The Early Devonian Rhynie hot spring system is the oldest known and is of the low sulphidation type. It extends for at least 1.5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments, plant-bearing sinters and andesitic lavas. The age of sedimentation and hydrothermal activity has been determined by palynological (Pragian) and radiometric (396 f 12 Ma) techniques. The outlier is a half graben with a complex stepped western margin.The Devonian rocks show intense hydrothermal alteration along the fault zone.The main alteration minerals are quartz, K-feldspar, calcite, hematite and illitic and chloritic clays. Multiple chert veining and brecciation are widely developed, and geyserite and vent material are also present. Pyrite occurs in veins and all alteration facies. Sinters and altered rocks contain high concentrations of Au, As, Sb, Hg, W and MO. Gold occurs in arsenian pyrite and as sub-micron particles in oxidized rocks.The fluid(s) responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water. However, incursions of high temperature (300-440°C) magmatic fluids occurred with SD -65960 and S'*O around +8.59~. 634S (pyrite) and initial *' Sr/%r ratios (vein calcite) lie mainly within the ranges +3.4%0 to +8.5% and 0.71138 to 0.71402 respectively. These data indicate that late Proterozoic Dalradian metasediments are a likely source for S and Sr but other sources are possible. 613C values for caliche and vein calcite imply derivation of carbon from non-organic sources.The Rhynie cherts were deposited from a low salinity fluid of probable meteoric origin (S1'Ochen +13.1% to +16.5%) which had interacted with the basement rocks and sediments (high Xe/Ar, Br/CI and I/Cl ratios). Plant-bearing chert yielded an 40Ar/39Ar ratio (292.1 f 0.6) significantly less than that of modem air and may be the first valid determination of a sample of ancient atmosphere.

A new geological map of the Early Devonian Rhynie Basin has been produced by traditional methods supplemented by trenching to bedrock and a ground magnetic survey. This shows that the basin margins are mostly fault-controlled and three trends are recognised: NE–SW, NNE–SSW and N–S. Three sets of open folds are distinguished with axial traces trending NE–SW, E–W and roughly NW–SE. The faults defining the basin margin and the folding may be related to basin formation within a regional strike-slip system of Early Devonian age.The stratigraphic succession comprises three mappable units: a lower mixed unit of sandstones, shales, conglomerates and andesitic lava (>700 m), a middle unit of laminated grey shale and siltstone (>300 m); and an upper unit of laminated sandstones and shales (>300 m). These correlate with the Tillybrachty Sandstone and Quarry Hill Sandstone Formations (lower unit), and the Dryden Flags Formation (middle and upper units).Small areas containing abundant chert float found outside the Rhynie SSSI may represent the surface expression of chert pods within the middle unit of laminated grey shale and siltstone, which also hosts the Rhynie cherts. The Windyfield cherts occur within the upper unit. No further centres of hydrothermal activity have been found in the northern half of the basin

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The NW‐SE trending Muglad Basin (SW Sudan) is one of a number of Mesozoic basins which together make up the Central African Rift System. Three phases of rifting occurred during the Cretaceous and Tertiary, resulting in the deposition of at least 13 km of sediments in this basin. Commercial hydrocarbons are sourced from the Barremian‐Neocomian Sharaf Formation and the Aptian‐Albian Abu Gabra Formation. The Heglig field is located on a NW‐SE oriented structural high in the SE of the Muglad Basin, and is the second‐largest commercial oil discovery in Sudan. The high is characterised by the presence of rotated fault blocks, and is surrounded by sub‐basinal structural lows. We modelled the geohistories of three wells on different fault blocks in the Heglig field (Heglig‐2, Barki‐1 and Kanga‐1) and one well in the Kaikang Trough (May25–1). The models were calibrated to measured porosity‐depth data, temperature and vitrinite reflectance measurements. Predicted present‐day heat flow over this part of the Muglad Basin is about 55 mW/m2. However, a constant heat‐flow model with this value did not result in a good fit between calculated vitrinite Ro and measured Ro at the wells studied. Therefore a variable heat‐flow model was used; heat flow peaks of 75, 70 and 70 mW/m2 were modelled, these maxima corresponding to the three synrift phases. This model resulted in a better fit between calculated and measured Ro. The source rock section in the Sharaf and Abu Gabra Formations was modelled for hydrocarbon generation in the four wells. Model results indicate that the present‐day oil generation window in the Hegligfield area lies at depths of between 2 and 4 km, and that oil and gas generation from the basal unit of the Abu Gabra Formation occurred between about 90 and 55 Ma and from the Sharaf Formation between 120 and 50 Ma. The results suggest that the oils discovered in the Heglig area have been generated from a deep, mature as‐yet unpenetrated source‐rock section, and/or from source rocks in nearby sub‐ basinal areas.

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SYNOPSISA ground magnetic survey and a programme of shallow drilling have enabled considerable revisions to be made to the position of the margin of the Insch mass at its eastern end and revealed that the distribution and attitude of the igneous rock units have been strongly influenced by shear zones trending ENE.New petrological information shows that, except in a restricted area, there is a lack of regular relationships between the cumulate igneous rock units and it is established that the major trends and discontinuities in the contoured magnetic map can be correlated with zones of shearing and mylonitization. Magnetic modelling indicates that the shear zones dip steeply (65-80°) to the south and that the mafic units extend no further than about 2 km in depth. Later faults on N or NW trends have also contributed to the disruption of the intrusion.These findings are discussed in relation to regional structure and it is suggested that such shearing is more important than has been recognized hitherto and may constitute a discrete and extensive episode of deformation about 470 m.y. in age.

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