Basin Filling Patterns on the Waipaoa Continental Shelf (NZ) Revealed by Mapping Tephra Beds in High-Resolution Seismic Records

Gerber, Thomas P.¹, Alan Palmer², Lincoln F. Pratson¹, Steve Kuehl³, J.P. Walsh⁴, Clark Alexander⁵, Alan Orpin⁶ (1) Duke University, Durham, NC (2) Massey University, Palmerston North, New Zealand (3) Virginia Institute of Marine Science, Gloucester Point, VA (4) East Carolina University, Greenville, NC (5) Skidaway Institute of Oceanography, Savannah, GA (6) NIWA, Wellington, New Zealand

We combine tephra analyses from sediment cores with a high-resolution chirp seismic grid to reconstruct basin filling during the post-glacial transgression and modern sea level highstand on a tectonically active shelf. The Waipaoa shelf is a forearc basin downwind from the Taupo Volcanic Zone, which has periodically ejected large volumes of silicic tephra during rhyolitic eruptions throughout the Quaternary. Six distinct and dated eruption tephras are preserved in the Holocene shelf basin fill. The oldest (7005 +/- 155 cal yr B.P.) marks the culmination of post-glacial sea level rise (~7000 cal yr B.P.), providing an approximate marker for the transition from transgressive to highstand conditions. Three younger tephras provide useful markers for the early highstand period. The entire post-glacial sequence overlies an unconformity interpreted as the LGM erosion surface. Uplift of anticlinal structures that define the outer shelf drives subsidence along a mid-shelf subbasin and has produced a small fault-bounded subbasin seaward of the anticlines. Correlation of tephra beds tied to seismic reflectors between cores in the depocenters and in condensed sections along anticline flanks shows that while both subbasins preserve relatively thick transgressive and early highstand deposits, late Holocene fill in the southern mid-shelf and outer shelf subbasins exceeds that in the northern mid-shelf. Differential subsidence in the southern mid-shelf landward of Lachlan anticline and more frequent sediment bypassing of the northern mid-shelf may explain the observations. The shelf architecture demonstrates that stratigraphic sequence generation following sea level rise on tectonically active shelves can be dominated by localized accommodation.