The mode of origin of some late diagenetic sandstone concretions from the Scottish Carboniferous

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Abstract
Synopsis: Field observations and textural analysis suggest that the carbonate concretions in the fluviatile Wall Hill Sandstones of southern Scotland are late diagenetic in origin since the carbonate of the concretions has clearly replaced a secondary silica cement in the enclosing sandstones. Carbonate crystallization seems to be mainly dependent on pH conditions and the problem of producing localized areas of varying pH within a sediment is discussed. Decaying organic material can produce local variations of pH and thus promote localized carbonate crystallization but this is considered an unlikely explanation in the present case. A possible mode of concretion formation is advanced involving fluctuation in pH around the threshold value necessary for carbonate crystallization which governs the number of nuclei of crystallization formed.