Correlation of fluvial terraces within the Hikurangi Margin, New Zealand: implications for climate and baselevel controls

A correlation of fluvial terraces is presented for eight non-glacial catchments of the eastern North Island, New Zealand, within the actively uplifting Hikurangi Margin. Using a combination of loess and tephra coverbed stratigraphy, and radiocarbon and OSL dating of fluvial deposits and loess coverbeds, we demonstrate correlation of four fill terraces, T1-T4. The available age constraints suggest T1=15-30 ka, T2=31-50 ka, T3=50-70 ka, and T4=∼115 ka, but the association and temporal link with loess deposits suggest correlation with cold periods, and thus the refinement of T2=late MIS 3 (31-40 ka), T3=MIS 4 (55-70 ka), T4=MI Substage 5b (∼90 ka), 5d (∼110 ka), or MIS 6 (∼140-160 ka). The ability to correlate terraces between catchments, plus the lack of independent evidence for tectonic triggering events, indicates terraces have probably formed in response to either climate (terrestrial) or baselevel (sea level) control. Climate control is indicated by the temporal link of post-glacial incision with re-establishment of forest cover, and of LGM aggradation with limited grass and shrub cover and periglacial processes. Aggradation due to increased sediment supply under reduced vegetation is dramatically demonstrated by formation of the Taupo Pumice Alluvium terrace in response to inundation by volcanic deposits (unwelded ignimbrite) following the 1.8 ka Taupo eruption, and the response to post-settlement (<500 yr BP) deforestation. The upstream limit of post-glacial baselevel control is recorded by a post-glacial sediment wedge burying older terraces in six catchments. In one catchment, the inner (landward) edge of the wedge is a mid-Holocene fill terrace, interpreted to have formed in response to the post-glacial sea level highstand, and thus is a baselevel-controlled terrace. Evidence defining the downstream extent of climate control during the LGM (and thus the upstream limit of baselevel-controlled lowstand incision) is now buried beneath the continental shelf, but limited fluvial features on the inner shelf (<20 km offshore), and LGM aggradation despite a range of shelf gradients indicate climate-induced aggradation probably extended at least to the present day inner shelf.