Reinhard Gaupp

On the origin of the southern Permian Basin, Central Europe.

Abstract A detailed study of the structural and stratigraphic evolution of the Southern Permian Basin during latest Carboniferous to Early Jurassic times, supported by quantitative subsidence analyses and forward basin modelling for 25 wells, leads us to modify the conventional model for the Rotliegend–Zechstein development of this basin. The Late Permian–Early Jurassic tectonic subsidence curves are typical for a Permian to Early Triassic extensional stage that is followed by thermal subsidence. However, a purely extensional model is extremely problematic because active faulting during this time is ‘minor’ and generally hard to document. Using inverse techniques to model the subsidence curves, we quantitatively show that a significant component of Late Permian and Triassic tectonic subsidence can be explained by thermal relaxation of Early Permian lithospheric thinning, and by delayed infilling of paleo-topographic depressions that developed during the Early Permian. In this interpretation, Stephanian–Autunian wrenching resulted in thermal destabilisation of the lithosphere, deep fracturing of the crust, disruption and erosion of its sedimentary cover and regional uplift of the area of the future Southern Permian Basin. Upon termination of wrench tectonics and associated volcanism, towards the end of the Autunian, the Southern Permian Basin began to subside in response to thermal contraction of the lithosphere. The evolving basin was isolated from the World oceans and had subsided possibly up to some 700 m below their level at the beginning of Upper Rotliegend sedimentation. After catastrophic flooding of this paleo-topographic depression at the beginning of the Zechstein, changing sea level, sedimentation and subsidence rates remained essentially in balance. Although the effects of Triassic rifting overprinted parts of the Southern Permian Basin, its overall subsidence pattern persisted well into the Jurassic. In contrast to the remainder of the Southern Permian Basin, Permian and Triassic crustal extension contributed significantly to the subsidence of the Polish Trough.

Provenance of Cretaceous synorogenic sandstones in the Eastern Alps: constraints from framework petrography, heavy mineral analysis and mineral chemistry

Abstract The detrital components of Cretaceous sedimentary rocks in the Northern Calcareous Alps reflect the early Alpine geodynamic evolution of the Austroalpine microplate. Two contrasting source areas are distinguished on the base of light and heavy mineral analysis. The first source area is located at the southeastern margin of the Austroalpine and is composed of Palaeozoic sediments and metamorphic rocks, Mesozoic carbonate rocks, and ultrabasic rocks derived from the suture zone of the Vardar/Meliata Ocean. The second source area is located in the northwest in a Lower Austroalpine position near the transpressive plate margin that juxtaposes Austroalpine and Penninic units. This source area comprises Palaeozoic low-grade metamorphic rocks including high-pressure (HP) rocks, late Palaeozoic (meta)sediments, Mesozoic carbonate rocks, and ultrabasic rocks from obducted slices of Penninic oceanic crust. Chemical analyses of detrital white mica, amphibole and garnet support the discrimination between the two source areas. Tourmaline chemistry calls for a significant amount of metasedimentary rocks in the source area. Granitoid rocks and high-grade metamorphic rocks are rare. Blue amphibole, phengite, and chloritoid composition suggest the erosion of lower blueschist facies rocks in the northwestern source area. We suggest a modified model of Austroalpine Valanginian to Coniacian tectono-sedimentary evolution which is based on (1) an onset of subduction of the Penninic Ocean no earlier than Late Cretaceous, (2) deposition of the analyzed sedimentary rocks in piggyback basins, and (3) a reconstruction of provenance as proposed in this study.

Structure-related geochemical (REE) and isotopic (K-Ar, Rb-Sr, δ18O) characteristics of clay minerals from Rotliegend sandstone reservoirs (Permian, northern Germany)

Abstract Euhedral illite cementing the gas-bearing sandstone reservoirs of the Rotliegend in the Niedersachsische rift system (northern Germany) was studied along a horst-to-graben cross-section to examine its chemical and isotopic characteristics. The data show that differentiated illite particles grew during a tectono-thermal event marked by distinct episodic hydrothermal activities along fault drains and in the poral space of sandstones at 210 Ma and at 195 to 190 Ma in horst positions, at 185 to 175 Ma in the nearby graben, and at 170 to 165 Ma in both the horst and graben. Varied REE distribution patterns and initial 87Sr/86Sr ratios (from 0.7124 to 0.7142) relative to illite-particle size outline a high chemical variability of the fluids during illite growth. The δ18O values of illite range from +20.0 to +12.4‰ and those of chlorite from +12.0 to +6.9‰. The changing δ18O values of the minerals most likely relate to varied δ18O values of the fluids, or to different water/rock ratios during crystallization. Differences among the REE and 87Sr/86Sr tracers and the δ18O values of different generations of illite also suggest a changing fluid chemistry relative to time. Significant positive Eu anomalies and negative Ce anomalies in the REE distributions of illite suggest feldspar alteration by the migrating fluids in oxidizing environments.

Cyclicity of Triassic to Lower Jurassic continental red beds of the Argana Valley, Morocco: implications for palaeoclimate and basin evolution

Abstract Cyclical playa deposits form a prominent part of the continental clastic succession of the Argana Valley, Western High Atlas of Morocco. The red beds formed in Triassic to Lower Jurassic times during rifting of the North American and African plates. Detailed stratigraphic work revealed asymmetrical, metre-scale cycles in mudstone-dominated successions that constitute the intermediate and upper portion of the basin fill. Sedimentary cycles commonly comprise ephemeral lake shales at the base, playa mudflat mudstones in the intermediate part, and both fluvial and aeolian sandstones at the top. Cycles of the Aglegal Member (T4) are mainly characterized by analcime-rich playa mudflat deposits displaying features indicative of palaeo-Vertisols. Cycles of the Sidi Mansour (T7) and Hasseine (T8) Members are represented by ephemeral lake or dry playa mudflat deposits grading into saline mudflat mudstones that are overlain by sheet flow and minor aeolian sandstones. Sedimentary cycles of the Imerhrane Member (T9) comprise sheet delta sandstones at the base overlain by ephemeral lake and dry playa mudflat mudstones. Upper cycle portions consist of aeolian dune and sand sheet sandstones. Cyclicity is attributed to palaeoclimatic and associated palaeohydrological fluctuations, probably within the Milankovitch frequency band. Sedimentary facies of each cycle provide evidence for a gradual climatic drying with time. The asymmetrical drying-upward cycles possibly record the precession cycle, since they are modulated by symmetrical (i.e., oscillating drying/wetting) cycles of a lower frequency indicative of eccentricity. A long-term change in palaeoclimate ranging from semi-arid conditions with seasonal precipitation (T4) towards an arid, non-seasonal climate (T8) is preserved within the cyclical units spanning a time interval of several million years. Individual cycles can be traced laterally for tens of kilometres. Correlative sections are almost identical in thickness, suggesting a phase of tectonic quiescence during their deposition. Sediment accumulation was not restricted to small-scale grabens, but took place continuously throughout the exposed area of the Argana Valley, implying that accommodation space has not been controlled by syndepositional tectonism.

Palaeoclimatic implications of Mio–Pliocene sedimentation in the high-altitude intra-arc Lauca Basin of northern Chile

Abstract This paper examines the Late Miocene to Pliocene sedimentary succession in the high-altitude, intra-arc Lauca Basin of northern Chile, and discusses the palaeoclimatic evidence recorded within the basin fill. Sedimentological and geochronological data allow the reconstruction of four stages of the fluvial to lacustrine basin fill from the Late Miocene (>6.4 Ma) to the Late Pleistocene. We interpret the sedimentary evolution to reflect changes in precipitation during a time interval of general aridity. Climatic changes exerted the dominant control on sediment accumulation. Volcanism, tectonism, changes in morphology, and sediment supply were factors of subordinate importance. Stage 1 comprises a clastic red bed sequence developed during the Late Miocene under relatively humid conditions. An abrupt change in depositional style occurred around 6.4 Ma, when ephemeral saline lake sedimentation prevailed (stage 2). This change in depositional style is considered to document the begin of a phase of desiccation, that lasted until the Early Pliocene (3.7 Ma), and which was succeeded by less arid climatic conditions during the Late Pliocene and Pleistocene (stages 3 and 4). The transition between stages 1 and 2 and the subsequent dry period is assumed to coincide with a global cooling event during the latest Miocene to Early Pliocene. Comparable sedimentary responses to this climatic change can be expected in adjacent basins of the Central Andes.

The impact of diagenetic fluid–rock reactions on Rotliegend sandstone composition and petrophysical properties (Altmark area, central Germany)

In the framework of the German R&D joint project CLEAN (CO2 large-scale enhanced gas recovery in the Altmark natural gas field), Rotliegend reservoir sandstones of the Altensalzwedel block in the Altmark area (Saxony-Anhalt, central Germany) have been studied to characterise litho- and diagenetic facies, mineral content, geochemical composition, and petrophysical properties. These sands have been deposited in a playa environment dominated by aeolian dunes, dry to wet sand flats and fluvial channel fills. The sediments exhibit distinct mineralogical, geochemical, and petrophysical features related to litho- and diagenetic facies types. In sandstones of the damp to wet sandflats, their pristine red colours are preserved and porosity and permeability are only low. Rocks of the aeolian environment and most of the channel fill deposits are preferentially bleached and exhibit moderate to high porosity and permeability. Although geochemical element whole rock content in these rocks is very similar, element correlations are different. Variations in porosity and permeability are mainly due to calcite and anhydrite dissolution and differences in clay coatings with Fe-bearing illitic-chloritic composition exposed to the pore space. Moreover, mineral dissolution patterns as well as compositions (of clays and carbonate) and morphotypes of authigenic minerals (chlorite, illite) are different in red and bleached rocks. Comparison of the geochemical composition and mineralogical features of diagenetically altered sandstones and samples exposed to CO2-bearing fluids in laboratory batch experiments exhibit similar character. Experiments prove an increase in wettability and water binding capacity during CO2 impact.

Miocene to recent history of the western Altiplano in northern Chile revealed by lacustrine sediments of the Lauca basin (18°15′–18°40′ S/69°30′–69°05′W)

The intramontane Lauca Basin at the western margin of the northern Chilean Altiplano lies to the west of and is topographically isolated from the well-known Plio-Pleistocene lake system of fluvio-lacustrine origin that covers the Bolivian Altiplano from Lake Titicaca to the north for more than 800 km to the Salar de Uyuni in the south. The Lauca Basin is filled by a sequence of some 120 m of mainly upper Miocene to Pliocene elastic and volcaniclastic sediments of lacustrine and alluvial origin. Volcanic rocks, partly pyroelastic, provide useful marker horizons. In the first period (6–4 Ma) of its evolution, the ‘Lago Lauca’ was a shallow ephemeral lake. Evaporites indicate temporarily closed conditions. After 4 Ma the lake changed to a perennial water body surrounded by alluvial plains. In the late Pleistocene and Holocene (2-0 Ma) there was only marginal deposition of alluvial and glacial sediments. The basin formed as a half-graben or by pull-apart between 10 and 15 Ma (tectonic displacement of the basal ignimbrite sequence during the ‘Quechua Phase’) and 6.2 Ma (maximum K/Ar ages of biotites of tuff horizons in the deepest part of the basin). Apart from this early basin formation, there has been surprisingly little displacement during the past 6 Ma close to the Western Cordillera of the Altiplano. Also, climate indicators (pollen, evaporites, sedimentary facies) suggest that an arid climate has existed for the past 6 Ma on the Altiplano. Together, these pieces of evidence indicate the absence of large scale block-faulting, tilt and major uplift during the past 5–6 Ma in this area.

Timing of fluid flow in a sandstone reservoir of the north German Rotliegend (Permian) by K-Ar dating of related hydrothermal illite

Abstract Authigenic illite is the most abundant clay mineral in Carboniferous and Rotliegend rocks of northern Germany. In spite of extreme present-day burial depths of 4580 to 5280 m, the Rotliegend sandstones are characterized by moderate permeabilities which are controlled, together with their reservoir qualities, by omnipresent illite. It is admitted that deep-crustal hydrothermal fluids, migrating along major fault systems, interacted with the Carboniferous Coal Measures and generated acidic fluids inducing primary leaching processes, mass transfers and concomitant pervasive clay authigenesis. The investigated samples belong to wells located in a horst-graben structure covering an area of 20 by 20 km. This restricted area allowed investigation of a three-dimensional distribution of fluid flows and related illitization in distinct sandstone reservoirs. The K-Ar ages of the < 2 μm fractions decrease from 210-180 Ma (11 analyses with a mean at 198 Ma) for illite in the horst position to 190-155 Ma (24 analyses with a mean at 177 Ma) for illite in the graben area up to about 4 km away from faults, suggesting a relationship between timing of illite formation and structural position of the host rocks. The decrease in the illite K-Ar ages from the horst into the graben allowed calculation of a migration rate of 0.2 μm per year for the illitization front in the Rotliegend sandstones towards the centre of the graben, which might be of use for prediction of the illite distribution in these rocks.

The molecular composition of dissolved organic matter in forest soils as a function of pH and temperature

We examined the molecular composition of forest soil water during three different seasons at three different sites, using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS). We examined oxic soils and tested the hypothesis that pH and season correlate with the molecular composition of dissolved organic matter (DOM). We used molecular formulae and their relative intensity from ESI-FT-ICR-MS for statistical analysis. Applying unconstrained and constrained ordination methods, we observed that pH, dissolved organic carbon (DOC) concentration and season were the main factors correlating with DOM molecular composition. This result is consistent with a previous study where pH was a main driver of the molecular differences between DOM from oxic rivers and anoxic bog systems in the Yenisei River catchment. At a higher pH, the molecular formulae had a lower degree of unsaturation and oxygenation, lower molecular size and a higher abundance of nitrogen-containing compounds. These characteristics suggest a higher abundance of tannin connected to lower pH that possibly inhibited biological decomposition. Higher biological activity at a higher pH might also be related to the higher abundance of nitrogen-containing compounds. Comparing the seasons, we observed a decrease in unsaturation, molecular diversity and the number of nitrogen-containing compounds in the course of the year from March to November. Temperature possibly inhibited biological degradation during winter, which could cause the accumulation of a more diverse compound spectrum until the temperature increased again. Our findings suggest that the molecular composition of DOM in soil pore waters is dynamic and a function of ecosystem activity, pH and temperature.

Mineral evolution and processes of ferruginous microbialite accretion – an example from the Middle Eocene stromatolitic and ooidal ironstones of the Bahariya Depression, Western Desert, Egypt

Peritidal ferruginous microbialites form the main bulk of the Middle Eocene ironstone deposits of the Bahariya Depression, Western Desert, Egypt. They include ferruginous stromatolites and microbially coated grains (ferruginous oncoids and ooids). Their internal structures reveal repeated cycles of microbial and Fe oxyhydroxide laminae. The microbial laminae consist of fossilised neutrophilic filamentous iron-oxidising bacteria. These bacteria oxidised the Fe(II)-rich acidic groundwater upon meeting the marine water at an approximately neutral pH. The iron oxyhydroxide laminae were initially precipitated as amorphous iron oxhydroxides and subsequently recrystallised into nanocrystalline goethite during early diagenesis. Organic remains such as proteinaceous compounds, lipids, carbohydrates and carotenoids are preserved and can be identified by Raman spectroscopy. The ferruginous microbialites were subjected to post-depositional subaerial weathering associated with sea-level retreat and subsurface alteration by continued ascent of the Fe(II)-rich acidic groundwater. At this stage, another iron-oxidising bacterial generation prevailed in the acidic environment. The acidity of the groundwater was caused by oxidation of pyrite in the underlying Cenomanian Bahariya formation. The positive iron isotopic ratios and presence of ferrous and ferric iron sulphates may result from partial iron oxidation along the redox boundary in an oxygen-depleted environment.