CYCLIC STRATIGRAPHY AND FACIES CHANGE ACROSS A 3RD ORDER BOUNDARY (UPPER BERRIASIAN) LOWER CRETACEOUS IN THE FRENCH JURA AND DORSET ENGLAND

Applying the Croll-Milankovitch orbital forcing model a 3rd order sequence boundary is defined near the top of the Purbeckian in both Dorset and the French Jura. Recognition of this boundary is based on patterns of facies change within the underlying cyclic structure, mapping of unconformities associated with the boundary and significant paleosol development below the boundary surface. This surface is well exposed at four localities in the French Jura: Mount Salève, Chapeau de Gendarme, Crozet, and Yenne. In Dorset it was observed and analyzed at two localities, Durlston Bay and Stair Hole. This boundary is illustrated by Jacquin et al. (1998) in SEPM Special Publication #60 and correlated between the Jura and Dorset. The boundary is defined by Jacquin et al. as a 3rd order sequence boundary, Be-7 (Berriasian) and dated at 138 ma. The 3rd order boundary occurs between the Vions and Chambotte Formations in France and is marked by a change from shale rich lagoonal facies with paleosols to more massive calcarenite beds deposited in higher-energy, more open marine paleoenvironments. At this 3rd order boundary an unconformity is indicated because 4th order (400 ka) sequences are truncated and 5th (100 ka) and 6th (20 ka) order sequences are missing. Thin section analysis of shales suggesting soils in the Vions Formation and the occurrence of high-energy peloids and echinoderm fragments in the Chambotte Formation indicates that a major sea-level event occurred at this boundary. In England the same 3rd order boundary occurs between the Chief Beef and Broken Shell Limestone Members. The facies changes across this boundary in England are similar to those in the equivalent interval in France with shales prominent below and more massive coarse grain calcarenite beds above. Evidence for paleosols in the shale rich interval below the 3rd order boundary in both the Jura and Dorset include, molecular weathering ratios determined by ICP-MS data, soil structure from thin-section analysis and clay mineralogy from X-ray diffraction.