ABSTRACT

The Corallian Group of the Dorset coast consists of four transgressive-regressive cycles. Detailed petrographic and cathodoluminescence studies supplemented by trace element analyses reveal that the diagenesis varies from cycle to cycle, and even within each cycle. Differences between the diagenetic sequences in the transgressive and regressive units are most clearly seen in the Osmington Oolite Formation. In the transgressive sequence, the neomorphism of skeletal aragonite with the preservation of original architecture, early columnar isopachous cement fringes, and the pervasive distribution of strongly ferroan calcite cements, together with the dominantly non-zoned dull luminescence, suggest a change from a marine phreatic into a low-oxygen, deeper-burial connate zone. The absence o grain over-packing before major cementation is primarily due to the presence of widespread early columnar isopachous cements.

In contrast, in the regressive sequence, extensive leaching of originally aragonitic grains with commonly collapsed micrite envelopes, pervasive mildly ferroan or non-ferroan calcite cements, and the dominantly zoned, bright luminescence indicate replacement of marine phreatic water by meteoric water, which dissolved unstable grains and precipitated carbonate cements in a sub-oxic environment.