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Petrology of a lower miocene polymict intracalcirudite from timor
Sedimentary Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
PETROLOGY OF A LOWER MIOCENE POLYMICT INTRACALCIRUDITE FROM TIMOR M. G. AUDLEY-CHARLES
Geology Department, Imperial College, London (Great Britain) (Received April 7, 1967)
SUMMARY
This study of an unusual limestone conglomerate extends the range of facies reported from ancient bahaman-type sedimentary rocks. More than forty different kinds of limestone clast have been identified in one thin section (6 cmz) of a polymict intracalcirudite. Three principal kinds of diagenesis can be recognised in these conglomerates: pressure solution, dolomitisation and dedolomitisation. The most important process was pressure solution which occurred relatively late in the formation of these rocks. The genesis of these conglomerates may be related to extensive reworking of penecontemporaneously deposited limestones, brought about by changes in sea level which resulted perhaps from epeirogenic or tectonic movements. Similar microfacies may be found in the bahamites of ancient island arcs and other unstable areas.
INTRODUCTION
Polymict intracalcirudites form a relatively uncommon microfacies of the Lower Miocene Cablac Limestone of eastern Timor. The Cablac Limestone is a bahamite facies (cf. BEAL~S, 1958) about 600 m thick which has recently been described and defined (AUDLEY-CHARLES, 1967). Among the various constituent microfacies of the Cablac Limestone the polymict intracalcirudites are of particular interest on account of their unusual composition and texture. They are intraformational conglomerates which rate as fine to medium rudites on FOLK'S(1959) classification. About 75 ~o of the intracalcirudite is composed of clasts which have long diameters between 1 mm and 5 mm; about 5 of the rock is composed of clasts which have long diameters between 5 mm and I0 ram; a few clasts with long diameters between 10 mm and 15 mm occur. Clasts less than 1 mm long diameter, which are mostly pellets and ooliths, contribute between 10 ~ and 15 ~ of the rock. The geometry of this microfacies is lenticular, beds vary in thickness between 1.5 m and a few cm. Sediment. GeoL, 1 (1967) 247-257
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The classification applied to the composition of the limestones of the Cablac Limestone is based largely on the system proposed by FoLK (1959), modified according to the suggestions of STAUFFER(1962). The grain size classification proposed by FoLrc (1959) is employed here.
COMPOSITION OF THE POLYMICT INTRACALCIRUDITES
The two most notable features of the composition of these intracalcirudites are the wide variety of limestone allochems, which make up most of these rudites (Fig.2), and the presence of terrigenous clasts. The most common allochems in these rudites are: pelspararenites, pelmicarenites, oospararenites, oomicarenites, ooliths, pellets, intramicarenites, micrite clasts, dolomitised micrite, dolostone, and bioclastic debris (of Lower Miocene age) chiefly calcareous algae and foraminifera with lesser amounts of echinoderms and bryozoans. Terrigenous clasts make a small but interesting contribution to these rudites. The most common terrigenous clasts are: eruptive rock fragments, calcilutites containing foraminifera (some of Cretaceous and some Lower Miocene age), radiolarian chert (probably Cretaceous), biomicarenites (probably Upper Triassic), detrital quartz and magnetite. Because the large majority of the clasts of these conglomerates are of rudite grade (greater than 1 mm diameter) they can be studied in detail. Certain highly characteristic allochems such as pelspararenites have been recognized in the polymict intracalcirudites which are indistinguishable from particular pelspararenites in the Cablac Limestone, this suggests that they are the source beds of the clasts in the polymict intracalcirudites. The composition of these calcirudites can be illustrated by considering one sample in some detail. The hand specimen (Fig.l) is a conglomerate, a fine to medium grade calcirudite. The colour is predominantly pinkish grey, but red, green and dark grey phenoclasts produce a mottled appearance. The phenoclasts are generally subangular to subrounded, a few are angular. The predominant type of clast is a pinkish grey calcilutite. Other common clasts are a reddish calcarenite and a dark yellow dolostone. Clasts of a dark grey fine grained igneous rock and a yellowish green serpentinitic rock are notable on account of their colour. The thin section of this conglomerate (Fig.2) is a polymict intracalcirudite. About 50 ~ of the rock is composed of allochems that have long diameters between 3.0 mm and 5.0 ram. The majority of the clasts are subangular to subrounded, but as pressure solution has been active along many grain boundaries the original shape and size of the grains cannot be determined precisely. The thin section (Fig.2) is composed of the following clasts: Pelspararenite - - clasts representing two distinct types can be recognised. Dismicrite clasts of five different kinds of dismicrite are present. Oospararenite - - clasts representing seven different kinds are present. Biomicarenite - - two clasts of a very similar kind are present. Dolostone one large clast present -
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Bioclastic allochems - - fragments of calcareous algae and bryozoans are present. Pellets m a n y pellets o f different size and texture are present as discrete allochems. Ooliths m a n y ooliths o f different size a n d texture are present as discrete aUochems. ~r,: Terrigenous clasts grains of radiolarian cherts, eruptive rocks and calcilutite with Globigerina sp. are present. -
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Fig.1. Acid-etched surface ofa polymict intracalcirudite. An oblique photograph of an etched surface of a polymict intracalcit'udite of the Cablac Limestone. Note the dolostone clasts standing proud. Other proud clasts are terrigenous, mainly cherts, igneous rocks and pyrite. Note the boundaries between the clasts marked by lines of proud minerals, mainly clay, chert, dolomite and pyrite.
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Fig.2, Photomicrograph of part of a thin section of a polymict intracalcirudite (ordinary light). Twenty-five different kinds of limestone clast have been identified (out of a total of 43 different kinds of clast) in this thin section for purposes of illustration. Note complex composition of many of the clasts.
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PEIROLOGY OF A MIOCENE INTRACALCIRUDITE FROM TIMOR
I 0
I 5
A
I lOmrn
E
Fig.3. Tracing of Fig.2 as an index to the clasts and textural features. A ~ Dolomite rhomb cutting micrite intraclast and pellet; B : Dolomite rhornb in pelmicarenite intraclast; C = Calcareous algal fragment; D : Three discrete micrite pellets; E = Fracturing with calcite vein along previously sutured clast boundaries; F = Matrix of mixed allochems and micrite; G : Pressure solution features. The following intraclasts are identified: 1 = Pelspararenite; 2 = Dolostone; 3 = Micrite; 4 = Micrite; 5 :- Dismicrite with one pellet; 6 = Chert terrigenous clast; 7 = Biodismicarenite; 8 = Oospararenite; 9 = Biomicrite; 1 0 : Pelspararenite; 1 1 = Oospararenite; 1 2 = Dolostone; 1 3 = Dolostone; 1 4 = Micrite; 1 5 : Dolostone; 1 6 ~ Intramicarenite almost an intramicrudite; 1 7 = Dolostone; 1 8 ~ Pelspararenite; 1 9 = Biomicrite; 2 0 = Biodismicrite; 2 1 ~ Oospararenite; 2 2 = Pelspararenite; 2 3 ~ Pelspararenite; 2 4 - - Oospararenite; 2 5 = Pelmicarenite. The dotted lines represent the post-consolidation fractures filled with veins of ferroan calcite. The very small clasts are not represented in this drawing.
T h e o r t h o c h e m i c a l c o n s t i t u e n t s o f these p o l y m i c t c a l c i r u d i t e s f o r m a v e r y insign i f i c a n t f r a c t i o n o f t h e w h o l e r o c k . T h e r o c k s are c h a r a e t e r i s e d b y s u t u r e d g r a i n b o u n d a r i e s ; t h e s e m a y be c o m p a r e d w i t h m i c r o s t y l o l i t i c c o n t a c t s , w h i c h a r e g e n e r a l l y r e g a r d e d as t h e r e s u l t o f p r e s s u r e s o l u t i o n b e t w e e n a d j a c e n t clasts (SLOSS a n d FERAV, 1948). T h i n s e c t i o n s r e v e a l l o c a l d e v e l o p m e n t s o f a little s p a r r y calcite c e m e n t e i t h e r in a g r a n u l a r m o s a i c o r as a d r u s y m o s a i c filling w h a t m u s t h a v e b e e n a n o r i g i n a l Sediment.
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cavity between the clasts. This sample (Fig. 1, 2, 3) also contains local developments of a micrite matrix mixed with comminuted allochemical debris (fragments of ooliths, pellets and shells) and terrigenous material. Fig.2 and 3 show part of the thin section in which the matrix can be seen to be confined to a few small regions between the clasts. Strictly this "matrix" is not within the definition of orthochemical constituent (FOLK, 1959, p.7-9), because it is highly unlikely that it was precipitated in the region where the clasts accumulated, on the contrary it was probably transported by currents and should therefore be regarded as a complex type of allochem. The nature of the clast boundaries is best seen by studying the etched surface of a previously polished face under the binocular microscope (Fig.l). The sutured boundaries between the clasts can be seen to be marked by a thin line of non-carbonate minerals mainly limonite, pyrite and various clay minerals,
DIAGENESIS
A variety of diagenetic processes have been recognised in the rocks of the Cablac Limestone. The most important diagenetic changes are calcite cementation, pressure solution, recrystallisation textures of various kinds, dolomitisation, dedolomitisation and silicification. In the polymict intracalcirudite microfacies three principal diagenetic processes are recognised: pressure solution, dolomitisation and dedolomitisation.
Pressure solution Generally most of the clasts of the polymict intracalcirudite are involved to some degree in this process as Fig.2 and 3 show. Many of these clasts are themselves particles of complex limestones (e.g., pelspararenite, oospararenite and biomicarenite) and it can be seen that the pressure-solution process has not respected the microstructure of these complex clasts, but cut across them indiscriminately (Fig.2 and 3, clasts 8, 18, 20, 21, 24 and 25). The concentration of relatively insoluble non-carbonate minerals such as clay, limonite and pyrite along the microstylolitic boundaries (Fig.l) is probably one result of the pressure-solution process. It is clear from the manner in which the sutured contacts between the clasts cut across all the other diagenetic textures that the pressure-solution process must have occurred relatively late in the formation of the rock. It may have resulted from the pressure of overburden as the rocks were buried beneath younger deposits. There are three main theories of the origin of stylolites in limestones (stylolites are the extreme case of sutured grain boundaries): (1) They are formed after consolidation of the rock by pressure solution (STOCKDALE,1922; DU~NINOTON, 1954). (2) The theory of SHAUB (1939, 1949) that they are formed before the rock consolidates by a "contraction-pressure process" related to movements of connate waters.
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(3) The theory of I~OKOVOVICH(1952) which depends on subaqueous solution of the original sediment early in its history. These three major theories on stylolite formation have been reviewed recently by MANrEN (1966) who concluded strongly in favour of STOCKOALE'S (1922) theory. One of Manten's conclusions was that the presence of clay, and other materials which modify permeability to water, in the limestone greatly affects stylolite development. The significant proportion of clay and other relatively impervious materials, concentrated along the clast boundaries, is clearly seen in Fig.1. These materials may have inhibited the development of microstylolites in these rocks. The pressuresolution process may have been curtailed so that only sutured grain boundaries formed. The microstylolites are only well developed in the polymict intracalcirudites, such features are rare in the calcarenites and absent in the calcilutites. The calcarenites, and to a greater extent the calcilutites, contain sylolites whose amplitude can be seen in hand specimen. The subject if grain size and its relation to pressure solution in rocks may need further investigation. After the pressure solution there was a later post-consolidation fracturing of these rocks that can be dated as probably Middle Miocene. This fracturing is a notable feature of the Cablac Limestone. The rocks are cut by numerous small cracks filled with a clear sparry calcite (Fig.2 and 3) which is nearly always ferroan calcite. This distinguishes it from the sparry calcite (Fig.2) of the cement in and around the clasts which is usually free from ferrous iron. The reticulate pattern often formed by these cracks is interpreted as the response of a hard, essentially massive limestone to the intensive fold movements that resulted in the overthrusting of great sheets of Permian and other strata onto Timor during the mid-Miocene orogeny (AuDLEY-CHARLES, 1965). Bedding planes are rare in this formation, but broad gentle folds have been recognised. One of the features of this formation is the development of large faults which form scarps. It has been shown that the Cablac Limestone was never deeply buried, it is therefore interesting to note that this massive limestone formation under orogenic pressure in these conditions reacted by fracturing rather than by folding. This brittle behaviour may be seen in some calcarenites where the detrital quartz grains have been fractured "in situ" after they have been cemented. Similarly some dolomite rhombs are found bordered by cracks filled with sparry calcite, or cut by a calcite vein which always follows the cleavage of the dolomite grain. Some of the polymict intracalcirudites appear to have responded to this orogenic pressure by fracturing along the sutured clast boundaries which had formed previously by pressure solution (Fig.2 and 3E); some cracks have cut indiscriminately across these "welded" clast boundaries. Dolornitisation
There are two principal ways in which the mineral dolomite occurs in the Cablac Limestone. One is as the exclusive constituent of dolostone (SHROCK, 1948) Sediment. Geol., 1 0967) 247-257
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clasts that occur in some of the polymict intracalcirudites; the other mode of occurrence is as discrete crystals (rhombs) of dolomite in some of the rudites, arenites and lutites of the Cablac succession. The distribution of dolomite in the polymict intracalcirudites may be seen in Fig. l where the etching by acid has left the dolomite standing proud. The parent dolostone rock of the clasts which occur in the intracalcirudites has not been identified in the field. The detrital clasts of dolostone (Fig.l, 2 and 3) are composed only of the mineral dolomite (except where the subsequent process of dedolomitisation has occurred). The texture of these clasts, that vary in grain size, colour, density of inclusions and zoning, is typically what CAYEUX (1935, pl. 25, fig.94-95 and pl. 26, fig. 101) called "dolomie grenue". Most of the grains appear to be moulded into each other, while a few crystals retain a tendency for euhedral outline, and a rare crystal shows well developed rhombic form. Some of the polymict intracalcirudites that contain clasts of dolostone (dolomie grenue) also contain discrete rhombs of dolomite which are clearly diagenetic and not detrital (Fig.l, 2, 3). Two types of dolomitisation are recognised in the intracalcirudites. One is the development of fairly large rhombs (generally about 0. l mm0.3 mm diameter) that have replaced various allochems, cement and clasts indiscriminately, cutting across clast boundaries, and earlier textural features. This kind of dolomitisation often shows some well developed rhombs, although generally they are incomplete and have many inclusions. The other type of dolomitisation seen in the polymict intracalcirudites usually consists of smaller rhombs that are restricted to certain clasts and never cut clast boundaries. Petrographically these rhombs are distinguishable from those associated with the other kind of dolomitisation described above; they generally do not have the strong relief and dark outlines with water-clear rims that characterise the rhombs of the first type. Although it cannot be proved from the available evidence, it is suggested that the second type of dolomitisation (small poorly defined rhombs generally occurring in micrite or pelmicrite clasts) developed earlier in the history of the rock and were present in the clasts before they were incorporated in these conglomerates. If this suggestion is correct it suggests that there was a very early phase of dolomitisation in the carbonate muds and pellet muds "in situ" before they were eroded to form clasts. The intensity of this kind of dolomitisation varies greatly from clast to clast. One clast of a biomicarenite in a polymict intracalcirudite has been dolomitised to a stage where only a few slivers of the original biomicarenite remain "floating" in what is now almost a clast composed of the mineral dolomite. Other kinds of dolomitisation are found in the different microfacies of the Cablac Limestone; one variety is known that contains dolomite crystals that are later than the sparite cement, and form up to 10 70 of the rock volume. Dedolomitisation
In the Cablac Limestone this diagenetic process has been seen only in the polySediment. GeoL, 1 (1967) 247--257
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mict intracalcirudites where it takes two forms: one in the dolostone clasts, and the other occurrence is in some of the discrete dolomite rhombs that cut the boundaries of different clasts.
Dedolomitisation of the dolostone clasts This can be seen in various stages of progress in different clasts within the same hand specimen. The most extensive development has resulted in the whole of the dolostone clast being altered to a fine granular mosaic of calcite with numerous dark dusty inclusions; the outlines of the anhedral and subhedral dolomite grains are preserved as "ghosts". This has been accomplished by many small calcite crystals growing together to pseudomorph the dolomite crystal. A similar phenomenon has been described by SHEARMANet al. (1961) in rocks from the French Jura. Other dolostone elasts in these Timor conglomerates have been less completely altered. These have suffered in one of two ways: either those crystals of dolomite in the core of the clast have been preserved while those near the surface have gone over to calcite, or else all of the dolomite crystals have been partially altered to calcite. This partial alteration is always centrifugal with the rims of the dolomite crystals being preserved. Dedolomitisation of the discrete dolomite rhombs Some of the discrete rhombs have been altered to a granular mosaic of calcite crystals that are much smaller than the original dolomite rhomb. This process is always centrifugal in these rocks, and appears to be similar to that described by SHEARMANet al. (1961).
MODE OF FORMATIONOF THESEPOLYMICTINTRACALCIRUDITES
It has been shown elsewhere (AUDLEY-CHARLES, 1967) that the Cablac Limestone accumulated in a shallow and very shallow marine environment, and that it is mainly a fore-reef formation. In many respects the lithology, microfauna and microflora suggest that the Cablac Limestone was deposited in conditions similar to those of the present Bahama Banks (ILLING, 1954; NEWELL, 1955; CLOUD, 1962). The presence of the variety of terrigenous clasts of Triassic, Cretaceous and Oligocene rocks in the polymict intracalcirudites indicates that the formation of the shallow carbonate banks (represented by the Cablac Limestone) was accompanied by the erosion of the older formations that probably formed island cores to these shallow banks which occupied the site of Timor. The polymict intracalcirudite microfacies does not seem to occur in the Bahama Banks. The grain size and composition of these rudites imply that they were formed in very shallow water where wave action was severe, because most of the intraclasts of which they are composed were produced by the erosion of various penecontemporaneously deposited facies of the Cablac Limestone which were compacted and cemented before they were eroded. Some of these limestones may have been dolomitised
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before erosion to form clasts which now occur in the polymict intracalcirudites. The formation of the polymict intracalcirudites implies that rocks deposited penecontemporaneously with the rudites were after cementation and consolidation brought within the influence of waves and currents that eroded them. The presence of these rudites at several horizons within the Cablac Limestone succession indicates that these changes in the environment occurred several times. These variations in environment are most easily explained by postulating changes in sea level. The movements of sea level must have been rapid because several took place within the time interval of the Lower Miocene. It seems most probable that such relative movements of sea level in the Timor region during the Lower Miocene were related to local epeirogenic or orogenic movements. In the Indonesian Archipelago the Miocene was a time of crustal instability, as the following remarks illustrate. The base of the Lower Miocene Cablac Limestone is an unconformity with basal conglomerates. This represents a marine transgression widespread in Indonesia, called the Bebulah transgression by BROUWER (1942), which is found in Sumatra, Java, Borneo, the Philippines and elsewhere (VAN BEMMELEN, 1949). ,:There was a major orogenic phase in Timor, called the Ramelauean orogeny (AUDLEY-CHARLES, 1967), which occurred during the Middle Miocene. During the Upper Miocene Timor was rapidly uplifted and eroded. Similar polymict intracalcirudites should be looked for in bahamites of ancient island arcs and other unstable areas. The interpretation of the formation of these rocks as the product of bahaman shoals in exceptionally unstable regions suggests that the apparent absence of this microfacies in ancient shelf deposits may be significant.
REFERENCES
AUDLEY-CHARLES,M. G., 1965. A Miocene gravity slide deposit from eastern Timor. Geol. MAR., 102 : 267-276. AUDLEY-CHARLES,M.G., 1967. The geology of Portuguese Timor. Geol. Soc. London, Mere., 4 (in press). BEALES, F. W., 1958. Ancient sediments of Bahaman type. Bull. Am. Assoc. Petrol. Geologists, 42 : 1845-1880. BROUWER,H. A., 1942. Summary of the geological results of the expeditions. Geol. Expedition Lesser Sunda Islands, 4 : 345-402. CAVEtJX,L., 1935. Les Roches Sddimentaires de France: Roches Carbonatdes. Masson, Paris, 463 pp. CLOUDJR., P. E., 1962. Environment of calcium carbonate deposition west of Andros Island Bahamas. U.S., Geol. Surv., Profess. Papers, 350 : 1-138. DUNN~NGTON,H. V., 1954. Stylolite formation post-dates rock induration. J. Sediment. Petrol. 24 : 27-49. FOLK, R. L., 1959. Practical petrographic classification of limestones. Bull. Am. Assoc. Petrol. Geologists, 43 : 1-38. ILLIrqG,L. V., 1954. Bahaman calcareous sands. Bull. Am. Assoc. Petrol. Geologists, 38 : 1-95. MArqa'EN,A. A., 1966. Note on the formation of stylolites. Geol. Mijnbouw, 45 : 269-274. NEWELL,N. D., 1955. Bahamian Platforms. Geol. Soc. Am., Spec. Papers, 62 : 303-316. PgolcoPovIcr~,N., 1952. The origin of stylolites, aT.Sediment. Petrol., 32 : 212-220. SHAtJB,B. M., 1939. The origin ofstylolites. J. Sediment. Petrol., 9 : 47-61. Sediment. Geol., 1 (1967) 247-257
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SHAUB, B. M., 1949. DO stylolites develop before or after the hardening of the enclosing rock? J. Sediment. PetroL, 19 : 26-36. Sr~EARS~AN,D. J., KrtouRI, J. and TAVIA,S., 1961. On the replacemertt of dolomite by calcite in some Mesozoic limestones from the French Jura. Proc. GeoL Assoc. London, 72 : 1-12. SHROCS:,R. R., 1948. Sequence in Layered Rocks. McGraw-Hill, New York, N.Y., 507 pp. SLOSS,D. D. and FERAY,D. E., 1948. Microstylolites in sandstone. J. Sediment. PetroL, 18 : 3-13. STAUFFER, K. W., 1962. Quantitative petrographic study of Paleozoic carbonate rocks, Cabello Mountains, New Mexico. J. Sediment. PetroL, 32 : 357-396. STOCKDAL~,P. B., 1922. Stylolites: their nature and origin. Indiana Univ. Studies, 9 : 1-97. STOCr,DALr, P. B., 1943. Stylolites: primary or secondary. J. Sediment. PetroL, 13 : 3-12. VAN BEMMELEN,R. W., 1949. The Geology of Indonesia. Government Printing Office, The Hague, 732 pp,
Sediment. GeoL, 1 (1967) 247-257
PETROLOGY OF A LOWER MIOCENE POLYMICT INTRACALCIRUDITE FROM TIMOR M. G. AUDLEY-CHARLES
Geology Department, Imperial College, London (Great Britain) (Received April 7, 1967)
SUMMARY
This study of an unusual limestone conglomerate extends the range of facies reported from ancient bahaman-type sedimentary rocks. More than forty different kinds of limestone clast have been identified in one thin section (6 cmz) of a polymict intracalcirudite. Three principal kinds of diagenesis can be recognised in these conglomerates: pressure solution, dolomitisation and dedolomitisation. The most important process was pressure solution which occurred relatively late in the formation of these rocks. The genesis of these conglomerates may be related to extensive reworking of penecontemporaneously deposited limestones, brought about by changes in sea level which resulted perhaps from epeirogenic or tectonic movements. Similar microfacies may be found in the bahamites of ancient island arcs and other unstable areas.
INTRODUCTION
Polymict intracalcirudites form a relatively uncommon microfacies of the Lower Miocene Cablac Limestone of eastern Timor. The Cablac Limestone is a bahamite facies (cf. BEAL~S, 1958) about 600 m thick which has recently been described and defined (AUDLEY-CHARLES, 1967). Among the various constituent microfacies of the Cablac Limestone the polymict intracalcirudites are of particular interest on account of their unusual composition and texture. They are intraformational conglomerates which rate as fine to medium rudites on FOLK'S(1959) classification. About 75 ~o of the intracalcirudite is composed of clasts which have long diameters between 1 mm and 5 mm; about 5 of the rock is composed of clasts which have long diameters between 5 mm and I0 ram; a few clasts with long diameters between 10 mm and 15 mm occur. Clasts less than 1 mm long diameter, which are mostly pellets and ooliths, contribute between 10 ~ and 15 ~ of the rock. The geometry of this microfacies is lenticular, beds vary in thickness between 1.5 m and a few cm. Sediment. GeoL, 1 (1967) 247-257
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The classification applied to the composition of the limestones of the Cablac Limestone is based largely on the system proposed by FoLK (1959), modified according to the suggestions of STAUFFER(1962). The grain size classification proposed by FoLrc (1959) is employed here.
COMPOSITION OF THE POLYMICT INTRACALCIRUDITES
The two most notable features of the composition of these intracalcirudites are the wide variety of limestone allochems, which make up most of these rudites (Fig.2), and the presence of terrigenous clasts. The most common allochems in these rudites are: pelspararenites, pelmicarenites, oospararenites, oomicarenites, ooliths, pellets, intramicarenites, micrite clasts, dolomitised micrite, dolostone, and bioclastic debris (of Lower Miocene age) chiefly calcareous algae and foraminifera with lesser amounts of echinoderms and bryozoans. Terrigenous clasts make a small but interesting contribution to these rudites. The most common terrigenous clasts are: eruptive rock fragments, calcilutites containing foraminifera (some of Cretaceous and some Lower Miocene age), radiolarian chert (probably Cretaceous), biomicarenites (probably Upper Triassic), detrital quartz and magnetite. Because the large majority of the clasts of these conglomerates are of rudite grade (greater than 1 mm diameter) they can be studied in detail. Certain highly characteristic allochems such as pelspararenites have been recognized in the polymict intracalcirudites which are indistinguishable from particular pelspararenites in the Cablac Limestone, this suggests that they are the source beds of the clasts in the polymict intracalcirudites. The composition of these calcirudites can be illustrated by considering one sample in some detail. The hand specimen (Fig.l) is a conglomerate, a fine to medium grade calcirudite. The colour is predominantly pinkish grey, but red, green and dark grey phenoclasts produce a mottled appearance. The phenoclasts are generally subangular to subrounded, a few are angular. The predominant type of clast is a pinkish grey calcilutite. Other common clasts are a reddish calcarenite and a dark yellow dolostone. Clasts of a dark grey fine grained igneous rock and a yellowish green serpentinitic rock are notable on account of their colour. The thin section of this conglomerate (Fig.2) is a polymict intracalcirudite. About 50 ~ of the rock is composed of allochems that have long diameters between 3.0 mm and 5.0 ram. The majority of the clasts are subangular to subrounded, but as pressure solution has been active along many grain boundaries the original shape and size of the grains cannot be determined precisely. The thin section (Fig.2) is composed of the following clasts: Pelspararenite - - clasts representing two distinct types can be recognised. Dismicrite clasts of five different kinds of dismicrite are present. Oospararenite - - clasts representing seven different kinds are present. Biomicarenite - - two clasts of a very similar kind are present. Dolostone one large clast present -
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Bioclastic allochems - - fragments of calcareous algae and bryozoans are present. Pellets m a n y pellets o f different size and texture are present as discrete allochems. Ooliths m a n y ooliths o f different size a n d texture are present as discrete aUochems. ~r,: Terrigenous clasts grains of radiolarian cherts, eruptive rocks and calcilutite with Globigerina sp. are present. -
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Fig.1. Acid-etched surface ofa polymict intracalcirudite. An oblique photograph of an etched surface of a polymict intracalcit'udite of the Cablac Limestone. Note the dolostone clasts standing proud. Other proud clasts are terrigenous, mainly cherts, igneous rocks and pyrite. Note the boundaries between the clasts marked by lines of proud minerals, mainly clay, chert, dolomite and pyrite.
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Fig.2, Photomicrograph of part of a thin section of a polymict intracalcirudite (ordinary light). Twenty-five different kinds of limestone clast have been identified (out of a total of 43 different kinds of clast) in this thin section for purposes of illustration. Note complex composition of many of the clasts.
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I 0
I 5
A
I lOmrn
E
Fig.3. Tracing of Fig.2 as an index to the clasts and textural features. A ~ Dolomite rhomb cutting micrite intraclast and pellet; B : Dolomite rhornb in pelmicarenite intraclast; C = Calcareous algal fragment; D : Three discrete micrite pellets; E = Fracturing with calcite vein along previously sutured clast boundaries; F = Matrix of mixed allochems and micrite; G : Pressure solution features. The following intraclasts are identified: 1 = Pelspararenite; 2 = Dolostone; 3 = Micrite; 4 = Micrite; 5 :- Dismicrite with one pellet; 6 = Chert terrigenous clast; 7 = Biodismicarenite; 8 = Oospararenite; 9 = Biomicrite; 1 0 : Pelspararenite; 1 1 = Oospararenite; 1 2 = Dolostone; 1 3 = Dolostone; 1 4 = Micrite; 1 5 : Dolostone; 1 6 ~ Intramicarenite almost an intramicrudite; 1 7 = Dolostone; 1 8 ~ Pelspararenite; 1 9 = Biomicrite; 2 0 = Biodismicrite; 2 1 ~ Oospararenite; 2 2 = Pelspararenite; 2 3 ~ Pelspararenite; 2 4 - - Oospararenite; 2 5 = Pelmicarenite. The dotted lines represent the post-consolidation fractures filled with veins of ferroan calcite. The very small clasts are not represented in this drawing.
T h e o r t h o c h e m i c a l c o n s t i t u e n t s o f these p o l y m i c t c a l c i r u d i t e s f o r m a v e r y insign i f i c a n t f r a c t i o n o f t h e w h o l e r o c k . T h e r o c k s are c h a r a e t e r i s e d b y s u t u r e d g r a i n b o u n d a r i e s ; t h e s e m a y be c o m p a r e d w i t h m i c r o s t y l o l i t i c c o n t a c t s , w h i c h a r e g e n e r a l l y r e g a r d e d as t h e r e s u l t o f p r e s s u r e s o l u t i o n b e t w e e n a d j a c e n t clasts (SLOSS a n d FERAV, 1948). T h i n s e c t i o n s r e v e a l l o c a l d e v e l o p m e n t s o f a little s p a r r y calcite c e m e n t e i t h e r in a g r a n u l a r m o s a i c o r as a d r u s y m o s a i c filling w h a t m u s t h a v e b e e n a n o r i g i n a l Sediment.
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cavity between the clasts. This sample (Fig. 1, 2, 3) also contains local developments of a micrite matrix mixed with comminuted allochemical debris (fragments of ooliths, pellets and shells) and terrigenous material. Fig.2 and 3 show part of the thin section in which the matrix can be seen to be confined to a few small regions between the clasts. Strictly this "matrix" is not within the definition of orthochemical constituent (FOLK, 1959, p.7-9), because it is highly unlikely that it was precipitated in the region where the clasts accumulated, on the contrary it was probably transported by currents and should therefore be regarded as a complex type of allochem. The nature of the clast boundaries is best seen by studying the etched surface of a previously polished face under the binocular microscope (Fig.l). The sutured boundaries between the clasts can be seen to be marked by a thin line of non-carbonate minerals mainly limonite, pyrite and various clay minerals,
DIAGENESIS
A variety of diagenetic processes have been recognised in the rocks of the Cablac Limestone. The most important diagenetic changes are calcite cementation, pressure solution, recrystallisation textures of various kinds, dolomitisation, dedolomitisation and silicification. In the polymict intracalcirudite microfacies three principal diagenetic processes are recognised: pressure solution, dolomitisation and dedolomitisation.
Pressure solution Generally most of the clasts of the polymict intracalcirudite are involved to some degree in this process as Fig.2 and 3 show. Many of these clasts are themselves particles of complex limestones (e.g., pelspararenite, oospararenite and biomicarenite) and it can be seen that the pressure-solution process has not respected the microstructure of these complex clasts, but cut across them indiscriminately (Fig.2 and 3, clasts 8, 18, 20, 21, 24 and 25). The concentration of relatively insoluble non-carbonate minerals such as clay, limonite and pyrite along the microstylolitic boundaries (Fig.l) is probably one result of the pressure-solution process. It is clear from the manner in which the sutured contacts between the clasts cut across all the other diagenetic textures that the pressure-solution process must have occurred relatively late in the formation of the rock. It may have resulted from the pressure of overburden as the rocks were buried beneath younger deposits. There are three main theories of the origin of stylolites in limestones (stylolites are the extreme case of sutured grain boundaries): (1) They are formed after consolidation of the rock by pressure solution (STOCKDALE,1922; DU~NINOTON, 1954). (2) The theory of SHAUB (1939, 1949) that they are formed before the rock consolidates by a "contraction-pressure process" related to movements of connate waters.
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(3) The theory of I~OKOVOVICH(1952) which depends on subaqueous solution of the original sediment early in its history. These three major theories on stylolite formation have been reviewed recently by MANrEN (1966) who concluded strongly in favour of STOCKOALE'S (1922) theory. One of Manten's conclusions was that the presence of clay, and other materials which modify permeability to water, in the limestone greatly affects stylolite development. The significant proportion of clay and other relatively impervious materials, concentrated along the clast boundaries, is clearly seen in Fig.1. These materials may have inhibited the development of microstylolites in these rocks. The pressuresolution process may have been curtailed so that only sutured grain boundaries formed. The microstylolites are only well developed in the polymict intracalcirudites, such features are rare in the calcarenites and absent in the calcilutites. The calcarenites, and to a greater extent the calcilutites, contain sylolites whose amplitude can be seen in hand specimen. The subject if grain size and its relation to pressure solution in rocks may need further investigation. After the pressure solution there was a later post-consolidation fracturing of these rocks that can be dated as probably Middle Miocene. This fracturing is a notable feature of the Cablac Limestone. The rocks are cut by numerous small cracks filled with a clear sparry calcite (Fig.2 and 3) which is nearly always ferroan calcite. This distinguishes it from the sparry calcite (Fig.2) of the cement in and around the clasts which is usually free from ferrous iron. The reticulate pattern often formed by these cracks is interpreted as the response of a hard, essentially massive limestone to the intensive fold movements that resulted in the overthrusting of great sheets of Permian and other strata onto Timor during the mid-Miocene orogeny (AuDLEY-CHARLES, 1965). Bedding planes are rare in this formation, but broad gentle folds have been recognised. One of the features of this formation is the development of large faults which form scarps. It has been shown that the Cablac Limestone was never deeply buried, it is therefore interesting to note that this massive limestone formation under orogenic pressure in these conditions reacted by fracturing rather than by folding. This brittle behaviour may be seen in some calcarenites where the detrital quartz grains have been fractured "in situ" after they have been cemented. Similarly some dolomite rhombs are found bordered by cracks filled with sparry calcite, or cut by a calcite vein which always follows the cleavage of the dolomite grain. Some of the polymict intracalcirudites appear to have responded to this orogenic pressure by fracturing along the sutured clast boundaries which had formed previously by pressure solution (Fig.2 and 3E); some cracks have cut indiscriminately across these "welded" clast boundaries. Dolornitisation
There are two principal ways in which the mineral dolomite occurs in the Cablac Limestone. One is as the exclusive constituent of dolostone (SHROCK, 1948) Sediment. Geol., 1 0967) 247-257
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clasts that occur in some of the polymict intracalcirudites; the other mode of occurrence is as discrete crystals (rhombs) of dolomite in some of the rudites, arenites and lutites of the Cablac succession. The distribution of dolomite in the polymict intracalcirudites may be seen in Fig. l where the etching by acid has left the dolomite standing proud. The parent dolostone rock of the clasts which occur in the intracalcirudites has not been identified in the field. The detrital clasts of dolostone (Fig.l, 2 and 3) are composed only of the mineral dolomite (except where the subsequent process of dedolomitisation has occurred). The texture of these clasts, that vary in grain size, colour, density of inclusions and zoning, is typically what CAYEUX (1935, pl. 25, fig.94-95 and pl. 26, fig. 101) called "dolomie grenue". Most of the grains appear to be moulded into each other, while a few crystals retain a tendency for euhedral outline, and a rare crystal shows well developed rhombic form. Some of the polymict intracalcirudites that contain clasts of dolostone (dolomie grenue) also contain discrete rhombs of dolomite which are clearly diagenetic and not detrital (Fig.l, 2, 3). Two types of dolomitisation are recognised in the intracalcirudites. One is the development of fairly large rhombs (generally about 0. l mm0.3 mm diameter) that have replaced various allochems, cement and clasts indiscriminately, cutting across clast boundaries, and earlier textural features. This kind of dolomitisation often shows some well developed rhombs, although generally they are incomplete and have many inclusions. The other type of dolomitisation seen in the polymict intracalcirudites usually consists of smaller rhombs that are restricted to certain clasts and never cut clast boundaries. Petrographically these rhombs are distinguishable from those associated with the other kind of dolomitisation described above; they generally do not have the strong relief and dark outlines with water-clear rims that characterise the rhombs of the first type. Although it cannot be proved from the available evidence, it is suggested that the second type of dolomitisation (small poorly defined rhombs generally occurring in micrite or pelmicrite clasts) developed earlier in the history of the rock and were present in the clasts before they were incorporated in these conglomerates. If this suggestion is correct it suggests that there was a very early phase of dolomitisation in the carbonate muds and pellet muds "in situ" before they were eroded to form clasts. The intensity of this kind of dolomitisation varies greatly from clast to clast. One clast of a biomicarenite in a polymict intracalcirudite has been dolomitised to a stage where only a few slivers of the original biomicarenite remain "floating" in what is now almost a clast composed of the mineral dolomite. Other kinds of dolomitisation are found in the different microfacies of the Cablac Limestone; one variety is known that contains dolomite crystals that are later than the sparite cement, and form up to 10 70 of the rock volume. Dedolomitisation
In the Cablac Limestone this diagenetic process has been seen only in the polySediment. GeoL, 1 (1967) 247--257
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mict intracalcirudites where it takes two forms: one in the dolostone clasts, and the other occurrence is in some of the discrete dolomite rhombs that cut the boundaries of different clasts.
Dedolomitisation of the dolostone clasts This can be seen in various stages of progress in different clasts within the same hand specimen. The most extensive development has resulted in the whole of the dolostone clast being altered to a fine granular mosaic of calcite with numerous dark dusty inclusions; the outlines of the anhedral and subhedral dolomite grains are preserved as "ghosts". This has been accomplished by many small calcite crystals growing together to pseudomorph the dolomite crystal. A similar phenomenon has been described by SHEARMANet al. (1961) in rocks from the French Jura. Other dolostone elasts in these Timor conglomerates have been less completely altered. These have suffered in one of two ways: either those crystals of dolomite in the core of the clast have been preserved while those near the surface have gone over to calcite, or else all of the dolomite crystals have been partially altered to calcite. This partial alteration is always centrifugal with the rims of the dolomite crystals being preserved. Dedolomitisation of the discrete dolomite rhombs Some of the discrete rhombs have been altered to a granular mosaic of calcite crystals that are much smaller than the original dolomite rhomb. This process is always centrifugal in these rocks, and appears to be similar to that described by SHEARMANet al. (1961).
MODE OF FORMATIONOF THESEPOLYMICTINTRACALCIRUDITES
It has been shown elsewhere (AUDLEY-CHARLES, 1967) that the Cablac Limestone accumulated in a shallow and very shallow marine environment, and that it is mainly a fore-reef formation. In many respects the lithology, microfauna and microflora suggest that the Cablac Limestone was deposited in conditions similar to those of the present Bahama Banks (ILLING, 1954; NEWELL, 1955; CLOUD, 1962). The presence of the variety of terrigenous clasts of Triassic, Cretaceous and Oligocene rocks in the polymict intracalcirudites indicates that the formation of the shallow carbonate banks (represented by the Cablac Limestone) was accompanied by the erosion of the older formations that probably formed island cores to these shallow banks which occupied the site of Timor. The polymict intracalcirudite microfacies does not seem to occur in the Bahama Banks. The grain size and composition of these rudites imply that they were formed in very shallow water where wave action was severe, because most of the intraclasts of which they are composed were produced by the erosion of various penecontemporaneously deposited facies of the Cablac Limestone which were compacted and cemented before they were eroded. Some of these limestones may have been dolomitised
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before erosion to form clasts which now occur in the polymict intracalcirudites. The formation of the polymict intracalcirudites implies that rocks deposited penecontemporaneously with the rudites were after cementation and consolidation brought within the influence of waves and currents that eroded them. The presence of these rudites at several horizons within the Cablac Limestone succession indicates that these changes in the environment occurred several times. These variations in environment are most easily explained by postulating changes in sea level. The movements of sea level must have been rapid because several took place within the time interval of the Lower Miocene. It seems most probable that such relative movements of sea level in the Timor region during the Lower Miocene were related to local epeirogenic or orogenic movements. In the Indonesian Archipelago the Miocene was a time of crustal instability, as the following remarks illustrate. The base of the Lower Miocene Cablac Limestone is an unconformity with basal conglomerates. This represents a marine transgression widespread in Indonesia, called the Bebulah transgression by BROUWER (1942), which is found in Sumatra, Java, Borneo, the Philippines and elsewhere (VAN BEMMELEN, 1949). ,:There was a major orogenic phase in Timor, called the Ramelauean orogeny (AUDLEY-CHARLES, 1967), which occurred during the Middle Miocene. During the Upper Miocene Timor was rapidly uplifted and eroded. Similar polymict intracalcirudites should be looked for in bahamites of ancient island arcs and other unstable areas. The interpretation of the formation of these rocks as the product of bahaman shoals in exceptionally unstable regions suggests that the apparent absence of this microfacies in ancient shelf deposits may be significant.
REFERENCES
AUDLEY-CHARLES,M. G., 1965. A Miocene gravity slide deposit from eastern Timor. Geol. MAR., 102 : 267-276. AUDLEY-CHARLES,M.G., 1967. The geology of Portuguese Timor. Geol. Soc. London, Mere., 4 (in press). BEALES, F. W., 1958. Ancient sediments of Bahaman type. Bull. Am. Assoc. Petrol. Geologists, 42 : 1845-1880. BROUWER,H. A., 1942. Summary of the geological results of the expeditions. Geol. Expedition Lesser Sunda Islands, 4 : 345-402. CAVEtJX,L., 1935. Les Roches Sddimentaires de France: Roches Carbonatdes. Masson, Paris, 463 pp. CLOUDJR., P. E., 1962. Environment of calcium carbonate deposition west of Andros Island Bahamas. U.S., Geol. Surv., Profess. Papers, 350 : 1-138. DUNN~NGTON,H. V., 1954. Stylolite formation post-dates rock induration. J. Sediment. Petrol. 24 : 27-49. FOLK, R. L., 1959. Practical petrographic classification of limestones. Bull. Am. Assoc. Petrol. Geologists, 43 : 1-38. ILLIrqG,L. V., 1954. Bahaman calcareous sands. Bull. Am. Assoc. Petrol. Geologists, 38 : 1-95. MArqa'EN,A. A., 1966. Note on the formation of stylolites. Geol. Mijnbouw, 45 : 269-274. NEWELL,N. D., 1955. Bahamian Platforms. Geol. Soc. Am., Spec. Papers, 62 : 303-316. PgolcoPovIcr~,N., 1952. The origin of stylolites, aT.Sediment. Petrol., 32 : 212-220. SHAtJB,B. M., 1939. The origin ofstylolites. J. Sediment. Petrol., 9 : 47-61. Sediment. Geol., 1 (1967) 247-257
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SHAUB, B. M., 1949. DO stylolites develop before or after the hardening of the enclosing rock? J. Sediment. PetroL, 19 : 26-36. Sr~EARS~AN,D. J., KrtouRI, J. and TAVIA,S., 1961. On the replacemertt of dolomite by calcite in some Mesozoic limestones from the French Jura. Proc. GeoL Assoc. London, 72 : 1-12. SHROCS:,R. R., 1948. Sequence in Layered Rocks. McGraw-Hill, New York, N.Y., 507 pp. SLOSS,D. D. and FERAY,D. E., 1948. Microstylolites in sandstone. J. Sediment. PetroL, 18 : 3-13. STAUFFER, K. W., 1962. Quantitative petrographic study of Paleozoic carbonate rocks, Cabello Mountains, New Mexico. J. Sediment. PetroL, 32 : 357-396. STOCKDAL~,P. B., 1922. Stylolites: their nature and origin. Indiana Univ. Studies, 9 : 1-97. STOCr,DALr, P. B., 1943. Stylolites: primary or secondary. J. Sediment. PetroL, 13 : 3-12. VAN BEMMELEN,R. W., 1949. The Geology of Indonesia. Government Printing Office, The Hague, 732 pp,
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