Abstract
The intracontinental Lower Miocene Cypris paleo-lake originated during progressive subsidence in the Sokolov Basin, part of the Cenozoic OhA (TM) e Rift, after the deposition of coal seams. The Cypris Fm. consists almost entirely of lacustrine clays with variable mineral composition and organic matter, where this succession is 70-120 m thick.
The main objective of this study was to interpret the geochemical history of the Lower Miocene Cypris Fm. using high-resolution, down-core geochemical records and study of the organic matter. This work revealed that the lower part of the lacustrine sediment sequence was deposited in a freshwater lake, in an open hydrological system.
An increase in the K/Zr and K/Ti ratios towards the upper part of the Cypris Fm. indicates a gradual increase in the pelitic fraction of the local sediments and/or a decline in input of volcanic material. Simultaneously, increasing Ca/K and Sr/K ratios indicate the precipitation of carbonates, predominantly dolomite and siderite.
In the upper part of the Cypris Fm., there is a significant increase in Na/K, Na/Zr, and Na/Ti ratios, suggesting increasing salinity (alkalinity) of the paleoenvironment in a closed hydrological system. Reaction between the Na-rich water and clastic components of the sediment in an alkaline medium gave rise to the formation of zeolites, mixed-layer clay minerals and smectite.
Abundant remains of aquatic organisms, especially algae, increased with greater salinity in the upper part of the Cypris Fm. This is reflected in the greater hydrogen index (HIRock Eval), and the growing proportion of liptinite group macerals of aquatic origin in the bulk organic matter.
During the entire history of sedimentation in the Miocene lake, repetitive changes in the sediment geochemistry occurred at both micro- and macroscales, and fluctuations of K/Ti, K/Zr, and Sr/Ca ratios over meters to tens of meters are observed. These changes probably reflect either long-term climate fluctuations during the Lower Miocene or oscillations caused by changes in the rate of subsidence of the basin floor.
Variations in the elemental composition of sediments can be used to correlate individual boreholes across the entire sedimentary basin.