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Journal of Sedimentary Research, 2008, v. 78, 335–356 Research Article DOI: 10.2110/jsr.2008.039 SIGNIFICANT VOLCANIC CONTRIBUTION TO SOME QUARTZ-RICH SANDSTONES, EAST JAVA, INDONESIA HELEN R. SMYTH,* ROBERT HALL, AND GARY J. NICHOLS. SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham, TW20 0EX, U.K. e-mail: [email protected] ABSTRACT: Quartz-rich sedimentary rocks are commonly assumed to be the eroded products of cratons or recycled orogens. However, active or eroded acidic volcanic regions can also be an important, but commonly overlooked, source of quartz. Cenozoic sandstones from East Java, Indonesia, illustrate this point. They are rich in quartz, and it has long been assumed that they are the product of erosion of a continental source. However, new work using a variety of provenance indicators shows that the sandstones contain a significant, previously overlooked, volcanic component. A number of factors have contributed to their character: quartz-rich source regions, eruptive volcanic processes, and tropical weathering. Ternary discriminant diagrams, such as QFL plots which use the ratios of quartz, feldspars, and lithic grains to interpret provenance from cratonic, volcanic, and recycled orogen hinterland, may mislead, particularly in tropical volcanic settings. The quartz from acidic volcanic sources is commonly overlooked because it is commonly assumed that quartz has a continental crustal source. Volcanic eruptive processes can lead to crystal enrichment in rapidly eroded ash and sediments. Intense chemical weathering can have considerable impact on the composition of sedimentary rocks by selectively removing labile minerals and lithic grains. The resulting deposits may be texturally immature but compositionally mature, and rich in resistant minerals such as quartz and zircon. In tropical settings the widely held view that quartz-rich sandstones are mature sediments representing multiple phases of recycling may in many cases be incorrect. INTRODUCTION This paper examines the provenance of tropical, Cenozoic quartz-rich sediments from East Java, Indonesia. These sandstones have long been of interest (e.g., Rutten 1925; van Bemmelen 1949), and in particular to the petroleum industry as several are proven hydrocarbon reservoirs (e.g., Soetantri et al. 1973; Soeparyono and Lennox 1990; Ardhana 1993). Therefore their source, geographic distribution, depositional environment, and characteristics are of importance in exploration. The provenance of these sandstones also provides important information on the geological evolution of Java and the surrounding region. If the East Java quartz-rich sandstones are plotted on traditional discriminant diagrams such as QFL (Quartz, Feldspar, Lithic grains) and QmFLt (monocrystalline Quartz, Feldspar, total Lithic grains) ternary plots (Dickinson and Suczek 1979), a ‘‘cratonic interior’’ provenance is indicated. However, detailed examination of the quartz grains in these sandstones, by optical, SEM, and SEM-CL techniques indicates that there is a variety of types including igneous, volcanic, metamorphic, hydrothermal vein, chert, and recycled sedimentary quartz. In particular, the volcanic contribution, which has previously been overlooked, is of great importance when considering the likely subsurface distributions of sandstones, and in interpreting the geological development of Java. The paleoclimatic location of the sediment source regions is also of importance. The study area is presently located just to the south of the equator (between 6u and 9u S), and the potential source regions are within * Present address: CASP, Department of Earth Science, University of Cambridge, 181a Huntingdon Road, Cambridge, CB30DH, U.K. Copyright E 2008, SEPM (Society for Sedimentary Geology) the equatorial belt, as they were throughout the Cenozoic (Hall 2002). Therefore, the influence of tropical weathering must be considered because of its well-documented effects on sandstone composition (e.g., Dosseto et al. 2006; Suttner et al. 1981). This paper briefly summarizes the characteristics that allow different types of quartz to be distinguished, drawing on published literature (e.g., Ingersoll 1984; Basu et al. 1975; Bernet and Basset 2005; Götte and Richter 2006) and studies of quartz from different rock types in the region (Table 1). The Cenozoic quartz-rich sandstones of East Java are then described. This is followed by a discussion of potential continental, metamorphic, volcanic, and sedimentary source areas with consideration of transport mechanisms, transport distances, and paleogeographical barriers. BACKGROUND TO THE STUDY AREA, EAST JAVA, INDONESIA The island of Java is located in a central position within the Indonesian archipelago (Fig. 1). It is situated on the southeast edge of the Eurasian Plate, and to the south of the island there has been subduction of the Indian–Australian Plate along the Java Trench since the middle Eocene (Hall 2002). The southeastern part of the Eurasian Plate is known as Sundaland (e.g., van Bemmelen 1949; Hamilton 1979) and is the Mesozoic continental core of SE Asia. West Java is underlain by Sundaland continental basement, but the rocks which constitute the basement of East Java have been interpreted to be accreted slivers of metamorphosed arc and ophiolitic rocks (Hamilton 1979; Miyazaki et al. 1998). These slivers were accreted to Sundaland during the Cretaceous. The southern part of East Java is now known to be underlain at depth by 1527-1404/08/078-335/$03.00 Miocene Southern Mountains Eocene Southern Mountains Setting ii iii iv v 1 3 1 2 vi i 1 + 3 2 Id. Code Type(s) Jaten Kresek Cakaran, Wungkal-Gamping Kali Songo, Nanggulan Sermo, ?Nanggulan? Lukulo, Karangsambung Member, Formation Latitude, Longitude Kali Lukulo river 7.54771S, section, near to 109.6694E Karangsambung Village, Kebumen, Central Java Type Section South of Gunung 7.76705S, Pendul, Klaten, 110.67146E Central Java Hillside exposure 7.8114S, on Gunung 110.63121E Cakaran, Klaten, Central Java Kali Songo river 7.72558S, section, 110.20059E Nanggulan, Yogyakarta Early Miocene Road cut near to 8.13847S, 19 6 1 Ma (zircon the village of 111.26913E U-Pb SHRIMP) Tulukan, Pacitan District, East Java Province ?Middle Eocene? ?Middle Eocene? Middle Eocene (NP16) Probably Middle Sermo Reservoir, 7.826S, Eocene Yogyakarta 110.108E (, 56.1 Ma zircon U-Pb SHRIMP) Middle Eocene (P10-11) Age Location Information Yellow/white, wellsorted and crystal-rich quartz sandstones. Interbedded with volcanic muds, lignites, and pumice-rich horizons. A series of quartz-rich sandstones and muddy interbeds, which overlie basal polymict conglomerates. Up section the sandstones become increasingly arkosic. Quartz-rich, yellow, laminated, occasionally channelized sandstones. Interbedded with organic-rich muds, containing abundant plant fragments. Quartz-rich sandstones and conglomerates with abundant plant fragments and coal interbeds. Granular, yellow, quartz-rich sandstones interbedded with channelized polymictic conglomerates. Crystal-rich quartz sandstones with tuffaceous interbeds. Outcrop Environment of Deposition Tidal flats or estuary. Free from input of fresh volcanic material. Thinly laminated Air fall deposition or bands of euhedral and epiclastic bipyramidal volcanic quartz reworking of crystals and shards. volcanic deposits on a shallow marine shelf. Sandstones are Close to an active dominated by volcanic and/or eroding quartz with pumice, acidic volcanic volcaniclithic fragments, source in a and plagioclase. Quartz terrestrial setting types include faceted possibly on a bipyramidal crystals, floodplain or skeletal or embayed, mangrove swamp. shards, and Primary air fall and microcrystalline epiclastic aggregates. reworking. Quartz arenites to Terrestrial to deltaic. sublitharenites rich in Gradual increase in metamorphic and volcanic material of quartz. Abundant fresh volcanic source up laths of plagioclase, and section. volcanic lithic fragments. Sublitharenites, composed Terrestrial setting. of metamorphic and vein No quartz and lithic fragments contemporaneous of chert, basalt, and schist. volcanic activity. Dominated by metamorphic and vein quartz. These are texturally and compositionally mature quartz arenites. The sandstones are Clastic shoreline sublitharenites, in a terrestrial to composed of metamorphic shallow marine and vein quartz and (intertidal) setting. lithic fragments of chert, Sediment supply basalt, and schist. Up from a volcanic section they become rich source increases up in plagioclase and volcanic section. lithic fragments. Thin section of the quartz-rich sandstones Description TABLE 1.—Description of the Cenozoic quartz-rich sandstones of East Java, Indonesia. Type 1 contain metamorphic quartz, Type 2 volcanic quartz, and Type 3 have a mixed-provenance metamorphic, volcanic and recycled sedimentary quartz (van Bemmelen 1949; Sartono 1964; Sumarso 1975; Ardhana 1993; Lunt et al. 1998; Lelono 2000; Smyth 2005). 336 H.R. SMYTH ET AL. JSR JSR Lodan Quarry, 6.834272S, Rembang Hills, 111.67713E East Java Middle Miocene (N8-9) Yellow quartz and Quartz is dominated by Unstable (slumping bioclastic rich metamorphic grains with and dewatering) sandstones overlying significant proportions of shallow marine basal channelized volcanic and recycled slope near to a conglomerates. sedimentary quartz. The fluvial source. Sandstones are sandstones contain overlain by laminated abundant reworked siltstones, mudstones, Eocene and Oligocene and volcaniclastic bioclasts. rocks. Brilliant white to Quartz arenites dominated Terrestrial to margin yellow, apparently by metamorphic quartz marine possibly at mature, clean quartz- with a significant a river mouth. rich sandstones, proportion of volcanic interbedded with quartz: melt inclusions, quartz-rich siltstones, shard shape. Abundant claystones, coal,and elongate euhedral zircons. thin shallow marine limestones. 7.12241S, 110.15894E Kali Lutut river section, Semarang, Central Java Early-Middle Miocene N5-N7, NN4, 19.5 6 1.5 Ma (zircon U-Pb SHRIMP) Ngrayong 3 Miocene Shelf Edge viii Lutut 3 Miocene Kendeng Depocentre vii Thin section of the quartz-rich sandstones Latitude, Longitude Type Section Age Member, Formation Type(s) Id. Code 337 a sliver of Gondwana continental crust (Smyth et al. 2007; Smyth et al. 2008) but there is no evidence to suggest that this fragment was exposed at the surface or available for erosion during the Cenozoic. The Cenozoic volcanic and sedimentary rocks exposed on land in East Java were deposited on this accreted basement. Java is a volcanic island and contains the products of modern and older Cenozoic igneous activity. The volcanoes of the modern Sunda Arc are distributed along the length of the island (Fig. 1). A second older arc is exposed in the Southern Mountains of East Java and runs broadly parallel to, and south of, the modern arc (Fig. 1). This volcanic arc, here named the Southern Mountains Arc, was active from the Eocene to the early Miocene (Smyth 2005). During much of the Cenozoic there was a deep basin, the Kendeng Basin (Fig. 1), located to the north of, and behind, the Southern Mountains Arc (de Genevraye and Samuel 1972; Smyth 2005). From gravity calculations the depocenter is believed to contain more than 10 km of sedimentary rocks (Waltham et al. 2008; C.J. Ebinger, personal communication 2005), which are locally exposed at the surface in a fold thrust belt. A shallow marine clastic and carbonate shelf, the Sunda Shelf, formed the northern limit to the Kendeng Basin (Fig. 1). In the Cenozoic sequences of East Java there are several quartz-rich sandstones (ranging from litharenites to quartz arenites) of middle to late Eocene and early to middle Miocene age (Fig. 2, Table 1). They appear to be compositionally mature, in as much as they are rich in quartz, but they are texturally immature, in that many of the grains are angular or euhedral. The sandstones also contain abundant zircons, of which many are euhedral elongate prisms, a grain form which is typical of volcanic zircons (Mange and Maurer 1992). The origin and provenance of these quartz-rich sandstones was one aim of this study. Rutten (1925) discussed early differences in views about the source of material in Neogene sedimentary rocks of Java. Despite long-lived volcanic activity, the contribution of volcanic material to the older Cenozoic sedimentary rocks of East Java has previously been considered to be relatively unimportant. However, during the course of this study it became clear that many rocks considered to be terrigenous siliciclastic rocks (e.g., de Genevraye and Samuel 1972; Lunt et al. 1998) have a significant volcanic component (Smyth 2005). To assess the importance of the volcanic contribution and identify possible source regions it was necessary to evaluate the compositions and characters of the East Java quartz-rich sandstones, and consider the processes which may have formed them. FORMATION OF QUARTZ-RICH SANDSTONES Setting Location Information TABLE 1.— Continued. Outcrop Description Environment of Deposition ORIGIN OF SOME QUARTZ-RICH SANDSTONES, EAST JAVA, INDONESIA Quartz-rich sandstones, and in particular quartz arenites (in which quartz exceeds 95%), are the subject of considerable, in many cases conflicting, discussion in the literature (e.g., Chandler 1988; Dott 2003; Johnsson et al. 1991; Potter 1978; Suttner et al. 1981; and references therein). The discussion of Dott (2003) addresses many of the common myths and misconceptions surrounding the formation of quartz arenites. Dott (2003) recalls the conventional wisdom of the mid-twentieth century which emphasized the importance of multiple sedimentary cycles in the production of quartz-rich sandstones. In compositionally and texturally mature sandstones, the polycyclic selective removal of less stable minerals by processes of abrasion was commonly thought to be the ultimate cause of maturation. However, during the later part of the twentieth century detailed investigations illustrated that quartz arenites could be the product of single sedimentary cycles and/or postdepositional diagenesis. Today, we know that there are numerous, potentially interlinked, factors which may contribute to the formation of quartz-rich sandstones, including source-area characteristics, chemical weathering, climate, topography and orogenesis, multicycling, sediment transport and storage pathways, and diagenesis and/or leaching (e.g., Akhtar and Ahmad 1991; Avigad et al. 2005; Dott 2003; Folk 1974; Johnsson 1990; Johnsson et al. 338 JSR H.R. SMYTH ET AL. FIG. 1.— Simplified geological map of East Java, showing the main geological subdivisions and stratigraphic units (adapted from Smyth 2005). Inset shows current plate-tectonic setting and location of Sundaland. 1988; Johnsson et al. 1991; Suttner et al. 1981). The following section provides an overview of some of the most important processes; the reader is referred to the references cited for more detailed discussion. Chemical Weathering Sediment maturity is mainly acquired through chemical weathering, as chemically unstable minerals are eliminated (e.g., Salano-Acosta and Dutta 2005). Therefore, in most cases the daughter product of recycled sandstone should be mineralogically more mature than its parent source rock. In a few rare cases the daughter sediment may be mineralogically less mature owing to the breakdown (physical or chemical) of lithic fragments or large unstable grains (Friis 1978; Solano-Acosta and Dutta 2005). There are a number of important controls on rates of chemical weathering, such as residence time, climate, and presence and thickness of a soil profile. It is generally accepted that tropical climatic settings have higher rates of chemical disaggregation of source rocks and the resultant daughter sediment than high-latitude settings. Topographic Relief and Single-Cycle Sediments Johnsson et al. (1991) describe the impact that topographic relief can have on the sediment produced by chemical weathering based on a case study from the Orinoco River drainage basin. In this example, sediments produced within areas of high, often steep, relief, such as orogenic terranes or parts of the elevated shield, are not as compositionally mature as sediments produced in areas of low, flat-lying topography. Johnsson et al. (1991) explain this in terms of sediment residence time and transportation efficiency. In the areas of high relief, sediment transpor- tation processes can remove weathered material as rapidly as it is produced. In these areas the soil profile is commonly very thin or absent. The sediments produced closely resemble the parent rock, because the chemical weathering process is incomplete and the unstable minerals and lithics remain. In contrast, in the low-relief areas and flat upland erosional surfaces of the Guyana Shield the weathering process is more prolonged. Here the weathering rate exceeds the rate at which sediment is removed and a thick soil profile is common. As a consequence, there is a long soil residence time, there is destruction of unstable grains, and the resulting sediments are rich in quartz and have little resemblance to their parent rocks. Diagenesis Diagenesis and deep leaching can lead to development of secondary porosity and contribute to quartz enrichment. Franca et al. (2003) suggest a number of factors are required to produce secondary porosity. These include uplift of at least one basin margin to produce a hydraulic head, down-dip fluid escape route to remove water, abundant rainfall to recharge meteoric waters, and long-term tectonic stability. Dissolution by acid formation waters is also known to lead to enrichment of quartz within sandstones by leaching. In the Middle Jurassic Brent Sandstones of northwest Europe nearly all the feldspar was dissolved from the sandstone without leaving a trace (Harris 1989). Volcanic Processes Volcanic processes are often overlooked in discussion of quartz-rich sandstones. In acid arc settings, crystal-rich deposits are common because JSR ORIGIN OF SOME QUARTZ-RICH SANDSTONES, EAST JAVA, INDONESIA 339 FIG. 2.—Stratigraphy of East Java showing the distribution of quartz-rich sandstones (adapted from Smyth 2005); inset map shows the geographic locations. Codes (i to viii) are explained in Table 2. 340 H.R. SMYTH ET AL. of the character of the erupted material and the sorting efficiency of the eruption mechanism. Quartz-rich ash deposits formed by Plinian eruptions may be abundant even at considerable distances from the erupting volcano (e.g., Rose and Chesner 1987; Carey and Sigurdsson 2000). They are unstable and rapidly eroded (unwelded loose deposits which commonly lack vegetation cover), and the non-quartzose material is rapidly destroyed by weathering and transport. Thus, volcanism can lead to the formation of quartz-rich sandstones during a single cycle by providing a volumetrically significant source in a very short time. It is clear that a number of processes could have contributed to the formation of the tropical quartz-rich sandstones of East Java. However, characteristics of the grains, such as shape, internal structure, and alteration would be expected to be distinctive and aid discrimination between them. In order to assess the sources and processes that formed the sandstones it is therefore important to identify the types of quartz they contain. The following section summarizes the basis for distinguishing different quartz types. VARITIES OF QUARTZ There are numerous varieties of quartz. Those discussed here include igneous (separated into plutonic, hypabyssal, and volcanic), metamorphic, hydrothermal vein, chert, and recycled sedimentary (Fig. 3). Early work by Folk (1956, 1974), Basu et al. (1975), and Donaldson and Henderson (1988) showed that each of these quartz varieties has distinctive characteristics (Table 2) that allow them to be distinguished optically and using SEM-CL (scanning electron microscope cathodoluminescence). In more recent years comprehensive discussions of quartz CL characteristics have been published which adds greatly to the data collected by optical examination alone (e.g., Demars et al. 1996; Seyedolali et al. 1997; Hickel et al. 2000; Bernet and Bassett 2005; and references therein). Grain Shapes, Types, Crystal Units, and Undulosity The shape of detrital quartz grains ranges from euhedral forms with clear, well-defined crystal faces to anhedral grains without crystal faces. The grain shape may be changed during burial diagenesis and pressure solution, producing concave, convex, and sutured contacts, and quartz overgrowths. The grains can be monocrystalline, polycrystalline or composite. The number of crystal units (Basu et al. 1975) that make up a polycrystalline grain depends on its origin. Grains from low-grade metamorphic rocks (Fig. 4) have the most numerous crystal units, highgrade metamorphic rocks have fewer, and plutonic rocks generally have only two or three crystal units per grain (Basu et al. 1975). Polycrystalline grains from a metamorphic source are generally composed of small, similar sized crystal units which often show similar orientations. Composite grains, which can form in sedimentary, volcanic, plutonic, and metamorphic settings, usually have more random crystal orientations and are variable in size. Suturing can result in composite grains, and in some examples other minerals such as feldspar are present. Distinction between composite and polycrystalline grains can therefore be made on the basis of composition, contacts, orientation of the crystal units within the grains, and extinction angles. Undulose extinction is caused by imperfections in the crystal lattice resulting from strain or impurities. The angle of undulose extinction (Basu et al. 1975) is typically less than 5u in plutonic quartz but increases to over 5u in metamorphic rocks (Fig. 4). Igneous Quartz There are two polymorphs of quartz (Deer et al. 1998) but only one, a or low-temperature quartz, with trigonal symmetry, is stable at surface temperatures (, 573uC). At temperatures greater than 573uC, the stable form is the high-temperature b quartz polymorph, which has hexagonal JSR symmetry (Deer et al. 1998). When b quartz cools it inverts to a quartz, and when cooling occurs rapidly, as in volcanic settings, the quartz may retain the hexagonal form of the high-temperature polymorph. The resulting grain shape is often bipyramidal, but some additional trigonal faces may be added during cooling. Conversely, when cooling occurs slowly, as in plutonic settings, the grains have the trigonal form of a quartz. Plutonic Quartz.—Plutonic quartz is milky white to translucent in hand specimen, and can have an anhedral (Fig. 3A) or euhedral shape with trigonal form. Plutonic grains often have straighter grain boundaries than those of metamorphic quartz, and the crystals may, rarely, be zoned when observed using cathodoluminescence, recording the history of growth or crystallization from the melt. Polycrystalline grains are uncommon, and those that do exist generally have fewer than three crystal units per grain (Basu et al. 1975). Healed fractures filled with silica are common in plutonic quartz (Seyedolali et al. 1997; Bernet and Bassett 2005) and may link along their length to open fractures. These fractures are uncommon in metamorphic quartz and are very rare in volcanic quartz (Seyedolali et al. 1997). In panchromatic images plutonic quartz appears light gray, and its microcracks and healed fractures are easily distinguished (Bernet and Bassett 2005). In many granitic rocks quartz is a late-stage mineral which infills the gaps between other minerals, and therefore is irregular in shape. The grains eroded from such rocks have an anhedral shape and may be composite. Fluid inclusions are especially common in quartz from granitic rocks (Shepherd et al. 1985) forming thin strings throughout the grains (Fig. 3A). The simultaneous growth of two minerals such as quartz and alkali feldspar producing a granophyric texture is a feature of plutonic origin. Such textures may be found in composite grains in sedimentary rocks but often cannot be recognized unless the section is stained for feldspar. Hypabyssal.—Quartz crystallized in high-level intrusions may have some characteristics similar to both plutonic and volcanic quartz. If the intrusion is very close to the surface, the grains may form large euhedral phenocrysts (Fig. 3B), and the hexagonal shape of b quartz may be preserved but is likely to be accompanied by additional trigonal faces. More commonly the grains have a trigonal form. The grains are typically clear and bright in thin section and are free from fluid inclusions, and commonly show signs of melt reaction (see below). Volcanic.—Volcanic quartz (Fig. 3C) can be very distinctive when fresh because it is commonly monocrystalline, and clear and bright in thin section (Leeder 1982). In some cases the hexagonal form of b quartz is retained after inversion to a quartz and the grains may have a bipyramidal form. Composite grains may occur and owing to their microcrystalline nature can easily be confused with authigenic chert (see below). The growth of microcrystalline or fibrous quartzo-feldspathic grains from a crystalline core, known as ocelli or an ocellar texture, is also a common feature of volcanic quartz (Fig. 3C.7). Volcanic quartz grain shapes include euhedral, rounded, or embayed forms, and shards may also occur with a distinct cuspate shape, formed by breakup of a pumice bubble wall or shattering of a fractured crystal (Fig. 3C.8). Rapid cooling usually results in clear, non-undulose quartz, but explosive eruption may lead to strain, causing lattice imperfections and undulose extinction. Volcanic quartz may have a well-developed zonation, visible using cathodoluminescence (Fig. 3C.6), and curved fracture patterns, which have the appearance of a cracked-tile, are also a common feature. Melt reaction features, including rounding, and formation of embayments (Fig. 3C.5) and skeletal grains, are common in both hypabyssal and volcanic quartz. Rounding of crystal edges is due to resorption (Donaldson and Henderson 1988), which is a consequence of a lack of JSR ORIGIN OF SOME QUARTZ-RICH SANDSTONES, EAST JAVA, INDONESIA equilibrium between the crystal and the melt (McPhie et al. 1993). As a quartz phenocryst bearing magma rises, SiO2 solubility in the melt increases as pressure decreases and quartz that was previously in equilibrium with the melt is partially resorbed (McPhie et al. 1993). Embayments are due to unstable growth, dissolution in the melt, or ‘‘gas bubble drilling,’’ which is a reaction with the melt as gas bubbles approach the crystal (Donaldson and Henderson 1988). Embayments are distinguished by their rounded shape from etching caused by corrosive formation waters or pitting because of transportation. Volcanic quartz grains may also develop a skeletal shape if the crystal edges form first, generating a framework or skeleton outline (Spry 1969); the faces between the edges form more slowly and in some cases are infilled by other minerals or can remain as voids. Melt inclusions are diagnostic of a volcanic origin and can readily be distinguished from fluid inclusions (Fig. 3C.9). Fluid inclusions are commonly small, , 5 mm (Shepherd et al. 1985), and form strings parallel to fractures within the grains. Melt inclusions, however, form at the time of mineral growth, can be much larger, up to 200 mm (Shepherd et al. 1985), and may be arranged along growing faces so that they are parallel to zonation in the crystal. Metamorphic Quartz Quartz of metamorphic origin (Fig. 3D) has several diagnostic characteristics including undulose extinction angles, healed fractures, indistinct mottling under cathodoluminescence, strings of fluid inclusions (often needle-like), and anhedral, sutured or irregular grain shapes and contacts (Basu et al. 1975; Donaldson and Henderson 1988; Demars et al. 1996; Peppard et al. 2001; Boggs et al. 2002). Metamorphic quartz is more commonly polycrystalline than plutonic quartz and the straining of the lattice during metamorphism and deformation results in higher angles of undulose extinction (. 5u) compared to plutonic quartz (Fig. 3D). Impurities and abundant fluid inclusions may cause metamorphic quartz to be milky white. Mortar texture, in which large strained quartz grains are surrounded by finely crystalline new quartz (Spry 1969), is commonly observed in quartz from metamorphic rocks. SEM-CL images of metamorphic grains are commonly mottled or patchy (Table 2). Shearing during metamorphism results in the alignment of crystal units and the development of foliation in polycrystalline or composite quartz grains. Pressure fringes, commonly composed of fibrous quartz, calcite, chlorite, or muscovite, are abundant in low-grade metamorphic rocks, and their shape is related to the original crystal around which they formed. Fringes may resemble the ocellar texture common in volcanic rocks but are rarely preserved after erosion and transportation (Spry 1969). Hydrothermal Vein Quartz Vein quartz commonly has a milky white color due to fluid inclusions (e.g., Tucker 2001). The crystal faces may be clear but can be distinguished from volcanic quartz by the abundance of fluid inclusions 341 and lack of concentric zoning. The crystals are in many cases elongate and columnar, in as much as they grow from a fixed point, often a fracture wall, into an open space or vug, a texture known as comb structure (Spry 1969). Crystal terminations at either end of the column are different, and if there is limited space within the fracture, the quartz may form equant crystals. Chert Chert is cryptocrystalline or microcrystalline quartz formed either by siliceous organisms such as radiolaria, diatoms, and sponges, or by secondary replacement, usually of limestones (Adams et al. 1984). In radiolarian and other biogenic cherts spherical and elongate skeletons can sometimes be distinguished, which allows easy identification (Fig. 3E). However, when the chert is fine grained or cryptocrystalline and does not contain any visible biogenic structures, it may be difficult to determine the original nature of the grain. When the chert forms as secondary replacement, it commonly has a radial fibrous growth texture, ‘‘chalcedonic quartz’’ (Adams et al. 1984), which may be very similar to spherulites which form in devitrified siliceous volcanic glass (Fig. 3E, F). The spherulites are ‘‘radiating arrays of crystal fibers’’ (McPhie et al. 1993), which consist of feldspar and quartz, and the staining of thin sections for feldspar and examination using SEM can assist with the distinction of from other varieties of quartz. Recycled Sedimentary Quartz Detrital quartz that has been through multiple cycles of erosion is commonly rounded and pitted, and may have brown corrosion rims (Fig. 3H). The grains usually lack the crystal faces common in hypabyssal and volcanic quartz. They may have quartz overgrowths, or a fringe of other minerals such as calcite. Fractures which formed during transportation are likely to be angular or irregular, and are open, in contrast to the curved fractures which occur in volcanic quartz or the healed fractures in plutonic quartz. Diagenetic quartz formed as overgrowths on grains may contain fluid inclusions which are very small, , 5 mm (Shepherd et al. 1985), and are readily distinguishable from the much larger melt inclusions found within volcanic quartz. CHARACTER OF THE QUARTZ-RICH SANDSTONES OF EAST JAVA The Eocene and Miocene quartz-rich sandstones from East Java in this study plot within the ‘‘recycled orogen’’ field on a standard Dickinson plot. They have been reexamined and subdivided on the basis of the types of quartz that they contain (Tables 1, 3). Fine to coarse sandstones were selected for point counting using the Gazzi-Dickinson method (Gazzi 1966; Dickinson 1970; Ingersoll et al. 1984), and the quartz types were identified using the criteria discussed above. A minimum of 300 grains were counted from each sample. Where the quartz variety could not be determined the grains were assigned to an ‘‘unknown’’ category (up to 2% R FIG. 3.—Characteristics of quartz types commonly found in sedimentary rocks. Examples selected from Sumatra and Java, Indonesia, Tasmania, and Luzon. The scale bar is 1 mm unless stated otherwise. A) Plutonic: 1. Anhedral grain with melt inclusion, strings of fluid inclusions, and slightly undulose extinction. 2. Large late-stage filling grain with healed fractures. 3. The individual crystals within this composite grain have variable size, orientation, and extinction. B) Hypabyssal: euhedral quartz phenocrysts from a high level intrusion. C) Volcanic: 1. Top left and right sketches of bipyramidal quartz. Lower sketch, bipyramidal grain with additional trigonal faces formed during cooling. 2. Photograph of a bipyramidal quartz. 3. SEM image of bipyramidal quartz. 4. Bipyramidal grains in a crystal-rich dacitic ash. 5. SEM image of embayed quartz. 6. SEM-CL image showing concentric zoning within a large quartz phenocryst. 7. Ocelli texture and rounded fractures. 8. SEM image of a shard of quartz. 9. Melt inclusions in quartz. D) Metamorphic and sheared: 1. Polycrystalline grains with numerous crystal units, and monocrystalline grains with undulose extinction and strings of fluid inclusions. 2. Sheared quartz. E) Chert: 1. Radiolarian chert. 2. Authigenic chert with radial fibrous growth pattern. F) Volcanic sphericules (McPhie et al. 1993) formed in devitrified siliceous volcanic glass. G) Volcanic quartz aggregates easily confused with chert in thin section. The grains in the photomicrograph of the left appear chert-like, but examination under SEM on the right shows the grains are aggregates of bipyramidal quartz grains. H) Recycled sedimentary: rounded grains, with etched surfaces and alteration halos. The grains contain numerous strings of fluid inclusions. 342 H.R. SMYTH ET AL. JSR JSR ORIGIN OF SOME QUARTZ-RICH SANDSTONES, EAST JAVA, INDONESIA 343 Chert Hydrothermal Metamorphic Volcanic Plutonic (Hypabyssal) Plutonic Quartz type Polycrystalline Undulosity is weak, , 5u Undulosity Inclusions Zoning Fracture SEM-CL textures (and panchromatic colors) Symmetry Other common textures Fluid Well Healed Blue-red, Randomly Trigonal Mineral inclusions, inclusions developed fractures may overlap oriented melt reaction common with microcracks or textures, volcanic healed cracks granophyric are observed in growth with all types of feldspar. plutonic quartz. Light gray CL. Rare zoning. weak, , 5u to May be Common Healed Blue Trigonal Melt reaction, will nonundulose free fractures (potential depend upon depth from preservation and cooling inclusions of hexagonal history. symmetry) Euhedral, Clear Microcrystalline Commonly Present Curved Blue Concentric Melt Hexagonal Skeletal grains, melt monocrystalline, nonundulose and inclusions aggregates may leading to zoning is very with possible reactions leading to composite and appear to be clear. Grains common in are crackedaddition of rounding of crystal aggregate grains also may have strong diagnostic polycrystalline tile pattern volcanic quartz. trigonal faces, melt present, as are shards undulosity in Homogeneous faces during embayments, with a distinctive case of lattice CL is also cooling ocellular texture cuspate shape. imperfection. commonly and bipyramidal obseverd in grain shape. volcanic quartz. CL light gray to black. Anhedral, sutured or Milky white to . 3 crystal units Undulosity is Fluid ____ Mortar texture, Absent Angular Blue-brown, Inhomogeneous irregular. translucent per grain. Most . 5u inclusions healed and may overlap patchy or pressure fringes, abundant in lowcommon, open with mottled CL foliations in grade metamorphic often polycrystalline and fractures plutonic rocks needlecomposite grains. like. Euhedral Milky white to ___ Undulose Fluid Not May be Variable in Trigonal Grains are commonly translucent inclusions concentric present some case elongate, comb common green texture. Commonly rounded or Variable ___ ___ ___ ___ ___ May be non- Black CL not ___ If radiolarian, anhedral in form. The luminescent easily identified spherical and quartz can be using SEM-CL elongate skeleton cryptocrystalline, may be visible. microcrystalline, or Radial fibrous fibrous growth. May be confused with volcanic sphericules or microcrystalline aggregates of bipyramidal quartz. Colour Anhedral (space Milky white to , 3 crystal units filling) + euhedral. translucent per grain Can be mono-, polycrystalline, or composite. Grain boundaries generally straighter than in metamorphic rocks Euhedral, Clear Not monocrystalline grains common Grains SEM-CL colours TABLE 2.—Types of quartz found in sedimentary rocks and their distinguishing characteristics (Spry 1969; Basu et al. 1975; Leeder 1982; Adams et al. 1984; Roedder 1984; Shepherd et al. 1985; Donaldson and Henderson 1988; McPhie et al. 1993; Demars et al. 1996; Seyedolali et al. 1997; Peppard et al. 2001; Boggs et al. 2002; Bernet and Bassett 2005). 344 H.R. SMYTH ET AL. JSR JSR Etching + surface alteration is a common feature of recycled grains. Pits can easily be distinguished from melt embayments. ___ Undulose with Strings of Lost due to Commonly Variable, can Grain variable angles very small diagenetic seen and be nonshattering depending on fluid overprint are luminescent history inclusions angular or if diagenetic , 5 mm irregular common open fractures Rounded, pitted Brown corrosion Depends on overgrowths may be rim original source observed. Lack of preserved crystal faces. Recycled sedimentary Inclusions Undulosity Polycrystalline Colour Grains Quartz type TABLE 2.— Continued. Zoning Fracture SEM-CL colours SEM-CL textures (and panchromatic colors) Symmetry Other common textures ORIGIN OF SOME QUARTZ-RICH SANDSTONES, EAST JAVA, INDONESIA 345 of grains counted). In addition to thin-section analyses, SEM examination was used was used to identify unknown minerals and grain surface textures. Additional analysis of polished thin sections by panchromatic SEM-CL and back-scatter imaging was also undertaken which aided the identification of grains by optical techniques. There are three types of sandstone: N N N Type 1: Quartz and other fractions (minerals, lithics, and matrix clays) are almost entirely metamorphic. Type 2: Quartz and other fractions are entirely volcanic. Type 3: Quartz and other fractions have a mixed provenance. These sandstones are essentially a mix of Types 1 and 2, with the addition of varying volumes of recycled sedimentary and plutonic quartz. A proportion of the plutonic quartz is considered to be hypabyssal. The principal features of the Eocene and Miocene quartz-rich sandstones are listed in Table 1. They are subdivided into four groups by area and age, and further subdivided into eight categories corresponding to specific locations identified by roman numerals in the text (i to viii). Type 1 Metamorphic Quartz-Rich Sedimentary Rocks Pre-middle Eocene sandstones (i, ii, iv) are the oldest sediments exposed on land in East Java. They are restricted to the western part of the study area, where they rest directly on the basement (Table 1, Figs. 1, 2, 5). They are terrestrial deposits (Smyth 2005), but they lack palynomorphs or any other fossils and so cannot be directly dated. However, they are overlain by a succession of well-dated middle Eocene strata (Lelono 2000). The pre-middle Eocene sandstones are dominated by material of metamorphic origin, lack fresh intermediate to acidic volcanic material, and are the only deposits identified in East Java that contain no evidence of contemporaneous volcanic activity. In the Type 1 sandstones quartz constitutes 44 to 87% of the total QFL count. These rocks are composed almost entirely of grains of vein quartz (Fig. 5) and polycrystalline quartz grains with numerous crystal units, suggesting a low-grade metamorphic origin. The remaining quartz is dominated by chert. Weathered laths of plagioclase feldspar and a few grains of very altered microcline feldspar form between 1 and 8% of the grains counted. The sandstones and conglomerates also contain lithic clasts of chert, basalt, quartz–mica schist, and phyllite, and fragments of quartzose vein material, all of which are lithologies that are typical of rocks found in basement exposures in East and Central Java (Wakita and Munasri 1994; Miyazaki et al. 1998). In addition, the clay mineralogy (serpentinite, illite, and chlorite) of cements, and clay interbeds also suggests erosion of such basement rocks (Smyth 2005). Type 2 Volcanic Quartz-Rich Sandstones The Type 2 sandstones containing only volcanic material are restricted to lower to middle Miocene strata of the Southern Mountains. These quartz-rich deposits are found in close proximity to the acid volcanic centers of the Eocene to lower Miocene Southern Mountains Arc. The best-exposed example is the Jaten Formation (vi), located near Pacitan (Table 1, Figs. 1, 2, 6). The presence of lignite, channel structures, and abundant rootlets, and the lack of marine fauna, indicate a terrestrial depositional setting, probably on the flanks of a volcanic center. In the Type 2 sandstones quartz constitutes 82.5 to 95% of the total QFL count (Table 3). The sandstones contain concentrations of coarse, bipyramidal quartz grains measuring up to 10 mm (Fig. 6). Other grains have distinctive volcanic features including perfect crystal faces, large melt embayments, skeletal grains, negative crystals, rounded fractures, CL zonation, and melt inclusions. The sandstones contain volcanic lithic fragments and laths of plagioclase feldspar which have volcanic textures such as melt inclusions and embayments. The heavy-mineral fraction of the 346 H.R. SMYTH ET AL. JSR FIG. 4.—Methods of distinguishing plutonic and metamorphic quartz. A) Classification of source by examining polycrystallinity and undulosity (redrawn from Basu et al. 1975). B) Distribution of true angles of undulosity in detrital quartz from plutonic and low-rank metamorphic sources (redrawn from Basu et al. 1975). Jaten Formation sandstones contains abundant fresh zircon grains which are elongate and have length-to-breadth ratios greater than 5, a feature which is common in grains of pyroclastic origin (Mange and Maurer 1992). These sandstones are interpreted to have formed from the products of a Plinian eruption of crystal-rich magma that deposited ash, which was sorted during flow or fall, or subsequently reworked by epiclastic processes. Several red siliceous beds crop out to the north of Pacitan in the Watupatok Formation. Previously these red beds were interpreted as deep-water sediments because they resemble cherts. In thin section some grains appear chert-like but do not contain radiolaria. SEM examination shows that the grains are composed of microcrystals of bipyramidal volcanic quartz held together with strings of silica (Fig. 3G.). Type 3 Mixed-Provenance Quartz-Rich Sandstones Middle Eocene and Miocene sandstones of mixed metamorphic, volcanic, recycled sedimentary, and plutonic provenance are distributed widely in East Java. Middle Eocene Quartz-Rich Sandstones of the Southern Mountains.— Directly above the oldest sedimentary rocks that include the Type 1 sandstones, there is a thick unit, which may exceed 500 m in thickness, of quartz-rich sandstones. These form the lower part of a well-dated sequence (Lelono 2000) of middle Eocene to lower Oligocene rocks (iii) known as the Nanggulan Formation (Table 1, Figs. 1, 2, 7). At the base the quartz-rich sandstones are fluvial to shallow marine deposits, and they pass upwards into a series of arkosic arenites which form the upper part of the formation and are fully marine turbidites (Lelono 2000; Smyth 2005). The quartz-rich sandstones in the lower part of the Nanggulan Formation are moderately sorted sublitharenites (Table 3) and have a mixed provenance with components of metamorphic, volcanic, plutonic, and detrital quartz. In these Type 3 sandstones quartz constitutes 54 to 81% of the total QFL count. A significant proportion (up to 47%) of the quartz is monocrystalline metamorphic grains, which are subrounded, with abundant strings of fluid inclusions, and undulose extinction. Quartz grains with a clear volcanic origin are also present, and their abundance increases up section from 14 to 35%. There are also some volcanic lithic grains that contain quartz. CL imaging confirms that many of the quartz grains are fragments of much larger zoned volcanic grains (Fig. 7G). In addition to these quartz types plutonic and detrital grains with quartz overgrowths have been identified using SEM-CL images, but these are present only in the lowermost parts of the formation. The sandstones also contain feldspar, predominantly fresh plagioclase with a small number of altered microcline grains. Schists make up the majority of the lithic grains; they are commonly small (, 0.5 mm) and well rounded. Zircons are the most common heavy mineral. The zircon population includes fresh euhedral grains, elongated prisms, and broken prisms with sharp terminations. SHRIMP U-Pb dating of grains yielded ages of 41.7 6 1 Ma and 42 6 0.9 Ma (P.J. Hamilton, personal communication 2005), similar to the middle Eocene (49 to 37 Ma) biostratigraphic age for the formation. The remaining zircon grains are anhedral and rounded with some evidence of zoning; these grains yielded much older U-Pb SHRIMP ages. This indicates reworking of some older igneous material but shows that at least some of the volcanic material was erupted contemporaneously. The identification of quartz types and the other provenance techniques indicate that the middle Eocene quartz-rich sandstones of the Southern Mountains contain predominantly two types of material: an older metamorphic component and a contemporaneous volcanic component. There is a clear increase up section in the percentage of volcanic quartz as metamorphic quartz decreases (Table 3, Fig. 8), indicating a change in the sediment supply. In addition there is igneous and recycled sedimentary material (average 31%). Miocene Quartz-Rich Sandstones of the Kendeng Basin.—The Miocene quartz-rich sandstones of the Lutut Beds (vii), Semarang (Fig. 1), are not shown on the geological map for the area (Thaden et al. 1975) because they are exposed only in small outcrops. They are interpreted to have been deposited on the southern margin of the Kendeng Basin, and have subsequently been deformed and moved northwards to their present-day position by thrusting (Smyth 2005). JSR 347 ORIGIN OF SOME QUARTZ-RICH SANDSTONES, EAST JAVA, INDONESIA TABLE 3.— Summary of the average percentages of each quartz type found within the quartz-rich sandstones of East Java. ID Sample QFL Q (% of QFL total) F (% of QFL total) L (% of QFL total) Quartz counts QMNU nof QMU nof QM , 5% QM . 5% PC , 4 PC . 4 Chert Volcanic* Detrital VolLithic Metalithic SedLithic Unknown** Number Quartz types Metamorphic from optical, Volcanic Recycled sedimentary SEM, and Plutonic SEM-CL Unknown imaging Quartz Metamorphic Volcanic categories Recycled sedimentary and chosen (%) plutonic ID Sample QFL Q (% of QFL total) F (% of QFL total) L (% of QFL total) Quartz counts QMNU nof optical (300 QMU nof were possible) QM , 5% QM . 5% PC , 4 PC . 4 Chert Volcanic* Detrital VolLithic Metalithic SedLithic Unknown** Quartz types Metamorphic from optical, Volcanic SEM, and Recycled sedimentary SEM-CL Plutonic imaging Unknown Quartz Metamorphic categories Volcanic chosen (%) Recycled sedimentary and plutonic I Jhs2KK2 I Jhs2KW1 I Jhs2KW2 I Jhs2KK38 71.0 4.1 24.9 5 0 6 101 5 57 49 3 3 0 70 1 0 300 76.0 2.7 17.7 3.7 0.0 76.0 2.7 21.3 77.5 6.9 15.6 2 0 1 93 5 59 86 3 11 0 9 31 0 300 53.7 1.7 42.7 2.0 0.0 53.7 1.7 44.7 44.1 39.9 16.0 2 0 0 110 4 92 73 0 3 1 7 8 0 300 69.7 1.0 28.0 1.3 0.0 69.7 1.0 29.3 68.9 7.6 23.5 26 0 13 7 6 1 2 59 0 34 2 1 0 151 6.6 78.8 2.0 12.6 0.0 6.6 78.8 14.6 87.4 3.8 8.8 12 1 15 114 10 54 4 0 7 0 83 0 0 300 84.0 4.0 3.7 8.3 0.0 84.0 4.0 12.0 81.1 8.7 10.9 31 0 11 46 7 96 60 11 13 2 6 17 0 300 49.3 14.7 30.0 6.0 0.0 49.3 14.7 36.0 56.5 19.7 23.8 57 2 39 111 11 22 16 14 5 1 20 2 0 300 51.7 24.0 7.7 16.7 0.0 51.7 24.0 24.3 54 19 23 39 0 56 42 7 13 12 35 35 31 19 11 0 300 24.7 35.0 19.3 21.0 0.0 24.7 35.0 40.3 78.5 0.5 21 1 0 1 37 4 122 65 0 10 0 19 41 0 300 59.3 0.3 38.7 1.7 0.0 59.3 0.3 40.3 VI Jhs2Pac21 VII Jhs2Lutut11 VIII Jhs1-012 VIII Jhs1-006 VIII Jhs1-008 VIII Jhs2Ngr5 82.5 10.5 7 2 0 0 0 0 7 0 219 0 69 0 0 0 2.4 97.6 0.0 0.0 0.0 2.4 97.6 0.0 64.6 0 35.4 22 0 10 99 16 47 26 13 9 10 20 28 0 55.3 15.0 21.0 8.7 0.0 55.3 15.0 29.7 83.2 13.1 3.7 64 3 61 74 12 10 2 16 17 0 4 1 0 34.5 30.3 7.6 27.7 0.0 34.5 30.3 35.2 81.7 10.1 8.2 19 1 14 59 34 82 9 22 37 0 16 7 0 52.7 13.7 17.7 16.0 0.0 52.7 13.7 33.7 96 1 2 53 1 20 77 20 60 9 9 33 0 15 3 0 51.0 20.7 15.0 13.3 0.0 51.0 20.7 28.3 95.5 2.2 2.3 29 0 35 42 14 81 9 12 32 0 30 16 0 51.0 13.7 19.0 16.3 0.0 51.0 13.7 35.3 IV V VI Jhs2Pendul1 Jhs2Kresek Jhs2Pac17A 81.1 0.5 18.4 16 0 23 79 19 69 10 1 6 3 72 1 1 73.3 6.7 5.7 14.0 0.3 73.6 6.7 19.7 40.4 53.4 5.6 32 97 0 0 2 1 0 162 0 1 0 0 5 32.7 65.0 0.0 0.7 1.7 33.2 66.1 0.7 95 0 5 0 0 0 0 0 20 0 214 0 66 0 0 0 6.7 93.3 0.0 0.0 0.0 6.7 93.3 0.0 II III III III IV Jhs2Sermo1 Jhs2NKS10 Jhs2NKS17 Jhs2NKS26 Jhs2JC5 Q (Quartz), F (feldspar), L (lithics), QMNU (quartz, monocrystalline, non-undulose extinction, no other features), QMU (quartz monocrystalline, undulose extinction, no other features and angle of extinction cannot be measured), QM , 5% (quartz monocrystalline , 5% undulose extinction), QM . 5% (quartz monocrystalline . 5% undulose extinction), QPC , 4 (quartz, polycrystalline , 4 crystal units), QPC . 4 (quartz, polycrystalline , 4 crystal units). In these Type 3 sandstones quartz constitutes 65% of the total QFL count. The quartz types within the sandstones include metamorphic, volcanic, recycled sedimentary, and plutonic (Table 3). The metamorphic quartz (55%) has undulose extinction, and both polycrystalline and monocrystalline types have been identified. The volcanic quartz (15%) is euhedral, contains melt inclusions, and appears clear and bright. The lithic fragments are diverse and include metamorphic, sedimentary, bioclastic, and abundant volcanic rocks (Fig. 7). The bioclastic lithic clasts contain fragments of reworked Eocene and Oligocene fossils. These sandstones contain three components: recycled Cenozoic sedimentary rocks, fresh contemporaneous acid volcanic rocks, and metamorphic rocks. These are the only quartz-rich sandstones on land in East Java which contain clear evidence of reworking of older Cenozoic sedimentary sequences (Smyth 2005). 348 H.R. SMYTH ET AL. JSR JSR ORIGIN OF SOME QUARTZ-RICH SANDSTONES, EAST JAVA, INDONESIA Miocene Quartz-Rich Sandstones of North East Java.—The Miocene quartz-rich sandstones of the Ngrayong Formation (viii), Rembang (Fig. 1), are well exposed in quarry sections along the flanks of the Lodan Anticline. The sandstones were deposited on the southern edge of the Sunda Shelf and pass onto the northern slopes of the Kendeng Basin (e.g., Ardhana 1993). The sandstones have been described as compositionally mature, clean, and quartz-rich and have previously been interpreted to be cratonic in origin and derived from Sundaland (e.g., Ardhana 1993; Sharaf et al. 2005). In these Type 3 sandstones quartz constitutes 82 to 96% of the total QFL count. Lithic grains are rare in these sandstones and range from 2 to 8% of the total QFL count. These sandstones are unconsolidated and lack clays, cement, and/or matrix. The sandstones are composed nearly entirely of well-sorted, often angular quartz grains. Because these sandstones are so rich in quartz they are commonly referred to as ‘‘glass sands,’’ and they are used extensively by the local ceramic industry. Thin-section and SEM-CL examination shows that between 13 and 30% of the quartz grains are very angular, are euhedral, and have melt inclusions and weak concentric zoning indicating a volcanic origin. The predominant metamorphic quartz (up to 55%) includes anhedral, polycrystalline and monocrystalline types with undulose extinction and strings of fluid inclusions. In addition, quartz grains of plutonic origin (up to 28%) and recycled sedimentary quartz (up to 19%) are also present. Feldspar is uncommon in the Ngrayong Formation; plagioclase is absent but a few grains of extremely weathered microcline feldspar have been identified. Zircon grains (Fig. 7) are abundant, and there are significant proportions of elongate prisms (43%), typical of pyroclastic zircons, or equant euhedral grains (25%), with the remaining 32% being moderately to well-rounded grains. The preservation of crystal faces in 68% of the zircons indicates that they were not significantly reworked. The presence of fresh volcanic quartz, angular but well-sorted grains, and pristine volcanic zircons raises questions about previous interpretations that these sandstones were derived solely from continental Sundaland (e.g., Ardhana 1993; Sharaf et al. 2005). SOURCES OF QUARTZ FOR EAST JAVA SANDSTONES This study indicates that there were three main sources of quartz for the Cenozoic quartz-rich sandstones of East Java: metamorphic rocks, acid volcanic material, and recycled sedimentary rocks. There were also contributions from plutonic sources. Metamorphic Source Rocks The most likely source areas of metamorphic material are the (1) Upper Cretaceous and older basement of East Java and (2) basement rocks on the edge of Sundaland such as those exposed in southeast Kalimantan and along the Karimunjawa Arch (van Bemmelen 1949). In East Java the basement rocks are observed only in small exposures in the western part of the study area at Karangsambung and Jiwo (Fig. 1). At these locations the lithologies exposed include mica and quartz-mica schists, basalts, cherts, serpentinites, metasediments, and a range of high pressure low temperature metamorphic rocks including eclogites, garnet amphibolites, and jadeite–quartz–glaucophane rocks (Wakita and Munasri 1994; Miyazaki et al. 1998). The rocks are thought to be the metamorphosed equivalents of ophiolites and arc rocks (Wakita and Munasri 1994; Miyazaki et al. 1998) accreted during Cretaceous 349 subduction along the Sunda margin. A subduction setting is supported by the occurrence of high pressure low temperature jadeite–quartz– glaucophane-bearing rocks within the Karangsambung Basement Complex (Miyazaki et al. 1998). These rocks were subsequently uplifted in the Late Cretaceous. The Cretaceous basement and the Eocene sedimentary sequence are separated by a regional angular unconformity. Based on the youngest ages of the cherts in the Karangsambung Basement Complex (Wakita 2000) and the ages of the oldest Eocene sedimentary rocks above the unconformity (Lelono 2000; Smyth 2005) the basement of East Java may have been uplifted and available for erosion for a period of around 30 My. The almost complete absence of Paleocene sedimentary rocks from Java and Sumatra suggests that southern Sundaland was an elevated region following Late Cretaceous collision of a continental fragment with the Sundaland margins (Smyth et al. 2008). This long period of weathering and erosion could have resulted in enrichment of resistant minerals such as quartz and zircons. The Karimunjawa Arch is an area within the shallow marine shelf north of Java (Fig. 8), which was elevated throughout most of the Cenozoic and was therefore a potential source of sediment (e.g., Cater 1981). The Karimunjawa Islands, which are found along the arch, are reported to contain exposures of pre-Cenozoic quartz sandstones and conglomerates, schist, and shale (van Bemmelen 1949) but the character of the basement in this region is not well known. The location of the Karimunjawa Arch immediately to the northwest of East Java means that material could have been transported a relatively short distance across the shelf into the Kendeng Basin. To the east of this drainage divide lie the Meratus Mountains of southeast Kalimantan. The rocks exposed in these areas are similar to those described from Java; they range in age from the Middle Jurassic to the Late Cretaceous and comprise chert, siliceous shale, limestone, basalt, ultramafic rocks, schists, and sedimentary volcanic rocks (Wakita et al. 1998). However, it is not clear whether these rocks were available for erosion during the early Cenozoic. Volcanic Source Rocks The Southern Mountains Arc in East Java is the closest and most likely source of acid volcanic material. The arc was active from the middle Eocene until the early Miocene (42 to 18 Ma) and formed the southern margin of the Kendeng Basin (Smyth 2005). Throughout the late Oligocene and early Miocene the volcanic activity in the Southern Mountains Arc was extensive, and produced by explosive Plinian-style eruptions. The deposits range from andesite to rhyolite, with an average SiO2 content of 67 wt% (Smyth 2005), and include thick mantling tuffs, crystal-rich tuffs, block and ash flows, pumice–lithic breccias, andesitic breccias, and silicic lava domes and lava flows. In the Southern Mountains Arc, there is a record of a major eruption towards the end of the period of arc activity. Extensive deposits of this eruption are widespread to the east of Yogyakarta; these were deposited in a short period, possibly during one eruptive phase, between 21 and 19 Ma (Smyth 2005). Due to its position at the southern margin of the basin, the arc would have supplied volcanic debris, including volcanic quartz, to the basin by pyroclastic flows, air fall from erupted ash columns and clouds, and epiclastic reworking. Ash-fall from large eruptions would have been distributed over an extensive area including the shelf area to the north of the Kendeng Basin. r FIG. 5.—Character of Type 1 quartz-rich sandstones. A) Photomicrograph of metamorphic grains from iv (Cakaran Member of the Wungkal–Gamping Formation) (scale: 1 mm). B) Photomicrograph showing metamorphic grains from basal i (Lukulo Member, Karangsambung Formation) (scale: 1 mm). C, D) BS (back scatter) and CL images of polycrystalline grains (Cakaran Member of the Wungkal–Gamping Formation) (scale: 200 mm). E) QFL plot showing the Type 1 sandstones. F) Triangular plot showing metamorphic, volcanic, and recycled sedimentary and plutonic quartz of the Type 1 sandstones. 350 H.R. SMYTH ET AL. JSR JSR ORIGIN OF SOME QUARTZ-RICH SANDSTONES, EAST JAVA, INDONESIA 351 Volcanic processes are extremely efficient sorting mechanisms (e.g., Walker 1972; Cas and Wright 1987). Crystal-rich, well-sorted deposits can be produced by single volcanic events, and by subsequent epiclastic processes. Concentration of crystals can occur in the magma chamber, during dome collapse, in the eruption column, or by reworking and weathering of tuffs, ash falls, and pyroclastic debris (Cas and Wright 1987). Examples of quartz-rich and other crystal-rich volcanic deposits include sediments in the Lower Permian Collio Basin of the Italian Alps (Breitkreuz et al. 2001) and in the marine Paleozoic basins of the Sarrabus region, SE Sardinia, Italy (Gimeno 1994). Ash particles within an eruption cloud fall out at different distances from the vent. This is dependent upon the terminal velocity of particles, which is determined by density and aerodynamic factors. Heavy lithic particles reach their terminal velocity and fall out close to the vent, but crystals and pumice are transported greater distances until they reach their terminal velocity and descend. This leads to ‘‘increased proportions of crystals further from the vent’’ (Cas and Wright 1987). This phenomenon is noted in the air-fall deposits of the Fogo A, Azores, and the 1980 Mount St. Helens eruptions (Cas and Wright 1987; Fisher and Schmincke 1984; Fisher et al. 1998; Kaminski and Jaupart 1998; Veitch and Woods 2001). Air-fall material is distributed over a wide area, it is commonly rapidly deposited and rapidly reworked from land into sea, and it is very likely to be mixed with sediment from other sources and redeposited. After eruptions in the tropics, lahars commonly carry large volumes of material downslope and mix ash and crystals with preexisting sediments. Due to post-eruptive reworking, mixing, and transportation, the paleocurrent indicators in sedimentary rocks in which this material is finally deposited may not necessarily provide information on the ultimate source of the material. For example, the quartz-rich Ngrayong Formation was deposited in a terrestrial to shallow marine setting on the edge of the Sunda Shelf and has numerous indicators of north-to-south sediment transport, including channels, cross-bedding, and the regional change in facies from terrestrial in the north to subsurface marine fans in the south (Ardhana 1993). The sediment is therefore interpreted to have been transported southwards from the shelf onto the shelf edge, and the source for this sediment has been assumed to be the continental rocks of Sundaland farther to the north (e.g., Ardhana 1993; M. Adams, personal communication 2001). The new provenance data suggest that the volcanic material, including quartz, zircons, and clays, in these sandstones were the result of ash-fall onto the Sunda Shelf. A significant component of this air-fall is now thought to be associated with the major eruption which occurred in the Southern Mountains Arc at around 20 Ma (Smyth et al. 2005; Smyth et al. 2007; Smyth et al. 2008). The air-fall deposits on the shelf were subsequently reworked, mixed with metamorphic and recycled sedimentary material, and redeposited on the shelf edge. This illustrates the importance of mixing and reworking of multiple sources of sediment, and indicates that the ultimate source for a significant proportion of the sediment was not only the Sundaland continent to the north but also the Southern Mountains Arc to the south. A. Karimunjawa Arch.—As discussed above, the Karimunjawa Arch was elevated throughout most of the Cenozoic and was therefore a potential source of sediment. Pre-Cenozoic quartz sandstones and conglomerates (van Bemmelen 1949) now exposed on Karimunjawa Island north of Java could have provided a source for abundant recycled sedimentary quartz. It is also possible that quartz-rich sandstones were deposited on the arch in the early Cenozoic and were subsequently removed. Recycled Sedimentary Source Rocks INFLUENCE OF TROPICAL WEATHERING AND USE OF DISCRIMINANT DIAGRAMS The Cretaceous and older basement of East Java is the closest source for recycled sedimentary material. In addition there are several other potential sources: B. Eocene sedimentary rocks on land, East Java.—The Miocene Lutut Beds, described above, contain a reworked Eocene and Oligocene fauna, sedimentary lithic grains, and recycled sedimentary quartz. In addition, they contain fresh contemporaneous volcanic and metamorphic material. The abundance of volcanic material strongly suggests that these sandstones are the product of Miocene uplift and erosion of lower Cenozoic volcanogenic rocks and older basement rocks in the Southern Mountains Arc. Plutonic Source Rocks Plutonic quartz does not occur in abundance in the sedimentary rocks of East Java, contributing only 9% of the total quartz in all of the quartzrich sandstones analyzed in this study. The closest granitic rocks to East Java exposed at the surface are the Cretaceous granites of the Schwaner Mountains of SW Borneo. These granites are known (van Hattum 2005; van Hattum et al. 2006) to have been elevated between the late Eocene and the early Miocene, providing material to the sedimentary rocks of northern Borneo. The Schwaner Mountains granites have yielded a small and distinctive range of isotopic ages including U-Pb ages from zircons (van Hattum 2005). Similar age populations of zircons are recognized in northern Borneo sedimentary rocks (van Hattum et al. 2006), but are not seen in the zircons of East Java (Smyth 2005; Smyth et al. 2007; Smyth et al. 2008). Other granites are located much farther away and include the Tin Belt of the Malay Peninsula and Sumatra, and the Tin Islands of the Sunda Shelf: Bangka, Billiton, and Belitung. Sediment produced by the erosion of these granites would require lengthy transportation by river systems, which in the case of Malay Peninsula or Sumatra would have exceeded 1500 km. During the Cenozoic there were also paleogeographic barriers in the Java Sea, including two elongate elevated ridges, the Karimunjawa and Bawean Arches (Fig. 8), which were emergent throughout most of the Cenozoic (e.g., Bishop 1980; Cater 1981). The Karimunjawa Arch separates the East and West Java Seas: to the east of the arch lower Miocene to Oligocene sedimentary rocks are marine but to the west most of the lower Miocene and all of the Oligocene sedimentary rocks are nonmarine (Cater 1981). These arches, and the narrow basins which flanked them, could have acted as sediment traps or barriers preventing detritus from Sundaland entering the East Java system. In addition a number of basins in the West Java Sea, such as the Billiton and Arjuna Basins, could have been sediment traps. Today, and throughout the Cenozoic, the source regions that provided sediment to East Java were located close to the equator (Hall 2002) and as r FIG. 6.—Character of Type 2 sandstones. A) Bipyramidal grain (scale: 1 mm). B) Photomicrograph showing poorly sorted sandstone of v (Kresek Member, Wungkal Gamping Formation). Note the two large clear, euhedral grains in the top right of the image (scale each box: 1 mm). C) Photomicrograph showing melt embayments from vi (Jaten Formation) (scale: 500 mm). D) Quartz shard (scale: 1 mm). E, F) SEM-BS and SEM-CL images of a grain showing melt embayments and concentric zoning (Jaten Formation) (scale: 700 mm). G) QFL plot showing the Type 2 sandstones. H) Triangular plot showing metamorphic, volcanic, and recycled sedimentary and plutonic quartz of the Type 2 sandstones. 352 H.R. SMYTH ET AL. JSR JSR ORIGIN OF SOME QUARTZ-RICH SANDSTONES, EAST JAVA, INDONESIA 353 FIG. 8.— Potential source areas for the quartz-rich sandstones. a result had a tropical climate. Tropical weathering can have a significant impact on the composition of sediment, and weathering processes can occur during erosion of the source rock, during transportation of sediment, after deposition, and/or when the resulting sedimentary rock is exposed at the surface. increase in quartz (Schulz and White 1999). This enrichment is the result of the selective removal of kaolinite. Unconsolidated volcanic deposits and sediments which are subject to tropical weathering are expected to break down much more rapidly than in lithified sediments or in granitic rocks. Tropical Weathering Discriminant Diagrams Tropical environments with high temperature and precipitation are sites of rapid weathering and erosion (e.g., White and Blum 1995; Dosseto et al. 2006). There are a number of well documented examples of the impact of tropical weathering on rocks in both the ancient and the modern record. In the Narmada Basin in India (Akhtar and Ahmad 1991) the Lower Cretaceous Nimar Sandstone, a quartz arenite, was produced by single-cycle weathering of a cratonic source. The quartz was enriched relative to feldspar and other labile constituents by the ‘‘humid tropical climate and a long residence time in the soil horizon’’ (Akhtar and Ahmad 1991). The resulting sediment is compositionally mature but texturally immature. In a modern example from the Guaba Ridge in the Luquillo Mountains of Puerto Rico, Schulz and White (1999) recorded an increase in quartz concentration in the soil profile developing above granitoid rocks. In the upper meter of the soil profile there is a 30% In tropical environments traditional assessments of maturity and provenance from discriminant plots such as those of Dickinson and Suczek (1979) that focus on the proportions of quartz, feldspar, and lithic fragments may be misleading. In these settings rapid, intense, and deep weathering break down any labile minerals, mineral aggregates, and many lithic fragments. The resulting sediment is rich in resistant minerals such as quartz and heavy minerals like zircon. As the unstable or weak fraction is removed the quartz content is enriched and the resulting sands contain a higher proportion of quartz than the source rock. Datasets on the composition of arc-related sediments (e.g., Dickinson and Suczek 1979; Marsaglia and Ingersoll 1992) do not include modern tropical environments, because there is a gap in samples collected around the equator between latitudes of 9.7u N and 16.5u S. Up to now few provenance studies have been published from tropical SE Asia and little r FIG. 7.—Character of Type 3 sandstones. A) Photomicrograph showing a bipyramidal quartz grain in the center bottom left (scale: 1 mm). B) Photomicrograph of the angular, clear quartz grains and opaque mineral grains (scale: 500 mm). C) Elongate volcanic zircons from viii (Ngrayong Formation) (scale: 100 mm). D) Quartz lithics from the vii (Lutut Formation) (scale: 1 mm). E) Weathered volcanic lithics from vii (Lutut Formation) (scale: 1 mm). F, G) SEM-BS and SEM-CL images of a quartz grain exhibiting incomplete concentric zoning, indicating that it is a fragment of a much larger grain (200 mm) (Nanggulan Formation). H) QFL plot showing the Type 3 sandstones. I) Triangular plot showing metamorphic, volcanic, and recycled sedimentary and plutonic quartz of the Type 3 sandstones. 354 JSR H.R. SMYTH ET AL. information from Indonesia, despite the size and the high sediment yields of this region at present (e.g., Milliman and Syvitski 1992; Milliman et al. 1999), and its importance as a region of abundant volcanic and tectonic activity throughout the Cenozoic (e.g., Hall and Smyth 2008). The abundance of volcanic quartz in East Java suggests that more data are needed from tectonically and volcanically active tropical regions such as Indonesia, and that discriminant plots should be considered in the light of climate at the time the sediment was eroded and deposited, as well as the present day. CONCLUSIONS Quartz can provide valuable provenance information. There are numerous potential sources of quartz in sedimentary rocks, and the middle Eocene to lower Miocene sandstones of East Java contain igneous (plutonic, hypabyssal, and volcanic), metamorphic, hydrothermal vein, chert, and recycled sedimentary quartz. However, each quartz type has distinctive characteristics which allow them to be distinguished, and combined with other provenance information, can provide critical information in interpreting sources, sediment pathways, and regional geological history. In East Java, at the base of the Cenozoic succession, immediately above the basement, the oldest sandstones are dominated by quartz of metamorphic origin, but this gradually decreases up section, through the middle Eocene, as volcanic quartz becomes more abundant. The increase in volcanic material records the initiation of volcanism in the Southern Mountains Arc and the early stages of arc growth from the middle Eocene to the early Oligocene. This early Cenozoic period of arc activity has previously been largely overlooked (e.g., Rutten 1925; van Bemmelen 1949; Hamilton 1988), partly because of the abundance of younger and more obvious volcanic products such as the ‘‘Old Andesites’’ (van Bemmelen 1949) and because its acid character means that the volcanic products produced by explosive eruptions are preserved mainly in sedimentary rocks. First-cycle volcanic quartz arenites are preserved only within or close to the arc. These quartz-rich sands were transported away from the arc and mixed with quartz derived from multi-stage erosion of local and more distant basement rocks. Farther away from arc, volcanic particles are interpreted to have fallen as ash onto the Sunda Shelf and into the Kendeng Basin. On the shelf the material would subsequently have been reworked, enriched in quartz, and mixed with material derived from uplifted basement blocks in the East Java Sea (Bishop 1980; van Bemmelen 1949) and redeposited on the edge of the Sunda Shelf. The sedimentary rocks of East Java are dominated by material eroded from (1) the basement, distributed by fluvial systems, and (2) from the Southern Mountains Arc, distributed by volcanic processes and subsequent epiclastic reworking. Quartz-rich sandstones are not necessarily the result of erosion of a cratonic or recycled continental crust in orogenic source regions. Active volcanism or eroded acid volcanic rocks may be an important but overlooked source of quartz. Volcanism can distribute quartz over a large area during explosive eruptions. Quartz may be concentrated by a variety of volcanic processes, and further enriched by reworking after eruption. In tropical settings weathering may further enrich sediments in quartz. The mixture of such quartz-rich material with the other sediment in terrestrial settings, elevated highs, or shallow marine settings may mean that the volcanic contribution is overlooked. The proportions of quartz types in sandstones may change within a sedimentary succession and can provide valuable information to aid interpretation of provenance and the geological evolution of an area. In tropical environments it is possible to rapidly enrich the quartz content of the sediment by the removal of labile minerals, lithic fragments, and clays. In such settings standard QFL discriminant diagrams may provide a misleading indication of provenance. ACKNOWLEDGMENTS The SE Asia Research Group at Royal Holloway University of London funded this project. Financial assistance for SHRIMP U-Pb analyses was provided by a grant from the University of London Central Research Fund and by CSIRO, Australia. 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