environments. They were exposed to a tropical climate. Thus, the bedding of the lacustrine shales shows an even lamination from layers of light silt to dark clay. The graded silts are correlated with the suspensions swept into the lakes by heavy rainfalls which are frequent in tropical summers. The clays are rich in particulated organic matter and most often associated with calcite individuals, so that biogenic carbonate precipitation by blooms of floating algae has occurred. The suspension input to the lakes was low during tropical winters. The lakes were shallow and eutrophic, lacked oxygen in the pelagic zone, and had no benthos.

Calcite quite often was converted to ankerite and siderite in the reducing conditions of the sediment. Thus, the organic matter is well preserved.

The chemistry of the sediments, characterized by major, trace and rare earth elements, is quite distinctive for each of the three types of paper shales, the siliciclastic ones, those containing carbonate and the pure carbonates.