Nozomi Iso

Age and height distribution of holocene transgressive deposits in eastern North Island, New Zealand

Abstract Holocene transgressive deposits are frequently exposed near the present-day coastline of the study area along eastern North Island, New Zealand. They occur in sites of former estuaries that were filled during the postglacial rise in sea level. We present one hundred radiocarbon dates of Holocene transgressive deposits from the study area, ranging in age from ca. 10,000 to 5500 yr B.P. Relative sea level curves up to ca. 6000 yr B.P. were reconstructed for six locations. The curves have similar slopes prior to about 7000 yr B.P., indicating that sea level rise was much more rapid than any tectonic uplift at that time. The postglacial rise in sea level in New Zealand is considered, in general, to have culminated at about 6500 yr B.P. but the upper limit ages of transgressive deposits in our study area vary from ca. 5500 to 7000 yr B.P. At sites where the uplift rate is high the postglacial transgression culminated rather earlier than ca. 6500 yr B.P., and at sites where there is subsidence or there is very low uplift the culmination is later than ca. 6500 yr B.P. Nine of fourteen dates from fossil trees in growth position, that grew in and were buried by estuarine silt, cluster in the age range ca. 8000–8400 yr B.P. These data support the view that there was a minor regression or stillstand in the eustatic sea level rise at that time. Eleven tectonic subregions are recognized in the study area on the basis of average uplift rate. Most of these subregions coincide with those established from the number and ages of younger Holocene marine terraces of probable coseismic origin.

Holocene marine terraces on the northeast coast of North Island, New Zealand, and their tectonic significance

Abstract As many as seven Holocene marine terraces are preserved between Raukokore River and Gisborne on the northeast coast, North Island, New Zealand. Six terraces up to 20 m above present mean sea level (a.m.s.l.) are dated at c. 300, 600–700, 900–1200, 1600–2000, 4500, and 6000 radiocarbon yr B.P. to the west of East Cape. Seven terraces are preserved up to 27 m a.m.s.l. near Pakarae River mouth, and the higher six terraces have radiocarbon ages of c. 1000, 1600, 2500, 3900, 5500, and 7000 yr B.P. The coastal region from Waiapu River to Tolaga Bay has only two to three marine terraces, the highest attaining a maximum height of c.8m. Sponge Bay Terrace is generally the highest preserved marine terrace, and it is underlain by more than 10 m of estuarine deposits that record the rapid rise of postglacial sea level. The terrace surface records the culmination of this sea‐level rise at 5500 yr B.P. or slightly younger in areas of low average uplift rate (<1.5 m/1000 yr) and c. 7000 yr B.P. in areas of high...

Origin and age of erosion surfaces in the upper drainage basin of Waiapu River, northeastern North Island, New Zealand

Abstract In the upper drainage basin of Mata River, a tributary of the Waiapu River, hilltop height accordances at two levels, 500–580 m and 560–640 m above sea level, are observed. Both hilltop height accordances are inferred to represent erosion surfaces developed over folded Neogene rocks. The lower erosion surface is estimated from geomorphological position relative to the Mangamaunu Upland and tephrochronology to have been formed during a period of 25 000–30 000 years from prior to c. 35 000 years ago to c. 12 400 years ago (late Otiran). This lower erosion surface is not an elevated peneplain but was originally formed at a high level similar to its present altitude. The lower erosion surface was probably formed by cryoplanation under cooler and slightly drier climatic conditions than present with a sparse vegetation cover. The age of the higher erosion surface is not known but it must be much older than c. 35 000 years.