Abstract The lower part of the Waikari Formation (Otaian) was sampled at 30 localities in North Canterbury. The field relationships, sedimentary structures, textures, and gross compositions of its constituent members are compared with the geographic variation in their grain size parameters (mean size, standard deviation, skewness, and kurtosis). From these data patterns of sedimentation are reconstructed for a basal Otaian depositional surface and for a second depositional surface a little higher in the Otaian stage. At the beginning of Otaian time sediments were accumulating in a single depositional basin, part of a broader continental shelf. The basin was oriented north-north-east and south-south-west, and was bounded by a land ridge on the west and a submarine ridge on the east. Probable tidal flat sediments were deposited in the extreme south-west of the study area and a short distance to the north-east shallowshelf muddy sand, rich in shell fragments was accumulating. Glauconitic sandy silt (mean siz...