Maps, stratigraphic logs, and age control data for river terraces in the eastern North Island

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Litchfield, N.J. 2003 Maps, stratigraphic logs, and age control data for river terraces in the eastern North Island . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2003/31 102 p.

Abstract: River terraces along the mainstems of nine river catchments in the eastern North Island have been compiled and/or mapped, described, and dated as part of a study of post-glacial river incision rates. The focus is mainly on the best preserved, last glacial maximum age Ohakea river terrace, but data has been collected for pre-Ohakea aggradation terraces in particular. Terraces are only preserved downstream of the greywacke ranges, and in all but the Mohaka and Waiapu catchments, project beneath, and are buried by, younger fluvial and marine deposits at the downstream end. Terrace aggradation deposits are generally greywacke or sandstone gravel, with minor sand and variably thick silt, averaging 5-6 m in thickness. Silt is especially thick in the Raukumara Peninsula area (Wairoa, Waipaoa, and Waiapu catchments) reflecting the greater proportion of Tertiary mudstone bedrock there. Tephra coverbeds are widely preserved, particularly the Kawakawa Tephra (except the Waiapu catchment), which also occurs within Ohakea aggradation deposits in a number of catchments. North of approximately Napier City, Rerewhakaaitu, Omatoroa, and Rotoehu tephras rest on Ohakea, Rata, and Porewa terraces respectively. Loess deposits are particularly thick in the south, and are best preserved near the coast in the north. Numerical ages have also been obtained from sparse radiocarbon dating and OSL dating, some of which is still in progress. Compilation of all available ages confirms regional correlation of aggradation terraces, as well as correlation with the glacial periods. An apparent younging of tread ages is at least partly (if not wholly) an artefact of the absence of tephra coverbeds in the south, and a bias in the number of OSL ages in the Ruamahanga and Rangitikei catchments. Future dating should therefore focus within the aggradation deposits, such as OSL dating of silt and sand beds, and identifying Kawakawa Tephra. (auth)

 CD contains 1:50,000 terrace maps and can be purchased with the print edition of this Science report, also through the Webshop.