Seismic stratigraphy of the Giant Foresets Formation, North Taranaki Basin

Abstract

The Plio-Pleistocene Giant Foresets Formation has been investigated in the offshore North Taranaki region (longitudes 172°E-174°E and latitudes 38°S-39°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 facies 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, variable continuity subhorizontal reflectors. Five continuous reflectors have been mapped, three within the formation and two bounding it. The three internal reflectors terminate towards the upper boundary by toplap and erosional truncation, and towards 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 deposition sequence. Results of mapping the prominent internal reflectors show that the foresets prograded from southeast to northwest as part of a large fan lobe. Well correlation data based on the physical character of well logs (sonic, density, gamma ray) and supported by lithologic and biostratigraphic data allowed division of the Plio-Pleistocene sediments (from the top to bottom) into four zones (A, B, C and D). These zones were deposited in from middle shelf to deep bathyal depths and each zone is represented by prominent seismic reflectors that are most clearly recognized in the Taimana-1 well. The result of this correlation shows that water depth during Opoitian (5.0 Ma) to Mangapanian (2.6 Ma) time deepened from southeast to northwest. During this period oscillation of sea level was followed by the movement of sediments from the south-southeast where erosion of uplifted areas occurred, then transport and deposition to the northwest. The fluctuation of sea level can be recognized from the displacement of the shelf edge shown by biostratigraphic and paleoenvironment analyses, which shown indicate clockwise rotation from the southeast to northwest with the pivot around the Tangaroa-1 site. Fluctuation of the sea level and the transport of sediment from the eroded area during Opoitian to Mangapanian times influenced the development of the progradational foresets facies of the Giant Foresets Formation. The geometry of the progradational foresets facies suggests that the rate of the sediment supply was higher than the basement subsidence. This facies is bounded at the bottom by a flooding surface and at the top by a correlable unconformity surface. The position and feature of this foresets facies within the regional paleogeography, where there is no evidence for incised valleys or rejuvenated streams during this period, suggests that this progradational foresets facies was deposited as a "highstand systems tract type II". Continuation of the shelf edge displacement to the northwest until the present day suggests that sediments in this shelf edge area are possibly a "shelf margin systems tract".