Thirteen time-stratigraphic associations of
the nannofossil Discoaster have been defined and used in the Miocene
Kutei Basin of eastern Borneo to establish a regional stratigraphic framework.
The methodology used is discussed and the fossils employed are figured and
annotated. Their aid in resolving the timing, stages and details of delta
construction is presented graphically.
Borneo, delta, Discoaster, Kutei,
Mahakam, Miocene, nannoflora, nannofossil, Neogene, stratigraphy, zonation
B.,
C.
(2005).- Discoaster
zonation of the Miocene of the Kutei Basin, East Kalimantan, Indonesia (Mahakam
Delta Offshore).- Carnets de
Géologie / Notebooks on Geology, Brest, Memoir 2005/01 (CG2005_M01)
Distribution chronostratigraphique des
représentants du genre Discoaster au Miocène dans le bassin de Kutei
(delta de la Mahakam, Est de l'île de Kalimantan, Indonésie).- Treize
associations chronostratigraphiques basées sur les espèces du genre Discoaster
ont été définies et utilisées pour le Miocène du basin de Kutei (Est de
Bornéo) pour établir un cadre chronostratigraphique régional. La
méthodologie utilisée et les nannofossiles utilisés sont figurés et
discutés. L'aide apportée par les Discoasters pour comprendre la mise en
place des différents systèmes deltaďques est présentée.
Le basin de Kutei est reconnu comme une
province pétrolière depuis les premières découvertes à terre à la fin du
19éme siècle. De 1970 à 1985 le cadre chronostratigraphique
régional n'a pu être précisé faute d'avoir des marqueurs
paléontologiques fiables. La prédominance des faciès deltaďques excluant les
formes planctoniques au profit de fossiles benthiques de large extension
chronostratigraphique.
A la fin des années 1980, Total utilise les
nannofossiles calcaires. Une méthodologie adaptée aux facies détritiques peu
favorable à la préservation des éléments planctoniques et au matériel
disponible (déblais de forage) été mise en śuvre.
Les premières investigations avaient
révélé la présence de représentants du genre Discoaster dans les
argiles de prodelta parfois localisées loin en arrière de la rupture de pente
correspondant à de faibles profondeurs d'eau. Ils étaient souvent les seuls
représentants de la nannoflore calcaire et dans la plupart des cas dispersés
dans un abondant matériel détritique (silts, matière organique, etc.).
Treize intervalles
chronostratigraphiques ont
pu être reconnus et sont décrits dans ce mémoire. Ils couvrent la presque
totalité du Miocène à l'exception de la partie basale (Aquitanien) non
rencontrée. Une comparaison avec les zonations classiquement utilisée (,
) est présentée. L'utilisation de ces
différents intervalles a permis d'établir un cadre chronostratigraphique
régional fiable en corrélant un grand nombre de puits largement disséminés
dans le bassin de Kutei.
Bornéo, delta, Discoaster, Kutei,
Mahakam, Miocène, Nannofossiles, Néogène, stratigraphie, zonation
Pembagian daerah Discoaster pada
lapisan Miocene dari Cekungan Kutei (Mahakam, Delta Offshore).- Tiga belas
assosiasi waktu stratigrafi dari fosil nanno Discoaster telah
didefinisikan dan digunakan pada lapisan Miosen di cekungan Kutei Kalimantan
Timur untuk menetapkan kerangka stratigrafi regional. Metoda yang diterapkan
akan didiskusikan dan fosil yang digunakan akan digambarkan dan dicatat.
Penggunannya dalam mengatasi masalah waktu (time), masa (stage) dan konstruksi
rinci dari delta di presentasikan secara grafik.
Cekungan Kutei sudah dikenal sebagai daerah
yang kaya minyak bumi semenjak penemuan minyak pertama didaratan pada akhir abad
ke-19. Selama 15 tahunan (1970-1985) suatu kerangka stratigrafi regional yang
lengkap untuk lapisan Neogen di cekungan Kutei belum bisa ditetapkan karena
kurangnya marker fauna yang bisa dipercaya. Kekurangan ini disebabkan oleh
jarangnya foraminifera plankton pada facies yang didominasi oleh delta dan
ketidak hadiran bentuk bentonik yang luas dengan nilai stratigrafi yang berarti.
Pada akhir tahun 80an Total mencoba
menggunakan fosil nanno kapuran untuk mengatasi masalah ini. Metoda baru
dikembangkan untuk memperhitungkan karakteristik penghalang pada sedimen
didaerah tsb: lempung, lanau dan pasir; dan kualitas dari material yang tersedia
untuk studi: utamanya, "cutting".
Penyelidikan pertama menemukan fosil nanno
dari genus Discoaster pada lempung berfacies "prodelta",
umumnya berada jauh kearah laut dari batas paparan. Mereka mewakili secara
unique dari plankton nanno kapuran yang ditemukan. Kehadirannya bersifat
sporadis dan contoh ini jarang yang menyebar dalam jumlah yang banyak pada
detritus halus.
Sekalipun demikian, 13 assosiasi waktu
stratigrafi terpisah telah dapat dikenal, didefinisikan dan didiskripsikan.
Mereka mewakili semua lapisan, kecuali lapisan terbawah Miosen yang tidak
ditembus oleh sumur pada daerah tsb. Hubungannya dengan zonasi fosil nanno yang
diangap sebagai "Standard" (, )
telah ditunjukkan. Kegunaannya sebagai fasilitasi penentuan hubungan umur dan
memungkinkan untuk korelasi sumur-sumur yang tersebar secara luas di cekungan
Kutei.
Borneo, delta, Discoaster, Kutei,
Mahakam, Miocene, nannoflora, nannofossil, Neogene, stratigraphy, zonation
The Kutei basin has been known as a prolific
petroleum province since the first discoveries onshore at the end of the 19th
century. For some fifteen years (1970-1985) a comprehensive regional
stratigraphic framework for the Neogene sequence of the Kutei basin could not be
established because of a lack of reliable faunal markers. The deficiency was
caused by a scarcity of planktonic foraminifera in the predominant delta facies
and the absence of large benthonic forms with significant stratigraphic value.
In the final years of the
"eighties" Total attempted the use of calcareous nannofossils to
resolve this problem. A new methodology was developed to take into account the
obstructive characteristics of the sediments in the region: deltaic clays, silts
and sands; and the quality of the materials available for study: for the most
part, "cuttings".
The first investigations found nannofossils
of the genus Discoaster in the shales of the "prodelta" facies,
commonly far seaward of the shelf edge. They were and are the unique
representatives of calcareous nannoplankton encountered. Their presence
is sporadic and specimens are always sparsely dispersed in a huge amount of fine
detritus.
Nevertheless, thirteen discrete
time-stratigraphic associations have been recognized, defined, and described.
They represent all but the lowermost stage of the Miocene, which was not reached
by wells in the area. Their relationships to the nannofossil zonation considered
"Standard" (, )
are indicated. Their use facilitated the determination of age relationships and
permitted the correlation of a number of wells scattered widely in the Kutei
basin.
The Tertiary Kutei Basin is in the eastern
part of the island of Borneo in the Indonesian state of Kalimantan. It occupies
more than 45,000 km2 onshore and offshore. The Mahakam delta, the seaward
extension of the basin, is often referred to as the "Lower Kutei"
Basin (Fig. 1
). There, sedimentation was continuous
throughout the Neogene, whereas in the onshore portion only strata of lower
Neogene age are present because of uplift late in Miocene times. This tectonic
activity resulted in the development of three structural axes offshore, oriented
slightly east of north (Fig. 2 ).
They are the sites of the main oil and gas fields of the Kutei basin. The Median
axis houses the giant Peciko and Tunu gas fields (
et alii, 2004). Trapping mechanisms are both structural and stratigraphic
and are enhanced by overpressured slope shales.
Overall, the sedimentary sequence of the
Mahakam delta Basin consists of very thick, generally regressive clastics
deposited without interruption from earliest Miocene times to the Present by the
Mahakam river in its center. Their thickness totals more than 10 km.
Consequently, the lithology is monotonously repetitive, a superposition of
deltaic cycles clearly visible on seismic sections (Fig. 3 ).
Progradation was interrupted at regular intervals, for short periods of
aggradation are intercalated in longer periods of strong progradation (Fig. 3 ).
The average thickness of a cycle ranges between 30 and 50 meters, and appears to
correspond to the length of the average step of regional subsidence.
In each cycle, the normal sequence of facies
(Prodelta, Deltafront and Deltaplain) are present (,
2003). Their lithology includes shales (Prodelta and Deltafront), sands
(Deltafront and Deltaplain), and coal (Deltaplain). In more distal locations on
the shelf, limestones and marine shales exist, but are always strongly affected
by deltaic influx, and, of course, are confined to the base of the deltaic cycle
in its initial transgressive phase.
One deltaic cycle represents the progressive
replacement of a column of water 30 to 50 meters deep. In general the
replacement consisted mainly of clay (80 per cent) and sand (20 per cent). When
deposition was relatively slow, most of these clastics remained on the shelf
(aggradation: shelf shale prone, Fig. 4 ).
When deposition was more rapid and more
abundant, most sediment deposited on the slope was clay while sand remained
trapped on the shelf (Progradation: Shelf sand prone, Fig. 4 ).
The clays built the shelf outward. In any case, the deltaic cycle begins with a
deposit of fine-grained clay produced by landward flooding. Identifying these
more marine intercalations is important because they are sites where
nannofossils may be present. During the "eighties" an important
side-wall coring program was launched to identify these potential regional
datums of maximum flooding surfaces. The moving average method was widely used
to help locate them. It is based on a quantification of sand content. The
average "Net to Gross" is first determined for a 50 meter interval.
Then the same calculation is made for another 50 meter interval but
"slid" from the previous interval by 10 meters. Thus a curve is
obtained, useful in defining a subregional "shale" event (Fig. 5 ).
In the Neogene sequence of the Kutei Basin some 20 of these regional flooding
events have been detected (Fig. 6 ).
Most of them have been dated using
Discoasters (Fig. 7 ),
including some far landward of the shelf break. Combining this direct
chronostratigraphic dating with electrofacies correlations permitted the
construction of a detailed history of subsidence in the Mahakam delta sub-Basin
(Median axis, Fig. 7 ).
However, no correlation with presumed third order cycles of sea level change
could be seen. Consequently, the large number of regional flooding events is
probably caused by rapid and repeated subsidence, for it is most difficult to
link any one of these events to the Vail et alii chart.
Progradation and aggradation and their
relative duration in time have been mentioned, but another point of interest is
the rapid change of the stacking pattern from place to place in a given interval
of time. This phenomenon is probably controlled by changes in the direction from
which deposition took place. Fig. 8
shows the change in the pattern of progradation–aggradation on the Median
axis, some 80 km long. This effect is important for it regulates the
characteristics of the shelf sediments. For example, in the south (Peciko area)
the interval MF 75/MF 85 has a large amount of progradation. In the north
progradation is much less evident. Consequently, the strata will be sand prone
in the south because flooding duration was reduced, whereas the northern portion
with well-developed flooding cycles is shale prone. Therefore it is difficult to
establish a hierarchy among the several flooding events in the Mahakam delta
region, and as mentioned, to identify the effects of changes in sea level.
As the usual chronostratigraphic markers
(foraminifera, spores and pollen) are rare or absent in most of the Kutei Basin,
for many years it was not possible to set up a comprehensive, integrated,
stratigraphic model for the whole of the basin. At the beginning of the
"eighties" nannofossils were used successfully by C.
to make stratigraphical correlations in a few selected wells. These initial
satisfactory results caused Total at the end of that decade to begin a complete
regional synthesis. To do this, exchanges of well data were negotiated with
other operators in the area (Vico and Unocal). These initial efforts ended
finally in a complete review of 70 wells. All of the rare slope facies
information was studied first, then all significant thicknesses of shale on the
shelf were sampled and studied (side-wall cores and cuttings). A large number of
thin beds of shale clearly identified by benthic foraminifera and electric logs
as typical of a prodelta facies contained scattered and generally poorly
preserved nannofossils, predominantly specimens of Discoaster. This
finding focused attention on the unique potential of Discoaster. But the
question immediately arose: How do we make full use of the sparse and often
poorly preserved populations of Discoasters, commonly less than 50 specimens per
sample?
No commonly used conventional methods would
be really efficient or satisfactory in collecting and concentrating the fossils
from this type of clastic, scantily fossiliferous sediments. The material
available was mainly cuttings, with some side-wall cores and occasional
conventional cores. Each level selected, (spacing 10 meters) was soft-washed
with water (for water-based mud), or with gasoline (for oil-based mud). Under a
binocular microscope the lithologies were described and cuttings selected in
accordance with electric log indications of rock type. Doubtful cuttings and
cavings were removed, leaving a selection of the principal, presumably valid
lithologies. Some 10 to 20 cutting chips were usually chosen (occasionally up to
50), crushed, and mounted with Canada balsam on a smear slide.
For a quantitative evaluation only specimens
of the genus Discoaster were counted. A minimum of 30 specimens was
considered an adequate representation for a chronostratigraphic determination
based on both identity and relative abundance. For that reason the number of
smear slides prepared for each sample was increased until the 30 specimens were
obtained (in one single slide). All of them were photographed. In the sparsest
intervals sometimes more than 50 smear slides had to be made in order to get the
required 30 in one individual slide. For side-wall and conventional cores the
results of several slides from the same piece or fragment were added together.
Unfortunately sometimes the specified 30 specimens are not obtained. However it
seems possible with around 20 specimens to determine the corresponding
association (see Pl. 8
for instance).
To improve the reliability of the
chronostratigraphic significance of sparse populations of Discoasters the
following procedure was followed:
First, a general chronostratigraphic chart
using commonly accepted criteria was established, using mainly Discoasters, but
including other calcareous nannofossils. The chart adopted the nomenclature of
the classical zone markers ( NN). To
construct the chart, wells in the most distal locations (slope facies) were
sampled, their nannoflora identified and studied in detail.
This work was then added to with data from
more landward wells. The result: a comprehensive survey of the distribution of
the calcareous nannoflora of the Kutei basin keyed to the standard zonation
proposed by and used by C.
in her pioneer work. Helpful more recent modifications of this standard zonation
were included (, 1973; ,
1985; ,
1984).
Most commonly, Discoasters are the only
nannofossils present in the sediments. Consequently, their identification,
associations and frequency are the bases of the chart: a statistical approach
using the relative abundance of the several species as a means of establishing a
chronologic succession.
In the Miocene strata
involved (Aquitanian
not reached), 13 discrete associations are recognized based on the presence and
relative abundance of index species. In the more landward wells where less than
50 specimens per slide occur and no other fossils are present, relative
abundance is the most useful means of determining age accurately.
Generally speaking, in deltaic environments
(including the slope facies) the integrity of the individual specimen is vital
to reliable identification. In lithologies rich in organic matter and lacking
carbonates, overgrowth does not occur. On the other hand, specimens are often
damaged and incomplete. Elongate slender forms are particularly susceptible, as
their delicate arms are seen to lack terminal portions critical to their
identification. However, some specimens from more marly (seaward) localities do
exhibit calcitic overgrowths and some of them have been illustrated (Pl. 8
).
In order to preserve homogeneity, all specimens shown in this study are from the
same facies, i.e. the upper slope.
Here is a review of the two well-known
nannofossil zonations (, 1973; ,
1971)
used by biostratigraphers working with Tertiary Nannofossils; some of the
modifications proposed by in 1984 have
been added. With the exception of the original definitions of zones that include
all calcareous nannofossils, the following discussion is confined to the zones
and subzones defined by their content of Discoaster species as indicated
by their author. A compilation is given in Fig. 9 .
(1971)
NN1: Triquetrorhabdulus carinatus zone
Interval from the last occurrence of Helicosphaera
recta to the first occurrence of Discoaster druggii. This interval has not been reached in the
Mahakam Delta sub-basin.
NN2: Discoaster druggii Zone
In the Neogene sequence of the Kutei basin,
the first Discoaster used for chronostratigraphic purposes is Discoaster
druggii ( et
1967b). The first occurrence of this marker is the main criterion for the
NN2 zone (first occurrence of D. druggii plus last occurrence of Triquetrorhabdulus
carinatus). Neither of these two markers has been observed in the area of
interest probably due to the fact that no strata of the lowermost Miocene have
been reached in the offshore Kutei basin.
NN3: Sphenolithus belemnos Zone
Interval from the last occurrence of Triquetrorhabdulus
carinatus to the last occurrence of Sphenolithus belemnos.
indicates the appearance of S. heteromorphus in the upper part
of the NN3 zone. It is the oldest stratigraphic entity found in the Mahakam
delta sub-basin.
NN4: Helicosphaera ampliaperta Zone
Interval from the last occurrence of Sphenolithus
belemnos to the last occurrence of Helicosphaera ampliaperta
indicated the appearance of Discoaster variabilis in the upper part of
the NN4 zone. In the Kutei basin Helicosphaera ampliaperta is not present.
NN5: Sphenolithus heteromorphus Zone
Interval from the last occurrence of
Helicosphaera
ampliaperta to the last occurrence of Sphenolithus heteromorphus, a well known common marker.
indicated among the common species: Discoaster variabilis, D. exilis, D.
nephados, D. trinidadensis, last occurrence of D. druggii and
first occurrence of D. brouweri. In the Kutei basin D. nephados
and D. druggii have never been observed and D. trinidadensis is
considered to be a morphotype of D. deflandrei.
NN6: Discoaster exilis Zone
Interval between the last occurrence of Sphenolithus
heteromorphus and the first occurrence of Discoaster kugleri. Common
species: D. exilis, D. trinidadensis and D. nephados. In the
Kutei basin D. kugleri has never been identified.
NN7: Discoaster kugleri Zone
Interval from the first occurrence of Discoaster
kugleri to the first occurrence of Catinaster coalitus. Common
species: those listed for zone NN6 plus Discoaster kugleri. D. trinidadensis
has its last occurrence and D. challengeri and D.
pseudovariabilis have their first occurrence in zone NN7.
NN8: Catinaster coalitus Zone
Interval from the first appearance of Catinaster
coalitus to the first occurrence of Discoaster hamatus. Common
species: those listed for zone NN7 plus Catinaster coalitus. The first occurrence
of Discoaster calcaris is in the uppermost part of the Catinaster coalitus
zone.
NN9: Discoaster hamatus Zone
Interval from the first to the last
occurrence of Discoaster hamatus. Common species: D. hamatus, D.
variabilis, D. challengeri and D. calcaris. D. exilis
seems to have its last occurrence, D. neohamatus and D.
pentaradiatus their first occurrence in the lower part of the D. hamatus
zone. D. pseudovariabilis has its last occurrence and D.
bollii its first occurrence in the upper part of the NN9 zone.
NN10: Discoaster calcaris Zone
Interval from the last occurrence of Discoaster
hamatus to the first occurrence of D. quinqueramus. Common species: those listed for the NN10 zone less D. hamatus. D. bollii
has its last occurrence in the lower part of the D. calcaris zone.
NN11: Discoaster quinqueramus Zone
Interval from the first to the last
occurrence of Discoaster quinqueramus. Common species: D. quinqueramus, D.
variabilis, D. challengeri, D. brouweri and D. pentaradiatus.
D. calcaris and D. neohamatus have their last occurrence. Rare
specimens of D. surculus occur throughout zone NN11.
In 1986
and proposed a revised version of the
original standard zonation. In that text no change is proposed for the Neogene
except the introduction of a subdivision in the NN11 zone based on the first
occurrence of Amaurolithus delicatus. But in the detailed chart they
provide, some changes to the original zonation are noted:
D. icarus has never been found in our
samples.
(1971b, 1973), low latitude coccolith biostratigraphic zonation
introduces
in this zonation the concept of "acme zone" corresponding to the
interval of time in which a given species is most abundant.
In 1980 in association with ,
proposed a revision of his initial zonation and introduced a code number.
A comparison between the two classical
standard zonations is provided by and
in 1986.
Triquetrorhabdulus carinatus Zone (CN1 = NN1+2)
Cycligargolithus abisectus Subzone (CN1a)
Interval from the last occurrence of Sphenolithus
ciperoensis to the last occurrence of Cyclicargolithus abisectus.
Common species: Discoaster deflandrei.
Discoaster deflandrei Subzone (CN1b)
From the last occurrence of
Cyclicargolithus floridanus
to the first occurrence of Discoaster druggii. Abundance of D.
deflandrei.
Discoaster druggii subzone (CN1c)
From the first occurrence of Discoaster druggii
to the first occurrence of Sphenolithus belemnos. Common species: D.
deflandrei, D. lidzi, D. druggii, D. sp. and D. variabilis.
D. lidzi has never been observed here
by this author. This form is relatively close to D. nephados (both
described originally by 1967).
Sphenolithus belemnos Zone (CN2 = NN3)
From the first to the last occurrence of Sphenolithus
belemnos and to the first occurrence of S. heteromorphus. Common species: Discoaster
aulakos and D. deflandrei
D. aulakos originally described by
1967, obviously is a specimen overgrown by calcite.
Helicosphera ampliaperta Zone (CN3 = NN4)
From the last occurrence of Sphenolithus
belemnos, the first occurrence of S. heteromorphus to the
end of the acme zone of Discoaster deflandrei and to the last occurrence of Helicosphera
ampliaperta. Reduction of the dominance of Discoaster deflandrei in favour of
long-rayed Discoasters such as D. exilis, D. signus
and D. variabilis.
Sphenolithus heteromorphus Zone (CN4 = NN5)
From the last occurrence of Discoaster
deflandrei and Helicosphaera ampliaperta to the last occurrence of Sphenolithus
heteromorphus. Decrease of the short-rayed Discoaster (deflandrei),
presence of D. exilis, D. moorei, D. signus and D.
variabilis.
We consider D. moorei as a
pentaradiate form of D. exilis.
Discoaster exilis Zone (CN5 = NN6+7)
Coccolithus miopelagicus Subzone (CN5a =NN6)
From the last occurrence of Sphenolithus
heteromorphus to the first occurrence of Discoaster kugleri. Common species: D.
exilis, D. variabilis, D. aulakos, D. deflandrei, D. braarudii, D. moorei and D. signus.
Discoaster kugleri Subzone (CN5b = NN7)
From the first to the last occurrence of Discoaster
kugleri to the first occurrence of Catinaster coalitus. Common species:
Discoaster aulakos, D. bollii, D. braarudi, D. challengeri,
D. kugleri and D. variabilis.
D. braarudii has never really been
observed in our samples. Various planar 6-rayed asteroliths have been found
often without any satisfactory specific name to propose for them.
Catinaster coalitus Zone (CN6 = NN8)
From the first occurrence of Catinaster
coalitus to the first occurrence of Discoaster hamatus. Common species: Catinaster
coalitus, Discoaster bollii, D. braarudii, D. challengeri, D.
exilis, D. pseudovariabilis and D. variabilis.
Discoaster hamatus Zone (CN7 = NN9)
From the first to the last occurrence of Discoaster
hamatus. Common species: Catinaster coalitus, Discoaster hamatus,
D.
bellus, D. bollii, D. braarudii, D. brouweri
(rare), D. calcaris, D. neohamatus, D.
prepentaradiatus and D. pseudovariabilis.
D. bellus has not been found / used,
we consider this species as a pentaradiate asterolith overgrown by calcite.
Discoaster neohamatus Zone (CN8 = NN10)
Discoaster bellus Subzone (CN8a)
From the last occurrence of Discoaster
hamatus to the first occurrence of D. neorectus. Common species: Catinaster
coalitus, Discoaster asymmetricus (rare, possibly a variant of D. bellus), D. bellus,
D. bollii, D. brouweri, D. intercalaris
(rare), D. loeblichii, D. neohamatus, D.
pentaradiatus, D. perclarus, D. prepentaradiatus, D.
pseudovariabilis and D. variabilis.
Discoaster neorectus Subzone (CN8b)
From the first to the last occurrence of Discoaster
neorectus to the first occurrence of D. berggrenii. Common species:
D. asymmetricus, D. bellus, D.
brouweri, D. challengeri, D. intercalaris, D. loeblichii,
D. neohamatus, D. neorectus, D. pentaradiatus (rare), D.
perclarus, D. quinqueramus (transitional form from D.
bellus) and D. variabilis.
We follow
and consider D. neohamatus as a form of D. calcaris / neorectus
ovegrown by calcite. D. loeblichii has never been observed; generally
speaking bifurcated asteroliths remain very rare in the uppermost part of the
Miocene of the Kutei basin.
Discoaster quinqueramus Zone (CN9 = NN11)
Discoaster berggrenii Subzone (CN9a = NN11a)
From the last occurrence of Discoaster
neorectus / first occurrence of D. berggrenii to the first
occurrence of Ceratolithus primus. Common species: Discoaster asymmetricus (rare),
D.
berggrenii, D. brouweri, D. challengeri, D. intercalaris,
D. pentaradiatus, D. quinqueramus, D. surculus and D.
variabilis.
Ceratolithus primus Subzone (CN9b = NN11b)
From the last occurrence of Ceratolithus
primus to the last occurrence of Discoaster quinqueramus. Common species: D.
asymmetricus (rare), D. berggrenii, D. brouweri, D.
challengeri, D. intercalaris, D. pentaradiatus, D.
quinqueramus, D. surculus and D. variabilis.
(1984)
In 1984, ,
in his work devoted to the biozonation of the Miocene, proposed some interesting
modifications, some of them based on accurate Discoaster species
descriptions. Most of these observations have been confirmed in our Indonesian
material. However, following (1984) and
(1985) we consider the distinction between Helio-Discoaster and Eu-Discoaster
unnecessary so we use only the name Discoaster. Below we maintain the
name Eudiscoaster only in the author's original designation of the zone
or subzone.
Triquetrorabdulus carinatus Zone (NN1 - NN2 zones)
From the end of the acme of Reticulofenestra
abisecta to the first occurrence of Geminilithella rotula.
Eu-discoaster deflandrei Subzone
End of the acme of Reticulofenestra abisecta and
the first occurrence of Discoaster druggii, presence of D.
deflandrei in the nannofossils assemblages.
Eu-discoaster druggii Subzone
First occurrence of Discoaster druggii
to the first occurrence of Helicosphaera ampliaperta, presence of Discoaster
deflandrei.
Helicosphaera vedderi Subzone
From the first occurrence of Helicosphaera
ampliaperta to the first occurrence of Geminilithella rotula.
Presence of Discoaster deflandrei and D. druggii.
Triquetrorhabdulus milowi Zone (upper NN2 - NN3)
First occurrence of Geminilithella rotula
to the first occurrence of Sphenolithus heteromorphus.
Triquetrorhabdulus martinii Subzone
First occurrence of Geminilithella rotula
to the last occurrence of Triquetrorhabdulus carinatus. Discoaster deflandrei
and D. druggii are present in the assemblages.
Sphenolithus belemnos Subzone
From the last occurrence of Triquetrorhabdulus
carinatus and/or the first occurrence of Sphenolithus belemnos to the
first occurrence of S. heteromorphus. Discoaster druggii and D.
deflandrei are present in the assemblages.
Helicosphaera ampliaperta Zone (lower NN4)
From the first occurrence of Sphenolithus
heteromorphus to the first occurrence of Discoaster exilis. D.
deflandrei and D. protoexilis are present in the association.
Sphenolithus heteromorphus Zone (upper NN4 – NN5)
First occurrence of Discoaster exilis
to the last occurrence of Sphenolithus heteromorphus.
Helicosphaera obliqua Subzone (upper NN4): Interval from the first occurrence of Discoaster exilis to the first occurrence of D. signus. Common species: D. deflandrei and D. exilis.
Eu-discoaster signus Subzone (upper NN4): Interval from the first occurrence of Discoaster signus to the last occurrence of Helicosphaera ampliaperta. Common species: Discoaster deflandrei, D. exilis, D. musicus and D. signus.
Helicosphaera perch-nielseniae Subzone (lower NN5): Interval from the last occurrence of Helicosphaera ampliaperta to the last occurrence of Helicosphaera perch-nielseniae. Last occurrence of Discoaster signus.
Helicosphera waltrans Subzone (middle NN5): Interval from the last occurrence of Helicosphaera perch-nielseniae to the last occurrence of Helicosphaera waltrans. Common species: Discoaster deflandrei, D. exilis, D. musicus, D. signus and D. variabilis.
Eu-discoaster musicus Subzone (upper NN5): Interval from the last occurrence of Helicosphaera waltrans to the last occurrence of Sphenolithus heteromorphus.
Eu-discoaster exilis Zone (lower NN6 - NN8)
Interval from the last occurrence of Sphenolithus
heteromorphus to the first occurrence of Discoaster calcaris / D.
bellus.
Helicosphaera walbersdorfensis Subzone (lower NN6): Interval from the last occurrence of Sphenolithus heteromorphus to the first occurrence of Helicosphaera orientalis and/or the first occurrence of Syracosphaera fragilis. Common species: Discoaster deflandrei, D. exilis, D. subsurculus and D. variabilis.
Helicosphaera orientalis Subzone (part of NN6): Interval from the first occurrence of Helicosphaera orientalis ond/or the first occurrence of Syracosphaera fragilis to the first occurrence of Helicosphaera stalis. Common species: Discoaster deflandrei, D. exilis, D. subsurculus and D. variabilis.
Helicosphara intermedia Subzone (upper NN6): Interval from the first occurrence of Helicosphaera stalis to the last occurrence of Reticulofenestra floridana and/or the first occurrence of Discoaster kugleri. Common species: D. bollii, D. deflandrei, D. exilis, D. subsurculus and D. variabilis.
Eu-discoaster kugleri Subzone (NN7): Interval from the first occurrence of Discoaster kugleri and the last occurrence of Reticulofenestra floridana to the last occurrence of Discoaster kugleri and/or the first occurrence of Catinaster coalitus. Common species: Discoaster bollii, D. deflandrei, D. exilis, D. micros, D. pansus, D. subsurculus and D. variabilis.
Eu-discoaster bollii Subzone (lower NN8): Interval from the last occurrence of Discoaster kugleri and/or the last occurrence of Helicosphaera walbersdorfensis and/or the first occurrence of Catinaster coalitus to the first occurrence of Discoaster calcaris and / or the first occurrence of D. bellus. Common species: Catinaster coalitus, Discoaster bollii, D. deflandrei, D. exilis, D. pansus, D. pseudovariabilis and D. variabilis.
Eu-discoaster calcaris Zone (upper NN8 - lower NN9)
Interval from the first occurrence of
Discoaster
calcaris / D. bellus to the first occurrence of Minylitha
convallis.
Eu-discoaster bellus Subzone (upper NN8): Interval from the first occurrence of Discoaster calcaris / D. bellus to the first occurrence of D. hamatus. Common species: Catinaster coalitus, Discoaster bellus, D. bollii, D. calcaris, D. deflandrei, D. exilis, D. pansus, D. pseudovariabilis and D. variabilis.
Eu-discoaster hamatus Subzone (lower NN9): Interval from the first occurrence of Discoaster hamatus to the first occurrence of Minylitha convallis. Common species: Catinaster coalitus, Discoaster bellus, D. bollii, D. calcaris, D. deflandrei, D. exilis, D. hamatus, D. pansus, D. pseudovariabilis and D. variabilis.
Minylitha convallis Zone (upper NN9 - lower NN11)
Total range of Minylitha
convallis.
Eu-discoaster pseudovariabilis Subzone (upper NN9): Interval from the first occurrence of Minylitha convallis to the last occurrence of Discoaster hamatus. Common species: Catinaster calyculus, C. coalitus, Discoaster bellus, D. bollii, D. brouweri, D. calcaris, D. deflandrei (rare), D. exilis (rare), D. giganteus, D. hamatus, D. pansus, D. pentaradiatus, D. pseudovariabilis and D. variabilis.
Eu-discoaster pentaradiatus Subzone (lower NN10): Interval from the last occurrence of Discoaster hamatus to the last occurrence of D. pentaradiatus and / or the first occurrence of D. misconceptus. Common species: Catinaster coalitus, Discoaster bellus, D. brouweri, D. calcaris (sporadic), D. giganteus, D. loeblichii, D. neorectus, D. pansus, D. pentaradiatus, D. pseudovariabilis, D. surculus and D. variabilis.
Geminithella rotula Subzone (upper NN10 - lower NN11): Interval from the last occurrence of Discoaster pentaradiatus and / or the first occurrence of D. misconceptus to the last occurrence of Minylitha convallis. Common species: Discoaster bellus, D. brouweri, D. calcaris (sporadic), D. giganteus, D. loeblichii, D. misconceptus, D. neorectus, D. pansus, D. pseudovariabilis, D. quinqueramus, D. surculus and D. variabilis.
Coccolithus pelagicus Zone (part of NN11)
Interval from the last occurrence of Minylitha
convallis to the first occurrence of the genus Amaurolithus. Common species:
Discoaster bellus, D.
brouweri, D. misconceptus, D. pansus, D. quinqueramus, D.
surculus and D. variabilis.
Amaurolithus primus Zone (part of NN11)
Interval from the first occurrence of
species of Amaurolithus to the first occurrence of Reticulofenestra rotaria. Common species:
Discoaster bellus, D.
brouweri, D. misconceptus, D. pansus, D. quinqueramus, D.
surculus and D. variabilis.
Reticulofenestra rotaria Zone (part of NN11)
Range defined by the range of
Reticulofenestra rotaria. Common species: Discoaster bellus, D.
brouweri, D. misconceptus, D. pansus, D. quinqueramus, D.
surculus and D. variabilis.
Calcidiscus leptoporus Zone (upper NN11 – lower NN12)
Interval from the last occurrence of
Reticulofenestra rotaria to the first occurrence of Ceratolithus rugosus.
Calcidiscus leptoporus Subzone A (NN11 upper): Interval from the last occurrence of Reticulofenestra rotaria to the last occurrence of Discoaster quinqueramus and / or the first occurrence of Ceratolitus acutus and / or the last occurrence of Triquetrorhabdulus rugosus. Common species: Discoaster bellus, D. brouweri, D. misconceptus, D. pansus, D. quinqueramus, D. surculus and D. variabilis.
A total of 13 associations have been
described from the Miocene of the Outer Kutei basin. The lowermost Miocene
(Aquitanian) has not been reached in this area.
- 2 ,
Figs. 10
- 11 )Age: Lower Miocene.
NN reference: Zones NN3, NN4, NN5.
Type level: Well-1,
2265 m (Sphenolithus belemnos, NN3)
+ Well-4, 3660 m (S. heteromorphus, NN4/5).
Description: 40% of the Discoaster population are
specimens of D. deflandrei et
1963. 40% of the population are specimens referred
to D. protoexilis 1984. The remaining 20% are pentaradiate forms
probably D. protoexilis.
Taxonomic comment:
We do not follow
in the use of the concept of Helio-Discoaster and Eu-Discoaster
(see previous discussion).
Discoaster
protoexilis: The specimens
observed in the Kutei basin are similar to the specimens described by :
Generally small asterolith, size between 5
and 15 µm.
The pentaradiate forms have been referred to
D. protoexilis because of the presence on the distal side of depressions
around the distal knob. These depressions do not exist in D. deflandrei.
Discoaster deflandrei:
As explained by ,
although the typical forms of the index species D. deflandrei and D.
protoexilis can be easily recognized, intermediate forms are common especially
when the overall poor state of preservation of the specimens is taken into
account and in particular when the central area is not clearly visible. On the
distal part a sometimes well developed knob is present, and on the flat proximal
side it is possible to see sutures between the arms (as illustrated by
and , Pl. 1, fig. 3
;
Pl. 3, figs. 1-2 ). In the
same paper these authors present good SEM illustrations of the proximal and
distal sides of typical D. deflandrei. In these illustrations no
depression exists on the distal side of D. deflandrei.
Chronostratigraphic comment:
In the Kutei basin area the classical marker
Helicosphaera ampliaperta has never been encountered. For this reason it is not
possible to delimit the NN3-NN4 and NN4-NN5 boundaries. Based strictly on the
presence of index species of Sphenolithus this association is placed in the
NN2-3 / 4-5 zones (presence of S. belemnos in Well-1, 2265 m and S.
heteromorphus in Well-4, 3660 m).
indicates that Discoaster protoexilis appears in his Helicosphaera ampliaperta zone (among
with Sphenolithus heteromorphus and S. belemnos. This zone is the equivalent of the upper
part of 's NN3 zone). In the Mahakam area,
the oldest sample studied in Maruat-1, 2265 m contained only S. belemnos, so the
first occurrence of Discoaster protoexilis must be in the NN3 zone below the
first appearance of Sphenolithus heteromorphus. The absence of long, bifurcate, rayed Discoaster
(D. variabilis / challengeri) allows us to restrict this association to the
lower part of NN4 ( 1971). It accords with
the position of the Helicosphaera ampliaperta zone of
(last occurrence of Discoaster protoexilis) if its D. exilis
(illustrated by forms with large bifurcated arms) corresponds pro parte to our D.
variabilis / challengeri group.
Proposed range of
association 1: NN3 -
lower NN4
- 4 ,
Figs. 12
- 13 )Age: Lower Miocene.
NN reference: Zones NN4-5.
Type levels: Well-4,
3355 m and 3051 m.
Description: 30% of the population is
Discoaster deflandrei. 70% of the population is "slender"
asteroliths comprised of:
Taxonomic comment:
In this interval a more diversified
association appears including relatively larger specimens with slender bifurcate
arms. They are related to the species D. variabilis and D.
challengeri (Any one asterolith is very much like its neighbour). From the
association found in Well-4, 3355 m to the association present in Well-4, 3051 m
the average size increases slightly (from 10 to 15 µm).
All the small specimens (less than
10 µm)
are referred to D. protoexilis. Their number decreases through time and
in the latest sample (Well-4, 3051 m) they have all disappeared.
Discoaster variabilis /
D. challengeri: These index species constitute the two poles of a population
composed of various morphotypes with numerous intermediate forms as explained by
and
1963 in their original description of D. variabilis.
Chronostratigraphic comment:
The presence of Sphenolithus heteromorphus indicates a
range from NN4 to NN5. For , Discoaster
protoexilis disappears before the last occurrence of Helicosphaera ampliaperta (NN4 lower).
But the distinction between lower and upper NN4 equivalents based on the first
appearance of Discoaster exilis is not obvious in our material. The presence of
about 20% of Discoasters belonging to the long rayed D. variabilis /
challengeri group suggests (following ) that
Association 2 is probably situated in the NN4 upper - NN5 zones. This is in
agreement with who indicates a decrease of D.
deflandrei during CN3 (NN4).
Proposed range of
association 2: upper NN4
– NN5.
,
Fig. 14 )Age: Middle Miocene.
NN reference: NN6-7 zones.
Type level: Well-5,
3660 m.
Description: 10%
Discoaster deflandrei, 50% D. variabilis / D.
challengeri, 40% D. brouweri.
Taxonomic comment:
Appearance of large slender non-bifurcate
specimens of Discoaster. We have assigned these large elongated non-bifurcated forms to D. brouweri. The arms of the specimens are slightly
bent. In many cases, the arms of these delicate forms are broken and it is
difficult to determine whether or not some of them are Discoasters with broken
bifurcated branches. Appearance of small massive forms very similar to D.
adamanteus et
1967.
Chronostratigraphic comment:
Following
and 1986, the last occurrence of D.
deflandrei allows us to place this association in the NN6 zone.
Unfortunately in the Kutei basin the zonal marker D. kugleri has
never been found and for this reason, in company reports this association is
often assigned the overall NN6 - NN7 interval (from last occurrence of Sphenolithus
heteromorphus to the first occurrence of Catinaster coalitus).
Proposed range of association 3:
lower NN6.
- 7 ,
Fig. 15 )Age: Middle Miocene.
NN reference: NN 6-7 zones.
Type levels: Well-1,
1403 m, Well-5, 2502 m.
Description: 45% Discoaster
exilis, 45% D. variabilis / D.
challengeri, 10% non-bifurcate specimens.
Taxonomic comment: We have referred to
D. exilis all asteroliths which satisfy the original description proposed by
and
As noted in the previous association, the
specimens observed are large (up to 20 µm)
D. adamanteus
et 1967: First appearance of typical
specimens of this small species, always sporadic in the samples
D. sp. aff. formosus: Appearance of asteroliths with six simple arms and a prominent star shaped
central knob (Pl. 6, fig. 26
;
Pl.
7, figs. 6-10
).
Pl. 6, fig. 16
may be a broken specimen of D.
exilis (to be compared with Pl. 6, figs. 2-3 ). The central area is hexagonal
with a stellate central knob. If these specimens are really unbifurcated they
are close to D. archipelagoensis described by
and 1976.
D. sp. aff. signus, some
specimens are close to D. signus 1971a
(Pl. 6, figs. 11-12 , to be compared with
1971a, Pl. 3, fig. 34). The central area is extremely reduced and the slender arms are bifurcate.
These bifurcations seem to be smaller in our material than in the type specimens
(broken?).
D. sp. aff. tuberis
1985 (Pl.
7, figs. 1-4
) illustrate asteroliths with six simple arms and a very
large prominent central knob. In the type species the arms are clearly
bifurcate. We do not follow who places
asteroliths with bifurcate arms and a prominent central knob in the species D.
musicus. It is difficult to reconcile the illustrations provided by
(1984: Pl. 33, figs. 14-17; Pl. 34, figs. 1-6) with the original illustrations (
1971: Pl. 3, figs. 3-4).
Chronostratigraphic comment:
This
association in similar to the previous one but differs in the high percentage of
large, elongate D. exilis specimens. D. formosus is reported by
and as restricted to the Sphenolithus
heteromorphus
zone. The specimens Discoaster aff. formosus are slightly younger. D.
deflandrei has disappeared. As D. kuglerii is absent, NN6 cannot be
segregated from NN7. But the change in the ratios of D. exilis and D.
variabilis suggests that Association 4 represents only the NN6 zone.
Proposed range of association 4: NN6.
- 9 ,
Figs. 16
- 17
- 18 )Age: Middle Miocene.
NN reference: NN6-7 zones.
Type levels: Well-2,
2821 m and 3398 m.
Description: 90% of the population is Discoaster
variabilis and D. challengeri.
Taxonomic comment:
The upper levels of this interval are
characterized by the presence of big asteroliths (up to 20 µm).
Chronostratigraphic comment:
Probably restricted to the NN7 zone due to
the presence of numerous bifurcate species (D. variabilis / challengeri).
This characteristic is shared with the next interval (NN8, Catinaster coalitus, see
hereafter).
Proposed range of association 5: NN 7.
,
Fig. 19 )Age: Upper Miocene.
NN reference: NN8 zone.
Type level: Well-2,
2747 m.
Description: 20% of the population is Catinaster
coalitus which has its first occurrence in this association; 30% of the distribution is large Discoaster
variabilis; 30% of the population is D. brouweri (small forms).
Taxonomic comment:
This association is characterized by the
presence of asteroliths with long, slender arm without clear bifurcations. The
end of the arm is curved. Due to relatively poor preservation it is often
difficult to distinguish between a simple curve, for instance D.
brouweri rutellus , and D.
calcaris in which the end of the arm turn
sharply in one direction, bifurcating asymmetrically.
reports the two in the same stratigraphic interval (Lengua Formation, Trinidad).
Chronostratigraphic comment: This
association is undoubtedly in the NN8 zone because of the appearance the zonal
marker Catinaster coalitus.
Proposed range of association 6: NN8.
,
Fig. 20 )Age: Upper Miocene.
NN reference: NN 9 zone.
Type level: Well-3,
2980 m.
Description: 30% Catinaster coalitus;
20% Discoaster pseudovariabilis; 20% D. variabilis (large
forms with simple bifurcation); 20% D. calcaris; D. hamatus (sporadic).
Taxonomic comment:
D. pseudovariabilis
et was found only in this association. Large
asteroliths (up to 25 µm):
Typical D.
calcaris, with a
long asymmetrical spine, occurs for the first time.
Chronostratigraphic comment:
The appearance of the zonal marker D.
hamatus confirms the placement of this association in the NN9 zone. In the Kutei
basin, D. pseudovariabilis seems to be restricted to the lower
part of the NN9 zone ( and
give NN8 through 9 zones as its full range.
First occurrence of typical
D.
calcaris.
Last occurrence of large
D.
variabilis.
The presence of D. hamatus and
Catinaster coalitus suggests a lower NN9.
Proposed range of association 7: lower NN9.
,
Fig. 21 )Age: Upper Miocene.
NN reference: NN 9 zone.
Type level: Well-3,
2550 m.
Description: 70%
Discoaster hamatus, 20% D. bollii, 10% D. calcaris.
Taxonomic comment:
Dramatic decrease of the bifurcate forms (D.
variabilis). After this interval bifurcate specimens occur only sporadically
in the samples.
Chronostratigraphic comment:
This association is characterized by the
acme of D. hamatus. D. bollii (slender forms) appears in this
association. Catinaster coalitus is no longer present.
The contrast between the lower and the upper
part of the NN9 is significant and it is easy to determine even from a
restricted number of specimens.
Proposed range of association 8: upper NN9.
,
Fig. 22 )Age: Upper Miocene.
NN reference: NN 10 - CN8a zone
Type level: Well-3,
2320 m
Description: 40%
Discoaster pentaradiatus (sensu
1984), 20% D. bollii, 30% D. calcaris.
Taxonomic comment:
D. pentaradiatus sensu
(D. quintatus sensu , see this
author for further details). The specimens of this pentaradiate asterolith
observed in the Kutei basin correspond to those described by :
Chronostratigraphic comment:
The absence of both D.
hamatus (NN9) and D. quinqueramus / berggrenii (NN11) proves an
NN10 age for this association. The restriction of D.
pentaradiatus (sensu ) to the lower part
of the NN10 zone accords with the restricted distribution proposed by
(Eu-discoaster pentaradiatus Subzone) and supports the subdivision of NN10.
Proposed range of association 9: lower NN10.
,
Fig. 23 )Age: Upper Miocene.
NN reference: NN10 – CN8b zone.
Type level: Well-3,
2160 m.
Description: 80%
Discoaster calcaris / D.
neorectus, 15% D. intercalaris.
Taxonomic comment:
Acme zone of D. calcaris / D.
neorectus.
Presence of D. intercalaris
1971b, typical stellate asterolith with:
D. perclarus, some delicate, slender,
bifurcate asteroliths occur sporadically in this interval (Pl. 14, fig. 26
). The
presence of this small species in the NN10 / CN8 is in agreement with its range
as proposed by , but the number of specimens
seen in the Kutei basin are too few for a valid assessment of the true range of
this taxon.
Chronostratigraphic comment:
D. intercalaris seems to be
restricted to the upper part of the NN10 zone. It has never been found with the
NN11 zonal marker (D. berggrenii) as reported by
1985.
As in the NN9 zone, a marked difference
exists between the lower and the upper levels of NN10. Even based on only a few
specimens it is relatively easy to distinguish them. The subdivision proposed
here corresponds to that proposed by to divide
CN8 (a&b), a separation based on the first occurrence of D. neorectus.
Proposed range of association 10:
upper
NN10.
,
Fig. 24 )Age: Upper Miocene.
NN reference: NN 11 zone.
Type level: Well-3,
2040 m.
Description: 60%
Discoaster calcaris / D.
neorectus, 30% D. berggrenii, 10% D. misconceptus
Taxonomic comment:
D. misconceptus
1984 is a pentaradiate asterolith characterised by:
D. berggrenii:
proposed that D. berggrenii is a morphotype of D. quinqueramus. He
also indicated that the number of more elongated specimens (D. quinqueramus) increased with time (upper NN11) but that the two share a common
stratigraphic range (NN11). In the Kutei basin the lower part of NN11 is
characterized by the presence of massive specimens of D. berggrenii with
no elongated asteroliths present, but we do not know if in a more fossiliferous
time equivalent stratum we will not find some elongate specimens corresponding
to D. quinqueramus. However we maintain that the two species are distinct
because we see a clear quantitative evolution during the uppermost Miocene and
that based partly on this evolution we can subdivide NN11 into several subzones.
Chronostratigraphic comment:
D. misconceptus appears in
the NN11 zone in the Kutei basin and this appearance is in agreement with the
first appearance proposed for it by .
D. berggrenii also appears for the
first time in this zone (CNN9 / NN11).
Proposed range of association 11: lower
NN11.
,
Fig. 25 )Age: uppermost Miocene.
NN reference: NN 11
zone.
Type level: Well-3,
1960 m.
Description: 40% Discoaster brouweri,
50% D. berggrenii /
quinqueramus, 10% D. misconceptus.
Taxonomic comment:
The 6-rayed specimens are almost entirely
single armed D. brouweri. The pentaradiate forms are a mix of D.
misconceptus (easily recognizable under cross polarized light) typical D.
berggrenii and D. quinqueramus (first occurrence of this slender
pentaradiate form). D. berggrenii and D. quinqueramus represent
two poles of a same population with all possible transitional forms between
them. Probably the two coccoliths belong to the same biological species. However
due to the clear quantitative evolution observed we have to maintain two
distinct typological species. It is frequent to observe distinct morphotypes on
the same coccolithophoraceae cell in other species.
Chronostratigraphical comment:
Transitional interval between a lower NN11 /
CN9 clearly identified by the appearance of D. berggrenii and an upper
NN11 / CN9 with D. berggrenii / quinqueramus and D. surculus.
Proposed range of association 12:
"middle" NN11.
,
Fig. 26 )Age: uppermost Miocene.
NN reference: NN11 zone.
Type level: Well-3,
1420 m.
Description: 80%
D. quinqueramus / berggrenii, 20% D. surculus.
Taxonomic comment:
Appearance of D. surculus, with
typical forms, "stellate knob, small ridges, slender rays clearly
trifurcated with the central spine extending beyond and downward from the outer
pair" ( and
1963).
With the exception of
D. surculus we
note a true collapse of 6-rayed Discoasters (80% of the population is 5-rayed).
Chronostratigraphic comment:
Both
and indicate the first occurrence of D.
surculus in their
NN11/CN9 zone(s) (uppermost Miocene). In the Kutei basin the first significant
occurrence of this species has a clear relationship with the upper portion of
the NN11/CN9 zone. These taxa together (D. surculus and D.
berggrenii /quinqueramus) are characteristic of the upper part of the NN11/CN9
zone(s).
From a quantitative point of view, as is the
case in the two previous zones (NN9 and 10) there is a marked distinction
between the lower and upper portions of NN11, supplemented by the
quasi-extinction of 6-rayed asteroliths in the upper part of the zone.
Proposed range of association 13:
upper
NN11.
& 27 )A general correspondence exists between the
associations in the Kutei basin described in this work and the
standard zonation. It is illustrated in Fig. 27 . Its correspondence with the
and zonations is illustrated by Fig. 9 .
It is important to keep in mind that in the
Kutei basin, two important factors strongly influenced our perception of the
distribution of Discoaster specimens in the sediments:
The conjunction of these two factors
enhances the contrast between the adjacent associations observed in the
sediments. Each fossiliferous association must
represent only a short chronostratigraphic interval.
The writers are grateful to Total, Pertamina
and Inpex for permission to publish on this topic. Many thanks to the
micropaleontologists who have reviewed and supplied helpful information, C. Muller,
M. Covington and B. Granier.
Special thanks to N.J. Sander
who perused the draft for niceties of English and to I. Umar
who provided the Indonesian abstract and summary.
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H.N. & J.A. (1967a).- Middle Tertiary calcareous nannoplankton of the Cipero section, Trinidad, W.I.- Tulane Studies in Geology, New Orleans, Vol. 5, no. 3, p. 93-131.
H.N. & J.A. (1967b).- Discoaster druggi nom. nov. pro Discoaster extensus et 1967, non 1967.- Tulane Studies in Geology, New Orleans, Vol. 5, no. 4, p. 220.
D. (1971a).- Discoaster evolutionary trends.- Micropaleontology, New York, Vol. 17, no. 1, p. 43-52.
D. (1971b).- Cenozoic calcareous nannofossils from the Pacific Ocean.- San Diego Society of Natural History, Transactions, Vol. 16, no. 14, p. 303-327.
D. (1973).- Low-latitude coccolith biostratigraphic zonation.- Initial Reports of the Deep Sea Drilling Project, College Station, Project 15, p. 685-703.
D. & H.N. (1969).- Some new and stratigraphically useful calcareous nannofossils of the Cenozoic.- Tulane Studies in Geology, New Orleans, Vol. 7, no. 3-4, p. 131-142.
(1988).- Calcareous nannofossils biostratigraphy and paleoenvironmental interpretation of the mediterranean Pliocene.- Utrecht Micropaleontological Bulletins, 36, 245 p.
M.V. (1985).- Calcareous nannofossil biostratigraphy of the Middle America Trench and slope, Deep Sea Drilling Project Leg 84.- Initial reports of the Deep Sea Drilling Project, College Station, Project 84, p. 339-361.
S. (1967).- Calcareous nannofossils from Neogene of Trinidad, Jamaica, and Gulf of Mexico.- Paleontological Contributions, Lawrence, Paper 29, 8 p.
S. (1969).- Correlation of Neogene Planktonic Foraminifera and Calcareous Nannofosils zones.- Gulf Coast Association of Geological Societies, Transactions, Miami, Vol. 19, p. 585-599.
Y., G., B.C. (1994).- Discovery of a giant in a mature deltaic province, Peciko, Indonesia.- In: Aspects of detailed regional exploration, 14th World Petroleum Congress, Stavanger, Poster 12, Session 2.
W.W., H.P., P.H., R.R. & J.E. (1967).- Calcareous Nannoplankton zonation of the Cenozoic of the Gulf Coast and Caribbean-antillean area, and taxonomic correlation.- Gulf Coast Association of Geological Societies, Transactions, San Antonio, Vol. 17, p. 428-480.
B. (2003).- Micropaleontological investigations in the modern Mahakam delta, East Kalimantan, Indonesia.- Carnets de Géologie - Notebooks on Geology, Maintenon, Article 2003/02, CG2003_A02, p. 1-21.
B., B.C., Y., I.M. & P. (2004).- The Peciko case history: Impact of an evolving geologic model on the dramatic increase of gas reserves in the Mahakam Delta.- In: M.T. (ed.), Giant oil and gas fields of the decade 1990-1999.- American Association of Petroleum Geologists, Memoir, Tulsa, 78, p. 297-320.
E. (1971).- Standard Tertiary and Quaternary calcareous nannoplankton zonation.- In: A. (ed.), Proceedings of the Second Planktonic Conference Roma 1970, Vol. 2, p. 739-785.
E. & M.N. (1963).- Calcareous nannoplankton from the experimental Mohole Drilling.- Journal of Paleontology, Tulsa, Vol. 37. no. 4, p. 845-856, pls. 102-105.
E. & C. (1986).- Current Tertiary and Quaternary calcareous nannoplankton stratigraphy and correlations.- Newsletters in Stratigraphy, Berlin - Stuttgart, Vol. 16, no. 2, p. 99-112.
E. & T. (1971).- Tertiary calcareous nannoplankton from the western Equatorial Pacific.- Initial Reports of the Deep Sea Drilling, College Station, Project 7, Part 2, p. 1471-1507.
C. (1974).- Calcareous Nannoplankton, Leg 25 (western Indian Ocean).- Initial Reports of the Deep Sea Drilling Project, College Station, Project 25, p. 579-633.
H. & D. (1980).- Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation ( 1973; 1975).- Marine Micropaleontology, Amsterdam, Vol. 5, p. 321-325.
K. (1971).- Durchsicht Tertiärer Coccolithen.- In: A. (ed.), Proceedings of the Second Planktonic Conference Roma 1970, Vol. 2, p. 939-980.
K. (1972).- Remarks on late Cretaceous to Pleistocene coccoliths from the North Atlantic.- Initial Reports of the Deep Sea Drilling, College Station, Project 12, p. 1003-1069.
K. (1977).- Albian to Pleistocene calcareous nannofossils from the western South Atlantic, DSDP Leg 39.- Initial Reports of the Deep Sea Drilling, College Station, Project 39, p. 699-823.
K. (1985).- Cenozoic calcareous nannofossils.- In: H.M., J.B. & K. (eds.), Plankton Stratigraphy.- Cambridge University Press, p. 427-554.
B. (1971).- Speculations on relations, evolution and stratigraphic distribution of Discoaster.- In: A. (ed.), Proceedings of the Second Planktonic Conference Roma 1970, Vol. 2, p. 1017-1037.
P. & K.P. (1976).- Late Miocene – Early Pliocene D. from Neill Island, south Andaman.- Geological Society of India, Journal, Bangalore, Vol. 17, no. 1, p. 37-44.
H. (1973).- Catalogue of calcareous nannoplankton from sediments of Neogene age in the eastern North Atlantic and Mediterranean Sea.- Initial Reports of the Deep Sea Drilling, College Station, Project 13, Part 2, p. 1137-1199.
H. & F. (1982).- The nannofossil assemblages of Deep Sea Drilling Project Leg 66, Middle America Trench.- Initial Reports of the Deep Sea Drilling, College Station, Project 66, p. 589-639.
S. (1984).- Calcareous nannofossils biozonation of the Miocene and revision of the Helicoliths and Discoaster.- Utrecht Micropaleontological Bulletins, 32, 271 p.
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Figure 1: Location map of the area studied, in red main gas fields: in orange other oil and gas fields. The circled numbers show the location of the reference wells.
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Figure 2: Mahakam Delta, simplified structural map at the so-called MF9 marker (Middle Miocene). Location of the main structural axes and the two giant gas fields (Tunu and Peciko): 1- Internal axis; 2- Median axis; 3- External axis (modified from et alii, 1994).
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Figure 3 (a): Mahakam Delta, Peciko field area: location of a seismic dip line showing overall deltaic progradation, the shelf, the shelf break and overpressured slope facies. The main seismic markers are related to the main flooding events (two of them are demarcated, MF2 and MF75).
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Figure 3 (b): Detail of the previous line showing the transition between an
aggrading and a prograding interval. In the aggrading interval corresponding to
a low influx of terrigenous sediment (see Fig. 4 )
transgressive limestones
are widespread on the shelf (strong reflections visible on the line). In the
prograding interval the transgressive limestones have disappeared (no strong
reflections).
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Figure 4: Geological interpretation of the seismic observations. The average thickness of a single deltaic unit is relatively constant (50 m) and corresponds to the pulses of regional subsidence. If the influx of sediment is heavy the deltaic unit will be strongly prograding and the shelf will be mainly sand prone (clays will be carried seaward to construct the slope). If the influx of sediment is relatively low, deltaic units will be aggradational and the shelf will be shale prone (most of the sediment will remain on the shelf). The geological interpretation of facies from electric logs: "A" and "B" of a single deltaic cycle shows graphically the marked change in sand content seaward and that the expression of the basal transgressive event (flooding surface) is not the same in a proximal and in a more distal location.
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Figure 5: Mahakam Delta, Peciko field area, geological characterization of the flooding events. Using the moving averages method (quantitative sand distribution) flooding events could be distinguished and correlated from well to well. The MF2 and MF75 markers previously identified on the seismic line are shown in green and red.
Legend:
MF (5.0): flooding surfaces (age in MY)
emw: density (equivalent mud weight)
40%: sand content, moving averages
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Figure 6: Mahakam Delta, Upper Miocene deltaic stacking pattern with the main flooding events detected seismically and in wells numbered in green.
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Figure 7: Mahakam Delta, Tunu and Peciko areas, Offshore Mahakam Area, subsidence rate of Median axis. The chronostratigraphic calibration is based on calcareous nannoplankton identified in the shales associated with flooding surfaces.
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Figure 8: Mahakam Delta, evolution of shelf break through time, comparison of the areas north and south of the Tunu field showing the huge differences in the aggradation / progradation pattern between north and south (after et alii, 2004). These differences are related to the changes in the rate of sedimentary influx through time.
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Figure 9: Distribution chart of Miocene Discoasters, and a comparison relating it to the two main standard zonations ( and ) and / Kutei basin.
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Figure 10: Well-1, 2265 m, association 1, NN 3. 1-5 & 7: Discoaster
protoexilis (5-6: pentaradiate forms, 7: small form with a reduced central
area, see Pl. 1, figs. 23 & 25 ), 8-13: D. deflandrei.
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Figure 11: Well-4, 3660 m, association 1, NN 4. 1-5: Discoaster protoexilis sensu stricto, 6-9: D. protoexilis with reduced central area, 10-14: D. deflandrei.
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Figure 12: Well-4, 3355 m, association 2, NN 5. 1-6: Discoaster variabilis / challengeri, 7-12: D. protoexilis "group", 13-7: D. deflandrei.
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Figure 13: Well-4, 3051 m, association 2, NN5. 1-9: Discoaster variabilis / challengeri, 10-13: D. deflandrei.
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Figures 14: Well-5, 3660 m, association 3, NN6. 1-3: Discoaster brouweri, 4-9: D. variabilis / challengeri, 10-12: D. deflandrei.
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Figure 15: Well-1, 1403 m, association 4, NN6. 1-5: Discoaster exilis, 6: D. brouweri, 7: D. adamanteus, 8-13: D. variabilis.
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Figure 16: Well-5, 2502 m, association 5, NN7. 1-10: Discoaster variabilis / exilis, 11: D. brouweri.
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Figure 17: Well-2, 3398 m, association 5, NN7. 1-9: Discoaster variabilis.
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Figure 18: Well-2, 2821 m, association 5, NN7. 1-4, 6-8, 10-14: Discoaster variabilis group, 5: D. adamanteus, 9: D. sp.
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Figure 19: Well-2, 2747 m, association 6, NN8. 1-7: Discoaster variabilis / exilis group, 8: D. sp., 9-13: D. brouweri, 14-15: Catinaster coalitus.
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Figure 20: Well-3, 2980 m, association 7, (lower) NN9. 1: Discoaster hamatus, 2-5: D. pseudovariabilis, 6-7: D. variabilis, 8-10: D. calcaris, 11-12: D. sp., 13: D. bollii, 14-16: Catinaster coalitus.
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Figure 21: Well-3, 2550 m, association 8, (upper) NN9. 1-5: Discoaster hamatus, 6-9: D. bollii, 10: D. calcaris, 12: D. variabilis.
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Figure 22: Well-3, 2320 m, association 9, (lower) NN10. 1-4: Discoaster calcaris, 5-9: D. pentaradiatus, 10-12: D. bollii.
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Figure 23: Well-3, 2160 m, association 10, (upper) NN10. 1-2: Discoaster calcaris, 3-7: D. neorectus, 8-10: D. intercalaris, 11: D. sp.
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Figure 24: Well-3, 2040 m, association 11, (lower) NN11. 1-6: Discoaster neorectus, 7: D. misconceptus, 8-11: D. berggrenii.
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Figure 25: Well-3, 1960 m, association 12, NN11. 1-3: Discoaster neorectus, 4: D. sp., 5-6: D. misconceptus, 7: D. quinqueramus, 8-10: D. brouweri.
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Figure 26: Well-3, 1420, association 13, (upper) NN11. 1-5: Discoaster quinqueramus, 6-8: D. berggrenii, 9-10: D. surculus, 11: D. brouweri, 12: D. sp.
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Figure 27: Mahakam area, general distribution of species of Discoaster.
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Plate 1: Association 1 (NN3).
Type level: Well-1, 2265 m.
Figs. 1-3, 6-9, 12, 24-25, 27: Discoaster protoexilis 1984, 6-rayed forms.
On the proximal side of the asterolith the knob is clearly visible (figs. 1-2, 6) and the elliptical depressions around a limited little knob on the distal side associated with median ridges (fig. 7). Most of them are relatively small (less than 10 µm). Note in figs. 23 and 25 a clear reduction of the central area foreshadowing D. variabilis / challengeri ?).
Figs. 4-5, 26-30: D. protoexilis, pentaradiate forms.
Figs. 10, 18-19: Intermediate form between D. protoexilis and D. deflandrei et 1954.
When the terminal branches of the arms are also bifurcate it is difficult to separate these forms from the most slender specimens of D. deflandrei. Note in fig. 18 the presence of depressions in this distal side. This characteristic is typical of D. protoexilis but not of D. deflandrei.
Figs. 11, 13-17, 20, 22-23, 26: D. deflandrei et 1954.
Note the variation in size (small forms, fig. 13, large form, fig. 14) and in morphology (massive specimen, fig. 17 and more "slender" specimens, fig. 14).
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Plate 2: Association 1 (NN 4).
Type level: Well-4, 3360 m.
Figs. 1-20: Discoaster protoexilis 1984.
The number of specimens increases (60% of the association). Forms of moderate size are always present (less than 10 µm).
A change in the size of the central area observed sporadically in the previous association (and shown on the plate) is confirmed, and the relative number of specimens exhibiting it increases (as compared to the previous plate). We note that the reduction of this central area (figs. 2, 6-7, for example) is associated with a well developed central knob that occupies the whole area.
Figs. 20-29: D. deflandrei et 1954. Note in figs. 22 & 23, the presence of prominent knob associated with well-defined ridges (proximal view). This variant was illustrated by 1974 (Leg 25, Pl. 7, fig. 3) reported from the NN5 zone.
Fig. 30: D. calculosus 1971a. "Compact form with short free length of the broad bifurcated tap, lack of any prominent central knob" ( 1971a). Specimens have been observed only rarely in our material.
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Plate 3: Association 2 (NN5).
Type level: Well-4, 3355 m.
Figs. 1-10, 18, 22-23: Discoaster protoexilis 1984. Note the very small size of the specimens and the great variability in their morphology (see fig. 8).
Figs. 11-17: D. variabilis et 1963 / challengeri et 1954 group? Appearance of relatively small forms with slender bifurcate arms. The central area varies greatly in size, so it is very difficult to differentiate the two species. A more or less heterogeneous population of asteroliths ranging from specimens with a relatively large central area (variabilis type) to specimens with a very small central area (challengeri type). Size is not stable: forms from very small (5 µm) to moderate in size (10 µm). Most of the small forms retain some "ancestral" aspects of D. protoexilis.
Figs. 18-21, 24-30: D. deflandrei et 1954. The relative poor preservation of the specimens does not permit a distinction among forms transitional between D. protoexilis (overgrowth) and D. deflandrei.
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Plate 4: Association 2 (NN5).
Type level: Well-4, 3051 m.
Figs. 1-20, 29: Discoaster variabilis et 1963 / challengeri et 1954. The average size of the specimens increases (up to 15 µm), probably because of the disappearance of the smallest asteroliths. As in the previous level, the size of the central area ranges widely: from narrow to broad. The bifurcations are also variable from acute angles (figs. 11-12) to obtuse angles (figs. 9-10), and from short ends (figs. 11-12) to long ends (figs. 7, 14). Some specimens have more than six rays (fig. 29).
Figs. 21-28, 30: D. deflandrei et 1954.
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Plate 5: Association 3 (NN6).
Type level: Well-5, 3660 m, limy sample, specimen with slight calcitic overgrowths.
Figs. 1-9, 14: Discoaster brouweri 1927. This association is characterized by the first occurrence of slender non–bifurcate Discoaster.
Figs. 10-13, 15-19: D. variabilis group.
Figs. 20-28: Forms intermediate between D. variabilis and D. deflandrei. Massive small forms with short arms and the ends of the bifurcations proximate.
Fig. 29: D. sp. cf. adamanteus.
This plate demonstrates the effects of a more calcareous milieu by the existence of forms with minor calcitic overgrowths. The general morphology and size are preserved but the details are obscured.
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Plate 6: Association 4 (NN6).
Type level: Well-1, 1403 m.
Figs. 1-4, 19, 19: Discoaster exilis. Large specimens (up to 25 µm) with a more or less well developed central area, long slender arms, weakly bifurcate.
Figs. 5-9: D. sp. aff. exilis. Close to the type specimens but differ in that the bifurcations are longer. Sometimes a web is visible between the ends of the bifurcation (fig. 8).
Figs. 13-14: D. sp. cf. exilis. Broken specimens.
Figs. 11-12: D. sp. aff. signus 1971a. Small central area close to the type, but without the prominent central knob. The bifurcations seem to be shorter.
Figs. 15-17: D. brouweri (?), fig. 16 could be a broken specimen of D. exilis or a form close to D. archipelagoensis et .
Figs. 20-25: D. variabilis (small forms). The ends of the bifurcations are flat with a web between them. Always, some of them are very similar to D. protoexilis (fig. 22).
Fig. 26: D. aff. formosus et . Small (less than 10 µm) in comparison to the type species described by et . Knob always present.
Fig. 27-28: pentaradiate forms (D. exilis / aff. signus variant).
Fig. 29: D. adamanteus.
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Plate 7: Association 5 (NN7).
Type level: Well-5, 2502 m.
Figs. 1-4: Discoaster sp. aff. tuberis 1985. All the specimens seen do not have a clear bifurcation at the ends of the arms.
Figs. 6-10: D. aff. formosus et . Sometimes it is difficult to discriminate between specimens of this species and broken specimens of D. variabilis. Small, with a well–developed star–shaped central knob and ridges extending from the knob to the rays.
Figs. 11-20, 25-30: D. variabilis et
Figs. 21-24: D. exilis.
Figs. 29-30: pentaradiate forms (exilis, variabilis).
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Plate 8: Association 5 (NN7).
Type level: Well-2, 3398 m.
Figs. 1-25: Discoaster variabilis group.
Illustration of a typical poorly preserved association with but few specimens of Discoaster.
Only 25 specimens were obtained but all of them are assignable to the D. variabilis group.
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Plate 9: Association 5 (NN7).
Type level: Well-2, 2821 m.
Figs. 1-4: Discoaster variabilis group. Large specimens appear (up to 30 µm).
Figs. 5-24: D. variabilis group (including form with weak bifurcations approaching D. exilis).
Figs. 25-26: D. sp. aff. formosus.
Figs. 27-28: D. sp.
Fig. 29: D. adamanteus.
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Plate 10: Association 6 (NN8).
Type level: Well-2, 2747 m.
Figs. 1-4, 8-10: Discoaster variabilis group, large forms (up to 25µm). Some of them with large bifurcations could be related to D. pansus (fig. 4). But these specimens are always rare and occur only where D. variabilis is abundant.
Figs. 5-7, 12: D. exilis et
Figs. 14, 15, 20: D. brouweri (sensu et 1963).
Figs. 13, 16-18: D. sp. cf. brouweri.
Figs. 21-22, 24: D. sp.
Fig. 23: D. sp. aff. tuberis
Fig. 25: D. sp. pentaradiate form.
Figs. 26-30: Catinaster coalitus et
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Plate 11: Association 7 (NN9 lower).
Type level: Well-3, 2980 m.
Figs. 1-4: Discoaster pseudovariabilis et . Huge specimens (up to 25/30 µm) with the typical trifurcation (with two dent-like tips) described by et
Figs. 5-8: D. variabilis et
Figs. 10-14: D. calcaris
Figs. 15-16: D. sp.
Figs. 17-18: D. hamatus et
Fig. 19: D. sp.
Figs. 20-21: D. bollii et . "Primitive" slender forms with a relatively small central knob.
Fig. 22: Catinaster aff. coalitus et (sensu and , large specimens).
Figs. 23-29: C. coalitus et
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Plate 12: Association 8 (NN9 upper).
Type level: Well-3, 2550 m.
Figs. 1-19: Discoaster hamatus et
Figs. 20-24: D. bollii et
Figs. 25-27: D. calcaris
Figs. 28-29: D. variabilis / exilis.
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Plate 13: Association 9 (NN10 lower).
Type level: Well-3, 2320 m.
Figs. 1-2: Discoaster variabilis et
Figs. 3-4, 20-29: D. pentaradiatus (sensu 1984).
Figs. 5-14: D. calcaris
Figs. 15-19: D. bollii et
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Plate 14: Association 10 (NN10 upper).
Type level: Well-3, 2160 m.
Figs. 1-2: Discoaster calcaris
Figs. 3-14: D. neorectus
Figs. 15-20: D. brouweri or broken D. neorectus?
Figs. 21-25: D. intercalaris
Fig. 26: D. sp.
Figs. 27-28: pentaradiate forms (D. calcaris / neorectus?).
Fig. 29: triradiate form.
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Plate 15: Association 11 (NN11 lower).
Type level: Well-3, 2040 m.
Figs. 1-5: Discoaster misconceptus
Figs. 6, 19-20: D. brouweri ?
Figs. 7-18: D. neorectus
Figs. 21-30: D. berggrenii
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Plate 16: Association 12 (NN11 "middle").
Type level: Well-3, 1960 m.
Fig. 1: Discoaster sp.
Figs. 2-10: D. brouweri (fig. 6: D. neorectus ?).
Figs. 11-23: mixed D. berggrenii and D. quinqueramus
Figs. 24-29: D. misconceptus
Fig. 30: quadriradiate form.
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Plate 17: Association 13 (NN11 upper).
Type level: Well-3, 1420 m.
Figs. 1-3: Discoaster surculus et , typical forms.
Figs. 4-5: D. sp. aff. surculus, similar to figs. 1–3 but the trifurcation is not clearly visible and the central spine appears to be missing.
Fig. 6: D. brouweri
Fig. 7: D. sp.
Figs. 8-9, 14, 19-20, 30: D. berggrenii
Figs. 10, 12: D. quinqueramus
Figs. 11, 13, 15-16, 18, 26-29: forms intermediate between D. quinqueramus and D. berggrenii.
TAXONOMIC APPENDIX
Discoaster species in alphabetical order of species names
Discoaster adamanteus et
1967a
Discoaster archipelagoensis et 1976
Discoaster aulakos 1967
Discoaster bellus et 1971
Discoaster berggrenii 1971a
Discoaster braarudi 1971a
Discoaster bollii et 1963
Discoaster brouweri
1927
Discoaster calcaris 1967
Discoaster calculosus 1971a
Catinaster calyculus et 1963
Discoaster challengeri et
1954
Catinaster coalitus et 1963
Discoaster deflandrei et
1954
Discoaster druggii et
1967b
Discoaster exilis et 1963
Discoaster formosus et 1971
Discoaster hamatus et 1963
Discoaster intercalaris 1971b
Discoaster kugleri et 1963
Discoaster lidzi 1967
Discoaster loeblichii 1971b
Discoaster misconceptus 1984
Discoaster moorei 1971a
Discoaster musicus 1959
Discoaster neohamatus et 1969
Discoaster nephados 1967
Discoaster neorectus 1971b
Discoaster pansus 1971b
Discoaster pentaradiatus
1927, emend 1984
Discoaster perclarus 1967
Discoaster prepentaradiatus et 1971
Discoaster protoexilis 1984
Discoaster pseudovariabilis et 1971
Discoaster quinqueramus 1969
Discoaster signus 1971a
Discoaster subsurculus 1967
Discoaster surculus et 1963
Discoaster trinidadensis 1967
Discoaster tuberis 1985
Discoaster variabilis et 1963