3. OBJETIVOS
5.4.1. Planta de tratamiento de agua potable
thin tephras
� ? / / / ..
• - - - - -Tephra
" " " " N1 29/934286F i gure 3 . 3 Strati graph i c col umns demonstrati ng l i thol ogic rel ationships of soi l -forming parent material s on upl ifted marine terraces near Hawera . The col umns s how ( i ) the cons i stent upper and l ower uni ts and marker l api l l i throughout the reg i o n , ( i i ) l oca l i ti es where the 20 , 000 year B . P . Aokautere Ash i s preserved and ( i i i ) the wi despread occurrence of underl yi ng dune sand nea r the coast, as shown at Turuturumokai Pa , Ba l l Road and Mokoia .
O'l ex:>
towards Mt Egmont , accompan i ed by an i ncreas i ng abundance and s i ze of l api l l i . The l ower un i t does not appear to thi cken appreci ably towards Mt Egmon t . Few l api l l i occur wi th i n the l ower un i t , except fo r an unnamed l api l l i preserved at the base of the uni t , whi ch i s an i mporta nt ma rke r h ori zon th roughout the H awera reg i on ( Fi g . 3 . 2 ) .
Th i s marker l ap i l l i res ts on a 2m th i ck ' s peck l ed ash ' near Mt Egmont , wh i ch th i ns rap i dl y towa rds Hawera ( Fi g . 3 . 1 ) and i s absen t
from the s ampl e s i te south-east o f Hawera . Here the marker l ap i l l i
res ts on l oess -l i ke materi a l whi ch becomes i ncrea s i ngly more sandy towards the underlyi ng dune sand . I t i s wi th i n th i s sandy materi a l , ( and unde r the ' s peckl ed ash ' where present ) that the Aokautere As h dateq ( NZ1 056A) at 1 9850 ± 310 yea rs B . P . i s preserved ( Fi g . 3 . 3 ) . The presence of the Aokautere Ash shows th at accumul ati on of the underlyi n g col d c l i mate dune sands ceased approximately 20 , 000 yea rs ago i n the Hawera regi on . Rel ati vel y th i n teph ra and l oess un i ts wi th i n the dune s ands i ndi cate that there may have been s eve ral dune
s and advances before 20 ,000 years B . P . Wood , i denti fi ed as ei ther
totara , P . h a l l i or from a l i gn i te
beneath the dune sands , has been dated ( NZ39 70B ) at > 43 , 900 years B . P . The dune sands a re absent from the o l d mari ne terraces , and the ,
marker l api l l i rests on e i ther a th i n uni t of ' s peck l ed as h ' o r l oess whi ch i n turn ove rl i es a <1 m th i ck pal eoso l ( Fi g . 3 . 3) .
Pa rameter Vari ation W i th Depth
Vari ati ons i n the g rai n s i ze paramete rs wi th depth ( Fi g . 3 . 4 ) confi rm the exi s tence of the two l i thol ogi cal un i ts recogn i zed on macros cop i c phys i ca l features . Parameter val ues remai n un i form
th roughout the upper un i t to 75 cm depth . Bel ow 75 cm depth the re i s a marked change i n parameter va l ues across the bounda ry i nto the l ower un i t where mean grai n-s i ze decreases , sort i n g i mproves and kurtos i s
Ap Bw1 Bw 2 · 2Cu 1 2Cu 2 0·2 0·4 0·6 0·8 1 ·0 1·2 1·4 M EAN ( M z ) .... ... ... 20fJm SORTING ( o-,> (m) 0·2 0 ·4 0 ·6 0 ·8 1 · 0 1 ·2 1 ·4 0·6 1·0 1·4 2·0 KURTOS I S ( KG )
Fi g . 3 . 4 Vari a t i on i n gra i n s i ze parameters for the Egmont l oam profi l e a s a functi on o f depth .
Val ues for the underl yi ng dune sand a re pl otted near the base of each col umn . "-J 0
val ues become much h i gher . I n the l owermost 30 cm of the l ower uni t , th e re i s evi dence o f mi xi n g wi th the coars e r , un de rl yi ng dune sand to produce a poorly sorted end-product wi th a di s ti nctly b i modal character s h own by the much l ower k u rtos i s val ues .
The c umul ati ve curves fo r s ampl es from the upper and l owe r uni ts and the dun e sand a re compa red , together wi th cu rves from a No rth Ameri can and a New Zea l and l oes s i n Fi g . 3 . 5 . A repres entati ve cumul ati ve curve fo r the l ower uni t ( curve B ) i s ·s i mi l ar i n s ha pe to both l oess curves ( C , D) and the dune sand curve ( E ) , i ndi cati ng thei r cognate aeol i an o ri gi n . I n contra s t , th e repre s entati ve cumul ati ve cu rve for the upper uni t ( A) s hows that i t i s much more poorl y sorted , i s pol ymoda l and has a coa rser gra i n s i ze more cons i s tent wi th nea r s o u rce vol can i c l asti c depos i ts s uch as , for examp l e , the I ng l ewood Tephra ( curve F) , a 4000- 5000 years B . P . tephra - fa l l from Mt Egmont. Q ua rtz
Tota l quartz conten ts wi th i n th e Egmont p ro fi l e vari ed from 30% i n the l ower un i t to 3% i n th e uppe r uni t . Th e re was a marked change i n q ua rtz abundance i n the s i l t fra cti ons across the bounda ry between th e two uni ts ( Fi g . 3 . 6 ) . The boundary i s i n te rp reted as refl ecti ng a change from condi ti ons of rel ati vel y ab undant i nter- regi ona l quartz s upply duri n g the accumul ation of the l ower uni t to a very res tri cted i n ter- regi ona l quartz s upply duri ng the a ccumul ati on of the upper uni t .
I n contrast to the s i l t fra cti ons the ve ry fi ne s and fracti on s howed no marked change i n quartz ab undance and th i s was uni formly l ow ,
< 1% ( Fi g . 3 . 6 ) . Quartz was rare or abs ent i n the coars er s and
fract i ons . Thes e data s uggest that the sand-s i ze fracti ons have been conti nuous l y i ncorporated i nto the profi l e from an i n tra- regi onal , q ua rtz defi ci ent source , p robabl y andes i ti c tep h ra domi nated .
1 00 7 5 ... z w 5 0 0 a: w Q. w > ... � ..J :J � 2 5 :J 0 F i g . 3 . 5 ,, ,'I , I , , I , I ,, . . , I . , I y ,. I ,, I , . / . I / . I ,, . I / . I ,, I ''F . . I / . I . I I I I I I E .' A • I 0 . , I . . I . . , I I I 2RJ
I
I
1 25,um 20J.!m G R A I N S I ZEGra i n s i ze cumul a ti ve curves for :
A , Egmont u pper un i t ( tephra ) ; B , Egmont l ower un i t ( tephr i c l oess ) ; C , Tokomaru s i l t l oam l C wg· ho ri zon ( after Pol l o k , 1975) ; D , North Amer i can l oess ( after Smi th , 1942 ) ; E , Aeo l i a n dune sand from benea th the Egmont profi l e ; F , I ng l ewood Tephra . The hatched area represents the range of quartz cumu l ati ve curves from the Egmon t profi l e . Curves are cal cul a ted on a c l ay free basi s .
I
Th e q ua rtz gra i n-s i ze cumu l ati ve curves fo r a l l s ampl es from the Egmont p rofi l e fel l wi thi n a na rrow fi el d ( cu rve G , Fi g . 3 . 5 ) . They a re s i mi l ar i n s h ape and s how a s i mi l ar grai n-s i ze di s tri buti on to the l oess curves , i ndi cati ng that th e quartz was trans ported i nto the profi l e by aeol i an p rocesses . The s i mi l ari ty of a l l q ua rtz c umul ati ve curves th rou ghout the profi l e s how that thi s tra ns port mechan i sm operated uncha nged duri ng the accumu l ati on of the enti re profi l e , be i ng i ndependent of both th e vo l cani c i nfl uences and changes i n q uartz s upp l y .
Abundance and P rovenance
Oxygen i s otope ab undance of q ua rtz from representati ve s ampl es of the upper and l ower uni ts was determi ned i n an attempt to defi ne th e
provenance of the q ua rtz i n the profi l e . S i mi l a r o 18o val ues we re obta i ned for the coarse and medi um s i l t fra cti ons i n both s ampl es ( Ta b l e 3 . 1 ) .
Tabl e 3 . 1
The pri nci pal d i fference occurs be tween th e fi ne s i l t
Oxygen i sotope abundance of quartz i s ol ated from sel ected s i ze fracti ons , Egmont l oam
Upper un i t Lower uni t ( teph ra ) ( tephri c l oess) *o 18o 0/oo o 18o 0/oo Coars e s i l t 1 3 . 6 ± 0 . 3 13 . 9 ± 0 . 3 Medi um s i 1 t 13 . 9 % 0 . 3 1 4 . 0 ± 0 . 3
Fi ne s i l t 14 . 5 ± 0 . 3 1 5 . 0 ± 0 . 3 Quartz from Parahaki rhyo l i te ( vol can i c ) 9 . 6 Cretaceous che rt ( auth i geni c ) 32
A p Bw1 Bw 2 2Cu1 2 Cu2 3C Fi gure 3 . 6 0·8 1 ·0 1 · 2 1 ·4
coa rse silt
0·2 0·6 2 4 6 8 5 1 0 1 5 20
QUARTZ P E RC E NT TOTAL S A M PLE
very fine sand 0 ·2 0·4 0 ·6 0·8 1 ·0 o · 4 1 · 2
Di stri buti on of quartz i n pa rti cl e s i ze fractions a s a functi on of depth . The quartz content of the underl yi ng dune sand i s s hown at the base of each col umn .
-....,J �
fra cti on and the other two s i l t fra cti ons . The data s u g gest tha t the fi ne s i l t fracti on , a mi no r component of the q uartz content of the profi l e , i s o f aerosol i c o ri gi n ( Re x et al ., 1 969 ) , whi l e th e coars e and medi um s i l t fracti ons are predomi nantly deri ved from mo re res tri cted regi ona l s o u rces , refl ected i n the l ower o 18o val ues .
The oxygen i sotope abundance data s how th a t the q ua rtz i n al l s i ze fracti ons ana lysed has nei the r an i gneous ( cf . Pa rahaki rhyo l i te , Tab l e 3 . 1 ) , nor an auth i geni c ( cf . Cretaceous ch e rt , Tabl e 3 . 1 ) o ri gi n . Th i s precl u des the quartz bei ng e i ther deri ved from andes i ti c teph ra
or by � s i tu formati on i n the s oi l .
Most of the fel ds pa r present is a ca l ci c andes i ne , p robab l y de ri ved from M t Egmont . Th e coa rs e s i l t fra cti on s hows fl uctuati ons i n fel ds pa r con tent i n the upper un i t ri s i ng to a h i gh e r , un i form l evel i n th e l ower un i t ( Fi g . 3 . 7 ) . The peaks i n the upper un i t are
s uggesti ve o f major andes i ti c erupti ve epi sodes . The l es s vari ab l e fel ds pa r content i n the l ower un i t i ndi cates l i tt l e materi al bei ng di rectly i nco rporated from e rupti ons .
I n the very fi ne sand fracti on there i s l i ttl e fe l ds pa r va ri ati on wi th depth , i ndi cati n g , as i n the q ua rtz , th at th i s fract i on was deri ved
i n tra- re g i on a l l y and i nco rpora ted at a s teady ra te i n to the p ro fi l e . Andes i ti c Gl ass
' Andes i ti c g l ass ' w i th i n the Egmont profi l e cons i s ts o f mi crol i tes of fel ds pa r , mafi c mi neral s and t i tanomagneti te i n a gl assy matri x . I t approxi mates to the g roundmas s i n andes i ti c rocks .
Andes i ti c g l ass abundance i n the l ower un i t i s at a cons i s tently
1 ow 1 eve 1 ( Fi g . 3 . 7 ) . I n the upper uni t th ere i s a s teady i ncrease i n abundance overp ri nted by major peaks i n both the coa rse s i l t and very
Ap Bw1 Bw 2 2Cu 1 2Cu 2 3C F i g . 3 . 7 0·2 0·4 o:6 0·8 1 ·0 1·2 1 ·4 0·8 1 ·6 5 1 0 1 5 20
RHYOLI T I C G L ASS P LAGIOCLASE
2 4 6 8 1 0
ANDESITIC GLASS PERCENT TOTA L SAMPLE
- -- - --- -- --- --- --- --- --- -- --- - - - ---· -
l
2 m]
1 0,450 :!: 100 y r B . P. ( NZ 31 53 B ) - TEPHRA c::J PEAT NGAERE S\n/AMP ( N 1 19/ 901469)Di s tri buti on of rhyol i ti c gl a s s , pl a g i oc l ase fel ds par and andes i ti c gl a s s as a functi on of depth . The sol i d l i nes represent the coarse s i l t fraction and the broken l i nes the very fi ne sand fracti on .
fi ne s and fracti ons ( Fi g . 3 . 7 ) , whi ch represent s peci fi c e rupti ve epi sodes . Di ffe rences i n peak pos i ti ons between the two s i ze fracti ons probably refl ect va ri ati on i n gra i n s i ze di s tri buti on of e rupted materi al s . A po i nt on the axi s of teph ra di s pers a l woul d tend
ra.ther one o'"'
to be domi nated by coarse r teph ra�than
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the l ateral and termi nal ma rgi ns of di s pe rs a 1 .Mafi c Mi nera l s
Th ree ma fi c mi neral s occur i n s i gni fi cant amo unts ; augi te , hornb l ende and hypers thene . I n th e coars e s i l t fracti on both augi te and hornbl ende s how a h i gher abundance i n the l owe r un i t , p robably as a res ul t of h i gher res i s tance to weatheri ng re l at i ve to andes i ti c gl ass i n the source area pri or to the acc umul ati on of th e l owe r uni t . Both mi nera l s show only one major e rupti ve peak i n the upper un i t ( Fi g . 3 . 8 ) , co rres pond i ng to the l owermost peak i n the fel ds pa r and andes i ti c gl a s s abundances . Au gi te and h o rnbl ende show l i ttl e va riati on i n the very fi ne s and fra cti on . Hype rs thene h as a s i mi l ar di stri buti on pattern to the andes i ti c gl ass ( Fi g . 3 . 8 ) .
Gl ass
Rhyol i ti c gl as s i s a mi nor component of th e mi nera l s presen t , b u t i ts vari a t i on wi th depth i s a record of the i n fl uence of rhyo l i ti c teph ras from the Central North I s l and . Two rhyo l i ti c gl ass abundance peaks occur i n the coarse s i l t fraction , both wi th i n the upper un i t , whi l st only one peak i s p resent , at the top o f th e uppe r uni t , i n the very fi ne s and fract i on ( Fi g . 3 . 7 ) .
Mi nor Mi nera l Consti tuents
Apa rt from q uartz , s evera l mi nor ( < 1%) mi n era l s occur wh i ch are