Quaternary geology of the Rangitata Fan, Canterbury Plains, New Zealand

(Inc. GST)
(Ex. GST)
Write a Review

Barrell, D.J.A.; Forsyth, P.J.; McSaveney, M.J. 1996 Quaternary geology of the Rangitata Fan, Canterbury Plains, New Zealand . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 96/23 67 p.

Abstract: The Rangitata fan is a low-angle alluvial fan of about 900 km2 extent that forms part of the Canterbury plains. The fan includes very extensive aggradation surfaces formed during and after the Otiran glacial maximum, and a terrace lined valley entrenched below the main surfaces in response to post-glacial degradation. Terraces which are incised at the head of the fan merge with older surfaces on the lower part of the fan, hence the ''main surface'' of the plains is a composite feature of diachronous age. Deposits exposed beneath the Rangitata fan surfaces are inferred to be of Otiran to Aranuian age, and within the Rangitata valley, no older deposits have been recognised beneath the Otiran deposits. Basement rock forms a series of ranges along the northern and western boundaries of the Rangitata fan because of tectonic uplift through the Quaternary. Several Otiran and post-Otiran fault traces occur along or near the range-front and attest continuing tectonic uplift. These tectonic features include sharp fault traces and broad flexures, which are inferred to be the expression of faulting beneath thick alluvial deposits. Geometry of the deformation indicates that the faulting is contractional (reverse or thrust) with no significant lateral movement; all deformation on the plains in northwest-side up. In addition to the range-front fault traces, a zone of faults and flexures extends across the Rangitata fan between two discontinuous sections of the range-front. The larger of these faults offsets a late Otiran surface by up to 12 m, and there is evidence for multiple faulting events. The Rangitata fan is the product of climatic, eustatic and tectonic influences upon the Rangitata fluvial system. Valley glaciation has affected discharge from the catchment by diverting ice into neighbouring catchments. Catastrophic glacial outburst floods are geomorphically significant events that may occur only during glaciations. Extent and height of aggradation deposits is attributed to the position of the glacial termini. A likely consequence of deglaciation is the formation of lakes that interrupt sediment movement through the system, and probably influence the extent of post-glacial incision on the plains. Eustatic sea level rise on the low-gradient Canterbury Bight shelf is predicted to induce only minor change within the relatively high-gradient fluvial systems unless the rise is attended by cliff-cutting, in which case fluvial incision is expected. The effect of tectonic deformation depends on conditions within the fluvial system; uplift across a degrading system may enhance downcutting whereas fault traces formed during major aggradation are likely to be buried. In the Rangitata system, about one-third of the fluvial incision at the head of the fan is attributed to tectonic uplift, while the remainder represents climatically induced changes on the fluvial system. (auth)