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Geological Society, London, Special Publications; 2003; v. 216; p. 95-107;
DOI: 10.1144/GSL.SP.2003.216.01.07
© 2003 Geological Society of London

Shallow Subsurface Sediment Mobilization

Tubular structures of northern Wairarapa (New Zealand) as possible examples of ancient fluid expulsion in an accretionary prism: evidence from field and petrographical observations

Béatrice Ledésert, Christophe Buret, Frank Chanier, Jacky Ferrière & Philippe Recourt

UMR Processus et Bilans des Domaines Sédimentaires, FR 1818 CNRS, USTL, SN5, 59655 Villeneuve d’Ascq cedex, France

The Cape Turnagain area is located on the inboard portion of the Hikurangi subduction margin, on the northern Wairarapa coast of the North Island of New Zealand. A 4.5 km long coastal section of sea cliffs of Mio-Pliocene sediments contains numerous tubular carbonate-rich concretions. Their morphology and petrographical observations suggest they were possibly formed by fluid flows of carbonate-rich water through a silty sediment. These tubular concretions could be fossil fluid expulsion structures similar to dewatering chimneys described offshore in New Zealand. The external diameter of the concretions observed in situ reaches 60 cm and internal canal up to 4 cm. Up to four canals are encountered in a single concretion. A positive relationship is observed between the chimney size and the number of canals or cumulative diameter of canals, suggesting that the size of the concretion is a function of the fluid which flowed through the plumbing network. The increased number of tubular concretions in upper Miocene siltstones compared to overlying Pliocene strata could be linked to a compressive event that caused overpressuring and the expulsion of fluids through the sediment pile.