Geological contours for groundwater modelling, South Canterbury

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2012the topic of this project commonly extend to many hundreds of metres depth.A major limitation of geophysical data is that geophysical surveys have been undertaken atonly a few localised places in the project area, and furthermore, the results do notnecessarily allow definitive conclusions to be drawn about subsurface geology.The interface between hard basement rock and the softer and more varied Cretaceous-Cenozoic rocks is far and away the most easily recognisable geological boundary in theproject area. In most cases, it is easily distinguished in geological exposures, from contrastsin landform character and in bore hole logs. Unfortunately, as is described in Section 3.1.2below, the same is not true of the Kowai Formation. It has a varied lithological character, andin many places is not distinctively different from the overlying and underlying formations.In a project that was a precursor to the present project, Jongens (2011) produced a structurecontour model of the boundary between Quaternary sediments and underlying olderformations beneath the Canterbury Plains between the Ashley and Rakaia rivers. Thatproject benefited from an array of hydrocarbon-exploration seismic survey lines and severaldeep (>1 km) exploration bore holes. However, no subsurface information of that density orquality is available in the area of the South Canterbury project reported here, and a differentmethodological strategy was therefore needed in order to achieve the aims of the project.The thicknesses of the Cretaceous-Cenozoic cover rock strata are well established at anumber of locations in the project area (measured sections; see Fig. 3 and Table 1).Although these thicknesses vary from place to place, they provide a good foundation forgeological interpolation and extrapolation, and form a major basis for the structure contourinterpretations presented in this report.The following sections outline the procedures used to define the structure contours for eachof the target geological features. Commentary on the reasoning behind aspects of thecontouring interpretations is provided in Appendix 1.3.2 Contouring the top of basement rockThe following approach was used to define contours on the top of the basement rock:• The position of the contact between the basement rock and overlying Cretaceous-Cenozoic rocks is mapped at numerous locations around the project area (see Fig.2a). In a few places the contact is directly exposed, but in many others its preciselocation is inferred with good confidence based on nearby rock exposures. Where amapped contact intersects a topographic contour defines a point on the structurecontour line corresponding to that topographic elevation. In other words, theintersection of a geological contact and a topographic contour directly pinpoints astructure contour. For a structure contour map of a particular geological feature, it isimportant to delineate the extent of the target feature, including where it lies at orabove ground (outcrop) as well as where it lies underground (subcrop) (Fig. 4).• Points (‘stratigraphic picks’; Fig. 3) were marked where distinctive geological strataboundaries intersect specific topographic contours. In the project area, the mostdistinctive geological stratum, apart from the contact between the basement rock andGNS Science Consultancy Report 2012/245 3
2012the overlying Cretaceous-Cenozoic rocks, is the Kekenodon Group limestone layer(eMk). An inherent asset of the hard limestone layer is that it commonly forms rockyoutcrop, and so can be detected and located accurately in many places within theproject area.• At each stratigraphic pick on a distinctive stratum, we used measured sectionstratigraphic thickness estimates to calculate depth to basement beneath thatstratum, and assigned an elevation to the basement surface (metres relative to sealevel) based on the topographic contour value at the pick site. To illustrate with anexample, the limestone layer is semi-continuously exposed between Hanging Rockon the north bank of the Opuha River through to Kakahu Bush. Measured sectioninformation indicates that in this general area, the top of the basement lies 400 mbelow the base of the limestone, and so where the base of the limestone crosses a100-m interval topographic contour, the stratigraphic pick is assigned an elevation ofthe top of the basement that is 400 m less than the elevation of that topographiccontour. In this way, numerous stratigraphic data picks were marked across theproject area, using a variety of distinctive stratigraphic boundaries (Fig. 3).• At each stratigraphic pick, we did not allow for the dip of the strata. Measuredstratigraphic sections are ‘true’ thicknesses, measured perpendicular to bedding. Truestratigraphic thickness equates to depth below the ground where the strata arehorizontal, but where strata are inclined, the vertical thickness is greater than the truethickness. However, for the most part, the strata of the project area are inclined atgentle (
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Geological contours for groundwater modelling, South Canterbury

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