The term 'hiatus concretions' was introduced for the first time by the late Ehrhard Voigt, a well-known German bryozoan specialist, in 1968 (originally as Hiatus-Konkretionen). Hiatus concretions are early diagenetic bodies that formed within the host sediment. In this respect, they are similar in composition to other concretions that are very common in siliciclastic deposits of different ages, some of which are known to contain fossils or minerals. Hiatus concretions, however, differ from conventional concretions in their complex post-diagenetic history, including exhumation on the sea-floor, colonization by various encrusting and/or boring organisms during a break in sedimentation, and final burial. Thus, the name 'hiatus concretions' refers to the fact that they indicate hiatal surfaces in sedimentary sequences (Fig. 1). It is known that hiatus concretions may have developed during very different time-spans: for example, within a part of one ammonite subzone (i.e. tens of thousands to a few hundreds of thousands of years) or much longer, during a time embracing more than a stage (i.e. several millions of years). The majority of the hiatus concretions known from the fossil record have carbonate cements, usually calcite, and these are the main focus of this article. Some hiatus concretions, however, are cemented by phosphate minerals.
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