PALEOPEDOLOGY OF THE MAMMAL BED, 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 the siliciclastics are marked by fossil roots and soil horizons. The Mammal Bed, located in the Lulworth Formation of the Purbeck Group, was studied from proximal to distal locations (west to east) in order to evaluate relative degrees of pedogenesis. In proximal areas (Lulworth Cove), the Mammal Bed contains pyrite-preserved root traces and a ped-like macrostructure. Marine fossils are absent. Limestone directly below the mammal bed is relatively shell free and contains clustered pyrite framboids. Samples from Mupe Bay, 2 km further east, show pyrite-preserved root traces, a blocky macrostructure, but no marine fossils. The microfabric is composed of oriented clays with parallel extinction, possibly as a function of relict bedding. In distal locations (Durlston Bay) the Mammal Bed was influenced by marine conditions, as evidenced by abundant shells of pelecypods, ostracods, and gastropods. Charophytes are present but rare. Pyrite-preserved shells and root traces are most abundant at the top of this paleosol at Durlston Bay and decrease down the profile. The microfabric has been overprinted by tectonics. Based on the presence of pyrite-preserved root traces and preservation of relict bedding, Mammal Bed paleosols likely formed under waterlogged and reducing conditions across the entire basin, although slightly oxidized framboids toward the top of the profiles may suggest a late stage oxidation event. Lateral changes in the Mammal Bed may be due to variation in tectonic activity with increased subsidence in the distal (Durlston Bay) locations. Differential subsidence between Lulworth Cove and Durlston Bay led to variations in soil development with a lateral decrease in soil macrostructure, but an increase in detrital organics, marine fossils and bed thickness.