IPA 88-11.24

PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Seventeenth Annual Convention, October 1988

BIOSTRATIGRAPHY IN INDONESIA: METHODS, PITFALLS AND NEW DIRECTIONS

J.T. van Gorse1 *

ABSTRACT

In applied oilwell micropaleontology zonations are used to identify relative age of rocks (biostratigraphy), while comparison of composition of fossil assemblages with modern fauna and flora distribution helps to determine depositional environments (biofacies). Since few explo- rationists are sufficiently familiar with this discipline to evaluate results and do re-interpretations without violating basic data, this paper attempts to review the various microfossil groups currently used in Indonesia, the facies ranges in which they can be applied successfully, and some common pitfalls.

Although local centers of micropaleontological activity have cmtributed little or no modifications to the ”world- wide,low-latitude zonations” published more than 15 years ago, new time scales and data published outside Indonesia justify an updated version of the Indonesian biostratigraphic correlation chart.

Errors or inaccuracies in, ”paleo” reports may result from the type of samples used, sample spacing, ”geolo- gical mixing” of fossils, laboratory or field contamination, biased picking of samples by technicians, misidentification of species or insufficient time spent on looking for marker species.

A significant new development that relies heavily on micropaleontological data is sequence stratigraphy. By combining biofacies trends, lithologies, well log patterns and age information, micropaleontologists are in a position to pick depositional sequences in well sections. If done correctly, this improves understanding of geo- logical history, interpretation of fossil-poor zones, allows prediction of lateral facies changes, and, since most seismic reflectors are sequence boundaries or ”condensed intervals”, facilitates integration with seismic data.

Analysis of microfossils is standard procedure in most wildcat wells in Indonesia. Since detailed biofacies analysis

* PT Stanvac Indonesia, Jakarta.

aids in recognition of depositional systems and high- frequency cyclicity (”parasequences”), microfauna or flora analyses should also be considered in field develop- ment wells for reservoir mapping and prediction studies.

INTRODUCTlON

(Micro-)Paleontology has two major applications in the analysis of sedimentary basins. The succession of evolu- tionary appearances and extinctions is used to determine the relative age of a sediment (biostratigraphy), and the knowledge of the environmental ranges of modern species can help interpretations of depositional environment (biofacies analysis).

Well sections usually contain intervals that can be dated accurately, and intervals with less diagnostic fossils. It is the way these ”poor-signal” zones are handled that cause most or‘ the errors and inconsistencies in biostratigraphy reports.

Most explorationists are not sufficiently familiar with the methods of biostratigraphy and biofacies analysis to evaluate their conclusions, and how to do re-interpre- tations without violating basic data. The intention of this paper is to narrow the communication gap by describing the methods of biostratigraphy and biofacies analysis, in which facies they can be applied, the common pitfalls, and a recommended method of integration of biostra- tigraphic and lithostratigraphic trends: sequence stratigraphy.

Although older rocks may be encountered in Eastern Indonesia, the discussion of zonations will here be restricted to the more commonly encountered Middle Eocene to Recent interval.

BRIEF HISTORY AND CURRENT STATUS OF BIOSTRATJGRAPHY IN INDONESIA

Significant geological investigations in Indonesia started around 1850, mainly on Java. In the early 1900”s molluscs were the main biostratigraphic tool. The older the rocks, the lower the number of still living species, and the

© IPA, 2006 - 17th Annual Convention Proceedings, 1988

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percentage of recent species was used to determine relative ages. In 1919 Martin, after a 1ifetime”s work on Java molluscs, concluded that Quaternary rocks are characte- rized by more than 70% recent species, Pliocene 50-7070, Young Miocene 20-50%, Old Miocene 8-20%, and in the Eocene no recent species could be found (see Van Bemmelen 1949, p.82). One has to realize that in order to arrive at a reliable percentage figure hundreds of species had to be identified (Martin recognized 1412 fossil mollusc species on Java alone !), and the analyst had to be familiar with both recent and fossil species. This kind of expertise hardly exists any more today, and mollusc biostratigraphy has become a ”lost art”.

A major step forward was the introduction of a larger foraminifera zonation by Van der Vlerk and Umbgrove (1927), the famous Letter-classification of the Far East Tertiary. It was primarily based on sections from East Kalimantan, but is now applied in the entire Indo-Pacific province. The ”letter stages” (biozones labelled Ta to Th) were defined by the ranges of about a dozen larger foram genera. Field geologists with a few days training and a handlens could recognize these forms and could now rapidly determine the age of larger foram-bearing beds. With minor modifications the zonation is still valid today (see Rutten in Van Bemmelen 1949, p.83-88, Marks 1957, Adams 1970, and Fig.1). However, it only works in shallow marine calcareous facies while non-marine and deeper marine formations remained undatable.

The 1930”s were a period of high industry and government-sponsored activity in geology and micro- paleontology in Indonesia. Many papers on larger forams appeared, with those on evolutionary trends of several genera by Tan Sin Hok deserving special mention. Oil companies started using smaller foraminifera for local correlation purposes, and also for facies analysis (De Sitter 1948). The momentum of rapid advances was abruptly terminated by World War 11.

Little or no progress was made until the development of the planktonic foraminifera zonations in the late 1950”s and 1960”s. Although some species now used as markers were first described from Indonesia (Globigerina binaiensis Koch 1935, Globigerina siakensis Le Roy 1939), and one of the first papers advocating the use of plank- tonic foraminifera as guide fossils was based on Central Sumatra material (Le Roy 1948), this took place almost entirely outside Indonesia. An early application in Indonesia is Bolli (1966) on material from the Bojonegoro-1 well, NE Java. Since the early 1970’s planktonic fora- minifera have been the major group used for dating deep water sediments.

The latest additions to the Indonesia biostratigraphy tool kit are the applications of spores and pollen (a broad zonation to date non-marine beds, developed from Brunei and Sarawak material, used by commercial and oil

company laboratories since the early 1970’s), and calcareous nannofossils. Although one of the first ever nannoplankton studies is from the Moluccas (Tan Sin Hok, 1927), the current zonation is also an ”imported” method, in use here since the late 1970’s as an additional method to date open marine deposits.

The present status of micropaleontology in Indonesia gives little reason for enthousiasm. Fewer and fewer oil companies operate laboratories locally. The bulk of their work is done by commercial laboratories, which do offer state-of-the-art services, but have not contributed any new data in the last decade, and very little before. This is partly due to confidentiality restrictions, but more fundamental problems are probably rapid personnel turnover (no development of expertise) and an apparent lack of ”tool- sharpening” research programs. The current business climate and the downturn in exploration forced service companies into further cost-cutting measures that are unlikely to lead to improvement in quality.

Local academic and government-sponsored centers do produce occasional good application studies (e.g. Adi- negoro 1973, Hartono 1969, Kadar 1975, 1986), but in general their field studies and mapping projects lack adequate biostratigraphic support. The occasional micro- fauna studies tend to be published in the Indonesian language in periodicals with very restricted circulation, not ideal for the dissemination of information and advancement of scientific knowledge.

Ample opportunities for interesting micropaleonto- logical research projects that would ”sharpen the tools” exist in Indonesia, but it requires dedicated professionals and (a world-wide phenomenon:) a mentality shift away from the idea that a discipline is old-fashioned if it does not involve computers or other fancy electronic equip- ment. Most of the geological story can still be deduced from direct observation of rocks and fossils.

WHAT GROUPS OF MICROFOSSILS?

Of the microfossil groups used in the Tertiary of Indonesia foraminifera are the most versatile, providing both age and paleoenvironmental information. They are protozoans, living in marine and marginal marine waters. Most species have a calcareous test, some benthic species have a test of cemented very fine sand or silt grains (agglutinated or arenaceous foraminifera). Size is generally between 0.1 and 1 mm, average between 0.3 and 0.4 mm (”fine sand” grade).

Almost 40000 species have been’named, but many of these are synonyms. Among the vast literature good introductory texts are Loeblich and Tappan (1964) and Haynes (1981), while Barker (1960) and Belford (1966) illustrate most of the species common in the Indonesian Neogene.

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Sample processing is relatively simple. Rocks are boiled in hydrogen peroxyde or detergent to desintegrate the clays, clay is washed off over a fine mesh sieve (63um), and foraminifera can be picked with a fine brush or needle from the remaining residue, or directly identified and recorded, under a 10 to 60x magnification stereo micros- cope. Foraminifera in cemented rocks (limestones, hard sandstones) need to be studied in thin sections.

Preservation is best in claystones. Calcareous tests may be dissolved in some daystones rich in organic matter (prodelta) or in rocks subjected to surface weFthering. Planktonic forams have thin, porous tests and live free- floating in the upper few 100 meters of the oceanic water column. They are abundant in deep marine deposits, unless significant carbonate dissolution takes place. In shallow marine deposits (50-100m or less) only impo- verished assemblages are found, age determination of which can be difficult. The most comprehensive illustrated texts on tropical Tertiary planktonics are Blow (1969, 1979), Stainforth et a1.(1975) and Kennett 8 Srinivasan (1983).

Species distribution changes with latitude (see Be 1977), but in the tropical belt more or less the same assemblages are found worldwide. The rapid succession of evolu- tionary appearances and extinctions in low latitudes allowed the development of a high-resolution zonation system, mainly between 1956 and 1969. Incursions of higher latitude (colder water) species into the tropical belt can be used to reconstruct paleoclimate changes (see Van Gorse1 & Troelstra 1981 for an example from Indonesia), or for local correlation purposes ("acme zones").

Smaller benthic (or benthonic, bottom-dwelling) foraminifera are found from brackish coastal waters down to the deepest marine environments. Composition of modern assemblages is known to change with water depth, salinity, substrate, etc.. Most species live on the sediment surface (epifaunal), some on sea-grasses (epiphytic; planoconvex Cibicides, Discorbis, etc.), some dig into the upper centimeters of unconsolidated mud (infaunal; some Rotalia, Ammonia, buliminid and bolivinid species).

In the early days of oilfield biostratigraphy benthic forams were the main tool for correlations. However, because of their strong facies dependency and generally long ranges, correlations based on benthic forams tend to be correlations of facies boundaries rather than time horizons. In certain areas they may still be useful local marker species, but their main value is for facies analysis (see below).

Larger foraminifera have large, complex calcareous tests (generally 2-5mm in diameter; giant variants of Cycloclypeus, Eulepidina and Nummulites more than 5 cm). External features or random thin sections generally allow identification at the species level, but oriented thin

sections through the embryonic stage are usually required for determination of species or degree of development within an evolutionary series. They live in shallow, clear, tropical or subtropical marine waters, and are often associated with coral reefs. Some are bottom dwellers, others live attached to sea grasses. Most, or all, have a symbiotic relationship with algae or diatoms and are thus restricted to the photic zone. In extremely clear water like the Persian Gulf the photic zone may extend down to 100-130m, but in most areas this is shallower, closer to 50m (see also Facies interpretation).

Unfortunately, specialists that can "do" larger forams are few, and, since for accurate results oriented thin- sections through a number of specimens are needed, the method tends to be time-consuming. No modern compre- hensive text on the group exists. References to most papers on Indo-Pacific Tertiary larger forams can be found in Adams (1970, 1983).

Calcareous nannofossils are extremely small calcite plates that cover unicellular, planktonic marine algae (cocco- lithophorids). They are useful for dating open marine beds, the same facies in which planktonic foraminifera occur, and with a similar zonal resolution (see Zonations). Used in conjunction with planktonic foraminifera, higher resolution may be obtained than with either method alone.

As a result of their small size (around 10 microns) only a very small amount of sample material is needed, which is useful in the study of sidewall cores. When working with cuttings, however, it becomes very critical which chips of rock are selected for processing: if recirculated s:diment is used, the floras studied may have little relation with the depth on the sample bag. A disadvantage of their small size is that traces of drilling mud on a sample may contaminate a sample significantly. Also field samples can easily be contaminated by dirty collecting tools, hands or natural processes (runoff, wind).

Processing of samples is fast and simple: a small amount of material is scratched off a rock sample (marl or clay), and, with some water smeared over a glass slide. Identification requires a transmitted light microscope with 400- lOOOx magnifications and with polarization and light contrast equipment. A Scanning Electron Microscope may be needed for the determination of some species.

Useful reviews of nannofossils are Hay (1977) and Perch-Nielsen (1985).

Spores and pollen from land plants and trees are the main group of microfossils used for dating alluvial and coastal plain deposits (see Zonations). Diversity in the Far Eastern region is extremely high; the number of Recent taxa alone is about 30000 (Haseldonck 1977). For many forms it is uncertain from which plants they were derived and what their ecologic and stratigraphic distribution is.

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Facies interpretations can be made, using relative abundances of upland, coastal and marine palynomorphs (palynofacies analysis; Haseldonckx 1974), but since these organisms were designed to be transported by wind or water, interpretation of these data is not always very precise.

Spores and pollen are generally not preserved in highly oxygenated facies, but are otherwise highly resistant and ire frequently found reworked from older deposits. They change colour from light-yellow to black when subjected to high temperature, so they are used as indicators of kerogen maturation. In overmature rocks they may become too dark to be identifiable.

Sample processing is elaborate, and includes treatment of rock with various acids, centrifuging and sieving. Due to the small size (mainly in the 20-40 micron range), cleaning of equipment can be difficult, and contamination of samples is not uncommon.

The microfossil groups discussed below are not routinely used for oil-well biostratigraphy or biofacies analysis in Indonesia. Ostracodes are small crustaceans with bivalve mollusc-like calcareous shells. They live in fresh, brackish and marine waters and can be observed in many samples, although rarely as frequent as foraminifera. Processing of samples is the same as for foraminifera.

They are good facies indicators (see for instance Peypouquet in Allen et al. 1979) and some species may have stratigraphic significance, but since the early papers of Le Roy (1939) and Kingma (1948) little work has been done in Indonesia.

Dinoflagellate cysts, made by extremely small planktonic algae, are a relatively new tool in biostratigraphy. They are used successfully in the Paleogene and Mesozoic marine beds in areas like the North Sea. For the Cenozoic a broad "world-wide'' zonation was proposed by Williams (1977), but there is no published information on Indonesian samples to test its validity here.

Sample processing is the same as for spores and pollen. The presence of common dinoflagellates in palynology slides is usually taken as evidence of a marine depositional environment, although some freshwater dinoflagellates are also known to make cysts.

Radiolaria are a group of small marine planktonic protozoans with intricate siliceous tests. They are rare in nearshore waters and are common mainly in open-ocean, high biological productivity regions. For this reason they will never become a widely used tool in oil-well biostra- tigraphy. In Indonesia the first study of fossil radiolaria is by Tan Sin Hok (1927). Very little additional work was done since. A commonly used zonation of the tropical Cenozoic is that by Riedel & Sanfilippo (1978).

Diatoms, another group of siliceous microfossils, are tests of unicellular brown algae, which live in a wide variety of (semi-) aqueous terrestrial and marine environments, but are restricted to the photic zone (generally lOOm or less). Some species are planktonic, others are bottom dwellers. Planktonic diatoms can be useful biostrati- graphic markers (see Bukry 1978 and Barron 1985 for low- latitude zonations), but they are found only in open oceanic deposits, where other, higher resolution dating tools are available (planktonic forams, nannoplankton). Little work has been done on fossil diatoms in Indonesia, although one of the early classic studies is from Java material (Reinhold 1937). Burckle (1982) is one of the very few more recent papers.

ZONATIONS

The Middle Eocene to Recent zonations currently used in Indonesia are shown on the accompanying chart. The applicability of a zonation depends on facies. In open marine environments planktonic foraminifera and calcareous nannoplankton are used, in shallow marine and deltaic series local zonations based on rotaliid benthic foraminifera may apply, in shallow marine carbonate facies larger foraminifera are the most important markers, and in coastal and alluvial plain environments spores and pollen are used.

Planktonic foraminifera. The zonation used most in Indonesia today is that by Blow (1969, 1979; see Table I), largely because of its easy-to-use N (Neogene) and P (Paleogene) number zones, but also because it uses relatively many stages of evolutionary lineages, and it is more adapted to the Indo-Pacific province than most other schemes, which are primarily based on Caribbean and Atlantic material. More than 40 zones can be recognized in the Cenozoic (average duration close to 1.5 m.y.). Other important papers on the subject, with similar zonations and additional information, are Bronnimann & Resig (1971), Stainforth et a1.(1975), Kennett & Srinivasan (1983) and Bolli & Saunders (1985).

Little used possible additional datum levels are coiling changes in some genera (see review in Bolli & Saunders 1985), and incursions of cold water species, reflecting climate changes.

Calcareous nannoplankton. The most widely used zonation in the low-latitude Cenozoic is that of Martini (1 97 l), with zones numbered in a similar fashion as used in planktonic foraminifera. Zones NN1 through "21 subdivide the Neogene, zones NP1 through NP25 represent the Paleogene. Another frequently used zonal scheme is that of Okada & Bukry (1980; CN and CP number zones). Varol(l983) proposed a modified scheme for the Miocene - Recent in Southeast Asia.

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Benthic foraminifera. Between about 1930 and 1960 these were used extensively by oil company paleontologists in Indonesia for zonations and correlations. In Central Sumatra, for instance, Stanvac paleontologists subdivided the Early Miocene in an upper Uvigerina 4 zone (deep marine), a middle Rotalia 5 zone (mainly prodelta) and a lower Elphidium 8 zone (mainly delta front-delta plain). Over short distances correlations based on these were not unreasonable, but they were facies correlations with no age significance, and where facies changed laterally the succession could not be recognized. Such benthic fora- minifera zonations are no longer used.

However, in the shallow marine realm benthic fora- minifera are usually the only common microfossils, and certain species probably do have age significance. Billman & Witoelar (1974) and Billman et al. (1980) proposed a zonation for the Late Miocene -Recent deltaic series in East Kalimantan. Soeka et a1.(1980) proposed six benthic foram zones in the mid-Miocene to Recent interval in the NE Java basin. Mohler (1946) and Djamas & Luwarno (1982) suggested that a small miliolid, Sigmoilina per- sonata, is a good marker species for Eocene beds in East Kalimantan.

Research on evolutionary patterns in smaller benthic foraminifera is called for, as it might give marker species in those shallow water facies that generally lack age- diagnostic microfossils, but are associated with reservoir rocks. One example of a possible lineage with stratigraphic significance is the spinose, small Asterorotalia group. Early forms with incipient spines are found in the early part of the Middle Miocene in Central Sumatra (Rotalia sumatrensis of Le Roy 1944). These probably develop into Asterorotalia rnultispinosa with 6-8 well-developed spines I Possibly from this stock, through a gradual reduction in the number of spines, the first A. subtrispinosa appears near the Middle-Late Miocene boundary (SoeKa et ai. 1980). Further development is towards a more pronounced triangular test outline (A. trispinosa; ?latest Miocene- Recent). Other evolutionary series can perhaps be established in the Pseudorotalia group (see also Billman et al. 1980), and other genera.

Larger foraminifera are the only microfossils used to date shallow marine carbonates. A zonation (The "East Indian Letter Classification") was introduced by van der Vlerk and Umbgrove (1927) and a modified version is still in use (see Tan Sin Hok 1939b, Rutten in Van Bemmelen 1949, and Adams 1970). Correlation of the letter zones with planktonic foraminifera zones and time scale on our zonation chart was compiled from Clarke & Blow (1969), Haak & Postuma (1975), Chaproniere (1984) and own observations. As the number of well documented co- occurrences of diagnostic planktonic and larger fora- minifera assemblages is limited, the calibration of both scales may need revision in the future.

Higher resolution in relative age dating may be obtained by using successive stages of well-studied evolutionary lineages (whether defined qualitatively of quantitatively). Pioneering work on Indonesian Cycloclypeus, Lepidocy- clina and Miogypsina was done at the Mining Department in Bandung by Tan Sin Hok (1932, 1936, 1939a and references therein). The better known lineages were reviewed in relatively recent papers on the Miogypsi- noides-Miogypsina group (Drooger 1963, Raju 1974), Lepidocyclina (Van der Vlerk & Postuma 1967, Ho Kiam Fui 1976, Van Vessem 1978), Cycloclypeus (MacGillavry 1962, Adams & Frame 1979), and .4ustrotrillina (Adams 1968). For ranges and references to descriptions of the Borelis Flosculinella Alveolinella series see Adams (1 970).

It may be noted that most of the major changes in larger foram assemblages (letter zone boundaries) are very close to or coincident with major sequence boundaries on the Exxon cycle chart (latest published version is by Haq et al. 1987). Major extinctions may be related to eustatic sea level falls, while diversification of faunas can be related to periods of prolonged sea level rise or highstand (Adams 1983, Seiglie 1978).

Spore-pollen. The zonation of the Oligo-Miocene is mainly based on evolutionary changes in Florschuetzia, the pollen of the Sonneratia mangrove (Germeraad et al. 1968, Morley 1977). The zones are very broad (close to 10 m.y. average), and, since transitional forms may occur near zonal boundaries, they can not always be determined unequivocally. Also, in environments away from mangrove belts Florschuetzia pollen are rare.

Little is known about Eocene spores-pollen in Southeast Asia; for the Pliocene-Quaternary interval local zonations have been proposed (Morley 1977).

Some local consultants claim to have a more refined zonation, but documentation has been withheld from independent testing.

FACIES INTERPRETATION Because the distribution of microfauna and flora

in modern environments is reasonably well known, microfossils are a useful tool for interpretation of depositional environment of sedimentary rocks. One of the first published application studies in oil wells from Indonesia is De Sitter (1941).

Foraminifera are the main tool for facies work. The main trends are increase in number and diversity from brackish to outer suoiittoral/upper bathyal environments and increase in the percentage of planktonic foraminifera with depth. Controls on distribution are primarily water salinity, temperature, substrate, light, nutrients and oxygen content. Some of these are interrelated, and related to water depth.

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A vast literature exists on the subject, mostly from the 1950’s and 60’s in the Gulf Coast and Gulf of California. Since then the focus of academic research turned to ocean- floor material, which has little application in oil explo- ration areas. Useful review texts are Murray (1973), Boltovskoy & Wright (1976) and Poag (1981).

Little work has been done on distribution of recent forams in the Indonesian archipelago. One early study on the Java Sea was aborted by World War I1 (Myers, 1945). Hofker (1978) published a paper on deep water material collected during the Snellius I expedition in 1929. Another paper dealing with deep water material is Frerichs (1970; Andaman Sea).

For distribution data from coastal and shelfal environ- ments some small-scale studies from Malaysia should be mentioned (Dhillon 1968, and others). Exemplary recent works in Indonesia are two oil company sponsored studies from East Kalimantan, one on the Mahakam delta (Allen et al. 1979) and one OR the Paternoster carbonate platform (Boichard et al. 1985).

Fig.2 summarizes the distribution of foraminifera and ostracodes in the Mahakam delta, compiled from Allen et al. (op. cit.) and our own examination of samples. It should be stressed that, like sediment distribution, fossil distribution patterns vary with delta type. The Mahakam delta is a large river and tide-dominated delta system, building out into deep water. Post-mortem transport of microfauna by tidal currents is common in the estuaries and on the delta front. In river- or wave-dominated deltas thiswould probably be less significant.

The Paternoster platform study describes a modern analogue of ancient foraminifera1 platform limestones and associated small reefs, useful for comparison with, for instance, the Baturaja Limestone in South Sumatra.

More studies of this nature on different types of recent coastal and shallow marine environments in Indonesia would greatly enhance our capability of detailed facies analysis in reservoir formations.

A summary of the general trends in modern tropical foraminifera distribution is:

Fresh water

this environment. No foraminifera with fossilizable tests are known from

Marginal marine (intertidal zone and areas of mixing of fresh and marine waters)

Salinity (or salinity fluctuations) is the dominant control on distribution in this zone (fig.3). Remarkably, very similar assemblages are found worldwide. The lowest salinity assemblages consist of small arenaceous forams only (Trochammina, Haplophragmoides, Miliammina)

and are found in environments like intertidal mangrove swamps and mud flats, upper estuary, lagoons or bays. In higher, but variable salinity waters (bay, lower estuary, mud flat, delta front) assemblages of mixed small arenaceous (as above plus Ammobaculites or Ammotium) and small calcareous forams (Elphidium, unkeeled Ammonia) are found.

Shallow marine (inner sublittoral; low tide30m) Nearshore environments with normal marine salinity

have low diversity assemblages, with 2 or 3 species making up more than 90% of the fauna. Species present depend mainly on the type of substrate (or the associated water turbidity; fig.3). On muddy substrates we find predomi- nantly Pseudorotalia, Ammonia and Nonion assemblages, on silty-sandy substrates Operculina-Elphidium, on rocky, high-energy substrates Baculogypsina and species with an encrusting or attached mode of life, and on carbonate substrates (reef flat, shallow lagoon) Calcarina and large miliolids (Mar,ginopora and Peneropfis).

In Miocene and older shallow marine carbonates or mixed calcareous-clastic systems larger foram assemblages are useful for facies interpretation (fig.4). Hallock & Glenn (1986) summarized data on the distribution of Recent larger foraminifera assemblages (which are poorer in species than Early Miocene or Eocene assemblages). Restricted platform facies assemblages are dominated by large peneroplids and soritids (Marginopora). On platform edges Baculogypsina, Calcarina and robust Aniphistegina are common. Shallow foreslopes may have common Amphistegina fessonii and alveolinids, deep foreslope and open shelf areas within the euphotic zone are characterized by flat, discoidal species of Amphis- tegina, Heterostegina and Cycloc[ypeus.

Similar facies distribution models can be made for older periods, although many taxa common in the older Tertiary are now extinct. In the Late Oligocene - Early Miocene, for instance, the shallowest assemblages are dominated by miogypsinids. Going deeper, dominant forms are Lepidocyclina and Spiroclypeus, and near the base of the photic zone Cycloclypeus dominates the assemblages (see Chaproniere 1975 and fig.4). In sheltered back-reef or lagoonal settings complex miliolids like Austrotrillina and alveolinids (Borelis, Flosculinelfa) dominate. In our experience ”back-reef” facies are rare in Miocene carbo- natesof Indonesia.

In the Eocene Alveolina-Orbitolites assemblages signify restricted, lagoonal conditions. Thick Nummulites are found near high-energy platform margins or shallow marine shoals. In deeper foreslope environments flat Nummulites, Spiroclypeus and Discocyclina are the dominant taxa.

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Deep marine Planktonic foraminifera are an important constituent

of deeper marine microfaunas. The percentage of plankto- nics in a foram assemblage increases with water depth, from about 20% in middle sublittoral (30-100m), 40-80‘70 in outer sublittoral (100-200m) to 90% or more in bathyal (deeper than 200m) eiivironments. However, there is a rather large variability in these numbers in different areas, depending on nutrient distribution, circulation patterns, etc..

Diversity of benthic foram assemblages increases with depth to maximum values in the outer sublittoral-upper bathyal realm (up to one hundred species or more) and decreases again at greater depths. Species composition also changes with depth. Common forms in the middle neritic zone are Cibicides praecinctus, Lenticulina and small Bolivina. and in the outer sublittoral zone Uvigerina, Bolivina, Lenticulina, Bulimina, Cassidulina, Siphonina, etc.. In the upper bathyal zone (200-1000m) many of the above genera are still found, but Sphaeroidina, Gyroidina, Pullenia, Globocassidulina and robust arenaceous species (Cyclammina) are also common. In the lower bathyal zone (1 000-4000111) Planulina wuellerstorfi, Oridorsalis umbo- natus, Laticarinina, Melonis pompiliodes, Sigmoilopsis and Karreriella are the most typical taxa. In the deepest oceanic environments (abyssal), below the Carbonate Compensation Depth (CCD), calcareous microfossils are dissolved and large arenaceous foraminifera are the only microfossils present (”flysch-faunas’ ’ with Bathysiphon, Ammodiscus, etc.). Samples just above the CCD tend to have many fragmented, partly dissolved calcareous microfossils.

Other criteria that may be used for depth interpretation are changes in size of various genera and changes in ornamentation with depth in Uvigerina (Pflum & Frerichs 1976).

The main controls on benthic foram distribution is probably not depth itself, but depth-associated changes in water mass characteristics, primarily temperature and salinity. Since these vary from area to area and may change with climate fluctuations, real ”isobathyal species” (species with the same upper depth limit world- wide; Bandy & Chierichi 1966) are rare or non-existent.

Other significant factors controlling distribution are substrate (grain size, organic matter content, sedimen- tation rate) and oxygen content of the sea water. Low diversity deep water faunas dominated by either Bolivina, Globobulimina, Chilostomella and/or Uvigerina tend to be associated with sediments with relatively high organic carbon content and coincide with areas of low oxygen bottom waters (Poag 1981, Miller & Lohmann 1982). Off California Bolivina from areas with low oxygen waters are larger and flatter than those from more oxygenated

areas (Douglas 1979).

In the late 1930’s Stanvac paleontologists MacGillavry and Thalmann noted the difference between Miocene ”Java faunas” (mainly marly sediments rich in genera like Nodosaria, Lenticulina, Gyroidina, Melonis, Planulina and Pullenia) and the ”Sumatra faunas” from the dark gray Telisa (Gumai) clays, in which the above genera are absent or rare, but contain rich Bolivina, Uvigerina and Bulimina. They are all deep water faunas, but the Sumatra assemblages either reflect deposition in a relatively low oxygen environment, in restricted back-arc basins behind the Proto-Barisan island chain, while the Java faunas reflect well-oxygenated open ocean environments, or they reflect higher deltaic mud influx (many of the Java species are comparable to ”delta-depressed” species with limited distribution off deltas; see Pflum & Frerichs 1976), or a combination of both.

When doing biofacies analysis paleontologists assume that fossil representatives of a species had the same ecological requirements as the modern ones. Despite some papers questioning this assumption, there is no real evidence that major shifts did take place. Three factors that do preclude highly precise paleo-waterdepth interpre- tation are: (1) not enough is known about Recent species distribution in Indonesian waters, (2) temperature-, salinity-, oxygen-, etc., stratification during non-glacial periods may have been different from the present, with associated shifting of fauna depth ranges, and (3) as a result of continuous evolution pre-Oligocene foram faunas have very few species in common with modern faunas; the older the fauna, the more difficult direct comparisons.

WHERE CAN WE GO WRONG ?

Micropaleontologists base their conclusions on the microfossil species observed in a sample. There are many reasons why those conclusions may be wrong or inaccurate.

They are outlined below and can be classified as:

samples containing species introduced from elsewhere (1 -3) conclusions not justified by observations (4) observed assemblages being atypical or incomplete (5 - 8) incomplete recording of microfossil content (9 -1 1) errors (12 -14).

In general, when important conclusions are based on one specimen from one sample it may be useful to check whether this occurrence is real or not. For subsequent re- evaluations a distribution chart is an indispensable part of a report. Also the use of negative evidence in phrases like “the absence of xx suggests an age older than yy”

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should be treated with suspicion as there are several reasons why a marker species may not be observed in a sample from beds of the right age.

Contaminated samples

rhe Imst frequently used samples in the oil industry are cuttings. These always contain a (variable) percentage of rock material from uphole (borehole cavings and recirculated fragments). Microfossils from a thin, rich zone will not only be conspicuous in samples from the zone, but also below it. In samples from fossil-poor or barren intervals all microfossils may be from overlying, richer zones. Thin marginal marine or non-marine beds between relatively rich marine shales are therefore hard to recognize. Figure 5 illustrates a case where two non- marine, barren intervals (around 5800’ and below 5950’; alternative interpretation based on log analysis and regional information) were originally interpreted as marine, based on the presence of caved marine micro- fauna. This is probably the most frequent error in oil-well facies and age analysis.

In order to minimize errors a complete suite of cuttings from the top of the hole should be examined. Highlight first downhole appearances; mistrust lower occurrences, especially when frequency diminishes. Describe lithologies and be alert for unusual combinations of fauna and lithology (like coaly sand and marine microfossils). Use core and sidewall cores if available.

If not handled properly, other types of samples may also be contaminated. Sidewall cores generally come with a certain amount of ”mud-cake”, which may contain microfossils from uphole or downhole and thus needs to be removed thoroughly before further processing parti- cularly for the extremely small nonnoplankton and spores/poIlen). Field samples collected with a dirty hammer or dirty hands may contain material from elsewhere.

Poor cleaning of laboratory equipment can introduce material from samples processed earlier. In processing for foraminifera it is good practice to rinse the sieves and bowls with a blue dye solution, which stains any remaining microfossil and rock fragments. Cleaning of centrifuges and other eq~tipment used in the processing of calcareous nannofossils and palynomorphs is more diffucult due to the small size of these fossils, and contamination is more frequent here.

Reworked microfossils

Microfossil assemblages eroded from older formations and redeposited, can lead to erroneous age and facies interpretations. However, they can often be recognized by their worn, abraded tests, darker colour, absence of fragile species, size sorting, and older age. They may give

useful information cn uplift hidory of the hinterland and on the source area of the clastics.

Uplift of the Kendeng zone in NE Java caused erosion of the rich Late Miocene-Pliocene Globigerina marls and deposition of locally abundant reworked deep marine foraminifera in the Pleistocene lacustrine and fluvial Pucangan and Kabuh Formations to the South. At first sight preservation is good, but close inspection reveals common dissolution pits. Also, most species are thick- walled Globorotalia and Pulleniatina; small fragile Globigerina are absent. Recent sediments of the Solo River delta North of Surabaya also contain such reworked Kendeng zone material (S.R. Troelstra pers.comm.).

Displaced microfossils

Shortly before or after death, fauna and flora may be displaced from their habitat. Spores and pollen were designed for transport and may be common in deposits far away from their place of origin.

Shallow marine foramifera are often found in deep water sediments, either displaced by gravity processes or or carried seaward by rafting on detached seaweeds.

Microfossils in the basal, sandy members of turbidites are displaced from upslope and may provide information on basin margin depositional systems (rotalid forams suggest deltaic facies, larger forams carbonate shorelines, etc.).

Shallow marine forams may also be displaced into marginal marine facies. Tidal currents may carry material several miles up a tide-influenced river mouth, both as bed load (size-sorted, large, abraded forms in sandy channel deposits) and in suspension (small, < 200 um, thin-walled, well-preserved species in muddy sediments; see Wang & Murray 1976). Waves frequently wash shallow marine fauna onto beaches. Foraminifera in beach deposits may be recognized by their rounded, polished tests. Bird droppings have been held responsible for dispersal of fresh water and marginal marine ostracodes.

Unusual preservation and size-sorting should alert the paleontologist to possible post-mortem transport when making paleoenvironmental interpretations. Unfortu- nately, in paleo-reports information on preservation is rarely recorded in sufficient detail for subsequent re- interpretation purposes.

Fades change

In older reports the highest or lowest occurrence of a species in a well section was often equated with its extinction or first evolutionary appearance level. This is reasonable when there are no indications of change in environment, but when tops or bottoms coincide with facies changes the age may be anywhere within the range of that species.

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Water depth Biogeography

Although foraminifera assemblages are very similar throughout the tropical climate belt, there are minor geographically controlled differences. Among the plank- tonic foraminifera, for instance, forms like Globorotalia margaritae, G. multicamerata and G. miocenica are good marker species in the Atlantic Province, but are absent or extremely rare in the Indo-Pacific. On the other hand, Pulleniatina and the Globorotalia tumida group are more frequent in the Indo-Pacific realm. Among benthic larger and smaller foraminifera biogeographic provinces are even more pronounced (e.g. Adams 1983). When working in a particular area a modified local scheme may give better results than a ”world-wide” zonation.

The compositio of planktonic foraminifera assemblages from relatively shallow marine facies (middle neritic or shallower) may be misleading (incomplete). In living faunas a distinction can be made between ”shallow- water” species, living predominantly in the upper 50m of the water column (Globigerinoides spp., small Globigerina), ”intermediate-water” species, living predominantly between 50 - lOOm (Orbulina, Neoglo- boquadrina, Pulleniatina), and ”deep-water” species, with adult stages living predominantly below l00m (keeled Globorotalia, Sphaeroidinella; ror more details see Be 1977). Many of the zonal mark-r species are intermediate and deep water forms, so althcdgh planktonics may be frequent in a sample from 50m water depth, diagnostic species may be absent for reasons mentioned above. For example, in the upper Telisa-Binio Formations in Central Sumatra are intervals with common Globigerinoides and Globigerina (Early Miocene or younger), but without Orbulina or Praeorbulina (Middle Miocene or younger). The absence of the latter could be taken as evidence of Early Miocene age, but since Orbulina is present in underlying, deeper water Telisa facies we know the age of the upper samples must also be Middle Miocene (or younger), and the assemblages are incomplete for facies reasons.

Restricted basins

Planktonic microfauna and flora have their greatest abundance and diversity in open oceans. The main controls on their distribution are temperature, salinity, nutrients and light. In marginal basins these parameters may differ, leading to anomalous assemblages. Some species may bloom, while others are absent or rare. For instance, in the Central Sumatra basin, which was linked to the Indian Ocean only through a few passages in the Proto-Barisan Mountain island chain, planktonic forams are abundant in the Early Miocene of the Telisa Forma- tion, but marker species like Globigerinatella insueta, Catapsydrax dissimilis, C. unicava and Globigerina binaiensis have never been found. On the other hand, a high-spired Globigerinoides (G. irregularis of Le Roy 1944), normally a rare species, is abundant in many samples. The same situation was observed by the author in the Early Miocene of the Red SeaRpCulf of Suez. The ”missing marker species” are probably deeper water species, which did not enter the basin from the Indian Ocean due to shallow sill depth, absence of a temperature stratification, or other environmental factors.

Marker species may thus be absent due to environ- mental conditions in marginal basins. Since most petroleum exploration is in such basins the use of absence criteria for age determination is dangerous.

Dissolution of calcareous microfossils

Partial or complete dissolution of calcareous and siliceous organisms may occur in deep ocean waters. At low latitudes significant carbonate dissolution starts below 3000m (lysocline), and below 4000-5000m (CCD; Car- bonate Compensation Depth) all carbonate is dissolved. Among planktonic foraminifera thin-walled Globigeri- noides and Globigerina dissolve before thicker-walled Globorotalia, Pulleniatina and Sphaeroidinella; among calcareous nannofossils the placoliths are more resistant to solution than the holococcoliths.

Dissolution may also take place in deltaic sediments (mainly prodelta or bay clays), triggered by the decom- position of organic matter, and in rocks that were exposed to subaerial weathering (fresh-water leaching). Fossils may still be recognizable as molds and casts in the rock.

”Residual” assemblages of arenaceous forams in prodelta clays may look like brackish water delta plain assemblages. Deep marine sediments, with partly dissol- ved assemblages ofresistent species only, may be assigned a wrong age if absence of some dissolution-prone species is used as evidence. For this reason a zonation based on dissolution-resistent planktonic foramifera was proposed by Orr 8 Jenkins (1977). However, very few oil wells penetrate such deep water facies.

Selective picking

Many centers of foram micropaleontology employ technicians to pick foraminifera from the washed sample residue. If not well-trained, theji have a tendency to select big, pretty specimens and ignore the small, inconspicuous (but possibly important) and broken ones. Paleontologists should therefore check the residues themselves, also because lithologies provide useful facies information.

Small sample

When microfossils are not abundant in a rock, a sidewall core with poor recovery (or parts of which need

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to be used for other analyses) or small cuttings samples (common in trade wells) may not contain enough material to yield rare, but diagnostic species. For such samples, if marine, calcareous nannoplankton are the most suitable tool for age determinations.

Rush jobs

Minor errors in depth of cuttings may arise from incorrect calculation of lag time by the sample catcher.

Drafting or typing errors

Rare, but nevertheless present in some reports, are errors in the presentation of basic data. (Re-)interpretation of such data obviouslv may lead to unfortunate errors.

In sections with continuous deposition it is found that the relative frequency of most species changes gradually near the top and bottom of its vertical range (see Fig. 6). Instantaneous catastrophic extinctions are extremely rare; observed abrupt changes usually signify a hiatus or facies change. Determination of ”first appearance” and ’7extinction” horizons thus depends to a large degree on the amount of time spent on looking for it.

Species misidentification

Obviously, when a species is misidentified stratigraphic conclusions based on its presence must be wrong. In all fossil groups morpholoyic differences between species may be very subtle and. only be obvious to a trained eye. Among the planktonic foraminifera, for instance, nobody will misidentify Orbulina (a useful marker also reliably identified in pre-1960”s reports), but it is more difficult to distinguish between homoeomorphs like Globigeri- noides ruber and G. subquadratus. There is an element of subjectivity in the distinction between successive stages of gradually evolving series like Globorotalia merotumida- plesiotumida-tumida or Neogloboquadrina acostaensis- humerosa-dutertrei (with consequences for the position of zonal boundaries in the N16-17-18 interval).

Identical problems are encountered in the determination of transitional specimens in evolutionary lineages of

calcareous nannofossils (Sphenolithus conicus- S. heteromorphus, etc.) and spores/pollen (Florschuetzia trilobata- F. levipoli).

Inexperience may lead to extreme cases of misiden- tification. One geologist reported abundant planktonic foraminifera in fluvial deposits in South Sumatra, but these were small, spherical siderite concretions.

Oolites are rare or non-existent in the Cenozoic of Indonesia. The ones seen by geologists in the white beach ”sands” of Sanur, Bali are tests of Baculogypsina sphaerulata, a spherical reef-flat foraminifer, and those from the Miocene marls of the Java Southern Mountains are planktonic foraminifera.

Sample mislabelling

Samples may be mislabelled in the collection or labo- ratory processing phase. If they are a long way out of sequence this can be detected, but if the difference is minor it may not be noticed and become a source of erroneous conclusions.

- . Summarizing, we may state that there are multiple

sources of possible error, but almost all can be avoided by careful work by qualified personnel in the collecting, processing, analysis and reporting phases. Geologists can minimize errors by providing the paleontologist with a good set of samples (see Appendix). As in many other fields the ”garbage in-garbage out” principle is also valid for micropaleontology.

NEW DIRECTIONS

The initial stage in the study of foraminifera was one of cataloguing, of species descriptions (the d’orbigny- Brady-Cushman era between about 1850 and 1950). The second stage was biostratigraphy, the search for marker species for dating and correlating wells or surface sections, starting with the benthic foram-based oilwell biostratigraphy in the 1930s through 1950s, followed by development of the planktonic foram zonations between 1955 and 1970. A third phase was the study of modern environmental distribution patterns, useful for facies analysis of sedimentary rocks (the Phleger-Bandy era, peaking in the 1950’~-60’s). Other groups went through a similar succession of stages, but at later dates.

In today’s ”mature” phase we see numerous inter- disciplinary studies involving microfossils. Emphasis may either be on paleobiology (modes of evolution, morphologic variability, etc.), chemistry (oxygen and carbon stable isotopes in carbonate shells; used primarily in paleoclimate studies), or paleoceanography (various methods for water mass characterization, from which circulation patterns and climate fluctuations can be deduced; documenting sea-floor spreading by dating sediments above oceanic crust). Microfossil data are also subjected to increasingly complex quantitative analyses (cluster analysis, quantitative correlation methods, etc.).

With a wealth of new sample material becoming available from the DSDP/ODP oceanic drilling programs and other cruises in the last 20 years, the literature on micropaleontology now seems to be dominated by ”academic” studies from the oceanic realm.

With the exception of the refinements of planktonic microfossil zonations most of these ”new” developments are only marginally relevant to oil industry application work.

Two relatively new geological application techniques

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involving results of micropaleontological analysis that are important are:

Geohistory analysis

The use of quantified micropaleontologic interpre- tations to construct subsidence curves, was named ”geohistory analysis” by Van Hinte (1978). If for a sufficient number of horizons in a well or surface section age (in millions of years) and paleobathymetry are known, a plot of time versus interval thickness plus paleo-water depth will produce curves that illustrate the subsidence history of these levels. These allow conclusions on the structural history of the basin, and, when combined with paleotemperature data, estimates of timing of hydro- carbon generation (e.g. the Lopatin method; Waples 1980).

”Kinks” in the curves may represent structural events, but may also be due to errors in age or paleobathymetry interpretation, flaws in the numerical time scale used, or eustatic sea level changes. The curves may thus also be used to check for anomalous biostratigraphic interpre- tations. Sophisticated curves include detailed paleo- waterdepth information, and corrections for eustatic sea level changes and compaction through time. Computer software to generate geohistory curves is now available from various sources.

Sequence (bio-)stratigraphy Recenc advances in seismic stratigraphy (Vail et al.

1977) and sedimentology suggest there is no such thing as continuous deposition. The sedimentary record consists of stacks of depositional sequences (genetically related, predictable successions of lithofacies; commonly shanowing-upward sequences; deposited in relatively short time periods), separated by sequence boundaries, re- presenting periods of erosion or non-deposition. Factors controlling this cyclic deposition are variations in eustatic sea level, relative rates of basin subsidence, and sediment supply.

In wells or outcrops cyclicity is particularly obvious in deltaic systems, thin ones being caused by distributary switching, thicker ones by sea level changes. An example describing the major Eocene to Recent cycles in Sarawak is Ho Kiam Fui (1978),

A sequence is composed of a predictable 3-dimensional arrangement of facies. Haq, Hardenbol and Vail (1987) published an idealized 2-dimensional model of a (clastic) depositional sequence. It is defined as the succession of sediments deposited during a complete eustatic sea level cycle, from a sea level fall to subsequent rise and ending with the next fall (see Fig.7). It can be subdivided into four ”systems tracts” (packages of sediments deposited during a particular part of the sea level curve): Lowstand

fan (rapid sea level fall), Lowstand wedge (sea level lowstand), Transgressive (sea level rise) and Highstand (relatively stable high sea level): A fifth unit, the Shelf margin wedge, forms during a slow drop in eustatic sea level.

A ”condensed section’’ separates the lowstand and transgressive systems tracts from the overlying prograding (downlapping) highstand systems tract. It corresponds to the period of maximum transgression (maximum landward extension of marine facies), during which most coarse clastics were trapped on the basin margin. In wells in basinal settings it can be recognized as a thin zone of very slow deposition, enriched in authigenic marine material (marine fossils, glauconite, phosphate). As it represents the deepest marine facies in the sequence, the best plankton assemblages for age dating are found around this interval.

The Haq et al. (1987) paper also contains updated versions of the Exxon Mesozoic and Cenozoic cycle charts (”Vail-curves”), which show the ages of sequence boundaries, thought to reflect globally isochronous eustatic sea level changes. Although there is good evidence that many of these sequence boundaries are indeed due to eustatic sea level drops and can be recognized world- wide, opposing opinions suggested that many boundaries on the chart may be the result of local tectonics or fluctuations in sediment supply, and are not isochronous world-wide events. Regardless of who is right on the time issue, sequences are the main genetic components of basin- fill, and sequence boundaries the principal correlation surfaces.

Cyclicity in rock successions can be observed at many scales. In the Central and South Sumatra basins thin transgressive- regressive ”parasequences” can be recog- nized (in deltaic series generally 2-8m thick), which are organized in larger ”seismic-scale” sequences of 50-2OOm thickness and of 1-3 million years duration. These sequences, in turn, are stacked in the large, basin-scale transgressive-regressive pattern with a fluvial-lacustrine- deltaic ”wedge base”, a marine ”wedge middle” and a deltaic-fluvial ”wedge top”, together comprising 25-30 million years.

Micropaleontologists can identify sequences in well sections, by combining biofacies trends with sample lithologies and well logs. If sufficiently thick, sequences may be picked from seismic stratigraphic analysis. Age dating is done on the appropriate facies in each sequence (deepest water for plankton), or, if diagnostic markers are absent, by using a global or local cycle chart.

Although there are no microfossils that are unique to certain systems tracts there are some indirect criteria that help identify position within a sequence: - common reworked shallow marine fauna in deep

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marine beds are suggestive of updip erosion and turbidite transport, and thus point to lowstand fan deposits;

- abundant large arenacous forams (”flysch faunas”) are thought to reflect relatively rapid mud deposition in a deep marine facies, which most likely occurs in the lowstand fan or distal lowstand wedge deposits;

- in the transgressive systems tract most clastics are trapped in aggradational alluvial and coastal plain deposits. Shorelines receive relatively little sediment, so these tend to be dominated by marine processes (waves, tides). Water turbidity is relatively low, and ”clear water” marine microfaunas like Cellanthus, Operculina, Amphistegina, larger foraminifera and sea-grass species like planoconvex Cibicides and Discorbis are most frequent here. Also glauconite-rich and calcareous beds, including reefal buildups, are most frequent in transgressive deposits.

- the highstand systems tract is characterized by relatively rapidly prograding shorelines. Influence of rivers and deltas is relatively high, and ”turbid water” marine foraminifera tend to dominate tend to dominate here (Ammonia, Pseudorotalia in the shallow, Uvigerina

and Bolivina in the deeper parts).

Figure 8 shows the idealized distribution of forami- nifera in a vertical section through a Middle Miocene sequence (13.8 to 15.5 million years). The approximate position of this section in the model is indicated on Fig.7. About 80% of the sequence is composed of the Highstand systems tract, an overall shallowing-upward package with minor breaks (parasequences). Evidence for shallowing- upward is: coarsening-upward log pattern, decreasing percentage of planktonic foraminifera, and benthic foraminifera assemblages changing from upper bathyal species at base to brackish water species near the top.

The Condensed Interval is around the zone of deepest facies and contains abundant glauconite. The thin shallow marine and fluvial packages between the condensed interval and the 15.5 Ma sequence boundary are inter- preted as the Transgressive and Lowstand Wedge systems tracts. The beds below the 15.5 boundary are from the shallowing-upward Highstand systems tract of an older sequence, which is truncated by erosion at 15.5 Ma.

Only the deep marine parts of the sequence can be dated with planktonic foraminifera. Species diagnostic of zones N8, N9 and N10 are present. Using the Exxon cycle chart numerical ages of the sequence boundaries can then be obtained.

It should be noted that some aspects of the example of Fig. 8 may not be typical of most real world sections: - all samples are cores or sidewall cores. Paleontologists

usually work with cuttings, containing cavings that obscure the fossil-poor zones around the sequence boundaries;

- not many sequences show the broad range of facies from upper bathyal to non-marine. This may be found only in high subsidence-high sedimentation rate areas. In general narrower ranges of facies are found. Where bathyal to non-marine facies transitions do occur they usually comprise several sequences;

- the relative thickness of the Lowstand, Transgressive and Highstand systems tract in a well section depends on the position in the basin. In settings farther basinward than the Fig.8 example lowstand deposits would become relatively thick, transgressive beds would be absent, and highstand deposits would become thin, incomplete shallowing-upward packages. Further landward the sequence boundary becomes a more pronounced erosional surface, lowstand deposits are absent (except for valley-fill sands), and the trans- gressive beds become relatively thick (see model of Fig. 7);

- smaller scale sequences (parasequences) may be more pronounced than in the Fig.8 example.

Summarizing, sequence (bio-)stratigraphy attempts to subdivide sections into genetically related packages, improving correlations and interpretation of fossil-poor zones, and allowing prediction of lateral facies changes. In addition to the traditional breakdown of a well section into zones and environments, ”sequence biostratigraphy” reports also indicate the position of sequence boundaries, their age in millions of years, and describe the nature of the sequences. Since most seismic marker horizons are sequence boundaries or condensed intervals (”downlap surfaces”), this method facilitates integration of fossil and rock data with seismic studies, and, in genera1,leads to a more meaningful stratigraphic framework. Detailed and accurate age and facies information from micropaleonto- logy is indispensable for establishing a reliable sequence framework.

EPILOGUE

This review attempted to demonstrate the possibilities of and the difficulties facing micropaleontological analysis. Opinions expressed herein are strictly the author”s personal views. Much is known, but more can be done to improve the tools. Zonations can be improved by testing the validity of locally significant marker species, acme zones, evolutionary lineage studies (particularly in shallaw marine benthic foraminifera), etc..

Facies analysis would benefit from additional studies on modern microfauna distribution in Indonesia, par- ticularly in marginal and shallow marine environments. Hopefully results of past and present work in this field by some government institutions will be made available to the scientific and industrial community.

New developments in stratigraphy suggest cyclicity in rock successions is much more common than thought

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previously, and in future stratigraphic studies the es- tablishment of a sequence stratigraphic framework will be indispensable. Micropaleontology should play an important role here in recognizing facies trends and dating sequences.

Remarkably, micropaleontology seems to be under- utilized in reservoir studies. Detailed biofacies analysis helps identify small-scale cyclicity (correlations) and types of depositional systems (prediction of reservoir geometry, continuity, etc.). For optimum results a suitable set of samples should be submitted. Thinly interbedded marine and non-marine facies tend to all be assigned a marine facies when only widely spaced cuttings are used. Good core or sidewall core control is a prime requirement in zones of above average interest.

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APPENDIX - SAMPLES AND SAMPLING PRO- GRAMS Sample types

Standard oil-well analyses are primarily done on cuttings samples, because they are the cheapest. They are also the least desirable because they contain recirculated and caved rock material from uphole in addition to the material ground up by the bit at the time of sampling. Typical errors arising from this are that intervals with little or no fauna below a rich interval are not recognized as such. Age and environment of the lower interval may be described as the same as the overlying interval, making it too young and too deep. Series, with interbedded shallow marine-nonmarine facies may be entirely des- cribed as shallow marine. Overall, facies interpretation tends to be too deep because deep water sediments tend to have the most abundant fauna and will obscure the weaker signal of the shallower water, less rich assem- blages. The only way to improve results is to also collect an adequate number of sidewall core or conventional core samples.

The advantage of sidewall-cores is that we have a sample, the position of which is known exactly (barring mishaps like well-site or laboratory mix-ups or misla- belling). Before processing all drilling mud should be removed to obtain uncontaminated microfossil assembla- ges. It is advisable to peel the core like a potato and inspect it for cracks into which mud might have penetrated. Disadvantages of sidewall cores are their small size (generally too small if more than one type of analysis is desired), and they only show one inch of section, which may or may not be representative of the interval.

Conventional cores combine the advantages of unconta- minated samples and sufficient quantity, but are expensive and thus cut only occasionally, and only in zones of economic interest.

Collection of surface samples in humid tropical Indo- nesia requires some care. Calcarous material is rapidly dissolved in the weathering zone, so samples should only be taken from fresh outcrops (new road cuts, quarries, stream beds), otherwise most of them will prove to be barren.

Rock types

Overall, clays or shales are preferable over any other rock type. Clastic sedimentation rates are generally low, allowing an enrichment of microfossils. The low energy of the environment allows settling of the clay-size calca- reous rlannofossils and palynomorphs from suspension, and post-mortem transport of microfossils is minor.

Isolated microfossils are obtained by deseggregation of the shales by boiling in detergent or hydrogen-peroxyde. Sieves, centrifuges or, less frequently, heavy liquids may then be used to concentrate microfossils from the shale residue.

Sandstones and conglomerates generally lack fine grains, and are thus very poor in any kind of microfossils. Marine or estuarine sandstones frequently contain fora- minifera, but, when cemented, isolated specimens are hard to obtain, and the forms present are generally transported and displayed from their original habitat.

Limestones may be rich in microfossils, but are gene- rally too much cemented to yield isolated specimens. Thin sections of the rock are required for their study. Random sections allow identification of the larger forams (if present) at genus level and determination of the "Letter- zone". Identification of planktonic foraminifera in thin section is usually impossible; only some unusual morpho- types may occasionally be recognized.

Dolomitized limestones are poor samples because fossils are usually altered beyond reqognition. Volcanic tuffs are unfossiliferous, except for occasional silicified wood.

Coals never contain calcareous microfossils. They may be rich in palynomorphs, but these are generally uniden- tifiable because of difficulties in separation from the abundant woody material. Dark clays are the preferred rocks for palynology.

Sampling program

A recommended sampling program for oil-well biostra- tigraphic analysis would include both cuttings and sidewall-cores (assuming little or no conventional core is available): sidewall cores to determine lowest occurrences of taxa in the well, cuttings for highest occurrences and

29 1

for material from rich, diagnostic zones not sampled in the sidewall coring program. Sidewall cores should preferentially be taken in clays.

on the ”vertical rate of change” in the well. In thick, rapidly deposited shale zones a 50” spacing may be adequate. In more condensed sections 20’ or 30’ intervals would be required. Where 20’-30’ thick deltaic shallowing- upward parasequences are stacked,‘sample spacing of 10’ or less may be needed to demonstrate that cyclicity.

Sample spacing should depend on the importance of a zone (economic, or for understanding basin history) and

292

Haq,Hardenbol& Vail (1987)

CHRONOSTRATI- GRAPHY

Ua

Blow (1969 1979j, Kennett & Srinivasan (1983) Martini (1971 j , Perch-Nielsen (1985)

PLEISTO. O 'lz

PLANKTONIC FORAMINIFERA CALCAREOUS NANNOPLANKTON ZONES I DATUM LEVELS ZONES I DATUM LEVELS

NN 20 21

NN 19

Gr Globorotalia Gs Globigerinoides 7 Pseudoemiliana lacunosa N 2S22

Discoaster brouweri --KT Discoaster penfaradialus -7 ' Discoaster surculus

-165- 4 Gr fruncatulinoides

, 4 Gr tosaensis NU ?fi PlACENZlAN

- 3.5-

ZANCLIAN

52- MESSlNlAN - 6.3-

-

Gr. margaritae Reticulofenestra pseudournbilica

Sphaeroidinella dehiscens Ceratolithus rugosus - C.acutus

Ceratolithus (Arnaraulithus) tricornulatus Discoaster asyrnrnetricus

lumide f Gr rnargaritae Discoaster quinquerarnus

eD Coiling change NN , Gr. plesiotumida Gr acostaensis

I N16 I TORTONIAN

-10.2-

SERRAVALLIAN

- 15.2- LANGHIAN - 16.2-

BURDIGALIAN

20 - -

A Gr. acostaensis

Gr siahensis 1 Ga. nepenthes

~phaero id ine l l ops i s subdehiscens

1 Gr fohsi 4 Gr. praefohsi

4 Orbulina suturalis

'5 14

Gr. ,ohsi 7 Gs. subquadratus N 13

N 12

" l o Gr. peripheroacuta

Praeorbulina glomerosa

N 7

f Catapsydrax dissirnilis 1 Globigerinatella insueta

N 5 7 Gs. prirnordius

7 Ga binaiensis

AQUlTANl AN Gr. kugleri

A Gs. trilobus

Globoquadrina dehiscens -25.2

CHATTIAN

Globigerinoides primordius

Gr. hugleri s.1.

Gr. opirna opima

-30-1 I RUPELIAN

Ga. angulisuturalis A Gr. opirna opirna

Pseudohastigerina spp

Ga. selii

Ga. tapuriensis

-36-1 Gr. cerroazuiensislHaritkenina 1 i~ Cribrohantkenina inflata

PRlABONlAN

-39.4-

Globigerinatheka serniinvoluta Cribrohanthenrna

Truncatulinoides rohrilGr.lehnerr

Orbulinoides bechrnanni Orbuiinoides beckrnanni

LUTETIAN 2, Gr. aragonensis

Discoaster quinqueramuslD.surculus

Discoaster hamatus

Discoaster hamatus Catinaster coalitus

Discoaster hugleri Cyclicargohthus floridanus

Sphenolithus heteromorphus

Helicopontosphaera arnpliaperia

Sphenolithus belemnos

NN 2

Discoaster druggii

1 Sphenolithus distentus

NP 24

I Sphenolithus ciperoensis

NP 23

Sphenolithus distentus

Reticulofenestra umbilica i Cyclococcolithus (Ericsonia) formosus

Discoaster saipanensis, D. barbadiensis

NP22

NP 21

NP 2o '1 Sphenolithus pseudoradians

1 Istrnolithus recurvus

t Chiasrnolithus oarnaruensis

NP 19

NP 18

NP 17

f Chiasrnolithus solitus,Discoaster bifax

NP 16 , Discoaster bifax

t Rhabdolithus gladius NP 15

7 Chiasmolithus gigas

293

Ma

-

- -

5 -

- - - -

10 - - - - -

15 - -

1 -

20 - -

- - -

25 - - - - -

30

-

-

35 -

- - -

40 - - - - -

45 -

LARGER FORAMINIFERA LETTER ZONE

Tg h

-- - - _ _

UPPER Tf

(T f3 )

_ _ - _ _ _

LOWER Tf (Tf 1-2)

- - - - - -

UPPER Te

(Te 51

LOWER Te (Te 1-4)

Td

Tc

Tb

Ta

Van der Vlerk & Umbgrove (19271, Adams (1970). Haak & Postuma (1975) Chaproniere (1984)

DATUM LEVELS

A Calcarina spengleri

7 Lepldocyclina spp

7 Kaiacycloclypeus

Miogypsina

Lepidocycllna (Nephrolepidma) 3 Austrotrillina howchlni

A Flosculinella bontangensis

3 Lepidocyclina (Eulepidina) Spiroclypeus spp

, Miogypsina s s Heierosiegina borneensis Miogypsinoides complanatus

-4 Spiroclypeus Miogypsinoides

7 Nummulites fichieli

Lepidocyclina (Eulepidinaj

7 Discocychna spp Pellatispira,Biplanispira

Pellatispira

7 Assihna spp Nummuliies lavanus Alveolina (Fascroliies) spp

ROTALl I D FORAM I NI FERA ZONES DATUM LEVELS

Calcarina

- 7 Ammonia pila - _ _ - Ammonia pila ‘=A’ ikebei j - - - _ _ 7 Asanoina globosa

Asanoina globosa

Peudorotaiia catilliformis - - , P catilliforms - - - -

1 Ayabei .- _ _ - -

Asieroroialia yabei

- 7 A umbonaia - -

Ammonia umbonafa

Billman & Witoelar (1974). Billrnan,Hottinger & Oesterle (1980)

SPORES-POLLEN ZONES DATUM LEVELS

( IN PLIOCENE-QUATERNARY LOCAL ZONATIONS ONLY)

-1- -I--- Florschuelzia

meridionalis subzone

E - --q F trilobata F irilobaia

subzone __I F meridionalis 1 F semilobaia

Florschuetzia IeviDoIi

- - - - - A F levipoli

Florschuetzia irilobaia

- -7 - - A F irilobata

(EOCENE POORLY DEFINED)

Gerrneraad,Hopping & Muller (1968); Morley (1977)

Correlation Chart-Indonesia (Cont.)

BR

AC

KIS

H W

ATE

R (

Tida

l del

ta p

lain

) F

smal

l are

nace

ous

spec

ies

only

(H

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phra

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des,

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min

a, M

iliam

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0

rare

, low

div

ersi

ty (s

moo

th C

ypnd

eis)

TID

AL

MA

RIN

E (m

arin

e m

ud fl

ats,

est

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mou

th)

Spe

cies

tole

ratin

g sa

linity

fluc

tuat

ions

: F:

M

ixed

are

nace

ous

(as

abov

e an

d A

mm

obac

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s)

and

smal

l cal

care

ous

fora

ms

(unk

eele

d

MA

RG

INA

L M

AR

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(sub

tidal

del

ta fr

ont,

uppe

r pr

odel

ta)

F:

smal

l cal

care

ous

bent

hics

onl

y (A

ster

orot

alia

tri

spin

osa,

Pse

udor

otal

ia, N

phid

ium

, N

onio

n)

0:

2 as

soci

atio

ns: i

nner

del

ta fr

ont f

ew, t

hick

-wal

led,

or

nam

ente

d sp

ecie

s (T

achy

lebe

ris,

Neo

cyth

eret

ta, e

tc.),

ou

ter d

elta

fron

t pla

tform

v$r

y ric

h, h

igh

dive

rsity

(C

ythe

rella

, H

emic

ythe

ridea

, Lox

ocon

cha,

P ar

acyp

ris)

10

20 K

m

OP

EN

MA

RIN

E (p

rode

lta a

nd d

eepe

r)

F. s

hallo

w m

arin

e sp

ecie

s as

abo

ve,

with

the

first

pla

nkto

nic

fora

min

ifera

0:

re1

low

div

ersi

ty, d

omin

ated

by

Par

ahth

e,

Mac

rocy

pris

, C

ythe

rello

idea

)

FIG

UR

E

2 - D

istri

butio

n of

For

amin

ifera

(F)

and

Ost

raco

da (

0) i

n th

e w

este

rn p

art

of t

he M

ahak

am d

elta

295

SALINITY

Hyposaline

Normal Marine

Hypersaline

0 %o

32 %o

36 %o

45 %o

Muddy Siltykandy Rockyheef

Miliammina Trochammina

Trochammina Ammobaculites

unkeeled Ammonia small Elphidium

Pseudorotalia Nonion

Operculina Cellanthus Quinqueloculina Amphistegina

Calcarina Baculogypsina

1

Quinqueloculina Peneroplis Sorites

FIGURE 3 - Dominant foraminifera in shallow water facies, as a function of salinity and substrate

296

PLATFORM/LAGOON RE E F/B A N K FORESLOPE OPEN MARINE

RECENT

Marginopora, Calcartna, Baculogypstria - - Robust Amphistegina, Heterostegina --

Flat Amphistegina and Operculina, Cycloclypeus - -- Sorites, Peneroplis , smal I m i liolids increasing planktonics - - - - t

Wave base . _ . . .

c _ _ _ . ( I 50-100 rn) . .

EARLY MlOCENE

Sphaerogypsina Tayamaia - - Miogypsina Miogypsinoides --

r o E d iepidocycltna flat - - jgiobuicis) Spiroclypeus (margaritatus) - ---

Austrotrihna. Bore//s Sorites Cycloclypeus - - -

EOCENE

-- small thick Nummulites L

giant N~immulites (1 cm or more)

flat Nummulites - -- -- Orbitolites, Alveolinidae thick Discocyclina flat -- -

I

FIGURE 4 - Models of Cenozoic larger foram distribution in carbonate facies

W

[r

-

13

14

-

15

16

.

- E D

SD

P S

ITE

292

(B

EN

HA

M R

ISE

, NW

PA

CIF

IC)

PLA

NK

TON

IC F

OR

AM

INIF

ER

A D

ISTR

IBU

TIO

N

v

I

k

LL

W

AB

UN

DA

NC

E

n

120

.9

v) C

a, s .a

0

130

14

0-

v) C

a,

0

c:

a, F Q

0.

0

3 a”

I

t .L m 6

.9 m 0

0

c 1

v) C

a,

0 9

c:

a, b

0

3

0 10

20

30

40 %

-

RA

NG

ES

v)

v) C

a,

a, u m C ‘C

a,

U

0

._ 2 .9

9

c

ZON

ATI

ON

D

ATU

M L

EV

ELS

N5

J. G

rkug

leri

Gq

prae

dehi

scen

s T

2 G

q al

tispi

ra

7 G c

iper

oens

is

2 G

q de

hisc

ens

A G

lobi

gerin

oide

s

N4

B

N4

A

P 22

(N3)

FIG

UR

E

6 -

Com

paris

on o

f re

lativ

e ab

unda

nce,

ran

ges,

dat

um le

vels

and

zona

tion

(mod

ified

from

Kel

ler,

1981

)

299

0

w z

L

52 g d

2 z Q

300

C

BENTHIC

BARREN

PLANKTONIC FORAMINIFERA DISTRIBUTION IN A DEPOSITIONAL SEQUENCE

I

SEQUENCE BOUNDARIES - PARASEQUENCE BOUNDARIES-----

$ PALEO-ENVIRONMENTS G.R. 1 SYSTEMS TRACTS (...) ~

cn 0 Z 0 s + Y Z

a 4

M_

- 0

- 0

- 5

- 0 - 10 - 40

c 50

C 40 - 50 - 70

C 70 - 80

.c 30

-10

- 5

+- 0

+- 0

T . + . *

! ! ! 1 ' l . ! + * I

+ I l l + + +

13.8 Ma W INTERTIDAL

INNER NERlTlC

w

INNER NERlTlC n z

i i + + * I I I ~ OUTER NERlTlC 1 I

i i i i i I l l i'i i c + + . + + + I I i i

- - __ - - - - - -- MIDDLE- OUTER NERlTlC

i i i i i / / I I t 7 t i I + * + + OUTER NERlTlC 1

i T j * * *

+ + . I l l I 1 . . i i

i i + *

+ + I I

I 1 + +

MIDDLE NERlTlC \ \ 4

_wy - 25 - 35 c 50 - 40

+ + + I I I I MIDDLE NERlTlC

cn Z + + + ;

+ + + I l l I l l . + + .

! Shallowing-upward c Position of sample (core, swc) Rare + Common 4k Abundant

FIGURE 8 - Typical distribution of foraminifera in a Middle Miocene depositional sequence (hypothetical well section; for approximate position in model see Fig. 7)