Carbonate content, compaction, and porosity are evaluated from a large number of samples using micritic marl-limestone alternations from Germany, France, and Italy. Compaction is measured directly by utilizing both deformed, originally cylindrical bioturbation tubes and steinkerns of ammonites. Additionally, a method is developed to indirectly evaluate the total compaction of the rock matrix by using early, selectively cemented burrows and fossils. Plots representing measurements of compaction versus carbonate content display distinctly non-linear relationships which show increasing compaction with decreasing carbonate content. These relationships are found to clearly correspond with the carbonate compaction law. The compaction law is introduced as a theoretical derivation for sediments and rocks which calculates the carbonate and non-carbonate content, compaction, and porosity. It is based on the assumption that the non-carbonate fraction standardized to the primary sediment-volume remains constant during carbonate diagenesis. The compaction law is used to calculate the most commonly found sediment/rock transformations (e.g. mechanical compaction, cementation, and chemical compaction) and to simulate the diagenetic processes of given examples. Decompaction formulas are developed to evaluate the porosity and carbonate content of the primary sediment. An example of calculating decompaction and determining the original composition of the sediment is given utilizing carbonates spanning the Cretaceous-Tertiary boundary at Gubbio (Italy). 相似文献
Pyritized plant tissues with well-preserved morphology were studied in rocks from Vanoise (western Alps, France) that experienced high-pressure, low-temperature metamorphic conditions in the blueschist facies during the Alpine orogeny. Organic and inorganic phases composing these fossils were characterized down to the nanometer scale by Raman microspectroscopy, scanning transmission X-ray microscopy and transmission electron microscopy. The graphitic but disordered organic matter composing these fossils is chemically and structurally homogeneous and mostly contains aromatic functional groups. Its original chemistry remains undefined likely because it was significantly transformed by diagenetic processes and/or thermal degradation during metamorphism. Various mineral phases are closely associated with this organic matter, including sulphides such as pyrite and pyrrhotite, carbonates such as ankerite and calcite, and iron oxides. A tentative time sequence of formation of these diverse mineral phases relative to organic matter decay is proposed. The absence of traces of organic matter sulphurization, the pervasive pyritization of the vascular tissues and the presence of ankerite suggest that the depositional/diagenetic environment of these metasediments was likely rich in reactive iron. Fe-sulphides and ankerite likely precipitated early and might have promoted the preservation of the fossilized biological soft tissues by providing mechanical resistance to compaction during diagenesis and subsequent metamorphism. In contrast, iron oxides which form rims of 100-nm in thickness at the interface between organic matter and Fe-sulphides may result from metamorphic processes. This study illustrates that it may be possible in some instances to deconvolve metamorphic from diagenetic imprints and opens new avenues to better constrain processes that may allow the preservation of organic fossils during diagenesis and metamorphism. 相似文献
Biomarkers, or the so-called molecular fossils, are used tentatively in the Eogene lacustrine stratigraphy study in the Jiyang
Sub-basin. Notwithstanding the fact that unidentified microfossils or amorphism and acritarchae are widely distributed in
lacustrine source rocks, molecular fossils are useful to identify the sources. It is helpful to reconstruct the palaeo-enviroment,
palaeo-ecosystem and compartmentalize the stratigraphic sequence by using molecular fossils with which the existence and types
of microbes including bacteria, archaea and certain algae can be identified. 相似文献
Burial compaction is one of several major obstacles to estimating palaeoprecipitation from depth to pedogenic carbonate in favourably preserved palaeosols. Palaeosols must be decompacted and the preburial depth to the pedogenic carbonate obtained. Vertic palaeosols may be particularly good candidates for palaeoprecipitation estimates, because of their increased likelihood of preserving clastic dykes, one of the best features for estimating burial compaction. Compaction estimates from clastic dykes and literature-based depth of burial estimates suggest vertic palaeosols undergo significantly less burial compaction than may be commonly assumed. Late Carboniferous vertic palaeosols, buried to 2·5–3·0 km, compacted to 93% of their original thickness. In contrast, clastic dykes in a nonpedogenic shale directly underlying one of the Late Carboniferous palaeosols records compaction to 70% of original thickness. Similarly obtained burial compaction and burial depth estimates for Early Carboniferous, Ordovician, and Proterozoic vertic palaeosols were used to test a burial compaction curve and equation specific to vertic palaeosols. Results suggest this ‘vertic-calibrated’curve and equation can be used to estimate burial compaction for vertic palaeosols lacking clastic dykes, but additional testing is needed. Naturally high bulk densities may have limited the compactibility of vertic palaeosols. Likewise, high initial bulk density and an abundance of swelling clays may have severely limited the transmissivity of some vertic palaeosols as they passed from pedogenic to burial environments. Upon burial these vertic palaeosols may have behaved as closed systems, which has implications for understanding their diagenetic modification. Additional efforts to understand burial compaction of vertic palaeosols also promises to improve our understanding of aquifer/aquiclude and hydrocarbon reservoir/seal relationships in sedimentary basins containing intercalated palaeosols. 相似文献
Carbonate concretions are common features of sedimentary rocks of all geological ages. They are most obvious in sandstones and mudstones as ovoid bodies of rock that protrude from natural outcrops: clearly harder or better cemented than their host rocks. Many people are excited by finding fossils in the centre of mudstone‐hosted concretions ( Fig. 1 ) but spend little time wondering why the fossils are so well preserved. While the study of concretions has benefitted from the use of advanced analytical equipment, simple observations in the field can also help to answer many questions. For example, in cliff sections, original sedimentary beds and sedimentary structures can be traced right through concretions ( Fig. 2 ): so it can be deduced that the concretion clearly formed after these depositional structures were laid down. In this article we explain how and where concretions form and discuss the evidence, ranging from outcrop data to sophisticated laboratory analyses, which can be used to determine their origins. The roles of microbes, decaying carcasses, compaction and groundwaters are highlighted. Concretions not only preserve fossils but can also subdivide oil, gas and water reservoirs into separate compartments. Figure 1 Open in figure viewer PowerPoint An early diagenetic carbonate concretion split in half to reveal an ammonite retaining its original aragonite shell, from the Maastrichtian of Antarctica. Image courtesy of Alistair Crame (British Antarctic Survey, NERC). Lens cap is 6 cm. 相似文献
The present study examines a fossil saprock–saprolite–laterite-profile beneath the sub-Cambrian peneplain in the Pan-African Roded Granite, Israel, with regard to structure and magnetic fabrics (anisotropy of magnetic susceptibility, AMS), and image analysis of compaction. The deformed granite shows two pre-weathering foliations, S1m (magmatic) and S2g (gneissic). Pre-Early Cambrian weathering comprised weathering-brecciation in saprock and saprolite, and chemical weathering with clay-formation in saprolite and laterite. During subsequent Phanerozoic burial the laterite was vertically compacted to 73% of its original thickness. In the laterite, compaction produced an unconformity-parallel cleavage (S3d) with increasing intensity towards the unconformity. Bulk susceptibility (κbulk) and anisotropy (P′) decrease from the unweathered granite into the saprolite, as a result of progressive magnetite breakdown, martitization and weathering-brecciation. In the laterite, an enrichment of haematite and relic Fe–Mg–mica lead to increased κbulk. Here, magnetic fabrics trace the compaction fabrics. The subhorizontal, compactional clay–/mica-fabric S3d defines a structurally weak and impermeable layer. The mechanical weakness of a clay-enriched weathering horizon with an unconformity-parallel, planar shape-preferred orientation, combined with the potentially overpressured state due to the sealing character of such a zone provides a viable explanation for the abundant localization of decollement horizons at or beneath basement-cover interfaces. 相似文献
Initially planar fault surfaces can be refracted by differential compaction, sudden changes in the displacement gradient taking place at the contact of different lithologies. As a result, the original displacement pattern, e.g. a cone-type in an idealized case, can be changed into a zigzag-type, especially when the initial dip angle of the fault and differential compaction are high. Compared with pre- and syn-faulting compaction, post-faulting compaction is more likely to change the initial displacement pattern along normal faults which were active near the sediment surface. Based upon the assumption that compaction is homogeneous vertical strain, the original fault geometry and displacement pattern can be estimated by means of decompaction and related geometric calculations. 相似文献
Fossils represent the only physical evidence for the existence of extinct life, and hold a vast potential to reconstruct organisms and ecosystems vanished a long time ago. Yet fossils are not as complete as they might appear in museum exhibits, documentaries or Hollywood blockbusters. Millions of years of fossilization have left their marks on the fossils, which might no longer resemble the condition of the organism when it was alive. A key challenge in palaeontology is therefore to restore and reconstruct the morphology of fossils. Luckily, novel digital visualization and reconstruction techniques offer powerful tools to bring extinct organisms back to life in unprecedented detail. 相似文献
The Middle Marker is a thin (3–6 m) sedimentary unit at the base of the Hooggenoeg Formation in the 3.4 Ga old Onverwacht Group, Barberton Mountain Land, South Africa. The original sediments consisted largely of current-deposited volcaniclastic detritus now represented by green to buff-colored silicified volcaniclastic rock and fine-grained gray chert. Black chert, possibly formed by the silicification of a non-volcaniclastic precursor, makes up a significant part of the unit. The Middle Marker is underlain and overlain by mafic and commonly pillowed volcanic flowrock. Although the original sediment has been replaced by and/or recrystallized to a microquartz, chlorite, sericite, carbonate and iron oxide mosaic under lower greenschist-grade metamorphism, sedimentary textures and structures are remarkably well preserved. Textural pseudomorphs indicate the primary volcaniclastic sediment consisted of a mixture of crystal, vitric and lithic debris. Middle Marker sediments were deposited as a prograding, cone-flanking volcaniclastic sedimentary platform in a relatively-shallow and locally current/wave-influenced subaqueous sedimentary environment. Available paleocurrent data indicate a largely bimodal, orthogonal distribution pattern which is quite similar to both ancient and modern shallow marine/shelf systems. Diagnostic evidence for tidal activity is lacking. As felsic volcanic activity waned, an extensive airfall blanket of fine-grained volcanic ash and dust was deposited in a low-energy subaqueous environment. The sedimentary cycle was terminated with a renewal of submarine mafic volcanism. Middle Marker volcaniclastic sediments accumulated in an anorogenic basin removed or isolated from the influence of continental igneous and metamorphic terranes. Although compositionally dominated by a volcanic source, Middle Marker sediments owe their final texture and sedimentary structures to subaqueous sedimentary rather than volcanogenic processes. 相似文献
The flow pattern within a slump in Permian marine rocks of the southern Sydney Basin, Australia, is recorded by folds and deformed fossils. Abundant brachiopod and bryzoan fossils in the slumped rocks are relatively undeformed, but fossil crinoid stems have been deformed by relative rotation of individual ossicles. Measurement of the strain indicates that the deformation of the crinoids is consistent with flexural flow folding within the slump. Previous models assume that curved slump fold axes remain parallel to the enveloping bedding surface of a slump sheet. Detailed measurements of the orientation of slump folds in this study found fold axes to be oblique to bedding, which is interpreted as a result of folds plunging downward towards the flanks of the slump or slump lobes. In the present model, fold axes are not generally parallel to the strike of the fold axial surface, and this can explain differences between the orientations of slump fold axes and axial surfaces when these are used as directional indicators of slump movement. 相似文献
Pyrite occurs both in normal clays and shales with a benthic fauna (Oxford Clay, England, and Lias ε, Germany) and in highly bituminous shales (Lias ε, Germany). In normal shales it is present in small quantities as early framboids, but more conspicuously as internal moulds of fossils, especially ammonites. The pyrite in these is petrographically varied; several types of internal sediments and chamber linings are described and illustrated by reflected-light and scanning electron microscopy. Most striking are pyrite stalactites, suspended from the roofs of ammonite chambers, which were later filled by calcite or baryte. Pyrite formed in reducing micro-environments, while the sediment generally was not wholly anoxic. Most pyrite pre-dates compaction of sediment, breakage of fossils and solution of shell aragonite. Variable rates and conditions of reduction of sea water sulphate are reflected in δ34S values ranging from ?55 to +44. Stalactites probably started to form when the ammonite chambers were partially gas-filled. In the bituminous Lias ε shales pyrite occurs abundantly as early framboids and micro-nodules. Larger nodules show a variety of forms, some of which post-date compaction of the sediment. Pyrite is not associated with the abundant flattened ammonites. δ34S values in shales are grouped about a mode near ?20. Pyrite formed over a long time-span, and throughout the sediment, not just in protected cavities. Contrasts in pyrite types can be related to differing depositional environments and organic contents of the shales. Pyrite is an important mineral in diagenetic mineral parageneses which can be deduced by studying fossil void-fillings and concretions, and which help define the diagenetic history of a shale. 相似文献
The Lower Permian Snapper Point Formation at its type locality in the southern Sydney Basin is interpreted as a regressive sequence of a linear clastic shoreline. Lithologies, sedimentary structures, and palaeocurrent patterns suggest a prograding barrier‐beach environment. Barrier foot, bar nucleus, bar crest, and back‐bar are distinguished. Abundant trace fossils aid the recognition of minor facies. The thickness of sediments deposited in the protected inshore environment may be explained by progradation into rising relative sealevel, but rates of sealevel rise or land subsidence were ultimately exceeded by the rate of sediment supply. Up‐sequence changes in the character of the sedimentation units and biofacies may therefore reflect an evolution from a barrier profile to an open mainland beach. 相似文献