LATERAL CONTINUITY AND DISCONTINUITY OF 100 KA ECCENTRICITY SEQUENCES WITHIN AN ORBITALLY FORCED CYCLIC HIERARCHY: THE PURBECK GROUP, LOWER CRETACEOUS, DORSET, ENGLAND

At least six distinctive stratigraphic units representing 100 ka sequences or their lower boundaries can be traced for up to 25 km in coastal sections of the Purbeck Group in Dorset. These include in ascending stratigraphic order: the Great Dirt Bed, quartz sandstone in the Hard Cockle Member, the Mammal Bed unconformity, the Freshwater Chert Bed, the Cinder Bed, the Scallop Bed and the Broken Shell Limestone. Within this context of marked lateral stratigraphic continuity other 100 ka sequences up to 3.5 meters in thickness comprising multiple precessional cycles (20 ka rock cycles or PACs) entirely disappear in lateral distances of less than a kilometer. Two examples of such lateral discontinuity of 100 ka sequences occur in the Mammal Bed 4th order sequence (400 ka) between the north and south outcrops at Durlston Bay, Dorset. The first example occurs immediately above the Mammal Bed unconformity (a 4th order sequence boundary). Each complete 4th order sequence (a product of long eccentricity) contains four 100 ka (short eccentricity) sequences labeled A, B, C and D in stratigraphic order. At the north outcrop the first or 'A' 100 ka sequence is 1.6 meters thick and contains parts of four 20 ka rock cycles. The sea-level low-stand phase of the first 20 ka cycle is the Mammal Bed proper. Less than a kilometer to the south, at a second exposure of this stratigraphic interval in Durlston Bay, this 100 ka sequence is entirely missing and the 'B' 100 ka sequence sits on the unconformity. The second example of discontinuity occurs at the top of the Mammal Bed 4th order sequence. This time in the south outcrop at Durlston Bay the 'D' 4th order sequence is well developed where it comprises three 20 ka sequences and is nearly 2 meters thick. This 'D' sequence and 2 meters of the overlying 'A' unit of the next 4th order sequence are nearly completely missing at the north outcrop where a few centimeters of clay (soil?) occurs at the 4th order boundary. A significant result of applying a genetic hierarchic stratigraphic model lies in the ability to recognize loss of specific allocyclic elements and their time implications.