Geological contours for groundwater modelling, South Canterbury

presence of the peneplain was confirmed in geological mapping by Farmer et al. (1975) priorto filling of Lake Opuha, who recorded quartz sand dipping very gently west at the easternmargin of the basin. The very gentle dip of both the peneplain and these quartz sedimentsimply that the basement would lie at shallow depths (>200 m) under the basin floor in SS2b.At the western margin of the SS2b sector, basement rock is exposed and no Cretaceous-Cenozoic rocks have been reported. The only logged bore hole of any depth in the basin(J37/0309) muddies rather than clarifies the situation as it reports gravel to 92 m depth,underlain by 30 m of basalt. No basalt rock has previously been reported in this area, and if itwere, for example, Geraldine Basalt, it would imply something of the order of 1 km ofCretaceous-Cenozoic rocks covering the basement at that location. As the weight of surfacegeological evidence at the basin margins favours basement at shallow depth, we tentativelyinfer that the basalt reported from J37/0309 is basement rock greywacke rather than basalt,and that it is overlain by gravels associated with the Quaternary sediments. The implicationsof this interpretation is that early in the Quaternary period, SS2b must have undergonesufficient uplift to allow erosion to strip off the Cretaceous-Cenozoic cover rocks, andsubsequently, the South Opuha River built a fan of fluvial sediments across the basin,burying the basement rock.The boundary between SS2b and SS2c to the south is inferred to be a major fault. At thewestern margin of the basin between the Opihi River and the South Opuha River, theCretaceous-Cenozoic rock sequence is exposed, and dips east into the basin at a moderateangle (between ~20° to ~40°). This rock sequence is also present on the eastern margin ofthe basin dipping west into the basin. A fault dislocation running across the basin is the onlyexplanation compatible with the geological interpretation proposed for sector SS2b, but if thatwere shown to be incorrect, it may remove the need for this fault.Sector SS2c is bounded to the west by major range-front fault systems, the Albury Fault andthe Hunters Fault/Opawa Fault system. To the east it is bounded by the Brothers Fault thathas uplifted the Brothers Range on the eastern side of the fault. An east-west trending fault isinferred where the Opihi River enters the Fairlie basin from the west. This fault accounts forthe marked contrast between homoclinally dipping Cretaceous-Cenozoic rocks north of theriver, and the generally flat Cretaceous-Cenozoic strata, with open folds, south of the river. Amajor fault, downthrown to the east and part of the Albury Fault System, is inferred to lie eastof this area of flat-lying Cretaceous-Cenozoic strata, in order to account for the considerablethickness of Cretaceous-Cenozoic strata filling this part of Fairlie basin, implied by the overallweight of geological evidence.The Fairlie/Cannington basin has been long interpreted to be a synclinal fold structure, theCannington Syncline (e.g. Gair 1959). The basement contouring is based largely on thesurface distribution of geological formations, stratigraphic picks and extrapolation ofmeasured bedding dips. The saddle in the basement contour pattern in the basin axis nearFairlie accommodates the results of the Langdale & Stern (1998) ‘Line 4’ gravity survey thatindicates basement at about 500 m depth under Fairlie. However, this is in conflict withmeasured section data that implies ~850 m of Cretaceous-Cenozoic sediments under theKowai Formation, which is the surface geological formation along the basin axis near Fairlie.The saddle in contours under Fairlie is the closest compromise that can be achievedbetween the stratigraphic and gravity data sets. Immediately south of Fairlie, measured dipsnear the entrance to the Opihi Gorge, and in Coal Stream guide the contour spacing.GNS Science Consultancy Report 2012/245 A-2