Ben Avraham and Emery 1973 - Structural Framework of Sunda Shelf

8/12/2019 Ben Avraham and Emery 1973 - Structural Framework of Sunda Shelf

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Structural Framework of Sunda Shelf

The American Association sf Patroleum Geologists BulletinV. 57, rto. 12 (December 19 73) , P. 2323 -236 6, 30 Figs., 1 Table

Z V I B E N - A V R A H A M a n d K. O . E M E R Y 'Wo o d s Ho l e , M as s a ch u s e tt s 0 2 5 4 3

A bs tr ac t The Sund a Shel f is one of the most extensivecont inenta l shelves of the wo r ld . In June-Ju ly 1971 a geo physical survey was conducted over the southern part ofthe shelf (Java Sea) . W at er d ep th , sed ime nt th ickness, an dthe grav i ty and magnet ic f ie lds were measured cont inuously . Expendable rad iosonobuoys permit ted se ismic re f ract ion measurements . These geophysica l data , supplemented by data f rom ear l ier tud ies o f the nor thern SundaSh e l f a n d g e o l o g i c d a t a f r o m l a n d a r e a s , p r o v i d e a c o m prehensive p ic ture o f the s t ructura l f ramework of the ent i re Shel f .

The seismic ref lect ion profi les show that the Sunda Shelfconsists of three major units: the northern Sunda Shelfb a s i n o l a r e a , t h e S i n g a p o r e p l a t f o r m , a n d t h e J a v a Se abaslnal area . In the nor thern un i t are two large sedimentary basins ( the Brunei and Gul f o f Thai land basins) separated by the Natuna Ridge. In the Java Sea ore severa lo ther basins separated by up l i f ts . The basins in the west ern Java Sea are approximate ly c i rcu lar and seem to re su lt f rom ten sional forces, wh ereas those in the easternJava Sea are nar row and long and appear to be the resu l to f comp ressionol forces. Radiosonobuoys re vea led smal lbasement features and resolved many st ra ta having d i f ferent seismic velocit ies.Faul ts are abundant throughout the Sunda Shel f andclear ly contro l the d is t r ibut ion and shapes of the basins .The faults str ike north-south in the western Java Sea andnor theast -southwest in the eastern Java Sea. A major d is cont inu i ty t rending nor th -south ( termed here the Natunarif t In the northern Sunda Shelf and the Bil l i ton depression in the western Java Sea) cuts the structures of the ent ire Sunda Shelf and continues southward across centralJava to the deep-ocean f loor .

Analys is o f magnet ic anomal ies shows that the reg ioncan be d iv ided in to severa l d is t inct magnet ic provincesthat do not everywhere fol low the major structural unitsmapped by the se ismic re f lect ion data . These magnet icprovinces co inc ide wi th cor responding provinces of l i th icuni ts . The grav i ty f ie ld over the centra l and southern SundaSh e l f o v e r a g e s a b o u t + 3 0 m g o l . l o c a l g r a v i t y a n o m a l i e swi t h r e l a t i v e a m p l i t u d e o f 1 0 - 2 5 m g a l a r e s u p e r i m p o s e don the reg ional background leve l . Al though the local grav i ty anomal ies were he lp fu l in reso lv ing the upper crusta lstructures, the cause for the high regionol gravity is unkn o wn .

Al l o f the geophysica l s tud ies serve to out l ine the d is t r ibut ion pat tern o f sed iment - f i l led basins and in terveningr idges and p la t forms beyond the leve l o f understandingreached by ear ly workers through in ferences based uponthe geolog y of the lan d areas , an d be yon d the scant publ icat ions of o i l companies wi th in the rest r ic ted areas oftheir concessions. These reg ion al results may be h elp fulboth for understanding the genera l s t ructure o f the SundaShel f and for denot ing the areas wi th the greatest fu tureo i l pote nt ia l . The main s t ructura l e lements ar e i n t e r p r e t e dat the resu l t o f past in teract ion between l i thospher icplotes.

INTRODU TION

The Indonesian Archipelago with its activevolcanism, seismicity, and orogenesis is amongthe most complex structural regions in theworld. Van Bemmelen (1954) stated that Indonesia is a most suitable region for the studyof mountain building processes, and manyother famous Dutch geologists such as H. A.Brouwer, J. H. F. Umbgrove, Ph, H. Kuenen,F . A. Vening Meinesz and others recognizedthat this area illustrates various stages in theevolution of orogenic belts. The impressive arcuate sweep of the Sunda Islands and the associated ocean trenches have been linked with theconcept of geosynclines since the beginning of 1973. Th e Am erican Association of Petroleum Geologists. All rights reserved.

'M anu scr ipt received, March 6, 1973; accepted, M ayI , 1973. Contr ibution No. 3066 of the Woods HoleOceanographic Inst i tut ion.^Wo ods Hole O ceanographic Inst i tution.This study was made possible by National ScienceFoundation Gram GA-27449, which provided for theshipboard studies under the direction of Emery. Ben-Avraham was supported partly by the same grant andpart ly by the Wood s Hole Oceanographic Inst i tut ionthrou gh a fellowship fund from Mobil Fou nda tion Incorporated. Both writers are indebted to the officers,crew, and scientific staff on cru ise 100 of R /V Chain tothe Java Sea. W. C. Pitman, M. Talwani, and W. R.Raitt provided unpublished geophysical data from thisregion. W. Hamilton gave us a copy of an unpublishedtectonic map of Indonesia and provided some of thebasement samples used in the study, with the permission of Atlantic Richfield Indonesia, Inc., and CitiesService Oil Co mp any. Shell Internation ale PetroleumMaatschappij , N. V., The Hague, Nether lands, provided gravity maps (thro ugh C. O. Bowin) from landareas. The work also benefited from data of oil companies and from discussions with many oil companygeologists. C. O. Bowin, N. M. Toksoz, E. T. Bunce,J. D. Phillips, W. B. Bryan, R. P. Von Herzen, J. G.Sclater, B. P. Luyendyk. J. C. Macllvaine, and ElazarUchupi reviewed parts of this manuscript and madevaluable suggestions. Discussions with S. Uyeda, W.

Hamilton, J. R. Heirtzler, S. T. Knott, and J. A. Growalso have stimulated various aspects of this study. H.Hoskins helped with the analysis of sonobuoy records,and K. E. Prada with the processing of the seismic reflection data.2 3 2 3


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2 3 2 4 Zv i Ben-Avraham and K. O. Emeryt he cen tu ry . In fac t , c r i t ica l observat ions f romthis region and especially from the S und a l and( the Sunda Shel f and adjacent i s lands) havebeen used in most theor ies of o rogen ies andt ec ton ics ; and such fundam enta l concep t s asthe tec togene and id iogeosyncl ine or iginal lyw ere in t roduced for th is area . However , newerviews concerning pla te tec tonics and sea-floors p r e a d i n g ( H e s s , 1962; D ie t z , 1961;V i n e andM a t t h e w s , 1963;Isacks et al, 1968; Le P ichon ,1968 ; and M o r g a n , 1968) indicate that c lass i ca l theor ies of o rogeny such as the geosyncl inalcycle that were developed for the S unda l andm a y be largely in er ror .

The Sunda Shel f was s tudied as p a r t of ap r o g r a m of r econna i s sance m apping of thes t ruc tu re of the con t inen ta l m arg in ofl easternA s ia tha t was begun in 1967 in the E as t C h inaS e a ( W a g e m a n et al., 1970) and con t inueds o u t h w a r d to the C hina B as in (E m eryand Ben-A v r a h a m , 1972) and the S o u t h C h i n a Sea( P a r k e et al, 1 9 7 1 ) . The latest l ink in the ser ies was the J a v a Sea, w h i c h was invest igatedaboard Woods H ole O ceanograph ic Ins t i t u t ion ' s R/V Chain dur ing pa r t s of J u n e andJuly 1971. Pre l im inary resul ts based on sh ip board ana lys i s of the m e a s u r e m e n t s w e r e repor t ed by E m e r y et al. ( 1 9 7 2 ) . A m ore thor ough s tudy of the resul ts wh ich includ ed se ismic reflection and ref ract ion (via r a d i o s o n o -buoys) prof i les , gravi ty , magnet ics , and b a t h y m e t ry the i r i n t egra t ion w ith pub l i shed in fo rm a t i o n and unpub l i shed da ta for the ent i reSunda Shel f and for the su r round ing l and andocean floor, and the appl ica t ion of pla te tec tonics to the s t ruc tura l syn thes is formed a doc to ra ldisser ta t ion for B e n - A v r a ha m ( 1 9 7 3 ) . T h i sis as u m m a r y r e p o r t on the s t ruc tu re of the JavaS ea and the o the r pa r t s of the Sunda Shel f andthe i r re la t ion to the s t ruc tu re of the adjacentdeep-ocean and land regions .D a t a for the Sun da Shelf (Ja va Sea andS o u t h C h i n a Sea) w ere supp lem ented by a fewgravi ty s ta t ions of V en ing Meinesz (1932 and1934) , som e g rav i ty and m agne t i c t r ave r ses byships in t r a n s i t R / V H. M. S. Cook of theH y d r o g r a p h i c S u rv e y , E n g l a n d ( G r a y , 1959-1 9 6 2 ) , Pioneer of U .S .C .G .S . , Oceanographero f N O A A , Vema and Robert Conrad of La-m o n t - D o h e r t y G e o l o g i c a l O b s e r v a t o r y 1 0aerom agne t i c p rof il es f rom pro jec t M A G N E T ,land g rav i ty m easurem ent s f rom S he l l In t e rna t iona le P e t ro l eum M aa t schapp i j (ob ta inedt h r o u g h C. O. Bowin) , ref lec t ion and ref ract ionstudies over smal l areas in the nor the rn S undaShelf (Dash, 1970, 1971;D a s h et al., 1 9 7 2 ) ,

and som e sam ples of basem ent rocks and seismic velocit ies from oil com pan ies ope ra t ing inthe region. Al though oil com pan ies have m ademany geophysical s tudies in the Sunda Shel fs ince about 1967 ( H u m p h r e y , 1970, 1971;T a n n e r and K e n n e t t , 1972) the resul ts are p r o p r i e t a ry and not avai lable outs ide the c o m p a nies for a synthes is of regional geology that isthe object of this study.G E O L O G I C S E T T I N G

P h y s i o g r a p h y and B ot tom M ate r i a l sT he S unda S he l f is a l a rge (1 ,8 50 ,000 sqk m ) sea genera l ly shal lower than 100 m (F igs .1, 2), in contras t wi th the r a the r com plex andi r r egu la r ba thym et ry of the s u r r o u n d i n g d e e p -

ocean f loor . Ear le (1845) was the first to rep o r t the extensive flat floor of the S u n d a Shelf.A b o u t 75 years later , after the resultsof the Si-boga E xped i t ion had b e c o m e k n o w n , M o l e n -graaff (1 92 1) ad vanc ed Ear le ' s conce pt and argued tha t the Sunda Shel f is a su b m e r g e d p e n e pla in that resul ted f rom eusta t ic changes in sealevel dur ing the latest Pleistocene glaciat ion.K u e n e n ( 1 9 5 0 , p. 482) and others suggestedtha t the cour ses of severa l submerged dra inagesystemscan be dis t inguished.T h e s o u t h e r n and western sides of the S u n d aShelf are b o r d e r e d by the Indones i an i s l and arc( the vo lcan ic Inne r B anda Arc of B rouw er ,1925) com posed of M a l a y a (in a b r o a d s e n s e ) ,S u m a t r a , and Java; many smal ler i s lands cont inue the arc b e y o n d the eas tern l imi t of the

shelf. S ou th of the island arc are a para l le lt rough , ano the r r idge hav ing a few sm a l l isl ands (the nonvo lcan ic O ute r B anda Arc ofB r o u w e r ) , and the deep na r row Java T rench .T h e n o r t h e r n b o u n d a r y of the Sun da Shel f consists of the Indo-C hina P en insu la , B orneo , andC elebes , all sepa ra t ed by deep bas ins that a lsocon t inue eas tof the shelf.

B ot tom sed im ent s ove r the shelf have beenm a p p e d by E m e r y and N i i n o ( 1 9 6 3 ) , E m e r y( 1 9 6 9 , 1 9 7 1 ) , K e ll e r and R i c h a rd s ( 1 9 6 7 ) ,van B aren and K i el ( 1 9 5 0 ) , and M o h r ( 1 9 3 8 ) .In genera l , the pa t t e rns show the p resence ofwidespread modern s i l t s and c lays in the Gulfo f T ha i l and and the axia l par t of the shelf betw een S um at ra , J ava , and B o r n e o , and det r i ta lsands a long most of the shores except those oft he nor the rn G ul f of T ha i l and , eas te rn S um at ra , and w es te rn B orneo . Mos t of the ou te rshelf is covered by coarse sands that probablyare rel ict from the la tes t g lac ia l epoch. Gravelbo t tom is c o m m o n a r o u n d the i s lands between


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Structural Framework of Sunda Shelf 2 3 2 7S um at ra and B orneo , and in the en t r ance toMalacca S t ra i t .

S t r a t ig raphyT he l and geo logy o f t he a reas su r round ingthe Sunda Shel f i s poor ly known in spi te of

near ly a century of work by many geologis ts ;accord ing to U m bgrov e (194 9) abou t 6 ,000publ ica t ions on the geology of Indonesia existed prior to 193 7. I t is imp ossible in theframework of th is s tudy to descr ibe the geologyof Indonesia in great deta i l ; for a summary ofgeologic knowledge of th is area up to 1949 thereade r i s r e fe r r ed to van B em m elen (1949) .S ince his work l i t t le has been added to the reg iona l know ledge . A lex ande r (196 2) andC hung (1968 ) r ev iew ed the geo logy o f t he Malay Peninsula , L iecht i et al. ( 1 9 6 0 ) c o m p i le dthe w ork o f t he R oya l D utch S he l l G roup inthe Br i t i sh Terr i tor ies in Borneo, and Hamil ton(19 72 a) p repa red a t ec ton ic m a p of Indon es i auti l izing the plate concept. In this section onlythe main highlights of the geology will begiven.The oldes t s t ra t igraphic uni t consis ts of crysta l l ine schis ts of unknown age that are productsof sedimentary deposi t s a l tered by regionalm e tam orph i sm , accord ing to van B em m elen(1949, p . 60) . The oldes t foss i l i ferous deposi t s

on Sunda land are middle and la te Paleozoicand have a very res t r ic ted dis t r ibut ion, mainlyon S um at ra , B orneo , and Malay P en insu la .U m bgrove (1938 ) sugges t ed tha t t hey o r ig i nated in a ner i t ic to l i t tora l envi ronment . Paleozoic s t ra ta of the Malay Peninsula are character ized by nor th-south para l le l be l t s of sedi ments of widely different facies. Several geologis ts descr ibed an extensive unconformity inupper Paleozoic s t ra ta associa ted wi th in tensefolding and volcanic ac t iv i ty {e.g., U m b g r o v e ,19 38 ) ; how ever K lom pe (195 4) ques t ionedthe val id i ty of such an unconformity .D ur ing the Mesozo ic E ra sed im enta ry be l t scon t inued to fo rm in the Malay P en insu la andnew ones evolved in Indonesia . The foss i l sc lear ly indicate a Mesozoic sea connect ion ( theT e thys S ea ) be tw een the p resen t Indones i anA rch ipe lago and the Medi t e r r anean r eg ion( v a n B e m m e l e n , 1 9 4 9 , p . 6 3 - 7 9 ) . P a l e o z o ic ,Mesozoic , and Cenozoic sedimentary bel t s inthe Sunda land region denote impor tant d i f fer ences in depth of deposi t ion. For example , the

ear ly Mesozoic s t ra ta of western Borneo andSumatra include l i t tora l and ner i t ic as wel l asdeep- sea f ac ie s (van B em m elen , 1949 , p . 22 5) .Al l of the Mesozoic and Cenozoic sediments on

the Ma lay P en insu la a re non m a r ine (C h ung ,1968 ) , i nd ica t ing tha t t he pen insu la has beenabove sea level s ince ear ly Mesozoic t ime. Ofthe larger i s lands , Java h as the fewest M esozoicsediments, consist ing of only a few small localit ies in cent ra l Java (va n Bem melen, 1949, p .6 0 3 ; U m b grove , 1938 ) w here p re -T er t i a ry sediments were recognized.

O rogen ic m ovem ent s dur ing the C enozo icEra gave the region i t s present physiography.About 75 percent of the surface of the i s landsconsis t of sediments and volcanic deposi t s ofC enozo ic age . E norm ous T er t i a ry vo lcan ic ac t iv i ty which occurred in most par ts of the Arch ipe lago p roduced m any m ixed o r i n t e rca l a t edtuffs, breccias, and lava flows. The Tertiary sediments are concentra ted in i sola ted oblong bas ins wi th high ra tes of sedimenta t ion on the i s l ands o f S um at ra , J ava , and B orneo . A ccord ingto van B em m elen (1949) , i n pa r t o f sou th S umatra the Neogene a lone has a th ickness of11 .5 km . U m bgrove (1938 ) gave these bas ins aspecia l name, " idiogeosyncl ines ."

The s t ra t igraphy of the Ter t iary bas ins i spar t icular ly in teres t ing, as the bas ins are s imi larto others on the Sunda Shelf. T he bas ins onland w ere desc r ibed by U m bgro ve (1 93 8 ) ,S c h u p p li ( 1 9 4 6 ) , v a n B e m m e l e n ( 1 9 4 9 ) ,S c h a u b a n d J a c k s o n ( 1 9 5 8 ) , W e e d a ( 1 9 5 8 a ,b ) , W e n n e k e r s ( 1 9 5 8 ) , H a i l e ( 1 9 6 3 ) , a n dK oesoem adina ta (1969) . O f f shore bas ins w eredesc r ibed by T od d and P u lung gono (1 97 1) ,K o e s o e m a d i n a t a a n d P u l u n g g o n o ( 1 9 7 1 ) , a n dC ree (1972) . T he bas ins d id no t o r ig ina te s i mul taneously , a l though some geologis ts arguethat the sedimentary sequences ins ide the severa l bas ins are s imi lar . According to Koesoem ad ina ta ( 19 69 ) , each bas in unde rw en t a m egacycle of sedimenta t ion beginning wi th at ransgress ion, fo l lowed immediate ly by bathyalcondi t ions , and terminat ing wi th a regress ion.

Marine Eocene rocks are res t r ic ted, wi th bes tdevelopment in the eas tern Java Sea , eas ternand nor thw es te rn B orneo , and poss ib ly nor th e rn S um at ra . T he E ocene in sou the rn S um at raand the western Java Sea i s represented by vol canic rocks . Dur ing the Ol igocene th ick del ta icc las t ic deposi t s accumulated in the westernJava S ea (T odd and P u lunggono , 1971) ,whereas mar ine condi t ions exis ted in eas ternJava and sou theas t e rn B orneo (Weeda , 1958 a ,b ) . T h ick ca rbona te depos i t s accum ula t ed dur ing the Ol igocene in southeas tern Borneo.The most widespread l i thologic uni t in thesouthern Sunda Shel f i s a lower Miocene ree-foidal l imestone. For the f irst t ime deposit ion


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2328 Zvi Ben-Avraham and K. O. Emerybecam e con t inuous th roughou t t h i s a rea (K oe-s o e m a d i n a t a a n d P u l u n g g o n o , 1 9 7 1 ) . C a r b o n ate deposi t ion was fol lowed by a regress ivephase dur ing which shales were deposi ted inthe deeper par ts of the western Java Sea bas ins ,and shale and carbonate rocks in the eas ternJava Sea . This regress ive phase in the eas ternJava Sea was associated with folding and uplif t .In the western Java Sea a shor ter second mar ine t ransgress ion can be recognized a t the endof the M iocene . L i t t le is kno wn abo ut the m id-Ter t iary s t ra t igraphy of the bas ins in the nor thern Sunda Shelf.

During P l io-P le is tocene t ime mar ine sedi m enta t ion con t inued ove r the en t i r e S undaShelf and gradual ly changed the f loor to a re lat ively flat plane.Igneous ac t iv i ty in the Indonesian Archipel ago occurred a lmost throughout i t s h is tory(van B em m elen , 1949 , 19 54 ) . T h i s ac t iv i typrobably was associa ted wi th major tec toniceven t s . S om e of the m os t consp icuous m ag-mat ic fea tures are the grani t ic bel t s which insom e a reas (e.g., M alay P en insu la ) a re ve rylong and con t inuous and c l ea r ly m ark the pos i t ions of o ld tec tonic bel t s . Not a l l the grani tesand other igneous rocks are ar ranged in bel t sand in many places re la t ions to tec tonic bel t sare not c lear . In t rus ive rocks range f rom large

bathol i t ic bodies to many smal ler s tocks , sheets ,and dikes . A wide range of composi t ions probably reflects relat ive posit ion with respect to tecton ic be l ts (H a the r ton and D ick inson , 1969 ;H a m i l t o n , 1 9 7 2 b ) . M e t a m o r p h i s m , b o t h r e g iona l and the rm al ( con tac t ) , i s w idespreadand af fected both sedimentary and igneousrocks .E a r t h q u a k e E p i c e n t e r s

The Indonesian Is land Arc i s a bel t of ac t iveear thquakes that separa te four major pla tes ofl i t h o s p h e r e ( F i t c h , 1 9 7 0 ) : I n d i a n O c e a n - A u s t ra l ian, Asian, Phi l ippine Sea , and Paci f icpla tes (F ig . 3) . Severa l o ther smal ler p la tes arepresent in the eas tern par t of the archipelago,but thei r boundar ies are subject to considerableinterp re ta t ion . T he Su nd a Shel f i s near lyaseismic and is par t of the Asian pla te . I t s h istory and s t ructure , therefore , are ra ther d i f fer ent f rom those of the adjacent regions .S E I S M I C R E F R A C T I O N AN D O B L I Q U E R E F L E C T I O N

G e n e r a lFo r ty obl iqu e ref lec t ion-ref ract ion profi lesm a d e w i th ex p e n d a b le A N / S S Q 4 1 r a d i o so n o -buoys in the Java Sea (F ig . 4) provided infor

mat ion on the depth to acoust ic basement andthe veloci ty of sound wi thin i t and a t var iousdepths in the over lying sediments . The sono-buoy prof i les are typical ly 10-25 km in range.T hey w ere ob ta ined dur ing norm al inc idencereflection profi l ing, when the range on the so-nobuoy open ed a t a speed of abou t 7 km /h ou r .T he sonobuoy hydrophone s igna l s w ere r ad iot ransmit ted to the ship , where they were recorded on a P rec i s ion G raph ic R ecorde r(P G R ) and on ana log m agne t i c t ape . T h e technique of us ing expendable sonobuoys and theveloci ty and depth ca lcula t ions f rom obl iqueref lec t ion and ref ract ion has been descr ibed byL e P ichon et ai. ( 1 9 6 8 ) a n d H o u t z et ah(1968) . Because of the shal low depth of water( 4 0 - 6 0 m ) , t h e s o n o b u o y h y d r o p h o n e w a svery near the bot tom. This proximi ty and the10-second source repet i t ion ra te a l lowed theresolution of relat ively small features in thebasem ent (F ig . 5) and the over lying sedim entsand the dis t inguishing of many s t ra ta (F ig . 6)with different velocit ies.

T he sha l low w a te r dep th and the long (50-75 msec) dura t ion of the outgoing pulse a l lowed determinat ions of wide-angle ref lec t iondata in only a few ins tances . W he re such datawere avai lable ihey were used to compute themean velocity between reflection interfaces ( interval veloci ty ; Le P ichon et al., 1968) forcom par i son w i th the hor i zon ta l p ropaga t ion ve locit ies obtained from the refraction data forref ract ion ar r ivals f rom the same inter faces . Adetailed description of the sonobuoy profi les isg iven by B en-A vraham ( in p ress ) w here t echnique of analysis also is described.The resul ts are combined wi th previously obta ined two-ship se ismic ref ract ion da ta f romthe deep-sea area south and eas t of the JavaS ea (R a i t t , 196 7) and from the nor the rn S unda

Shelf (D ash , 1970 , 1971) to s tudy the re la t ionof the shelf to the deep sea.Resul ts

Java SeaSeven grou ps of ref rac t ion v eloci t ies have been identif ied in the Java Sea (Table1 ) , a l though the complete success ion of veloci t ies was detected in no single location. Thecomputat ions show some layers to be very th in ,probably as a resul t of d iscont inuous sedimenta t ion, fo lding, and faul t ing. Because the sourcepu l se l eng th w as 50-75 m sec , t he m in im umresolut ion of lay er th ickness i s abo ut 40 m.A compar ison between veloci ty logs of somewel ls in the sedimentary bas ins of the west JavavSea with son obu oy profiles ov er these basin s


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2 3 3 0 Zvi Ben-Avraham and K. O. Emery

Fio. 4Posit ion of traverses and geophysical measurements. Large numbers 1-22 identify traverses shown inFigures 11-17; numbers 23-29 identify traverses ( f rom Parke et al, 1971) used to construct the three-dimensionalmodel of Figure 20. Other l ines are f rom Parke el al. (1971) . Small numbers indicate posit ions of radiosonobuoys.Wide l ines marked A~H identify positions of seismic and magnetic profiles shown in Figure 19.

(B en-A vraham , in p ress ) show s a r easonab lecorre la t ion. Poss ibly the main reason that therefraction profi les do not show as many veloci ty discontinuit ies as the well logs is the existence of two l imestone layers whose se ismic velocity is so high as to cause velocity inversion.Such invers ions cannot be detected by the se ismic ref ract ion process .Sonobuoy refraction velocit ies l isted in Table1 appear as his tograms in F igure 7 . Veloci t iesof the order 1.8-2.3 k m / s e c w e r e t h e m o s tf requent ly observed in the ent i re area , wi thhigher velocit ies tending to be confined to certa in geographic locat ions . Refract ion veloci t ies

o f 2 .5 -4 .2 km /sec a re a lm os t absen t be tw eenS um at ra and B orneo (F ig . 7 ) , bu t t hey appea rin the eas tern Java Sea between Borneo andJava. On the other hand, veloci t ies h igher than5 km/sec are rare in the eas tern Java Sea , butthey a re m ore com m on fa r the r nor th , be tw eenBorneo and Sumatra . The regional l imi ts probably are due to differences both in basementtype and in the mater ia l of the sedimentary col um ns .The acoust ic basement can be represented byV-, V^ or Vg (Table 1) , depending on the area .The average refraction velocity of V, which isconfined almost entirely to the area between


9/44

Structural Framework of Sunda Shelf 2331

0

1 -

0 km 10FIG. 5Sonobuoy station 4, on line 5 (Fig. 4) between Sumatra and Borneo. Symbols are: D, direct arrival:V, computer velocity; R, oblique reflection; m, multiple arrival (Fig. 6). Figure shows, on left, sonobuoy profile andtracing of it, and on right, concu rrent norm al inciden ce reflection profile and interpretation of it: 4.69 and 5.74obviously originate from same horizon; 5.74 is result of propagation upslope and 4.69 downslope. True basementvelocity thus lies between these two values. It is indicated as 5.22 km/sec, (V,,) in Table 1. No te abrup t changein slope between 5.74 and 4.69 lines.B orneo and S um at ra , i s 6 .27 km /sec w i th as tandard devia t ion of 0 .56. According to Birch(1960) these veloci t ies for rocks a t pressure of500 bars (appropr ia te to the depth of V, , ) aretypical of a wide var ie ty of igneous and meta-m orph ic rock types; how ever , the highest veloci t ies (as in son obu oys 3 an d 5) m ost l ikely aref rom bas i c , u l t r abas i c , and m e tam o rph ic rocksra ther than f rom grani t ic rocks . V. , which averages 5 .02 km/sec , a lso probably i s character ist ic of several rock types but is mainly representa t ive of grani te and a lower Miocene l imes tone and dolomite . Sonobuoy 7, between thetwo grani t ic i s lands of Bangka and Bi l l i ton, recorded a very pro m inen t ref ract ion ar r ivalf rom the shal low basement . I t has a veloci ty of4 .70 km /sec , w hich shou ld r epresen t t he g ra nit ic basement. This velocity is sl ightly lowerthan is typical for grani tes . S imi lar ly , sonob uoy20, ove r the K ar im u ndjaw a a rch , w hich is p robably a grani t ic body, has a basement veloci ty of

4 .83 k m /s ec . In th is s ta t ion, how ever , thegran i te ma y hav e a velocity as high as 5 .95km /se c (T ab le 1 ) . In the deep sed im enta ry ba s ins Vs can belong to a lower Miocene l imes tone, the Batu Radja L im estone (T od d andPulu ngg ono , 197 1) in the western Java Sea ,and to the upper K udjung L im es tone (C ree ,1972) far ther eas t . The average veloci ty valueof the ref ract ion ar r ivals f rom what appears onthe normal incidence records to be the lowerM iocen e l imestone i s in good agree me nt w i ththe velocit ies measured in wells for this horizon(B en-A vraham , in p ress ) . V , , w i th an ave ragevalue of 4 .09 km/sec , a lso i s f rom the lowerMiocene l im es tone .

Vg and Vj refraction velocit ies of 3.04 and2.59 km/sec , respect ively , were observedmainly in the eas tern Java Sea , whereas theyare a lmost absent far ther west and nor th (F ig .7 , Table 1) . These veloci t ies may be due tolocal d is t r ibut ion of shal low-water g lauconi t ic


10/44

23320 ,

Zvi Ben-Avraham and K. O. Emery

FIG. 6 Sonobuoy station 13 southeast of B illiton Island in area having no major b aseme nt lows or highs. Symbolsare sam e as for F igure 5. Arrival with Kg correspo nds to strong reflector w hich probably is lower M iocene limestone. Discontinuity in this arrival results from some deeper structures that intrude limestone horizon. Otherarrivals are very clear and continuous, indicating well-stratified layers.M . 19 - 4 0n_Ji

b 21^ 0

lOrJOLH

Refraction Veloc ities in l(m/secFIG. 7Histograms of sonobuoy refraction data from Java Sea. Stippled entries represent velocities of doubtfulaccuracy. Top: eastern Java Sea (sonobuoys 19-40); middle: west and north Java Sea (sonobuoys 1-18); bo t tom:entire Java Sea.


11/44


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sands and clays of early Miocene age accordingto published stratigraphic sections (Todd andPulunggono, 1971; C ree, 197 2). V V. and V^are abundant throughout the Java Sea andtypify sediments of Mio-Pliocene and Plio-Pleistocene age.Northern Sunda ShelfThe preliminary results of some two-ship seismic refraction studies using explosives, made by the Imperial College of London with the cooperation of various

countries in the region, are given by Dash(1970, 1971) and Dash et al. (1970). Threeprofiles around Natuna Island penetrated to themantle at a depth of about 20 km. Comparisonof the crustal structure of the northern SundaShelf with typical oceanic and continentalstructures shows it to be intermediate (Fig. 8).In fact, the crustal thickness resembles that ofsome of the smaller ocean basins, such as theGulf of Mexico and the Colombia Basin in the


12/44

2334 Zvi Ben-Avraham and K. O. Emery

KM 0

10 -

1 5 -

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T T T T8.1(4)FIG. 8Comparison of velocity structure of northernSunda Shelf with those of continents and ocean basins;( / ) Raitt , 1963; (2) Raitt , 1967; ( i ) Dash, 1971; {4)Edgar et al., 1971.

C ar ibbean S ea (E w ing et al., 1 96 0) . T he m a indifference is in the water depth, which is a fewtens of meters on the shel f and severa l k i lometers in these ocean bas ins . The crus ta l s t ructurein most marginal bas ins i s re la ted c losely tothat of the adjacent main ocean bas in , exceptfor the comparat ively greater th ickness of low-veloci ty sediments in the former . As Menard(1967) has s ta ted, the essent ia l character is t icof a normal oceanic crus t i s tha t i t inc ludesLay er 3 mater ia l wi th a veloci ty ne ar 6 .7 k m /sec and a th ickness of about 5 km. I f th is layeris so represented, the veloci ty and/or th icknessof other layers would be considered modif icat ions of an otherwise normal oceanic crus t .Dash showed that crus ta l veloci t ies of 6 .7 and6.8 km/sec are present in some prof i les ,whereas others have a lower value of about 6 .2km /sec . T he c rus t a l - l aye r t h i ckness w as g iven

only for one profi le where a layer with 6.2km /s ec veloci ty is 15 km thick (F ig . 8; Da sh,1971) . A l though D ash ' s da t a ind ica t e the c rus ta l s t ructure to have character is t ics in termedia te between cont inenta l and oceanic crus t , theyare insuff ic ient to determine whether SundaShelf crus t or iginated as cont inenta l or oceaniccrus t .Veloci ty Cross Sect ions

Two s t ructura l sec t ions based on se ismic-ref rac t ion informat ion are shown in F igures 9(nor th - sou th ) and 10 (nor thw es t - sou theas t ) .The data in the deep sea are f rom Rai t t (1967)and on the shelf from the sonobuoy profi lesand D ash (1971) . T he sec t ion o f F igure 9 i snorm al to four "r id ges" one south of thet r ench , one sepa ra t ing the Java T rough f romthe Java T rench , Java I s l and , and the K ar i -mundjawa arch. The oceanic sect ion south ofJava conta ins two most conspicuous fea tures .(1 ) T he c rus t nor th o f t he Java T rench i smuch thicker than the crus t south of i t ; andnor th o f t he Java T rench the unconso l ida t edand semiconsol idated sediments are muchthicker than south of the t rench. (2) Theoceanic crus ta l layer . Layer 3 (wi th veloci ty6 .9 -7 .2 km /s ec ) , ex tends ac ross the t r ench towi thin 50 km of Java and seems to cont inuebeneath the i s land of Java (Rai t t , 1967) . Nor thof the i s land the sonobuoy informat ion i s l imi ted to the upper crus ta l s t ructure , to the lowerMiocene l imestone in the bas ins , and to thepos tu l a t ed g ran i t i c l aye r ove r the K ar im und-jawa arch. The sect ion c lear ly demonst ra tes thedifferences in penetrat ion capabil i ty and in resolut ion of two-ship ref ract ion and sonobuoytechniques . Whereas the la t ter can give deta i ledinform at ion for the sedim entary colum n, theformer essent ia l ly averages the s t ructure overthe range of the profi le and is necessari ly insens i t ive to deta i led s t ructures ; i t does , however ,pene t r a t e m uch deeper .

The s t ructura l sec t ion f rom the F lores Seaalong the Sunda Shel f to Natuna Is land (F ig .10) i s approximate ly a t r ight angles to the previous sect ion. F rom a normal oceanic sect ion inthe F lores Sea (S ta t ion R , j ) the crus ta l th ickness appears to increase abrupt ly westward onthe evidence of the two-ship prof i les ( the sonobuoy data s imply indicate uni formi ty of the uppe r c rus t a l t h i ckness ) . T he s t ruc tu ra l t r ans i t i onfrom the oceanic basin to the shelf is unclear.In the Natuna Is land area , the mant le i s a t adepth of 20 km. The main crus ta l layer in theN a tuna a rea has a ve loc i ty o f 6 .2 km /sec and


13/44


S 10I20

R24 R25 R261 5 1 T r o u g h ^ , _ _ - . - r f ?% ' *^_ ,_ / P : - - ,

, 5 9 -6~l

1817 1920I II I4 7 2 _

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21 15 2214I I I I

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532 'fo/ greater scale)

_60 160 - ' 60... -1 90 = -'''2 35 ;_'" '

\ 4 3 ? - - - ' \ 398 ,

F I G . 9 Seismic structure section ac ross Java Sea , Java Trough, Java Trench, and nor thern Whar ton bas in .Dots represent sonobuoy stations and dots marked with R represent two-ship refract ion stat ions f rom unpublished report by Raitt (1967). Sonobuoy results are shown also at greater (expanded) scale .

near Tioman Island a velocity of 6.8 km /se c, Seawasa more sophisticated version of the sys-wbich is similartothat of Layer3. tern described by Kno tt and Bunce (1 96 8) .Two different sound sources were operated sep-SEISMIC REFLECTION arately and together (fired simultaneo usly) at

Procedure various times during the cruise; one, an under-The seismic profiler system used in the Java water spark discharged 85,000-95 ,000 joulesS.W Nolimi

s * nomn 1 3 2 4 5 6 13 14 21 2 4 27 29 32 30 Ri4 R H S R'>C .s S.E Tiomon ii r ii iI I II I II I0 , - ^~ _ 5 - = -c -I I I I II

I

3.0 9065 27

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at motm * )O r . ' - J , '

II4 5 ' - - - , 2 4 j- . - ^ . _ _ , .

, S 2 ? . > 3 3 - 2 55 1 4 5 5 ^

Fio. 10Seismic structure section from F lores Sea across Java Sea to Tioman and Natuna islands. Symbols sameas in F igure 9. Stations at Tioman and Natuna islands are f rom Dash (1971) .


14/44

2 3 3 6 Zvi Ben-Avraham and K. O. Emeryof s tored energy, the other an ai r gun of 40,120, or 300 cu in. discharged 1,800 psi of compressed ai r . Seismic s ignals were received ontwo 30-m l inear arrays, band-pass f i l tered between 20 and 50 Hz, and recorded on threePrec i s ion Graphic Recorders opera t ive a t 5-and 2.5-second sweep rates . In addi t ion, abouttwo thirds of the t raverses were recorded unfi l -tered on magnet ic tapes.

The analog prof i les over the shelf are verynoisy, therefore, the normal incidence ref lect ion data were reprocessed at the laboratorythrough a s imple spat ial f i l ter . The analog magne t i c t ape record ings were f i l t e red a t 18%-250-Hz band-pass and digi t ized by using the shotinstant of each successive outgoing pulse tostart a 10-bit digitizer driven by a stable 512-Hz osc i l l a tor . A running summat ion of fourshot s was made and d i sp layed on an incremental x-y-z plot ter . An average of four gave abouta two- fo ld reduc t ion in random backgroundnoise whi le adding the horizontal ly coherent ret u rns . Sloping layers with dip up to 3 wereadded with less than 3-dB at tenuat ion due tophase shif t (at 30-Hz signal center f requency) .These processed data were used to edi t theshipboard in te rpre ta t ion and proved mos t useful for detect ing the basement and removingnoise.

Resul tsJava SeaThe seismic prof iles (Figs. 1 1 -17) show that the Java Sea can be divided intothe Singapore plat form (or Sunda shield) inthe northwest , and the basins and r idges in thesouth and eas t (Fig . 29) .The Singapore p la t form has a sha l low basement and is overlain by a thin sedimentarycover (F ig . 1 1) . Th e basem ent appears to consis t of smal l "bodies ." Sonobuoy 1 (Fig. 11)

indicates that a thickness of as much as 1.5 kmof sediments can be present between the baseme nt bodies ; how ever , the basem ent could becont inuous and composed of a rough sur face(as shown by the pre l iminary in te rpre ta t ionaboa r d s h i p Em er y et al., 1972) . The l i nespacing of the profiles is not close enough tode te rmine whether the basement bodies a reequir t imensiona or elong ate in plan. Tw ot rougns conta in ing more than 800 m of sedi ments are the most prominent features in thisa rea : the Bangka depress ion which s t r ikesnorthwest-southeast paral lel wi th the coast ofSumat ra (Fig . 29) ; and the Bi l l i ton depress ion( F i g s . I L 18) which runs nor th-south para l le lwith the west coast of Borneo.

South of the is lands of Bangka and Bi l l i ton,in the western Java Sea, several deep basinswith sedime nts thicke r than 1 km ca n be recognized (Figs . 12 , 13) : South Sumat ra (onl a n d ) , Sunda , Wes t Java (par t ly on l and) , Bi l l i ton, and Tjer ibon (par t ly on land; nomenclature a f t e r Todd and Pulunggono, 1971) . Theland bas ins were descr ibed by Umbgrove(1938) as short - l ived marginal basins containing thick sediments that are weakly folded(" id iogeosyncl ines") . The bas ins a re separa tedby h igh a reas , the Lampung h igh (on l and) ,the Ser ibu p la t form, and the Kar imundjawaarch. The Seribu plat form (prof i le 7) and par tof the K arim und jaw a arch (profi le 14) appe aras fault blocks on the reflection-profiler records .

Detai led seismic coverage of the area between Java, Sumatra, Bangka, Bi l l i ton, and theKar i m und j awa a r ch ( Todd and Pu l unggono ,1971) shows that deep north-south faul ts separate the basins f rom the upl i f ts . The s t raightnor th-south coas t of Sumat ra be tween Bangkaand Java, which s t r ikes at 45 to the l ineat ionof the island itself, may mark the posi t ion of amajor faul t separat ing the La m pu ng high fromthe Sunda basin. This area has been dr i l led ext ens ive ly ( Hu m p hr ey , 1971 ) , and Todd andPulun ggon o ( 19 71 ) presented some dri ll logsand cross sect ions that show the metamorphicand igneous basement to be more than 3 kmdeep in the Sunda basin and about 2.5 km deepin the West Java basin.

An important feature shown by the seismicprofiles is a group of strong discrete reflectorsthat can be t raced throughout most of the basins. This group is identified as the Batu-Radja,a lower Miocene reefoid l imestone plus thin dolomi te (Todd and Pulunggono, 1971) tha t a l sois kno wn on southern Sum at ra and w es te rnJ ava (va.T\ B emm elen , 1949, p . 1 16 ) . I ts aver age seismic veloci ty determined by the sonobuoy profi les is 5 .0 km /se c (Tab le 1) . Th e importance of this horizon is i t s widespread dis t r i bution in the Java Sea and its value as a t imemarker , as i t seems to be cont inuous throughthe several basins (Fig. 7) and over some ofthe highs. This layer covers the Ser ibu plat form(F ig. 12, profi le 7) and par t ly covers the K arimu ndjaw a arch (Fi g. 13. prof iles 10, 12 ) . Inlarge par ts of the sedimentary basins this lowerMiocene l imestone is the acoust ic basement(F igs . 12, 13) for the energy of the reflectionprofi l ing equipment . At the southern end ofprof i le 6 (Fig. 12) dr i l l hole data (Todd andPulun ggon o, 19 71) indicate the t rue basement


15/44

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16/44


3 8' S 1 0 8 ' 1 5 ' E

r s , , S e r i b u R o t f o r m f~- West Java Basm5* 34 ' s lo e * 00 ' e

5 5 4 ' S 1 0 8* 0 0 ' E 3*04 ' S 109* 10 ' E

FIG. 12Interpretive geophysical profiles 6, 7, and 8 across western Java Sea. L designates Batu Radja, lowerMiocene limestone. Basement configuration on left of profile 6 and in profile 7 was inferred from cross sectionsreported by Todd and Pulunggono (1971) and gravity anomalies. Other symbols are same as for Figure 11.to be more than 3 km deep, about double thepenetration recorded by the seismic profile.

Several other prominent reflectors are visibleabove the Batu Radja Limestone in the sedimentary basins (e.g., on the right-hand side ofprofile 10, in the Billiton basin) and over theSingapore platform (e.g., in profile 1 at the B illiton depression). Most of these reflectors areso limited in area that they cannot be correlated from one profile to another. They may represent different lithologies or changes in physical properties of the sediments.As basin-fill material above the lower Miocene limestone generally dips away from thehigh areas (the Singapore platform, the Lampung high and the Karimundjawa arch; Figs.12, 13), these uplifts probably served as

sources of the sediments. Locally some of thestrata dip in one direction and other strata ofthe same section dip in another direction, ascan be seen in profile 7 (Fig. 12) where theBatu-Radja dips east and west away from theSeribu platform. In the upper part of the section the layers dip westward away from theKarimundjawa arch, and at the very top thelayers have a strong eas* dip away from theLampung high. This may indicate that somemovements occurred along the faults that border the highs during various periods of the basins' evolution. On the south side of the Karimu ndjawa arch in profile 14 the relatively steepdips indicate continuous uplift during the period of basin filling. Many structural discontinuities such as small folds, faults, and angular


17/44


6 3 2 ' S t 0 9 2 3 ' E 3- 10 ' S 109* '7 ' E

6 1 7 S 11 0* 4 2 ' E 3 * 3 8 ' 5 l i e 47 ' E

6 * 2 1 ' S 1 1 1 *5 1 ' E 3 * 4 3 ' S 1 1 1 * 4 9

FIG. 13Interpretive geophysical profiles 10, 12, and 14 across western and central Java Sea. Symbols are sameas for Figure 12.

unconformities exist in the basin fill. Except forthe Batu Radja Limestone, a given reflectinghorizon can be followed only a short distance.In the eastern Java Sea (Figs. 14-17) basinsand ridgelike features also are present, but theircharac teristics, dim ensions,' and orientationsdiffer from those in the western Java Sea. Fivedominant uplifts are the Meratus and PulauLaut ridges which extend from the MeratusMountains on Borneo and the Pulau Laut Island southwest into the Java Sea, the Karimundjawa arch which extends east and northeast from the western Java Sea into its easternpart, the Bawean arch, and the Madura Ridgewhich extends east from Madura Island toKangean Island and the Flores Sea. Except forthe Madura Ridge, which strikes east-west, all

the ridges strike northeast-southwest. The Karimundjawa arch is transitional, as it strikes generally east-west in the western Java Sea butbends northeastward in the eastern Java Sea.All these uplifts are relatively long and narrowand, except for the Karimundjawa arch, appearto be anticlines. The seismic profiles (Figs. 14,15) clearly demonstrate that these structuresare plunging; the Meratus, Pulau Laut, andprobably also the Bawean ridges plunge southwest and the Madura Ridge east. The PulauLaut ridge and the Bawean arch may be partsof one continuous structure or two separate enechelon structures. If these two uplifts join,their junction is east of Bawean Island near so-nobuoy station 27 (profile 16). A detailed seismic survey of an area north of eastern Java and


18/44


* r j " i- s n r 5 9 '

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^ ^ ^ ' ^ ' ^ M o d u r o R dqeBasir ' T- m ^cZZ.4 3 S I I 4 ' 4 2 E 1? S 40 4 ' E

FIG. 14Interpretive geophysical profiles 16, 17, and 18 across central and eastern Java Sea. Fine-dashed lineshows Bouguer gravity anomaly. Horizontal bar with P designates part of original recording for profile 18 reprodu ced in Figu re 18. Oth er symbols are same as for Figure 12.M ad ura (Cre e , 1972) show ed the exis tence ofa few other s t ructura l h ighs para l le l wi th thePulau Laut r idge , but they are shor t and couldnot be seen because of the wide spacing of ourseismic reflection l ines.As in the western Java Sea, the uplif ts in theeas t are separa ted by deep sedimentary bas ins :the B aw ean T rough (o r t he Mur iah T rough o fC ree , 1972) ; t he K angean bas in and the Madura bas in . The bas ins in the eas tern Java Seadiffer from those in the west . Here the basinsare narrow and long, whereas those in the westare wide and more c i rcular in shape. The reflection and refraction profi les show that themater ia l in terpre ted to be lower Miocene l imes tone i s preserve d in th is area (F igs . 14, 1 5) .As in the western Java Sea, i t is a very strongref lec tor and local ly i s the acoust ic basement(F ig . 16) . C ree r epor t ed the p rom inen t occur rence of a l imestone hor izon of probably ear ly

Miocene age in some wel ls nor th of eas ternJava and Madura . T h i s s t r a t ig raph ic pos i t i oncorre la tes wi th that of the Batu-Radja L imes tone in the western Java Sea . Cree termed thisl aye r "upper K udjung L im es tone , " bu t w e usethe name Batu Radja to reduce confusion.S t ra ta in the eas tern bas ins are more deformed than those in the western bas ins . Theyconta in broad folds wi th axes para l le l wi th thet rends of the main s t ructura l uni ts . Many faul tsa l so a re p resen t . A m ong the m os t im por t an tfeatures in th is area are the unconformit ies anddips of the sedimentary layers. On both sides ofthe Meratus and the Pulau Laut r idges the sedi mentary layers dip parallel with the ridge flanks(prof i les 18 , 20, 21) , and the updip ends of thestrata, as well as the r idges themselves, aret runcated by the sea f loor . The spacing betweenthe reflectors on the ridge flanks is un iform anddoes not increase away from the ridge crests.


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FIG. 15 Interpretive geophysical profiles 20, 21 , and 22 across eastern Java Sea. Horizo ntal ba r with Pdesignatespart of original recording for profile 20reproduced in Figure 18.Symbols aresameas for Figure14.This indicates that these two r idges formed re l a t ively recent ly , and subsequent tothe hor i zonta l deposi t ion of thel aye r s . Thet im e of thiseven t is difficult to da te , but it m us t be post -ea r ly Miocene , as thel ime stone lay er (profi le2 1 ) was uplif ted and t i l ted wh en the ridgesw ere fo rm ed andsubsequen t ly t runca ted .TheB aw ean a rch mayh a v e a similar historybutonly oneprofile (14) p rov ided da ta . In c o n t ras t theM a d u r a R i d ge hasbeen upl i fted con t inuously dur ing thepe r iod of basin filling, as

the sediments dip steeply and th icken awayfrom ther idge (F ig . 18 ) . The seismic profilesindicate that the M a d u r a R i dg e was uplif tedvery recently ortha t it still isac t ive . B order ingthe Madura bas in on thesouth arefour basem ent r idges ; thelarges t oneis thepresent vol canic arcon which the i s landsof Bal i , Lombok,and Sum baw a are s i tuated (F ig . 17 ) . The oth err idges probably t rend eas t -west para l le l wi ththe island arc.A s a w hole thetec tonic pat tern of the west-


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FIG. 16Schematic north-south cross section across East Java basin on Java, Java Sea, and Barito basin onBorneo. Based on seismic profiles 14 and 16 (Figs. 4, 13, 14), and on van Bemmelen (1949, Fig. 293), Weeda(1958a,b), Koesoemadinata and Pulunggona (1971), and unpublished information from drillholes.em Java Sea seems to resul t f rom tensionalforces, whereas that of the eastern Java Seawas formed by compress ion .

One of the most interesting features of theeastern Java Sea is the relat ion between thestructures and the shelf break. The Pulau Lautand the Meratus r idges more or less paral lel theshelf edge (Fig. 29) , but the Kangean basinstr ikes obl iquely to i t , and the Madura Ridgeand the Madura basin cut the shelf break perpendicular to i ts t rend and cont inue into theFlores Sea. The s t ructural sect ion (Fig. 10)shows that the crust beneath the Flores Sea isoceanic and that i t thickens abrupt ly westward.This suggests that the Madura basin is underlain by oceanic crust wi th a thick sedimentarycolumn over the Java Sea but wi th thinner sediments over the Flores Sea.

Northern Sunda ShelfThe most extensivework in the northern Sunda Shelf was done byP a r k e et al. ( 1 9 7 1 ) a b o a r d R / V F.V. Hunt. Inthe present study, two of their seismic profiles,not previously publ ished, were analyzed and incorporated with the previously publ ished resul ts(Fig. 29) . In addi t ion, some smal l -scale s tudieswere repor ted by Dash (1970, 1971) and Dashet al. (1970, 1972) . Pre l iminary resu l t s of offshore explorat ion were discussed by Koesoemadina ta and Pulunggono (1971) . In th i s sec t ionthe s t ructural e lements in this area wil l be summarized briefly.A major r idge separates two large sediment-fil led basins, the Gulf of Thailand basin on thewest , and the Brunei basin on the east . TheGulf of Thai land basin is about 1,100 km longand averages 200 km wide at the 2-km isopachdepth. In this basin large folds and faults havebeen recognized. In i ts widest par t between lat .41 5 ' and S^SO'N a 300-km-long zone of intensive folds trends east-west and appears to bedue to tectonism. The folds have side dips of5-10 and a re t runca ted by an unconformi ty

that is 0.5-1.0 km below the sea f loor . In someplaces these folds are associated with steep normal faul ts that cannot be mere s lump planes,because they are beneath the flat shelf with nofree s lope toward which s lumps or s l ides couldmove. Several folds in this zone have the character is t ics of diapirs . These diapir ic int rusions,if they exist , may consist of shale, as indicatedfrom some ofl 'shore dr i l l ing resul ts (Koesoemadina ta and Pulunggono, 1971) . In the southernpart of the Gulf of Thai land basin northeast-t rending faul ts paral lel the bo un dar y be tweenthe basin and the Singapore plat form.Th e B runei basin is even larger than th eGulf of Thailand basin, with a total length of

about 1,500 km and an average width of 200km at the 2-km isopach depth. I t appears to bepart of the Northwest Borneo geosyncl ine onBorneo. The par t of the basin on land was descr ibed by Liecht i et at. ( 1 9 6 0 ) , F i tc h ( 1 9 6 1 ) ,Wi l ford (1961) , Hai le (1963) and o thers . TheBrunei basin has two major t rends: the westernpart t rends northwest paral lel wi th the Gulf ofThai land basin, and the eastern par t t rendsnortheast subparal lel wi th the northwest coastof Borneo. As in the Gulf of Thai land basin,folds and faults exist but here they probablytrend northwest. Offshore dril l ing in this basinreached a basemen t of low-grade metam orphicrocks , including phyl l i tes of Cretaceous-Eoceneage.The Gulf of Thai land and Brunei basins aresepara ted by the prominent Natuna Ridge , a d i rect cont inuat ion of the Semitau zone in Borneo (van B emm elen, 1949, v. 1, p. 33 1) thatplunges northwest . Faul ts wi thin the r idgeprobably t rend paral lel wi th i ts s t r ike (Fig.1 9 ) . Along the northeastern f lank of the Na

tuna Ridge a major faul t ( the Sarawak faul t )extends the ent i re dis tance from central Borneointo the Sunda Shelf (F ig. 19; Ya nsh in, 1966 ;Tj ia , 1970; Dash et al, 1972) .


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Madura L Madura RidgeBasin

8 4 1 ' S 116 4 0 ' E 6 24 S 116 3 6 E150 lim

FIG. 1 7 Interpretive geophysical profile 22 where it crosses deep water between J ava Sea and L ombok. Symbolsare same as for Figure 14.The southern and the northeastern boundaries of the Gulf of Thailand basin abut platforms, one of which is the Singapore platformon the south. The Khorat-Con Son platform onthe north seems to be a direct continuation beneath the sea floor of the basement rocks ofSouth Vietnam. It is characterized by a roughtopography and becomes deeper on the south.The Khorat-Con Son platform and the NatunaRidge are separated by a major discontinuitytermed the Natuna rift (Figs. 19, 20) that

crosses the entire northern Sunda Shelf and becomes narrower southward. It seems to be connected with the Billiton depression on the Singapore platform.

M A G N E T I C S ProcedureMagnetic data over the Sunda Shelf were recorded by various ships and by Project MAGN ET aircraft of the U. S. Naval OceanographicOffice (Fig. 21). Except for the data of R/VF. V. Hunt, all the magnetic data were digitizedand the regional field based upon the referencefield of Cain et al. (1968) was removed. Thedata of R/V F. V. Hunt was reduced graphi

cally as described by Parke et al. (1971). Inthe construction of the magnetic anomaly map(Fig. 21), the regional field was reduced by150 gammas to provide a better fit to the ob-


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served da ta . Tempora l var ia t ions were cons idered in none of the anomaly computa t ions .Interpretation

The Sunda Shelf is near the magnet ic equator . Under such condi t ions , where the magnet ic

field direction is at a low angle of inclination,the secondary f ield induced by a body of highsusceptibili ty in the earth's field must be opposed mainly to the direct ion of the ear th 'sf ield along a plane of observat ion above thebody. Thus, the resul t ing total intensi ty anom-


23/44

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24/44


Fio. 20Photograph of three-dimensional cardboard model of continuous seismic reflection profiles taken innorthern Sunda Shelf (Parke et ah, 1971). Broad arrow at top indicates north, and depth in km is marked bysmall horizontal bars on edges of profiles. Heavy dashed lines indicate position of Natuna rift. Profiles 23-29 ofFigure4.aly must be nega t ive . Another fea ture of m a g net ic bodies near the magnet ic equa tor concerns two-dimens iona l i ty . If a two-dimens iona lbody st r ikes north-south, it s hou l d p r oduce noanomal ies , because the c o m p o n e n t of magnet i za t ion a long the body cannot affect the fieldoutside the b lock as the lines of force neveremerge from the sea floor. This is one reasonwhy east-west sea-f loor spreading near theequator produces only low-ampl i tude anomal ies( M c K e n z i e and Sclater , 19 71 ) .

Analys i s of magnet ic anomal ies in t e rms ofthe shape and magnet ic proper t i es of the sourceis difficult, partly because of the complexi ty ofthe field arising from distribution of dipoles ,and par t ly because both remnant and inducedc o m p o n e n t s may be present in the source .However , severa l model s tudies were based onthe magnet ic anomal ies over the Sunda Shelf,an d in genera l the magnet ic da ta were found tobe very helpful in the s t ructural s tudy especial ly where the ba s em en t is shallow and itss t ruc ture could not be resolved properly by theseismic ref lect ion method, and in showing differences between basement types in var iouspar t s of the shelf. The magnet ic anomal ies provide li t t le help for est imat ing the depth to themagnet ic basement , because their wave lengths

and ampl i tudes appear to be m o r e a funct ion ofthe var ied l i thology of basement rocks thantheir depth, unl ike areas in which the basementis of uniform l i thology. Also it is not clear ifthe l ength /width ra t io of the basemen t i r regularit ies is sufficiently larg e to assume two-di mens iona l i ty . There are more magnet ic prof i lesthan ei ther seismic or gravity profiles, and thusin some areas the magnet ics serves as the onlysource of informat ion about the basements t ruc ture .

N a t u r e of Magnet ic Anomal iesThe magnet ic anomaly prof i le map (Fig. 21)shows that this area can be divided into severaldis t inct magnet ic provinces (Fig. 22) tha t denote cor responding provinces of li thic units.Province 1 has large isolated a nom alies w ithampl i tudes of 4 0 0 - 7 0 0 g a m m a s and wave lengths of 4 0 - 7 0 km; mos t of the anomal iesare negat ive. This type of anomaly general ly isassociated with r idges and the use of two-di mensional models appears just i f ied, as illust rated for Na t una Ri dge (Fig. 23).Thes e m od

els are genera ted by r idges paral lel wi th thet r end of the Na t una Ri dge, and they show thatthe r idge plunges northwest more s teeply thanis indicated by the seismic reflection profiles


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Structural Framework of Sunda Shelf 2 3 4 7

FIG. 21Total magnetic anomaly profiles plotted along all traverses. Most of data north of 2N were taken byR /V F. V. Hunt of the U.S. Naval Oceanographic Office (Parke et al, 1971) and south of 2' 'N by R/V Chainof Woods Hole Oceanographic Insti tution (Emery et al., 1972). Supplementary traverses from other ships (VER / V Vema a n d C O N R / V Conrad of Lamont-Doherty Geological Observatory; COH.M.S. Cook of Hydro-graphic Survey, England [Gray, 1959-1962]; PI Pioneer of U.S . Coas t and Geod etic Survey) and from airplanesof Project MAGNET (three-digit numbers) were used.

(F ig . 19) . At the southern par t of the r idge,the configuration of the top of the model blockgeneral ly ref lec ts the acoust ic basement topography, but on the nor th , the magnet ic bodymust be deeper , so that most of the acoust icbase me nt ma y be a cora l reef or o ther no n-magnet ized body. The model a lso expla ins thelarge negat ive anomal ies of about 800 gammas wi th very sharp gradients associa ted wi ththe Saraw ak fault (Fi g. 19, profi les C, D, E) .Th e provinc e 1 type of anom aly a lso is present in the eas tern Java Sea (prof i les 14-22) in

associa t ion wi th the Bawean arch, the Meratus ,Pulau Laut , and the Madura r idges and ther idge on which the i s lands of Bal i and Lombokare s i tuated. Oth er provinc e 1 anom al ies are in

the F lores Sea and in the western Java Seaalong the Bi l l i ton depress ion, a long a majornor th -south faul t , and a long the southeas t p ar to f t he L am pung h igh (F ig . 29) . T he h igh sus cept ibi l i t ies required by the models (F ig . 23)suggest that the s t ructura l e lements accompanied by these anomalies consist of basic or ul-t rabasic bodies .

Magne t i c p rov ince 2 has sha rp anom al i e swi th high ampl i tude and very shor t wavelengths, indicating that the source is very closeto the sea f loor . Anomaly ampl i tudes are 400-8 00 gam m as , bu t som e reach 1 ,500 gam m as .These anomal ies are mainly in the nor thernpar t of the S ingapore pla t form, where theyprobably are associa ted wi th smal l basement


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234 8 Zvi Ben-Avrah am and K. O. Emery

FIG. 22Magnetic provinces over Sunda Shelf. Typical magnetic anomaly from each province is shown in legend.

bodies (profi le 1, Fig. 11). C a lcu la t ions by Pete rs ' ha l f -s lope me thod (Pete rs , 1949) on som eof the more e longate anomal ies indicate adep th to m agne t i c basem ent of 200 m belowsea level , in good agreement wi th the seismicin t e rp re t a t ion . A few wide negat ive anomal iess imi lar to those in m agne t i c p rov ince 1alsoarepresen t in th is p la t form, but the s h a r p a n o m a lies of p rov ince 2 are supe r im posed u pon th em .The la t ter a lso are p resen t above the P u lauL au t R idge , w here they may be caused by basalt flowson the r idgetop.

Magne t i c p rov ince 3 is cha rac t e r i zed by veryb r o a d low ma gnet ic anom al ies ( less than 50g a m m a s ) or n o n eat all.T h i s p rov ince occup iesa m a jo r pa r t of the nor the rn S und a S hel f andthe China bas in far ther nor th . It also is presenta r o u n d the islands of B a n g k a and Bil l i ton. Thissm ooth m agne t i c p rov ince is a t t r ibuted to several different factors. In the nor the rn S undaShelf, it may be caused by the great depth of

bur i a l of m agne t i c basem ent or by a regionalm e tam orph i sm tha t dec reased the m agne t i za t ion of the basem ent rocks . In the Ch ina bas in ,w hich is under l a in by an oceanic crus t , thesm ooth p rov ince may reflect sea-floor spreadingdur ing a long per iod of cons t an t geom agne t i cf ie ld polar i ty . Around the i s lands of B a n g k aand Bil l i ton, it is the resul tof the low suscept i bility of the widesp read grani t ic basem ent .

Magne t i c p rov inces 4 and 5 are not so welldefined as the o the r p rov inces . P rov ince 4,w hich is ove r the cen t r a l and eas t e rn Java Sea,t h e K h o r a t - C o n Son p l a t fo rm , and smal l par tsof the no r the rn Sun da Shelf is cha rac t e r i zed bya low-level f ield with isolated magnetic anomal ies having shor t wavelengths , sharp gradients ,and var ied ampl i tudes of less than 300 g a m m as . T hese anom al i e s are related to smal l near-surface magnet ic bodies . In the cent ra l JavaSea, they are p resen t on both sides of the K a r i -mundjawa arch (Prof i les 10, 12) and p robab ly


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0 100 200 kmI . ... .. 1 . . I

Fio. 23Natuna Ridge magnetic and reflection profiles (Profile B and E of Fig. 19). Top curve is simulatedprofile generated by block model at bottom, middle curve is observed profile, bottom is seismic profile. Simulatedprofiles were calculated using modified version of computer program described by Talwani and Heirtzler (1964).Strike of model block is N60W, magnetic inclination 0, geomagnetic field is taken to be 40,000 gammas, and rocksusceptibility is .03 for left profile and .06 for right profile.

indicate the presence of d ikes a long the faul tedf lanks of the arch . There i s no ma jor ch ange inthe overa l l magnet ic character is t ic between theKar imundjawa arch and the Bi l l i ton bas in , suggesting that the arch consists of rocks with verylow magnet ic suscept ibi l i ty , such as grani te .T he K hora t -C on S on p la t fo rm , w i th s im i l a rmagnet ic character is t ics , probably a lso i s granite, as are the i s lands on the pla t form (H i ldeand Engle , 1967) . The genera l low-level magnet ic f ie ld of province 4 over the sedimentarybasins (m ainly ov er the Bi ll iton bas in) impl iesthat igneous rocks are deep, conf i rming the evi dence from the seismic profi les.

P rov ince 5 i s t he m os t p rob lem at i c one . T heboundar ies between this province and provinces1, 2 , and 4 are not abrupt , as though this province may conta in e lements f rom both provinces1 and 4 . The province i s in the western andno r ther n Java Sea , in the area between B orne o,S um at ra , and Java , ove r bo th the S ingaporepla t form and the Sunda and West Java bas ins .T he m agne t i c anom al i e s have r a the r l a rge am p l i tudes (200-600 gam m as) , bu t s l i gh t lysmal ler tha n those in provin ces 1 and 2 , andw ave leng ths (10-30 km ) tha t a re shor t e r t hanthose in provinc e 1 but larger th an tho se inprovince 2 . Thei r d imensions are s imi lar to

those r epor t ed by E m ery et al. (1970) in theA t l an t i c O cean fo r ocean ic m agne t i c anom al ies , and are the only anomal ies over the ent i reSunda Shel f tha t resemble oceanic magnet icanomal ies . The exis tence of th is magnet ic province indicates a possible difference between thebasem ent m a te r i a l under the S unda and Wes tJava bas ins and that under the Bi l l i ton bas in .Contras ts in the character of the magnet ic f ie ldover the S ingapore pla t form are apparent betw een the nor the rn pa r t (w i th m agne t i c p rovince 2) and the southern par t (wi th magnet icp rov ince 5 ) , i nd ica t ing cor respond ing con t ras t sbetween the l i th ic uni ts under lying these areas .A contour map of the magnet ic anomal ies inthe Java Sea i s shown in F igure 24. Only magnet ic anom al ies of provinces 1 and 4 have longenough wavelengths for contour ing a t th isscale . The map shows a genera l nor theas t -southwest t rend in the eas tern Java Sea , whichis approximate ly para l le l wi th the major s t ructura l e lements in th is area . In the western andnor the rn Java S ea , anom aly w ave leng ths a retoo shor t for contour ing and the t rend of themag net ic anom al ies , if any, i s un kn ow n. Aprominent magnet ic fea ture exis ts between Bi l l i ton Is land and Borneo in the southern par t ofthe Bi l l i ton depress ion. Also shown in F igure


28/44

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29/44

Structural Framework of Sunda Shelf 235124 are contours of magnet ic anomal ies in theInd ian O cean sou th o f Java and S um at ra . T hemap demonst ra tes the di f ference between thena tu re o f t he ocean ic m agne t i c anom al i e s andthose over the shelf. Whereas i t usual ly i s cons idered that magnet ic anomaly l ineat ions in theocean or iginate f rom the upper basal t ic par t ofthe oceanic crus t and represent s t r ips of rocksthat were formed a t the center of mid-oceanr idges dur ing per iodical ly revers ing magnet icf ie lds of the ear th (Vine and Mat thews, 1963) ,the anomalies over the shelf mainly reflect thes t ructura l e lements beneath i t . The poss ibi l i tystill exists that a few areas of this large shelfwere formed by a m echa nism of sea-f loorspreading, but the density of our data is insuffic ient for such a s t ructura l inference .

The magnet ic anomal ies south of the JavaTrench t rend most ly eas t -west and do notfol low the curve of the Indonesian Is land arc .This suppor ts the idea that they or iginatedfrom an eas t -west mid-ocean r idge. A majordiscont inui ty in the magnet ic-anomaly pat ternexis ts between the area south of Java and thatsouthwest of Sumatra . Also shown in the map(F ig. 24) i s a zone, more than 500 km long, oflarge eas t -west magnet ic anomal ies a t la t . 430 'S. This zone is associated with high heat f lowand w as in t e rp re t ed by V acqu ie r and T ay lo r(19 66 ) to be a f r ac tu re zone . P ro jec t M A G N E T f l ight 56 3B shows a large mag net ic ano m a ly a t 4 30 'S in sou theas t S um at ra (F ig . 21) ,abou t 500 km eas t o f t he anom aly m apped a tsea . Perhaps a large eas t -west s t ructure a longthis la t i tude i s cont inuous f rom the deep seaac ross sou the rn S um at ra .G R A V I T Y

H is to ry an d P roc edureG rav i ty obse rva t ions in the Indones i an A r

chipelago s tar ted wi th the c lass ica l surveyaboard subm ar ines by V en ing Meinesz in the1920s and 1930s (V en ing Meinesz , 1932) . A tthat t ime about 30 gravi ty pendulum s ta t ions inthe Java Sea showed that th is area i s character ized by "posi t ive anomaUes varying around 30m g a l" (V en ing Meinesz , 1948 , p . 3 6) . T h e va r ious gravi ty maps of the Indonesian Archipel ago that were publ ished over the years basedon V en ing Meinesz ' da t a (.e.g.,V e n i n g M e i n esz , 1932 ; Daly , 1940 ; W ool lard and S t range,1962) conta in no contours over the Java Sea ,because the contour in terval was 50 mgal . Themain so urce of la ter data i s the survey by R /VChain ( E m e r y et al., 1972) , p lus some l inesmade by other ships cross ing th is area .

T he m os t im por t an t r e su l t o f V en ing Meinesz ' work was the discovery of a bel t of in tensenega t ive anom al i e s 100-200 km w ide pa ra l l e l ing the tec tonic arc over the inshore s lope ofthe deep t rench south of Java and Sumatra .This d iscovery led Vening Meinesz to the wel l -know n theory o f dow nbuck led t ec togenes .Most tec tonics theor ies to expla in the s t ructura le l em ent s in the Indones i an A rch ipe lago w ereengendered by th is negat ive gravi ty-anomalyzone. Today, the gravi ty minimum is expla inedas due to under thrus t ing of l i thospher ic pla tesin accordance wi th the pla te tec tonics theory(e.g., B ow in , 1972) . Many m ore da ta ove r theIndones i an T rench w ere ob ta ined dur ing theIn te rna t iona l Ind ian O cean E xped i t ion , m a in lyby the Scr ipps Ins t i tu t ion of Oceanography andby the Uni ted S ta tes Coast and Geodet ic Survey (U S C G S ) . A de ta i l ed desc r ip t ion o f t hegravi ty f ie ld over the Indonesian Archipelagoand i t s explanat ion are given by Bowin andB e n - A v r a h a m ( in p r e p . ) .

On land the most extensive gravi ty surveyhas been m ade by S he l l In t e rna t iona le P e t ro l eum Maa t schapp i j , N . V . , T he H ague , N e the r l ands , and affi l iated companies. Recently, somelocal surveys were made by Japanese sc ient is t son Java, Bal i , and Krakatau i s lands (Yoko-y a m a a n d H a d i k u s u m o , 1 9 6 9 ; Y o k o y a m a a n dS u p a r t o , 1 9 7 0 ; Y o k o y a m a et al. 1 9 7 0 ) .Fo r the present s tudy, a com pi la t ion w asmade of al l the available gravity data over theSunda Shel f and adjacent land areas (F ig . 25) .T he g rav i ty m easurem ent s aboard R /V Chainwere done wi th a vibra t ing-s t r ing gravi ty meter(Wing , 1969 ; B ow in et al, 19 72 ) . T he d ig it a lreadout a t 5-minute in tervals was adjus ted forthe Eotvos accelera t ion caused by the ship ' seas t -west veloci ty and compared wi th the Inter nat iona l Gravi ty F orm ula of 1930 to get free-

a i r anomal ies . Bouguer anomal ies were ca lculated on an assumption of an infinite slab correct ion (densi ty 2 .67 g/cu cm) and sea-waterdensity of 1.03 g/cu cm. The gravity profi lesof Vema 19, obta ined wi th an Ask ania-G rafs table pla t form meter , were suppl ied by M.T a lw an i o f L am ont -D oher ty G eo log ica l O bse r vatory; the data reduct ion was done in a manner s imi lar to that for the R/V Chain da ta . R educed data were obta ined f rom al l the othership t racks . Over the shel f the Bouguer andfree-air anomalies are quite similar, owing tothe shal low water depth s (4 0 -6 0 m ) , so in th isreport only the profi les for free-air anomaliesare presented wi th the ref lec t ion and magnet icprofi les (F igs . 11 -1 7 ) . The Bouguer anom aly


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Structural Framework of Sunda Shelf 2353prof i les are presented only where the water i sdeep and the Bouguer anomalies differ significant ly f rom the f ree-a i r anomal ies .

Regional G rav i tyThe gravi ty f ie ld over the Sunda Shel f aver

ages abou t -1-30 mgal , in accord anc e wi th th eor iginal descr ipt ion of Vening Meinesz . Thisposit ive f ield now is recognized as one of themain features of the earth 's gravity f ield. A regional f ree-a i r m ap (Talw ani , 1970 ) ca lcula tedfrom a spher ica l harmonic expansion of theear th ' s gravi ty f ie ld by Kaula (1966) shows agravity high associated with the entire Pacificmargin and especia l ly wi th the i s land arcs a longthe western margin . The f ie ld over the SundaShelf i s among the highest in the western Pacific. S imi lar resul ts were obta ined wi th f ree-a i r anom aly av erages over 20 X 20 squaresf rom sur face sh ip m easu rem ent s (L e P icho nand T a lw an i , 196 9) . T a lw an i (1 97 0) o fferedtwo poss ible explanat ions for th is h igh-gravi tyf ie ld throug ho ut the western marg in of the Pacific. One is that the island arcs themselveshave large posi t ive anomal ies , and, because thesate l l i te-der ived gravi ty anomal ies have longwavelength, these large anomal ies contr ibute tothe ent i re region. This does not expla in thehigh-gravity f ield observed by ships over theJava Sea . A no the r explanat ion i s that the gravi ty high might be caused by greater rock densi t ies associa ted wi th a descending l i thospher icpla te in the mant le , an explanat ion that i s suppor ted by the long-wavelength anomal ies thatmust reflect deep sources.

Java SeaLocal gravi ty anomal ies super imposed on theregional background level have re la t ive ampl i tude o f 20-30 m ga l and va r i ed g rad ien t . T he

ma in po tent ia l of the gravi ty an om al ies i s in theinformat ion they can give about s t ructures ofthe basement , in par t icular beneath a deep sedi mentary bas in . Analys is of gravi ty anomal ies i squi te d i f ferent f rom that of magnet ic anomal ies , as the theoret ica l backg rou nd is considerably simpler. A body having a specific densitycon t ras t p roduces the sam e g rav i ty anom aly nomat ter where i t i s on the ear th ' s surface and,therefore, i t is not subject to variat ion with lat itude or or ienta t ion as i s a magnet ic anomaly.On the other hand, gravi ty in terpre ta t ion lacksthe great geologic s impl ic i ty of the magnet icmethod because of the def ini te magnet ic cont ras t a t the surface of the basement . The basement surface may or may not be the surface of

densi ty contras t f rom which gravi ty anomal iesor iginate .Over the eas tern and cent ra l Java Sea thet r ends o f t he B ouguer anom al i e s (F ig . 25) ares imi lar to the s t ructura l t rends . In the westernand nor the rn Java S ea the t r end o f t he anom al ies , i f any, i s unclear . Al though the area as awhole is sl ightly out of isostat ic equil ibrium,the s t ructura l e lements have re la t ively smal lgravi ty anomal ies and seem to be i sos ta t ica l lycompensated. A poss ible reason for th is i s tha tmost of the shelf has been inactive since earlyTer t iary t ime. S ince the ac t iv i ty s topped, sedi ments bur ied the tec tonic re l ief and produced aflat shallow sea floor through which only a fewof the s t ructura l h ighs (e.g., P u lau L au t R idge)pro t rude .

In most of the sedimentary bas ins of theJava Sea the seismic reflection profi les did notpenet ra te to basement . At the south end of profile 6 ( Fi g. 12) a steep dro p in gravity of 15m ga l s w as obse rved nea r t he nor the rn m arg inof the Sunda bas in . Oi l company dr i l l holes(T odd and P u lunggono , 1971) ind ica t e a difference in depth between acoust ic and t ruebasement of 1.5 km. With an infinite-slab approximat ion th is i s consis tent wi th a densi tydi f ference of 0 .24 g/cu cm between the t ruebasement and the sediments . I f one assumes adensi ty of 2 .48 for the acoust ic basement (alow er Miocene l im es tone , T ab le 1 ) t he com puta t ion indicates that th is anomaly i s consis tentwith a density of 2.20 for the material belowthe acoust ic basement and 2 .50 for the

Ben Avraham and Emery 1973 - Structural Framework of Sunda Shelf

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Transcript of Ben Avraham and Emery 1973 - Structural Framework of Sunda Shelf