PALEOPEDOLOGY OF THE BACON HOLE AND WORBARROW TOUT LOCALITIES, LOWER CRETACEOUS PURBECK GROUP, DORSET COAST, ENGLAND

The Purbeck Group is a succession of interbedded limestones and siliciclastics deposited under increasingly humid conditions during the Lower Cretaceous (Berriasian). Periods of ancient soil formation within siliciclastic intervals are marked by fossil roots and horizons. In order to understand vertical climatic changes within the study area, four individual paleosols are compared at progressively higher levels in the section between two separate localities. The two localities are correlated using the regionally persistant Cinder Member. Observations of thin sections reveal that the oldest profile (Worbarrow Tout 2) is composed of a fine-grained marl with small detrital organic flecks and horizontal rhizolithic root traces filled with carbonized material. The two middle profiles (Worbarrow Tout 6 and Bacon Hole 1) preserve root traces defined by partially oxidized and pristine pyrite framboids, relict bedded siliciclastic-rich matrix with mottled zones and shell fragments. The uppermost profile (Mupe Bay 3) preserves some relict bedding, jarosite, and gypsum, which possibly formed due to base level drop and subsequent oxidation of pyrite. The overall trend suggests a transition from drier conditions (Worbarrow Tout 2), to a variable wet/dry environment (Worbarrow Tout 6 and Bacon Hole 1) and finally to waterlogged soil conditions (Mupe Bay 3).