Plio-Pleistocene Boundary in Jakarta :
Eko Yulianto 1,*
L.M. Hutasoit 2
H.
Pindratno 3
W. S. Sukapti 4
K.
Hirakawa 5
1 Research Center for Geotechnology, Indonesian Institute of Sciences, Jl. Sangkuriang Bandung 40135 Indonesia.
2 Department of Geology, ITB, Jl. Ganesha 10 Bandung 40132 Indonesia.
3 Mining service, DKI Jakarta, Jl. Raya Jatinegara Timur 55 Jakarta 13310
4 GRDC, Jl. Dr. Junjunan 236 Bandung 40174 Indonesia.
5 Graduate School of Environmental Earth Science, Hokkaido University, Sapporo-shi, Kita-ku, Kita 10, Nishi 5, Japan 060-0810
Abstract
Plio-Pleistocene
boundary in Jakarta has been reviewed based on Palynology and Foraminifera
data. This review involves palynological data of the Blok-M, the Meruya and the
Tongkol cores and existing Foraminifera data, which provided in the report of
Dinas Pertambanngan DKI Jakarta (1997) and LGPN-LIPI team (1983). Based on
those data it is known that boundary of Plio-Pleistocene is varied in depth. In
Blok-M core the boundary lies on about 162 m depth, which characterized by
disappearance of Stenochlaeniidites
papuanus above 162 m level. This
disappearance coincides with Graminae abundance that shortly continued above
the boundary. The occurrences of Stenochlaeniidites
papuanus and Podocarpus imbricatus in the sample at 68.00 m in the Meruya core
imply that the age of the sample might be included within Plio-Pleistosen so
that the boundary lies on 68 m in this core. Meanwhile Globigerina cf. venezuelana in sample 79,70 m of the Situ Babakan core
shows the boundary in this core lying on 79,70 m at the most since the
disappearance of this species took place at Middle Pliocene. In the Tongkol
core the boundary probably lies on shallower depth than 227 m that indicated by
the occurrence of Globigerinoides
obliquus. In the Cengkareng core
the occurrence of Globigerinoides obliquus and Globigerinoides ruber in sampel 105-106 m refers to the age of
sample not older than Early Pliocene and showing that boundary of
Plio-Pleistocene lies on shallower depth than 105 m. Depth variation of the
boundary possibly reflects relief of basement due to structural setting of the
area.
Keywords: Plio-Plistocene, boundary, palynology, foraminifera, Jakarta,
1. Introduction
Plio-Pleistocene boundary in Jakarta is still obscure. Research on this subject has been being going on since the boundary and its characteristics are important information in planning and development of the area mainly due to environmental carrying capacity and natural resources assessment. From previous research it was assumed that the boundary lies at about 300 m depth and becomes lower to the south (Soekardi & Purbohadiwidjojo, 1975). A preliminary stratigraphic subdivision based on water-well logs has also been established for the Jakarta artesian basin, in which Quaternary lithologic units have been distinguished. The thickness of the Quaternary in the Jakarta artesian basin generally exceeds 250 m and locally exceeds 300 m (Soekardi and Koesmono, 1973). Attempts have been made to correlate the stratigraphic units with the eustatic rise and fall of the sea-level during the Pleistocene. Because of the absence of radiometric and biostratigraphic dates, however, these attempts are not reliable. Core profiles taken by Dinas Pertambangan DKI Jakarta (LPM ITB, 1997) show some volcanic layers at levels shallower than 300 m. These profiles appear to be similar to the Tertiary Genteng Formation which outcropped around Jakarta. This paper presents a study on Plio-Plistocene boundary on the basis of palynological analysis. Some foraminifera data were also reviewed to examine wider regional setting of the boundary.
2. Method
The samples for palynological analysis were taken from three cores Blok-M, Tongkol and Meruya (Figure 1). Four samples collected from Blok-M core at 162.5 m, 153.45 m, 152.3 m and 36.6 m depths; seven samples from Tongkol core at 116.2 m, 124.4 m, 143.5 m, 159.8 m, 174.0 m, 188.6 m and 234.6 m; six samples from Meruya core at 48.0 m, 68.0 m, 117.0 m, 134.0 m, 136.3 m and 147.8 m. Those levels were defined, since the Plio-Pleistocene boundary was predicted to stand within the range of those levels.
Samples were then treated with 10% KOH, swirling technique to remove sand, mix of HCl and HNO3, treated with 40% HF. Heavy minerals were removed by using ZnCl2 s.g 2.2 prior to acetolysis treatment. All pollen and spore grains were counted. Ages of samples and the Plio-Pleistocene boundaries are deducted based on the occurrence of index pollens. Frequencies for all pollen and spore types were calculated on the total counts of pollen grain and presented in pollen diagrams (figure 2, 3, 4). There was no new sample treatment for foraminifera analysis. Foraminifera data were obtained from the report of Dinas Pertambangan DKI Jakarta (LPM-ITB, 1997) and LGPN-LIPI team (Hehanussa & Djoehanah, 1983). These two reports provides un-analyzed planktonic foraminifera data concerning on age analysis. These data will be reviewed to determine ages of samples and the Plio-Plistocene boundaries.
Fourty-four types of fossil pollen and spores are recognized in this core. Fourty taxa are identified to genera or families, and 4 types are difficult to identify exactly. Although most of taxa present virtually, some of them such as Avicennia, Rhizophora, Sonneratia alba and Achrostichum of mangrove elements and Engelhardtia, Quercus, Castanopsis/Lithocarpus, Macaranga, Graminae, Cyperaceae have substantial values. However their occurrences are not consistent in all samples. In the lowest sample at 162.5 m, Graminae and Cyperaceae dominate the pollen composition with their total value more than 74%. Avicennia and Macaranga have also relatively high values. Among the ferns, Acrostichum has the highest value up to almost 4%. Stenochlaeniidites papuanus, which is absent from modern mangrove ecosystem in western Indonesia, also presents in this core. Although Graminae and Cyperaceae decrease in the sample at 152.3 m, their total values remain high. The value of Avicennia declines, while the mangrove element inclines following the progressive increasing of Rhizophora. Some montane taxa such as Quercus and Castanopsis/Lithocarpus also show significant values. Chenopodiaceae identified only presents in this sample has a high value up to almost 13%. The highest value of Macaranga and Acrostichum also take place in this sample. Rhizophora and mangrove are strongly represented in the sample at 153.45 m. Engelhardia value inclines and reaches its maximum in this sample. In contrast, Quercus, Castanopsis/Lithocarpus and Macaranga values are lower than those of the lower samples. Graminae value falls to less than 2% and Cyperaceae is absent. Among the ferns, Stenochlaena areolaris and Acrostichum show substantial representations. The uppermost sample of the Blok-M core at 36.6 m bears a few palynoforms, less than 30 grains. Spores dominate the sample to 67%. Graminae, Cyperaceae, Macaranga and Calamus represent pollen elements. An index fossil, Stenochlaeniidites papuanus, presents in the sample at 162.5 m.
The diversity of Palynoform in the Meruya core is higher than in the Blok-M core. It bears 53 taxa consisting of 43 pollen and 10 spores, identified to genera or families, and 8 types are difficult to identify exactly. Different from the Blok-M core that just bears a few taxa in substantial values, most of taxa in this core have sporadically substantial value in any particular levels of samples. The lowermost sample at 147.8 m is characterized by high value of spores particularly Asplenium and Polypodiaceae. Among the pollens Graminae, Sabal type and Potamogeton have high values. Mangrove element is definitely absent from this sample. Some other taxa significantly present are Cyperaceae and Macaranga. Graminae, Sabal, Eugenia and Potamogeton disappear from the pollen assemblage of sample at 136.3 m while spores in common and particularly Asplenium and Polypodiaceae substansially decline than those in the sample at 147.8 m. Macaranga, Rubiaceae, Castanopsis/Lithocarpus slightly increased. Excoecaria, Blumeodendron, Canthium, Elaeocarpus, Palmae, Verbenaceae, Compositae, Podocarpus sp, Stenochlaena areolaris are among the taxa which absent in the sample at 147.8 m. Mangrove element is virtually present and represented by Sonneratia caseolaris. Two index fossils present in this core namely Stenochlaeniidites papuanus and Podocarpus imbricatus. Stenochlaeniidites papuanus presents in the sample at 117.4 m and 68.0 m while Podocarpus imbricatus presents in the sample at 68.0 m.
The
Tongkol core bears 42 taxa consisting of 28 pollen taxa and 8 spores identified
to genera and families and 6 types difficult to identify. Spores dominate the
pollen assemblages with the values more than 60% in all samples. However total
palynomorf in all samples are less than 200 grains. The highest total pollen
and total spores are 120 and 66 grains consecutively, both in the sample at 174.0 m. Even there are only 8 grains of spores present in sample
234.6 m. No index fossil found in this
core.
It was reported that Gs. Obliquus occurs in TKL 227-228 m, Situ Babakan 79.70 m, Cengkareng 105-106 m and Cengkareng 108-109 m. Gs. ruber presents in Cengkareng core at 105-106 m while G. cf. venezuelana presents in Situ Babakan core at 79,70 m (Hehanussa & Djoehanah, 1983; LPM-ITB, 1997).
4.1
Plio-Pleistocene boundary in pollen record
In general pollen and spores contents in the samples does not refer to a definite level of the Plio-Pleistocene boundary. In Java, Plio-Pleistocene boundary is palynologically characterized by the last appearance of mangrove element, Stenochlaniidites papuanus and the first appearance of montane element, Podocarpus imbricatus (Rahardjo et al., 1994). In the Blok-M core Stenochlaeniidites papuanus is present only in the sample at 162.5 m, while Podocarpus imbricatus is absent in all samples. Stenochlaeniidites papuanus is also present in the Meruya core at 68.0 m and 117.4 m, while it is absent in the Tongkol core. Therefore the occurrences of Stenochlaeniidites papuanus and Podocarpus imbricatus in the sample at 68.0 m of the Meruya core imply that the age of the sample might be included within Plio-Pleistosen. The occurrence of Stenochlaeniidites papuanus in the sample at 162.5 m of the Block-M core and its absence from samples 153.45 m, 152.3 m and 36.6 m coupled with Graminae abundance in the sample at 162.5 m and 153.45 m is interesting in relation with Plio-Pleistocene boundary in this core. It is well known that Graminae extensively grows in dry climates condition where the light reaches the forest floor. The absence of Stenochlaeniidites papuanus from samples at 153.45 m, 152.3 m and 36.6 m is possibly an indication that its last appearance took place coinciding with the deposition of sample at 162.5 m. However the single specimen occurrence of Stenochlaeniidites papuanus causes some doubts whether or not it is reworked fossil. Even if it is reworked-fossil, roughly it still refers that the Plio-Pleistocene boundary lies on the depth shallower than 162 m in this core. But we think its single occurrence is the matters of the abundance of the tree in the forest and the distance between the depositional sites to the mangrove forest. Mangrove pollen values are obviously dominant in samples taken from surface sediment of modern mangrove forest (Caratini & Tissot, 1985; Blasco et al, 1973 in Thanikaimoni, 1987; Lorente, 1986). According to Thanikaimoni (1987) surface sediment of modern mangrove forest bears about 50% at the least. The farther the distance from the forest to the sea the lower the value. Thus low value of mangrove element in this sample possibly indicates a distantly depositional environment to the forest. Meanwhile Graminae abundance in those deepest samples is an obvious indication of dry climates, which lasted during their deposition. That last appearance of Stenochlaeniidites papuanus associated with Graminae abundance is a characteristic of Plio-Pleistocene Boundary in Java. Result of Plio-Pleistocene boundary studies at Mojokerto area, East Java (Rahardjo, 1999) shows such similar characteristic. Dry climates of Plio-Pleistocene period gave a chance for Graminae to grow extensively. As in those two areas, in the Blok-M core Graminae abundance continued shortly after the disappearance of Stenochlaeniidites papuanus. Therefore the Plio-Pleistocene boundary may be within 153-163 m level. Abundant mangrove pollen in samples of 162.5 m, 153.45 m, and 152.3 m is obvious in this core. Rhizophora, Avicennia and Acrostichum aureum indicates mangrove depositional environment and its surrounding areas. Because those samples were taken from more than 150 m depth, sea-level should stand at about that level once samples deposited. It means sea-level was greatly lower than today. If it was not, great subsidence after deposition should have taken place. Global sea-level curve shows Plio-Plistocene as one of four peaks of glacial periods in Quaternary. Therefore the abundance of mangrove pollens may ascribe to sea-level drop in this instance. This condition supports to the possibility of Plio-Plistocene boundary standing at about 162 m in Blok-M core. Therefore it is concluded that Plio-Plistocene boundary lies on about 162 m depth in Blok-M core.
4.2
Review of foraminifera data
The occurrence of Gs. Obliquus in TKL 227-228 m, Situ Babakan 79.70 m, Cengkareng 105-106 m and Cengkareng 108-109 m implies to the ages of the samples are not younger than Early Pliocene in respect to Bolli et al (1985). The occurrence of Gs. obliquus and Gs. ruber in samples of the Cengkareng core is an indicative of Early Pliocene for 105-106 m sample. So that Plio-Plistocene boundary lies at a depth shallower than 105 m in Cengkareng core. Meanwhile Gs. obliquus and G. cf. venezuelana which occur in sample of Situ Babakan 79,70 m is indicative for Middle Pliocene age. So Plio-Plistocene boundary of Situ Babakan core should lye at a depth shallower than 79,70 m. In TKL 227-228 m sample Gs. obliquus is the only index fossil. It is concluded the Plio-Plistocene boundary lies shallower than 227 m in TKL core.
4.3 Implication
on south-north section of Jakarta
Pollen
and foraminifera data show us that Plio-Pleistocene boundary in Jakarta does
not lye on a similar level nor simply sloping in a particular direction. It
varies in depth place to place (table 1). Structure configuration in which
Jakarta and its surrounding areas have been evident to be in low and high
structures shows that the basement configuration may control in some parts to
this boundary level undulation. Jakarta basin is controlled by NE-SW and
NW-SE structures. In southern part, Tertiary isopach contour shows ESE trends
with some NS faults cross on it. Blocked-structures developed in ENE-WSW trends
in northeastern part (Suwiyanto,
1978; Soekardi and Purbohadiwidjojo, 1979; Subagio and Untung, 1994; Suwiyanto,
1997). Pollen and foraminifera data
lend supports to that structural configuration. Considering the position of the
cores, a north-south section puts the cores respectively as follows: Cengkareng core, Tongkol core, Meruya
core, Blok M core and Babakan core. Reconstruction of Plio-Pleistocene
boundaries in this section allow recognizing two low and two high structures.
Babakan and Meruya cores stand on low structures while Blok M and possibly
Tongkol and Cengkareng cores stand on high structures (Figure 5). The boundary
in those cores lies at the levels less than 300 m. This contradicts argument
that the boundary in general lies at the depths more than 250 m and locally
more than 300 m as argued by Soekardi and Koesmono (1973). It also rejects previous argument that
Plio-Pleistocene boundary in Jakarta sloping and deepening to the north
(Soekardi & Purbohadiwidjojo, 1979). Although basement configuration
may ascribe to be the main factor in the boundary undulation, quaternary
tectonic also seems to contribute some influences. The occurrences of wood
fossils dated 38.000 yr BP in the river terrace sediment, 25 m above the modern
river surface at Depok and some marine terraces (LPM-ITB, 1997) may indicate
quaternary tectonic uplift. Some seismicities in the last decades lend support
to the possibility of quaternary tectonic uplift that may influence the
boundary undulation by fault reactivation. On the other hand compaction and
sediment consolidation due to over pumping of ground water and loading by
surface construction may be the other considerable factors.
6. Conclusion
In four cores i.e. Cengkareng, Situ Babakan, Tongkol, Meruya
and Blok-M the Plio-Plistocene boundary varies in depth. As a whole the
boundary lies shallower than 300 m, a level that previously suggested being the
Plio-Pleistocene boundary in Jakarta. In Tongkol,
Cengkareng and Situ Babakan cores the Plio-Pleistocene boundary could not be
defined exactly but it lies respectively within the depth shallower than 227 m,
105 m and 80 m. While in Meruya and Blok M cores the boundary respectively lies
on 68 m and 162 m. The undulation of the boundary may ascribe mainly to
basement configuration and quaternary tectonic as well.
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* Coresponding author. Present address Laboratory of Geoecology, Graduate
School of Environmental Earth Science, Hokkaido University; Kita-ku, Kita 10,
Nishi 5, Sapporo 060-0810, Japan; Fax:+081-11-7064867; e-mail:[email protected]