The Geology of the Matemateaonga Formation, Central Taranaki, New Zealand

Abstract

The Matemateaonga Formation consists of up to 1500m of mainly shallow marine sandstone of latest Miocene to early Pliocene (late Tongaporutuan-Opoitian) age which was deposited over both Taranaki and Wanganui Basins. The formation outcrops over an area of approximately 3,500 km2 across central Taranaki. The outcrop is bordered in the east by the Ruahine Range and the Central Volcanic Zone and to the west by Mt. Taranaki and its associated ring-plain deposits. The site of deposition was a broad and essentially planar, rapidly subsiding basin which lay to the west and north of active compressional tectonic zones. Sediment supply was sufficient to maintain paleoenvironments at or near sealevel and build a north-northwest prograding shelf wedge. The Matemateaonga Formation is distinctive from other Tertiary shallow shelf deposits in New Zealand because of its wide expanse, relatively undeformed nature and lack of evidence for syn-depositional tectonism. Tectonism, although important in providing a voluminous sediment supply, played a secondary role in the distribution and character of the Matemateaonga Formation. A formal definition of the formation introduces a composite stratotype, boundary stratotypes and several lectostratotypes. The depositional history of the formation is presented and is based on facies distribution and interpretations, paleocurrent indicators, provenance determinations and a consideration of the major external controls on sedimentation. Eight facies are recognised and interpreted with respect to a terrigenous shelf model. Facies range from intertidal to outer shelf deposits from a range of water depths of about 100m. To maintain this shallow water deposition, sedimentation rates for the Matemateaonga Formation must have been approximately 0.6 m/kyr, which is around an order of magnitude greater than the average for similar terrigenous shelf sediments deposited elsewhere over similar time spans. The facies architecture is interpreted to represent the response to rhythmic relative sealevel changes. The estimated duration of the unit-stratotype of the Matemateaonga Formation (2.45-2.85 million years) is comparable to third order sealevel changes as defined by Vail and Sangree (1988). The depositional setting of the formation therefore makes it useful for studying sealevel changes of this and higher orders of periodicity. It is shown that the changing accommodation potential of Taranaki-Wanganui Basin may have been as an important factor as tectonism and third order sealevel change in producing differences in bulk characteristics of formations in the latest Miocene and early Pliocene. However, a minimum of eleven fourth order sealevel cycles are interpreted from a series of transgressive lags found in a composite section of the formation. Using the time span for the unit-stratotype, these give an average cyclicity of between 220-260,000 years per sealevel cycle. Sandstones from the Matemateaonga Formation are poorly sorted and angular feldsarenites derived from a quartzo-feldspathic sedimentary provenance. Mineralogical homogeneity for the formation suggests that much of the type area had the same provenance. Geochemical discriminants and petrography show that in the type area sediments are closely linked to the Triassic-Cretaceous Torlesse terrane and to Tertiary rocks directly underlying them. To the west of the type area, sediment compositions reflect the different Mesozoic basement terranes that make up the underlying structural grain of New Zealand.