Diagenesis, porosity and permeability in the Corallian Beds (Upper Oxfordian) from the Harwell Research Site, South Oxfordshire, UK
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Abstract The Corallian Beds beneath the Harwell Research Site, south Oxfordshire, are a highly variable sequence of sandstones, mudstones and limestones. The more permeable lithologies of the upper part of the sequence constitute an important local aquifer. Diagenetic and post-lithification processes have strongly influenced the porosity, permeability and mineralogy of the aquifer rocks. Calcite cementation reduced the porosity and permeability of the sandstones and limestones in the upper part of the aquifer. Cementation occurred in at least three stages ranging from early diagenesis to post-compactional diagenesis. Late-stage dissolution of calcite took place along fractures and bedding discontinuities, restoring the porosity and permeability of these sediments and developing secondary porosity and permeability where original clastic carbonate was removed. Invasion of the Corallian porewaters by waters charged with CO 2 is postulated as a mechanism by which calcite was removed. At the base of the aquifer, early diagenetic dissolution of biogenic silica created a high secondary porosity. Silica was reprecipitated in the dissolution voids and matrix as opal-CT. Authigenic smectite and zeolites are also associated with opal-CT. These phases are believed to have precipitated from pore-waters rich in ions derived from the alteration of volcanogenic detritus associated with the Corallian sediments.Keywords:
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Abstract Measurements of natural cosmogenic 32 Si ( t 1/2 ~ 140 years) in tropical deltaic sediments demonstrate for the first time that most 32 Si is present in rapidly formed authigenic clays and not biogenic opaline silica (bSi). The burial of bSi in deltaic and continental margin sediments has likely been greatly underestimated because of diagenetic alteration of bSi to clay, little of which dissolves in the classically used operational bSi leach. Rapid reverse weathering reactions during early diagenesis must be considered as a significant pathway of reactive Si storage in deltaic deposits. Based on 32 Si, actual storage may be 2–3 times the best recent estimates extrapolated from diagenetic models or attempts to modify operational bSi methods to include authigenic clay (~900 µmol/g versus ~250 µmol/g). Measurements of natural 32 Si inventories in sediments and initial specific activities in biogenic silica provide a means to independently constrain the marine Si cycle.
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The effect of diagenesis upon natural remanent magnetization (NRM) has been studied at two sites within a single bed of dolomite from the Monterey Formation in Santa Barbara County, California. Paleomagnetic direction demonstrates that the NRM is primary at one site and remagnetized at the other site. Magnetic separates from samples carrying a primary NRM yielded detrital magnetite. Separates from samples carrying secondary NRM yielded an unusual form of authigenic magnetite. Evidence suggests that the formation of the authigenic magnetite accompanied late stage dolomite diagenesis. Correlating the remagnetization direction to the paleomagnetic history of the region gives a probable age of 0.72–2.47 m.y.b.p. for both of these diagenetic events.
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Paleoecological interactions among fossil microorganisms have garnered significant interest within the paleobotanical community; however, an understanding of the early diagenesis of associated plant material is of critical importance when assessing putative body fossils of fungi and bacteria.Structures preserved within permineralized petioles of the Carboniferous fern Botryopteris tridentata Felix (Scott) have been interpreted as the earliest remains of Actinobacteria found in association with vascular plants, but re-examination of the specimens indicates instead that these biomimetic structures (BMS) are authigenic carbonate minerals.Using spinning disk confocal microscopy, we generated monochromatic luminescence maps of BMS found within the phloem cells of Botryopteris.Luminescence was captured at wavelengths of 665 nm, consistent with an interpretation of these structures as disordered dolomites, an inference subsequently corroborated with energy-dispersive X-ray spectrometry (SEM-EDS).The presence of high-magnesium carbonates within Botryopteris is suggestive of an early anaerobic stage of plant tissue degradation characterized by metabolic activities of sulfate-reducing bacteria.Anaerobic biodegradation may also have been performed by chytridiomycetes, and we interpret larger (5-8 mm) unicells found within the specimens as fossils of chytrid zoosporangia.Understanding microbial contribution to the early diagenesis of plants preserved within calcium carbonate concretions (coal balls) is dependent upon both characterizing diversity of microbial communities within fossil plants, and elucidating the geomicrobiological parameters of mineralization.As such, this study underscores the necessity of integrating geomicrobiology with plant taphonomy in investigations of the microbial component of ancient ecosystems.
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