CORRELATIVE PALEOSOL PROFILES AT CYCLE BOUNDARIES ACROSS A DIFFERENTIALLY SUBSIDING BASIN MARGIN: THE PURBECK GROUP (BERRIASIAN), LOWER CRETACEOUS, DORSET, ENGLAND

Paleosols are developed at cycle boundaries at all levels of a four-tiered hierarchy of cycles and sequences of cycles in marginal marine to freshwater facies of the Purbeck Group in Lower Cretaceous rocks in Dorset, England. This hierarchic cyclic structure is recognizable at 6 localities across 23 km of the Dorset coast from Durlston Bay to Durdle Door. Individual components of the cyclic structure are correlated by integrating the cyclic patterns with seven key marker beds that are laterally persistent at all localities. The thickness of the Purbeck Group markedly decreases from over 100 m in eastern sections to 40 m in the equivalent western sections. This decrease in thickness is attributed to decreasing subsidence toward the western margin of the Wessex Basin during the time of deposition of the Purbeck Group. In these sections paleosols are preserved on sequence and cycle boundaries. Based on the presence of jarosite, aquic features and abundant plant remains, these paleosols are classified as sulfahemists, sulfaquepts, and sulfaquents. Similar modern soils are referred to as acid sulfate soils (ASS), and display the same jarositic root traces and organic accumulations that characterize the Purbeck paleosols. The paleosols analyzed in this paper are traced from high to low subsidence areas based on their position within the correlative cyclic structure. They represent parts of four separate landscapes that change markedly in their pedogenic structure and composition as a function of lateral position within the basin and reflect the interaction of elevation, relative subsidence, water-table fluctuations, and possibly their position within the cyclic hierarchy.