The Eocene to Oligocene boundary and paleoclimatic indications based on calcareous nannofossils of Tonasa Formation, South Sulawesi, Indonesia

The biostratigraphy of the Tonasa Formation in the Jeneponto Regency of South Sulawesi, Indonesia, is still poorly known, and there are barren ages, such as much of the Oligocene to Early Miocene. The Tonasa Formation is well exposed along the coast of the Jeneponto Regency, in which the Karama area consists of the most important outcrops of this formation which in this area consists of interbedded marl and limestone. Our study focuses on the biostratigraphy of the Karama area section A based on nannofossil. Samples were collected by measured stratigraphy methods and then subjected to investigation using smear slides. The assemblages of species were determined by semiquantitative analysis. Data analysis obtained three nannofossil datums (boundaries): The First Occurrence (FO) of Sphenolithus pseudoradians NP19/NP20), the First Occurrence of Sphenolithus distentus (CP.16/CP.17), and the Last Occurrence (LO) Sphenolithus predistentus (NP.23/NP.24. The zonal boundary was determined based on calcareous nannoplankton; the Late Eocene to Middle Oligocene boundary of the Tonasa Formation was found in this section. Interestingly, throughout this period, the marker species in this section is Sphenolithus . In addition, the presence of Sphenolithus , Discoaster , and Zygrhablithus bijugatus indicated that the basin was in warm water condition.


Introduction
Indonesia is located between three major plates: the Pacific Plate from the east, the Indo-Australia Plate moving to the north, and the Eurasia Plate, which is relatively passive.They have been actively moving, have caused complex geological conditions, and they have strongly influenced the geological history.One of the results of the collision of the three plates is the formation of Sulawesi Island with its unique K-shaped outline.The consequences of this geological condition are reflected in the stratigraphic setting that is found on Sulawesi Island.
The stratigraphic sequence of the southern arm of Sulawesi is from the Late Cretaceous to the present (Van Leeuwen 1981;Sukamto 1982).One of the widely studied formations is the Tonasa Formation that developed during the Cenozoic era.This formation has significant benefits and is one of the most beautiful karst topography in the world.However, its stratigraphic succession has not been recorded in some area.For example, the Oligocene to Early Miocene strata are poorly exposed in the Jeneponto area (Wilson and Bosence 1997), although the carbonate succession in this area ranges from the Late Eocene to the Middle Miocene (Sukamto and Supriatna 1982).and from the Middle Eocene to the Early Miocene based on nannofossil assemblages in the Barru area (Farida et al. 2022a).A stratigraphic correlation of the Tonasa Formation was obtained between the Pangkajene and Jeneponto areas.
The age of the Tonasa Formation in Karama area is latest Eocene (P17) based on planktonic foraminifera (Supardi and Barianto 2017).Preliminary studies documented the abundance and diversity of nannofossils in the Karama Traverse (B) area (Farida et al. 2022b).Other sites of the Tonasa Formation have a good stratigraphic succession record.Therefore, we were interested in examining the Eocene/Oligocene boundary based on calcareous nannofossil assemblages in the Jeneponto area.High-resolution biostratigraphy with nannofossils can provide the ages of rocks with higher precision and is one of the most powerful biostratigraphical tools in carbonate sediments (Agnini et al. 2017).This applies also to the investigation of the paleoenvironment, paleoclimate, paleoceanography, and other aspects (Perch-Nielsen 1985;Persico and Villa 2004;De Vargas et al. 2007;Villa et al. 2008;Ali 2009;Sato and Chiyonobu 2009).The biozonation of calcareous nannofossils during the Paleogene has been proposed by some authors.For this study area, we used calcareous nannofossils zonation as proposed by Martini (1971), and Okada and Bukry (1980).The Tonasa Formation consists of interbedded limestone and Globigerina marl (Sukamto and Supriatna 1982).It is well-exposed in the Karama area, which we call the Karama A and Karama B sections.The study area in Karama A section (Fig. 1) mainly consists of interbedded limestone and marl.
One of the most important components of carbonate rock is nannofossils, which were primary producers.They are useful as a tool for determining the biostratigraphy, paleoceanography, and paleoclimate of marine sediments (Perch-Nielsen 1985;De Vargas et al. 2007;Sato and Chiyonobu 2009).The distribution patterns of nannoplankton are strongly related to surface water temperature and nutrients (Imai et al. 2015;Kanungo et al. 2017).The Cenozoic climate development is characterized by the transition between Greenhouse and Icehouse conditions, particularly at the Eocene/Oligocene boundary (Zachos et al. 2001;Fornaciari et al. 2010).Climatic changes in the Cenozoic are accompanied by tectonic and biotic events.The most obvious change is the temperature decrease during the Eocene -Oligocene, which caused a decrease in biodiversity (Berggren and Phrotero 1992;Zachos et al. 2001).A major deterioration in global climate occurred through the Eocene and Oligocene, which was characterized by long-term cooling in both terrestrial and marine environments (Villa et al. 2008).In addition, the presence of certain species indicated a specific water condition.For example, Discoaster was considered an indicator of warm oligotrophic waters, being a lower photic zone species (Bukry 1973;Aubry 1992;Farida et al. 2012;Imai et al. 2015).

Geology of the study area
Carbonate rocks are widely distributed in the southern arm of Sulawesi (Fig. 2), thereby indicating that this area was under marine conditions.The carbonate platform was dominated by foraminifera and a ramp-type southern margin, and subsidence was the dominant control of accommodation space on the Tonasa Carbonate Platform (Wilson and Bosence 1997).The Tonasa Formation is one of the most widely distributed and has a thickness of 1750-3000 m (Sukamto 1982;Sukamto and Supriatna 1982).
Regionally, the Tonasa Formation is composed of partly layered and massive limestone, coral bioclastic, and calcarenite with Globigerina marl intercalation (Sukamto and Supriatna 1982).This formation discordantly overlies the older volcanic sediments of the Camba Formation (Sukamto 1982;Sukamto and Supriatna 1982).Carbonate platforms in the north and south of South Sulawesi are separated by the Camba Formation.
Stratigraphically, the southern part of Sulawesi is composed of rock formations from the Mesozoic to the Cenozoic.Tertiary-aged rocks are most widely distributed in this area.The Tertiary stratigraphy of the western part of South Sulawesi is divided into (1) the Tonasa Formation that was deposited interfingering with the Early Eocene Malawa Formation, and (2) the Camba Formation that was deposited above the Tonasa Formation during the Middle to Late Miocene.Carbonate development was terminated by the influx of volcaniclastic materials.In the eastern part of South Sulawesi, the Tonasa Formation interfingered with the middle part of the Salo Kalupang Formation around the Middle Eocene, and an unconformity was found at the upper part of the Tonasa Formation with respect to the younger rock formations (Sukamto 1982;Sukamto and Supriatna 1982;Wilson and Bosence 1996) (Fig. 3).The study area is situated in the southernmost part of the Tonasa Formation and is included in western of South Sulawesi, where spot-like outcrops are found in the Jeneponto area.Wilson and Bosence (1996).

Samples and methods
A systematic calcareous nannofossil analysis was conducted starting from field data collection, sample preparation, and determination of the different species composing the assemblage.Samples were collected at each layer of the Karama A section by using measured stratigraphy methods at interbedded limestone and marl.A total of 23 layers were sampled and prepared using the smear slide method with a cover glass of 24 mm × 24 mm in size.Observation under a polarized microscope with 1000× magnification was carried out to recognize the species present in the assemblage (Bown 1999; Farida at al. 2019).The age was determined based on the First Occurrence (FO) and the Last Occurrence (LO) of marker species, following the standard zonation by Martini (1971), Okada and Bukry (1980), and also datum by Perch-Nielsen (1985).The paleotemperature could be analyzed based on the presence of species known to flourish under a specific climate.The semi-quantitative method was used to obtain the nannofossil abundance based on Kapid and Suprijanto (1996), using the four categories scheme: Abundant (> 15%), Common (10% <n<15%), Few (1%<n<10%), and Rare (<1%).biostratigraphy based on analysis of FO and LO and paleotemperature identification is discussed in the following paragraph.

Biostratigraphy
Biozonation schemes were used to determine biostratigraphy of the Tonasa Formation in the Karama A section on the basis of calcareous nannofossils from the NP zonation of Martini (1971), the CP Zonation of Okada-Bukry (1980), and the age correlation of Perch-Nielsen (1985) to examine the age of calcareous nannofossils (datums).The results of the biostratigraphy of the study area are (Fig. 6) as follows:

Paleoclimatic Indication
Calcareous nannofossils are known as a good tool to reconstruct paleoclimate, paleoenvironment, paleoceanography, or paleoecology.The presence of calcareous nannofossils that live in a typical climate indicates the climatic conditions when these rocks were deposited.For instance, Discoaster, Sphenolithus, and Zygrhablithus bijugatus, typically lived in warm water conditions.These species are present and almost abundant from the bottom to the top of the Karama A section, although their diversity declined and some species decreased in abundance.
As mentioned above, Discoaster is one of the typical species that lived in warm water.In the study area, Discoaster is observed almost all throughout the Karama A section, even though they are not abundant, and the numbers tended to decrease and finally disappeared (Fig. 5A).However, the presence of these species indicates that the basin was under warm water conditions and associated with the lower photic zone.Besides that, a few to rare Coccolithus pelagicus are also present throughout this section, and the low abundance of this species indicated warm water conditions.

Discussion
A previous study investigated the biostratigraphy of the Tonasa Formation in the Karama area using planktonic foraminifera and identified a Late Eocene (Supardi and Barianto 2017).The determination of the calcareous nannofossil datum may use different approaches.The present study refers to the zonation proposed by Martini (1971) and Okada and Bukry (1980) and also refers to the age correlation proposed by Perch-Nielsen (1985).Therefore, the age of the Tonasa Formation in the Karama area (section A) from the calcareous nannofossils is Late Eocene -Middle Oligocene.Wilson and Bosence (1997) reported that Oligocene strata of the Tonasa Formation in the Jeneponto area were not known.Beside that, the age determination of the Tonasa Formation using nannofossils has already been conducted in Barru area, and yielded a Middle Eocene to Early Miocene age (Farida et al. 2022a).From the

A B
results of this study, we will develop our understanding, knowledge, and research experience, especially in placing bioevent datums into the framework of the stratigraphic sequence in this area.Therefore, the Tonasa Formation in this area is located approximately in the middle part of the Tonasa Formation in the regional stratigraphic framework.
The calcareous nannoplankton shows a clear latitudinal distribution, related to the specified tolerance at different temperatures (Malfino and McIntyre 1990;Melinte 2004).As previously mentioned, previous researchers found that global climate cooling occurred through the Eocene and Oligocene.These events occurred globally and affected marine organisms' life when the Tonasa Formation was deposited.Although the changes of nannofossil assemblages in the study area did not show a sharp collapse, the trend of assemblages shown in the number of species and specimens tended to decrease in Oligocene compared to the Eocene (Fig. 5A, B).Some Discoaster and Sphenolithus are poorly preserved, which is why determining the species is difficult.Therefore, preservation has an effect on species quantification.Additionally, diagenetic processes such as overgrowth also make the identification of species impossible.The existence of Discoaster as a typical warm water species is important for the reconstruction of seawater temperature.However, it is not found in all layers and is not abundant.Therefore, we assume that the conditions of the studied area experienced a decrease in temperature, thus reducing the number of Discoaster individuals.Coccolithus pelagicus as typical cold-water species (Floresh et al. 2005) became rare to absent in low-latitude regions (Sato et al. 2004), the distribution of this species shown in Fig. 5A.Sphenolithus are generally found common to abundant in low to middle latitudes (Fornaciari et al. 2010).Sphenolithus and Zygrhablithus are Oligotrophic taxa, present under warm-water conditions (Aubry 1998;Agnini et al. 2007;Villa et al. 2008).In addition, Zygrhablithus bijugatus, Sphenolithus, and Discoaster show warm water affinity, and they became less abundant during the Oligocene.However, they increased again in the upper part of the Middle Oligocene (Fig. 5A, B).This event shows that despite the warm carbonate environment, the temperature decreased, which caused a decrease in the number of species, both in the diversity and in the number of specimens.Therefore, we concluded that water cooling occurred.

Conclusion
In this study, we identify three calcareous nannofossil datums, which are the FO of Sphenolithus pseudoradians (NP.19/NP.20),FO of Sphenolithus distentus (CP.16/CP.17),and LO Sphenolithus predistentus (NP.23/NP.24).The age of the Tonasa Formation in the Karama area (section A) is Late Eocene to Middle Oligocene.The diversity and number of specimens tend to decrease from the Eocene to the Oligocene but some increased again in the upper part of the Middle Oligocene, i.e.Discoaster, Sphenolitus, and Zygrhablithus, and the presence of these species indicate that the climate was under warm water to water cooling conditions through the Late Eocene to the Middle Oligocene.

Figure 1 .
Figure 1. A. Map of South Sulawesi-Indonesia.B. Map showing the location of the study area, Karama A section (from Bakosurtal 1991).C. Outcrop of the Tonasa Formation composed of interbedded between limestone and marl.

Figure 2 .
Figure 2. Geological map of the south arm of Sulawesi (modified from Wilson and Bosence 1996).

Figure 3 .
Figure 3. Stratigraphic comparison between western South Sulawesi and eastern South Sulawesi.The Tonasa Formation was deposited from the Middle Eocene to Early Miocene in western South Sulawesi.Modified fromWilson and Bosence (1996).

Figure 5 .
Figure 5. A. Distribution of nannofossils from bottom to the top of the section, some species became extinct and others appeared.B. Distribution of calcareous nanofossils from bottom to top of the section, some species disappearing and others appearing during the Oligocene.

Figure 6 .
Figure 6.Biostratigraphic column of the Karama A section showing calcareous nannofossil datums from the Late Eocene to the Middle Oligocene.Scale bar: 10 μm.