ABSTRACT: Tide-Influenced Sedimentation in the Late Cretaceous Pakawau Group, Tarankai Basin, Northwest Nelson, New Zealand

WIZEVICH, MICHAEL C., Research School of Earth Sciences, Victoria University, Wellington, New Zealand

Earliest deposition in the Taranaki Basin, spanning late Cretaceous to early Tertiary time, was confined to a series of fault-controlled subbasins. Upper Cretaceous coal-bearing sequences contained within the subbasins are considered the source of petroleum in the Taranaki Basin. Except for the Pakawau Basin in the Northwest Nelson region of the South Island, New Zealand, the subbasins exist only in the subsurface. Sediments of the Pakawau Group include thick conglomeratic and coal-bearing sequences that are primarily of terrestrial origin, but marine to marginal marine sediments are now recognized in the North Cape Formation within the onshore part of the basin.

The North Cape Formation consists of relatively mature sandstones with discontinuous conglomerate and fine-grained units, but no extensive coal beds. Analyses of the fine-grained sediments revealed marine dinoflagellates, which are frequently fragmented and abraded. Sedimentary structures, including mud-draped cross beds, bidirectional ripple cross-laminations, and occasional bioturbation, indicate tide-influenced deposition. Detailed facies analysis of the North Cape Formation reveals three distinct facies associations: proximal, intermediate and distal. The latter represents a high-energy, tide-influenced paralic environment, most likely estuarine. Further evidence for tide-influenced sedimentation is found in the more proximal associations. These findings indicate deposition in a t de-influenced braid delta-system.

The North Cape Formation, recognized in the subsurface throughout the Pakawau Basin (about 40 by 100 km) is up to 1 km thick (several hundred meters in Northwest Nelson) and exhibits acceptable reservoir quality. This study of the Pakawau Basin sediments can be used as a model for sedimentation in other subbasins in the Taranaki Basin, with implications for future hydrocarbon exploration.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)