Volcanic ash beds of the lower Waikato basin, North Island, New Zealand

Transcription

1 New Zealand Journal of Geology and Geophysics ISSN: (Print) (Online) Journal homepage: Volcanic ash beds of the lower Waikato basin, North Island, New Zealand W. T. Ward To cite this article: W. T. Ward (1967) Volcanic ash beds of the lower Waikato basin, North Island, New Zealand, New Zealand Journal of Geology and Geophysics, 10:4, , DOI: / To link to this article: Published online: 14 Feb Submit your article to this journal Article views: 353 View related articles Citing articles: 32 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 21 November 2017, At: 23:35

2 No VOLCANIC ASH BEDS OF THE LOWER WAIKATO BASIN, NORTH ISLAND, NEW ZEALAND W. T. WARD* Soil Bureau, Lower Hurt (Received for publication 10 September 1965) ABSTRACT This report describes volcanic ash beds that occur north of Otorohanga and west of Tirau in the central part of the North Island of New Zealand. The beds lie above one another as a mantle on the land surface, and form the parent material for many modern soils. Many of the beds, before burial by younger ash, were exposed to processes of soil formation and, as a result, the deposits include many fossil soils. Although their significance to agriculture is very well appreciated, the value of the beds for correlating terrestrial Pleistocene deposits and erosion surfaces is not widely recognised. A prominent break' occurs within the ash deposits and marks a lengthy time interval. The name Kauroa Ash Formation is proposed for the beds below this break, and for the younger beds above it the name Hamilton Ash Formation is used. The beds of Hamilton Ash Formation post-date the early Pleistocene ft terrace but pre-date the middle Pleistocene ft terrace. INTRODUCTION Previous Work Volcanic ash was first mapped north of Otorohanga by Grange (1931) during an examination of those beds which contributed to the parent material of the modern soil. The mixture of andesite and rhyolite ash which formed the topmost layer in the Waikato district was named "Mairoa Shower". This bed, a brown ash about 2 ft thick, had previously been recognised by Taylor (1930) in the Mairoa district, west of Te Kuiti. Its source was at first suspected to lie in the Rotorua-Taupo volcanic field (Grange, 1931), but was later considered to be Mt Egmont in Taranaki (Grange and Taylor, 1932). The name "Hamilton Shower" was given, by Grange and Taylor (1931), to the soil-forming ash bed that mantles the hills about Hamilton City. Taylor (1933) described a cutting, beside the Great South Road near Otorohanga, which exposed a sequence of ash beds differing in colour, texture, and mineral content. This exposure is important for correlation, and is redescribed below. The author recognises eight ash deposits in the lowest of four beds described by Taylor. In the course of a soil survey, Grange et al. (1939) studied the distribution of soils developed on Hamilton and Mairoa ash in the northern part of Waipa County, between Waikato and Waipa Rivers. They noted that several *Present address: CSIRO Soil Mechanics Section, Syndal, Victoria. N.Z. /1 Geol. Geopbys, 10 :

3 1110 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10 ash beds occurred underneath the Hamilton Shower and referred to these as the "older Hamilton Showers". The occurrence of Mairoa and Hamilton ashes in the Northern Waikato was recorded during later reconnaissance soil surveys of that district, and a broad division between Mairoa ash and Hamilton ash (partly covered by Mairoa) was shown in a map published in 1954 (N.Z. Soil Bureau). Nomenclature Originally, the terms "Mairoa Shower" and "Hamilton Shower" were applied to single ash beds. This usage did not persist for long; by 1931 Grange and Taylor had shown that Mairoa Shower contained a number of separate beds, and the terms "Mairoa ash" and "Hamilton ash" appeared as alternatives (see Taylor and Grange, 1939, p. 16). "Hamilton ash" was first used as a synonym for Hamilton Shower, but was later extended to include several ash beds of which Hamilton Shower was the highest member. These changes are confusing in the literature, and in some cases it cannot be decided whether an author is referring to a single bed or a set of beds. It is to be noted that bedding within the ash is made evident principally by the occurrence of fossil soils. Shower bedding cannot usually be distinguished in the field and other primary variations in colour, grain size, mineral content, and consistence, for example, are seldom recognisable. Weathering is so far advanced that in some instances the original ash has been completely transformed to soil. In consequence the usual lithological bases for separating rock-stratigraphic units are not normally available. The fossil soils occur at particular levels in the ash however, and each soil has clearly developed from the surface of a new ash deposit. The soils evidently record periods of non-deposition. Rock-stratigraphic units in the ash can therefore be recognised if the fossil soil mantles are accepted as boundary criteria, and for mapping to proceed these fossil soils must be regarded simply as the weathered parts of particular rock-stratigraphic units. In attempting to apply the rules of geological nomenclature to the naming of volcanic ash deposits in the Rotorua and Gisborne districts, Vucetich and Pullar (1964) (see also Pullar, 1967) used the term "member" for a discernible shower bed which is lithologically distinct over a wide area, and the term "formation" for a sequence of beds (or members) capped by either a soil or a fossil soil. This usage permitted convenient subdivision of Holocene ash deposits that had accumulated rapidly, with few periods of quiet long enough for soils to develop. If "formation" and "member" were applied in the present area, however, in the way they are used by Vucetich and Pullar (Ioc. cit.), the number of formations defined would be little less than the number of ash beds and the nomenclature would be cumbersome. To avoid this difficulty the following nomenclature is proposed: Formation: This term, coupled with a geographic name, will be used for each of two distinct sequences of volcanic ash beds that are separated by a lengthy erosion interval. Both sequences include several fossil soils. Member: A single bed of a formation. The term includes any sailor fossil soil developed on the bed. Variant: As a result of variations in kind of weathering, some of the beds described below occur in different forms in different localities. For clear description,

4 No.4 WARD - VOLCANIC ASH BEDS 1111 a term is needed to separate these forms, and for this purpose it is proposed to use "variant", "Variant" is not used as a synonym for "soil", Similar soils (or fossil soils) are found on particular variants, however, Although the thickness and uniformity of each ash bed in the field suggests that it was the product of a single eruption, this was not necessarily the case. Weathering might have kept pace with accumulation, and most beds might, in fact, include the products of more than one shower. It is possible that each is the product of many eruptions. The actual situation is uncertain and for this reason it is preferred to use the term "bed" rather than "shower" for the mapping unit in this report. The geographic names given to some units are taken from localities where the beds are well exposed. DISTRIBUTION AND CHARACTERISTICS OF THE ASH BEDS Field observations show that undulating to hilly terrain, i.e., land with slopes ranging from 0-25, is mantled with volcanic ash. The ash has been mostly stripped from the steep country (slopes greater than 25 ), and is restricted to the crests of ridges in areas where mudstones are exposed. Slopes developed on these rocks, Mangakotuku Siltstone and Whaingaroa Siltstone of Kear and Schofield (1959), are unstable and are rapidly denuded of ash as a consequence of repeated slumping. Natural exposures and road cuttings generally reveal an ancient surface cut on non-volcanic rocks buried by bedded ash deposits of nearly uniform thickness. This is so even on slopes of 20-25, but here the apparent regular depth of each ash bed may be misleading and may result from the fact that cuttings generally run almost at right angles to the slope. Nevertheless, a few sections cut parallel to moderately steep slopes show only slight thickening of the ash down-slope. The small variation in thickness of individual ash beds in relation to slope suggests that fresh ash falls were rapidly fixed by vegetation and provides some support for the view that the ash accumulated intermittently as thin shower deposits. The contact between one ash bed and the next, like the unconformity at the base of the sequence, is smooth and regular. As Vucetich and Pullar (1964) have stated, this does not imply at) absence of forest at the time of the eruptions. In fact, the fossil soils developed on the ash all indicate a forest cover, and some which are podzolised imply the presence of podocarps or other mer-formers, Charred fragments of wood, common in the ash beds of the Rotorua and Taupo districts (Baumgart, 1954), do not occur in the deposits of the lower Waikato basin except at one site. Other evidence of forest overwhelmed by ash, such as local disturbance of the beds by overturned trees, was rarely observed, and apparently the ash was cold when it fell. Ash-Rich Alluvium All exposures examined are incomplete. No cutting shows the full sequence of ash beds, and a bed present in one cutting might not appear in the next, a few chains away. This patchy distribution of individual ash beds is attributed to erosion and, as ash resists erosion when protected by

5 1112 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10 vegetation, this erosion occurred presumably at the time of ash fall, or soon afterwards. Contemporary alluvial deposits have a high content of ash, and where alluvium rich in ash forms a thin bed it can be confused with subaerial ash deposits. Some of these thin alluvial deposits can be recognised by the soils developed on them. Brown granular loams developed on ashrich alluvium are not easily distinguished from brown granular loams developed on subaerial ash, but yellow-brown loams differ in some respects. Three features appear to be correlated with kind of parent material: (1) the topsoil developed on the alluvial deposit has a light, fluffy texture when kneaded in the hand; (2) the subsoil formed on this parent material is pale yellowish brown rather than brown; and (3) weathered profiles on alluvium have the appearance of young soils even where the soil is strongly leached. Ash-Rich Colluvium In many places, including three of the sites described, a sediment crowded with firm, grey, sand-sized aggregates of clay truncates the bedding planes in an otherwise conformable sequence of ash beds. Unlike other beds, which frit away to form a characteristic exposure, this sediment resists weathering in cuttings, so that it commonly preserves the impressions made by the implements used to excavate the cutting. Correlation of exposures shows that deposits like this occur at different levels in the ash sequence and at one locality, Site 40 (Fig. 1), this material crops out at two levels. Characteristically, the sediment thickens rapidly downslope, from a few inches to over 10 ft in places (Fig. 2), and does not occur on flat surfaces undisturbed by erosion, where subaerial ash is generally best preserved. The first cutting on a road might not expose ash of this kind, but the next cutting might consist wholly of it. Such a sequence occurs along the Te Kauwhata Waerenga Road. The material occurs from the lowest to the greatest height above sea level examined. In places near Ruawaro it is crudely water-sorted and contains fragments of siltstone, the basement rock in this locality. The erratic distribution, water-sorting, inclusions of basement rock, repetition of the sediment at various levels, and marked overlap of other beds are features that are not well explained by supposing that the deposit is a subaerial ash, but seem to be adequately explained if the material is regarded as colluvium derived by sheet and rill erosion of the ash beds, or formed by soil creep. In this report the material is subsequently referred to as "colluvial ash". MORPHOLOGY AND MAPPING CRITERIA Although some ash beds of this survey were mapped by previous workers, diagnostic features for most have not hitherto been described. Because of the advanced stage of weathering, few beds retain any original characteristics, and features developed by weathering must be taken into consideration in recognising the units. Most beds contain distinctive morphological features, but a few have so many features in common that certain identification in the field is difficult. In these cases the position of the bed in the sequence is the best guide, and for this reason the beds were recorded in stratigraphic order.

6 No.4 WARD - VOLCANIC ASH BEDS 1113 SCALE e i I MIles \ ~..~;. FIG. I-Sites (numbered) of volcanic ash exposures described in text. Geology-IS

7 1114 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10 Comparison of the resulting stratigraphic sketches facilitated correlation of the beds in different exposures, and beds that could be confused with others were for the most part easily identified. For satisfactory correlation of the ash deposits, many outcrops were examined to test the worth of different features of the beds as differentiating criteria. Old sections proved to be of more value than fresh exposures, for they have allowed the ash beds and the fossil soils developed on them time to develop a characteristic weathered appearance'. Colour and Colour Patterns Variations in colour and consistence mark off the separate ash beds, and are the principal features that serve to distinguish them. The colours of weathered ash beds range from red (Munsell colour notation 2'5YR 5/8) through yellowish brown and brown to white, but reddish yellow (7'5YR 6/8), strong brown (7'5YR 5/6) and very pale brown (10YR 7/4) predominate. Most beds have a single colour, but slight variations in shade are common. Some beds have characteristic colour mottling; mingled shades of very pale brown and strong brown, for instance, are common in beds H4a, H4c, and H4e. Abrupt changes of colour often mark the contact between successive ash beds, and some beds consist of weathered ash that changes colour (e.g., from strong brown to yellowish brown) when crushed. FIG. 2-Site 3I, Huntly-Rotowaro Road. The massive bed thickening down the slope is considered to be coil uvial ash derived by erosion of the accumulated shower deposits. (N56/ )

8 No.4 WARD - VOLCANIC ASH BEDS 1115 Consistence This property is related to the degree of cohesion of the weathered ash. As a diagnostic criterion it proved in many cases to be of more value than colour, and for this reason a special terminology was adopted for its description, as follows: Friable: The particles of weathered ash adhere weakly to one another so that a sample held in the hand can be crushed with gentle pressure. Where there is difficulty in extracting a hand specimen, the ash is described as fragile. Earthy: The ash resembles a loam and will generally break up when a large sample is dropped to the ground, some of the material crushing completely and some remaining in aggregated form. Blocky: Blocky ash can be broken only with difficulty, but samples are easily separated from the mass because natural aggregates of various sizes, separated from each other by fissures, are well developed and have flat surfaces. Stony: Stony ash requires a sharp blow with a hammer or spade to separate a sample, and at first glance, where the exposure is incomplete, it is easily confused with a Tertiary siltstone. Smooth fissures generally divide stony ash into large blocks and, in places, fissures parallel to the surface of the ash separate tabular plate-like masses. Where this is the case the ash is described as platy. In some outcrops stony ash is so tough that little impression is made with the hammer. Clay Veins In some ash beds conspicuous pale grey veins of clay occur, mostly parallel to the surface of the ash (Fig. 3). Although they are not present everywhere in the bed, the veins were once thought to be confined to the Hamilton Shower (N. H. Taylor, pers. comm.), but in this survey it was found that they occur in other beds, and near Lake Whangape (N52/616862) and at Te Kauwhata (N52/677938) they are well developed in thick, uniform deposits of colluvial ash. Generally, the veins occur 18 in. to 3 ft from the surface of the soil, where they commonly lie close to the junction of two ash beds. Roots extending as far as the veins seldom penetrate them, except where there are narrow fissures. A dense mat of roots is commonly found on top of the veins or is enclosed by them where they occur in the modern zone of soil formation. Analyses (Grange et a~/., 1939) show that the veins are of the same composition as the enclosing material (except for a lower content of iron), and indicate that they are composed of a kaolin material which was identified as kaolinite by Grange et al. and later as halloysite by Dr L. D. Swindale (pers. comm.). Halloysite also forms the clay fraction of the enclosing soil (Fieldes, 1955). The veins branch and in many places have vertical offshoots. In some localities they cross the junctions between ash beds. These observations, and the occurrence of the veins in colluvial ash, show that they cannot represent original variations in ash composition. The association of veins with root mats suggests that they mark zones of reduction produced by decaying organic matter. This view is not easily disproved, although the veins occur where organic matter and root mats are absent, and vice versa. However, the veins are restricted to well defined areas in the field and the hypothesis does not account for this. It seems more likely that the veins are related to the processes of soil formation, and that their distribution is controlled by

9 1116 N.Z. JO URNAL OF GEOLOGY AND GEOPHYSICS VOL.lO FIG. 3- Rotowaro Variant (Mairoa Ash Member), Chu rch Road, Te Rapa, showing prominent clay veins, in. from the surface of the ground. The ro unded nodules on the pale massive horizon consist of iron and manganese oxides, formed by weathering. Th e measuring stick (30 in. long) is marked in inches. (N56/ ) some factor other than parent material. It is significant that modern soils with veins in the subsoil are also characterised by a blocky or massive horizon in the topsoil (Fig. 3), and the better developed the blocky horizon, the better developed are the veins. This is well shown in a road cut near Lake Whangapc (N52/572874), and the association of the veins with the blocky horizon demonstrates that they have a common origin. In the exposure cited, and in many other localities, the degree of blockiness in the soil varies rap idly over short distances. Where kauri tree stumps are found, the degress of blockiness and the degree of veining become less with increasing distance from the stumps. This evidence shows that the veins are not primary features but are the result of weathering, and are related to the kind of vegetative cover. As the modern soils that contain them were developed under mixed kauri and podocarp forest, it is probable that veins in fossil soils indicate a past forest cover of this kind. Clay A ggregates Clay aggregates which resemble those in colluvial ash occur in a few of the weathered subaerial ash beds, notably H3a, and can be used as a means of identifying them. Larger aggregates shaped like plant roots occur in other beds (Fig. 4). They are thought to be casts of root channels and are termed "rootlet pseudomorphs" in this paper.

10 No.4 WARD - VOLCANIC ASH BEDS 1117 Thin coatings of reddish brown clay, or "cutans" (Brewer, 1957), coat fissures and root channels in some blocky ash beds. Nodules of clay, mostly the size of garden peas, are commonly developed at the junction of successive beds. They are composed of halloysite (Dr L. D. Swindale, pers. comm.). Large, manganese-stained, creamy white aggregates of halloysite (Fig. 5) are characteristic features of some friable horizons, and a heavy coating of this material occurs at the base of Ohinewai Ash Member, Te Uku Variant (see below). According to Fieldes and Swindale (1954), these forms of halloysite develop by the combination of free silica with amorphous hydrous oxides produced by weathering of basic volcanic glass. Gravity Sorting Gravitational sorting of mineral constituents has been observed in the field only in Ohinewai Ash Member (see below). SUBDIVISION OF THE ASH SEQUENCE It is sometimes difficult to decide whether a particular weathered stratum in the ash sequence is an ash bed or a fossil soil. Even where there is abundant evidence that a bed is a soil it should be separated from the bed below, as this might not be the material which has given rise to it. If there had been sufficient time, the true parent ash might have been entirely consumed by weathering, as has happened in places in the lower Waikato basin where, for example, the fossil soil (H7) developed on Tikotiko Ash ' '' ~ '... ~::'; 7.;;'.: ':-.~. '.,...n:... ~..."y.;:~. ~- }"' ~ ~ ~...' ~ " "!."f~~r",...:.. ~ ~t, ~ ~7.r:. ". " ~~';;~~f < :~",,;>~, ' '1', ~,..~. lv~ ',t!'if" ' 4':' ~. ~i"'f Yo 11.",,' ' ~:i, v.~~. ~:~. ~.:-~J'.... ",,""~.,..;:~ ' ~ <'" ;",..-if'cj' '..,"f.... ~~"'. 'J/,"'- '" ~~, :m~,:"v;':..cit.~'r'" J~~.... ~ / \.::. ":./. Et~ >-:,::.:'..o:~'. ~ ~~ ':.~.~ ~ ~ ~v " '. --;,.. ~ ~r!':;a.+ ~ '7 ~"" \. ~,,",_,. 1 ~ ~ ~...~ ~ ~...,.. 0 I... '" 'I. '. ". ' -:~.. ', ~ ~,-:. ~'.--' '~.J, J.' j.~ t 2'! \tt~.~.:~ "} rr : l '...:~ +-c_ ~~. ;jl'.. ~ I 1'.A ;;l';~-~"~ e.:... \,. _..)... :;.y",.-.,,- ( r'/-::-:- : ".... '!,"',;. ' " ':' " ~4".... ~ f' ~.... _...."t,..y:... _ ' ~~N"'_~.. ' "" "'-,.....,;,...,. " ~~,""'lo#.. ~ t,..,-. _ or I ~). ;,.,,~.. "~.!...&J '..... ~~. ;~.~,f~~...~~. 'O,.,~", '.' :"~ f! " -f. ' ; :E:i!'~' _ ':I,_~~ "' ~ f"t'l :,,\.,..~-"~ i ~ fj,.~~~~ - i" ~A 4~?J"';'. "; -;;':"". 4 ::" 1':' I, "..-,ji<.,. ;J/J1;t;:,.~',. ~'~J~.. ~ :~ ~v, "... ''f'':. L "., """~. \ >.... "j' ~.:CI'- ' ''' rj' v...,~!, ~..... _..... ~_.~..... ; ' "I"...01(' JTi~'~ ' :p" >:~.r,..'~ '~.I'lr~~ ' ~ ";' '''' ~!'' ~""' ~"'~I.;.~*~,.1'.:.~ : ". /~.,. ' t " ~ '.:J ~('r~ :<:".~':r.. Q:t:.. < ':;".!!."':~ ~':ff1;.-' '''''',....',. r,'. J.,'.r:.r-., i'~"""' ''' ~,' '. 1jl,..l: "'..... ~-, :;. ~. ";><I'~ ' ~".1":, " r ~ '.' '".~.~" '...? " -, - ' ~ ' f.-j:~";:'~"" rt:.,+'... ~::;..!'ri"fi'.;.1 ';>'.) ' "', N',011'.=. ;.').. If{. :-;l~~':; :'- ' '''' i - ~- -.:1' ~~:..~... ijp ~..~... I' :'JO'" Y/."1fi.~ld~ t: ( ::y.::'i!'f,'iif.,tj. \~;r. ;i'~: ;,....,.,~.~,. 1'A'..._,..e-,.- ;<>' ",#" -; 11. ' 'j: ". A.,.., +>' ~,, <hrl" v ;..) )'. r,~. ~j:;~':'j'."~!.f ; ~ l ' ~'., "f. ~ '. 'if -. r,! ' '' "r-~ ~ ~.. lit '. "~-4lr:A ~" J ~ '." ".'. rl':. ~',. 'f,~-... ~~': ' tl.~l. ~ ~i'. ~.','. '.. /.-'~ ""-~.. "11".:.,.~.,.~'::'. ~/'.... c... 4 ~ FIG. 4-Rootlet pseudomorphs in Bed H5, at Ginns Road, Rotowaro (Site 28). The samples identified as hajloysite were taken from this locality. (N56/584723).

11 1118 N'Z. JOURNAL OF GEOLOGY AND G EOPHYSICS VOL. 10 F IG. 5-Rounded aggrega tes of creamy white halloysite, Ngaru awahia- W aingaro Road (Site 3i(). W hen broken open they are found to be stained wit h manganese oxide. A th ick layer of hall oysite coats the base of the bed in which they occur (Te Uku Variant, Ohinewai Ash Member). The measuring tape shows 7 in. (N 56/ 56R645.) Member ( H6) commonly rests directly on an unrel ated bed H5 or H4 ( see Figs. 8 and 9). To avoid confusion, each recognisably distinct layerash bed or fossil soil-was mapped separately in the field. Stratigraphy In the lower Waikato basin the beds occurring above a prom inent and persistent unconformity* (Fig. 6) can be easily correlated between outcrops. Detailed correlation of the beds below this unconformity is exceedingly difficult, and the correct sequence of these beds is unknown because exposures are few and too scattered to be of usc. The lack of outcrops is due partly to concealment of the early beds by the later deposits, and partly to the stripping away of the first beds deposited before the second formation accumulated. In this account the beds below the major unconformity in the sequence of ash beds are named "Kauroa Ash Formation" and the subsequent beds are given the name "Hamilton Ash Formation ". The stratigrap hy and nomenclature of the beds are summarised in Table 1. *In this report the term " unconformity" is restricted, following Hay ( 1959 ), to " erosional disconformities and angular discordances between successive falls". N o earth movemen ts are impl ied.

12 No.4 WARD - VOLCANIC ASH BEDS 1119 Kauroa Ash Formation The name "Kauroa Ash Formation" is proposed for the 17 oldest ash beds exposed in an outcrop at the entrance of Woodstock Farm (N64/437420), beside the Hamilton-Raglan Highway, Ii miles north of Kauroa, a locality 4 miles south-east of Raglan. The lowest and oldest ash bed here rests OIU colluvial ash. The beds are light in colour and appear to be mainly rhyolitic, but at Ahuroa Trig. (N55/392797) other beds which are correlated with the formation interfinger with basaltic ash derived from Ahuroa scoria cone. Only the uppermost beds of Kauroa Ash Formation show in most exposures. Detailed descriptions of the Woodstock section and of the thick sequence exposed at Kaawa (N51/392838) and near Ahuroa Trig. are given in Fig. 7. Weathered fragments of mudstone, up to half an inch in diameter, are incorporated in two of the subaerially deposited beds of the Woodstock section. Other fragments of Tertiary rocks lie near the base of the lowest of three Kauroa ash beds at Te Uku (N46/464416, Fig. 6). These sedimentary fragments were presumably torn from the throat of the volcanic source at the time of the eruptions which produced the beds, and suggest that the volcanic FIG. 6-Unconformity between Kauroa ash beds and Hamilton ash beds, 1 mile west of Te Uku, Hamilton-Raglan Highway (Site 42). The lowest Kauroa ash bed exposed rests on weathered basalt. It contains fragments of mudstone and siltstone. Ten beds of Hamilton Ash Formation (above unconformity) are well developed here. The lowest is Ohinewai Ash Member, Te Uku Variant (note prominent white nodules of halloysite). The modern soil is formed on Mairoa Ash Member, Rotowaro Variant (in shade). The scale (top left) is 30 in. long. (N64/ )

13 1120 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10 centre was not far distant. The second bed at Te Uku contains a volcanic bomb, 1 ft in diameter and composed of basalt, that has crumpled the bedding of the ash. The bomb cannot have travelled far because of its weight, and was probably derived from one of the scoria cones in Okete Valley nearby. However, the occurrence of Kauroa ash in places remote from the small scoria cones, and its wide distribution, indicate that the beds were not derived from these small eruptive centres but from another more vigorous source whose location is unknown. Interfingering of Kauroa ash beds with basaltic ash at Ahuroa scoria cone shows that the Kauroa ash was deposited when Ahuroa cone was active. Ahuroa cone, according to Kear (1959), is contemporary with the Middle to Upper Pleistocene Franklin Basalts (Schofield, 1958). Waiterimu Ash Member "Waiterimu Ash Member" is the name given to the top member of Kauroa Ash Formation exposed in a road cut at N52/761833, by the Ohinewai - Te Hoe Highway 1 mile east of Waiterimu Road. (Waiterimu is 8 miles south-east of Te Kauwhata.) At this locality the bed is 70 in. thick and rests on clays of Puketoka Formation (Battey, 1949). The bed consists of strong brown friable ash with scattered halloysite nodules, and grades up into dull red earthy ash. Prominent olive clay veins, marking the base of a well developed fossil soil, occur 40 in. above the base of the bed. The fossil soil* is described as follows: (topsoil) 8 in. olive grey clay; firm; moderately developed medium nutty structure; abundant prominent manganese nodules; indistinct boundary 10 in. olive grey clay; firm; weakly developed medium nutty structure; many manganese nodules; indistinct boundary 12 in. olive grey clay; massive; many distinct medium red mottles, with prominent olive clay veins at the base. The fossil soil has the essential features-s-ciay veins, manganese nodulesof the modern soil. Hamilton clay loam (Grange et al., 1939), a brown granular loam, and is considered to have developed under similar conditions, especially with respect to native vegetation. No beds of Kauroa Ash Formation were observed higher in the sequence than Waiterimu Ash Member, which is, therefore, assumed to be the youngest bed of the formation. The bed is well exposed in many localities in the lower Waikato basin. In any outcrop the Kauroa ash bed, developed at the unconformity separating the Kauroa and Hamilton ash formations, is commonly stony or platy and, its colour being in general olive grey, it superficially resembles a Tertiary siltstone. Similar stony beds also occur at other levels in Kauroa Ash Formation. On examination they mostly prove to be fossil topsoils, and when a specimen is broken, a nodular fracture, resembling that given by turf when broken across, is obtained. Other relics of the ancient soil such as tunnels excavated by soil organisms and cavities resulting from root *The terms used in the soil descriptions are mostly those recommended in the Soil Survey Method (Taylor and Pohlen, 1962).

14 No.4 WARD - VOLCANIC ASH BEDS 1121 Site 6 (Kaawa) N51/ Site 10 (Ahuroa) N55/ Site 40 (W..oodstock) N64/ H2 H2 H2 (with HI) Waiterimu Ash Member { 36 ~ s~ platy platy olive brown stony ash with olive stony ash olive grey stony clay veins grading to yellowish brown friable ash with discontinuous purple 45" bualticash 48" crushing to Stf\lOng brown rootlet pseudomorphs firm pale brown 10" earthy ash With nutty st ru ctu re and cucans 3Y re.sh fossil soil yellow blocky ash witheutans boundary obscured by dily vems S'o"y.ol...rev / firm yellow ash, f,rm yellow ash with brown 13'" dull red yellow earthy ash 60 fossil soil dull brown earthy 21'" firm yellbrmasslveash 24" weathered reddish brown except at base platy yellow blocky ash mostly stony olive grey R weathered dull red. Manganese 20" dull yellow blocky ash wuh cucans 24" olive brown earthy prominent clay nodule\ concretions blocky ash, tending friable. 16" brown yellow blocky as\ dull yellow blocky ash 21" low-volume/weight. cutans, clear boundary with cvcans. friable at base Ir halloysite nodules at base Hony,gradin& to yellow 16' brown yellow blocky as~n\ blocky ash with cutans reddish fossil soil reddish fossil soil 14' small halloysite and man ganese eencrencns 20" yellow blocky ash with cutans dull yellow massive to coarse blocky ash brieht yellow blocky ash with red fossil soil pink ash with red and dull yellow fossil soil friable to Iu-m yellow blocky ash bdy. with manganese stains pale yellow with red s~~n~o~i'~i: Ht~rnI~;h~~ \ olive brown stan)' yellowish \ brown ash with faint cutans 42 Irregular prominent Mn stains at middle distinct bdy. prom, white mottles, dull red y!liow blocky ash especially below.with reddish soil and manganese concretions Similar to HI distrnct Mn - stained cdv 30" 13' dull yell. indistinct br,flrmearthy Mn stains r-tn-. stamed bdy 10' dull'yel1br prorrunent 'pale yellow ash 8" :!'i}ds~~i~ow ;b~fjs ash with rare 12' yell. br, blocky with rare mudstone pebbles 4'" en. rra morse s yell. blocky, pink at base,bdy 14" with red clay nodules grey brown earthy ash With 30" clay veins at base and dull red scams 71''' R weathered basalt FIG. 7-Principal sections, Kauroa Ash Formation. HI-Huntly Variant} 'A. H2-Te Uku Variant Hamilton sh Formation R-Colluvial ash Geology-19

15 1122 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10 TABLE I-Stratigraphy and Nomenclature of the Ash Beds r Mairoa Ash Member [ 'I L r H8 H Bed 2 H7 H9-Dunmore Variant (formerly Mairoa Shower) H8-Rotowaro Variant (formerly Hamilton Shower) -soil Hamilton Ash Formation Kauroa Ash Formation ~ Oh mewai i. Ash ~ L Member ~ [ Tikotiko Ash Member O 0 r l r w aiterimu Ash 1 Member L H6 H H4e H4d H4c H4b H4a H3b HI H3a Several ash beds, not correlated H2 I I -soil -soil -soil -soil H2-Te Uku Variant HI-Huntly Variant Unconformity L I Unconformity

16 No.4 WARD - VOLCANIC ASH BEDS 1123 decay, are also evident. Some of the fossil topsoils are not stony or platy like those described, but are friable or earthy, and there is a consistent relationship between the condition of the fossil topsoil and the condition of the ash bed immediately overlying it. Where stony and platy fossil soil horizons occur at the top of the Kauroa deposits they are everywhere overlain by Ohinewai Ash Member, Te VIm Variant (Hamilton Ash Formation), and stony or platy fossil soil horizons within the Kauroa deposits are always followed by a friable bed which resembles Te Uku Variant in weathering status. Where friable and earthy fossil topsoils occur they are overlain by Ohinewai Ash Member, Huntly Variant, or by a similar earthy bed. These relationships suggest that leachates produced by weathering of friable ash beds, like Te Uku Variant, permeate the buried soils below and bring about their lithification. Hamilton Ash Formation The name "Hamilton", first applied to the soil-forming ash bed at Hamilton City (Grange and Taylor, 1931), has since been extended by usage to include the underlying beds, at times differentiated by the term "older Hamilton Showers" (Grange et al., 1939). It is desirable to give formal recognition to this usage and, for this reason, the name "Hamilton Ash Formation" is proposed for all these beds above the Kauroa Ash Formation. For convenient reference the beds of Hamilton Ash Formation are identified by numbers, except for "Bed 2" (Taylor, 1933), which was recognised in only a few localities and scarcely merits another symbol. Three members of the formation are given geographic names. Figs. 8 and 9 record the sequence of beds of Hamilton Ash Formation at 52 sites in the area examined. They show that no single outcrop exposes all the beds of the formation. The most complete section (Site 42, Fig. 10) is exposed in a road cutting on the Hamilton--Raglan Highway (N64/464416), 1 mile west of Te Uku. This exposure (Fig. 6) has the beds clearly separated, and is proposed as the type locality for many of the beds. Ohinewai Ash Member The name "Ohinewai Ash Member" is given to the basal member of Hamilton Ash Formation. The bed is well exposed at Ohinewai, 5 miles north of Huntly. It is strongly weathered and occurs in two dissimilar forms, here named Huntly Variant and Te Uku Variant. Huntly Variant, the form developed at Ohinewai, best preserves the features of the parent Ohinewai Ash Member, especially in the lower part. It has a pale colour and stands out against both Te Uku Variant and all other beds of Hamilton Ash Formation. It is easily visible at a distance and forms an excellent marker bed for the unconformity separating Kauroa and Hamilton ash formations. Huntly Variant Huntly Variant (HI in Figs. 8 and 9) is exposed above Waiterimu Ash Member (Kauroa Ash Formation) in a road cutting 300 yd east of Ohine-

17 1124 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10 wai Railway Station (N52/677833). The volcanic ash sequence at this locality is described in Fig. 8, Site 25. Huntly Variant is characteristically a pinkish grey, firm, blocky ash with prominent white veins and yellowish red stains, grading upwards to brownish grey silt loam. Locally, as at Site 1, and where the bed is thin, the veins and stains are absent and the bed is pale yellow. The thickness varies from 10 to 40 in. and weathering is not far advanced below the middle of the bed. Corroded crystals of quartz and weathered hornblende and augite, imbedded in a clayey matrix stained with manganese, are abundant in the basal 2 in. of the bed. In places this crystal fraction is 4 in. thick, and grades imperceptibly into the overlying fine-grained material. The lower boundary is sharp. Because of the association of the crystalline fraction with the overlying fine-grained material and the gradation between them, it is considered that they were erupted together, and that the ash suffered gravitational sorting while airborne. On a hillside near Huntly (N56/657734) current bedding is well developed in the coarse fraction, and also in the overlying fine material to a height of 6-10 in. The current bedding lies parallel to the hill slope and is thought to be due to rain wash at the time of ash fall. At Ohinewai the bed is succeeded in one part of the section by a thick (5 ft) lens of alluvial sand. The following description, recorded in an exposure on the Huntly-Ruawaro Road (N56/618717), is of the fossil soil formed on Huntly Variant: (topsoil) 10 in. brownish grey silt loam; firm; moderately developed medium and fine nutty structure with distinct cast granules; indistinct boundary 13 in. reddish yellow clay loam; firm; weakly developed medium prismatic structure; many distinct strong-brown allophane cutans in fissures; indistinct boundary on pink micaceous clay; firm to very firm; massive; abundant prominent red and black clay nodules, with allophane cutans in fissures. Te Uku Variant Te Uku Variant (H2) is strongly weathered, and the original structure of the parent Ohinewai Ash Member is almost completely destroyed. The variant is best exposed on the roadside at N64/464416, 1 mile west of Te Uku (Site 42). It is a strong brown to yellowish red, friable ash, with prominent, fragile rootlet pseudomorphs and large halloysite nodules near the base. The base is generally cemented to a depth of a few inches with manganese-stained halloysite which encloses abundant crystals, the gravitationally sorted, heavy fraction of the parent Ohinewai Ash Member. In a few places, e.g., at Site 23 (N55/408740), the bed is extremely rich in allophane, which has a high content of loosely held water and becomes moist on handling. Analyses (Dr L. D. Swindale, pers. comm.) show that the rootlet pseudomorphs of Te Uku Variant are composed of halloysite. Other beds which resembles Te Uku Variant are the fossil soils H3b, H4b, H4d, and H5, but none of these have the thick deposits of halloysite found in Te Uku Variant. H3b, H4b, and H4d were described as "waxy pans" by Taylor (1933), but their origin as soils is indicated by their similarity to Te Uku Variant and by the occurrence of rootlet pseudomorphs in each bed.

18 \V ARD - VOLCANIC ASH BEDS 2J l4" ]4" I H5 38" 12 ;-14 12"~4/10 H3a 12" H3a,...10' HI 14' H2/~;' 40" 16 H "1 H9 12"1 H6 Hl I, ". ~Iightly stony K fr-iable 'j;i;ry HS n, )2 H7 ",~"I H9 20'1 H9,12 H4 \ IS" / '/9"; H3a \ \, /"'2rl H5 S{ HI \\ 13" /1 //1 r: \~\\18"~4 9"'~l' 1,,\ I' 1// blocky '24" /,~ H2 ~/ K \\ H4 I~;~: Ii' (,,,' nrthy'''' IS" I 40" K Stony K stony ~ R 36 \ v -, \~ '\"r H3a ~~ HI platy K 21 /';;,1 H7 HS rn~" H9.../8 H7 18' 18" H6... HS 22 20'1 H7 31"1 H4 ->2'0 12" HS 20" H4 10 II ta m 12 H9 20" 20" H9 H9 /~:.n \~o"r:l 20"r: /' --- '- 18 H H7 H6 30 IS H6,,~: H7...-:s J UA r... ~ 18"1 I 28" I Hl HS HS...1:' 14 HS H4 ", I 1;- II /1 earthy II HI H4 //40' /'31 K 18 i ", 48" 1/ I 40 \ \32" \ I H9 \ '. \p \ I H7 ", Hl. I / / earthy ~ H2 HI K K \~\ 23 ~:' Hh _--- 12t.M3'\ \\ 28" 12" H2 22" H2 \\10' H4e ----~---- \i~,~~,:,mhy, ~ K \S \I~" H3a \r la" I /,/16' 24 H9,/,I H5 I J2 II stony I K / earthy 25,, H9 \22 """\~4 \\ friablel\\14:~3' K \ 2J( H2 4B 2rl sto ny <,.J~ LOCATION I N51/ N51/46484~ 2 N51/ N51/ N51/ N55/ N51i N55/ N51/ N51/ N51/ N5 I,: N51i NSS/ N51/ N51/ N51/ NSli N55; N55/4<l8740 II NSI/ N5S/ NSI/ S NS2i N51/ N55i I H: Hamilton Ash formation K: Kaurca Ash Fcrmauon fl.:colluvial Ash I mm I.,",,'.Z.}l c-«. Geophys. 10(4), 1967 FIG. 8-Stratigraphic columns For explanation of symbols HI to H9, seetable l.

19 ~I g</~,;j:t o~ ~~ ~~ ~ I : \)">ITin,~~ ~a:li'i~ i '[I\ <I ~ite=[:::~>i~/ [ ::::: ~ :I I: i ~'" ><: ~ '".. ~ 6]---:-!~:'~~~ c~v ~ \.i \j~ i Dr:J~ JS<i---[fil--~--U ;:;:; J: I T I I :' '" "" ;; gatl:! i!~ I',~ ~ i""'t ~~v ~ITt "~" d~li :'~'J~" [ ~ [di~1 +\I(:\)I iir~ ~~ ~'-~I llri~"""'~ {:ir;t-.>----,--- " g. ~DTI -~-----:-i\ ~ Ef:~:~"'~i'--:'I.. ~ \.. ~-,~.. ;"":::...~..:>,-...

20 No.4 WARD - VOLCANIC ASH BEDS 1125 The extreme friability and the grading soil horizons of Te Uku Variant are features possessed by modern yellow-brown loams, which also contain abundant allophane, and the variant is considered to be a fossil yellow-brown loam. A description of the fossil soil from Site 50 (N74/714968) is: (topsoil) 7 in. yellowish brown silt loam; firm; moderately developed medium and fine blocky structure; aggregates embayed and corroded; indistinct boundary 14 in. brownish yellow, heavy silt loam; friable; weakly developed medium blocky structure; pale cutans in fissures; diffuse boundary 8 in. brownish yellow silty clay loam; firm; massive; pale cutans in fissures; sharp boundary on pale yellow halloysite pan; stained with manganese; with. abundant crystals. Where Te Uku Variant is developed, it usually replaces completely the parent Ohinewai Ash Member, but unweathered parts of the parent bed are exposed with Te Uku Variant at Otorohanga (Sites 50 and 51), Waikorea Junction (Site 23), and Glen Afton (Site 27). At these localities small nodules of halloysite, mostly less than half an inch in diameter, lie at the interface between weathered soil and unweathered ash. Identical nodules, similarly placed on surfaces of weathering, also occur at other levels in the sequence of ash beds. At Woodstock (Site 40), and beside the Ngaruawahia-Waingaro Road (Site 34), Te Uku Variant and Huntly Variant are developed side by side in single exposures, at the same stratigraphic level. In these exposures the boundaries between the variants are abrupt. However, the basal crystal fraction of each variant continues without interruption from one variant to the other, and this confirms that the two variants are simply differently weathered forms of a single ash bed. The distribution of Huntly and Te Uku variants is associated with position in the landscape. Huntly Variant occurs on the low country, and Te Uku Variant occupies the high land. Beds H3a and H3b Bed H3a is the lowest of a group of ash beds with blocky structure. It differs from other blocky beds in that its colour is dull yellowish brown (loyr 6/4). Furthermore, the bed contains innumerable small scattered aggregates of greyish white halloysite, no larger than coarse sand. In some localites these aggregates are missing. Generally, the bed is thin, with an average thickness of 12 in. Bed H3b is a brown, friable ash apparently rich in allophane and is sharply separated from H3a. Manganese-stained halloysite nodules occur at the base. The bed is normally 4-6 in. thick, but is missing in very many exposures. In most localities where it is absent the level at which it would be expected to occur is marked by a line of halloysite nodules. H3b is well developed above H3a at Otorohanga (Site 51), which is proposed as the type locality for both beds. H3b resembles H2, and is considered to be fossil soil developed on H3a. Geology-20

21 1126 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10 Beds H4 and H5 The ash beds labelled H4a, H4c, and H4e resemble one another very closely, and cannot be separated in the field unless the intervening fossil soils H4b and H4d are present. In most localities the beds have been mapped together. H5, which succeeds bed H4e, has the characteristics of a fossil soil, and is commonly well developed at the expense of the H4 beds on which it is formed by weathering. The H4 beds are all thin and their total thickness, on an average, is 26 in. H4a is a pale brown (10YR 7/4), blocky ash, generally with strong brown clay skins on the aggregates and in fissures. The bed is sharply separated from H3b by small clay nodules, but where H3b is absent it is difficult to separate from H3a. H4b is a thin, strong brown, friable ash with nodules of halloysite at the base, and rootlet pseudomorphs scattered throughout. It is rarely developed, but is clearly exposed at Te Uku (Site 42), where it is 6 in. thick. Site 42 is proposed as the type locality for both H4a and H4b. H4c, a pale brown, blocky ash with distinct yellowish brown and strong brown cutans, resembles H4a and is mapped with it, unless H4b is present. Its thickness varies from 6 to 12 in., and it is best exposed at Site 34 (N56/568645), here proposed as the type locality. H4d strongly resembles H4b. It is rarely developed, and is not exposed at Te Uku. Its position in the sequence is well shown at Site 34 (N56/568645) on the Ngaruawahia-Waingaro Road, where the bed is 10 in. thick. H4d is a soil formed on H4c, and the weathered interface between the two beds is marked by small clay nodules. H4e, the topmost of the blocky ash beds, is very like H4a and H4c. It occurs above H4d at Site 34. In other places it is separated from H4c by a narrow zone of clay nodules. The bed is typically 6-10 in. thick, and is well shown at Te Uku (Site 42). H5, which has an average thickness of 24 in., is a yellowish brown to strong brown earthy ash and is well exposed at Te Uku. It becomes somewhat friable at the surface on exposure and contains numerous small halloysite nodules and rootlet pseudomorphs (Fig. 4). Fragments of charcoal occur at the top of H5 at N56/657716, near Huntly. Tikotiko Ash Member Tikotiko is 3 miles south-east of the Glen Murray. Tikotiko Ash Member includes a fossil soil (H7 in Figs. 8 and 9) and its parent ash bed (H6). H6 is well exposed at Otorohanga (Site 50), where shower banding is clearly marked in the lower part of the bed. This exposure is proposed as the type section. H6 is an earthy ash, and is readily distinguished from other beds by its characteristic greyish brown colour. Bed H7, a fossil soil, is characteristically developed above H6 at Te Uku. In many localities it completely replaces the parent bed, which has been entirely consumed by weathering. The fossil soil resembles the modern soil, Naike clay loam, developed under broadleaf forest on Mairoa Ash Member,

22 No.4 WARD - VOLCANIC ASH BEDS 1127 Rotowaro Variant. H7 is a compact, strong brown ash with fine blocky structure, and weathers to dark brown on exposed faces. Generally, it may be distinguished from other beds by a marked change from dark brown to bright yellowish brown when the material is crushed. On exposed faces bed H7 rapidly frits away. Outcrops of other beds, including the parent ash, H6, generally have steep slopes, but the outcrop of bed H7 is normally a gently sloping shelf. Scattered halloysite nodules mark the junction of H6 and H7 in places. The following description is of the fossil soil: (topsoil) 5 in. dark brown clay loam; firm; moderately developed medium granular structure; diffuse boundary 8 in. brown clay; firm; moderately developed fine blocky structure; diffuse boundary on brown to yellowish brown clay; firm; weakly developed blocky structure; grading to H6. Mairoa Ash Member The name "Mairoa Shower" was given by Grange (1931) to the soilforming ash bed, a brown silt loam, mapped by Taylor (1930) near Mairoa, west of Te Kuiti. In 1933 Taylor identified the bed near Otorohanga, at the locality here described as Site 50. Mairoa Shower of Grange and Taylor is given the symbol H9 in this paper. The thickness of the bed was found to be very uniform, averaging 24 in., the same thickness as at Mairoa (Taylor, 1930). Another ash bed, later named Bed 2, was described by Taylor at Mairoa in Taylor stated that Bed 2 graded into the overlying Mairoa Shower, and recognised it below Mairoa Shower at Otorohanga. The bed was identified elsewhere, in the present survey, immediately below H9. Unlike the contacts between other ash falls, the boundary with Mairoa Shower is everywhere a gradual one. This gradation between the beds and their invariable association suggest that Mairoa Shower and Bed 2 are parts of a single deposit rather than separate showers, but Taylor did not discuss this possibility. In this survey no evidence was found to support their separation, and they are consequently presumed to be different parts of one deposit, "Bed 2" being the least-weathered basal part. The soil-forming ash bed at Hamilton dty was given the name "Hamilton Shower" by Grange and Taylor (1931). No type section was nominated, but laboratory studies made in 1939 (Grange et al., 1939) were of samples taken from Church Road, Te Rapa, and this locality is accepted as type locality. Site 44 (Fig. 9) gives details of the stratigraphic sequence. The lenses of friable material which occur locally in the topsoil above the bed identified as Hamilton Shower (H8 in this report) at Church Road, and in other places between Hamilton and Taupiri, were considered by previous workers to be remnants of Mairoa Shower (N. H. Taylor, pers. comm.), and on this evidence Grange and Taylor (1931) concluded that Hamilton Shower was older than Mairoa Shower. Recognition of the friable lenses as Mairoa ash is based on slender evidence, however, and rests wholly on the texture, the type of clay mineral, and the friability of the material. It is possible that the situation of the

23 1128 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10 friable lenses in a zone of intense humification has prevented the development of other more reliable diagnostic features. Thicker deposits of friable ash occur above Hamilton ash in places between Hamilton and Huntly, and at Rukuhia Soil Research Station, particularly in depressions on the hills, but none of these deposits, which grade through a broad transitional zone into Hamilton ash below, were found to be typical of Mairoa Shower. At Pukekapia, for example, the friable ash is pale and contains small nodules of halloysite. Ash which could be identified with confidence as Mairoa Shower was not seen above Hamilton Shower in this survey. At Mairoa, on the roadside quarter of a mile north of Mairoa School, and at Ngapaenga Road, north of the trig. station, Mairoa Shower rests on Tikotiko Ash Member (Beds H6 and H7). Exposures between Hamilton and Ngahinepouri, where Hamilton ash is reported to lie below Mairoa ash (Taylor and Grange, 1939), show that Mairoa Shower here also rests directly on Tikotiko Ash Member, and Hamilton Shower does not intervene between these two beds at N65/827268, near Te Awamutu, an exposure similar to that figured by Taylor and Grange (1939, p. 16). In localities where "typical Mairoa Shower" and "typical Hamilton Shower" are both well distributed no trace of Mairoa ash was found above Hamilton ash, even on gentle slopes where' the ash beds are usually best preserved. At Rotowaro golf course, Naike, and Tikotiko, for example, Hamilton ash occurs on undulating land, whereas Mairoa ash lies at higher levels on adjacent hilly land. If it is assumed that the two beds are the products of different eruptions, this is difficult to explain. It cannot be supposed that erosion is selective, removing Mairoa Shower where Hamilton Shower is present, and vice versa, and the possibility that the beds are not separate showers, but differently weathered forms of the same parent bed, must be examined. Fieldes (1955) has concluded that clays derived by weathering from rhyolitic or andesitic ash pass, in time, through the sequence: allophane to halloysite to kaolinite. These changes in crystal structure are hastened by seasonal wetting and drying. Thus Hamilton "Shower", which today consists predominantly of halloysite (Fieldes, 1955), has previously passed through a stage when the dominant clay mineral was allophane. In other words, Hamilton "Shower" has once resembled the Mairoa bed, in which allophane is dominant. Describing the soil developed on Hamilton Shower at Church Road, G. D. Smith (pers. comm.) drew attention to thick, continuous clay coatings on soil aggregates and in pore spaces below a depth of 12 in. His observations were confirmed by Taylor (pers. comrn.), who showed also that the topsoil aggregates of Hamilton clay loam were corroded. The same features are to be found in other localites where friable, allophane-rich material overlies Hamilton Shower of Grange and Taylor (1931), such as Rukuhia Soil Research Station (N65/806421), Pukekapia (N56/634755), Rototuna (N56/797546), Horsham Downs (N56/770570), and beside the Great South Road in Taupiri Gorge (N56/684696). In all of these places soil particles in the friable horizons are embayed, and the fragments of quartz embedded in them are washed clean. In the same profiles clay skins are visible in the friable material immediately above Hamilton Shower, and

24 No.4 WARD - VOLCANIC ASH BEDS 1129 are prominent in Hamilton Shower. These observations suggest that the clay skins of Hamilton Shower are the product of weathering of the overlying, friable, allophanic material, and the field evidence supports the hypothesis that the allophane of the parent ash is converted, by weathering, to halloysite, which, it would appear from the distribution of the clay skins, is then washed to the base of the bed, where it accumulates. In time this process would bring about the complete alteration of the parent allophane bed to halloysite. To summarise, those exposures that appear to show Mairoa "shower" above Hamilton "shower" exhibit a continuous gradation between the one type of material and the other, and this gradation is consistent with the laboratory evidence that weathering of allophane leads to the production of halloysite. It follows that exposures showing friable allophanic material (i.e., Mairoa "shower") above dense halloysite (i.e., Hamilton "shower"), as for instance the one at Church Road, can be regarded as incomplete stages in the transformation by weathering of the one material to the other. Thus the materials hitherto identified as Hamilton and Mairoa showers might only represent different stages of weathering of a single ash bed. They need not be the result of separate ash eruptions as has been believed. If the change from allophane to halloysite is assisted by seasonal wetting and drying, then the distribution of the Mairoa ash, and of the Hamilton ash, might be controlled by climate, in so far as it affects the soil moisture regime in the field. There is, in fact, an apparent correlation of ash morphology with climate in the lower Waikato basin, where Hamilton ash occurs mostly in areas below 400 ft, with rainfall less than 55 in. and a short summer drought; and Mairoa ash lies generally at greater elevations with higher rainfall, in areas where summer droughts are neither frequent nor severe. In transitional zones Mairoa ash lies in depressions where the moisture supply is better than on the adjoining ridges occupied by Hamilton ash. It is to be noted that the distribution of Huntly and Te Uku variants of Ohinewai Ash Member follows the same pattern as the distribution of the Hamilton and Mairoa beds, with like materials occurring in the same general areas. Thus where the allophane-rich Te Uku Variant occurs it is normally followed higher in the sequence by Mairoa ash. Mairoa ash is more widespread than Te Uku Variant however, and descends to lower levels. where it lies above Huntly Variant. If the morphology of the weathered ash is determined by climate, it follows that there was less rainfall in the lower Waikato basin, and droughts were experienced at higher elevations, when Ohinewai Ash Member was exposed to weathering. The field observations detailed above do not support the view that the Hamilton and Mairoa beds are separate showers, but are satisfactorily explained if the beds are regarded as differently weathered forms of the same parent ash; thus it is proposed that the Hamilton bed is derived from Mairoa Shower by weathering and elutriation of clay. The separate occurrences of the Hamilton and Mairoa beds and their jumbled pattern in the field both appear to be satisfactorily explained by this hypothesis. The processes of weathering that are presumed to be responsible for the conversion of Mairoa ash to Hamilton ash are worth further study, particularly in the laboratory, and, for this purpose, sites where the changes are considered to be incomplete might be particularly valuable. The Church Road locality,

25 1130 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10 where only shallow patches of allophane remain, is considered to display a late stage in the weathering process. Stages that are less complete, and presumably earlier, show in outcrops at Rukuhia Soil Research Station (N65/806421), Pukekapia (N56/633775), and near Rototuna (N56/ Bed 2 (Taylor, 1933) is here considered to be the lowest part of Mairoa Shower (Grange, 1931). These beds, that is Mairoa Shower and Bed 2, together with Hamilton Shower (Grange and Taylor, 1931), are regarded as different weathering forms of the same parent ash bed, for which the name "Mairoa Ash Member" is proposed. The parts of Mairoa Ash Member formerly known as Mairoa Shower and Bed 2 are here named "Dunmore Variant", and the name "Rotowaro Variant" is given to the bed formerly called Hamilton Shower, the name "Hamilton" being reserved for the complete sequence of ash beds. The names for the variants are taken from localities in Raglan County where they are well developed. Dunmore Variant Dunmore Variant is well exposed in a road cutting 1 mile south of Otorohanga (Site 51, N74/714964), here proposed as the type locality. The basal part (Bed 2, Figs. 8 and 9) is a pale yellow earthy ash with fragments of pumice, visible to the naked eye, in southern outcrops. It has a nodular fracture, and fossil worm casts and burrows are dearly evident in places. These indicate that the bed was weathering to form soil as it accumulated. Taylor (1933) noted hypersthene, augite, and hornblende in the hand specimen. Bed 2 is prominently developed in the southern part of the area examined, where Mairoa Ash Member is thick. It was not separated from the remainder of Mairoa Ash Member north of Site 37, at N56/657706, near Huntly. At this locality the pumice fragments are very small (2 mm in diameter) and rare. Pumice fragments do not occur in Bed 2 at Site 49 (N57/200517). Here fragments of crystalline quartz are abundant. The upper part of Dunmore Variant (H9, Figs. 8 and 9) is a friable to fragile, allophane-rich ash, of light weight, with a fragile medium blocky structure. It is brownish yellow to dark brown. Rotowaro Variant Rotowaro Variant (H8) is a compact, stiff, strong brown day, consisting dominantly of halloysite. Reddish stains and prominent day veins occur in places in the lower part, but these are not essential features of the bed. Near Walton (Site 48, N47/190530) the lower part of Rotowaro Variant consists of pale yellow, firm clay loam with abundant crystals. This part of the bed is 14 in. thick, and contains prominent brown clay skins close to the base. It has a superficial resemblance to Bed 2. A similar outcrop is located at Tauwhare (Site 45, N65/940485). At Church Road, Te Rapa, Mairoa Ash Member is incompletely weathered to Rotowaro Variant, and the parent ash at this exposure is strongly affected by other processes of weathering, as the massive topsoil horizon, the manganese nodules, and the clay veins in the soil indicate (see Fig. 3). The exposure is therefore unsatisfactory as the type section. Mairoa Ash Member is completely weathered to

26 No.4 WARD - VOLCANIC ASH BEDS 1131 Rotowaro Variant at the Rotowaro golf course (N56j609673), and the massive horizon and clay veins of the Church Road site do not obscure the outcrop there; this site, therefore, is proposed as the type section. CORRELATION OF THE BEDS OF HAMILTON ASH FORMATION WITH THOSE MAPPED BY PREVIOUS WORKERS Taylor (1933) figured a composite column showing the succession of beds in the Mairoa district. This column was based mostly on the outcrop near Otorohanga, which is redescribed in this report (Site 51). It is reproduced in Fig. 10 with the present author's sketch, of Site 51. According to Taylor, the Intermediate Beds and Bed 5 shown in his column do not occur at Otorohanga. His description, supplemented with information given by Grange (1931), allows the following correlation: (I) Bed I (Taylor) is equivalent to Bed H9 (this report), the upper part of Mairoa Ash Member, Dunmore Variant. (2) Bed 2 is so mapped in this survey. It represents the lower part of Mairoa Ash Member, Dunmore Variant. (3) Bed 3 incorporates Bed H6 of this report. It is possibly equivalent to Tikotiko Ash Member. (4) Bed 4 includes Ohinewai Ash Member (HI and HZ), H3a and H3b, the H4 beds and H5. (5) The "waxy pans" described by Taylor as parts of his Bed 4 are the beds identified here as H3b, H4b, and H4d. Fig. 10 includes a new composite column, based mostly on the sections exposed at Otorohanga and Te Uku. SOURCE The probable eruptive source of the Hamilton ash beds is presumed to be either Mt Egmont, to the south-west, or a centre in the Rotorua-Taupo volcanic field, to the south-east, but the thickness of the individual Hamilton ash beds when plotted on the map gave no clear indication of its direction. Presumably the original thicknesses of the beds have been affected by weathering. To eliminate random effects the measurements were grouped, and averages taken as follows. Taking the ash beds one at a time, a circle centred on Mt Egmont was drawn so that half the observations lay inside, and half outside, the circumference. Average thicknesses for the two sets of observations so formed are given in Table 2. The records were then re-examined bed by bed, and two different groups were defined for each bed by an arc of a circle, centred now on the Rotorua Taupo volcanic field. Table 2 shows that the average thickness of most beds in the group far from Mt Egmont is not very different from the average thickness in the group near to Mt Egmont. It gives no support to the suggestion of Grange and Taylor (1932) that Mt Egmont was the source of Mairoa Shower. However, there is a marked increase in thickness towards the Rotorua-Taupo volcanic field, indicating that the eruptive source lay in this direction. The observations do not permit the preparation of reliable maps showing the thickness of the separate ash beds.

27 F, 30" OTOROHANGA SECTION H9 I I k// " I Bed 2 4S' I H6 26' HS ---- TAYLORS COMPOSITE COLUMN o ao II oe Q 0 Bed I : 26" brown silt loam Bed 2: 6"whitish fine sand f8~ brown silt loam Jl Intermediate Beds [not represented at 6" whitish fine sand Otorchanga (Taylor, 1933: 1"98)] Bed 3 : 36 H reddish brown day loam white tubes and rods (Gibbsite) A COMPOSITE COLUMN FOR THE AREA NORTH OF OTOROHANGA It H7~ H9 ~ i, r-tatroa Ash Member ~' H8 Bed2_ H6 } Tikotiko Ash Member 10 II IS IS " H4e 6" H4b 10" H4' 6' H3b 16" H3, 2S" HI + H2 J2-60' stony Kauroa alluvium (Hamilton Hills Fmn HealY,I94S \ \\ \ \\ \ \ \ \ \\ Bed 4 : 96" dull brown day foam Bed S : 120 H yellowish coarse sand (Missing from the Otorohanga section (Tayler 1933 : 197)) 10 II IS IS HS \ 'tio I Pale yellow nodules (Kaolinite) FIG. 10-Sequence of volcanic ash beds at Otorohanga (Taylor, 1933), correlated with ash beds identified in the present survey, and a new composite column, based mostly on the Otorohanga and Te Uku Sections \,).l N z N 0' c ::00 Z :> r< o "'1 o~ r- o ~ :> Zt:l o~ o '"C ::c ~ Q ~ r'... o \

28 No.4 WARD - VOLCANIC ASH BEDS 1133 TABLE 2-Hamilton Ash Beds Grouped According to Distance from Mt Egmont (Columns I and 2) and from the Rotorua-Taupo Volcanic Field (Columns 3 and 4). (Average Thickness in Inches.)* Ash Bed ISites C'o", '0 Sites Far from ISites Close to ISites Far from Mt Egmont Mt Egmont Rotorua-Taupo Rotorua-Taupo Mairoa Ash Membet (Bed 2 + H8 + H9) Bed 2 + H9 ("Mairoa Shower") H8 ("Hamilton Shower") Tikotiko Ash Member (H6 + H7) H H H5 + H H H4 (all beds) H3a Ohinewai Ash Member (HI + H2) I H2 (Te Uku Variant) HI (Huntly Variant) All beds together *The method of grouping is explained in the text (page 1131). AGE OF THE VOLCAN;.IC ASH BEDS Alluvial terraces east of Ohinewai and near Te Uku are considered to be contemporary with the Main Monastirian eustatic high sea level of Brothers (1954) (D. Kear, pers. comm.), and contemporary terraces in the Glen Murray and Rotongaro districts (Ward, 1960) are not covered by beds of Hamilton Ash Formation. They consist of ash-rich alluvium and evidently post-date the Hamilton ash beds. At Rangiriri, in trenches at the redoubt, and in drillholes at Te Kauwhata Horticultural Research Station (N52/652926), Hamilton ash occurs beneath alluvium, which rises to higher levels than the Main Monastirian terrace. This alluvium forms a high terrace, now maturely dissected, that appears to be graded to the Tyrrhenian high sea level of ft identified in 1954 by Brothers (Ward, 1960). The dissected terrace is widespread near Rotongaro and Pukekapia where erosion has locally exposed areas of Hamilton ash. i I

29 1134 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10 In this area and near Glen Murray the Hamilton ash rests on surfaces developed by erosion of an older terrace, ft above sea level, considered to be equivalent to the erosion surface which Brothers correlated with the Milazzian high sea level of Europe. These field relationships show that the Hamilton ash beds were deposited following deep erosion of the "Milazzian" terrace and before the construction of the "Tyrrhenian" terrace, and that, as subaerially deposited ash lies below the 25 ft contour, sea level at the time was evidently low. Hamilton Ash Formation is therefore considered to be early Pleistocene, and is correlated with the Antepenultimate (Mindel) Glaciation. ACKNOWLEDGMENTS I thank Dr E. J. Searle, University of Auckland, and Messrs J. D. McCraw and ]. G. Bruce, Soil Bureau, for their thorough criticism of the text, and Mr H. S. Gibbs, Soil Bureau, for help with preparation of the draft for publication. REFERENCES BATTEY, M. H. 1949: The geology of the Tuakau-Mercer area, Auckland. Trans. R. Soc. N.Z. 77: BAUMGART, I. L. 1954: Some ash showers of the central North Island. N.Z. Jl Sci. Tecbnol. 35: BREWER, R. 1957: Cutans: Phenomena of natural surfaces in soil profiles. Proc, 2nd Aust. Conf. Soil Sci., Melbourne, 1, Paper 32: 1-7. BROTHERS, R. N. 1954: The relative Pleistocene chronology of the South Kaipara district, New Zealand. Trans. R. Soc. N.z. 82: FIELDES, M. 1955: Clay mineralogy of New Zealand soils. Part II. Allophane and related mineral colloids. u.z. Jl Sci. Techno]. 37: 336B-50B. FIELDES, M.; SWINDALE, L. D. 1954: Chemical weathering of silicates in soil formation. N.Z. [l Sci. Tecbnol. 36: 140B-54B. GRANGE, L. I. 1931: Volcanic-ash showers. N.Z. It Sci. Tecbnol, 12: GRANGE, L. I.; TAYLOR, N. H. 1931: Reconnaissance soil survey of the central part of the North Island. N.Z. Geol. Suru. 25th A. Rep. (J930-3J): : The distribution and field characteristics of bush-sick soils. In N.Z. Dep, Sci. lndustr. Res. Bull. 32. Bush Sickness: GRANGE, L. I.; TAYLOR, N. H.; SUTHERLAND, C. F.; DIXON, J. K.; HODGSON, L.; SEELYE, F. T. 1939: Soils. N.Z. Dep, Sci. lndustr. Res. Bull. 76. Soils and Agriculture of Part of Waipa County, pp HAY, R. L. 1959: Origin and weathering of late Pleistocene ash deposits on St Vincent, HEALY, ]. B.W.I. J. Geol. 67: : Geology of the Karapiro district, Cambridge. N.z. Jl Sci. Tecbnol. 27: KEAR, D. 1959: Stratigraphy of New Zealand's Cenozoic volcanism north-west of the volcanic belt. N.Z. Jl Geol. Geopbys. 2: KEAR, D.; SCHOFIELD, J. C. 1959: Te Kuiti Group. n.z. It Geol. Geopbys. 2: NEW ZEALAND SOIL BUREAU, 1954: "General Survey of the Soils of North Island, New Zealand." N.Z. Soil Bur. Bull. No.5 (n.s.), PULLAR, W. A. 1967: Volcanic ash beds Waikato. district. Earth Science Journal 1: