Lower Miocene paleogeography and molluscan taxonomy of Northland, New Zealand.

Abstract

The lower Miocene paleogeography of Northland, New Zealand, consists of an elongate, north to south-tending marine trough bordered by a Mesozoic basement ridge to the east and an active volcanic arc to the west. During upper Waitakian and Otaian times, the trough contained deep-water: sediments from Kaipara to North Cape whilst in upper Otaian times, overstepping by the western volcanic arc in con junction with regional uplift in the Hokianga area created separate sedimentation patterns for North Cape and Auckland Kaipara regions. Detailed stratigraphic and paleoecologic studies on upper formations of the Parengarenga Group, North Cape reveal that a submarine canyon-fan complex was deposited in upper Otaian and Altonian times. Assessment of individual, richly fossiliferous localities in the Waioha Shellbeds Member, Pahuape Formation; Tahuna Pebbly Mudstone Member, Paingatai Formation and Waiheuheu shell lenses, Paratoetoe Formation all contain displaced, shallow-water faunas originating from a variety of substrates and are mixed with deeper-water elements from soft substrates. One fossil biocoenose from Pokere Mudstone Member, Paratoetoe Formation is dominated by carnivorous gastropods and the fauna is an example of resource partitioning of lower trophic groups. The mode of deposition of flysch of Poroporo Member, Paratoetoe Formation is discussed. Mapping and regional stratigraphy of the Waimamaku Valley, Hokianga was carried out but due to the paucity of the good outcrops, only limited detailed stratigraphy of lower Miocene Otaua Group (Waiwhatawhata Conglomerate and Waitiiti Formation) was achieved. A number of facies in the latter formation were recognised. Localised conglomerates and coarse sandstones overlain by fine-grained deposits indicate rapid down-sinking of the area in upper-Waitakian and Otaian times. Weak, tractive currents sorted deep-water muds. Coarser material and faunas were introduced from time to time by various gravity-induced, redepositionary mechanisms. Detailed assessment of individual faunal localities in the Waitiiti Formation reveal that scant fossil biocoenoses probably reflect subtle variations of the muddy substrate. Thanatocoenoses from redeposited units show origins from shallow water biotopes similar to those encountered at North Cape. The abundance and diversity of molluscan minutiae from pebbly mudstone lithologies suggests intricate ecologic relationships dependent on diverse, and mostly non-preservable lower trophic groups in the neritic zone. A few unusual faunal associations indicate the establishment of communities in deep-water utilising hard substrates of redeposited clastics. A detailed taxonomic study on molluscs, particularly minutiae from Waitiiti Formation, Otaua Group and Tahuna Pebbly Mudstone, Paingatai Formation, Parengarenga Group has established the presence of 107 new species.