Cyclic Stratigraphy Of Two Intervals At The Boundaries Of A 3rd Order Sequence (Lower Cretaceous, Berriasian) In The French Jura, France And The Dorset Coast, England

Abstract

Marked facies changes occur at and just above 3rd order boundaries in Lower Cretaceous carbonate platform deposits of the French Jura and the Dorset Coast. In contrast, the boundaries of cycles just below 3rd order boundaries show smaller facies changes and the facies are very restricted. Two studied intervals (lower and upper) in these carbonate platform deposits are near the lower and upper boundaries of a 3rd order sequence (Anderson, 2000; 2001a & b). These intervals are described in terms of facies and their cyclic stacking pattern and are correlated within each region. The studied intervals are well exposed at four localities in the French Jura: Saleve, Chapeau de Gendarme, Crozet, and Yenne, and two on the Dorset coast: Durlston Bay and Stair Hole. Correlation involves using unconformities interpreted from sequence stratigraphy techniques, biostratigraphy from previous literature, distinct marker beds such as paleosols or freshwater beds, and the interpretation and matching of the hierarchic cyclic structure at each locality. The interpretation of sequences and the stacking pattern at each locality is based on the Milankovitch orbital forcing model. The degree of facies change within a sequence and at sequence boundaries is a function of the magnitude of the sea-­level rise. Cycles and sequences usually are characterized by shallowing upward facies with more massive limestone, usually calcarenite, beds near the base and marl, shale, or paleosols toward the top. This pattern of facies change is seen throughout each locality and in cycles of every scale in the Milankovitch hierarchy. In each region cycles near the basin margin are missing or highly truncated, whereas in the most complete sections (type sections) located more basinward a more complete Milankovitch stacking pattern is preserved. Paleosols tended to form near major cycle boundaries and as a consequences are associated with formational and member boundaries. Paleosols are classified using USDA (1998) and Mack et al. (1993) standards by examining macro- and microanalysis, molecular weathering ratios determined from ICP-MS data, and clay mineralogy from X­ray diffraction. Four paleosols were sampled. Two of these paleosols are from the study interval in the French Jura and two are sampled from the study interval in the Dorset coast. The analysis of the paleosols in England (samples D3-5 and Dl-6) indicate moderate development and are classified as Inceptisols. Whereas the paleosols in the French Jura (samples R-2 and S-19) are poorly developed and are classified as Entisols.