origin of grain-coating chlorite by smectite...

Clay Minerals" (1994) 29, 681~92

O R I G I N OF G R A I N - C O A T I N G C H L O R I T E BY S M E C T I T E T R A N S F O R M A T I O N : AN E X A M P L E F R O M M I O C E N E

S A N D S T O N E S , N O R T H S U M A T R A B A C K - A R C B A S I N , I N D O N E S I A

B . H U M P H R E Y S * t , S. J . K E M P t , G . K . L O T T t , B E R M A N T O * , D . A . D H A R M A Y A N T I * AND I . S A M S O R I *

*PPPTMGB 'LEMIGAS', Jalan Ciledug Raya, Cipulir-Kebayoran Lama, Jakarta 12230, Indonesia, and tBritish Geological Survey, Keyworth, Nottingham NG12 5GG, UK

(Received 27 May 1993; revised 18 September 1993)

A B S T R A C T : Grain-coating chlorite cements commonly occur within sandstones of late Middle and Upper Miocene age deposited in the North Sumatra back-arc basin. Chlorites from the Lower Keutapang Member contain Ca (maximum 0.75 wt% oxide) and show textural evidence for direct precipitation on grains. However, crystals are subhedral, showing curved faces and often ragged edges, and show a tendency to merge together. In overlying beds of the Upper Keutapang Member, grain-coating chlorite-smectite (20% smectite) cements display an identical morphology but are more siliceous, have a lower octahedral occupancy and contain higher total (Na + Ca + K). It is proposed that chlorite cements in the Keutapang Formation originated as smectite-rich cement rims whose initial precipitation was related to the breakdown of volcanic detritus in the sediments. Transformation to chlorite occurred subsequently during burial, facilitated by a high geothermal gradient in the back-arc basin.

Gra in-coa t ing and pore-fill ing chlori te cements are commonly found in deltaic and shallow mar ine sands tone reservoirs of Ter t iary age in wes te rn Indones ia (Lot t & Sundoro , 1990). This paper describes grain-coat ing chlori tes f rom sand- s tones depos i ted in a back-arc e n v i r o n m e n t in the Nor th Sumat ra Basin in Indones ia . In this bas in there is much evidence for der iva t ion of sedi- men t s in par t f rom c o n t e m p o r a n e o u s volcanic deposi ts on the volcanic arc or by direct ash falls. Previous studies have d o c u m e n t e d the c o m m o n occurrence of grain-coat ing chlori te and smect i te cements in arc-re la ted basins (e.g. Gal loway, 1974; Lee & Klein, 1986). This paper discusses the role tha t c o n t e m p o r a n e o u s volcanism and high geo thermal gradients may play in the forma- t ion of some types of chlori te cement in arc- re la ted basins.

T E C T O N I C E V O L U T I O N

The Nor th Sumat ra Basin at its sou the rn limits is b o u n d e d by the Bar i san Moun ta ins (Sumat ran 'volcanic arc ') to the SW, the s table Sunda Shield to the N E and E and the A s a h a n Arch to the SE.

The s tudy area is the P E R T A M I N A UEP-1 acreage (Fig. 1).

The Nor th Sumat ra Basin lies in a back-arc e n v i r o n m e n t which m o d e r n global tectonics inter- pre t as result ing f rom subduc t ion of the N E moving Indian Ocean plate b e n e a t h the Sunda Shield. In Nor th Sumat ra , subduc t ion was at t imes obl ique and may have effected m o v e m e n t along the Sumat ran Fault System.

The first impor t an t event in the fo rmat ion of the back-arc basin was the Ol igocene-ear ly Mio- cene deve lopmen t of N-S or ien ted g rabens associated with tens ional stresses occurr ing throughout the western par t of the Sunda Shield in Pa leogene t imes (?44-30 Ma according to Moulds , 1989). Post-ear ly Miocene normal , reverse and strike-slip fault ing followed. Upli f t of the Bar isan Mounta ins , which began in the mid- Miocene (p lanktonic foraminifera l zone N12 or N13) according to C a m e r o n etal. (1980), is t hough t to have b e e n due to compress ive forces associated with r ight lateral m o v e m e n t s along the complex NW-str ik ing t r anscur ren t Sumat ran Fault System. Substant ia l eros ion accompan ied uplift of the moun ta in range, shedding sed iment into the back-arc basin until Pl io-Pleis tocene

�9 1994 The Mineralogical Society

682 B. Humphreys et al.

NORTH ~; )

2 ~ ~ R E A SUNDA SHIELD

~0~ PEN INSULAR

4~ "roDA /

JNOl AN

FIG. 1. Location of the study area within the North Sumatra Basin.

reverse faulting and large-scale folding uplifted the reservoir sandstones to their present configu- ration of anticlines and synclines.

Since the late Eocene , Sumatra has been the site of a periodically active volcanic arc. Tertiary volcanic rocks are concentrated along the W coast of Sumatra but their means of emplacement remains conjectural; several possible explana- tions were summarized by Rock et al. (1982).

S T R A T I G R A P H Y

A new stratigraphic scheme for the North Suma- tra Basin in the study area has been proposed recently by Kirby e ta l . (1993), modif ied from

earlier proposals by Kirby et al. (1989) and has been adopted here (Fig. 2).

All sandstones studied are of Middle to Upper Miocene age, part of the Keutapang Formation which includes all substantial sandstone beds of this age derived from the western flanks of the basin; most of the sandstones were deposited during delta progradation but sands reworked from abandoned deltas at sea-level lowstands are also included within the Keutapang Formation. A basin-fill shale occurring in the N of the study area (the Securai Member) divides the Keutapang Formation into an Upper and a Lower Member. The Besitang River sandstones are an important hydrocarbon reservoir within the Lower Keuta- pang Member.

Age Ma

1.7 F_~L E I'C~"

5-1

STRATIGRAPHiC NOMENCLATURE S (afterKirbyetal. 1993) N S

lulu Rayeu Fm.

Seureula Fro.

. . . . a pang Mbr . -~- s

Baong Fm.

~Taniung Pura Mbn Belumoi Mbr~ Peutu F ~ -

BRs * Besitang River sandstones

LITHOLOGY

] VOLCANICS

] PREDOMINANTLY SANDSTONES

] PREDOMINANTLY MUDSTONES

] LIMESTONES

] PREDOMINANTLY CONGLOMERATES

] DOLOMITE

] LOW GRADE METAMORPHICS

] HIATUS

FiG. 2. Stratigraphic nomenclature and lithologies for the Tertiary succession in the study area.

Grain-coating chlorite from srnectite

D E P O S I T I O N A L E N V I R O N M E N T

Sedimentation in the back-arc was dominated by delta progradation from the basin margin in the W, with pro-delta basin-fill mudstones deposited further offshore in the E. The deltaic sandstone deposits become increasingly fine-grained in distal lobes. In addition, eustatic sea-level changes exerted a strong control on sedimentation. Indi- vidual deltaic progradations can be matched with highstands of sea-level during the Middle to Upper Miocene (by reference to the curve of global sea-level change published by Haq et al., 1987), whereas reworking and offshore transport of the deltaic deposits occurred during lowstand sea-level phases (Kirby et aL, 1993).

The Upper Keutapang Member comprises two main progradational deltaic units separated by a mudstone interval. Foraminiferal assemblages indicate that the sandstones were deposited in shallow inner neritic to intertidal environments in the SW of the study area, deepening towards the E and NE (R. J. Morley, pers. comm.) . Indi- vidual sandstone beds, showing upward coarsen- ing cycles on gamma-ray log profiles, are probably the product of migration of mouth bars. Wave and/or tide reworking may have generated sand aprons at the front of the delta.

The Lower Keutapang Member comprises essentially three prograding delta lobes and one lowstand fan deposit, the Besitang River sandstones, the latter sourced by reworking of the

683

lowermost delta sequence. The Besitang River sandstones were deposited by a variety of gravity- induced processes at the delta front; semi-continuous slumping or collapse and spontaneous liquefaction are believed to have been the main processes of emplacement. Storm wave and biogenic reworking of some sands were also important (for further details see Kirby et al., 1993). These lowstand fan sandstones were therefore deposited in deeper water than the deltaic sandstones; in distal areas of deposition intercalated mudstones contain foraminiferal assemblages indicative of outer neritic to upper bathyal environments, but sedimentary structures in the sandstones indicate shallower water nearer to the former delta front.

S A N D S T O N E P E T R O G R A P H Y

Compositionally, sandstones from the Besitang River sandstones (Lower Keutapang Member) are mainly sublitharenites (Fig. 3), characterized by significant amounts of glauconite which may form up to 7.5% of the total mineralogy, a lithic suite containing a high proportion of volcanic rock fragments, the occurrence of chrome spinels in heavy mineral assemblages, and their frequent cementation by a grain-coating chlorite (chamosite) cement. These sandstones contain a high proportion of Na-feldspars, many of which are pure albites (Abloo). The K-feldspars are a

Fe203 + MgO

GRE ~_ ; , , , , ~ A R K , , , , OSE

Na20 K20

Quartz

SUBAR~

/;7 Feldspar

~.~_ SUBLITHARENITE

ARENITE / / / ! t

Rock Fragments

�9 Upper Keutapang Member

+ Besitang River sandstones (Lower Keutapang Member)

Fic. 3. Geochemical and compositional classifications for the chlorite-bearing sandstones.

684

subordinate component of the feldspar assemblage of these sandstones.

Sandstones from the Upper Keutapang Member reservoirs have more variable compositions (Fig. 3) and are readily distinguished from older sandstones by their feldspar assemblage, particularly by the presence of detrital calcic plagioclase grains which are not detected lower in the succession, and by the frequent occurrence of detrital smectite and sporadic occurrence of kaolinite cement. The feldspar assemblage is dominated by K-species with subordinate amounts of Na-feldspars and an impersistent but nevertheless diagnostic component of Ca-plagioclase. Only trace amounts of glauconite occur in places in the Upper Keutapang Member sandstones. The sandstones usually have high matrix contents and the most variable framework grain compositions. They also have the richest and most diverse heavy mineral assemblages in the basin-fill sequence with up to fifteen detrital, non- opaque species recognized (Morton et al., 1994). In terms of whole-rock geochemistry, the Upper Keutapang Member sandstones have higher K20 and A1203 values, reflecting greater proportions of detrital K-feldspar and detrital clay than occurs in the older sandstones.

S E D I M E N T P R O V E N A N C E

Sand distribution maps and sediment thickness patterns deduced from seismic interpretation (Kirby et al., 1989) and wireline log correlations indicate that sediments in the back-arc basin were largely if not wholly sourced from the Barisan Mountains during deposition of the middle to late Tertiary succession.

Heavy mineral assemblages in the Besitang River sandstones (Lower Keutapang Member) have many features in common with those of the Upper Keutapang Member (Morton et al., 1994) indicating a continuous supply of sediment from the same or a similar source. On the basis of rock fragments, the heavy mineral assemblages and feldspar types, the Keutapang Formation as a whole was derived from a terrain comprising granitic, metasedimentary and extrusive volcanic lithologies. In addition, the Lower Keutapang Member contains chrome spinel (forming up to 21% of the non-opaque heavy mineral assemblage) indicative of an input from an ultrabasic

B. Humphreys et al.

lithology or ophiolite complex (Morton et al., 1994).

Volcanic detritus in the Keutapang Format ion

Volcanism associated with the Sumatran arc probably occurred throughout the mid to late Miocene (Rock et al., 1982). The Upper Keuta- pang Member sediments were sourced in part from contempor~ineous ash falls and the erosion of older lavas. Smectite, which often forms a dominant component of the Upper Keutapang Member mudstones and of the detrital matrix of intercalated sandstones (Fig. 4), is frequently an indication of volcanic provenance; it is character- istic of detritus derived from the argillization of intermediate and mafic volcanic rocks and sediments derived from volcanic arcs (e.g. Hathon & Underwood, 1991).

000oc /t

o 0 5o I oo ~50 2o 0 250 30 0

F16.4. X-ray diffraction traces of <2 p~m oriented mounts of detrital clay assemblage, Upper Keutapang Member.

Co-Kc~ radiation.

Grain-coating chlorite f rom smectite

Evidence for erosion or weathering of lavas is indicated by the presence of Ca-plagioclase grains (Trevena & Nash, 1981); andesine and labrador- ite form, on average, 14% of all feldspars analysed in the Upper Keutapang Member although amounts vary greatly between wells. The maximum recorded anorthite component in detrital plagioclase grains is 61 mole %. Further, the occasional detection of the heavy mineral diaspore is indicative of a tropically weathered basaltic source (Morton et al. , 1994).

The extent of contemporaneous volcanism in northern Sumatra was reported by Rock et al. (1982). Miocene volcanics comprising porphyritic andesitic to dacitic tufts, calc-alkaline basalts, agglomerates and tufts are widely distributed to the SW of Lake Toba in Sumatra (and are possibly concealed below the Pleistocene Toba tufts), especially W of the Sumatran Fault System. Precise age determinations for these volcanics are not yet available, but a phase of volcanism occurred in the mid to late Miocene (Rock et al. , 1982). Tephra layers of late Miocene age have been recorded in deep-sea sediments W of the Sumatran arc (Ninkovich, 1979). It is likely, therefore, that the source area for the North Sumatra Basin sediment-fill was periodically mantled by fine-grained volcanic ash and lavas. Either clays sourced from these volcanic rocks could have washed into the back-arc basin or air-fall volcanic ash could have been deposited directly and altered syndepositionally to smectite in the basin. Probable ash layers have been noted in slabbed cores through the Upper Keutapang Member; these have a dominantly smectite composition. Tuffaceous material is known to occur in the Keutapang Formation (Cameron et al. , 1980).

In the Besitang River sandstones, it is also strongly suspected that clays of volcanic origin were a significant component of the sediment at the time of deposition. Volcanic rock fragments are common (Fig. 5), comprising subhedral to euhedral lath-like feldspar crystals, sometimes zoned, within a fine-grained groundmass. Elec- tron microprobe analyses indicate that the feldspars are pure or very nearly pure albite. These fragments may be basaltic lavas in which the feldspars have altered to albite during burial diagenesis, or they may be fragments of spilites (basalts in which the feldspars have been pseudo- morphed by albite following a phase of hydroth- ermal metamorphism). In either case, smectitic

685

clays could have formed at source and been transported to the basin along with other detritus.

B U R I A L H I S T O R Y

Following deposition, subsidence at the basin margin and continued sedimentation, locally punctuated by short hiatuses, allowed burial of the Miocene sandstones to considerable depths in a relatively short time. The maximum burial depth assessed from deposition rates and pre- served thicknesses of each sequence was in the order of 1.85 km for the Upper Keutapang Member and 2.5 km for the Besitang River sandstones (Lower Keutapang Member). This was achieved during the latest Pliocene or early Pleistocene (at the end of a period of relative tectonic quiescence), immediately preceding the phase of tectonism which folded and uplifted the sandstone reservoirs to their present depth range.

Sandstone samples used in this study range in depth from 950 to 1500 m (Upper Keutapang Member) and 1650 to 1850 m (Besitang River sandstones). Thus uplift in excess of 1 km may have affected the oldest samples examined.

High geothermal gradients based on bottom hole temperature measurements have been recorded from the North Sumatra Basin during drilling for oil and gas. In the PERTAMINA UEP-1 acreage, current geothermal gradients range from 35.4-62.7~ (Aadland & Phoa, 1981). In all likelihood, rates of heat flow would have been further elevated during the Oligocene- early Miocene phase of N-S oriented rift graben development in North Sumatra; higher geothermal gradients occur during the early rifting stage of basin evolution (Lee & Klein, 1986). A mean geothermal gradient of 44~ for the study area based on present day data should, therefore, be viewed as a minimum value when considering the palaeothermal gradients that affected Mio- cene sediments during their burial in the basin.

S A M P L I N G A N D A N A L Y T I C A L T E C H N I Q U E S

Core samples were studied from six wells in the Upper Keutapang Member sandstones and from four wells in the Lower Keutapang Member (Besitang River sandstones) reservoir. All core


FIG. 5. Backscattered scanning electron micrographs of grain-coating chlorite. (A) Chlorite coating a volcanic rock fragment; note absence of the chlorite at grain contacts. (B) Chlorite showing similar features to (A) but developed on surface of a glauconitic grain. (C) & (D) Examples of chlorite growth upon volcanic fragments composed of albite laths in a

fine groundmass.

samples analysed are from oil zones. Clay mineralogy was studied using a Philips PW1700 series X- ray diffractometer. Gold-coated rock chips were examined using a J E O L 35CF scanning electron microscope to determine clay morphologies. Composit ional analyses of clay cements were determined using a Cambridge Instruments electron microprobe. The limitations of such analyses have been discussed by Humphreys et al. (1989). A seawater and lignosulfonate drilling-fluid had been used in each of the wells examined.

C H L O R I T E C E M E N T C H A R A C T E R I S T I C S

Besitang River sandstones (Lower Keutapang Member)

X-ray diffraction (XRD) traces show weak, low intensity first and third order but strong second

and fourth order reflections (Fig. 6A). The first order basal spacing is typically 14.2 A and the second order at 7.11 ,~i. Authigenic chlorites with markedly enhanced even-order reflections have been illustrated by, amongst others, Curtis et al. (1984), Whittle (1986) and Humphreys etal. (1989). Hillier & Velde (1992) have reasoned that authigenic chlorites with structures characterized by sharp even-order and broad odd-order reflections contain interstratified 7 ]k layers. Thus many of the published examples are strictly interstrati- fled species, either chamosite-berthierine or cba- mosite-kaolinite, according to Hillier & Velde (1992). However , as discussed later, this proposal is unlikely to apply universally.

The grain-coating chlorite forms a honeycomb pattern of slightly curved, interlocking crystal platelets lying perpendicular to the host grain (Fig. 7). Individual crystals show ragged outer edges or smooth stepped edges and are typically

Grain-coating chlorite from smectite 687

healsd 5500C

oo so 10o ~so 200 2so 30o oo 50 ~oo 15o 200 ~so 300

A o20 B

FIc~. 6. X-ray diffraction traces, <2 ~m oriented mounts of authigenic clay assemblages; (A) Besitang River sandstones (Lower Keutapang Member); (B) Upper Keutapang Member. Co-Kc~ radiation.

<0 .3 ~tm thick; crystals often appear to merge. Coatings that have lifted away from the host grain or where the host grain has been dissolved show an outward increase in crystal size from 2 to 5-6 pore maximum size concomitant with an increasingly euhedral form (Fig. 7D). There is no evidence for growth of the grain-coating cements in this study by nucleation on an early tangential clay coating as proposed by Purvis (1990), or on a substrate of detrital clay such as a clay cutan (Moraes & De Ros, 1990; Pittman et al., 1992).

Coexisting with these grain-coatings are occasional pore-filling chlorite cements, comprising discrete platy and ' roset te ' clusters of crystals. Individual crystal plates are typically straight and rarely ragged in appearance. These pore-filling crystals show variable sizes; they may be comparable to the grain-coating chlorites or larger, up to 10-15 ~tm across, and thicker, up to 1 ~tm, than the grain-coating cements.

Electron microprobe analyses show that the grain-coating cements are chamosites with a relatively uniform composit ion (Figs. 8, 9). The Fe/(Fe + Mg) ratios vary from 0.5 to 0.61. Electron microprobe analyses record small but

persistent amounts of Ca (max. 0.75 wt% oxide) and often Na (max. 0.45 wt% oxide) in these cements. The Si contents are usually >6 formula positions and octahedral occupancy is low compared with typical detrital chlorite analyses. Small quantities of K are sometimes recorded (Table 1); these could result from illite or K-feldspar contamination but similar or higher K-contents are found within the structure of grain-coating chlorites from the Upper Keutapang Member . The wt% oxide totals are always low, frequently <78 compared to the expected total of c. 86 for chlorite (Table 1). Precision of analysis is con- sidered good and the chemical characteristics of these cements accord with the findings of Hillier & Velde (1991) that diagenetic chlorites are compositionally distinct from metamorphic chlorites.

Upper Keutapang Member

Grain-coating chloritic cements also occur in the younger Upper Keutapang Member sandstones. X-ray diffraction analyses on sandstones with low detrital matrix contents reveal an


FI6.7. Secondary electron scanning electron micrographs of grain-coating chlorite from the Besitang River sandstones. (A) & (B) Complete envelopment of framework grains by chlorite cement. Note the later, minor development of quartz overgrowths shown in (A). (C) High magnification view showing curved crystal plates, some with ragged edges.

(D) Chlorite-coating within pore-space showing outward increase in the crystal size.

Mg AI

FIG. 8. Chlorite cement compositions in Besitang River sandstones. Analyses, made on carbon-coated polished thin-sections, were obtained using a Cambridge Instru- ments Microscan 5 electron microprobe with a Link Systems AN10000 energy dispersive X-ray analyser. The

electron beam was focused to ~2 ~tm.

'1

ing

e)

0 0.1 0.2 03 0.4 0.5 0.6 0.7 o.B 0.9 1.0

Oc~ahedral Fe/(Fe + Mg)

FIG. 9. Electron microprobe data for chlorite cements in Besitang River sandstones; compositional fields after Hayes (1970) and Curtis etal. (1985). Trioctahedral chlorite structural formulae were calculated on the basis of 28

oxygens (or 36 anions) per unit-cell.

Grain-coating chlorite from smectite 689

TABLE 1. Representative electron microprobe analyses of chlorite cement (wt%) and structural formulae.

1 2 3 4 5 6 7 8 9 10 11 12

SiOz 25.529 24.491 25.260 25.111 25.534 23.464 23.929 22.435 26.591 26.222 28.109 28.091 Al~O3 15.372 15.784 15.464 15.948 15.609 14.716 13.625 14.586 13.034 11.488 11.850 13.215 FeO 27.044 25.378 24.613 25.125 25.090 24.631 23.518 23.700 17.(/35 19.537 19.791 17.(137 MgO 10.181 8 .952 9.729 9.375 9.318 8.957 8.711 8 .443 5.237 2.865 2.911 5.343 CaO 0.197 (/.233 0.615 0.750 0.352 0.192 0 .406 (I.37(t 0 .196 0.191 - 0.339 Na20 0.355 - (/.4(18 - 0.238 - 0.822 0 .684 (t.618 0.937 K20 - 0.156 - 0.123 - - 0.311 1.366 0.809 0.496 Total 78.678 74.837 76.245 76.310 76.263 71.961 70.189 69.535 63.225 62.354 64.087 65.457

Numbers of ions on the basis of 28 oxygen equivalents Si 6.110 6.117 6.181 6.132 6 .429 6.117 6.351 6 .036 7.461 7 .694 7.911 7.599 AI(IV) 1.89(I 1.883 1.819 1.868 1.751 1.883 1.649 1.964 (1.539 0 .306 0.(189 0.401 AI(VI) 2.446 2 .764 2.641 2.722 2.751 2 .639 2.613 2.662 3.771 3 .667 3.842 3.812 Fe 5.413 5.301 5.037 5.131 5.135 5.370 5.220 5.333 3.997 4 .794 4 .658 3.854 Mg 3.632 3.333 3.549 3.413 3 .399 3.481 3 .446 3.386 2 .190 1.253 1.221 2.155 Ca 0.051 0 .062 0.161 0 .196 (I.(192 0 .054 (I.116 0.107 0.059 0.060 - 0.098 Na 0.165 - 0.193 0.113 - - 0.447 (I.389 0 .337 0.491 K - - 0.049 0.039 - - 0.111 0.511 0.291 0.171

Oct. ll.49 11.40 11.23 11.27 11.29 11.49 11.28 11.38 9.96 9.71 9.72 9.82 Fe + Mg 9.045 8.634 8.586 8.544 8 .534 8.85l 8 .666 8.719 6.187 6.047 5.879 6.009 Fe/(Fe + Mg) 0.6(I (I.6l 0.59 0.6(I 0.6(I 0.61 (t.6(I 0.61 0.65 0.79 0.79 0.64

Key: 1-8, Lower Keutapang Member grain-coating cements; 9 11, Upper Keutapeng Member grain-coating cements (analysis 10 shows minor contamination from host feldspar grain) and 12, Upper Keutapang Member ?authigenic pore- filling clay.

authigenic clay assemblage dominated by chlorite with kaolinite and illitic phases. The 7 ~ reflection is broad and less intense, whereas the 14 reflection is very broad and subdued, and shows some expansion upon glycol solvation (Fig. 6B). These characteristics are indicative of an interstratified chlori te-smecti te species containing up to 20% smectite. Nonetheless , there are no obvious differences in morphology be tween this grain-coating cement and that in the Besitang River sandstones.

Electron microprobe data on selected samples has shown these cements to contain higher Na

contents (up to 0.94 wt% oxide) and Si contents (>7 .4 formula positions) than in the Lower Keutapang Member (Table 1). In addit ion, Ca and K are recorded and octahedral totals are depressed.

T R A N S F O R M A T I O N O F S M E C T I T E T O C H L O R I T E

Recent studies of diagenetic chlorites in sandstones have shown that they are composit ionally distinct from metamorphic chlorites. In particu-

lar, they are more siliceous, contain less total (Fe + Mg) and display a lower octahedral occupancy (Curtis et al., 1984, 1985; Humphreys e ta l . , 1989; Hillier & Velde, 1991). There is debate whe ther recorded composi t ions are a real feature of pure diagenetic chlorites (Hillier & Velde, 19911, or are due to interstratified phyflosi- licate phases. The existence of inheri ted 7 A layers in chlorites which appear to have progressively recrystallised from a percursor 7 ,~ ber thier ine , for example in Jurassic sandstones from the Nor th Sea (Hillier & Velde, 1992; Ehrenberg , 19931, is unlikely to apply here . Indeed , in the authors '

exper ience, authigenic chlorites from a variety of deposit ional and diagenetic envi ronments usually show a marked discrepancy in s trength of the odd- order and even-order reflections, irrespective of their postulated origin; transmission electron microscopy studies are really required to resolve the true structure of these cements . At the present t ime, the authors feel that chemical data may provide more reliable clues to the true nature and origin of chlorite cements . In this case study we feel there are several lines of evidence indicating a precursor smecti te phase.

69O

(1) Electron microprobe analysis always detected small amounts of Ca, and in some cases Na, and Si values are slightly elevated compared to most detrital chlorites. Several other authors document grain-coating chlorites that are Si-rich relative to detrital chlorites or even pore-filling chlorites (e.g. Pittman, 1988). Although the impersistent detection of Na may reflect salt contamination following dry storage of the core, it could equally be a real component of the clay structure. Persistent levels of Ca would appear to reflect a non-chlorite component to the clay. Although lower wt% oxide totals may reflect the inclusion of pore-space in the volume analysed by the electron beam, the existence of bound water in the clay structure would have a similar effect. The combination of a persistent Ca content, elevated Si concentration and depleted oxide totals is consistent with a small relict component of smectite in the clay structure. This postulated smectite component in the Besitang River sandstones is too small to induce swelling of the 14 .~ reflection on glycol solvation. However, chlorite cements in the Upper Keutapang Member show moderate swelling on glycolation.

(2) The curved, sometimes crinkly, appearance and merging together of platelets is reminiscent of the crystal morphology of an interstratified chlorite-smectite or a corrensite clay coating (Tomp- kins, 1981; Helmold & van de Kamp, 1984; Purvis, 1990). This morphology could have been inherited from a precursor smectite-rich clay.

Helmold & van de Kamp (1984) noted that the morphology of grain-coating chlorite can superfi- cially resemble chlorite-smectite, but that chlorite could be distinguished by discrete platelets whereas these cannot be resolved in chlorite-smectite. Also, chlorites were found by these authors to lack Ca.

(3) There is circumstantial evidence that other mineral phases have been affected by diagenesis during rapid burial under high geothermal gradients. Loss of smectite from the detrital clay mineralogy is apparent with depth in the stratigraphic succession, together with albitization of detrital plagioclase grains and leaching of heavy mineral grains such as garnet, staurolite and diaspore (Morton et al. , 1994).

C O N C L U S I O N S

This study shows the potential relationship between clay diagenesis and an environment of

B. Humphreys et al.

contemporaneous active volcanism in two main respects.

(1) The development of grain-coating cements may be related to abundant volcanic material in the sediments at the time of deposition. Volcanic ash and glass are metastable and breakdown rapidly to clay, usually of smectitic composition (e.g. Naish et al. , 1993). Grain-coating smectite is frequently recorded as an early diagenetic cement in volcaniclastic sandstones (e.g. Galloway, 1974; Davies et al. , 1979), usually precipitated on grains as evidenced by its absence at grain contacts. Comparable features are exhibited by the chlorite cements in the Besitang River sandstones (Fig. 5). Furthermore, the increase in crystal size of the grain-coating chlorite with distance from the host grain suggests crystal growth rather than replacement (Fig. 7).

(2) Despite textural evidence for direct precipitation, we believe that there are strong indica- tions from our study of a transformation from grain-coating smectite to chlorite during burial diagenesis and that this was facilitated by high geothermal gradients due to proximity to the active Sumatran volcanic arc. The importance of high heat flow in controlling sandstone diagenesis in back-arc basins has previously been stressed by Galloway (1974) and by Lee & Klein (1986) and in areas of rifting and crustal thinning by Trevena & Clark (1986). The occurrence of chlorite with some interstratified smectite in the Upper Keuta- pang Member, and the persistent Ca content and 'smectite-like' morphology of the grain-coating chlorite in the Besitang River sandstones is logically explained by a precursor smectite. The transformation from smectite to chlorite during burial diagenesis has been recorded by a number of authors from various basins. Dunoyer de Segonzac (1970) drew attention to the likely transformation of smectite to chlorite via an interstratified chlorite-smectite phase during burial diagenesis. Helmold & van de Kamp (1984) postulated that grain-coating chlorites altered from a precursor chlorite-smectite phase. These authors recorded a decrease in the expandability of extant chlorite-smectite clays in tb~: sequence studied as did Chang et al. (1986). There is also considerable evidence for this ~eaction occurring in thermally metamorphosed volcaniclastic rocks (e.g. Inoue & Utada, 1991). Curtis etal . (1985) drew attention to the chemistry of 'swelling chlorites' from Alaskan sandstones, which these authors tentatively suggested to be an inter-

Grain-coating chlorite from smectite

media te stage within a diagenet ic progress ion f rom t r ioc tahedra l smecti te to chlorite. The 'swelling chlor i tes ' f rom Alaska conta in small but significant concen t ra t ions of Ca, Na and K, low oc tahedra l totals and high Si values in the i r s t ructural formulae . The i r chemist ry has clear similarit ies to the composi t ion of the cements f rom the U p p e r Keu tapang M e m b e r (Table 1).

In summary , it is p roposed tha t the origin of the chlori te cemen t has been a two-phase process: initial prec ipi ta t ion of a gra in-coat ing smecti t ic clay on f r amework grains and subsequen t t rans- fo rmat ion to chlori te dur ing burial unde r a high geo the rmal gradient .

Interest ingly, exper imenta l l abora tory studies have p roduced grain-coat ing, platy boxwork coatings with composi t ions of 7 ~ ber th ie r ine and t r ioc tahedra l smecti te , whereas non-expandab le 14 A chlori te (chamosi te or c l inochlore) has not yet been synthesized at low t empera tu re s in the labora tory (Small etal . , 1992). These au thors suggested tha t 14 A chlori tes compris ing large discrete crystals are likely to be direct precipi- tates. However , they did not preclude an origin re la ted to t r ans format ion of a precursor 7 ,~ chlor i te or smecti te phase for less euhedra l , grain- coat ing chlorites.

A C K N O W L E D G M E N T S

The British Overseas Development Administration is thanked for funding the British Geological Survey to undertake a training programme in basin analysis, which included tuition in clay mineral analysis, at the Indonesian Government's Research Centre for Oil And Gas Techno- logy (LEMIGAS) in Jakarta. Drs Bona Situmorang and Mujito at LEMIGAS are thanked for instigating and directing the North Sumatra basin study and for arranging sampling trips to North Sumatra. Dr Abdul Muin and Sudarno Slamet of the LEMIGAS Geological Services Unit are thanked for logistical support and for allowing free access to analytical facilities. Thanks are also due to Ramli Djaafar for allowing sample collection from the PERTA- MINA core store at Pangkalan Brandan, North Sumatra. Information on the regional tectonics and stratigraphy of the North Sumatra Basin is based largely on the work of Drs Gary Kirby and Bob Morley, respectively, and their contribution to this study is gratefully acknowledged. The helpful comments of two anonymous referees were appre- ciated. This paper is published by permission of the Directors of PERTAMINA and LEMIGAS, and the Director, British Geological Survey (NERC).

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