Reinhard F. Sachsenhofer

Thermal History of Sedimentary Basins

Geoscientists have always been interested in temperature values, as the crucial role of temperature in various geological, geochemical, and geophysical phenomena was recognized relatively early. However, this interest was long limited to aspects such as temperature distribution in the earth’s crust and mantle, melting temperatures of different rocks, equilibrium temperatures of various metamorphic mineral assemblages, and temperature dependency of some geophysical parameters. Almost all of these are related to thermal conditions in the deeper parts of the earths crust. Low temperature fields at shallow depth have been of less concern. It is interesting to recognize that all these studies sought to define either the present distribution of temperature or the maximum temperature reached at some time in the geological past. In other words, temperature distribution as a steady state case was the subject of interest.

Petrographic composition and depositional environments of the Pliocene Velenje lignite seam (Slovenia)

Abstract A Pliocene lignite seam up to 160 m thick occurs in the Velenje depression in northern Slovenia. It forms part of a non-marine transgressive sequence. Petrographic data, together with ash and sulphur content of samples from two boreholes (one located in the central, and the other in the marginal part of the seam) have been used to study vertical and lateral coal facies variations. In the central part, the lignite seam overlies fluvial sediments. Facies critical macerals and petrographic indices suggest that the lower (ash rich) part of the seam (unit I) accumulated in a wet forest swamp in an alluvial plain setting. Later this environment was replaced by a dry (telmatic) forest swamp (unit IIa). According to our interpretation, a decrease in tree density in units IIb and IIc reflects a change to a bush moor. The transition from unit II to unit III is characterized by a rapid water table rise, which resulted in the establishment of a wet (limnotelmatic) forest swamp. Within unit III, the tree density decreases and the degree of gelification increases towards the top of the seam suggesting a transition to a fen type environment. Another rapid water table rise ended peat formation in the eutrophic, topogenous mire, and established open lacustrine conditions. A calcium rich environment resulted in high pH values and is responsible for high sulphur lignites (up to 4% on a dry basis). In the upper part of the seam, the pH values were controlled by the amount of the inflowing lacustrine water. Only thin, ash-rich topotelmites accumulated along the northern margin of the mire. There, the seam originated in a wet forest swamp and was separated by limnic and fluvial sediments from other organic rich horizons. Due to its closer vicinity to the pre-Tertiary carbonates, the depositional environment was probably even more alkaline. Therefore, the lignite shows a higher degree of gelification and is even more sulphur rich (up to 5.5% on a dry basis).

Paleoenvironmental implications from biomarker and stable isotope investigations on the Pliocene Velenje lignite seam (Slovenia)

A Pliocene lignite seam up to 160 m thick occurs in the Velenje basin (Slovenia). The seam originated in a topogenous mire and evolved within a non-marine, transgressive setting. Differences in soluble organic matter yield and hydrocarbon content of borehole samples from the lignite are related to differences in the composition of free lipids of microbial origin and/or hydrocarbons derived from the biogeochemical degradation of plant tissue. Variations of the redox conditions within the mire are reflected by pristane/phytane ratios. The abundance of terpenoid biomarkers indicates the predominance of gymnosperms over angiosperms, which is consistent with palynomorphic spectra dominated by pollen of the Sequoia-Taxodium-Metasequoia plant community rather than by angiosperms. Evidence is also provided that the content of land plant derived biomarkers and the preservation of plant tissue is controlled by the input of resin-rich, decay-resistant conifers. Sections of the seam characterized by a high degree of gelification of humotelinite (gelification index) show high contents of hop-17(21)-ene but low hopane concentrations. The results suggest that the gelification of plant tissue may be governed by the activity of anaerobic rather than aerobic bacteria. Despite the minor variation in the proportions of gymnosperms versus angiosperms in the peat-forming vegetation, a general influence of the floral assemblage on carbon isotopic composition of the coals (δ13C=−25.3 to −27.0‰) is proposed. Carbon cycling during biogeochemical decomposition of plant tissue by bacteria is assumed to affect the δ13C value of the lignite. Petrographic and geochemical data of gelified and ungelified fossil wood provide evidence that gelification may be governed by microorganisms (e.g. anaerobic bacteria) different from those responsible for decreasing cellulose contents during early diagenetic, aerobic degradation of wood. Based on the molecular composition of terpenoid biomarkers, the wood fragments are identified as derived from gymnosperms. The relative proportions of saturated versus aromatic hydrocarbon fractions of fossil gymnosperms display a general tendency towards lower values in gelified wood remains. This indicates that bacteria involved in gelification of plant tissue may also be involved in aromatisation of diterpenoid hydrocarbons. The chemotaxonomical classification of the macrofossils as gymnosperms is corroborated by the mean carbon isotopic compositions of the macrofossils (δ13C=−24.5‰) and the extracted cellulose (δ13C=−22.0‰). The higher isotopic difference of about 2.5‰ between cellulose and fossil wood, compared to that found in modern trees, can most probably be explained by the smaller effect of 13C discrimination for cellulose when compared with wood during decomposition [Chem. Geol. 158 (1999) 121]. Compared with the coals, the δ13C values of wood and extracted cellulose are affected to a minor extent by microbial activity.

Depositional environment of the Late Miocene Hausruck lignite (Alpine Foreland Basin): Insights from petrography, organic geochemistry, and stable carbon isotopes

Abstract Lignites and fossil wood from two boreholes in the late Miocene Hausruck district (Alpine Foreland Basin, Austria) were investigated with respect to organic carbon and total sulfur contents, ash yields, maceral composition, organic geochemistry, and stable carbon isotope ratios. The lignites from the two sampled profiles differ in sulfur contents and petrography-based facies indicators (gelification index [GI], tissue preservation index [TPI]). The results point to drier and more acidic conditions in the part of the mire sampled at drill site Lukasberg, whereas the lignites from Kalletsberg formed under near neutral conditions due to a raised (ground) water table. The different depositional environments are supported by the paleogeographic positions of the boreholes: borehole Lukasberg is located within a paleo-valley, whereas borehole Kalletsberg is located at the deeper part of a basin situated south and southwest of the Hausruck area. Environmental changes with time within the mire are confirmed by the molecular composition of the soluble organic matter (SOM). Differences in pristane/phytane ratio, isomerisation of hopanes at position 17, concentrations of hopanes, and in the ratio of hopanes to hop-17(21)-enes are related to the extent of gelification of plant tissue, sulfur content and pH values, caused by differences in (ground) water table. The variable contents of individual terpenoid hydrocarbons derived from land plants indicate that angiosperms and conifers contribute to peat formation in the Hausruck district in variable proportions. Overall, angiosperm biomarkers slightly predominate. The tissue preservation index (TPI) within the Kalletsberg profile is probably governed by the proportion of decay-resistant conifers, as indicated by the correlation between TPI and biomarker composition of the coals. Correlations between δ 13 C values of the lignites (between −24.9‰ and −27.4‰) and their non-hopanoid terpenoid composition indicate the major influence of changes in peat-forming vegetation on the carbon isotopic composition of the coals in the Hausruck district. Carbon isotope data of macrofossils are consistent with their taxonomical classification as conifers ( δ 13 C between −22.7‰ and −25.1‰) and angiosperms ( δ 13 C between −25.5‰ and −26.6‰), respectively. The δ 13 C data of the extracted cellulose reveal clear differences between fossil wood from gymnosperms (average δ 13 C=−21.0‰) and angiosperms (mean δ 13 C=−23.4‰).

Organic geochemistry of the Lower Miocene Oberdorf lignite (Styrian Basin, Austria): its relation to petrography, palynology and the palaeoenvironment

Abstract In the Oberdorf trough at the northwestern margin of the Styrian Basin (Austria), an up to 36-m thick Lower Miocene (Ottnangian) lignite seam occurs within the fluvial Koflach–Voitsberg Formation. Previous results indicate that the lignite originated in a topogenous mire within a wet-forest swamp palaeoenvironment. Differences in soluble organic matter (SOM) yield of borehole samples from the lignite seam are related to free lipids present in plant waxes or resins and the presence of thermally labile polymers indicated by the contents of liptinite macerals (i.e., resinite, suberinite, and liptodetrinite). The n -alkane distribution patterns are dominated by high-molecular weight lipids with marked odd over even predominance, originating from plant waxes. A contribution of algae to the even numbered short-chain lipids, observed in considerable amounts in the footwall sediments is proposed. Terpenoid biomarkers characteristic for conifers and angiosperms were detected, consistent with palynomorphic spectra which are dominated by forest swamp pollen (Taxodiaceae–Cupressaceae) and pollen from plants living in a mixed mesophytic forest. Excellent relationships between higher plant biomarker composition and palynologic data indicate that the organic geochemical results reflect the varying inputs of coniferales vs. angiosperms. The data further imply that cuparene may be used for taxonomical differentiation of coniferales families. Organic geochemical data provide evidence that the content of land plant-derived biomarkers is controlled by the input of resin-rich conifers. Concentrations of hopanoids, dominated by hop-17(21)-ene, generally increase towards the top of the lignite, a section characterised by high extents of aromatisation of triterpenoid biomarkers. The results suggest that the aromatisation of angiosperm-derived biomarkers may be governed by the activity of anaerobic rather than by aerobic bacteria. However, changes in the depositional environment and palynologic facies during peat formation probably influenced the origin and fate of angiosperm-derived triterpenoids. The results indicate how biomarker composition may contribute to the evaluation of environmental changes. Cross-correlations of organic geochemical parameters with palynological data and petrography-based facies indicators will provide further insights in the peat-forming vegetation and biogeochemical activities in the mire.

Subsidence analysis and quantitative basin modelling in the Styrian Basin (Pannonian Basin System, Austria)

Abstract We present the results of subsidence analysis and of quantitative basin modelling using isostatical and flexural models for basin evolution along four cross-sections in the Styrian Basin, the westernmost subbasin of the Pannonian Basin System. Subsidence analysis reveals a first Ottnangian-Karpatian synrift phase. Our local isostatic models predict crustal stretching values up to 1.3 and subcrustal stretching values of 1.6 for this event. Stretching factors of a minor Sarmatian extension phase are below 1.04. The termination of subsidence during the Pannonian and a rapid Quaternary uplift phase can be explained by major changes in the regional stress field. A W-E cross-section through the northern Furstenfeld Subbasin provides a key for the understanding of the dynamics of basin formation. It crosses a narrow Karpatian rift basin, the metamorphic core complex of the Penninic Eisenberg Window and shows an eastward tilting of the easternmost part of the basin during Pannonian times. Uplift of the Penninic window can only be modelled with an extremely weak lithosphere (equivalent elastic thickness (EET) ⪡ 2 km), whereas a best fit between observed and modelled tilting is obtained with an EET value of 5 km. These results suggest that the lithosphere was extremely weak during the onset of basin evolution in Ottnangian-Karpatian times, probably caused by high extension rates and high heat flows associated with Karpatian to early Badenian magmatic activity. Subsequent cooling led to a pronounced increase in flexural rigidity. An EET of 5 km fits well with estimations in other parts of the Pannonian realm. Depth-dependent rheology models based on palaeo-heat flow estimates indicate a similar increase in lithospheric strength with time. The impact of Plio-Pleistocene volcanism on rheology appears to be relatively modest, which can be explained by a deep position of the magma chamber for this event.

A new evaluation of palaeo-heat flows and eroded thicknesses for the Carboniferous Ruhr basin, western Germany

Palaeo-heat flows and thicknesses of eroded Carboniferous overburden in the coal-bearing Ruhr basin (western Germany) were estimated using one-dimensional basin modelling techniques. Thermal and burial histories for 11 localities were calibrated by comparing measured and calculated vitrinite reflectance data, based on the assumption of pre-tectonic maturation. In contrast to former studies, the kinetic EASY%R0 approach applied for the calculation of vitrinite reflectance. Lopatins TTI algorithm was used in addition for one well in order to demonstrate the impact of the selection of the algorithm for vitrinite reflectance calculation on the simulation results. Simulated palaeo-heat flows for the time of maximum burial (most probably in the Stephanian) range from 64 to 83 mW/m2 and decrease towards the south. These heat flows are lower than values computed during earlier studies, but are still relatively high in comparison to recent foreland basins. The regional distribution of palaeo-heat flows indicates that the crust beneath the Ruhr basin was relatively thin and thickened towards the south at the end of the Carboniferous. After the Stephanian, 2200–3500 m of Carboniferous overburden were eroded. An observed southward increase in eroded thicknesses is caused by an uplift of the area studied, which decreased towards the north, at the end of the Variscan folding. Large thicknesses of deposited (and later eroded) Carboniferous sediments have to be postulated to explain the maturity data even along the present southern margin of the Ruhr basin. This indicates that the late Carboniferous sediments originally extended far to the south. Geological arguments suggest that the deposition of Carboniferous sediments in the entire Ruhr basin continued until the Stephanian.

Palaeogeography, subsidence and thermal history of the Neogene Styrian Basin (Pannonian basin system, Austria)

Abstract The Neogene Styrian Basin represents an extensional structure on top of a crustal wedge, which was moving eastward during the final stages of the Alpine orogeny. Eastward extrusion was a consequence of continental escape and extensional collapse within the Eastern Alps. In a simple model the evolution of the Styrian Basin can be subdivided into an Early Miocene (Ottnangian to Karpatian) synrift and a subsequent postrift phase. Basement subsidence rates during the synrift phase reached 30 cm/100 yr and more than 2000 m of limnic to marine sediments were deposited. The coincidence of the synrift phase and the formation of pull apart structures along strike-slip zones bordering the eastward moving wedge is evidence for the close genetic relation between extrusion tectonics and basin formation. A Miocene magmatic phase is related to subduction along the Carpathian front. Magmatic activity started during the synrift stage and continued into the postrift stage. Heat flow in the vicinity of the Miocene volcanoes increased dramatically (> 300 mW/m2). A second volcanic phase producing basalts in Plio-Pleistocene times had only little influence on the heat flow pattern. Limnic to marine sediments of the synrift stage are separated by a major unconformity from Middle to Late Miocene (Badenian to Pontian) marine to limnic/fluvial postrift sediments. Sedimentation during the postrift phase was controlled by marine ingressions during Middle Miocene (early Badenian and Sarmatian) times from the south and a subsequent regression with a decline in salinity. Subsidence rates during the postrift stage were mostly below 2.5 cm/100 yr. In Pliocene times subsidence was replaced by uplift, which resulted in erosion of a few hundred metres of sediment. Present-day high heat flows are a consequence of thinned crust beneath the Styrian Basin.

The origin of hydrocarbon source rocks in the Austrian Molasse basin (Eocene-Oligocene transition)

Organic-rich rocks (Schoneck Formation) from five wells representing a section across the northern paleo-slope of the Austrian Molasse Basin were studied using a multi-method approach including petrography, X-ray diffraction, organic and inorganic geochemistry, and C- and N-isotopes of the organic material. Organic matter accumulation commenced during the latest Eocene and was dependent on variations of currents, salinity, organic productivity, and nutrient supply. Initial marl deposition (and phosphorite-formation) under dysoxic conditions was followed by organic-rich mud deposition associated with benthic and photic zone anoxia. This paleoceanographic situation resembles the first isolation of the western Central Paratethys. Deposition of organic-rich rocks terminated during lower Oligocene time when changing current patterns and decreasing salinity caused a breakdown of water column stratification. Even though the Schoneck Formation is an excellent source rock, major differences exist in the petroleum potential of the Schoneck Formation deposited in upper and lower basin slope settings. The Schoneck Formation from the upper slope is characterised by TOC contents up to 12% and HI values between 500 and 600 mgHC/gTOC. In the lower slope settings, TOC values are generally below 2%.

Parameters determining the carbon isotopic composition of coal and fossil wood in the Early Miocene Oberdorf lignite seam (Styrian Basin, Austria)

Abstract Petrographical and geochemical data of gelified and ungelified fossil wood from the Early Miocene Oberdorf lignite seam (Styrian Basin, Austria) provide evidence that the early diagenetic, aerobic degradation of wood by fungi may be followed by further decomposition under reducing conditions by the activity of anaerobic bacteria. Based on the molecular compositions of terpenoid hydrocarbons, the wood fragments in the lignite are identified as gymnosperms. This result is corroborated by the mean isotopic composition (δ13C=-24.2‰) of the macrofossils. The isotopic difference between fossil wood and extracted cellulose of most samples (3.9–2.5‰) is higher than in modern trees, which can be explained by the smaller effect of 13C discrimination during decomposition of cellulose when compared with wood. The mean carbon isotope values found for gymnosperms and coals (−24.2‰ and –24.7‰, respectively) confirm the results from biomarker analyses indicating that the peat-forming vegetation of the Oberdorf seam was dominated by gymnosperm taxa. Minor variation in the relative contributions of gymnosperms and angiosperms to coal deposition is of negligible influence on the isotopic composition of the coals. Cross-correlations between δ13C of the coals, soluble organic matter yield, and the proportions of liptinite macerals of higher-plant origin imply that the carbon isotopic composition may be affected by the contents of plant lipids because of their δ13C values ranging from 5 to 10‰ less than whole-plant tissue values. An isotopic shift of the coals towards more negative values due to the activity of prokaryotes (i.e. anaerobic bacteria) in organic matter degradation is proposed. The results obtained from the Early Miocene Oberdorf lignite indicate that the carbon isotope ratios of the coals are primarily affected by varying contributions of different parts of whole-plant tissue, due to their different isotopic and molecular compositions (e.g. epicuticular leaf waxes, resins, wood) and their different decay-resistance against the early diagenetic changes involved in organic matter decomposition. Carbon cycling during anoxic decomposition of plant-derived organic matter is assumed to affect the δ13C values of coal. δ13C values of wood and extracted cellulose are affected only to a minor extent.