ARTEFACTOS SANITARIOS

The borehole information was supplied by the Waikato Regional Council. It originally came in a text format but turning the text logs into a graphic representation made them more efficient to process (Figure 3.4). Having the bore- hole data allowed the geology of the Onewhero region to be further constrained and this was particularly useful for the two-dimensional gravity modelling.

Figure 3.3 is the legend for the borehole illustrations throughout this chapter are as follows:

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Figure 3.4: The locations of boreholes in the Onewhero region that are investigated and interpreted in this chapter.

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Onewhero-bore 1 (OB1) (Table 3.1; Figure 3.5) is located to the southwest of the Onewhero crater (Figure 3.4). A fine, grading to coarse, limestone is present at the end of the bore. This unit is approximately 30 m thick and probably represents the Waimai Limestone of the Aotea Formation. This unit is known to grade from a flaggy limestone to a glauconitic grey sandstone and then to the grey siltstone of the Akatea Formation (Carter Siltstone), and this also occurs in this borehole (Edbrooke, 2001). The Carter Siltstone is inferred to have been deposited in an outer shelf environment with low sedimentation rates. This has allowed glauconite to form. This is also observed in the borehole description. Overlying the Carter Siltstone are layers of alluvial gravels and silts capped by a 15 m thick volcaniclastic clay.

Table 3.1: Waikato Regional Council, OB1.

Figure 3.5: OB1 visual log.

Start depth (m) End depth (m) Lithology Description

0 15 Clay Volcanic clay

15 28 Silt Brown-white silts

28 36 Gravels Silty gravels

36 38.7 Silt Blue-grey silts

38.7 76 Siltstone Glauconitic siltstone

76 79 Sandstone Sandstone

79 92 Limestone Coarse limestone

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Onewhero-bore 2 (OB2) (Table 3.2; Figure 3.6) is located in the south-west of the Onewhero crater (Figure 3.4). This bore is difficult to interpret due to a lack of detail. A >47 m layer of limestone is present starting from 35 m in depth. This unit has probably been misinterpreted as there are no known limestone units in either the Te Kuiti or Waitemata groups that reach thicknesses greater than 30 m. The Carter Siltstone of the Te Akatea Formation and the Whaingaroa Siltstone are both highly calcareous siltstone that can be observed with a sandy limestone at their base (Edbrooke, 2001). I believe the limestone actually represents one of these formations, probably the Waingaroa Siltstone as this was mapped on the inner slopes of the Onewhero maar by Waterhouse (1968). Resting on the limestone unit is an 8 m thick layer of clay that could be a weathered cap of limestone or represent the land surface before the eruption. Above this is a sandstone layer that is probably composed of reworked tuff that was eroded off the steep inner crater wall after the eruption. OB2 is capped by a 15 m layer of clay sediment that probably consists of post-eruption material and lake infill.

Table 3.2: Waikato Regional Council, OB2.

Figure 3.6: OB2 visual log.

Start depth (m) End depth (m) Lithology Description

0 17.37 Clay Peaty, sandy, very soft

17.37 25.91 Sandstone Soft to hard

25.91 35.05 Clay -

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Onewhero-bore 3 (OB3) (Table 3.3; Figure 3.7) lies in the north-east of the crater (Figure 3.4). A >3 m thick sandstone deposit lies at its base; this is likely a reworked tuff deposit. Overlying the sandstone are bands of fine gravels 3 – 4 m thick. These gravels probably represent crater-rim failure, in the form of slumps and debris flows. Pirrung et al. (2003) outlined a detailed stratigraphic description of a pre- to post-eruption maar infill. Gravel layers in the maar infill were suggested to be deposited days to decades after the eruption. The gravels were interpreted as being syn-/post-eruptive rockfalls, debris flows and slumps. Overlying the gravels are silty clay sediments that are interpreted here as post-eruption lake infill. A distinctive peat layer occurs 3 m below the surface. This would have formed during a period of anoxia in the area. The peat layer only occurs along the edge of the crater and this makes sense as vegetation would be concentrated in these areas. Swamp deposits are a common constituent of the sedimentary infill of a maar and they usually represent the end of a maar lakes life (Pirrung et al., 2003; Houben et al., 2013).

Table 3.3: Waikato Regional Council, OB3.

Start depth (m) End depth (m) Lithology Description

0 3 Clay -

3 4 Peat -

4 44 Clay Green-brown silts

44 56 Gravel Fine, banded, cemented

56 59 Sandstone -

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Onewhero-bore 4 (OB4) (Table 3.4; Figure 3.8) is located on the southern slopes of the Klondyke cone (Figure 3.4) and comprises a repeating succession of interbedded scoria and basalt deposits that is approximately 40 m thick. The interbedded basalt and scoria represents a cycle of dry eruption styles from strombolian to hawaiian. Strombolian eruptions typically have higher gas contents and the resulting deposit is scoria. Hawaiian eruptions produce viscous basalt lavas with low gas contents (Mangan and Vergniolle, 2000). Overlying these deposits is a 45 m thick basaltic ash layer that is most likely associated with the Kauri Road cone that occurs nearby, to the south of the Klondyke cone.

Table 3.4: Waikato Regional Council, OB4.

Start depth (m) End depth (m) Lithology Description

0 2 Clay Basaltic

2 46 Ash Basaltic

46 86 Basalt/scoria Interbedded

86 88 Silt White

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Onewhero-bore 5 (OB5) (Table 3.5; Figure 3.9) is located on the southern outer slopes of the Onewhero maar’s ejecta ring (Figure 3.4). The bore reveals a 10-12 m thick basalt lava flow that overlies basalt-rich, sandy volcaniclastic sediment. The flow lies halfway between the Klondyke cone and the basalt flow that mantles the southern inner tuff slope of the Onewhero crater. A thin layer of ash overlies the basalt flow and this is capped with a layer of weathered clay.

Table 3.5: Waikato Regional Council, OB5.

Figure 3.9: OB5 visual log.