Clays and Clay Minerals, Vol.46, No. 2, 2(M-214, 1998.
G E O C H E M I S T R Y A N D P A R A G E N E S I S OF H E U L A N D I T E C E M E N T S IN A
M I O C E N E M A R I N E F A N - D E L T A S Y S T E M OF THE P O H A N G B A S I N ,
REPUBLIC OF K O R E A
JIN HWAN NOH
Department of Geology, Kangwon National University, Chuncheon 200-701, Republic of Korea
A b s t r a c t - - I n the Pohang area of Korea, heulandite occurs as cements in conglomerate and sandstone of
a Miocene marine fan-delta system resting on Eocene dacitic volcanics. Three types of heulandite cements
are distinguishable in the fan-delta sediments on the basis of texture, chemical composition and authigenic
mineral association. The earliest type I heulandite (Si/(AI+Fe): 3.5-3.8) occurs as microcrystalline (1030 p~m) in situ crystallites that replace volcanic matrix and are intermixed with early-formed smectite.
Type II heulandite (Si/(AI+Fe): 3.2-3.6) occurs as medium-grained (30-60 ixm) crystal aggregates rimming intergranular cavities. Type III heulandite (Si/(AI+Fe): 3.6-4.1) is the last to form and is a composite
phase of beulandite--clinoptiloite, which occurs as unusually coarse (50-200 p,m) single-crystal cement
associating with late-formed smectite and hematite.
These characteristic heulandite cements were formed by alteration of volcaniclastic sediment during
shallow burial (burial temperature: 4 0 - 6 0 ~ and uplift in marine pore fluid diluted by meteoric water.
Sr isotope data for heulandite II (8751"]868r: 0.706565-0.706598) and heulandite III (875r/86Sr: 0.7073470.707432) indicate that the pore fluid was progressively mixed with meteoric water during burial and
uplift but the Ca in the pore-filling heulandites has been derived mainly from dissolution of carbonate
cements. Heulandite III, heulandite-clinoptilolite, was formed with an unusual coarsening and chemical
zoning at somewhat diluted and disequilibrium conditions caused by the migration of oxygen-rich meteoric water during or after uplifting.
Key Words----Burial, Fan-delta, Heulandite Cement, Heulandite-Clinoptilolite, Meteoric Water, Sr Isotope, Uplift, Volcaniclastic.
INTRODUCTION
Zeolite o c c u r r e n c e s in s e d i m e n t a r y r o c k s are rem a r k a b l y similar in that they are c o m m o n l y f o u n d as
i n s i t u alterations o f the glassy m a t r i x o f p y r o c l a s t i c
rocks. T h e m o s t c o m m o n type o f zeolites that occurs
in v o l c a n i c l a s t i c r o c k s is the h e u l a n d i t e - g r o u p , that is,
h e u l a n d i t e a n d clinoptilolite, o f w h i c h the u p p e r r a n g e
o f t h e r m a l stability t e n d s to b e less t h a n a b o u t 100 ~
(Iijima a n d U t a d a 1971; S u r d a m a n d B o l e s 1979; N o h
a n d B o l e s 1993). G r o u n d w a t e r control o f clinoptilolite f o r m a t i o n has b e e n r e c e n t l y r e p o r t e d in tuffaceous rocks undergoing very low-temperature (27-55
~ burial d i a g e n e s i s ( L a n d e r a n d H a y 1993). In m o s t
cases, h o w e v e r , the h e u l a n d i t e - g r o u p zeolites f r o m
tuffaceous r o c k s are usually f o u n d as v e r y f i n e - g r a i n e d
( m o s t l y less t h a n 10 p.m) aggregates, c o m m o n l y acc o m p a n y i n g smectite (Hay 1966; S u r d a m a n d B o l e s
1979; N o h a n d B o l e s 1989; L a n d e r a n d H a y 1993;
N o h a n d B o l e s 1993). It is well k n o w n that h e u l a n d i t e
a n d clinoptilolite are nearly isostructural b u t v a r y i n g
in c h e m i c a l c o m p o s i t i o n (Gottardi a n d Galli 1985). A n
a s s o c i a t i o n o f h e u l a n d i t e a n d clinoptilolite in the s a m e
r o c k or t h e i r single c o m p o s i t e phase, s u c h as a z o n e d
crystal, h a s n o t b e e n f o u n d in a n y r o c k system, t h o u g h
a c o m p o s i t i o n a l l y z o n e d clinoptilolite was r e p o r t e d in
the B a r s t o w tufts ( S h e p p a r d a n d G u d e 1969).
C o a r s e - g r a i n e d s e d i m e n t s are o f s o m e interest bec a u s e they h a v e initially h i g h p o r o s i t y - p e r m e a b i l i t y
Copyright 9 1998, The Clay Minerals Society
w h i c h c a n allow c o n s i d e r a b l e m a s s transfer. H o w e v e r ,
v e r y little is k n o w n a b o u t the d i a g e n e t i c m i n e r a l facies
as well as zeolite o c c u r r e n c e in c o a r s e - g r a i n e d sedim e n t s o f alluvial fan a n d f a n - d e l t a systems. Various
a u t h i g e n i c h e u l a n d i t e s w e r e r e c e n t l y identified in the
middle Miocene conglomeratic formation of a marine
f a n - d e l t a s y s t e m in the P o h a n g area, Korea. A n unusual characteristic o f the d e p o s i t is that h e u l a n d i t e
a n d a c o m p o s i t e p h a s e o f h e u l a n d i t e a n d clinoptilolite
o c c u r as c o a r s e - c r y s t a l l i n e i n t e r g r a n u l a r c e m e n t s in
the n o n - g l a s s y , b u t partially volcaniclastic, c o n g l o m erate a n d sandstone. T h i s study illustrates a n e w m o d e
o f occurrence, p a r a g e n e s i s a n d g e o c h e m i s t r y o f the
heulandite and heulandite-clinoptilolite cements from
a f a n - d e l t a system, u s i n g textural, h i g h - t e m p e r a t u r e X ray diffraction ( X R D ) , c h e m i c a l a n d Sr isotope analyses.
MATERIALS AND METHODS
R e p r e s e n t a t i v e s a m p l e s o f c o n g l o m e r a t e a n d sandstone w e r e collected f r o m r o a d s i d e c u t t i n g s t h r o u g h
t r e n c h i n g o f o u t c r o p surface to a d e p t h o f a b o u t 2 0 30 cm. S o m e u n c o n s o l i d a t e d s a m p l e s w e r e o b t a i n e d
b y i m p r e g n a t i o n w i t h q u i c k - c u r i n g e p o x y in the field.
F o r the isotopic a n a l y s e s o f h e u l a n d i t e c e m e n t s , purified s a m p l e s w e r e o b t a i n e d by: 1) d i s a g g r e g a t i n g o f
s a m p l e s b y light c r u s h i n g a n d ultrasonic v i b r a t i n g in
distilled water, 2) o b t a i n i n g a 1 0 0 - 3 0 0 m e s h size frac204
Vol. 46, No. 2, 1998
Heulandite cements in fan-delta system
:::::::::::::::::::::::
205
LEGEND
I
[ Au.uv,u.
POHANG
FORMATION
>rr
<~-- ~
~ 6 # & ". .~-o;
": -~
~ : ~ . ?*'(~i.
. . , , . ,r
9
%
"
, F " ,,~ ~
CHUNBUK
FORMATION
RHYODACITE
.
uc=..MI
.~
~ .*E..~..
.~,.;~:~
~ ~0
~
~
FORMATION
_
.,p 9 'o
SOUTH KOREA
~'.-.-.~.~..~%..:.
-'~ : ' ~,?o'
: %..~~""t~a:i. ~;?,~~,,~::~,;';~
!~'~
"
~
~ ~ ~ - ~
t 3+',0'.
Pusan
0
I
Figure l.
100km
I
Geologic and index maps of Pohang area.
tion by wet sieving, 3) heavy liquid separation by dibromomethane (CH2Br2, S.G.: 2.48) and 4) further purification by hand-picking under the binocular microscope. Separation quality of the samples was optically
checked by the preparation of grain sections with immersion oil. All the samples treated with heavy liquid
and immersion oil were washed by acetone, 1:1 solution of ethanol and water and liquid soap, followed
by drying at the temperature less than 40 ~ in an
oven.
Petrographic observations are based on more than
200 thin sections of sandstones and sandy matrix of
conglomerates. Identification of bulk mineralogy and
concentrated authigenic phases such as heulandite and
smectite were based on XRD analyses. A phase transition of heulandite at elevated temperatures up to 600
~ was traced by using high-temperature XRD apparatus at the heating rate of 10 ~
Heulandite and other authigenic minerals were analyzed by an electron microprobe with 5 wavelength
dispersive spectrometers (WDS) at conditions of 15
kV and 10 nA with a beam size of 5 t~m. The 875r86Sr
ratios were measured at Korea Basic Science Institute
on a VG 54-30 thermal ionization mass spectrometer.
In addition, strontium concentrations for heulandite,
carbonates and volcanic rocks were determined by an
atomic absorption spectrophotometric analysis.
GEOLOGIC SETTING AND LITHOLOGY
The Pohang Basin located in the southeastern margin of the Korean Peninsula, that is, the Pohang area,
consists of the Miocene Yeonil Group (Figure 1). The
Pohang Basin has undergone rapid subsidence (about
700 m/my) during middle Miocene time and was uplifted in the late Miocene (Chough and Barg 1987;
Hwang and Chough 1990). The basement of the basin
is composed of Cretaceous granite, hornfelsic metasedimentary rocks (Banyawol Formation) and Eocene
extrusives (K-Ar age: 41.77-46.24 Ma) of dacitic
composition (Noh 1994). The Eocene volcanics
(rhyodacite, dacite and subordinating tufts) underlie
and are in contact with Yeonil Group by faulting and
unconformity.
The Yeonil Group dips gently eastward and consists
of the Chunbuk Formation and overlying Pohang Formation. The thickness of the Miocene sedimentary
rocks has been estimated at between 600 and 800 m.
The Chunbuk Formation, whose thickness ranges from
300 to 500 m, consists mostly of coarse-grained epiclastic units, including agglomerate, gravelstone, conglomerate and pebbly sandstone. The formation was
deposited as a fan-delta (Chough et al. 1990; Hwang
and Chough 1990). Stratigraphy and lithofacies of the
fan-delta system were recently documented by Hwang
(1993). The proximal facies of this system is nonmar-
Noh
206
Clays and Clay Minerals
Table 1. Different heulandite cements and their characteristic features.
Type
Heulandite I
Heulandite II
Heulandite III
Crystal size
10-30
ixm
30-60
~m
50-200
p~m
Texture
Microcrystalline random
aggregates
Cavity-rimmings & interstitial fillings
Single-crystal intergranular cement
Si/(A1+Fe)
Comments
3.8-3.8 Rare, in situ crystallites of glassy matrix, residuals in
calcite cement
3.2-3.6 Abundant, undulatory extinction, sometimes with smectite coatings
3.6-4.1 Common, mostly zoning, composite phase of heulandite
and clinoptilolite, smectite inclusion common, sometimes with hematite coatings
ine and coarser-grained. At least 3 small-scale fans are
locally superimposed to form composite discontinuous
stratigraphic units of terrigenous sediments. The overlying Pohang Formation is composed of fine-grain pelagic to hemipelagic marine sediments.
Sediments of the Chunbuk Formation were undoubtedly derived from the Cretaceous basement rocks
and the Eocene volcanic masses that are exposed in
the western margin of the study area (Noh 1994). Outsized coarse clasts consist mostly of the Cretaceous
granite and metasedimentary rocks such as hornfels.
Pebble- to cobble-sized detritus of volcanic origin are
also found in the fan-delta system, but their presence
is locally limited and uncommon.
In comparison to the low volcanic proportion in
pebbles and cobbles of conglomerates, sandstones and
sandy matrix of conglomerates of the fan-delta formation usually contain higher amount of volcanic detrims. Volcanic rock fragments made up mostly of the
Eocene rhyodacite, dacite and basalt are commonly
included in sandstones and sandy matrix of conglomerates, together with some phenocryst fragments such
as corroded quartz and euhedral feldspar grains indicative of volcanic origin. But the proportion of these
volcanic detritus is no more than 10% in point-count
analysis, In addition, no type of pyroclastic material,
such as glass shards and pumice fragments, is found
in these fan-delta sediments.
Sandstones and sandy matrix of conglomerates of
the fan-delta system range in composition from lithic
arkose to feldspathic litharenite. These are characterized by a subequal proportion of quartz and feldspar
and a dominance of alkali feldspars, made up of orthoclase, sanidine and perthite, over plagioclase. Mafic
mineral grains are nearly absent, but rare biotite flakes
are locally common.
The Pohang Formation, 2 0 0 - 3 0 0 m thick, is composed mainly of mudstone and shale. Thin beds of
channelized sandstone are embedded locally in the formation, but are very limited in occurrence. Siliceous
mudstone whose matrix chiefly consists of opal-CT
and authigenic quartz is dominant in the formation.
Several units of siliceous shale, which usually contain
abundant plant fossils, are intercalated in the formation, and are commonly interbedded with the mudstone in the upper part of the formation. Dolomite con-
cretions ranging from 1-3 m in diameter are common
in the mudstone units and occur in more than 10 horizons throughout the formation. Diatomaceous beds
are sparsely interbedded in the middle to upper part of
the formation.
O C C U R R E N C E OF H E U L A N D I T E A N D O TH ER
AUTHIGENIC MINERALS
Heulandite occurs as coarsely crystalline (0.01-0.2
mm) cement in conglomerate and sandstone of the
Chunbuk Formation. Compared to other heulandite occurrences in tufts and volcaniclastic sandstones (Hay
1966; Surdam and Boles 1979; Noh and Boles 1993),
the heulandite from the fan-delta system is unusually
coarsely crystalline. The heulandite cements are commonly found in all the coarse-grained fan-delta facies,
but the zeolite is locally absent in reworked channelized sandstone facies and transitional facies of the
fan-delta system. Smectite and calcite instead occur
there as the main authigenic minerals.
Heulandite is mostly found as coffin-shaped crystals
or crystal aggregates within intergranular pore spaces.
The heulandite cements can be divided into 3 types on
the basis of texture, crystallinity and the mode of occurrence: 1) early-formed microcrystalline (10-30
txm) aggregates (heulandite I), 2) intermediate-formed
(30-60 Ixm) crystals-rimmings (heulandite II) and 3)
late coarsely crystalline (50-200 p~m) intergranular cements (heulandite III) (Table 1). The 3 types do not
show any striking lateral zoning in relation to lithology
and sedimentary facies, although vertical zoning could
not be checked due to lack of available core samples.
Heulandite I is relatively rare, compared to heulandite 1I and III. The heulandite I is found as in situ
replacements and/or precipitates at the expense of interstitial glassy and clayey matrix (Figure 2A). Petrographic observations indicate that dacitic to rbyodacitic groundmass materials appear to be major constituents of the original matrix in the zeolite-containing
sandstone and conglomerate. Other pyroclastic substances such as glass shards are not found in the matrix. The complexity in the composition of the original
matrix seems to be related to weathering effects and
diverse sediment input source prior to deposition.
However, despite the fact that the fan-delta sediments
are not typical volcaniclastic deposits, the mode of oc-
Vol. 46, No. 2, 1998
Heulandite cements in fan-delta system
207
Figure 2. Photomicrographs showing the mode of occurrence of heulandite cements (heulandite I and II) in conglomerates
and sandstones in crossed nicols (Scale bars are 0.1 mm). A) Fine-grained heulandite (h) cements (heulandite I) formed at
the expense of incipient of glassy matrix (g, dark) between detrital grains of quartz (q) and volcanic rock (r). B) Heulandite
(h, heulandite I) remnants replaced by calcite (c) cements: quartz (q) K-feldspar (k). C) Crystallization of medium-grained
heulandite (h, heulandite II) rimming framework grains of K-feldspar (k), quartz (q) and clinozoisite (z). D) Cavity-filling of
heulandite (h, heulandite II) within deformed biotite flakes (b), and detrital grains of quartz (q) and volcanic rock (v).
currence o f heulandite I is similar to those reported in
volcaniclastic sandstones from other areas (Surdam
and Boles 1979; N o h and Boles 1993). The heulandite
I occurs locally as remnants in the calcite-cemented
sandstone and conglomerate (Figure 2B), which implies an early formation of the heulandite.
Heulandite II, the most c o m m o n type in the study
area, occurs as rims and fillings o f interstitial cavities
(Figure 2C). S o m e t i m e s it occurs as a coarse-grained
open-space filling within d e f o r m e d m i c a flakes (Figure
2D). Heulandite II usually does not a c c o m p a n y smectite, but if smectite is present in the heulandite aggregates, it occurs as a late coating on the heulandite (Figure 3A), This indicates that heulandite II formed prior
to the late-formed smectite precipitation. Equigranular
euhedral crystals that exhibit undulatory extinction are
characteristic of heulandite II.
Heulandite III, which is compositionally corresponding to a heulandite-clinoptilolite phase, occurs
as v e r y coarse-crystalline cements (Figure 3B). The
heulandite III has a broad crystal size distribution
(0.05-0.2 ram), c o m p a r e d to the other types of heulandite. It is unusual and characteristic in the m o d e of
heulandite occurrence that the heulandite HI sparsely
cements detrital grains as a large single-crystal c e m e n t
(Figures 3B, 3C, 3D). The heulandite III crystallizes
and perches onto the smectite coating, c o m m o n l y
along the outline o f detrital grains (Figures 3B, 3C),
though the smectite coatings tend to pinch out b e t w e e n
the heulandite and detrital grains. In addition, smectite
is sometimes found as an inclusion within the heulandite cement. This type o f heulandite sometimes occurs in association with pigment-like hematite or with
a s m e c t i t e - h e m a t i t e mixture. The heulandite usually
shows an optical zoning with nearly isotropic feature
in the inner part of the each crystal (Figures 3C, 3D).
Calcite, dolomite, pyrite, smectite and g y p s u m are
identified as the early-formed authigenic phases in the
208
Noh
Clays and Clay Minerals
Figure 3. Photomicrographs showing the mode of occurrence of heulandite cements (heulandite II and III) in conglomerates
and sandstones in crossed nicols (Scale bars are 0.1 mm). A) Cavity-filling of medium-grained heulandite (h, heulandite II)
coated with smectite: Nicols are subparallel. B) Coarse-grained heulandite (h, heulandite III) sparsely filling the interstitial
pore space (dark) outlined by smectite coatings (bright): detrital quartz (q), plagoclase (p), volcanic rock (v) and unknown
igneous rock fragments (r). C) A large heulandite (heulandite III) cementing detrital grains of quartz (q), K-feldspar, volcanic
(v) and unknown rock fragments: Note the lower interference color in the central part, compared to the outside of the heulandite
crystal. D) A single-crystal cement of heulandite (heulandite III) filling the detrital grains of quartz (q) and rock fragments
(r) coated with the mixture of smectite and hematite. Note the pinching out of smectite coatings within the interfaces of the
heulandite and detrital grains.
heulandite-deficient sandstones and conglomerates of
the C h u n b u k F o r m a t i o n . In a d d i t i o n to the earlyf o r m e d s m e c t i t e a s s o c i a t i n g w i t h h e u l a n d i t e I, the latef o r m e d smectite, o f w h i c h the p r e c i p i t a t i o n stage is
i n t e r m e d i a t e b e t w e e n h e u l a n d i t e II a n d III, is also
f o u n d in the z e o l i t e - b e a r i n g samples. G y p s u m occurs
separately f r o m calcite a n d pyrite. T h e early c a r b o n a t e
c e m e n t s o f e i t h e r micritic or c r y s t a l l i n e calcite are
u b i q u i t o u s t h r o u g h o u t the formation. In the m u d s t o n e
a n d shale o f the C h u n b u k F o r m a t i o n , s m e c t i t e a n d
o p a l - C T c o n s t i t u t e the m a j o r a u t h i g e n i c phases.
H e u l a n d i t e w a s not identified in the P o h a n g Formation. Instead, the o p a l - C T + smectite is the d o m i n a n t a u t h i g e n i c a s s e m b l a g e in the siliceous m u d s t o n e
a n d shale. E i t h e r g y p s u m or pyrite occurs as m i n o r
a u t h i g e n i c phases. M i c r i t i c d o l o m i t e a c c o m p a n y i n g
s m a l l a m o u n t s o f pyrite is characteristic o f the car-
b o n a t e c o n c r e t i o n s w i t h i n the m u d s t o n e s . Calcite a n d
o p a l - C T f o r m late vein-fill in the e a r l y - f o r m e d dolom i t e m a t r i x o f septarian concretions.
HEULANDITE CHEMISTRY
H e u l a n d i t e - c l i n o p t i l o l i t e is a solid solution series in
t e r m s o f c r y s t a l - c h e m i c a l aspects, h a v i n g significant
d i f f e r e n c e s in Si/A1 ratio a n d c a t i o n c o n t e n t (Alietti
1972; B o l e s 1972; G o t t a r d i a n d Galli 1985). H o w e v e r ,
the zeolite g r o u p b e h a v e s differently u p o n heating, depending on chemical composition. This has been used
to d i s t i n g u i s h h e u l a n d i t e f r o m clinoptilolite ( M u m p t o n
1960; B o l e s 1972; Alietti et al. 1974).
C h e m i c a l c o m p o s i t i o n o f m a j o r e l e m e n t s o f the h e u landite was o b t a i n e d b y a n e l e c t r o n m i c r o p r o b e analysis. T h e data w e r e s c r e e n e d in a c c o r d a n c e w i t h the
s u g g e s t e d b a l a n c e error criteria ( E % < 10%) b y Got-
Vol. 46, No. 2, 1998
209
Heulandite cements in fan-delta system
Table 2. Representative electron microprobe analyses (wt%) of heulandites and their chemical formulae.
H e u l a n d i t e 111
Heulandite 1
H e u l a n d i t e II
Core
Rim
l- 1
1-2
1-3
II- 1
11-2
II-3
111-1
111-2
111-3
Ill-4
SiO2
Ai203
Fe203t
MgO
SrO
CaO
Na20
K20
Total
64.47
14.32
0.04
1.62
0.11
4.84
0.13
0.68
86.21
62.40
14.51
0.00
1.48
0.31
4.96
0.11
0.92
84.69
60.92
15.59
0.10
1.20
0.00
6.16
0.16
0.79
84.92
64.40
15.69
0.32
1.80
0.50
4.72
0.29
1.02
89.01
63.75
15.77
0.04
1.58
0.24
4.87
0.19
0.11
87.55
63.33
15.74
0.25
1.64
0.79
4.57
0.24
1.32
87.88
66.25
13.86
0.13
1.66
0.17
4.33
0.18
0.68
87.25
66.96
13.92
0.00
1.33
0.17
4.89
0.30
0.70
88.41
61.82
13.83
0.08
1.30
0.00
4.17
0.34
1.21
82.75
64.11
14.27
0.06
1.38
0.00
4.48
0.61
1.20
86.15
Si
A1
Fe
Mg
Sr
Ca
Na
K
Si/(AI+Fe)
E(%)~
28.56
7.48
0.01
1.07
0.03
2.30
0.11
0.38
3.82
2.80
28.28
7.75
0.00
1.00
0.08
2.41
0.10
0.53
3.65
1.88
27.60
8.33
0.03
0.81
0.00
2.99
0.14
0.46
3.30
2.48
27.88
8.14
0.10
1.16
0.13
2.19
0.24
0.56
3.42
6.30
27.83
8.15
0.08
1.07
0.20
2.15
0.21
0.74
3.38
4.46
28.92
7.13
0.04
1.08
0.04
2.02
0.15
0.38
4.03
5.16
28.92
7.09
0.00
0.86
0.04
2.26
0.25
0.39
4.08
0.31
28.59
7.54
0.03
0.90
0.00
2.07
0.31
0.71
3.78
8.90
28.53
7.49
0.02
0,92
0.00
2.14
0.53
0.68
3.80
2.15
72 oxygens
27.98
8.16
0.01
1.03
0.06
2.29
0.16
0.62
3.43
8.19
t Total iron.
:~ E(%): balance error.
tardi and Galli (1985), and are s u m m a r i z e d in Table 2
and Figure 4. T h e c o m p o s i t i o n a l ranges o f the 3 heulandite types are d i s t i n g u i s h a b l e f r o m e a c h o t h e r on
the basis o f S i / ( A I + F e ) and ( N a + K ) / ( N a + K + C a + M g )
ratios (Figure 4).
H e u l a n d i t e I has the l o w e s t c o n c e n t r a t i o n o f alkalies, and its S i / ( A I + F e ) ratio ranges f r o m 3.5 to 3.8.
H e u l a n d i t e II is less silicic, r a n g i n g in S i / ( A I + F e )
f r o m 3.2 to 3.6 a n d r a t h e r r i c h e r in ( N a + K ) /
( N a + K + C a + M g ) . B o t h t y p e s o f the heulandite gen-
erally s h o w great variability in the S i / ( A I + F e ) ratio
f r o m s a m p l e to s a m p l e and p r e d o m i n a n c e o f C a as
e x c h a n g e a b l e cation, w h i c h are typical c h e m i c a l characteristics o f heulandite. In the heulandite I and II,
h o w e v e r , no c h e m i c a l variability was d e t e c t e d within
the scale o f a crystal.
Heulandite III is quite different in c o m p o s i t i o n c o m p a r e d w i t h the other t y p e s o f heulandite. T h e h e u l a n d ite n I s h o w s striking c h e m i c a l z o n i n g w i t h i n a crystal,
r a n g i n g f r o m S i / ( A I + F e ) ratio o f 3.6 o n the rim to 4.1
0.5
-
0.4
D
....
~
A
o)
a~
§
tJ
§
o
03
[]
[]
/9
§
Z
o
I~ e
9 1-=~
9
0.2
1.
~,
eO /
9
o
i:la i~:10 o
[]
[]
[]
-I-
W
Z
o.1
0
3
i
I
I
I
I
I
I
I
I
I
I
31
32
33
34
35
36
37
38
39
4
41
42
Sil(Al+Fe)
Figure 4.
A correlation of Si(AI+Fe) vs. alkali abundance in the heulandite cements.
210
Clays and Clay Minerals
Noh
1'o
2'0
3'o
40
CuKa 28
Figure 5. Comparison of XRD patterns of heulandite-clinoptilolite (heulandite III) at different temperatures: A) (020) reflection of natural phase of heulandite (heulandite A) and/or clinoptilolite, B) (020) reflection of high-temperature phase of
heulandite (heulandite B).
in the center (Figure 4). Most analyses for the heulandite correspond to the compositional range of silicic
heulandite, but some crystal center analyses are more
silicic, ranging up to more than 4.0 in the Si/(AI+Fe)
ratio, compatible with the clinoptilolite composition.
The silicic part within a coarsely crystalline sample of
the heulandite is comparable with the lower birefringent portions of the crystals. In addition, with increasing Si/(Al+Fe) ratio, considerable increase in ( N a + K ) /
( N a + K + C a + M g ) , mainly in K abundance, occurs in
the heulandite III (Figure 4).
XR D analysis on the heat-treated heulandite III at
350 ~ during 12 h results in a major shift of (020)
reflection from phase A (8.94 ,~) to phase B (8.26 .~).
But this high-temperature phase transition was not
completed at 450 ~ (Figure 5). A weak reflection at
8.94 A, that is, (020) of a clinoptilolite phase, was
sustained up to 600 ~ This reflects that the heulandite
III forms a composite phase of heulandite and clinoptilolite, and the silicic lower-birefringent part of the
heulandite III corresponds not to a heulandite but to a
clinoptilolite phase. This type of heulandite is similar
to the clinoptilolite crystals, showing uniform core and
progressively zoned rim, found in the zeolitic tuff of
the Barstow Formation (Sheppard and Gude 1969).
But the heulandite-clinoptilolite is quite different in
the modes of chemical zoning and occurrence of the
zoned clinoptilolite from the Barstow Formation.
D I A G E N E T I C FEATURES A N D P A R A G E N E T I C
SEQUENCE
The Chunbuk Formation has undergone shallow
burial corresponding to a m a x i m u m burial temper-
ature ranging between 4 0 - 6 0 ~ based on authigenic mineral facies and fission track thermochronology
(Noh 1994; Shin and Nishimura 1994). Shallow
burial and the presence of volcanic detritus in permeable sediments has resulted in crystallization of
3 types of heulandite cements in the fan-delta deposits. Paragenetic relationships among these heulandite phases cannot be deciphered directly, because they do not occur together in a specimen.
However, some textural evidence and authigenic
mineral associations make it possible to determine
their paragenetic sequence (Figure 6).
The earliest generation of heulandite I can be inferred from the close association with the early-formed
smectite together with unaltered glassy clayey matrix,
and from its presence as remnants replaced by crystalline calcite cement (Figure 2B). In addition, the
crystallization mode of heulandite I, that is, in situ
alteration of the glassy matrix, suggests an early formation of the heulandite I. Compared to the heulandite
I, heulandite II appears to have formed after the dissolution of carbonate cements during the later stage of
burial diagenesis. This can be inferred from the fact
that the heulandite II occurs as cavity-fillings associated with secondary porosity in the pore space of carbonate-free samples. The presence of clean boundaries
between rimming heulandite II and detrital grains,
without any early-formed diagenetic phases such as
smectite, may also reflect the total dissolution of carbonate cements, either micritic or crystalline calcite,
including small amounts of dolomite, prior to crystallization of heulandite II. Heulandite II may have
Vol. 46, No. 2, 1998
Heulandite cements in fan-delta system
Diagenetic Phase & Event
Burial Stage
211
Uplift Stage
smectite
heulandite I
heulandite II
heulandite III
calcite
mlc~c
crystalline
dolomite
gypsum
pyrite
hematite
compaction
dissolution of carbonates
Figure 6.
Paragenetic sequences of heulandite and other authigenic phases.
formed after slight compaction caused by the shallow
burial, because the heulandite fills the interstitial
cracks of dilated mica flakes.
Evidences that heulandite III formed after heulandite II is that late-formed smectite and hematite coatings
postdate heulandite II but predate heulandite III (Figure 3). Smectite or the mixture of smectite and hematite, which frequently coats over the heulandite II,
occurs between heulandite III and detrital grains, and
less commonly, occurs as remnants within the heulandite III. The highly expandable smectite is frequently associated with reddish pigment-like hematite,
which is presumably supergene in origin, forming typical coatings along the outline of detrital grains. Thus,
the late-stage smectite coatings that formed prior to
the precipitation of heulandite III appear to have
formed during the uplift stage of the Chunbuk Formation. In addition, the unusual mode of occurrence
of heulandite III, such as large single crystals, together
with characteristic chemical zoning, may reflect that
the heulandite III was formed during or after the uplift
at somewhat diluted fluid conditions, presumably
caused by the migration of meteoric water. This can
be inferred from the general relation between nucleation and solution composition that as supersaturation
decreases, both the rate of nucleation and the number
of nuclei produced decrease, and the resulting precipitate tends to be coarser-grained (Walton 1967; Boistelle 1982; Morse and Casey 1988).
S T R O N T I U M ISOTOPE C O M P O S I T I O N A N D
FLUID S O U RCES FO R H E U L A N D I T E
CEMEN TA TI O N
Calcium carbonates, plagioclase and volcanic material are generally important Sr sources in sandstone
cements and commonly control the concentration and
isotopic composition of Sr in pore fluids during diagenesis (Stanley and Faure 1979). Among these Sr
sources, detrital calcium carbonates are obviously insignificant for the understanding of heulandite cementation in the area because of their absence in the fandelta formation. In addition, the plagioclase is not important as a major source of Sr in the heulandite cements, because the input of plagioclase detritus in the
sediments is minimal and, if present, very sodic
(Ab98.96An2.4), and the plagioclase alteration such as
albitization and dissolution is lacking due to the shallow burial and extensive early cementation of authigenic carbonates. In the heulandite cements, considering the lithology, sedimentary environment and the
paragenetic sequence of diagenetic phases, volcanic
detritus and early-formed authigenic carbonates may
play major roles to control the concentration and isotope compositions of Sr in the heulandite cementation.
The fluid composition at the time of heulandite cementation has resulted from the successive reactions
of seawater with the unstable detrital volcanics and
early-formed carbonates such as carbonaceous fossil
tests and micritic calcite cement including calcite con-
212
Noh
Clays and Clay Minerals
0.7105
0.7095
~
o
0.7085
=.. 0.7075
~
o
o
,~ volcanic rock
]
Q
-\
oo
0.7065
o marine source o
dilution
I
9 heulandite II i
9 heulandite III ]
D mictitic calcite
o mollusca loss
0.7055
0.7045
0.7035
0.7025
~
~
~
200
400
600
800
~
~
=
=
1000
1200
1400
1600
1800
Sr(ppm)
Figure 7. A diagram showing 87Sr86Sr ratio vs. Sr concentrations for heulandites, volcanic rocks and micritic carbonate
cements and their correlation with the reported middle Miocene mollusca data (Woo and Park 1993).
cretions. These volcanic detritus and early-formed carbonates are considered as major Sr sources in the heulandite cements.
A m o n g 3 types of heulandite cements, the Sr isotopic composition of heulandite I is presumably influenced largely by the Sr source of volcanic substance,
because the Sr concentration of seawater entrapped as
initial pore water at the time of deposition must have
been too low to affect the Sr isotopic composition of
heulandite I. Unfortunately, however, purified samples
of the heulandite I could not be obtained for Sr isotope
analysis. The Sr of marine source fixed into the earliest
authigenic phases, that is, micritic carbonate phases
and carbonaceous fossil tests, m a y significantly influence the formation of later-formed heulandite II and
III. The concentrations and 87Sr86Sr ratios for the heulandite II and HI, volcanic rocks and early-formed carbonates have been analyzed to evaluate the sources of
fluid during heulandite cementation. These analyses
are shown in Figure 7, together with values reported
for unaltered mollusca by W o o and Park (1993).
The analyzed Sr isotope ratios (0.706565-0.707432)
of 4 samples of heulandite II and III range nearly in
the m i d d l e b e t w e e n v a l u e s o f v o l c a n i c rocks
(0.704072-0.705298) and fossil mollusc ( 0 . 7 0 8 6 7 0 0.708698). The Sr isotope data of micritic calcite cement (0.707801-0.708032) are slightly lower than
those o f the unaltered mollusc. As shown in Figure 7,
each sample of both heulandites has similar values in
Sr isotopic composition. Heulandite II is slightly lower
(875r86Sr ratio: 0.706565-0.706598) in terms of the Sr
isotope composition and higher ( 3 4 7 - 3 0 4 ppm) in Sr
concentration c o m p a r e d with heulandite III (875r86Sr
ratio: 0.707347-0.707432, Sr concentration: 152-179
ppm). These relations suggest that the chemistry of
pore fluid during formation o f heulandite cements was
influenced by both marine and volcanic sources. The
marine source of Sr was fixed together with Ca as
micritic carbonate cements, and then, as the result
from the dissolution of carbonate cements and carbonaceous fossils, it in part contributed to the formation
of heulandite II and III at the later stage o f diagenesis.
The dissolution of these carbonates in sandstones can
be caused by migration of meteoric water during burial
and uplift (Mathiesen 1984; Surdam et al. 1984; R e m y
1994). The entrapped seawater in sediments may be a
primary source of Sr of marine origin, but its concentration of Sr is too low to significantly affect the fluid
composition o f pore fluid to control the heulandite cementation.
Considering the basin geology, lithology and the Sr
analyses, 2 potential sources for Sr of volcanic origin
can be postulated to understand the Sr sources o f the
pore fluid relating to the heulandite cementation.
These are caused by a subsequent alteration of volcanic detritus during burial and an input by the migration of meteoric water f r o m tuffaceous volcanic rocks
overlain by the fan-delta formation. Coarse-grained
texture of the sediments and later dissolution of the
early-formed carbonates m a y provide a favorable condition for an active migration o f meteoric water into
pore fluid, presumably at the stage of the formation of
heulandite II and III. The higher 87Sr86Sr ratio and lower Sr concentration of heulandite III, c o m p a r e d to
those of heulandite lI, may indicate that meteoric water migration and subsequent carbonate dissolution
have occurred progressively during the formation of
these pore-filling zeolite cements. In addition to the
alterations of volcanic detritus in sediments, the meteoric groundwater circulated within tuffaceous dacitic
Vol. 46, No. 2, 1998
Heulandite cements in fan-delta system
v o l c a n i c s s e e m s to p r o v i d e a d d i t i o n a l c h e m i c a l c o m p o n e n t s s u c h as Si a n d AI n e c e s s a r y for h e u l a n d i t e
f o r m a t i o n into the pore fluid w i t h i n sediments.
A c o r r e l a t i o n o f S r isotope data a n d Sr c o n c e n t r a tions o f the h e u l a n d i t e II a n d III w i t h t h o s e o f v o l c a n i c
a n d m a r i n e sources m a y reflect that the h e u l a n d i t e III
was m u c h m o r e i n f l u e n c e d b y m e t e o r i c w a t e r a n d
f o r m e d at s o m e w h a t diluted c o n d i t i o n in fluid c o m position. In addition, the l a t e - f o r m e d s m e c t i t e coating
a n d h e m a t i t e p r e c i p i t a t i o n p r i o r to the h e u l a n d i t e III
indicate that the h e u l a n d i t e III was p r o b a b l y f o r m e d
d u r i n g or after uplifting o f the f a n - d e l t a f o r m a t i o n
f r o m o x y g e n - r i c h m e t e o r i c water. T h e d i l u t i o n o f p o r e
fluid i n c u r s i o n o f m e t e o r i c w a t e r m a y suppress nucleation, r e s u l t i n g in u n u s u a l coarse c r y s t a l s a n d c h e m i cal z o n i n g o f the h e u l a n d i t e III. T h e successive diss o l u t i o n d u r i n g uplift o f e a r l y - f o r m e d m a r i n e c a r b o n ate c e m e n t s i n e v i t a b l y p r o v i d e d a m a r i n e S r c o m p o n e n t i n the diluted pore water. A s a result, h e u l a n d i t e
III s e e m s to h a v e a slightly h i g h e r 875r/86Sr ratio t h a n
h e u l a n d i t e II.
SUMMARY AND CONCLUSIONS
H e u l a n d i t e w a s d i a g e n e t i c a l l y f o r m e d as in situ alterations ( h e u l a n d i t e I) a n d pore-fillings ( h e u l a n d i t e II
a n d III) in the c o n g l o m e r a t e a n d s a n d s t o n e o f the fandelta system. T h e first type o f h e u l a n d i t e , h e u l a n d i t e
I, occurs as r e p l a c e m e n t s o f v o l c a n i c a n d c l a y e y m a trix. H e u l a n d i t e II, the m a i n type o f the h e u l a n d i t e
c e m e n t s , w a s f o r m e d as fillings in i n t e r g r a n u l a r cavities after the c o m p a c t i o n a n d c a r b o n a t e d i s s o l u t i o n
d u r i n g burial. T h e h e u l a n d i t e III, heulandite--clinoptilolite, w h i c h occurs as c o a r s e - c r y s t a l l i n e single-crystal
c e m e n t , is a c o m p o s i t e p h a s e o f h e u l a n d i t e a n d clinoptilolite, s h o w i n g a c h e m i c a l z o n i n g r a n g i n g f r o m
3 . 6 - 4 . 1 in S i / ( A I + F e ) ratio.
H e u l a n d i t e f o r m a t i o n in the f a n - d e l t a s e d i m e n t s was
initiated b y the i n t e r a c t i o n o f v o l c a n i c detritus a n d ent r a p p e d s e a w a t e r d u r i n g burial. H i g h p o r o s i t y of
c o a r s e - g r a i n e d s e d i m e n t s o f the f a n - d e l t a s y s t e m m a y
h a v e facilitated the alteration o f v o l c a n i c m a t r i x to
h e u l a n d i t e I d u r i n g the early stage o f diagenesis, despite the l o w - t e m p e r a t u r e ( 4 0 - 6 0 ~
d i a g e n e t i c reg i m e a n d the epiclastic i n p u t o f silicic v o l c a n i c detritus in sediments. D u e to the l o w e r a b u n d a n c e o f volcanic detritus a n d the lack o f v o l c a n i c glass s u c h as
glass shards, c o m p a r e d to typical v o l c a n i c l a s t i c sandstones, this alteration r e a c t i o n c o n t i n u e d sluggishly
a n d i n c o m p l e t e l y t h r o u g h o u t the diagenetic regime.
E a r l y c e m e n t a t i o n o f micritic c a r b o n a t e s that o c c u r r e d
p r i o r to burial also p l a y e d a role in retarding the alt e r a t i o n b y p r e v e n t i n g the fluid access to v o l c a n i c detritus in the b e g i n n i n g stage o f burial diagenesis.
T h e l e a c h e d p o r e fluid f r o m the alteration o f volcanic detritus to h e u l a n d i t e I p r i m a r i l y p r o v i d e d nece s s a r y ions for the l a t e r - f o r m e d pore-filling h e u l a n d i t e
II a n d III. T h e h i g h porosity o f the f a n - d e l t a s y s t e m
213
m a d e it p o s s i b l e to p r o v i d e a d d i t i o n a l c o m p o n e n t s
s u c h as Si a n d A1 n e c e s s a r y for the later f o r m a t i o n o f
h e u l a n d i t e c e m e n t s , largely b y the m i g r a t i o n o f m e teoric w a t e r p e r c o l a t i n g f r o m t u f f a c e o u s v o l c a n i c
b a s e m e n t t h r o u g h residual v o l c a n i c detritus in sediments. B a s e d o n the p a r a g e n e t i c s e q u e n c e s o f authig e n i c p h a s e s a n d t h e i r Sr isotope data, the m a j o r c a t i o n
in the pore-filling h e u l a n d i t e c e m e n t s , Ca, w a s largely
d e r i v e d f r o m the d i s s o l u t i o n o f i n c i p i e n t c a r b o n a t e cements.
T h e f r e q u e n t r e c h a r g e o f m e t e o r i c w a t e r into the
f a n - d e l t a f o r m a t i o n a n d s u b s e q u e n t d i s s o l u t i o n o f carb o n a t e c e m e n t s c o n t r o l l e d t h e later f o r m a t i o n o f
coarsely crystalline heulandite cements. During burial
a n d uplift o f the f a n - d e l t a system, m i x i n g a n d d i l u t i o n
o f pore fluid w i t h p e r v a s i v e m e t e o r i c w a t e r r e s u l t e d in
the f o r m a t i o n o f characteristic h e u l a n d i t e II a n d III o f
the f a n - d e l t a formation. T h e u n u s u a l c o a r s e a n d characteristic s i n g l e - c r y s t a l c e m e n t a t i o n o f h e u l a n d i t e III
appears to b e related to the s u p e r g e n i c f o r m a t i o n o f
h e u l a n d i t e d u r i n g or after uplifting at d i l u t e d p o r e water c o n d i t i o n s w i t h o x y g e n - r i c h m e t e o r i c water. A
c h e m i c a l z o n i n g o f the h e u l a n d i t e IH m a y i n d i c a t e that
the heulandite--clinoptilolite p h a s e f o r m e d at a dise q u i l i b r i u m c o n d i t i o n a n d the a m b i e n t fluid c o m p o s i tion at that t i m e o f c r y s t a l l i z a t i o n o f the h e u l a n d i t e HI
h a d c h a n g e d w i t h d e c r e a s i n g i n Si/A1 ratio.
ACKNOWLEDGMENTS
This work has been financially supported by the Yonam
Foundation of Korea. Financial support was also provided in
part by KOSEF (KOSEF 94-0703-04-01-3). I am greatly indebted to J. R. Boles, R. L. Hay, and T. L. Dunn for their
critical readings of the manuscript and helpful comments. I
am grateful to K. H. Park and I. S. Lee of the Korea Basic
Science Institute for their cooperation in isotope analysis. I
also thank D. Pierce for his assistance with electron microprobe works.
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(Received 23 May 1997; accepted 26 August 1997; Ms.
97-044)