The opinion is widely held that each limestone bed in minor limestone-shale cyclic sequences, however modified subsequently by diagenesis, reflects an environmental event. On the assumption that this is true, attempts have recently been made to relate limestone-shale cycles to climatic variations affected by “Milankovitch” orbital cycles. A test is proposed and illustrated mainly with reference to the Blue Lias Formation of Dorset, England, that appears to indicate that many limestones can be produced solely by rhythmic unmixing of CaCO3 during diagenesis. Unless such limestones can be clearly distinguished from those that record genuine environmental signals, orbital cycle analysis based on such sequences will give meaningless results.

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