Late Pleistocene and Holocene Seismic Stratigraphy of an Active Forearc Basin, Waipaoa Continental Shelf, New Zealand

Thomas P. Gerber¹, Lincoln Pratson¹, Steve Kuehl², Lila Gerald², J.P. Walsh³, and Clark Alexander^4
1 Duke University, Durham, NC
² Virginia Institute of Marine Science, Gloucester Point, VA
³ East Carolina University, Greenville, NC
⁴ Skidaway Institute of Oceanography, Savannah, GA

As part of the MARGINS Source-to-Sink program, we report 1800 km of single fold chirp seismic data collected over 2200 km2 that encompass the entire Waipaoa continental shelf, a forearc basin located along the northeast margin of North Island, New Zealand. In gas-free sediments, the chirp FM pulse (.5-7.2 kHz) penetrated the post-glacial/Holocene mud belt (40-50 m maximum thickness) to a regional unconformity that is inferred to be the time-transgressive LGM erosion surface. The imaged strata are focused in three primary depocenters delineated by isopaching above this unconformity. Two depocenters lie along the mid-shelf and contain synclinal growth strata that taper seaward against the flanks of two anticlines that form the deformation front of the Hikurangi subduction margin. The anticlines are structurally offset, with the northern tip of the southern (Lachlan) structure separating the two depocenters. The offset has created a gap in the anticlinal barrier that connects the mid-shelf with a third major depocenter seaward of the Lachlan structure. Tectonic influence on the margin is strong and evidenced by onlap-offlap relations in the mid-shelf growth strata. Chronostratigraphic control and deposition rates from shallow (3m) vibracoring and deeper piston coring (30m) are forthcoming, but our preliminary seismic reconnaissance suggests the following: (1) the Waipaoa continental shelf has been an efficient sink for terrestrial sediment over most of the Holocene, and (2) the outer shelf depocenter indicates either recent escape of sediment sourced directly from the Waipaoa River or high influx of pelagic mud from the proximal East Cape Current.