Palaeozoic stromatoporoid diagenesis: a synthesis

Palaeozoic stromatoporoids, throughout their 100-million + year history (Middle Ordovician to Late Devonian and rare Carboniferous), are better preserved than originally aragonite molluscs, but less well-preserved than low magnesium-calcite brachiopods, bryozoans, trilobites and corals. However, the original mineralogy of stromatoporoids remains unresolved, and details of their diagenesis are patchy. This study of approximately 2000 stromatoporoids and the literature recognises three diagenetic stages, applicable throughout their geological history. Timing of processes may vary in and between stages; some components are not always present. Stage 1, on or just below sediment surface, comprises the following: micrite filling of upper gallery space after death, then filling of any remaining space by non-ferroan then ferroan calcite in decreasing oxygen of pore-waters; partial lithification of associated sediment from which stromatoporoids may be exhumed and redeposited, evidence of general early lithification of middle Palaeozoic shallow-marine carbonates; microdolomite formation, with the Mg interpreted to have been derived from original high-Mg calcite (HMC) mineralogy (likely overlaps Stage 2). Stage 2, short distance below sediment surface, comprising the following: fabric-retentive recrystallisation (FRR) of stromatoporoid skeletons forming fabric-retentive irregular calcite (FRIC), mostly orientated normal to growth layers, best seen in cross-polarised light. FRIC stops at stromatoporoid margins in contact with sediment and bioclasts. FRIC geometry varies, indicating some taxonomic control. Evidence that FRIC formed early in diagenetic history includes syntaxial continuation of FRIC into some sub-stromatoporoid cavities (Type 1 cement), although others were pre-occupied by early cement fills (Type 2 cement) formed before FRR, preventing syntaxial continuation of FRIC into cavities. Likely contemporaneous with FRIC formation, stromatoporoids in argillaceous micrites drew carbonate from adjacent sediment during reorganisation of argillaceous micrite into limestone–marl rhythms that are also early diagenetic. Stage 3, largely shallow burial, comprises the following: dissolution and silicification, but these may have occurred earlier in stromatoporoid diagenetic histories (more data required); burial pressure dissolution forming stylolites.

Aseismic Refinement of Orogenic Gold Systems

Orogenic Au deposits have contributed the majority of Au recovered globally throughout history. However, the mechanism that concentrates Au to extremely high bonanza grades in small domains within these deposits remains enigmatic. The volume of fluid required to provide extreme Au endowments in localized occurrences is not reflected in field observations (e.g., in the extent of quartz veining or hydrothermal alteration). Detailed optical, scanning and transmission electron microscopy, nanoscale secondary ion mass spectrometry, and 3-D neutron tomography have been used to investigate the processes responsible for development of anomalously high grade ore (upward of 3% Au) found in quartz veins at Fosterville gold mine (Victoria, Australia). Distinct textural settings of visible Au include (1) Au concentrated along pressure solution seams associated with wall-rock selvages, (2) as nano- to microscale dusty Au seams parallel to pressure solution seams, and (3) in microscale tension fractures perpendicular to stylolitic seams. The distribution of Au in arsenopyrite and pyrite hosted within pressure solution seams changes as a function of the extent of deformation. Sulfides in highly deformed pressure solution seams exclusively host Au as nano- to micrometer-sized clusters within features associated with corrosion and brittle failure, whereas sulfides in mildly deformed pressure solution seams have Au bound in the crystal structure. It is proposed that Au supersaturation in fluids introduced during seismic periods led to the deposition of abundant Au nanoparticles in quartz-carbonate veins. Subsequent pressure dissolution of vein quartz and carbonate during interseismic intervals allowed for episodic increase in the Au/quartz ratio and permitted liberation and migration of Au nanoparticles, promoting Au grain growth in favorable textural settings. Galvanic corrosion and brittle fracturing of auriferous sulfides during the interseismic period allowed additional remobilization and/or enrichment of sulfide-hosted Au. Repetition of this mechanism over the time scale of deposit formation acted to concentrate Au within the lodes. This Au ore upgrading model, referred to as “aseismic refinement,” provides a new insight for the genesis of ultrarich Au mineralization and, based on textures reported from many Au deposits, may be a globally significant component in the formation of orogenic Au deposits.

Diagenesis of Xing’anling Oil Reservoir in Sudeerte Area

In order to explore the types of diagenesis in Sudeerte area, through core and thin slice identification, it is clear that the Xing’anling oil layer is mainly affected by compaction, cementation, andrecrystallization. The compaction is mainly early compaction, late pressure dissolution is not obvious; cementation presents two forms of crystalline quartz and amorphous opal; recrystallization is manifested as the conversion process of opal t-ochalcedony quartz.

Partitioned tectonic shortening, with emphasis on outcrop-scale folding and flattening, Pindos fold-and-thrust belt, Peloponnese, Greece

Shortening estimates for fold–thrust belts seldom take into consideration outcrop-scale folding, especially folding related to prethrusting layer-parallel shortening (LPS) and flattening. The Pindos belt of the Peloponnese contains products of Maastrichtian to Paleocene tectonic shortening amenable to assessing strain partitioning. Shortening there initiated with LPS, including outcrop-scale folding, which was superseded by thrusting and macrofolding, with both macromechanisms producing additional outcrop-scale folding and (or) form-modification of initial LPS outcrop-scale folds. Skourlis and Doutsos (2003. International Journal of Earth Sciences, 92: 891–903) concluded that total shortening in this region was 68%, the summation of ∼15% LPS, ∼6% fault-related macrofolding, and ∼47% thrust–slip translation. But even this degree of shortening may be underestimated when outcrop-scale folding is considered. Two limestone-dominated Cretaceous formations and one Cretaceous–Eocene formation of limestone and mudstone display spectacular outcrop-scale folding produced by true multilayer behavior, internal buckling, and disharmonic quasi-flexural folding. Fold forms vary from chevron buckling of multilayers to more rounded folding of true multilayers to disharmonic flattened folds in pseudo-bedded limestone. Ramsay analysis of layer thickness variations across individual disharmonic folds underscores the disharmonic fold forms and intense degree of pressure dissolution generated fold flattening. Line-length shortening for well exposed “packages” of LPS outcrop-scale folding is as high as 30%, but overlying and underlying panels may be more modestly folded and (or) flattened, suggesting that shortening transfer mechanisms may have caused some sections to be bypassed. Achieving more meaningful shortening estimates of the outcrop-scale folding within an entire progressive deformation will require understanding of the partitioning of distribution of this folding regionally.

Origin and distribution of dolomite in Permian Rotliegend siliciclastic sandstones (Dutch Southern Permian Basin)

ABSTRACT Dolomite is a common and volumetrically important mineral in many siliciclastic sandstones, including Permian Rotliegend sandstones (the Slochteren Formation). These sandstones form extensive gas reservoirs in the Southern Permian Basin in the Netherlands, Germany, Poland, and the UK. The reservoir quality of these sandstones is negatively influenced by the content and distribution of dolomite. The origin and the stratigraphic distribution of the dolomite is not yet fully understood. The aim of this study is to identify the origin of carbonate. The main methods used to achieve those aims are a combination of thin-section petrography, scanning electron microscopy (SEM and EDX), and XRD analyses. The present study shows that the typical dispersed occurrence of the dolomite is a consequence of dispersed detrital carbonate grains that served both as nuclei and source for authigenic dolomite cement. The dolomite cement formed syntaxial outgrowths and overgrowths around detrital carbonate grains. The study also shows that dolomite cement, often in combination with ankerite and siderite, precipitated during burial after mechanical compaction. Most of the carbonate grains consisted of dolomite before deposition. The carbonate grains were affected by compaction and pressure dissolution, and commonly have no well-defined outlines anymore. The distribution of dolomite cement in the Rotliegend sandstones was controlled by the presence of stable carbonate grains. Due to the restricted and variable content of carbonate grains and their dispersed occurrence, the cement is also dispersed and the degree of cementation heterogeneous. Our findings have important implications on diagenesis modeling. The presence of detrital carbonate excludes the need for external supply by any large-scale advective flow of diagenetic fluids. By knowing that the carbonate source is local and related to detrital grains instead of being externally derived from an unknown source, the presence of carbonate cement can be linked to a paleogeographic and sedimentological model.

Pressure inverse solubility and polymorphism of an edible γ-cyclodextrin-based metal–organic framework

An edible metal–organic framework, γ-CD-MOF-1, has been obtained in a new trigonal form. The γ-CD-MOF-1 high-pressure dissolution as well as the cubic and trigonal polymorphs are closely related and regulated by the guest framework interactions and adsorption in the pores.