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UPPER JURASSICAND CRETACEOUSCONGLOMERATES INTHE TINUI—AWATOITOIDISTRICT, EASTERNWAIRARAPA (NOTE)M. R. Johnston a & P. R.L. Browne ba New Zealand Geological Survey, DSIR ,Nelsonb New Zealand Geological Survey, DSIR ,Lower HuttPublished online: 11 Oct 2012.

To cite this article: M. R. Johnston & P. R.L. Browne (1973) UPPERJURASSIC AND CRETACEOUS CONGLOMERATES IN THE TINUI—AWATOITOIDISTRICT, EASTERN WAIRARAPA (NOTE), New Zealand Journal of Geology andGeophysics, 16:4, 1055-1060, DOI: 10.1080/00288306.1973.10555241

To link to this article: http://dx.doi.org/10.1080/00288306.1973.10555241

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No.4 1055

TIPPER JURASSIC AND CRETACEOUS CONGLOMERATES IN THE TINUI-A W A TO ITO I DISTRICT, EASTERN

WAIRARAPA (NOTE)

M. R. JoHNSTON

New Zealand Geological Survey, DSIR, Nelson

and P. R. L. BROWNE

New Zealand Geological Survey, DSIR, Lower Hutt

(Receit·ed 15 September 1970; rel'ised 22 September 197 2)

AssrRACT

Eight pebble conglomerates, ranginb in age from Upper Jurassic to Upper Cretaceous, are present in the Tinui-Awatoitoi district, eastern Wairarapa. The commonest pebbles are of subangular to rounded greywacke, argillite, sandstone and siltstone ~imiiJr to, and no harder than, the Upper Jurassic and younger rocks exposed elsewhere in the district. The other pebbles are of small, well-rounded, highly indurated, commonly highly polished, greywacke, quartzite and distinctive igneous rock types. The commonest art! considered to have been derived from rocks cropping out in the district, and the llthers to be either !eworked from conglomerates older than Upper Jurassic, or derived from distant masses of igneous rocks being eroded from the Jurassic to late Tertiary.

INTRODUCTION

Conglomerates of Upper Jurassic and Cretaceous age, contammg well rounded and commonly highly polished pebbles, crop out in the Tinui­Awatoitoi District (Sheet N159 and pt Sheet N158, NZMS 1; Fig. 1) and elsewhere in the eastern Wairarapa (Ongley 1935). In this district the majority of the pebbles are greywacke and sandstone, a few (generally less than 10%) are of distinctive igneous or metamorphic rocks. The pebbles were examined to determine compositional differences between conglomerates of different ages, and, if possible, their origin. Numbers refer to samples lodged in the petrology collection, N.Z. Geological Survey, Lower Hutt.

STRATIGRAPHY

A good record of sedimentation, from Upper Jurassic to Recent, in the Tinui-Awatoitoi district is interrupted by a number of unconformities, some associated with conglomerates (Johnston in press). The stratigraphic positions and lithologies of the conglomerates sampled are given in Fig. 2, and their localities shown in Fig. 1.

N.Z. ]o11mal of Geology and Gwph)'ric.r 16 (4): 1055-60

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1056 N.Z. JoURNAL OF GEOLOGY AND GEOPHYSICS VoL. 16

5 , 0 miles

•'a' ' ' ' ' ' ' 1 '<\ •'a km

D Upper Tert1ary- Quaternary

m lower Tert1ary- Upper Cretaceous

D lower Cretaceous - Upper Jurassic

4B ConQior.nerate beds, collected from locahty 1nd1Cated by attitude symbol

y' Face known J(. Attitude Vert1cal

Y Face unknown /

FIG. 1-Generalised map of Tinui-Awatoitoi district and adjoining areas. Localities of samples named from 1 to 5C.

CONGLOMERATES

The conglomerates sampled occur either as thin lenses, (e.g., conglomerate 2), or continuous beds, some of which can be traced over 3 km, (e.g., con­glomerate l). The matrices in which the pebbles occur vary in induration ; in conglomerate l, the oldest, it is a hard greywacke (so defined on petrographic criteria) of the Torlesse Supergroup and in conglomerate 5C, one of the youngest, it is a soft glauconitic sandstone.

The conglomerates contain two types of pebbles: (a) Subangular to rounded pebbles. These are the most common, forming 90% or more of the total pebbles in all conglomerates except 1 and 2. They are lithologically similar to the rocks underlying the conglomerates. (b) Well-rounded pebbles (Table 1). These pebbles are smaller than type a, and are commonly highly polished. Several have small, random, nailhead scratches. In all but 1 and 2 they form small proportions of the conglomerate pebbles. Although none were counted, it appeared that conglomerates 5A, 5B

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No.4

LITHOLOGY

Well-bedded, alternating sandstone and s1ltstone commonly crossbedded, with thick grit and conglomerate beds.

Massive mudstone and siltstone with numerous concretions; local basal conglomerate

Indurated grey siltstone with scattered graded beds; basal conglomerate

Thick greywacke with m•nor conglomerate, grit and graded beds.

Graded greywacke and argill1te beds, with scattered thick greywack and mmor conglomerate and sp1hte; rare hmestone

e

lenses

NoTE- TINUI CoNGL:)MERATE

CONGLOMERATE FORMATION GROUP

5A

:r~•o.:.~

58 % •r.J~o

I

ToM,; } TINUI

ocal unconform•ty

Springhill

~ I

MANGA-

ocal unconformity PURUPURU

Maringi

Ooaoo3aoo regional unconformity

2~ Taipo

TORLESSE SUPERGROUP

1 0~o0oaa • Waewaepa

FIG. 2-Stratigraphic position of the conglomerates.

1057

N.Z. STAGE OR AGE

Haumurian

Piripauan

Teratan

Mangaotanean

Arowhanan

Ngaterian

Motu an

Urutawan

Lower Cretaceous

Uppermost Jurassrc

Upper Jurassic

and 5C had significantly fewer pebbles of this type than conglomerates 3, 4A and 4B. Small rounded pebbles also occur in Tertiary conglomerates and recent gravels in the district.

Most of the well-rounded pebbles (Table 1) are of rhyolite, ignimbrite, tuff, and relatively rare plutonic rocks; sedimentary pebbles are common locally. Many of these pebbles typically have their ferromagnesian minerals altered to chlorite and minor pyrite, and the groundmass to quartz, calcite and epidote. Quartz veins are present in a few pebbles.

Similar non-sedimentary pebbles occur in Mesozoic conglomerates which outcrop in the Taitai district, East Cape (Bartrum in Morgan 1928), Evans Bay, Wellin_cston (Lauder 1962) and east of Palliser Bay (Sallas & McKay 1906). However, a Jurassic conglomerate at Kawhia on the west coast of the North Island contains pebbles of keratophyre, andesite, and high-grade schist which MacDonald (1954) thought were derived from Paleozoic rocks similar to those of the South Island.

CoNCLUSIONS

Two types of pebbles are distinguishable in the conglomerates cropping out in the Tinui-Awatoitoi district. The subangular to rounded pebbles, con­sisting dominantly of greywacke, argillite; sandstone, and siltstone, were

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Con

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TAB

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dist

rict

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59/4

7679

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8/40

1687

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1\:1

59/4

9079

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ge

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m­

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ick

----------~--~

NZ

GS

P

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co

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n

um

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(m)

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14

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met

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(mm

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70

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120

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1060 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VoL. 16

probably derived from rocks cropping out at no great distance. The well­rounded pebbles have no obvious local source. These pebbles

(a) are hard and commonly highly polished; (b) are predominantly metamorphosed volcanics and compositionally

different from rocks cropping out on the east coast of the North Island;

(c) occur in all conglomerates sampled, and have also been found in those of Cenozoic age ;

(d) although harder than the subangular to rounded pebbles, are con­sistently smaller in size.

Whether the well-rounded pebbles were derived from distant masses of volcanic and granitic rocks being eroded from the Jurassic to late Tertiary, or whether recycling of pebbles from older to younger conglomerates took place has not yet been proved.

ACKNOWLEDGMENTS

The writers thank Mr T. L. Grant-Taylor, Drs D. Kear, M. G. Laird, R. P. Suggate, W. A. Watters, and A. Wodzicki, N.Z. Geological Survey, and Mr W. R. Lauder, Victoria University of Wellington, for constructive criticism of the manuscript.

REFERENCES

JOHNSTON, M. R. (in press): Sheet N159 and pt Sheet N158-Tinui-Awatoitoi '"Geo­logical map of New Zealand 1: 63 360". N.Z. Department of Scientific and Industrial Research, Wellington.

LAUDER, W. R. 1962: Notes on the greywackes near Wellington City. N.Z. Joumal of Geology and GeophyJicJ 5 (4): 626-9.

MAcDoNALD, H. A. H. 1954: Petrography of some Jurassic conglomerates at Kawhia, New Zealand. TralmtctionJ of the Royal Society of New Zealand 82: 223-30.

MORGAN, P. G. 1928: Pebbles of Taitai Conglomerate. In 0NGLEY, M., MACPHERSON, E. 0.: The geology of the Waiapu Subdivision, Raukumara Division. N.Z. Geologica/ Surt·ey Bulletin 30: 53.

ONGLEY, M. 1935: Eketahuna Subdivision. Annual Report of the N.Z. Geological Sun•ey 29: 1--6.

SaLLAS, W. )., McKAY, A. 1906: "The Rocks of Cape Colville Peninsula, Auckland, New Zealand". Vol. 2. Government Printer, Wellington. 215 p.

© Crown Copyright

A R. SHEARER, GOVERNMENT PRINTER, WELLINGTON, NEW ZEALAND-1971

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