THE CYCLIC STRUCTURE OF THE PURBECK GROUP, LOWER CRETACEOUS, DORSET, ENGLAND

Applying the 'Milankovitch' orbital-forcing model, the Purbeck Group at the type locality in Durlston Bay is divisible into meter-scale rock cycles and cycle sets (sequences). The Purbeckian, 100 meters of marginal marine, brackish and freshwater facies and soils, is divisible into about fourteen 400 ka, 4th order, sequences. Each 4th order sequence comprises a set of four 100 ka, 5th order, sequences. These in turn consist of from one to five 20 ka, 6th order, rock cycles (PACs). Sixth order cycles are produced by precession and are grouped into sets by eccentricity modulation of the precessional signal. All levels of the cyclic hierarchy are recognized by asymmetry in their patterns of facies distribution. Larger facies changes occur at the bases of cycles and early in sequences (sets of cycles) while smaller facies changes occur at cycle boundaries higher in sequences of cycles. In the Purbeck Group coarse-grained skeletal limestone is the dominant facies at the base of cycles and low in cyclic sequences while fine carbonates, shale and soils characterize the upper parts of cycles and sequences. Magnetostratigraphic and biostratigraphic (ostracod and charophyte zonation) work by others indicates that the Purbeck Group in Dorset largely is correlative with the Berriasian Stage and represents 5-7 million years of deposition in the earliest Cretaceous. Thus average stratigraphic accumulation rates of 1.4 - 2.0 meters per 100 ka can be calculated. Assuming this time and thickness constraint and the existence of a three-tiered orbitally forced hierarchy, the recorded rock cycles and the hierarchy of cycle sets only can be matched to the Milankovitch mechanism in one way. Tracing of correlative parts of this three-tiered hierarchy toward the basin margin (westward along the Dorset coast) demonstrates that most 5th order sequences are (in part) preserved but may be condensed (or amalgamated) into single limestone shale couplets, less than a meter thick, sometimes topped by soils.