Parent materials of Tirau silt loam

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New Zealand Journal of Geology and Geophysics ISSN: 0028-8306

Pullar & K. S. Birrell To cite this article: W. A. Pullar & K.

Zealand Journal of Geology and Geophysics, 16:3, 677-686,

10431389 To link to this article: https://doi.org/10.

Submit your article to this journal Article views: 218 Citing

1 New Zealand Journal of Geology and Geophysics ISSN: (Print) (Online) Journal homepage: Parent materials of Tirau silt loam W. A. Pullar & K. S. Birrell To cite this article: W. A. Pullar & K. S. Birrell (1973) Parent materials of Tirau silt loam, New Zealand Journal of Geology and Geophysics, 16:3, , DOI: / To link to this article: Published online: 14 Feb Submit your article to this journal Article views: 218 Citing articles: 12 View citing articles Full Terms & Conditions of access and use can be found at

2 No PARENT MATERIALS OF TIRAU SILT LOAM W. A. PuLLAR Soil Bureau, DSIR, Rotorua, New Zealand and K. S. BIRRELL Soil Bureau, DSIR, Lower Hutt, New Zealand (Received 29 November 1972) ABSTRACT The parent materials of Tirau silt loam (a moderately leached yellow-brown loam/humic Andosol) at the Reference Site near Tirau have been established with the aid of a correlation line carried westwards over the Mamaku Plateau from Rotorua where the individual tephras can be identified. However, supplementary laboratory tests were needed to establish the identity of some tephras that have been removed by erosion over an intermediate part of the correlation line. In addition to air-fall tephras, several beds of loessial material were recognised. These have originated from rhyolitic tephras that were erupted towards the end of the last glaciation, and lie immediately above the tephras from which they have been derived. Although the tephras recognised are all rhyolitic, frequent additions of andesitic ash from sources to the south have altered the physical and chemical properties of the soil so that it resembles a yellow-brown loam rather than the expected yellow-brown pumice soil. No evidence has been found for downward movement of clay within the described profile, and alterations to the designations of the soil horizons are suggested to take account of the tephras that have been identified. INTRODUCTION In the description of the profile of Tirau silt loam (a central moderately leached yellow-brown loamjmoderately enleached alvic soiljhumic Andosol) at the Soil Bureau Reference Site on Hetherington Road near Tirau (N.Z. Soil Bureau 1968) the parent material is given as "Tirau Ash" (N.Z. Soil Bureau 1954). It is also stated that in the topsoil there are thin contributions of Taupo Pumice and Kaharoa Ash. Vucetich & Pullar (1969) considered "Tirau Ash" to be a composite parent material containing the ejectamenta of eruptions from both Taupo and Okataina Volcanic Centres. On the Mamaku Plateau, these authors also described beds that resembled air-fall tephras, but which lacked distinctive paleosols and basal layers. They considered these beds to be of loessial origin, but derived from tephras by processes of erosion and redeposition. During extensive mapping of pyroclastic cover deposits in the central North Island (Pullar & Birrell 1973) "Tirau Ash" could not be established as a separate entity. Accordingly, to clarify the nature of the soil parent materials at the reference site, it was decided to study sections in a transect westwards across the Mamaku Plateau from near Rotorua, where the tephra stratigraphy was well established, to the reference site at Tirau. N.Z. Journal of Geology and Geophysics 16 (3): Gcology-27

678 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VoL. 16.&. Otanewainuku 640m To Taupo FIG. 1-Map of Rotorua-Tirau highway showing the locations of the five sites on the transect across the Mamaku Plateau.

3 678 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VoL. 16.&. Otanewainuku 640m To Taupo FIG. 1-Map of Rotorua-Tirau highway showing the locations of the five sites on the transect across the Mamaku Plateau. FIELD EXAMINATI:JN OF SECTDNS Sections were examined at five sites commencing at the Tarukenga Deviation near Ngongctaha (Site 1), and finishing 20m north of the Soil Bureau Reference Site on Hetheringtov Road near Tirau (Site 5), as shown on Fig. 1. Intermediate sites were on the Rotcrua-Hamilton highway except for site 4 on Tapapa Road. At site 1 (Tarukenga Deviation) the following tephras were identified from the surface downwards: Taupo Pumice, Rotakawau Ash, Whakatane Ash, Mamaku Ash, Rotoma Ash, Rotorua Ash, Rerewhakaaitu Ash, Okareka Ash, Te Rere Ash, and Oruanui Ash. Materials showing the features of the "loessial" beds noted by Vucetich & Pullar (1969) on the Mamaku Plateau were seen between Rerewhakaaitu Ash and Okareka Ash and also between Okareka Ash and Te Rere Ash. These beds will be referred to subsequently as "post-okareka loess" and "post Te Rere loess" respectively. A similar material was also noted between Te Rere Ash and Oruanui Ash. At site 1, the tephras and "loess" beds rest on undifferentiated terrace and fan deposits that include lake beds. Figure 2 shows the stratigraphy of the tepht"as and "loess" beds at each site. Taupo Pumice and Rotorua Ash persist from site 1 to site 5, but Rotokawau Ash, Whakatane Ash, Mamaku Ash, and Rotoma Ash are all absent at sites 4 and 5. At site 2, and for a distance of 11 km westwards along the highway, all tephras and "loess beds" below Rotorua Ash are missing, the Rotorua Ash resting directly on the Mamaku Ignimbrite. The missing formations and beds below Rotorua Ash, however, reappear at site 3, and, with the exception of "post-te Rere loess" and Te Rere Ash, continue to site 5 on Hetherington Road where the section down to the base of Oruanui Ash is telescoped to almost one-quarter of its thickness as sc:en at site 1. "Loess" occurs between Okareka Ash and Oruan:.1i Ash at site 4 but is absent at

4 No. 3 PULLAR & BIRRELL- T!RAU SILT LOAM 679 L '~o 'L ~3~L ~37km ~ tp Taupo Pumice 1 ::k ~~~~~~~ ~:hh ~ Mamaku Ash ~ RotomaAsh ::; rr Rotorua Ash <J) rk Rerewhakaaitu Ash ~ w "' ~ Ok Okareka Ash Te Te Rere Ash Ou Oruanui Ash Pal Paleosol Lap Lapilli LEGEND Mn Mangaoni Lapilti Formation Re Rotoehu Ash H Undifferentiated brmvn tuffs ~IH Mamaku Ignimbrite And Andesitic Ash L Loess Basal layer ~ Lake beds 2.5Y 4/4 s I 0 49 Q.95 NBB/372215, 21Om SITE3 N75/ m SITE~ N76/589126, 535m NBB/273253, 125m *Soil texture abbreviations as in Taylor & Pohlen (1962) FIG. 2-Correlation sections Tirau to Tarukenga. (Locations shown m Fig. 1.) site 5. Thus at site 5 the following tephras and "loess" bed are considered to be present: Taupo Pumice, Rotorua Ash, Rerewhakaaitu Ash, post Okareka loess, Okareka Ash and Oruanui Ash. All of these rest on an unnamed bed which may include some Mangaoni Lapilli Formation, followed by Rotoehu Ash and "undifferentiated brown tuffs" (Pullar & Birrell 1973). At the sites examined, the identity of Rerewhakaaitu Ash, Okareka Ash, and Te Rere Ash was established by the lithology of their basal layers. The basal layer of Rerewhakaaitu Ash is of ash grade (American Geological Institute 1960) and of grey colour, that of Okareka Ash is also of ash grade, pale yellow, with marked shower bedding, while Te Rere Ash is of ash grade, yellowish, but with no shower bedding. At site 4, the basal layer of Rerewhakaaitu Ash is discontinuous and it is absent at site 5. However, at site 4, below the basal layers of Rotorua Ash and above the level of the basal lay.er of Rerewhakaaitu Ash are numerous pale yellow grains of rhyolitic ash which are still present at site 5, although much fewer in number. Accordingly, the boundary between Rerewhakaaitu Ash and the underlying post-okareka loess at site 5 was placed where these yellowish grains of Rerewhakaaitu Ash disappeared. Oruanui Ash at sites 1, 3, 4, and 5 is a pale grey silt with a faint mauve tint, as seen at other exposures in the Rotorua and Bay of Plenty areas. The basal layer is very thin, sandy, and yellowish-grey. Some chalazoidites can be seen at site 5 a short distance above the basal layer.

5 680 N.Z. JoURNAL OF GEOLOGY AND GEOPHYSICS VoL. 16 The unnamed bed below Oruanui Ash at site 5 bears a close resemblance to the silty bed commonly seen immediately below Oruanui Ash at sites south of Rotorua and in the Bay of Plenty. However, at site 5 the clay content is unusually high and the inclusion of some Mangaoni Lapilli Formation which can be seen in strongly weathered form below Oruanui Ash near site 3, is possible. The sand mineralogy of this unnamed bed is very similar to that of Oruanui Ash, except for the presence of traces of andesine feldspar and biotite. At site 5 the Rotoehu Ash and the underlying "undifferentiated brown tuffs" with which Hamilton Ash Formation (Ward 1967) is tentatively correlated, resemble numerous other exposures of these beds in the eastern Waikato region. While the upper contact between a loess layer and the basal layer of an overlying tephra is usually well defined, the lower contact between the loess layer and the paleosol of an underlying tephra is much more difficult to see in the field. Differences in colour, texture, and consistence tend to be subtle, and laboratory data, likewise, show only small differences (Tables 1, 2 and 3). This is particularly the case when there has been a general addition of more easily weathered parent material to the section. At site 1, where there is thought to be only rhyolitic parent material in the section, both "post-okareka loess" and the paleosol of Okareka Ash are described as firm, massive, slightly sticky silt loams (for definitions, see Taylor & Pohlen (1962)). The paleosol, however, is browner than the loess, has a slightly lower dry bulk density, a higher total porosity, and a slightly higher organic carbon content. "Post-Te Rere loess" at this site is a firm, massive, slightly sticky silt loam, but the paleosol of Te Rere Ash is a non-sticky sandy loam. Differences in colour, dry bulk density, and organic carbon content are similar to those seen between "post-okareka loess" and the paleosol of Okareka Ash. At site 3, the "post-okareka loess" is considered to be of local origin. Field differences between "post-te Rere loess" and the paleosol of Te Rere Ash are the same as at site 1. At site 4, the "post-okareka loess" is a rather uniform material which is of a greyer colour than the underlying paleosol of Okareka Ash. It is distinctly firmer than the paleosol and contains large, weakly-coherent, rounded aggregates. At site 5, the "post-okareka loess" can be distinguished from the paleosol of Okareka Ash only by its greater degree of aggregation and slightly finer texture. Field colours and other laboratory data do not contribute any consistent distinguishing criteria for these two materials at this site. NOTES ON LABORATORY DATA (For methods see Birrell & Pullar 1973; and Gradwell 1972.) Clay minerals At site 1, the tephras down to and including Rerewhakaaitu Ash are difficult of access and have not been specifically examined for clay minerals.

6 z 0 <,).> TABLE 1-Location and description of samples. Field or Lab.No. BP367 BP368 BP369 BP362 SB7681E BP370 BP371 BP354 BP373 BP374 BP282 BP366 T729 T728 T730 Parent Material Site Position in section Rotorua Ash (rr) Tirau (site 5) em from surface Rerewhakaaitu Ash (rk ), in part Tirau (site 5) 8-15 em below rr Post-Okareka loess Tirau (site 5) 0-7 em below rk Post-Okareka loess Tirau (ai te 5) 7-15 em below rk 11 Tirau Ash 11 Tirau (site 5) (II) C horizon Tirau ail Okareka Ash ( Ok) Tirau (site 5) em below rk Oruanui Ash (Ou) Tirau (site 5) 0-8 em below Ok Unnamed Tirau (site 5) 1-5 em below Ou Rotoehu Ash (Re) Tirau (ei te 5) 5-12 em below unnamed bed "Undifferentiated brown tuffs" Tirau (site 5) 2-10 em below Re Rotorua Ash (rr) Earthquake Flat (N85/797882) o-8 em below wh Post-Okareka loess Tarukenga Deviation (site 1) em below rk Okareka Ash ( Ok) Tarukenga Deviation (site 1) 8 em below- post-okareka loess Post-Te Re're loess Tarukenga Deviation (site 1) 39 em below Ok Te Rere Ash ( Te) Tarukenga Deviation (site 1') 5 em below post-te Rere loess Colour+ Description 10YR 6/4 V.friable gran.f.sl, non-sticky 10YR 6/5 Friabie nutty sil1 non-sticky 10YR 5/8 Friable nutty ail, s.l. sticky 10YR 5/6 Friable nutty ail, sl. sticky 10YR 5/8 Friable nutty ail, sl. sticky 10YR 5/8 Firm gran. ail, sl. sticky 10YR 7/4 Firm massive ls, sl. sticky 10YR 5/3 V.firm massive cyl, sticky 10YR 5/IJ. Firm massive h.sil 1 sticky 7.5YR4/IJ. V.firm nutty cy, sticky 7o5YR4/IJ. Friable gran.sil 1 non-sticky 10YR 6/3 Firm massive sil, sl. sticky 10YR 5/4 Firm massive f.sl1 sl. sticky 10YR 6/3 Firm massive sil 1 sl. sticky 10YR 5/4 Firm massive f. sl, non-sticky '1::1 c t"' t"' >?:' I<' tj::j :;;?:' trj t"' t"' I..., ~ (/) r: >-! b > ~ wh = Waiohau Ash; rk = Rerewhakaai tu Ash; According to the Munsell Colour Notation G' 00 1-'

7 ~ N.Z. JoURNAL OF GEOLOGY AND GEOPHYSICS VoL. 16 TABLE 2~Clay mineral and chemical data on samples. Field or Parent Clay Clay Organic Tamm-extractable Lab. No. Material Minerals % Carbon AI % Fe% % BP367 Rotorua Ash Allophane BP368 Rerewhakaaitu Allophane, Ash (+) BP369 Post-Okareka loess Allophane, BP362 Post-Okareka loess Allophane, SB7681E "Tirau Ash" Allophane, BP370 Okareka Ash Allophane, BP371 Oruanui Ash Halloysite, a\lophane BP allophane BP354 Unnamed Rotoehu Ash Halloysite Halloysite, BP374 "Undiff. brown Halloysite tuffs" BP282 Rotorua Ash Allophane BP366 Post-Okareka loess Halloysite, allophane T729 Okareka Ash Allophane, T728 Post-Te Rere loess Halloysite, allophane T730 Te Rere Ash Allophane, Over a wide area, only allophane has been found in the clay fractions of these tephras. The "post-okareka loess" and "post-te Rere loess" contain (dominant) and allophane, whereas in the paleosols of Okareka Ash and Te Rere Ash, the order of abundance of these clay minerals is reversed (Table 2). At site 5, only allophane is found in the horizons derived from Rotorua Ash. Allophane is dominant, with minor in the bed that contains Rerewhakaaitu Ash and in the "post-okareka loess" and the Okareka Ash paleosol. In Oruanui Ash, and deeper tephras at site 5, is dominant. Aluminium and iron At site 1, medium amounts of these elements were extracted from the paleosols of Okareka Ash and Te Rere Ash by Tamm's acid oxalate reagent, but smaller amounts from the loessial layers derived from them (Table 2). More aluminium than iron was generally extracted. At site 5, the amounts extracted from Rotorua Ash, Reo:ewhakaaitu Ash, "post-okareka loess", and Okareka Ash were higher than for any of the beds at site 1, particularly for aluminium. One aluminium value for "post-okareka loess" was higher than the value for Okareka Ash paleosol, the other was the same.

8 No.3 PULLAR & BIRRELL - T!RAU S!L T LOAM 683 TABLE 3-Physical data from core samples. Parent Site Position in Dry bulk Total Material section density porosity g/ml % Rerewhakaaitu Tirau (site 5) em below rr Ash(+) Post-Okareka Tirau (site 5) em below rr loess Post-Okareka Tirau (site 5) em below rr loess Post-Okareka Tirau (~ite 5) em below rr loess Okareka Ash Tirau (site 5) 10 em below loess 0 85 Unnamed bed Tirau (site 5) 1-5 em below Ou Post-Okareka Tarukenga Devn em bdow rk loess (site 1) Okareka Ash Tarukenga Devn. paleosol (site 1) Post-Te Rere Tarukenga Devn. 39 em below Ok 1 07 loess (site 1) Te Rere Ash Tarukenga Devn. 8 em below loess 0 96 (site 1) Oruanui Ash Tarukenga Devn. below Te Rere Ash (site 1) Oruanui Ash and Rotoehu Ash at site 5, give hi_gher values for extractable aluminium than at other sites (Birrell & Pullar 1973), but the value for the "undifferentiated brown tuffs" bed is in agreement with those for soils derived from Hamilton Ash Formation (N.Z. Soil Bureau 1968). Sand mineralogy No mineralogy of sand fractions was done for sites on the transect. However, results are available for the reference profile (N.Z. Soil Bureau 1968). To a depth of 75 em the following minerals are reported in approximate order of abundance: glass, andesine feldspar, hypersthene, magnetite, and augite. Physical properties The "post-okareka loess" at site 5 contains more clay than the paleosol of Okareka Ash, and it also contains more clay than "post-okareka loess" at site 1 (Tables 2 and 3). Dry bulk density values for "post-okareka loess" and Okareka Ash at site 5 are similar and lower than for either of these beds at site 1. At site 1, "post-te Rere loess" and Te Rere Ash show a close resemblance to "post-okareka loess" and Okareka Ash respectively in dry bulk density. Total porosity of Rerewhakaaitu Ash and "post-okareka loess" at site 5 is greater than for "post-okareka loess" and Okareka Ash at site 1. Oruanui Ash at site 1 has a similar dry bulk density to Te Rere Ash and Okareka Ash at the same site. The unnamed bed below Oruanui Ash at site 5 has a slightly higher dry bulk density than Oruanui Ash at site 1.

$16 PARENT MATERIALS OF SOIL PROFILE AT SoiL BUREAU REFERENCE SiTE FOR TIRAU SILT LOAM Rotorua Ash has been found to contain only allophane in the clay fraction, Rerewhakaaitu Ash also contains only$

9 684 N.Z. JoURNAL OF GEOLOGY AND GEOPHYSICS VoL. 16 PARENT MATERIALS OF SOIL PROFILE AT SoiL BUREAU REFERENCE SiTE FOR TIRAU SILT LOAM Rotorua Ash has been found to contain only allophane in the clay fraction, Rerewhakaaitu Ash also contains only allophane, but is always present in Okareka Ash. The presence of in the reference profile will therefore indicate material originating from Okareka Ash. At site 5, this occurs at about 33 em from the surface (Tables 1 and 2) in the bed containing Rerewhakaaitu Ash which must therefore be mixed with "post-okareka loess". At the reference site, has not been found above 50 em from the surface (Ruth E. Symes, Soil Bureau, pers. comm.). Hence the base of Rotorua Ash is deeper at the reference site than at site 5. Sample SB 7681E (60-75 em) from the reference site agrees in dry bulk density and total porosity with the bed containing Rerewhakaaitu Ash and with the post-okareka loess at site 5. The values of bulk density and total porosity for sample SB 7681E are different from those for the overlying horizons, which a.re typical of air-fall rhyolitic ash (N.Z. Soil Bureau 1968). The day content of sample SB 7681E is in line with its being derived from "post-okareka loess" rather than from Okareka Ash. Rotorua Ash is a Holocene rhyolitic tephra that has not been found completely free from contamination by andesitic ash. However, studies of other Holocene rhyolitic tephras that can be obtained free or almost free of andesitic ash have shown that values of Tamm-extractable aluminium exceeding 2% indicate marked contamination by andesitic ash, and that for the late Pleistocene rhyolitic tephras, values exceeding 0 5% also indicate marked contamination (Birrell & Pullar 1973). Accordingly, at site 5, it can be concluded that the section down to and including Oruanui Ash contains significant amounts of andesitic ash (Table 2). At the reference site, the presence of andesitic ash down to a depth of 75 em can be inferred from the mineralogy of the sand fractions and from Tamm-extractable aluminium values comparable with those at site 5. Othe.r data that support the presence of andesitic ash in the reference profile are : 1. The corrected clay contents of the A 11 and AB horizons are 29% and 26% respectively (Birrell 1966; and in prep.). These values are higher than that of sample BP 282 (Table 2) which represents Rotorua Ash only slightly contaminated by andesitic ash, but are close to those recorded for Rotorua Ash and other rhyolitic tephras at sites where such tephras are strongly contaminated (Birrell & Pullar 1973). 2. Stronger allophane field tests (Fieldes & Perrott 1966) for all beds at site 5 down to and including Oruanui Ash than generally recorded elsewhere for the same beds. 3. Fairly strong chromas in the soil colours at site 5, particularly for "post Okareka loess" and Okareka Ash. At site 1, the chromas of these two beds are much weaker. In the light of the above discussion, the amount of andesitic ash in the loesses and late Pleistocene tephras at site 1 will be less than at the reference site.

10 No.3 PULLAR & BIRRELL- TIRAU SILT LOAM 685 CONCLUSIONS The morphology of the reference profile at Tirau shows a series of known rhyolitic tephras which can be correlated with the same members where they are exposed east of the Mamaku Plateau, together with a loessic layer derived largely if not wholly from Okareka Ash. Frequent accretions of andesitic ash from sources to the south have altered the physical and chemical properties of the soil so that it has been classed as a yellowbrown loam (N.Z. Soil Bureau 1963). However, data in Table 2 gives no. evidence for downward movement of clay or sesquioxides within the described profile. Hence the horizon denoted as AB (20-30 em) might preferably be denoted as AC, and the (II)B horizon (33-50 em) as C, both these horizons being derived from Rotorua Ash. The (II)C horizon (61-77 em) appears to be correctly designated and consists largely of post-okareka loess plus possibly some Rerewhakaaitu Ash. From the thickness of Rotorua Ash, post-okareka loess and Okareka Ash found at the reference site, it is likely that these parent materials will be found, along with variable amounts of andesitic ash, in the profiles of other soils occurring in the eastern Waikato district. It is likely, however, that Rerewhakaaitu Ash will disappear westwards. In the absence of andesitic ash, a distinction between Okareka or T e Rere Ash paleosols and their respective loesses might be made on the basis of colour, bulk density, and organic carbon content. AcKNowLEDGMENTS The authors express their thanks to Mr B. K. Daly for analyses of Tamm extracts, to Mr V. A. Weatherhead for mineralogical analyses of sand fractions, and to Mr M. W. Gradwell and Dr R. ]. Jackson for physical measurements on core samples. Thanks are also due to Mr ]. A. Pollok for assistance in the field, and to Mi~s C. Prendergast for drawing Fig. 2. REFERENCES AMERICAN GEOLOGICAL INSTITUTE 1960: "Glossary of geology and related sciences", with supplement. 2nd ed. American Geological Institute, Washington p, BIRRELL, K. S. 1966: Determination of clay contents in soils containing allophane. Part 1. Subsoils. N.Z. Journal of Agricultural Resectrch 9: BIRRELL, K. S.; PuLLAR, W. A. 1973: Weathering of palaeosols in Holocene and late Pleistocene tephras in central North Island. N.Z. Journal of Geology and Geophysics (this issue). FIELDES, M.; PERROTT, K. W. 1966: The nature of allophane in soils. Part 3. Rap'd field and laboratory test for allophane. N.Z. Journal of Science 9: GRADWELL, M. W. 1972: Soil Bureau laboratory methods. C. Physical analysis. N.Z. Soil Bureau Scientific Report 10. N.Z. SoiL BuREAU 1954: General Survey of the Soils of North Island, New Zealand. N.Z. Soil Bureau Bulletin 5. N.Z. SOIL BuREAU 1963: Soil Map of the North Island, New Zealand 1: N.Z. Soil Map 49, (reprinted 1972). This map also accompanies N.Z. Soil Bureau Bulletin 26 ( 1), N.Z. SOIL BuREAU 1968: Soils of New Zealand. Part 3. N.Z. Soil Bmea11 Bullet;n 26 (3).

11 -686 N.Z. JoURNAL OF GEOLOGY AND GEOPHYSICS VoL. 16 PULLAR, W. A.; BIRRELL, K. S. 1973: Age and distribution of Late Quaternary pyroclastic and associated cover deposits of the central North Island, New Zealand. N.Z. Soil Survey Report 2. TAYLOR, N. H.; PoHLEN, I. J. 1962: Soil Survey Method. N.Z. Soil Bureau Bulletin p. VucETICH, C. G.; PuLLAR, W. A. 1964: Stratigraphy and chronology of late Quaternary volcanic ash in Rotorua, Taupo and Gisborne districts. N.Z. Geological Survey Bulletin 73. Pt 2. Pp VucETICH, C. G.; PuLLAR, W. A. 1969: Stratigraphy and chronology of late Pleistocene volcanic ash beds in central North Island, New Zealand. N.Z. foumal of Geology and Geophysics 12: WARD, W. T. 1967: Volcanic ash beds of the lower Waikato Basin, North Island, New Zealand. N.Z. Journal of Geology and Geophysics 10:

Rotoehu ash and the Rotoiti Breccia Formation, Taupo Volcanic zone, New Zealand

New Zealand Journal of Geology and Geophysics ISSN: 0028-8306 (Print) 1175-8791 (Online) Journal homepage: https://www.tandfonline.com/loi/tnzg20 Rotoehu ash and the Rotoiti Breccia Formation, Taupo Volcanic