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The mid-Cenomanian non-sequence is described in terms of its stratigraphical position, regional impact and importance in Cenomanian stratigraphy. After its discovery in the site investigation for the Channel Tunnel and its location within the chalk succession of the Dover-Sangatte area, it has been traced throughout the United Kingdom and northern France. In stratigraphically complete successions it is coincident with the base of the Rotalipora cushmani Taxon Range Zone, although in many areas there is a major hiatus (= sequence boundary?) at this level. In more marginal successions, the non-sequence is characterized by the presence of reworked, phosphatized macrofossil assemblages in which the actual date of deposition can only be determined by the non-phosphatized macrofaunal elements or the microfossils extracted from the enclosing sediments. In Dorset/Devon and northern France the Cenomanian successions are condensed, and horizons with reworked macrofaunas have been investigated using a variety of micropalaeontological techniques (thin-sections, processed residues, acid reductions, etc.). In these areas the mid-Cenomanian non-sequence becomes one of the most important features of the succession and marks a major hiatus in many localities.

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