Seismic stratigraphy of the Giant Foresets Formation, offshore North Taranaki western platform

Soenander, H.B.

Provenance of sediments in Taranaki Basin-an assessment from heavy minerals - 4th New Zealand Oil Exploration Conference: 207-233


The Plio-Pleistocene Giant Foresets Formation has been investigated in the offshore North Taranaki region (longitudes 172 degrees E-174 degrees E and latitudes 38 degrees S-39 degrees S), using seismic reflection profiles and exploration well data. The formation is divided into four seismic facies above a prominent seismic reflector dated as base Pliocene. The topset facies is characterized by subparallel continuous reflectors, the progradational foresets by subparallel continuous reflectors in a clinoform pattern, the degradational foresets facies by chaotic offlapping low-amplitude reflectors, and the bottomset facies by moderate-amplitude, subhorizontal reflectors of variable continuity. Five continuous reflectors have been mapped, three within the formation and two facies bounding it. The three internal reflectors terminate towards the upper boundary by toplap and erosional truncation, and the lower boundary (base Pliocene) by downlap. Termination of the three internal reflectors toward both upper and lower boundaries suggests that the latter are correlative unconformities bounding one depositional sequence. The mapping results of the prominent internal reflectors show that the foresets prograded from southeast to northwest as part of a large lobe. The geometry of the formation suggests it is a "high stand system tract" which developed during early Opoitian (5.0 Ma) to Mangapanian (2.6 Ma) time when the rate of the sediment supply was higher than the rate of basement subsidence.