Anoxic events during sedimentation of a Palaeogene diatomite in Denmark

A diatomite, about 60 m thick, of late Palaeocene‐early Eocene age crops out in northern Jutland, Denmark. The diatomite is locally termed ‘Moler’. Frustules of marine diatoms constitute c. 65% (by weight) of the diatomite, and clay minerals, chiefly montmorillonite, make up the remainder. Slight variations in the relative supply of diatom frustules and clay minerals are preserved undisturbed in laminated diatomite, while lamination is partly destroyed by burrowing organisms in weakly laminated diatomite and obliterated by total bioturbation in structureless diatomite; these three facies alternate throughout the sequence. The presence or absence of infaunal burrowing organisms is interpreted as a record of the content of dissolved oxygen in the water above the sediment‐water interface and hence of the position of the redox potential discontinuity. Interspaced in the diatomite are 179 identifiable layers of volcanic ash. These ash layers provide a means of precise lateral correlation. They show that levels of laminated diatomite may be followed throughout the basin and therefore that changes between anoxic and oxic conditions occurred simultaneously across the area. The laminated diatomites may consequently be interpreted as representing short‐term anoxic events, of which twelve have been recognized.