Diagenetic Grains Research Articles

Ancient Oil as a Source of Carbonaceous Matter in 1.88-Billion-Year-Old Gunflint Stromatolites and Microfossils

The 1.88-billion-year-old Gunflint carbonaceous microfossils are renowned for their exceptional morphological and chemical preservation, attributed to early and rapid entombment in amorphous silica. The carbonaceous matter lining and partly filling filamentous and spherical structures is interpreted to be indigenous, representing thermally altered relicts of cellular material (i.e., kerogen). Here we show that stromatolitic black cherts from the Gunflint Formation, Schreiber Beach, Ontario, Canada, were saturated in syn-sedimentary oil. The thermally altered oil (pyrobitumen), which occurs in the stromatolites and intercolumn sediments, fills pores and fractures, and coats detrital and diagenetic grain surfaces. The occurrence of detrital bitumen grains in the stromatolites points to the proximity of shallow seafloor oil seeps and hence the possible existence of chemosynthetic microbes degrading hydrocarbons. We suggest that hydrocarbons that migrated through the silicifying stromatolites infiltrated semi-hollow microbial molds that formed following silica nucleation on the walls or sheaths of decayed cells. Upon heating, the hydrocarbons were transformed to nanoporous pyrobitumen, retarding silica recrystallization and enhancing detailed preservation of the carbon-rich microfossils. Hydrocarbon infiltration of silicified microbes offers a new explanation for the preservation of the Gunflint microfossils and may have played a role in the formation of some of Earth's oldest microfossils.

Overpressure development in sedimentary basins induced by chemo-mechanical compaction of sandstones

Abstract Mechanisms of overpressure build-up resulting from chemically-induced compaction are investigated by considering that intergranular pressure-solution, controlled by temperature and effective stresses, is the dominant process of chemical deformation in sandstones. The numerical simulations are performed using a thermo-poro-mechanical tool based on the finite element method specifically devised to deal with sedimentary basin modeling. At the material level, purely mechanical and chemo-mechanical deformations are respectively addressed by means of plastic and viscoplastic constitutive components. Porosity data of the Middle Jurassic Garn Formation from the Haltenbanken area of the Mid-Norwegian Continental Shelf are taken as reference for calibration of the sandstone model to be used on synthetic cases of a siliciclastic basin in oedometric conditions. Two situations are proposed involving different depositional sequences of sandy and shaly sediments with the aim to assess the permeability effect in the numerical model. The results have shown that early overpressure development in low permeability formations preserves sandstone porosities by reducing effective stresses and thus retarding pressure-solution compaction, whereas higher effective stresses associated with permeable depositional environments may lead to important chemo-mechanical deformation, resulting in low porosity sandstones and significant overpressure generation at later stages of basin history. An additional case is finally analyzed to investigate the consequences of pressure-solution inhibition due to diagenetic grain coating in a sandstone reservoir. The simulation resulted in porosity preservation and lower overpressure values, showing that a sedimentary basin submitted to an important level of chemo-mechanical compaction can present substantially higher overpressure distribution than basins where this phenomenon did not take place. The paper highlights the importance of integrating stresses, fluid pressure and temperature to represent the porous material evolution in order to describe the geological state of sedimentary basins.

Enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the Triassic Skagerrak, Central Graben, North Sea, UK

Abstract Current understanding of porosity preservation in deeply buried sandstone reservoirs tends to be focused on how diagenetic grain coatings of clay minerals and microquartz can inhibit macroquartz cementation. However, the importance of overpressure developed during initial (shallow) burial in maintaining high primary porosity during subsequent burial has generally not been appreciated. Where pore fluid pressures are high, and the vertical effective stress is low, the shallow arrest of compaction can allow preservation of high porosity and permeability at depths normally considered uneconomic. The deeply buried fluvial sandstone reservoirs of the Triassic Skagerrak Formation in the Central Graben, North Sea, show anomalously high porosities at depths greater than 3500 metres below sea floor (mbsf ). Pore pressures can exceed 80 MPa in the upper part of the Skagerrak Formation at depths of 4000–5000 mbsf, where temperatures are above 140°C. The Skagerrak reservoirs commonly have high primary porosities of up to 35%, little macroquartz cement and variable amounts of diagenetic chlorite grain coats. This research sheds light on the complex controls on reservoir quality in the fluvial sandstones of the Skagerrak Formation by identifying the role of shallow overpressure in arresting mechanical compaction and the importance of chlorite detrital grain coatings in inhibiting macroquartz cement overgrowth as temperature increases during progressive burial.

A complex investigation of building sandstones from Saxony (Germany)

The present paper provides a methodology for the investigation and characterization of building sandstones. This analytical scheme was designed for distinguishing mature arenites, which in general show very similar properties and are difficult to distinguish. This is shown for Cretaceous sandstones from various occurrences in Saxony (Germany), which have been used for centuries as building materials. The procedure is mainly based on the combination of macroscopic rock description, thin section polarizing microscopy (phase composition, texture, grain-size distribution) and cathodoluminescence (CL) microscopy (quartz types, feldspar and kaolinite content) coupled with image analysis, scanning electron microscopy (accessories, pore cement, diagenetic grain surface features), and analysis of pore space data. Sometimes, additional data from X-ray diffraction or chemical analyses (major and trace elements) can be used. Especially in the case of quartz rich arenites, CL is a powerful tool for provenance analysis. The detailed analysis of sandstone material in most cases allows us to assign historically used building material to a specific sandstone occurrence. These results are important for both interpreting the weathering behaviour of the building material and the conservation, reconstruction and stone replacement of historical monuments.

The worm gut; a natural clay mineral factory and a possible cause of diagenetic grain coats in sandstones

Using a series of experiments, we show that the formation of clay minerals can occur biologically via the processes of sediment ingestion and excretion by worms. Specimens of lugworm, Arenicola marina, were fed a mixture of coarse grained quartz sand and unweathered, finely crushed Icelandic basalt in experimental tanks that simulated an intertidal, shallow-marine, sedimentary environment. Faecal casts as well as the starting material and samples from a control tank, collected and separated into < 2 μm fractions, were analysed using X-ray diffraction and Fourier Transform Infrared spectroscopy. The faecal samples were found to have lost plagioclase feldspar due to dissolution and to contain new clay minerals (kaolinite, illite and a 14 Å clay) not present in the original or control samples. The experiments show that macrobiotic ingestion processes induce rapid clay growth. These clays coat sand grains and may be the precursors to porosity-preserving chlorite coats found in sandstones.

Microscopic scale characterization of ancient building sandstones from Saxony (Germany)

Sandstone has been a traditional building material all over the world for centuries and is still used for this purpose today. Because of the various geological origin, sandstones may differ in petrography and mineralogy, which mainly determine their technical properties and weathering behavior. Therefore, a careful investigation of sandstone building materials is required and a complex analytical scheme was developed for this purpose. Sample material from important quarry regions of the Elbe Zone (Saxony, Germany) was investigated, which supplied material for a lot of famous buildings in the historic city center of Dresden, in Meißen and in the whole state of Saxony. The complex study included macroscopic rock description and detailed investigations by polarizing microscopy (phase composition, texture, grain size distribution), cathodoluminescence (CL) microscopy (quartz types, feldspar and kaolinite content), scanning electron microscopy (SEM; accessories, pore cement, diagenetic grain surface features), and pore-size distribution by Hg porosimetry. In a case study, mineralogical and technical properties of building sandstones from the Meißen cathedral (Saxony, Germany) were investigated and compared with material from potential historical source quarries. The results of the present study allowed to assign unequivocally historically used material to specific sandstone occurrences, and provide a comprehensive basis for the interpretation of weathering damage on the historical monuments. These data are useful for current conservation and reconstruction activities.

Investigations into provenance and properties of ancient building sandstones of the Zittau/Görlitz region (Upper Lusatia, Eastern Saxony, Germany)

Abstract Mineralogical and technical properties were investigated of building sandstones from ancient monuments of the Zittau/Görlitz region (Germany) as well as material from potential source quarries. The complex study included macroscopic rock description and detailed investigations by polarizing microscopy (phase composition, texture, grain size distribution), cathodoluminescence (quartz types, feldspar and kaolinite content), scanning electron microscopy (accessories, pore cement, diagenetic grain surface features) as well as the analysis of open macroporosity (total water uptake) and the pore size distribution function (Hg porosimetry). For the first time, mineralogical and technical data were obtained for building sandstones of the Zittau region. The results not only confirmed earlier conjectures concerning different source areas for the ancient building sandstones of the Zittau and Görlitz area but also allowed the unequivocal assignment of historically used material to specific sandstone occurrences. The data obtained provide a comprehensive basis for the interpretation of weathering damage suffered by the historical monuments and give useful hints for their successful conservation and reconstruction.

Magnetic remanence in the Chalk of eastern England: an unusually resistant VRM?

AbstractA single component, natural remanent magnetization (NRM) is carried largely by pseudosingle domain magnetite in the Cretaceous Lower Chalk and Red Chalk of eastern England. The Red Chalk also records the same direction in haematite. Most of the ferro-magnetic minerals occur as primary clastic or early diagenetic grains. A stable remanence component is resistant to demagnetization, and is carried by both magnetite and haematite. Nevertheless, it has a steep inclination close to the present Earth's field and it is too steep for the previously reported palaeolatitude of these rocks at the time of sedimentation. A postglacial slump breccia scatters the ChRM but also provides some evidence of viscous, partial magnetic overprinting during slumping. Despite its resistance to thermal and alternating field demagnetization the characteristic remanent magnetization (ChRM) is probably a young Bruhnes epoch viscous remanent remagnetization (VRM).

Primary copper sulphides from the Kupferschiefer, Poland

Finely dispersed, primary, copper sulphide mineralization from the zones of Cu-S paragenesis in the Zechstein copper-bearing shale from the Fore-Sudetic Monocline, Poland, was studied. It is represented mostly by small spherules and framboids (5 to 10 μm in size) and framboidal clusters of digenite, chalcocite and minor covellite, dispersed throughout the whole shale but especially abundant in the organic-rich laminae. Detailed microscopic and chemical (microprobe) descriptions of this mineralization are given and a comparison with pyrite framboids is presented. Digenite usually dominates over chalcocite among dispersed mineralization, in contrast to that observed in larger diagenetic grains. The rarity of copper sulphide framboids, in comparison with spherules, can probably be explained by the fast process of their homogenization. The textures, chemistry and some different features from pyrite framboids suggest that the copper sulphides in Fe-poor zones of the Kupferschiefer are primary precipitates. The results of the study suggest that the beginning and probably the main stage of the ore mineralization process took place during the sedimentation and/or early diagenesis of the Kupferschiefer.

Diagenetically formed ooids and intraclasts in South African calcretes

ABSTRACTCertain South African calcrete (caliche) beds contain structures that very closely resemble normal marine ooids and intraclasts. The ooids/intraclasts are actually non‐marine in origin, and were formed by an in situ process of calcretization and selective neomorphism. The term ‘diagenetic ooids/intraclasts’ is thus applied to distinguish these grains from marine ooids/intraclasts. Quaternary calcretes contain the only known ooids/intraclasts in any South African Cretaceous or Cainozoic rocks.The diagenetic grains are believed to have formed in the following manner. Rain water percolates through an unlithified calcareous sand deposit, dissolving carbonate along its downward path. This carbonate‐laden water is eventually checked in its descent and drawn upward somewhat by capillary action. Evaporation and soil suction in the uppermost zones of the sediment body cause precipitation of a concentric coating of carbonate mud around individual grains (incipient ooids) or composites of grains (incipient intraclasts). As this dissolution‐precipitation cycle is repeated the micritic coatings thicken and locally push grains apart. At this stage three things can happen: (a) sparry calcite may be introduced as an intergranular cement (very rare in these calcretes), (b) wet carbonate mud may fill the pore spaces and lithify to micrite (common), or (c) wet carbonate mud may fill the pore spaces and neomorphose to microspar (common). The last process causes ooids/intraclasts to stand out distinctively, since they are opaque, micrite‐coated grains set in a fabric of clear microspar. This selective neomorphism of the micritic mud in pore spaces, rather than the micritic mud forming ooids/intraclasts, is probably a function of rapidity of lithification. The micrite of the coatings must have lithified quickly, before neomorphic crystal enlargement could occur, whereas lithification of the intergranular mud was delayed long enough for the growth of microspar crystals.