Evolution of the North Horowhenua Coastal Depositional System in Response to Late Pleistocene Sea Level Changes
The convergent tectonic setting of New Zealand has lead to the development of a series of anticlines and troughs resulting from folding and faulting of basement greywacke in southwest North Island. The most extensive of these is the Kairanga Trough spreading from the Horowhenua to the Manawatu, which lies between the uplifting Tararua Range and subsiding South Wanganui Basin. This trough was a major depocentre for fluvial and shallow marine strata during the Quaternary. By utilising a 280m deep borehole from the Kairanga Trough, this thesis investigates how climate and sea level variations affected sedimentation in the north Horowhenua District. This borehole has recorded a near continuous record of climate and sea level change for the last 340ka. The lower part of the core is a marine sequence representing progressive infilling of the Kairanga Trough during 5th order (c.100ka) glacioeustatic fluctuations, which consequently produced 4 marine cyclothems. Transgressions and subsequent highstand periods are represented by shallow marine sediment, which were followed by fluvial aggradation during lowstand periods, then marine planation during subsequent transgressions. Cycle 1 developed during OIS 9 (340-300ka). Cycles 2 and 3 both formed during OIS 7 as a result of two closely spaced highstands centred around 245ka (OIS 7c) and 200ka (OIS 7a), which were separated by a period of lower sea level around 225ka (OIS 7b) that produced a disconformity. Cycle 4 formed during the Last Interglacial transgression (OIS 5e) and represents an incised valley fill. Progradation of a coastal strandplain and alluvial plain representing the latter stages of infilling of the Kairanga Trough with coastal and terrigenous sediment during the mid to late Last Interglacial and Glacial Periods is recorded in the sediment composing the top part of the borehole.