An Integrated Stratigraphy of Tauranga Harbour

Abstract

Tauranga Basin is a Quaternary basin filled with successions of terrestrial and coastal sediments. The largest feature of Tauranga Basin is Tauranga Harbour, a 570km2 estuary. This is home to the Port of Tauranga, New Zealands largest export port. Studies conducted following an initial study by Davis and Healy (1993) have found complex distributions of the sediments within the dredged shipping channel. This research used 86 vibracores and limited seismic data to identify four main facies within the shallow subsurface of the dredged channel and establish their distribution. Four facies were identified within the shallow subsurface: the Volcanic Sand facies, a silt to fine sand volcaniclastic succession; the Silt facies, a silt to very fine sand lacustrine deposit; the Coastal Sands facies, a medium to coarse coastal sand deposit; and the Pumiceous Sand Facies, a medium to coarse sand succession with abundant pumice.

Within the dredged shipping channel, the Volcanic Sands and Silt facies outcrop at the surface, and appear as ridges in the subsurface. Draped over these ridges is the Coastal Sands facies, which in some cores is overlain by Pumiceous Sands. The Volcanic Sands facies represents a succession of tephra deposits, with some reworking between depositional events. In the Maunganui Roads region of the harbour this is overlain by the lacustrine Silt facies, formed from the reworking of the Volcanic Sands. The region was then inundated by the Holocene marine transgression, leading to the deposition of the Coastal Sands facies, sourced from offshore. This Coastal Sands facies is then overlain in areas by Pumiceous Sands, small mobile sand waves that exist within the shipping channel. These are formed following the erosion of ignimbrite cliffs around the harbour, with some input from the outside the harbour. This represents a change in source for sedimentation in the harbour.

An updated model of the paleogeographic development of the harbour is presented, using data from this and other studies. This model shows the presence of a braided river flowing through Stella Passage, before the development of a lake and the subsequent Holocene marine transgression leading to the evolution of the sand spit and Matakana barrier island.