AMALGAMATION AND CONDENSATION OF 400 KA AND 100 KA ORBITALLY FORCED SEQUENCES AT A BASIN MARGIN: THE PURBECK GROUP, LOWER CRETACEOUS, DORSET, ENGLAND

Applying the ‘Croll-Milankovitch’ orbital-forcing model, the Purbeck Group is divisible into a hierarchy of meter-scale rock cycles and cycle sets (sequences). At Durlston Bay the Purbeck Group consists of 100 m of marginal marine to freshwater facies comprising four 2 Ma (3rd order) sequences, each divisible into 400 ka (4th order) sequences. These in turn consists of four 100 ka (5th order) sequences that each contain up to five 20 ka (6th order) rock cycles. Sixth order cycles are the product of the precessional signal and are grouped into sets by eccentricity modulation of the strength of this signal. Sequences are recognized by asymmetrical patterns in the distribution of facies and the magnitude of facies change at cycle boundaries. This cyclic structure can be traced from Durlston Bay westward to Stair Hole by integrating cyclic stacking patterns with traceable key beds. At Stair Hole the Purbeck Group as a whole has thinned to 40 m and 400 ka sequences within it have thinned from more than 10 m at Durlston Bay to less than 2 m. In this distance entire 100 ka sequences up to 3.5 m thick disappear while others are condensed into limestone-shale couplets less than 1 m thick. Loss of section is a result of decreasing subsidence toward the western margin of the Purbeck Basin. Paleosols are common at all sections and all occur at cycle or sequence boundaries. Correlation of the cyclic hierarchy makes it possible to identify specific cyclic elements lost in basin margin areas and to compare pedogenic development between cycles in high and low subsidence areas.