THE CYCLIC STRUCTURE OF THE CINDER MEMBER 4TH ORDER SEQUENCE: THE MIDDLE PURBECK GROUP, DORSET, ENGLAND

Applying the hierarchic 'Milankovitch' orbital forcing model, the Cinder Member of the Durlston Formation is shown to be the product of the 100 ka periodic variation in eccentricity. It (the Cinder Member) is the second 100 ka sequence (5th order) in a 400 ka set of sequences. This 4th order sequence at Durlston Bay comprises four 5th order sequences arranged in an asymmetric shallowing-upward pattern, in which the second 5th order sequence contains the least restricted or most marine facies. The third and fourth 5th order sequences are progressively shallower or more restricted. Each 5th order sequence in turn consists of up to five 20 ka, rock cycles thought to be the product of precession (6th order). The overall 4th order sequence thus episodically first deepens and then shallows. Likewise the earlier 6th order cycles in each 5th order sequence are more carbonate-rich (deeper) while the upper, shallower or more restricted parts of these sequences are characterized by an increasing abundance of shale. Because the rate of addition of accommodation space (by differential subsidence) across the basin margin in Dorset is low, during the Lower Cretaceous, soils form at the tops of many cycles even in the zone of greatest subsidence (Durlston Bay). Toward the basin margin (westward along the Dorset coast from Durlston Bay), where subsidence is lower, 6th order cycles at the tops of 5th order sequences are missing (not deposited) or are amalgamated into single limestone-shale couplets topped by soils. The persistence of a hierarchic cyclic structure across a spectrum of near shore facies in the middle Purbeck Group, even if incomplete, reinforces the interpretation of Milankovitch orbital forcing as a primary process in building the stratigraphic record.