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| Home > Publications database > Geochemical indicators of paleoenvironmental and paleoclimatic change in ancient and recent lake deposits : facies models, facies distributions and hydrocarbon aspects |
| Dissertation / PhD Thesis/Book | PreJuSER-37418 |
2003
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/158
Report No.: Juel-4026
Abstract: Eocene lacustrine sediments (Es4 and Es3 member of the Shahejie Formation) from the Western Depression of the Liaohe Basin, NE China were examined in order to perform a systematic facies classification and to show the influence of facies variations an the source rock potential. Facies changes were reconstructed by applying a multidisciplinary approach, including mineralogy (XRD), organic petrology and organic geochemistry. The Liaohe lakes passed from alkaline to freshwater stages because of varying climatic conditions and tectonic movements. Fluorescing amorphous and lamalginite-rich organic matter (OM) was deposited in a restricted, hydrological closed-lake environment (playa and shallow lake), during Gaosheng (GS) time in the whole basin and during Dujiatai (DJT) time in the basin centre (Lei and Gao area). A high primary production dominated by cyanobacteria, low fluvial input, and paucity of terrestrial vegetation in an arid climate resulted in the deposition of type I OM. Water column stratification with anoxic, probably saline bottom waters existed which is consistent with the biomarker parameters determined in this study. Especially laminated samples from the GS sub-unit show high total organic carbon (TOC) and hydrogen-index values. They are excellent source rocks and have the potential to generate high-wax petroleum at high subsurface temperatures. The very narrow distribution of activation energies point to a mainly polymethylene kerogen structure which is supported by the pyrolysate composition. During lower DJT time intense evaporation led to the formation of magnesium and sodium rich brines with dolomite and analcime precipitation aided by evaporatioe pumping. In contrast, clay mineral-rich samples of the Shale I and Shale II sub-units as well as those of the Es3 member were deposited during lake-level highstands in a hydrological open freshwater environment in a more humid climate. They are mainly non-laminated and Botryococcus alginite predominates. Sediments in the marginal Shu-Du area were laid down in a fan-delta environment during DJT time, which is corroborated by the presence of abundant detrital minerals (feldspars, quartz) and coaly terrigenous particles. Variations in the composition of macromolecular OM in maar lake sediments from late Quaternary deposits of the Westeifel volcanic field, Germany (Lake Holzmaar, Lake Meerfelder Maar) and South China (Lake Huguang Maar) were studied, applying analytical pyrolysis and organic petrology. Investigations an the lipid OM were carried out for comparison. The objective of this work was to find suitable parameters which mirror the local environment and climate at the time of OM incorporation into the sediment. Parameter based an methoxyphenols together with the character and the relative abundance of terrigenous particles can be used to show variations of land plant input to the maar lakes through time. Lignin derived components increased during the amelioration of the climate, reflecting forestation of the landscape around Lake Meerfelder Maar after the last glacial retreat. A reverse trend towards the Younger Dryas cold event shows a reduction of the forest. During the Lateglacial and Younger Dryas period, high-reflecting reworked terrigenous OM from the geological substratum was introduced to the Eifel Maar lakes by soil erosion in an open, tree-less vegetation. In contrast, fresh and low-reflecting terrigenous OM predominates in the Bolling/Allerod and Holocene sequence. Beside a higher plant source, the phenols in most of the pyrolysates originate from the microbial reworking of proteinaceous tyrosine moieties or from a direct contribution of cyanobacteria. In Lake Huguang Maar, the $\delta^{13}$C_{TOC}$ parallels the isotopic enrichment of the plant-wax derived long-chain n-alkanes and the botryococcene biomarkers. This mirrors vegetation changes and specific environmental conditions which affected the algal biomass. Especially the significant transition from lithozone 4 to ithozone 3 is consistent with a rapid change from an ecosystem with relatively high primary productivity and C$_{3}$ vegetation towards a very stable ecosystem with a low primary productivity and a dominant input of C4 grasses because of drier climatic conditions and low atmospheric CO$_{2}$ levels. This is supported by the positive isotopic excursions in lithozone 3, a lower 4-methylguaiacol to p-vinylphenol ratio in lithozone 3 compared to lithozone 4 and microscopic as well as palynological data. One possibility for the very heavy $\delta^{13}$C values of the botryococcenes in lithozone 3 is that Botryococcus braunii changed its CO$_{2}$ demand towards a bicarbonate source. The comparison with other studies implies that downcore variations in Lake Huguang Maar are largely controlled by fluctuations of the summer and winter monsoon activity.
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