Three zones of chert beds showing a new type of deformation structure--chert "dikes"--have been found in the Senonian Sayyarim Formation in southern Israel. The "dikes" occur as a series of parallel bulges protruding both upwards and downwards from chert beds, deforming and intruding adjacent layers. The height of the "dikes" varies from a few millimeters to a few meters, while distances between adjacent structures vary between a few millimeters to a few tens of centimeters. The trend of the "dike" structures, being consistent on a regional scale (N70E), is roughly parallel to the facies boundaries within the discussed sequence. Petrographic study indicates that the structures were formed by plastic deformation followed by non-plastic brecciation. Structural relationships, such as rupturing of carbonate beds by the chert "dikes," indicate that lithification of siliceous sediments may he rather slow compared to other sediments. It is suggested that the dike-like structures are formed by an increase in volume of the siliceous sediment during diagenesis, the expansion being regionally controlled.

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