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IPA05-G-175

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirtieth Annual Convention & Exhibition, August 2005

HIGH RESOLUTION SEQUENCE STRATIGRAPHY AND DIAGENESIS IN CARBONATE ROCKS,

WONOSARI FORMATION, YOGYAKARTA: AN OUTCROP ANALOG FOR MODELING CHALKY LIMESTONE RESERVOIR DISTRIBUTION

Usman Jauhari* Budianto Toha**

ABSTRACT The importance of outcrop data in the development of carbonate sequence stratigraphy and diagenesis is highlighted by a study of the Middle Miocene carbonate of the Wonosari Formation, Yogyakarta. Four carbonate sequences are studied for use as analogs to model the distribution of chalky limestone reservoirs. Depositional and diagenetic facies have been correlated allowing a model of facies development and chalky limestone genesis and distribution in response to relative sea level changes to be reconstructed for the study area. The Wonosari Formation comprises five main depositional facies as follows: (1) Reef indicated by coral framestone to bafflestone. (2) Reef Mound indicated by red-algal bindstone. (3) Near Reef indicated by branching-coral-fragment

rudstone to floatstone. (4) Near Reef Mound indicated by rhodolith rudstone

to floatstone. (5) Inter-reef Lagoon indicated by foraminiferal

mixed skeletal packstone, wackestone to mudstone.

Sequence boundaries can be continuously traced in outcrops and show different physical expressions in each sequence. Two types of sequence boundary can be identified. The first is represented by an irregular surface and unconsolidated lithoclasts and the second is represented by calcrete and paleosols. Rapid rises in relative sea level during transgressions result in retrogradational patterns. Catch-up or prograding patterns develop during highstand in * ExxonMobil Oil Indonesia Inc. ** University of Gadjah Mada

relative sea level. At least four periods of relative sea level fall are recognized in the area which exposed the carbonate platform and resulted in alteration of hard limestone to porous and friable chalky limestone. The chalkification is proven to play an important role in the enhancement of secondary porosity in limestone. Vertical distribution of chalky limestone in a sequence is always bounded in its upper part by calcrete and paleosols and in its lower part by gradational changes from chalky into hard limestone. It does not develop below sequence boundaries characterized by irregular surface and unconsolidated lithoclasts. INTRODUCTION Carbonates are major reservoirs throughout the world. Porosity preservation and modification may be controlled by subaerial exposure and fresh water diagenesis during relative sea level fall. One product of reservoir quality is chalky limestone. It is defined as limestone with porous and friable characteristics in hand specimens (Reijers and Hsu, 1986) and dominated by microspar matrix in microscopic appearance (Jordan and Abdullah, 1992). Because of its characteristics, it is an important carbonate reservoir. Chalky limestone is a proven hydrocarbon bearing reservoir in the giant Arun field and accounts for 75% of the total porosity (Jordan and Abdullah, 1992), and also in other Tertiary reefs in Indonesia and the Philippines (Friedman, 1975; Longman, 1981). An understanding toward facies development in carbonate rocks and chalky limestone genesis in response to relative sea level changes is a prerequisite in order to model reservoir distribution. Miocene limestone of the Wonosari Formation is well exposed in outcrop in the Gunung Sewu area, Yogyakarta and gives an opportunity for a detailed

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study to establish an analog for carbonate deposition and diagenesis in response to relative sea level changes that can be applied to other subsurface areas. The limestones are put into sequence-stratigraphic framework to develop stratigraphic and diagenetic models. The study covers an area of approximately 3.5 km by 5 km in the Gunung Sewu area, Yogyakarta Province, Java (Figure 1). This paper is intended to demonstrate models of lithological and depositional environment, facies geometry of the Wonosari Formation in relation to relative-sea-level changes, and also the relationship between relative-sea-level changes and chalky limestone generation and distribution. The models can contribute in the development of carbonate sequence stratigraphy and diagenesis and serve as analogs for other carbonate reservoirs. METHODS This work consists of two main sources of data; outcrop and laboratory. Outcrops are examined in detail by measuring over a total of 1,115 m of mostly continuous, stratigraphic section in the study area. The stratigraphic section consists of east, middle and west sections (Figure 1) representatively crossing the study area. The three sections serve as a basis for identification and correlation of lithological, depositional and diagenetic facies, which in this study, particular attention is given to chalky limestone. Additional spot checks to the outcrops are undertaken in areas between the three sections. Laboratory data include thin sections and petrophysical properties analyzed for selected rock samples. Thin sections are examined to determine age and microscopic composition. Larger benthic foraminifera, supported by planktonic foraminifera, are used for age determination. Microscopic compositions are examined for facies identification and classification by applying the Embry and Klovan classification (1971) and interpretation of both depositional environment and diagenetic process. Consistency in petrographic and field data collection is achieved by having one geologist (the author) examining all the data. Porosity and density are measured in order to demonstrate the effect of diagenesis with respect to petrophysical properties. This is performed in the Soil Mechanic Laboratory, Civil Engineering Department, Gadjah Mada University.

GEOLOGIC SETTING The study area is located in Gunung Sewu, Southern Mountain Basin, Yogyakarta. Limestone of Miocene age in this basin is referred to as the Wonosari Formation (Toha et al., 1994). The limestone distribution in this area appears to be controlled by the position of pre-existing highs which formed during the Late-Oligocene by uplift and tilting of extensive tectonic blocks, which were subsequently truncated by erosion in the Late-Oligocene and Early-Miocene (Bolliger and De Ruiter, 1975). RESULTS Age Carbonate rocks in the study area contain larger forams Lepidocyclina (Nephrolepidina) with the parameter of F5, Cyclocypeus annulatus, and planktonic forams Orbulina Universa. The suites of microfossils indicate a Middle-Miocene age (Chaproniere, 1981; Blow, 1979). Facies of Depositional Environment According to regional geology, limestone of the Wonosari Formation is deposited on pre-existing high areas and surrounded by deep areas. It is only limitedly distributed on several pre-existing high areas and does not vastly cover all paleoshelf area and suggests patchy in its distribution. The geomorphology indicates that the carbonate platform of the Wonosari Formation is a patch reef complex or isolated carbonate platform rather than a ramp or rimmed shelf platform (Jordan (1998) and Handford and Loucks (1993)). Therefore the depositional environment for a patch reef complex as modeled by Jordan (1998) and James (1983) is selected and applied for the limestone of the Wonosari Formation. Five main depositional facies have been identified in the study area; reef, reef mound, near reef, near reef mound and inter-reef lagoon. Each facies is characterized by a particular lithofacies (Figure 2) as follows: Reef - characterized by coral framestone to bafflestone. The framestone contains 20-25 cm diameter head corals, 1 meter diameter massive corals and also encrusting corals. The bafflestone contains branching corals with finger diameters ranging from 2-7 cm. The corals are found encrusted and stabilized

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by red algae which build a rigid framework. Branching corals and head corals indicate a shallow water environment just below wave base and above wave base, respectively (James and Bourque, 1992). Therefore, the massive to head-coral framestone indicates a higher energy environment than branching-coral bafflestone. This depositional environment is symbolized by red color in Figures 3 and 4. Some of encrusting red algae have undergone recrystalization into bladed calcites with crystal length until 20 cm. These calcites are well known as batu lintang by villagers. Some of the massive and head corals have also undergone recrystalization which results in difficulties in differentiating between framestone, wackestone or mudstone. The major characteristic of framestone is a massive structure, which is very dense and hard in hand specimens whereas wackestone and mudstone are parallel-bedded in sedimentary structure and lighter and softer in hand specimens. Reef mound – characterized by encrusting-red-algal bindstone which is identified as having digitate, rhodolith and stromatolite structure. Thin sections reveal that the red algae are commonly in symbiosis with encrusting forams. Because of the dominance of encrusting red algae and lack of corals, this lithofacies is interpreted as reef mound. This facies is distinguished from reef by a dominance of encrusting organisms (James, 1983). The reef mound facies indicates an environment below base of large metazoan growth and above the lower limit of mound growth (James and Bourque, 1992). Therefore, reef mound indicates a deeper environment than reef. This depositional environment is symbolized by violet color in Figures 3 & 4. Some of encrusting-red-algal bindstone has undergone recrystalization and is difficult to identify in the field. Fresh outcrops and thin sections can help identify the presence of encrusting red algae and differentiate between bindstone and other lithofacies. Near reef – characterized by branching-coral-fragment rudstone to floatstone. Coral fragments are identified and interspersed in a wackestone to mudstone matrix. The fragments indicate products of both biological and physical breakdowns of coral reef and a depositional environment closely adjacent to the reef. This facies is distinguished from reef by the dominance of fragments rather than frame builders. This depositional environment is symbolized by blue color in Figures 5 to 10.

Near reef mound – characterized by rhodolith rudstone to floatsone. Rhodoliths are identified as fragments, sand to pebble size and interspersed in packstone. The fragments indicate results of biological and physical breakdown of encrusting-red-algal reef mound. The packstone matrix contains robust and thin, larger forams especially Amphistegina and Lepidocyclina, small benthonic forams (Milliolid forams), phylloid algae debris and mollusks. Near reef mound facies is distinguished from reef mound by the dominance of fragments to insitu, encrusting organisms. The abundance of rhodolith fragments indicates that this environment is close to reef mound. This depositional environment is symbolized by blue color in Figures 5-10. Inter-reef lagoon – is characterized by foraminiferal-mixed-skeletal packstone, wackestone to mudstone and this facies dominates in the study area. The inter-reef lagoonal facies is a deeper environment of deposition located among patch reefs (Jordan and Abdullah, 1992; Jordan, 1998). This facies contains microfauna including larger forams, planktonic forams, phylloid algae debris, red algae debris, small benthonic forams, mollusks and echinoderms. The shallow inter-reef lagoonal facies is white in color and contains abundant robust and thin larger forams (i.e. Lepidocyclina, Amphistegina), whereas the deep inter-reef lagoon contains abundant planktonic forams (i.e. Orbulina, Globigerina, Globorotalia). Argillaceous content also increases basinward and alters rock color to be brownish white. This depositional environment is symbolized by yellow color in Figures 3 to 10. Parallel bedding structure dominates this facies and indicates a low energy environment below wave base. Un-continuous reddish to brownish clay stringers with 2 – 10 cm in thickness are usually found parallel and inserted into the bedding. Sequence Boundaries Four sequences of carbonate rocks are identified in the field. Sequence boundaries bounding each sequence can be traced in outcrops and show two different physical expressions. The first type is represented by an irregular surface and or unconsolidated lithoclasts. In the case where unconsolidated lithoclasts are absent, the irregular surface is directly overlain by the following sequence. The unconsolidated lithoclasts range in size from 2 mm to 50 cm and have numerous and large inter-

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fragment spaces (Figure 11 A-B). This type of sequence boundary characterizes SB 1 and 2. The second type is characterized by paleosols and calcrete. The paleosols are 20 to 50 cm thick, brown to gray in color, friable, discontinuous and associate with calcrete. Thin section of paleosols shows the presence of both rhizoliths and alveolar texture which indicate plant’s root penetration into carbonate rocks during subaerial exposure (Esteban & Klappa, 1983). Calcrete is 20 to 40 cm thick, nodular to laminated, hard, impermeable and blankets underlying exposed carbonate rocks (Figure 11 C-E). It is formed by evaporation of calcium hydrogen carbonate dissolved in the pore water rising by capillary pressure (Flugel, 1978) and indicates dissolution and reprecipitation of CaCO3 in the phreatic and vadose meteoric zone (Esteban and Klappa, 1983). This type of sequence boundary characterizes SB 3, 4, and 5. Sequence 1 Figure 3 shows lithofacies successions of sequence 1 measured in the east section. The sequence boundary (SB 1) in this section is characterized by an irregular surface with overlying unconsolidated lithoclasts. Calcrete and paleosols are not identified. Figure 4 shows a reconstruction of facies development in sequence 1 in response to relative sea level changes. Overall, sequence 1 is dominated by a well developed reef facies in the shallow area (eastern area) and inter-reef lagoon in the basinward area (western area). Carbonate sedimentation of sequence 1 starts during transgression over older carbonate rocks and paleo-highs by relative sea level rise. The transgressive systems tract comprises 12 meters of sequence 1 in the shallow area, thins to 4 meters in the middle section and downlaps and pinches outs onto a sequence boundary in the basinward area. The basinward thinning and thickening in the shallow area suggests a retrogradational pattern of deposition. During the transgressive systems tract, reef facies which is identified as coral framestone, coral bafflestone and encrusting-red-algal bindstone, flourishes in the shallow water area. The association of corals and encrusting red algae indicates a shallow, normal marine environment. The facies change to reef mound toward the middle area, which are identified as encrusting red algal bindstone. This last lithofacies indicates a slightly deeper environment in the middle area than in the eastern area.

This transgressive systems tract is overlain by a highstand systems tract and separated from it by a maximum flooding surface (MFS). The MFS is identified from the contacts between reef and reef mound facies in the east section and between reef mound and inter-reef lagoon in the middle section. These contacts mark the end of deepening upward facies successions during transgression and the start of shallowing upward facies successions during highstand. As the rate of relative sea level rise slows down the highstand systems tract develops, reef and inter-reef lagoon facies prograde basinward (western area) over the previous transgressive system tract. Two smaller-scale shallowing upward cycles (parasequences) develop during highstand. The highstand systems tract comprises 148 meters in the thickest part of sequence 1 in the shallow area (eastern area) and thins to be 45 meters in the basinward area. Reef facies dominate in the shallow area and onlap onto the paleo-high of volcaniclastic deposits and change to inter-reef lagoonal facies basinward. Sequence 2 Figure 5 shows the lithofacies successions in sequence 2 from measurement in the east section. Like SB 1, SB 2 in this section is characterized by an irregular surface with unconsolidated lithoclasts. Calcrete and paleosols are not identified. Figure 6 shows a reconstruction of the facies development in response to relative sea level changes in sequence 2. It retrogrades over sequence 1 and is dominated by well-developed, inter-reef lagoon facies. The transgressive systems tract is 10 meters thick in the shallow area (eastern area), thins to be 3 meters thick in the middle area, downlaps and pinchs out onto sequence boundary 2 basinwards (western area). This thickness difference indicates a retrogradational pattern during transgression. Lithofacies of a sandy micrite, rich in quartz grains, with cross-bedding like structure in the shallow area (eastern area) records both reworking of terrigenous remnants and sedimentation during transgression. The percentage of quartz grains in this lithofacies decreases towards the middle area in which oncoidal floatstone develop. The MFS is picked on contacts in the east section between sandy micrite and phylloid-algal wackestone and in the middle section between oncoidal floatstone and large-foraminiferal wackestone. These contacts

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mark a slowing down in relative sea level rise and the commencement of the higstand systems tract over the previous transgressive systems tract. During the highstand, inter-reef lagoon and near reef mound facies slightly prograde basinwards (western area). At least, four, smaller-scale shallowing upward cycles (parasequences) develop during highstand. Each cycle contains inter-reef lagoon facies capped by near reef-mound facies. The highstand systems tract comprises of approximately 94 meters of sequence 2 in the shallow and basinward areas. Sequence 3 Figure 7 shows the detailed lithofacies succession of sequence 3 from measurements in the east section. It unconformably overlies sequence 2 and the sequence boundary (SB 3) is characterized by paleosols and calcrete overlying an irregular surface. The sequence boundary is well developed and is easy to identify in the eastern and middle area but difficult to trace in the western area. This might be caused by the fact that relative sea level only exposes the eastern and middle area but not the western area. Figure 8 shows the depositional model for sequence 3 in response to relative sea level changes. The sequence progrades over sequence 2 and is dominated by well-developed reef mound, near reef mound and inter-reef-lagoon facies. The transgressive systems tract is represented by 4 meters of rhodolith floatstone in the shallow area (eastern area) which downlaps and pinchs out onto a sequence boundary (SB3) basinwards (western area). Thickening towards the shallow area indicates a retrogradational pattern in sedimentation during transgression. This lithofacies grades upward into a large-foraminiferal packstone which suggests a deepening upward pattern. The MFS is picked on a contact in the east section between rhodolith floatstone and large-foraminiferal packstone. During highstand reef mound facies surrounded by near reef mound and inter-reef lagoon progrades basinwards (western area). At least two smaller scale shallowing upward patterns (parasequences) develop. The first cycle contains inter-reef lagoon grading upward into near reef mound and capped by reef mound facies. The second cycle is not complete because its upper part is truncated by a sequence boundary (SB 4). The

highstand systems tract comprises 82 meters in thickness of sequence 3. Sequence 4 Figure 9 shows the detailed facies succession of sequence 4 measured in the east section. It overlies sequence boundary 4 (SB 4) which is characterized by paleosols and calcrete overlying an irregular surface. Like SB 3, SB 4 is well developed and easy to identify in the eastern and middle area but is difficult to trace in the western area. This might be caused by the fact that relative sea level only exposes the eastern and middle area but not the western area. Figure 10 shows a reconstruction model of facies development in sequence 4 in response to relative sea level changes. It progrades over sequence 3 and is dominated by well-developed near reef mound and inter-reef lagoon facies. The transgressive systems tract is a 9 meter thick rhodolith rudstone to floatstone in the eastern area and downlaps and pinchs out onto sequence boundary (SB 4) in the most westerly part of the study area. The thickening towards the shallow area indicates a retrogradational pattern in sedimentation during transgression. This lithofacies grades upward into large foraminifera packstone to wackstone which suggests a deepening upward pattern. The MFS is picked on contacts between rhodolith rudstone to floatstone and large-foraminiferal packstone to wackstone. During highstand, near reef mound facies surrounded by inter-reef lagoon substantially progrades basinward (western area). At least, three smaller scale shallowing upward patterns (parasequences) develop. Each cycle is characterized by grading upward from inter-reef lagoon into near reef mound facies. The highstand systems tract comprises 76 meters in thickness of sequence 3 in the shallow area (eastern area) and slightly thins to be 71 meters basinward (western area). Chalky Limestone Genesis and Distribution The five sequence boundaries which are traced and delineated in the study area suggest periods of subaerial exposure due to relative sea level fall. These exposures allow meteoric water to infiltrate and alter the outcropped carbonate rocks due to diagenisis. An important product of the diagenesis in this area is chalky limestone.

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Chalky limestone in the field is bright white in color, friable, porous, powdery and has a grainy texture like sandstone. Bedding, which is a common sedimentary structure in limestone, is difficult to identify in chalky limestone because of obliteration by diagenisis (Figure 12). Loose or chalky characteristics are products of partial dissolution by meteoric water of Mg calcite cement in hard limestone (Aissaoui, 1988). Thin sections reveal the presence of uniform silt size and rhombic shaped microsparite crystals among skeletal grains and micro pores (intercrystalline porosity) among the crystals (Figure 12). Rhombic shaped microsparite is a characteristic product of high Mg micrite recrystalization by meteoric water. The recrystalization removes Mg++ from high Mg micrite, changes it to low Mg microsparite, and enables platy crystals of high Mg micrite to recrystalize to rhombic shaped microsparite (Longman, 1981). All of the texture changes produce a chalky limestone with porous, friable, powdery and grainy features in outcrop. This process of dissolution and recrystalization highlights the importance of subaerial exposure and meteoric water in generation of chalky limestone. Figure 5 shows how petrophysical properties change from chalky limestone into hard limestone. It is clearly seen that chalky limestone is a porous interval which grades downward to tight hard limestone. In the study area, chalky limestone is well developed below SB 3, 4 and 5 which are characterized by calcrete and paleosols, but does not develop below SB 1 and 2 which are characterized by an irregular surface and unconsolidated lithoclasts (Figure 11). Probable reasons why chalky limestone always associate with calcrete are suggested as follows: 1. It is a very dense, impermeable and laminated

layer which isolates the underlying limestone from marine influences during deposition of the overlying limestone. Its role is very significant in preserving and protecting chalky limestone from re-cementation by sea water during transgression (Rahardjo, 2003, pers. comm.).

2. It is formed by evaporation of calcium hydrogen

carbonate dissolved in pore water rising by capillary pressure (Flugel, 1982) and indicates dissolution and reprecipitation of CaCO3 in the phreatic and vadose meteoric zones (Esteban and Klappa, 1983). Its existence suggests that the underlying lithofacies is susceptible to dissolution

and recrystallization by meteoric water and, thus, indicates the presence of chalky limestone (Figure 11C).

Unlike calcrete, unconsolidated rock fragments have large inter-fragment spaces and, are therefore unable to preserve chalky limestone from marine re-cementation during transgression. In this case, multiple diagenesis or cementation can occur. The fragments reflect remnants of undissolved parts of exposed carbonate rocks and suggest that the exposed lithofacies is less susceptible to dissolution and recrystallization by meteoric water. Figure 13 shows the vertical distribution of chalky limestone in the study area. Chalky limestone develops below SB 3, 4 and 5 and does not develop below SB 1 and 2. In each sequence, loose, porous and friable characteristics in chalky limestone grade downward from the exposure surface into hard limestone. The base of vertical distribution of chalky limestone indicates the lowest penetration of phreatic water zone during subaerial exposure. Reconstruction of chalky limestone generation in each sequence is shown in Figure 6C, 8C, 10C. CONCLUSIONS 1. At least five exposure surfaces indicating

sequence boundaries are identified in the Middle-Miocene carbonate of Wonosari Formation. These sequence boundaries are expected to occur in age-similar subsurface carbonate rocks in other areas.

2. Chalkification is proven to play an important role

in the enhancement of secondary porosity in limestone. It changes, almost totally, the original texture and sedimentary structure into a relatively homogenous, porous, loose, powdery and grainy texture like sandstone. Therefore significantly increasing the reservoir properties of the original rock.

3. Chalky limestones are predicted to exist in

subsurface carbonate rocks in other areas and can be expected to be present below sequence boundaries with associated calcrete.

ACKNOWLEDGEMENTS This publication is part of the author’s master thesis

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in University of Gadjah Mada (UGM). I am grateful to Prof. Sukandarrumidi, Ir. Wartono Rahardjo, Dr. Subagyo Pramumijoyo and Ir. Sugeng Wijono M.S. for valuable discussions. This paper also benefited from reviews by both Prof. Daniel Lehrman from University of Wisconsin during his visit in the UGM campus and Mr. Keith Maynard as IPA editor. My special thanks go to Sidiq, Asmoro, Wisnu and Anton for helping me in collecting data in the field. REFERENCES Aissaoui, D. M., 1988. Magnesian Calcite Cements and Their Diagenesis: Dissolution and Dolomitization, Mururoa Atoll. in Tucker, M. E., and Bathurst, R. G. C., (eds.). 1990. Carbonate Diagenesis. Blackwell Scientific Publications. Oxford. p. 152-155. Blow, W. H., 1979. The Cenozoic Globigerinida. Leiden, E. J. Brill, 1413 p.

Bolliger, W., and De Ruiter, P. A. C., 1975. Geology of the South Java Offshore Area, Proceeding of the 4th Annual IPA Convention, p. 67-81.

Chaproniere, G. C. H., 1981. Australian Mid-Tertiary Larger Foraminiferal Associations and Their Bearing in The East Indian Letter Classification. B.M.R. Jour. Austral. Geol. And Geoph., v. 6. p. 145-151. Embry, A.F., and Klovan, E. J., 1971. Absolute Water Depth Limits of Late Devonian Paleoecological Zones. Geology Rdsch. 61/2, Stuttgart. in Reijers, T. J. A., and Hsu, K. J.1986. Manual of Carbonate Sedimentology: A Lexicographical Approach. Academic Press London. 301 p. Esteban, M., and Klappa, C. F., 1983. Subaerial Exposure Environment. in Scholle, P. A., Bebout, D. G., and Moore, C. H., (eds.). Carbonate Depositional Environments. AAPG Memoir 33. Tulsa, Oklahoma. p. 1-92. Flugel, E. 1978. Microfacies Analysis of Limestones. Springer-Verlag. New York. 632 p.

Friedman, G. M., 1975. The making and unmaking of limestones or the downs and ups of porosity. J. Sedim. Petrol., v. 45, p. 379-398. Handford, C. R., and Loucks, R. G., 1993. Carbonate Depositional Sequences and Systems Tracts-Responses of Carbonate Platforms to Relative Sea-Level Changes, in Loucks, R. G., and Sarg, J. F., (eds.). Carbonate Sequence Stratigraphy Recent developments and Apllication: AAPG Memoir 57. p. 3-41. James, N. P., 1983. Reefs. in Scholle, P. A., Bebout, D. G., and Moore, C. H., (eds.). Carbonate Depositional Environments. AAPG Memoir 33. Tulsa, Oklahoma. p 345-462. Jordan, C. F. and Abdullah, M., 1992. Arun Field -Indonesia, North Sumatra Basin, Sumatra. in Foster, N. I., and Beaumont, E. A., (eds.). Stratigraphic Traps III. Atlas of Oil and Gas Field. AAPG. p. 1-39. Jordan, C. F., 1998. The Sedimentology of Kepulauan Seribu: A Modern Patch Reef in the West Java Sea, Indonesia. IPA. 81 p. James, N. P., and Bourque, P. A., 1992. Reefs and Mounds. in Walker, R. G., and James, N. P., (eds.). Facies Models. Ontario. p. 323-347. Longman, M. W., 1981. Carbonate Diagenesis as a Control on Stratigraphic Traps. AAPG Education Course Note Series ≠ 21. Rahardjo, W., 2003. Personal Communication. Reijers, T. J. A., and Hsu, K. J., 1986. Manual of Carbonate Sedimentology: A Lexicographical Approach. Academic Press London. 301 p. Toha, B., Purtyasti, R. D., Sriyono, Soetoto, Rahardjo, W., Pramumijoyo, S., 1994. Geologi Daerah Pegunungan Selatan, Suatu Kontribusi. dalam Prosiding : Geologi dan Geoteknik Pulau Jawa Sejak Akhir Mesozoik Hingga Kuarter. Jurusan Teknik Geologi. Fakultas Teknik. UGM. hal 19-36.

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Figure 1 - Topograhic basemap showing the study area in the Gunung Sewu, Yogyakarta Province. Three

stratigraphic measurement paths are indicated by green, blue and red lines.

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Figure 3 - Detailed lithofacies successions in sequence 1 from measurement in the east section. This section

contains a deepening upward pattern in the lower part and two shallowing upward patterns in the middle and upper parts. See Fig. 1 for location of this section.

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Figure 5 - Detailed lithofacies successions in sequence 2 from measurement in the east section. This section

contains one deepening upward pattern in the lower part and three shallowing upward patterns in the middle and upper part. Petrophysical properties (density and porosity) are also shown. High porosity chalky limestone grades downward into tight hard limestone.

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tiona

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ttern

dur

ing

high

stan

d is

cle

arly

sho

wn.

Sub

aeria

l exp

osur

e pu

nctu

ates

dep

ositi

on o

f seq

uenc

e 2

and

enab

les

met

eoric

wat

er

to in

filtra

te a

nd d

iage

netiz

e ha

rd li

mes

tone

to c

halk

y lim

esto

ne.

310

Figure 7 - Detailed lithofacies successions in sequence 3 from measurement in the east section. TST is

represented by a deepening upward pattern whereas HST is represented by at least two shallowing upward patterns.

311

Figu

re 8

- M

odel

for

dep

ositi

on o

f ca

rbon

ate

rock

s in

seq

uenc

e 3.

The

thr

ee m

ain

sect

ions

use

d fo

r re

cons

truct

ion

are

indi

cate

d.

Sequ

ence

3 p

rogr

ades

ove

r se

quen

ce 2

. R

etro

grad

atio

nal

patte

rn d

urin

g tra

nsgr

essi

on,

catc

h-up

and

pro

grad

atio

nal

patte

rn

durin

g hi

ghst

and

is c

lear

ly s

how

n. S

ubae

rial

expo

sure

pun

ctua

tes

depo

sitio

n of

seq

uenc

e 3

and

enab

les

met

eoric

wat

er t

o in

filtra

te a

nd d

iage

netiz

e ha

rd li

mes

tone

to b

e ch

alky

lim

esto

ne.

312

Figure 9 - Detailed lithofacies successions in sequence 4 from measurement in the east section. TST is

represented by a deepening upward pattern whereas HST is represented by at least three shallowing upward patterns.

313

Figu

re 1

0 -

Rec

onst

ruct

ion

mod

el f

or d

epos

ition

of

carb

onat

e ro

cks

in s

eque

nce

4. T

he t

hree

mai

n se

ctio

ns u

sed

for

reco

nstru

ctio

n ar

e in

dica

ted.

Seq

uenc

e 4

prog

rade

s ov

er s

eque

nce

3. R

etro

grad

atio

nal p

atte

rn d

urin

g tra

nsgr

essi

on, c

atch

-up

and

prog

rada

tiona

l pa

ttern

dur

ing

high

stan

d is

cle

arly

sho

wn.

Sub

aeria

l exp

osur

e pu

nctu

ates

dep

ositi

on o

f seq

uenc

e 4

and

enab

les

met

eoric

wat

er

to in

filtra

te a

nd d

iage

netiz

e ha

rd li

mes

tone

to c

halk

y lim

esto

ne.

314

Figure 11 - Two types of physical expression of exposure surface. (A) & (B) Type 1 is characterized by

irregular surface and unconsolidated lithoclasts (red arrow). Hammer is for scale. (C) & (D) Type 2 is characterized by paleosols and caliche (red arrow) overlying irregular surface (dash line). Chalky limestone develops below this type of exposure surface. 160-cm jacob stiff is scale for (C). Increment of black color in scale bar in (D) is 5 cm. (E). Photomicrograph of paleosols showing rhizoliths or root molds (red arrow). Microscope Magnification is 400X.

Figure 12 - Outcrop and thin section of chalky limestone. (A) It is characterized by porous, friable, powdery

and grainy texture like sandstone in outcrop. Bedding commonly found in limestone is obliterated by chalk feature that results in homogenous appearance. Black in jacob stiff is 20 cm. (B) & (C) Abundance of uniform silt size, rhombic shape microsparites and micropores (intercrystalline porosity) (patcy light areas) characterize this limestone in thin section. Microscope magnification in (B) and (C) is 400X and 1000X.

315

Figu

re 1

3 -

Mod

el o

f ve

rtica

l di

strib

utio

n of

cha

lky-

limes

tone

in

the

stud

y ar

ea.

Cha

lky

limes

tone

dev

elop

s be

low

SB

3,

4 an

d 5

char

acte

rized

by

calc

rete

and

pal

eoso

ls b

ut is

abs

ent b

elow

SB

1 a

nd 2

cha

ract

eriz

ed b

y irr

egul

ar s

urfa

ce a

nd u

ncon

solid

ated

lit

hocl

asts

.