Hummocky Cross Stratification

Oct 7,2008

The photographs show a rectangular calcite cemented 0.15 m thick block of black laminated very fine grained sandstone interbanded with and cut by white bands and burrow fillings representing bioturbated clays deposited between offshore marine storms in the Eocene London Clay Formation. The block was dug up recently at a depth of 18 inches (c.0.5 m) in the front garden of Symonds Avenue in Eastwood, Essex, previously mapped by the British Geological Survey officer G.W. Green as basal Claygate Beds of Eocene age when it was a grassy field in 1972. More recently an excavation in the pavement there showed laminated orange sands and brown clays at the same depth and altitude (38 m above mean seal level). The occurrence of flint pebbles in the loose sands made it look like the local Quaternary Brickearth deposits showing similar banding elsewhere. Having a calcite cemented block of the sands, which include one 5 mm diameter quartz grain in the side, confirms the mapping of these sands as an uncontaminated Eocene storm deposit within the upper part of marine London Clay Formation. The way-up of the block was unrecorded and unclear, with fungal spores on the flatter side suggesting it was once on top in the soil and the cutting of the black sands by white clay-filled Eocene marine burrows (Chondrites and some larger ones) suggesting it is actually below. The other side of the block is absent and the adjacent part is fully bioturbated. It is probably the top of a rapidly deposited storm sand layer buried within offshore marine clays. The sandy sedimentary structures look like Hummocky Cross Stratification (HCS).