The most extensive phase of dissolution occurred immediately after the precipitation of the non-luminescent subaerial vadose cements. Textures, visible only with cathodoluminescence, provide evidence of recurrent periods of fabric dissolution. Precipitation of vadose cements was contemporary with subaerial alteration and micritization of the limestone. This needle-fibre calcite represents the final stage of vadose cementation. Subaerial vadose cements comprise two generations of non-luminescent cement, followed by a brightly luminescent generation which occasionally shows an acicular habit. A spar cement stratigraphy has been established for the calcrete profile. Cathodoluminescence reveals that these cements pre-date the late pore-filling meteoric phreatic cements which occur throughout local Asbian lithologies. The profile contains a unique series of early pore-filling vadose cements which only occur down to 1 m below the palaeokarstic surface. The upper surface of the profile is defined by a laterally discontinuous palaeokarstic surface and by laminated calcareous crusts which developed within the underlying limestone. An attempt is made to reconstruct the paleohydrology of the study area in an attempt to explain the nature and distribution of the cements.A laterally extensive calcrete profile has been identified in the Late Asbian (Lower Carboniferous) shallow marine shelf limestones of the Llangollen area, North Wales. Very light oxygen values in veins are attributed to increased temperatures associated with deep burial. These changes occurred over an area at least the size of the study area. The pattern of changes seen in successive cement zones is explained in terms of changing pore water chemistry and isotopic composition with time. The values typical of a specific cement zone are thought to relate to the pore fluids from which it was precipitated. The successive cement zones do not show a progressive trend in caroon and oxygen values however, in different samples successive cement zones do show the same pattern of changes in carbon and oxygen values. The carbon isotopic composition of the allochems and cements pre-dating the exposure of the top of the Oolite Group is related to their distance below the exposure surface at the top of the Oolite Group and thus seems to reflect meteoric alteration. Carbon and oxygen stable isotope analysis of allochems and cement zones highlights the distinction between Areas 1 and 2 and are used to try and identify the nature of the pore waters affecting the Oolite Group during its diagenesis. The presence of solution surfaces is used to illustrate the constantly changing nature of the pore waters that have affected the Oolite Group. Solution affects that are previously unreported are described. four solution events are identified in Area 2. Using CL it is possible to identify solution surfaces, on a micron to millimetre scale, within the cements. Oolomitisation prevents correlation of cements in Area 1 over a distance of more than 1/2 km. It is possible to correlate the cement zones of Area 2 along the outcrop for a distance of 8 km and also with cements in the overlying Llanelly formation. Area 1 : the Pwll-y-cwm and Blaen Onneu Oolites in the Clydach area, in which Zones 1, 2b and 6 are present, Area 2 : the rest of the Oolite Group, in which Zones 2a, 3, 4, 5 and 6 are present. Two distinct diagenetic areas are recognised on the basis of the distribution of the cement zones. The age of these zones relative to the exposure of the top of the Oolite Group is established. Each zone has a characteristic luminescence. Seven cement zones are identified using stained specimens. CL makes it possible to trace the growth of crystals and monitor changes in crystallographic form during growth. The calcite cement crystals show chemical zonation through staining but cathodoluminescence (CL) has proved the most useful method for displaying internal features of calcite crystals. Cement types, solution events, dolomitisation, concretions and other authigenic minerals are described. The diagenetic history of the Oolite Group from Pwll du to Blaen Onneu has been unravelled.
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