Diagenesis of the oolite group between Blaen Onneu and Pwll Du, Lower Carboniferous, South Wales
The diagenetic history of the Oolite Group from Pwll du to Blaen Onneu has been unravelled. Cement types, solution events, dolomitisation, concretions and other authigenic minerals are described. 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. CL makes it possible to trace the growth of crystals and monitor changes in crystallographic form during growth. Seven cement zones are identified using stained specimens. Each zone has a characteristic luminescence. The age of these zones relative to the exposure of the top of the Oolite Group is established. Two distinct diagenetic areas are recognised on the basis of the distribution of the cement zones. 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. 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. Oolomitisation prevents correlation of cements in Area 1 over a distance of more than 1/2 km. Using CL it is possible to identify solution surfaces, on a micron to millimetre scale, within the cements. four solution events are identified in Area 2. Solution affects that are previously unreported are described. The presence of solution surfaces is used to illustrate the constantly changing nature of the pore waters that have affected the Oolite Group. 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 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. 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 values typical of a specific cement zone are thought to relate to the pore fluids from which it was precipitated. The pattern of changes seen in successive cement zones is explained in terms of changing pore water chemistry and isotopic composition with time. These changes occurred over an area at least the size of the study area. Very light oxygen values in veins are attributed to increased temperatures associated with deep burial. An attempt is made to reconstruct the paleohydrology of the study area in an attempt to explain the nature and distribution of the cements.