Abstract:
This study is of Pliocene temperate shallow marine sediments that are widespread in southern Hawke's Bay, New Zealand, along the boundary between the Indian and Pacific plates. The area of deposition was characterised by active compressional tectonics to the east and a major zone of dextral transcurrent faulting to the west, with sedimentation occurring in a narrow north-east/south-west trending fault-controlled basin.
The strata consist of siliciclastic mudstone and sandstone (Mangatoro Formation or undefined), and several barnacle-dominated limestone units which have been collectively termed Te Aute Limestone. A revision of the stratigraphy as part of this study has led to the definition of Te Aute Group as all marine sediments overlying the Late Miocene-Pliocene regional unconformity, and underlying, mostly with angular unconformity, the Kumeroa Formation of southern Hawke's Bay, and the Petane Formation or Kidnappers Group of central Hawke's Bay. The Te Aute Group is divided into six formations. In order of decreasing age they are: Mangatoro Formation; Kairakau Limestone; Kokopeka Sandstone; Awapapa Limestone; Raukawa Mudstone (Argyll Sandstone Member); and Te Onepu Limestone.
Facies analysis of the Te Aute Group led to the definition of 14 Facies Associations which are interpreted in terms of the terrigenous shelf sedimentation model. Facies range from intertidal to outer shelf deposits and include widespread development of subtidal sand sheets, sand waves, and sand ridges containing giant foreset beds 40m high. Facies interpretation indicated a ranqe in water depth about 100m on the shelf during Te Aute deposition with large-scale cyclic alternations from shallow carbonate to deeper water terrigenous deposits. A depositional history of the group is presented based on the facies interpretation and a consideration of the major external controls on sedimentation.
δ18O measurements were made on mainly benthic foraminifera in two near-complete sections that span most of the Te Aute Group to assess the importance of sea-level change due to changing ice volume. However, the data could not be satisfactorily explained by variations in ice volume and temperature change shows no obvious relationship with sea-level variations inferred from the facies analysis. Local changes in relative sea-level can be accounted for tectonically, especially in view of the active setting of the basin. Though local debris flow and slump deposits are indicative of tectonic activity, the main evidence is from rapid variations in formation thickness.
Carbonates of the Te Aute Group, like other temperate carbonates, are dominated by metastable magnesium calcite with only rare preservation of aragonite. Main diagenetic processes include: compaction; dissolution; neomorphism; replacement; and cementation. Compaction was the single most important process in the diagenesis of the group with initial porosities of 45 – 70% reduced up to 40% by volume. Pressure solution with intergranular interpenetration of bioclasts, and later stylolite development with increasing overburden, is considered the major process in the supply of calcium carbonate.
