Holocene Evolution of the Upper Western Channel within Tauranga Harbour

Abstract

The Tauranga Harbour is a mesotidal lagoon that is actively infilling with sediment. The southern basin of the harbour is important from both ecological and socio-economic standpoints. An understanding of sediment dynamics is necessary for the management of the harbour. Previously, the Tauranga Harbour Sediment Study (THSS) analysed the terrigenous flux of sediments into the harbour. It identified predominantly silt-sized sediment yields, from catchments, which remains confined to entry points into the harbour, or get exported out to the open coast. However, mapping of the tidal inlet and parts of the Western Channel through to Rangiwaea Island, has identified that accretion involves sand-sized sediment. The presence of eroding cliffs has provided speculation that sediment is primarily derived by local source erosion, as opposed to terrestrial or marine inputs. However, little is known about the sediment dynamics through the central harbour region. This thesis involved seismic reflection surveying through the Western Channel, from Rangiwaea Island to Matakana Point, utilising a Knudsen Sub-Bottom Profiler that operates on a chirp sonar system. Through the seismic analysis, patterns of sandwave occurrence were analysed to discover how sediment dynamics varied along the Western Channel. Additionally, three fault sites were identified in the seismic profiles. Two of these faults occur parallel to a previously mapped fault at Omokoroa, where doming has been suggested. The third fault occurs in the southeast where subsidence has been identified. Vibracoring was utilised to collect intact, contiguous, and undisturbed cores through the field area. Sand is identified as the primary contribution to ongoing sedimentation in the harbour. A general coarsening trend of sedimentary texture is observed from the central intertidal flats through the upper Western Channel towards the tidal inlet. This pattern is disrupted where current amplification or close proximity to a sediment source is associated with the accretion of coarser sediment to form sandwaves. Rates of sedimentation through the Western Channel over the last 7,200 years, ranged from 0.0482 mm/yr approaching the tidal flats, to 0.436 mm/yr where extensive sandwave were identified. A sedimentation rate of 0.0977 mm/yr was calculated within the channel where no sandwaves were present. The primary source of sediment appears to be local erosion of coastal cliffs, with sedimentation rates strongly correlating to erosional sites.