PALEOECOLOGY OF LAKE SEDIMENTS: CASE STUDY FROM MIOCENE OF THE CZECH REPUBLIC (EUROPE)

A project on lake sediments in the Sokolov Basin (system of Tertiary rift basins) combines various disciplines in order to characterize paleoclimatic and paleoenvironmental changes. Its depositional history began in Late Eocene and spanned until Early Miocene. Its sedimentary sequences consist of fluvial, lake deposits and coal seams. Current project is focused on the upper part of the basin sequence – Cypris Fm. that is represented by lake deposits.

The drill core representing 70 m deep section was sampled in 20 cm intervals for rock magnetic analyses. Based on bulk magnetic susceptibility (MS) and anisotropy of magnetic susceptibility (AMS) data we are able to divide the studied section to three segments showing distinct changes in magnetic mineralogy. The upper segment (0–15.5 m) reveals relatively low MS (< 5×10^-3 SI), very high AMS degree and oblate magnetic fabric indicating a dominance of pyrhotite. Middle segment (15.5–37 m) is characteristic by increased MS values (up to 18×10^-3 SI), relatively lower AMS degree and oblate magnetic fabric given by greigite presence. In the contrary to upper segments, the sediments in lower segment (37–75 m) show very low MS (except on narrow horizon 49–50 m), lower AMS degree and inverse magnetic fabric most probably carried by siderite. Changes in magnetomineralogy reflect depositional and post-depositional processes, which controlled the iron mineral formation.

For pollen analyses the samples were collected in 50 cm intervals. The lower part of the sequence is characterized by highly diversified pollen association that contains termophilous and halophile elements. The upper part of the formation is characterized by abundant occurrence of green algae Botryococcus. It is accompanied by Pinaceae pollen. Many elements present in the lower part of the formation do not occur here (Restionaceae). Spores of ferns are rare.

Diversified pollen associations in the lower part of the formation indicate a near-shore environment during the initial stages of lake formation. Uniform association in the upper part of the formation dominated by green algae and conifers pollen corresponds to later stages of the lake evolution. It represents an off-shore facies of much more extensive lake.

This research was supported by Grant Agency of the CR (GA205/09/1162) and by the AS CR, v.v.i. (Z30130516).