The Hamilton Basin has long been thought of as an area of simple geologic structure with two major faults recognised to the west and east of the Basin; the Waipa and Kerepehi Faults. However, recent identification of deformation signatures within the Basin suggests that the geological structure of the Hamilton Basins may be far more complex than previously recognised. At the time of this study, 26 potential zones of interest associated with tectonic deformation had been identified within the Hamilton Basin. This study investigates two of these identified areas.
The first zone of interest runs through a hill section on Kay Road, near Horsham Downs in northern Hamilton (Kay Road field site). The field area was created during construction of the Waikato Expressway, through excavation of a hillslope. Consequent exposure of geology showed extensive deformation of the Walton Sub-group (Puketoka & Karapiro Formations) and the Kauroa Ash Formation, through uplift, offset and displacement of bedding. This deformation varied within the field area, with displacement of beds ranging in size from a few millimetres in the Walton Sub-group, to > 7 m of displacement within the Kauroa Ash Formation. The total throw of the Kay Road field area was calculated at 7.41, with the field area appearing down-thrown towards the south. This down-throw was associated with extensive, steeply dipping normal faulting, and associated displacement of geologic units.
Geological investigation of this field site along with detailed unit descriptions and structural information measurements were used to generate geologic models using Leapfrog® Geologic Modelling software. Geologic Models of the Kay Road field area confirmed initial field observations; that normal faulting was extensive within at the Kay Road field site, and that deformation at Kay Road is confined to a time period older than the Rangitawa Tephra (0.35 Ma). This was illustrated by the irregular distribution of strata within the Kay Road field site, extensive intrusion structures present in the field area, and the undisturbed nature of the Rangitawa Tephra unit.
The second field site studied an inferred fault running parallel to Osborne Road in the northern boundary of Hamilton City (Osborne Road field site). Initially, this was discovered through observation of a ridgeline in study of LiDAR information of the Hamilton Basin. The presence of this ridgeline then led to study of field geomorphology, soil distribution and soil electrical resistivity properties within the Osborne Road field area. Study of these phenomenon is thought to corroborate a hypothesis that reverse faulting is present within the Hinuera Formation (c. 16 ka). This is evidenced by a 2 m concave slope break running through the field site parallel to Osborne Road, alongside irregular soil distribution within the field area. This irregular distribution shows an extensive presence of coarser, sandy units in the east of the slope break, with finer grained silty-sand and silty-clay units found to the west of the slope break. A zone of mixed sand and silts was found between these two units, that is thought to have been created in the reverse fault roll over zone, a phenomenon commonly associated with reverse faulting. Due to the non- invasive investigation approach that was undertaken at Osborne Road, faulting cannot be definitively confirmed; however significant evidence was found within the field area, suggesting it is highly likely that recent deformation is present.
Evidence suggesting deformation is present within the Hamilton Basin is extensive, with this investigation detailing evidence of both normal and reverse faulting. These discoveries were found in close proximity to one another, suggesting that deformation within the Hamilton Basin has been far more extensive and complex then previously recognised. Through detailing of offset bedding, alongside extensive study of geomorphic and deformation structures within the Hamilton Basin, this study corroborates hypotheses first presented by Kleyburg (2015) and Moon and de Lange (2017), that suggest tectonic deformation is present within the Hamilton Basin.
These discoveries contradict widely accepted literature, that suggests that evidence of deformation and faulting has been limited to the outer extents of the Hamilton Basin, and that the Hamilton Basin is an area that has been subject to very little to no tectonic deformation (McCraw, 1967; Kamp & Lowe, 1981; de Lange & Lowe, 1990; Stirling et al., 2002; McCraw, 2011; Stirling et al., 2012). The presence of such extensive tectonic deformation in the Hamilton Basin is thought to contradict the low national seismic hazard rating that Hamilton City currently holds (Stirling et al., 2002; Stirling et al., 2012). For this claim to be validated however, further study and definitive evidence is required.
