How do thrombolites form? Multiphase construction of lacustrine microbialites, Purbeck Limestone Group, (Jurassic), Dorset, UK

AbstractThis paper examines how non‐marine thrombolites are formed through a complex, multiphase process of microbial framework construction, erosion, cementation, recrystallization and episodes of internal sedimentation. Recognition of such phases of thrombolite construction provides a framework for the interpretation of the fluctuating environmental conditions leading to their formation. Microbialite frameworks are examined in detail from the Purbeck Limestone Group and their affinities and palaeo‐environmental significance assessed. Three types of thrombolite, one stromatolite and a leolite are described and interpreted. The thrombolite frameworks include: a peloidal mesoclotted type, a thrombolite constructed by the filamentous alga Cladophorites and a type with concentrically laminated micritic mesoclots. Physical and chemical erosion led to extensive early cavity formation within the frameworks. Early calcite rim cements with associated spherulites then developed over the microbial frameworks and these were reworked into cavities. Frameworks were also replaced by chalcedonic quartz and calcite spherulites. Internal sediments comprise peloids, intraclasts and brackish‐water molluscs and ostracods, together with their debris. The thrombolites grew in moderate‐energy to high‐energy shallow, lacustrine, microbial mounds whereas stromatolites occurred in deeper‐water settings. A brackish‐water, lacustrine setting is indicated by the preserved macro‐biota, microbes, absence of charophytes and syndepositional evaporites, and negative stable carbon and oxygen isotope ratios. Strontium isotopes suggest that the carbonate‐rich waters were fed from erosion of Mid–Lower Jurassic limestones on the western basin margin with possible mixing with waters from nearby uplifted Upper Jurassic limestones and with Late Jurassic seawater. The research indicates that non‐marine thrombolites have a complex, multiphase origin resulting in a diverse succession of textures and structures relating to microbially induced and influenced construction, dissolution, cementation, recrystallization and mineral replacement which have not been previously recorded and indicate the major differences between marine and non‐marine thrombolites.</jats:p

Influence of extensional faults and relay ramp on palaeotopography and lacustrine carbonate facies: Purbeck Limestone Group (Upper Jurassic - Lower Cretaceous), Wessex Basin, UK

AbstractLacustrine carbonate facies distribution is controlled by multiple environmental parameters including climate, hydrology, and tectonic setting, but few published models address this complexity. In this study, seismic and borehole data, integrated with outcrop logging, correlations, and facies models, are used to create a new tectono‐sedimentary model demonstrating how extensional faults, linked by a relay ramp, control distribution of lacustrine carbonate facies in the Upper Jurassic to Lower Cretaceous Purbeck Limestone Group (Wessex Basin, UK). Accumulation occurred in half‐graben sub‐basins south of two extensional east‐west faults, with widespread subaerial emergence of footwall blocks to the north. The lacustrine limestones of the lowest unit of this Group are characterised by in‐situ microbial mounds within bedded inter‐mound packstones‐grainstones. Mounds occur in three depositional intervals separated by paleosols. The distribution of facies indicates more brackish‐water conditions shoreward to the west, and more hypersaline conditions basinward to the east. The relay ramp hosts extensive microbial carbonate buildups formed in response to carbonate‐rich waters sourced from the northern limestone footwall blocks that fed into extensive shallow‐water areas on the low‐angle relay ramp slope.</jats:p

Discriminating between the origins of remotely sensed circular structures:carbonate mounds, diapirs or periclinal folds? Purbeck Limestone Group, Weymouth Bay, UK

Many sedimentary rock successions contain plan-view circular structures, such as impacts, diapirs and carbonate build-ups. When remotely sensed, it can be difficult to discriminate between their formation mechanisms. Here we examine this problem by assessing the origins of circular structures imaged in high-resolution multibeam bathymetric data from Weymouth Bay, UK. The imagery shows 30–150 m across, concave-down structures within the upper Purbeck Limestone Group on the southern limb of the Purbeck Anticline. Similar structures have not been identified in the extensive outcrops around the bay. The morphology and geological setting of the structures are consistent with three different interpretations: carbonate mounds, periclinal folds and evaporite diapirs. However, none of these structures has been previously recorded in the upper Purbeck Limestone Group outcrops of this internationally renowned geological region. We apply a scoring system to 25 features of the circular structures to discriminate between these three alternative interpretations. This analysis indicates that evaporite diapirs are the least likely and carbonate mounds the most likely origin of the structures. The presence of carbonate mounds revises the upper Purbeck palaeofacies distribution in its type area and provides an analogue for the exploration for hydrocarbon reservoirs in lacustrine mounds

A arqueologia dos fermentados: a etílica história dos Tupi-Guarani

O consumo de bebidas fermentadas é geralmente negligenciado pela literatura arqueológica, que trata a questão como tema de interesse secundário (recreativo) na história das populações humanas. Entretanto, a literatura etnográfica das sociedades indígenas das terras baixas sul-americanas indica exatamente o oposto: é o alimento vegetal sólido e não alcoólico que tende a possuir um papel secundário na vida cotidiana e ritualística de diversos coletivos. Os dados arqueológicos aprofundam temporalmente essa relação entre o ser humano e os fermentados. Além disso, os vasos cerâmicos arqueológicos utilizados para o preparo e consumo desses fermentados são fundamentais para a compreensão de processos e eventos históricos que modelaram a dispersão de uma série de grupos pelo continente

Comparative sedimentology of Late Jurassic, lacustrine, microbial mounds (Purbeck Limestone Gp,Wessex Basin, UK

A central tenet of Ginsburg’s concept of comparative sedimentology was the use of modern analogues in understanding ancient sedimentary features of limestones; “The vitality of the comparative approach is attributed to the continuing comparisons of recent and fossil examples.” This presentation explores the benefits, and also the pitfalls, of this approach through new research on the environment of deposition of microbial mounds in the non-marine Purbeck Limestone Group from its type locality in the Wessex Basin, UK. Mound-forming, porous limestones have been described from the lowermost Purbeck limestones since the early nineteenth century and variously interpreted as fresh water spring tufa deposits or hypersaline lagoon stromatolites. Current research establishes these accumulations as in-situ microbialite mounds (up to 4m high and 20m across) that occur within bedded inter-mound peloidal packstones-grainstones. The microbialite mounds are located in three lacustrine sequences separated by three paleosols. The microbialite mounds reveal complex and irregular shapes in part due to their association with tree remains. They are constructed by a Microbialite facies (Stromatolite, and Thrombolite sub-facies) and Burrowed Boundstone facies. Commonly, mounds initiated around trees and branches during flooding of lake waters over a vegetated landscape. The Burrowed Boundstone facies initially forms a microbially-bound casing around trees when the trees were still upright. Contemporaneously, the Stromatolite sub-facies was deposited on the lake floor. Subsequently, the Thrombolite sub-facies forms the main framework for the mounds. Inter-mound facies onlap and interfinger with the mounds indicating that deposition occurred during the development of the thrombolite framework. A sharp transition is then recorded above these three sequences to evaporite bearing strata that form the (non-mounded) overlying beds of the Purbeck limestones. The mounds are comparable in structure and microfacies with several different present-day examples of mounds from lacustrine settings (fresh-water, brackish-water, hypersaline) as well as hypersaline marine settings. This reaffirms the view that thrombolite fabrics are not diagnostic of depositional environment. However, the associated molluscan and ostracod faunas are indicative of fresh to brackish waters, and the absence of charophytic algae and in-situ evaporites indicates a brackish-water setting for these mounds. This is supported by preliminary C and O stable isotope data from adjacent intermound sediments. No exact modern-day analogue is known for the Purbeck mounds but they have many similarities in their sedimentological development with microbialite mounds from the fresh-water Laguna Bacalar, Mexico. Regional geology, local seismic data and mapping of mounds indicate they occur in an extensional sub-basin within the Wessex Basin and that the mounds preferentially occurred in shallow, fault-controlled marginal sites including a relay ramp. So what? Thrombolitic mounds, in general, are not indicative of any specific depositional environment. Petrographic, palaeontological and isotope data indicate that the microbial mounds in the Purbeck limestones accumulated in a brackish-water setting. Local extensional tectonic structures control the occurrence of the Purbeck mounds to shallow lake margins. In lacustrine successions sharp vertical facies transitions can occur between brackish and hypersaline lake strata.Peer Reviewe

Broken Beds but better science: using multiple hypotheses to interpret geological data

AbstractIn this study, we propose a multiple hypotheses approach to improve interpretations of limited remotely sensed datasets, such as sparsely exposed outcrops, subsurface datasets, or planetary objects using semi‐quantitative scoring and ranking of observable features. This method is demonstrated using an outcrop example from the Broken Beds of the Upper Jurassic–Lower Cretaceous Purbeck Limestone Group exposed along Britain's Jurassic Coast. Four published hypotheses regarding their origin are refined, represented in matrix form, scored and ranked based on carefully selected outcrop features. Semi‐quantitative scoring utilises knowledge of likely processes governing the occurrence of a range of features, some of which might be ignored or down‐played to favour a single hypothesis. Furthermore, by integrating expertise from different sub‐disciplines (e.g. basin analysis, sedimentology, diagenesis), we also consider the combined evidence of multiple features. This new method results in an interpretation that favours a multi‐process origin for the Broken Beds due to evaporite dissolution, overpressure release and tectonic folding, with identified uncertainty, all useful to guide further data collection.</jats:p

Genomic structure of the human TATA-box-binding protein (TBP)

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