VARIATIONS IN THE CYCLIC STRUCTURE OF A 4TH ORDER SEQUENCE ACROSS A BASIN MARGIN: PURBECK GROUP, LOWER CRETACEOUS, DORSET, ENGLAND

The 4th order (400 ka) sequence from the 'mammal bed' to the top of the Cherty Freshwater member of the Lulworth Formation thins from 5.8 to 2.5 meters on a transect across the Purbeck basin margin in Dorset. Applying the hierarchic 'Milankovitch' orbital forcing model to the thickest section at Durlston Bay reveals that this interval is divisible into four 5th order (100 ka) sequences. Each of these comprises up to five, 6th order (20 ka) cycles. Facies sequences in each order of the cyclic hierarchy are arranged asymmetrically. The lower parts of both rock cycles and sequences are more carbonate rich and the upper portions are richer in shale. Carbonate is deposited in response to sea-level rise while shale is deposited in response to subsequent sea-level stability or fall. Purbeck facies in this interval represent a complex of marginal marine, brackish and fresh-water deposits as well as soils. Soils occur at four horizons: 1. the base of the study interval, 2. the 'mammal bed', 3. the top of the second 5th order sequence (the fern bed) and 4. at the top of the 4th order sequence. The number of 6th order cycles in each 5th order sequence decreases westward toward the basin margin. In the first 5th order sequence, the number of cycles decreases from 4 to only a soil. In the second and third 5th order sequences the number of cycles decreases from 4 to 3 and in the fourth 5th order sequence from 1 to only a soil. This lateral decrease in the number of 6th order cycles is due to loss of accommodation space and the resulting amalgamation of cycles toward the basin margin. Recognition of a hierarchic cyclic structure in Purbeck facies and patterns of cycle loss at the basin margin, reinforce the concept of orbital forcing as a fundamental stratigraphic process.