1 The Indication of Mid Miocene Climatic Optimum (MMCO) from Cibulakan Formation, 2 Bogor Basin, Indonesia 3 R. Kapid1), W.D. Santoso1), B. Ikhsan1), M.A. Jambak2), and D.E. Irawan1)* 4 1) Department of Geology, Institut Teknologi Bandung, Indonesia. 5 2) Department of Geology, Trisakti University, Indonesia 6 *Correspondence author: Dasapta Erwin Irawan ([email protected] cc 7 [email protected]) 8 Abstract 9 Nannoplankton analyses have been proven to identify rapid climate shifts in sedimentary records. Here, 10 we took 58 samples from Early Miocene to Late Miocene sediments of Cibulakan Formation (Bogor 11 Basin, Indonesia), to evaluate climatic benchmarks for Middle Miocene Climate Optimum in the tropical 12 region. The Helicosphaera carteri and Umbilicosphaera jafari were counted to identify the salinity signal. 13 We identify seven-biostratigraphy zones of Cibulakan Fm, indicating the change of seawater temperature 14 and salinity. We identify that warmer temperature in Middle Miocene as the effect of MMCO. The 15 temperature increased until Late Miocene triggered by global increasing temperature at Pacific Ocean and 16 widely distributed flow of terrestrial water at North West Java Basin. 17 Keywords: Mid Miocene Climatic Optimum (MMCO), nannoplankton, temperature, salinity, Cibulakan 18 Formation 19 20 Introduction 21 Mid-Miocene Climatic Optimum (MMCO) was a global climatic event during Middle Miocene featuring 22 increasing global temperature (You et al., 2009; Hansen et al., 2003). The CO2 content in the atmosphere 1 23 increased with the increasing temperature during MMCO period. The impact of MMCO was widely 24 distributed and associated with 60C of temperature warming in the mid latitude region (Flower and 25 Kennet, 1994). Nannoplankton is very sensitive to temperature change. The number of population 26 increases when the seawater temperature rises from 50C to 80C (Haq, 1973). 27 Information of MMCO in the tropical region (low latitude) is rare, especially in long continuous section 28 of Middle Miocene sediments. This study aims to identify the impact of MMCO in Early - Middle 29 Miocene in Cibulakan Formation using nannoplankton observation. 30 The research area is situated in Cileungsi River, Bogor, as a part of North West Java Basin (Martodjojo, 31 2003) (Figure 1A.). This basin was formed by the collision of the Eurasian Plate with the Indian 32 Australian Plate during Late Cretaceous to Early Eocene (Hamilton 1979; Martodjojo, 2003; Hall, 33 2002; Netherwood, 2000). The trend of regional structures are east - west, parallel to the Java subduction 34 (Martodjojo, 2003). 35 On Early – Late Miocene, Cibulakan Formation was deposited in Bogor Basin in back arc environment 36 (Martodjojo, 2003) (Figure 1B). The formation consists of interbedded of claystone and sandstone, and 37 minor insertion of limestone (Arpandi and Padmosukismo, 1975; Suherman and Syahbuddin, 1986). This 38 formation had a conformity contact with Parigi Formation in the upper part, and unconformity contact 39 with Jatibarang Formation in the lower part (Martodjojo, 2003). Moreover, Cibulakan Formation had a 40 interfingering contact with deep water Jatiluhur Formation (Abdurrokhim, 2016). 41 Transgressive phase took place during Cibulakan Formation deposition. Sea level rose during Early 42 Miocene that drowned the Jatibarang Formation and shifted the environment, from terrestrial and volcanic 43 environment to transitional deposit (Arpandi and Padmosukismo, 1975). At the bottom part, 44 The Cibulakan Formation was deposited in paralic environment and close to active 45 delta progradation (Atkinson et al., 1993). Trangressive phase continued to middle part 46 of Cibulakan Formation and the environment gradually changed from paralic to shallow water with a 2 47 significant influx of fresh water into the basin. At the upper part, offshore bar sediments was formed. It 48 contained claystone to bioturbated silty claystone and calcarenite limestone (Abdurrokhim, 2016). 49 50 Figure 1. (A). Structure map of West Java (Modified from Martodjojo, 2003). The research area (red box) is located 51 in the Bogor Through. (B) Schematic cross section of West Java SW – NE (Modified from Netherwood, 2000). 52 Cibulakan Formation infill back-arc basin setting in Bogor Through (red box). 53 54 Materials and Methods 55 Measuring section and samples collecting were performed during fieldwork from the outcrop of 56 Cibulakan Formation exposed in the Cileungsi River, Bogor . We collected 58 samples at a samping 3 57 interval of around x m. The sampling was focused on the claystone and limestone layers. The outcrop 58 condition was good to represent the sedimentology dynamic, and to cover the short term of 59 nannoplankton ecological changes. We observed in total of 5 km of outcrop to understand the succession 60 profile (Figure 2). 61 The smear-slides have been prepared from the untreated samples, in order to preserve the original 62 composition (Kapid, 2003). Samples were crushed and the resulted powders were smeared in a glass 63 sample plate. Then, we use Canada Balsam and glass cover for the plate. The analyses have been 64 performed using a polarizing light microscope Nikon Alphashot YS2-H at 1600x magnification. Both 65 qualitative and quantitative studies have been performed. Quantitative methods were completed using 66 Field of View (FOV) Method (Kapid 2003) following taxonomic identification from Perch-Nielsen 67 (1985) and Young (1998). Samples were identified to the species level if possible, then they were grouped 68 according to their stratigraphic range, to create a stratigraphic framework. 69 70 Figure 2. Sampling locations in Cileungsi River (Modified from Effendi et al., 1998). Green color shows Cibulakan 71 Formation area. 72 Results and Discussions 73 Cibulakan Formation in the research area was formed in the offshore environment, as indicated by thick 74 offshore shale deposit and capped by bioclastic limestone. The vertical section shows Cibulakan 75 Formation profile (from bottom to top) that can be divided into three sequences, which is bordered by 76 bioclastic limestone. Sequence I is characterized by 350 meters thickness of claystone, followed by 500 77 interlamination of thin sandstone and claystone, and wackestone to packestone limestone at the top of 78 sequence (Figure 3). Limestone layer contains biota fragments of coral and foraminifera, which indicate a 79 transgressive carbonates at the offshore. 4 80 81 Sequence II contains of 1250 m thin layers of claystone and sandstone at the bottom, and closed with 82 3000 m of thick limestone and claystone. The concentration of biota fragments in limestone in Sequence 83 II is more intensive than the one in Sequence I (Figure 4). This change is interpreted as the impact of sea 84 level rise. Sequence III shows layers of claystone and sandstone as the representation of regressive 85 sediment (Figure 5). 86 5 87 88 Figure 3. Sequence I at Cibulakan Formation; (A) Section of Sequence I at Cibulakan Formation in the research 89 area. (B) Claystone at the bottom of Sequence I with Cruziana sp. ichnofossil (location of photograph is bordered 90 by black box at sedimentation profile). (C) Interbedded sandstone and claystone at the middle of Sequence I 91 (location of photograph is bordered by blue box at sedimentation profile). (D). Limestone at the top of Sequence I 92 with coral (CR) and large benthic foraminifera (LBF) fragments (location of photograph is bordered by red box at 93 sedimentation profile). 6 94 95 Figure 4. Sequence II at Cibulakan Formation; (A) Section of Sequence II at Cibulakan Formation in the research 96 area. (B) Interbedded sandstone and claystone at the middle of Sequence I (location of photograph is bordered by 97 black box at sedimentation profile). (C). Limestone at the top of Sequence II with intensive encrusting algae (Alg) 98 and large benthic foraminifera (Fr) fragments (location of photograph is bordered by red box at sedimentation 99 profile). 100 101 7 102 103 Figure 5. Sequence III at Cibulakan Formation; (A) Section of Sequence III at Cibulakan Formation in the research 104 area. (B) Interbedded sandstone (SS) and claystone (CS) at Sequence III (location of photograph is bordered by red 105 box at sedimentation profile). 106 107 Cibulakan Formation in the research area was deposited during Early – Late Miocene, based on 108 nannoplankton biozones by Martini (1971). We observe many calcareous nannoplankton events. First 109 Appearance Datum (FAD) and Last Appearance Datum (LAD) from nannoplankton fossil can be used to 110 divide biostratigraphy event which correlates with age and stratigraphy succession. Several 111 nannoplankton species which act as index fossils consist of Sphenolithus belemnos, Helicosphaera 112 vederii, Sphenolithus heteromorphus, Discoaster challengeri, Catinaster coalithus, and Discoaster 113 neohamatus (Figure 6). 114 There are seven biostratigraphy zones in Cibulakan Formation (Figure 7): 115 Sphenolithus belemnos zone 116 This zone is bordered by LAD (Last Appearance Datum) of Sphenolithus belemnos, from RBK – 58 to 117 RBK – 45 in claystone layer. Sphenolithus belemnos zone equal with NN-3 (Martini, 1971) and relates 118 with Early Miocene, around 17.95 mya or older (Gradstein et al., 2012). 8 119 Sphenolithus belemnos – Helicosphaera vederii zone 120 Partial zone is marked by interval from extinction of Sphenolithus belemnos and FAD (First Appearance 121 Datum) of Helicosphaera vederii. This zone can be observed