Patterns of sedimentation during early Otaian (Early Miocene) time in North Canterbury, New Zealand

Abstract

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 size 3.6 φ-5.7 φ) accumulated everywhere on the basin slopes, while calcareous slightly sandy silt (mean size 5.6 φ-7.0 φ) rich in Foraminifera accumulated in the basin centre. Subsequently, traction currents periodically active from both sides of the depositional basin, spread thick, non-graded beds of slightly silty sand (mean size 3.5 φ-4.0 φ) over the silt of the basin centre. About the same time, or a little earlier, silty sand (mean size 4.1 φ-4.4 φ) and sandy silt (mean size 4.8 φ-5.1 φ) rich in ferromagnesian minerals and rock fragments, were derived from the north, and were deposited along the northern limits of the basin by persistent traction currents and wave action. Light as well as heavy mineral compositions show that the sediments were derived from mica schists of the Haast Schist Group and from the indurated geosynclinal sandstone and mudstone of the Mesozoic Torlesse Group.