ohinewai tephra formation; a c. 150000-year-old tephra marker in new...

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NZ. [ournal of Geology and Geophysics Vol. 21, No.1 (1978): 71-3 Ohinewai Tephra Formation; a c. 150000-year-old tephra marker m New Zealand C. G. VUCETICH, K. S. BIRRELL, AND W. A. PULLAR* Department of Geology, Victoria University of Wellington, and *Soil Bureau, DSIR, Rotorua, New Zealand ABSTRACT The stratigraphic rank of Ohinewai Tephra Formation is proposed for Ohinewai Ash Member (basal member of Hamilton Ash Formation) at the type section and newly defined reference sections at Te Kuiti, and Tirau, and at Sapphire Springs, Tauranga, and Opape in Bay of Plenty. An eruptive source near Taupo is indicated. An estimated age is c. 150000 years B.P. The weathered form of Ohinewai Tephra at these reference sections has the general character of Te Uku Variant but with variations depending on the proportion of allophane to halloysite. Ohinewai Tephra is commonly contrasted with older rhyolitic tephras in which the proportion of allophane to halloysite is much lower. STRATIGRAPHY Ohinewai Ash Member was defined by Ward (1%7) from a section near Huntly as the lowermost member of Hamilton Ash Formation mapped by him throughout the Waikato-South Auckland area. It was characterised by the distinctive form of the lowermost well weathered coarse ash unit that, in turn, contrasted sharply with an underlying strongly weathered paleosol, exclusively that of Waiterimu Ash Member of Kauroa Ash Formation. Ward also recognised two variants of Ohinewai Ash: Huntly Variant in which the weathered part is a firm, blocky, greyish ash; and Te Uku Variant in which this part is reddish-yellow and friable and contains, in addi- tion to allophane, prominent nodules of halloysite and a thin firm basal layer dominated by halloysite. The two variants have been considered by Tonkin (1970) and Pain (1975) as separate tephras. We do not agree with their views. . We have traced Ohinewai Ash as a distinctive tephra marker near Tirau (N66/231241; 1971) northwards to Selwyn, near Te Poi (N66/337334; 1971), to Waihi Beach (N53 & Pt 54/412926; 1965), and eastwards from Sapphire Springs (N57/369709; 1974) to Tauranga (N58/640598; 1965), Ohiwa (N69/599212; 1969), Opape (N70/845217; 1970), and Torere (N70/915255; 1970). Diagnostic criteria of Ward (1967) for Te Uku Variant, but not for Huntly Variant, are met at most of the above sites. Likewise, we have identified a previously unnamed tephra in the Te Kuiti district as Ohinewai Tephra using diagnostic criteria for Te 'Uku Variant. At Walker Road, 7 km east of Te Kuiti (N83/743809; 1971), a 1·6m thick prominent tephra occurring near the base of a 7 m thick cover bed section may be traced laterally for 30 m (Fig. 1). The profile is: O' 5m light yellow brown silty clay, firm, massive, slippery consistency distinct lower boundary (paleosol); equivalent Member H3a. Received 5 October 1976, revised 12 August 1977 5 m multiple unit comprismg bright yel- low brown, friable silt loam (high allophane) as "eyes" enclosed within creamy-white, firm, massive, clay Te Uku (halloysite); distinct lower boundary. Variant 0·10 m creamy white sandy clay, firm, with manganese streaks near base, coarse grained and dominated by corroded quartz crystals. (This layer here shows rudimentary bedding). The weathered form and basal character of this tephra closely resemble that of Te Uku Variant previously described by Ward for his Otorohanga site 50, now in- accessible. These features are well exposed at Ward's site 47 at Te Kawa. At Walker Road the immediately overlying O' 5 m layer meets the description of Ward's Member H3a. At Mile Peg 324 near Tirau (recently exposed on Rotorua-Hamilton Highway; N66/231241) a 1·6m thick tephra has the characteristics of Ohinewai Ash, Te Uku Variant: 1· 0 m pale grey, heavy silt, firm with prom- inent 2 em diameter halloysite con- cretions and with bright yellow brown, friable silt loam "eyes". O' 6m white basal coarse layer, strongly streaked black, with high corroded quartz content. This 1· 6 m thick layer occupies a mid-slope position and is considered overthickened. A corrected thickness is 1·2 m. At Sapphire Springs, Katikati, (N57/369709) a prom- inent O' 7m thick tephra comprises 0·6 m of yellow brown friable clay loam with creamy white halloysite- rich lobes and nodules resting on 1m white firm coarse ash streaked with manganese (Fig. 2). This tephra is assumed to be Ohinewai Ash. A lower contact to a 3 em thick grey cemented layer passing to a red cemented paleosol is a further diagnostic criterion

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NZ. [ournal of Geology and Geophysics Vol. 21, No.1 (1978): 71-3

Ohinewai Tephra Formation; a c. 150000-year-old tephra marker mNew Zealand

C. G. VUCETICH, K. S. BIRRELL, AND W. A. PULLAR*

Department of Geology, Victoria University of Wellington, and *Soil Bureau, DSIR, Rotorua,New Zealand

ABSTRACT

The stratigraphic rank of Ohinewai Tephra Formation is proposed for Ohinewai Ash Member(basal member of Hamilton Ash Formation) at the type section and newly defined referencesections at Te Kuiti, and Tirau, and at Sapphire Springs, Tauranga, and Opape in Bay of Plenty. Aneruptive source near Taupo is indicated. An estimated age is c. 150000 years B.P.

The weathered form of Ohinewai Tephra at these reference sections has the general character ofTe Uku Variant but with variations depending on the proportion of allophane to halloysite.Ohinewai Tephra is commonly contrasted with older rhyolitic tephras in which the proportion ofallophane to halloysite is much lower.

STRATIGRAPHY

Ohinewai Ash Member was defined by Ward (1%7)from a section near Huntly as the lowermost memberof Hamilton Ash Formation mapped by him throughoutthe Waikato-South Auckland area. It was characterisedby the distinctive form of the lowermost well weatheredcoarse ash unit that, in turn, contrasted sharply with anunderlying strongly weathered paleosol, exclusively thatof Waiterimu Ash Member of Kauroa Ash Formation.Ward also recognised two variants of Ohinewai Ash:Huntly Variant in which the weathered part is a firm,blocky, greyish ash; and Te Uku Variant in which thispart is reddish-yellow and friable and contains, in addi­tion to allophane, prominent nodules of halloysite and athin firm basal layer dominated by halloysite. The twovariants have been considered by Tonkin (1970) andPain (1975) as separate tephras. We do not agree withtheir views.

. We have traced Ohinewai Ash as a distinctive tephramarker near Tirau (N66/231241; 1971) northwards toSelwyn, near Te Poi (N66/337334; 1971), to WaihiBeach (N53 & Pt 54/412926; 1965), and eastwardsfrom Sapphire Springs (N57/369709; 1974) to Tauranga(N58/640598; 1965), Ohiwa (N69/599212; 1969),Opape (N70/845217; 1970), and Torere (N70/915255;1970). Diagnostic criteria of Ward (1967) for Te UkuVariant, but not for Huntly Variant, are met at most ofthe above sites. Likewise, we have identified a previouslyunnamed tephra in the Te Kuiti district as OhinewaiTephra using diagnostic criteria for Te 'Uku Variant.At Walker Road, 7 km east of Te Kuiti (N83/743809;1971), a 1·6m thick prominent tephra occurring nearthe base of a 7 m thick cover bed section may be tracedlaterally for 30 m (Fig. 1). The profile is:

O' 5 m light yellow brown silty clay, firm,massive, slippery consistency distinctlower boundary (paleosol); equivalentMember H3a.

Received 5 October 1976, revised 12 August 1977

1· 5 m multiple unit comprismg bright yel­low brown, friable silt loam (highallophane) as "eyes" enclosed withincreamy-white, firm, massive, clay

Te Uku (halloysite); distinct lower boundary.Variant 0·10 m creamy white sandy clay, firm, with

manganese streaks near base, coarsegrained and dominated by corrodedquartz crystals. (This layer here showsrudimentary bedding).

The weathered form and basal character of this tephraclosely resemble that of Te Uku Variant previouslydescribed by Ward for his Otorohanga site 50, now in­accessible. These features are well exposed at Ward'ssite 47 at Te Kawa. At Walker Road the immediatelyoverlying O' 5 m layer meets the description of Ward'sMember H3a.

At Mile Peg 324 near Tirau (recently exposed onRotorua-Hamilton Highway; N66/231241) a 1·6mthick tephra has the characteristics of Ohinewai Ash,Te Uku Variant:

1· 0 m pale grey, heavy silt, firm with prom­inent 2 em diameter halloysite con­cretions and with bright yellow brown,friable silt loam "eyes".

O' 6 m white basal coarse layer, stronglystreaked black, with high corrodedquartz content.

This 1· 6 m thick layer occupies a mid-slope positionand is considered overthickened. A corrected thicknessis 1·2 m.

At Sapphire Springs, Katikati, (N57/369709) a prom­inent O' 7 m thick tephra comprises 0·6 m of yellowbrown friable clay loam with creamy white halloysite­rich lobes and nodules resting on O· 1 m white firmcoarse ash streaked with manganese (Fig. 2). Thistephra is assumed to be Ohinewai Ash. A lower contactto a 3 em thick grey cemented layer passing to a redcemented paleosol is a further diagnostic criterion

72 NZ. JOURNAL OF GEOLOGY AND GEOPHYSICS, VOL. 21, 1978

FIG. 1-0hinewai Tephraat Walker Road, Te Kuiti,(N83/743809; 1971)showing prominent palecoloured lobes (halloysitedominant) and discretedark coloured "eyes"(allophane dominant).The basal coarse layer ofthis tephra is lensoid inform and provides a dis­tinct lower contact to athick blocky paleosol with­in tephric loess. The scaleis 1 m long.

(Ward 1967). At this site two welJ weathered thinnerbut clearly definitive tephras lie paraconformably overOhinewai Ash. This bedding pattern is repeated atTauranga, but here Ohinewai Ash rests on a darkbrown paleosol streaked with manganese.

East of Tauranga there are few reliable outcrops.Access to coastal cliffs at Little Waihi is impossibleand at Ohiwa difficult. At Opape, 15 km east of Opotiki,an excelJent section shows O· 5 m of yelJow brownfriable silt loam containing small halloysite noduleswith a thin but definitive coarse base resting on darkbrown clay (paleosol). This tephra meets the diagnosticrequirements for Ohinewai Ash. Because of its nowknown much greater distribution it is here proposed toraise the stratigraphic rank of Ohinewai Ash to tephraformation at the type section near Huntly, and also atreference sections in roadcuts that expose an overlyingparaconformable, subhorizontalJy bedded, sequence(2-5 m thick) of tephras and loess beds younging toRotoehu Ash (Vucetich & PulJar 1969).

SoURCE OF OHINEWA1 TEPHRA

Along an arc of radius c. 9Dkm linking Te Kuiti,Otorohanga, and Tirau the thickness of Ohinewai Tephrais respectively 1· 6 m, I· 4 m, and 1· 2 m (corrected).North and north-east from Tirau this tephra thinssystematicalJy; Te Poi, O' 8 m; Sapphire Springs, O' 7 m;Waihi Beach; (N53 & Pt 54/412926; 1%5), o·35 m;Tauranga, 0·4m; Ohiwa, 0·4m; Opape, 0·5m. Thethickness pattern precludes an eruptive source withinthe Rotorua and Okataina volcanic centres and suggestsa more southern source, possibly near Taupo. All thick­ness measurements arc considered minimal. The overlyingdefinitive paleosol (Ward's H3a Member) has beenexcluded because this soil is not proven to have been

developed within Ohinewai Tephra. On these un­corrected thickness plots a tephra volume in excess of100 krn" is indicated.

Ward (1967) was unable to attribute a source forOhinewai Ash. His thicknesses were measured withinthe limited are, Otorohanga northwards (and westwards)to Huntly.

AGE OF OHINEWAI TEPHRA

Ohinewai Tephra is not yet dated by the fission­track technique, but zircons are currently being extractedwith this intention. An age of between 120000 and210 DOD years B.P. for Hamilton Ash Formation wasdeter.nned by Chappell (1975) from his correlationof Bay of Plenty coastal terraces (BOP 2, BOP 3,BOP 4). Our independent estimates of the age ofOhinewai Tephra fall in this range.

1. Ohinewai Tephra overlies paraconformablyand is appreciably younger than terrace-forming thicktephras in Bay of Plenty, the Pahoia Tuffs (Pullaret al, 1973) and Little Waihi Formation (Chappell1975). These two are provisionally correlated as un­welded tephra-flow (pyroclastic flow) and tephra-fallcorrelatives, distal to Mamaku Ignimbrite and MatahinaIgnimbrite (Vucetich, Birrell &Pullar in prep.). Theseignimbrites are provisionally fission-track dated as200000 and 270000 years B.P. respectively (B. P. Kohnpers. comm.).

2. For Walker Road, Te Kuiti, an age estimateof c. 120000 years assumes a constant "positive netaccretion rate" of 65 mm per thousand years for"weathered" cover beds (calculated for measured thick­nesses between time planes), Rotoehu Ash, KawakawaTephra, and present day groundsurface. "Weathered"cover beds here include grossly altered fine grained

V UC ETICH et al. - OHINEWAI TEPHRA FMN

FIG. 2-0hinewai T ephra at Sapphire Springs( N n / 369709; 1974) show ing disti nct bedd ing in thelower most coarse unit (basal ) and a prominent palecoloured lobe (dominantly halloysite) contras ted wi ththe dar k uppermost unit ( containing allophane). Inthe middle of the section, below a thick promi nentlybedded teph ra, lensoidal cavities mark the base of twostrongly weathered rh yolit ic teph ras. Rotoehu Ash isconspi cuous as a white layer almost at the top of thesection. The scale is O· 9 m long .

andesitic tephra, dust (loess), and rhyoli tic ash butexclude ap preciably thick definitive rhyolit ic tephra layers(the products of short term eruptive episodes ) .

T his age estimate is in keeping with the per iod ofc. 100 000 years that W ard ( 1972) estima ted for theweather ing of Hamilton Ash Formation.

3. Th e form of weather ing of Ohinewai Tephraat Te Kuit i, Tirau, and Bay of Plenty localit ies has thecharacter of Ward's T e Uku Variant. At Te Kuiti theallophane zone has a very high natural mois ture content

73

and very low bulk density; at Tauranga the natura lmoistur e cont ent is lower and dry bulk density higher .T he basal halloysite layer is generally thicker in theBay of Plen ty area. T he weathered form of OhinewaiTephra depends on the proportion of allophane tohalloysite in the colloida l fraction. In the localitiesmentioned the wea theri ng character common ly diffe rsfrom older rhyolitic tephra, e.g ., Wai terimu Ash, inwhich the propor tion of allophane to hall oysite isappreciably lower. The Mount Curl Tephra dated asc. 230 000 years B.P. (Mi lne 1973) remains to beidentified wi thin these older teph ras, notably PahoiaTuffs and Littl e Wai hi Format ion.

4. W ith in Rang itikei Basin an unnamed, white,fine gra ined, rhyoliti c tephra 25-40 em thick is recog­nised to be younger than Mount Curl Tephra andprobably older than Burnadian loess (J. D . G . M ilnepers. comm.). This unnamed tephra with an est imatedage of c. 180000 years has a thickness in keeping with apredicted southern distribution of Ohinewai Tephraand may be a correlative.

From the above age estimates ranging from c. 120000to c. 180 000 years, an age of c. 150 000 years isinferred for Ohinewai Tephra,

REFERENCES

CHA PPELL, J. M. A. 1975 : Upper Qu aternary warpingand up lif t rates in the Bay of Plen ty and west coast,North Island, N ew Zealand, z-.;.2. Journal ofGeology and Geophysics 18 : 129-55 .

MILNE,.J. D. G. 1973 : Mount Curl Te phra, a 230000­year-old marker bed in N ew Zealand , and itsimpli cations for Q uatern ary chronology. N .Z.[ournal of Geology and Geophysics 16 (3):519-32.

PAIN, C. F. 197 5 : Some tep hra deposits in the south­west Waikato area, N orth Island , New Zealand.N.2 . Journal of Geology and Geopbysics 18 :541-50.

TONKIN, P. J. 1970 : Contorted strati fication within claylobes in volcanic ash beds, Raglan-Hamil ton region,New Zealand. Earth Science [ournal 4 (2 ) : 129- 40.

VUCETlCIJ, C. G.; PULLAR, W. A. 1969: Stratigra phyand chronology of late Pleistocene volcanic ashbeds in centra l North Island, New Zealand . N .2.Journal of Geology and Geophysics 12 : 784-837.

PULLAR, W . A .; BIRRELL, K. S.; H EINE, JANICE C.1973 : Explanatory note to accompany N.Z. SoilSurvey Report s 1 and 2; Age and dis tributi on ofLate Qu aternary pyroclastic and associated coverdeposi ts of centra l No rth Island, N .Z.

WARD, W . T . 1967 : Volcanic ash beds of the lowerWaikato Basin , N orth Island, New Zealand . N.2.Journal of Geology and Geophysics 10 : 1109- 35.

1972 : Ple istocene ash in the Wa ikato Basin,ages implied by changes in sea level. N .2. Journalof Geology and Geopbysics 15 : 678-85.