Hiatus concretions and hardground horizons in the Cretaceous of Zululand (South Africa)

Horizons of bored concretions, 'hardgrounds' or 'hiatus concretions' of European authors, occur at several levels in the Aptian to Coniacian marine sediments of Zululand (South Africa). They show signs of a complex burial and excavation history, are bored by lithophagous bivalves and encrusted by a variety of epizoans, while vacated borings have been secondarily occupied by nestling bivalves and other organisms. Recognition of these horizons provides insight into substrate conditions, bathymetry and diagenetic history, whilst explaining the incomplete nature of the Cretaceous succession in the area.