A new species of the medullosan seed fern genus, Pachytesta, is described from specimens discovered in coal balls from Iowa. The specimens used in this study were located in coal balls, which are concretions of ancient permineralized peat. Anatomical study of the new species was made possible through the use of mounted cellulose acetate peels and photomicroscopy. The ovule is approximately 5.0-5.5 cm long and 2.4 -2.8 cm wide at its mid point. The integument contains a sarcotesta with 12 vascular bundles, a sclerotesta of radiating fibers and variable thickness, and an endotesta consisting of a thin cuticle. The primary ribs of the ovule are commissured. Based on the anatomical features studied, Pachytesta sp. A. is most similar to Taylor’s (1965) P. olivaeformis, P. composita, P. illinoensis group and is potentially a member of this group.

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No description available.

Late diagenetic, fault-controlled dolomitisation has received much interest as it is an important host for MVT-mineralisation and hydrocarbons, and an excellent proxy for fluid flow and reaction in carbonate systems. The source of fluids of sufficient volume and the correct chemistry to explain the volume of dolostone is much debated. Recent work has shown how seawater convection along deep-seated crustal lineaments is focused in zones of structural complexity. Since dolomitisation is favoured where there is a precursor high magnesium calcite or dolostone, it is possible that such a process is a critical precursor to the formation of these late diagenetic dolostones from evolved brines during extension and transpression. In the Pennine Basin and North Wales, UK, late diagenetic fault/fracture controlled dolostones developed on the margins of Mississippian carbonate platforms that grew on the rotated footwalls of normal faults and a basement of Lower Palaeozoic metasediments. Conceptual models for their formation focus on expulsion of fluids from Serphukovian-Bashkirian sediments within adjacent hanging wall basins, by compactional dewatering or rupture of overpressured compartments and seismic pumping. This project aims to determine the source, composition and drive mechanism of fluids that formed a large (~60km2), non-stratabound dolostone body exposed within the Viséan sediments on the southern margin of the Derbyshire Platform, through a combined regional sedimentological, diagenetic and structural framework using multiscale, interdisciplinary techniques. Techniques include field observation, transmitted light and CL analysis, bulk major and trace element analysis including rare earth elements, stable isotope (oxygen/carbon), and strontium isotope analysis. The Derbyshire Platform underwent burial, several episodes of fluid-flow, and multiple phases of diagenetic overprinting. The products of fluid circulation in this area consist of dolomitisation and Mississippi Valley-type (MVT) mineralisation, affecting the carbonates of the Lower Carboniferous (Viséan) succession. Dolomitisation on the Derbyshire Platform is aligned to deep-seated basement faults and extrusive, intraformational volcanic beds, and five dolostone phases have been identified. These are present as matrix replacive and cement phases that are spatially and temporally related to deep seated structural lineaments. It is proposed that stratabound, early post-rift dolomitisation resulted from the geothermal convection of a mixed meteoric-seawater that interacted with the Viséan extrusive and intrusive volcanics on the Derbyshire Platform, providing additional magnesium for dolomitisation. This previously undescribed model of dolomitisation is key to explaining the anomalously large quantity of dolomitisation observed on the Derbyshire Platform and has implications to other carbonate platforms where dolomitisation is interpreted as fault-controlled. Subsequent phases of dolomitisation are fault-controlled, with each phase becoming increasingly confined to fractures. Timing of dolomitisation is interpreted to be a Carboniferous event, with later mineralisation also being of late Carboniferous in age, with basin de-watering on to the platform via faults/fracture systems and the development of pockets of overpressuring. Illite-smectite clay transformations within Viséan basinal sediments provided the necessary magnesium required within select fault/fracture systems. Consequently, burial calcite cements and MVT mineralisation was precipitated within fractures and dissolution-enhanced secondary porosity, with fluids derived from the overlying Namurian succession which also acted as the seal. This project provides a step-change in our ability to predict the location of late diagenetic fault/fracture controlled dolomitisation in rift basins by demonstrating the importance of dolomitisation by mixed meteoric-seawater on platform margins to the localisation of late diagenetic dolostone bodies. It also highlights the complex interplay between basin kinematics, host rock permeability and timing of fluid supply through episodic fault reactivation, connecting platforms to basin compartments, which ultimately controlled the positioning of dolostone geobodies on platform margins. This has implications to the exploration of both minerals and hydrocarbon within dolostone hosts, and will inform studies of fluid transfer and reaction in carbonate systems.

The stratigraphic relationships and sedimentary environments of the Silurian, Devonian and Lower Carboniferous lithologies in the Keel area are deduced and described. This was accomplished by the analysis of over 10.000M. of diamond drilled core from the area. The oldest rocks are Silurian shales with turbidites. Deposition was in forearc basins in an active subduction zone on the north margin of lapetus. The area was one of net erosion in the Lower Devonian following the continental collision which closed lapetus. The Upper Devonian of the area is represented by the Microconglomerate lying unconformably on the Silurian. This lithology was produced by braided streams draining the immediate Keel area. The earliest Carboniferous sediments are an assemblage of sandstones and conglomerates. The Lower Quartz Sandstone was laid down in sandy braided streams. The drainage basin of the Keel rivers was considerably larger by this time and sediment was derived from western Ireland. Uplift in western Ireland made quartz pebbles and higher stream velocities to transport them available. These pebbles formed the Quartz Pebble Conglomerate deposited in pebbly braided streams. Lowering of the source area by erosion resulted in reduced sediment grain size. Sandy braided streams again dominated the Keel area and deposited the Upper Quartz Sandstone. Owing to the sea transgressing from the south marginal marine sediments were then deposited in the area, these being represented by the Lower Mixed Beds. During the advancing transgression facies belts were moving northwards. Lagoonal (Navan Micrite) deposits were laid down behind a barrier complex now represented by the Upper Mixed Beds. To seaward of the barrier shallow marine calcareous sediments were deposited on a homoclinal ramp. These are represented in the Keel area by the Shaley Pales and the Bioclastic Limestone Unit. Further to seaward Waulsortian type mudmounds were deposited on the deeper ramp. Owing to the transgressive regime existing in the Lower Carboniferous each facies belt moved northwards over the Keel area. At the close of the Courceyan partial subsidence of the ramp took place and basinal/slope shales with turbidites (Calp) were deposited as lateral equivilants of the shallower Oakport Limestone on the surrounding shelf remnent.

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No description available.

No description available.

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No description available.