Major erosion surfaces in the basal Wealden Beds, Lower Cretaceous, south Dorset

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Abstract
The transition from evaporitic and lagoonal Purbeck Beds into the fluviatile basal Wealden Beds (Ryazanian-Valanginian) has in the past been interpreted as a continuous regressive succession. Mupe Bay and Bacon Hole, in south Dorset, are the only localities in the Wessex Basin of southern England at which the transition strata are well exposed and not extensively faulted. On the basis of facies analysis the lowermost Wealden Beds are interpreted as a regressive-transgressive-regressive package. Environments fluctuated between lagoon, inner-lagoonal shoreline and fluvial flood plain. The overlying succession is remarkable for the presence of two major erosion surfaces which, based on field geometries and facies relationships, are interpreted as unconformities. The lower unconfor­mity is overlain at Mupe Bay by sediments of lagoonal or lacustrine origin. The upper unconformity is expressed as the celebrated Mupe Bay palaeo-oilseep, an oil-cemented conglomeratic sand, overlain by a thick succession of fluvial sediments. A level near the basal sand and the two erosion surfaces are treated as candidate sequence boundaries (WBl-WB3), with the thin intervening ?lagoonal deposits representing peaks of transgression. The pattern of one minor (conformable) sequence boundary followed by two major (unconformable) sequence boundaries bears a strong resemblance to the Early Cretaceous sequence stratigraphy suggested in recent ‘global cycle charts’. However, biostratigraphical calibration of the sections is currently inadequate to allow close correlation with candidate sequence boundaries in other areas.

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