Nannofossil Distribution and Age of Kendeng Zone In Kalibeng River Section of Kedungringin , Plandaan Area , Jombang , East Java

Abstract This study was carried out at Kalibeng River Section, in Kedungringin area, a site with lithology suitable for a nannoplankton research. Methods used in this research include performing a stratigraphical measurement section at the field, collecting thirty-two samples, and preparing the samples with the smear slide method using a polarizing microscope with 1000x magnification, and preparing several rock samples using SEM analysis. The analysis of nannofossil resulted in twelve genera and forty-three species. The identified genera are: Calcidiscus, Coccolithus, Ceratolithus, Discoaster, Gephyrocapsa, Helicosphaera, Pseudoemiliania, Reticulofenestra, Rhabdosphaera, Sphenolithus, Syracosphaera, and Umbilicosphaera. The presence of these genera indicates that Kalibeng River Section have abundant nannofossils and based on the nannofossil analysis, from older to younger, the studied stratigraphic sequence indicates a more detailed age determination as follows: the Marl Unit of Kalibeng is NN10-NN18 (Middle Miocene to Pliocene), Calcareous Sandstone Unit of Sonde is NN19-NN20 (Pliocene-Pleistocene), and Calcareous Claystone Unit of Sonde is NN20-NN21 (Pleistocene).


Background
Research of fossil is one of the most important tools for the study of sedimentary rocks and basins. First, the succession of evolutionary appearances and extinctions provides age control, which is critical for understanding basin evolution and validation of geological concepts. Second, fossils are useful for interpretation of local paleoenvironment, paleoclimate, and also reconstruction of paleobiogeographic (Van Gorsel et al., 2014).
The stratigraphic analysis shows that the Kendeng Zone is a young fold-thrust belt, which was only active within the Pliocene. In this zone, deep marine volcaniclastic sediments of Early Pliocene and older ages have been uplifted and thrusted northwards since mid-Pliocene time (Lunt, 2013, in Berghuis et al., 2019. The Kendeng Zone is the southern part of East Java basinal system. The sedimentary evolution of this basin indicates that the East Java basinal system was formed as a continuation of extension around the Makassar Strait to the east, and not as a back-arc basin. This interpretation is based on tectonically pulsed east to west trend in crustal thinning, subsidence, and trans-gression (Lunt, 2019) The studied area is occupied by a sedimentation sequence consisting mostly of fine-grained marine sedimentary rocks that contain abundant microfossils with the age from Miocene to Quaternary. A nannoplankton research in the Kendeng Zone has been done by Choiriah et al. (2001), but only limited to the Bengawan Solo River in Ngawi. The results of previous study in Ngawi section show that the nannofossil contained within Kalibeng Formation consists of eighteen genera and forty-nine species, while Klitik/ Sonde Formation has eight genuses and twelve species. The age of Kalibeng Formation is Late Miocene to Late Pliocene (NN12-NN18), and Klitik (Sonde) Formation is of Early Pleistocene (NN19-NN20) The purpose of this study is to determine the distribution and the age of the youngest marine sediments in Kendeng Zone based on nannofossil found in the Kalibeng River Section, and to correlate it to the similar result of the Ngawi Section.

Stratigraphic Setting
The studied area, which is part of Kalibeng River in the Kedungringin and surrounding area, Plandaan Subregency, Jombang, East Java, is situated in the Kendeng Zone. This part of the river is classified as the type locality of Kalibeng Formation since it is located in the Kalibeng River. According to Pringgoprawiro (1983), the order of the lithostratigraphic formations in the Kendeng Zone, from the oldest to the youngest is as follows: Pelang Formation (Tomp), composed of marl and claystone with intercalation of calcarenite; Kerek Formation (Tmk), comprising interbedded marl, limestone, and sandstone; Kalibeng Formation (Tmpkk), consisting of marl with intercalations of sandstone {with Atasangin Member (Tmpa) made up of breccia, sandstone, and claystone}; Sonde Formation (Tpso), composed of marl, calcareous claystone, tuff, and sandstone {with Klitik Member (Tpk) consisting of limestone}; Pucangan Formation (Qtp), composed of tuffaceous breccia, conglomerate, and tuffaceous Sandstone; Kabuh Formation (Qpk), consisting of coarse tuffaceous sandstone with cross-bedding sedimentary structure, claystone, and conglomerate; and Notopuro Formation (Qpn), comprising tuff, interbedded with tuffaceous sandstone, volcanic breccia, and conglomerate ( Figure 1).
Sedimentary rocks found in the studied area contain a large amount of microfossils, including nannoplankton which is very valuable to support the objective of this study. Nannofossil is a kind of algae which belongs to the Haptophyceae class with a tiny size (0.5-10 µm). In the living state, this organism has planktonic characteristics.

Field Data Sampling
This step consists of making a measurement section and detailed rock sampling to better understand the changing and development of nannoplankton, as well as determining the age of each rock layer ( Figure 2).
A large amount of nannofossils was collected from a small sample due to its tiny size. Rock samples that contain the fossils can be obtained from outcrop, drill core, or cutting.
Nannoplankton is very small to identy macroscopically, that is why an optical (polarizing) microscope with a high magnification (1000 X), or even Scanning Electron Microscope (SEM) is needed. The nannofossil analysis was performed using a polarizing microscope by making "smear slides" that could be prepared in a few minutes.
The nannofossil analysis was done using a polarizing microscope with 1000x magnification, in parallel and cross Nicol views. The

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photomicrographs of fossils were taken using a microscopic camera (Moticam) connected to a laptop or computer. The names of genus and species were referenced from previous researchers (Martini, 1971;Okada and Bukry, 1980;Perch-Nielsen, 1985;Aubry, 1985). The appearance of nannofossil species in each rock sample is the main data of this study. The photomicrographs of nannofossils within the researched area were taken using a polarizing microscope in Petrographic Laboratory UPN "Veteran" Yogyakarta and Scanning Electron. Microscope of Sedimentology Laboratory, ITB.

Result and Analysis
Stratigraphically, the Kalibeng River Section in the studied area is composed of Marl Unit of Kalibeng Formation, Calcareous Sandstone Unit  of Sonde Formation, and Calcareous Claystone Unit of Sonde Formation (Figure 3). The analysis of nannofossil in the studied area reveals that within a total of thirty-three samples of rock thirteen genera and forty-three species of nannofossil (Table 1 and Figure 4) are identified.
The nannofossils found in Marl Unit of Kalibeng Formation are represented by sample D.1 to D.24 consisting of twelve genera with thirty-nine species (Table 1)        The occurrence result of twelve genera and forty-three species indicates that the age of Marl Unit of Kalibeng is NN10-NN18, Calcareous Sandstone Unit of Sonde is NN19, and Calcareous Claystone Unit of Sonde is NN20-NN21, according to the classification of Martini (1971).
The index fossil used to indicate age in this research is the first and t h e last appearance of species Ceratolithus acutus, Discoaster asymmetricus, Pseudoemiliana lacunose, Discoaster surculus, and Sephyrocopsa oceanica.

Discussion
The studied area belongs to the type locality of the Kalibeng Formation in Kendeng Zone, with the order of stratigraphy from older to younger consists of Marl Unit of Kalibeng Formation, Calcareous Sandstone Unit, and Calcareous Claystone Unit of Sonde Formation (Figure 3).
Result of the previous study carried out by Choiriah (2001) in Bengawan Solo River of Ngawi Section, shows the presence of nineteen genera and fifty-two species. Kalibeng Formation contains eighteen genera and forty-nine species, while Klitik and Sonde Formations have eight genera and twelve species. Kalibeng Formation is of Late Miocene to Late Pliocene (NN12-NN18) in age while, Klitik (Sonde) Formation is of Pleistocene (NN19-NN20) ( Table 2).
Results of the study from Kalibeng River Section of Kedungringin, Plandaan Area, Jombang, indicate the occurrence of twelve genera and forty-three species. Kalibeng Formation (Marl Unit) containing twelve genera and thirty-nine species, sends to indicate that the age of this formation ranges from Middle Miocene to Late Pliocen (NN10 to NN18).
While Sonde Formation (Calcareous Sandstone Unit) is of Pleistocene (NN19) and Calcareous Claystone Unit of Klitik Formation is Pleistocene (NN20 to NN21) in age (Table 2 and Figure 5).
The results of the study show the differences in age, especially for the Sonde/Klitik Formation. Based on the nannofossil analysis, the Klitik/ Sonde Formation is of Pleistocene, which is younger than the age based on the foraminifera analysis (Pliocene). The correlation of the same formation based on foraminifera biostratigraphy with nannofossil is shown in Figure 5.

Conclusion and Recomendation
The analysis results conducted during this study identify the presence of twelve genera and forty-three species, showing this section has abundant nannofossils. The age of Marl Unit of Kalibeng Formation is NN10-NN18 (Middle Miocene to Pliocene), Calcareous Sandstone Unit of Sonde Formation is NN19-NN20 (Pleistocene), while the age of Calcareous Claystone Unit of Klitik Formation is NN20-NN21 (Pleistocene).
The distribution of nannofosil in the Kalibeng River, Jombang, indicates fewer genera and species than that of Bengawan Solo River, Ngawi. Kalibeng River, Jombang, has twelve genera and forty-three species, while the Bengawan Solo River, Ngawi Section, has nineteen genera and fifty-two species.
The correlation results show that the age of Kalibeng Formation in Jombang is older, indicating NN10−NN18 (Middle Miocene to Late Pliocene) compared to the Kalibeng Formation in Ngawi which is NN12 to NN18 (or Late Miocene to Late Pliocene).