Identification and Stratiraphic Position of Mollusk Type Locality at West Progo Stage

The location of the discovery of mollusk fossils on the island of Java is spread in various places. One location is in the Kulon Progo region known as West Progo beds. However, due to the lack of studies of mollusk fossils in the Kulon Progo region, this resulted in a lack of understanding of the location of the discovery. This study was intended to re-record the location of fossil molluscs discovery in the Kulon Progo region, with the aim of contributing to the stratigraphic arrangement in Kulon Progo. Research methods include literature studies, field investigations and laboratory analysis. The literature study includes libraries of the Dutch colonial era regarding the location of the discovery of mollusk fossils. Field studies in the form of stratigraphic measurements and sampling. Laboratory investigations include petrographic observations and identification of micro and macro fossils. The results of the investigation successfully re-identified the Kembang Sokkoh and Spolong locations which are two types of locations on the West Progo beds. Based on the lithological characteristics of the two locations included in the Jonggrangan Formation, with the Lower Miocene age based on an analysis of the fossil content of the molluscs.


Introduction
During the Dutch colonial period, geological investigations on Java were always helped by the discovery of fossil mollusks in various places. These discoveries spread almost in all physiographic zones on Java. One of the locations of the discovery was in the Kulon Progo area, which Martin (1919) referred to as West Progo beds. There are two important locations that represent the age of the Early Miocene, namely in Kembang Sokkoh and Spolong (van den Hoek Ostend, et al, 2002).
Research on the use of mollusk fossils in Indonesia is still little done, some of which have already done include Pandita and Zaim (2009), Pandita, et al (2013), Kase et al. (2008), and Kase, et al. (2015). This condition results in fossils of mollusks not being an important asset in the utilization in the field of geology. One of the important things in utilization is biostratigraphy.
Mollusk fossils have not been utilized in geological aspects, especially biostratigraphy, because there is still little information about various places containing fossil mollusks (Pandita, et al, 2013. Based on the old literature review, it is estimated that there are more than 20 locations of fossil mollusk discoveries on Java (Martin, 1919;van Es, 1931). These locations are spread in West Java, Central Java, Yogyakarta and East Java. In the Yogyakarta region these locations are located in Kembang Sokkah and Mount Spolong, which represent the West Progo level (Martin, 1919) The stratigraphic position of each location needs to be reviewed again both in terms of biostratigraphy and lithography. One interesting location to study is the West Progo stage. The location needs to be identified again both from the stratigraphy and also the location coordinates, because there is no coordinate point listed when the location was examined (Martin, 1919). The lack of clarity about the location has an uncertain stratigraphic position.

The Objectives
This research is intended to re-identify the location of the type of mollusk fossil discovery that represents the West Progo stage. In addition, it is also intended to record fossils of mollusks and microfossils that can still be found. While the final goal is to find out the description of the stratigraphic position of the location

Methods
The research method carried out in the form of field investigations and laboratory investigations. Field investigations were carried out in the form of measuring stratigraphy, rock and fossil sampling. In laboratory activities it consists of observing thin rock section, identifying macro and micro fossils.
After conducting field and laboratory investigations various analyzes were carried out. The analysis carried out in the form of rock type analysis, age analysis and depositional environment. The last stage is a synthesis of the stratigraphic position of the West Progo type location.

Physiography
The physiographically of the study area are located in the Kulonprogo Mountains sub zone (van Bemmellen, 1949) (Figure 1). This sub zone is restricted to the western part with the South Serayu Mountain Zone while in the east it is bounded by the Yogyakarta plain. In the north it borders the central depression zone of Central Java, while in the south with the South Coast of Java (van Bemmellen, 1949).
The Kulonprogo Mountains Zone itself is a dome that extends north-south. Some researchers think the formation of this dome was controlled by tertiary volcanic activity (van Bemmellen, 1949). However, a number of other researchers such as Sopaheluwakan (1994) andSoeria Atmadja, et al. (1991) assume that the formation of this dome is related to the existence of twice the tectonic phase in this area.
The oldest formation found in Kulonprogo is the Nanggulan Formation. This formation is estimated to have formed in the Middle Eocene based on fossil content of Nummulites and Discocyclina (van Bemmelen, 1949). The depositional environment is estimated to begin in the transition area transformed into a shallow sea.
Unconformity overly the Nanggulan Formation deposited by the Old Andesite Formation. The naming of this formation was proposed by Bemmelen (1949) for all volcanic rocks in Sumatra and Java that formed in Late Paleogene to the Early Miocene. A number of name revisions to this formation were submitted by many authors relating to naming that did not follow the rules of the Indonesian Stratigraphic Code. Purnamaningsih and Pringgoprawiro (1981) proposed two names namely the Kaligesing Formation for units characterized by sediments in the sea, and the Hamlet Formation for volcanic rocks formed on land. Rahardjo, et al. (1995) gave the name of the Kebobutak Formation because of the lithological characteristics that are similar to the rock units found in the Southern Mountains of East Java. This formation is composed of volcanic rocks in the form of andesite breccia with andesite lava. This formation is thought to be deposited in two different environments, namely the sea and land facies. This rock is thought to have formed in the Oligocene -Early Miocene. The thickness of this formation is estimated at 660 m (Rahardjo, et al., 1995). Overly the Old Andesite Formation is unconformity deposited by the Jonggrangan Formation. The lower part of the Jonggrangan Formation is composed by conglomerates which are overlain by tuffaceous marbles and side sandstones with lignite inserts. The upper direction of the Jonggrangan Formation transforms into layered limestone and coral limestone. This formation is expected to form in the Early Miocene and at the bottom of the Formation with the Sentolo Formation. The thickness of the Jonggrangan Formation is estimated at 250 m (Rahardjo et al. 1995). Interfingering with the Jonggrangan Formation is deposited by the Sentolo Formation. In the Giripurwo area, conformity contact found between the Sentolo Formation at the top and the Old Andesite Formation at the bottom. The lower part of the Sentolo Formation is a base conglomerate which is boarded by tufaceous marl with a thin layer vitric tuff. This rock upward gradually turns into layered limestone rich in Foraminifera. From the results of the planktonic foraminifera analysis several previous studies concluded that this formation was estimated to have formed during the Early Miocene to the Pliocene (N7 -N21). The thickness of this formation is estimated at 950 m (Rahardjo et al., 1995).
Unconformable overly the tertiary-aged rocks are quarter volcanic breccias. This volcanic breccia originates from Old Merapi Mountain and Young Merapi Mountain. The distribution of this unit is in the Yogyakarta basin which extends between the River Progo in the west to the Opak River in the east (Rahardjo, et al., 1995).

Data and Analysis
Field investigations were carried out at three locations namely KS01, KS02 and KS05 located on the eastern slope of Mount Kelir, Kulonprogo (Figure 2). Site selection is based on written information from Martin (1919) which describes the location of discovery of mollusk fossils from the West Progo level.   Rahardjo, et al. (1995), green is the Jonggrangan Formation, Brown Color of the Old Andesite Formation.

KS01 Location
The KS01 location is in the Kembang Soka area with geographic coordinates of 7O46 '5.47' LS and 110O6 '59.7' BT. Outcrops of ± 15 m thick show very strict lithological changes where the lower part of the igneous rock is brownish red, while on top of it is brownish sandstone (Figure 3).
The outcrop conditions at the bottom experience weathering which is controlled by oxidation. In handspecimen observation fresh gray color, porphyritic fanatic rock texture, with massive structure, mineral composition include pyroxene, plagioclase feldspar, k-feldspar, hematite, hornblende, quartz, with a period of basic alteration minerals, which are interpreted as altered basal porphyry. Frozen rocks have undergone purple alteration at the bottom.
At the top there are brownish white sandstones. The sandstone is resembled by fossils of Architectonica molluscs. Mollusk fossils cannot be taken because the location has become a tourist spot for the Soka Kembang waterfall.
Based on a literature review of the location of the type of mollusk fossil discovery in the Kulon Progo region (Martin, 1919), it is likely that this KS01 location is the location of the Sokkah Flower. This is in accordance with the description of the location of Martin (1919) which illustrates that one location type of the Lower Miocene level is on the slopes of Mount Kelir in the village of Kembang Sokkoh.

KS02 Location
The location of KS02 is north of Kembang Soka, located on the edge of the road that connects Jonggrangan village to the Sermo reservoir, located at coordinates 07O45'38.8 "LS, 110O07'41.5" BT ( Figure  4). At this location stratigraphic measurements are carried out, with a thickness reaching almost 50 meters.
From the results of measured stratigraphic measurements can be found several lithological variations from young to old layers ( Figure 5). At the bottom, calcarenite is found with lignite inserts and carbonate sandstones. A number of mollusk fossils can be found in this location such as Haustator subulata which becomes an index at the level of West Progo (Oostingh, 1938).
In the middle it changes to calcarenite with the insertion of a crystalline limestone. Mollusk fossils are still found with unfavorable and cracked preservation conditions. Lignite inserts are also found in this section.

KS05 Location
The KS05 location is to the east of Kiskenda Cave and to the west of Jonggrangan Village, known as Sibolong hill. Located on the edge of the road that connects Jonggrangan village with Kiskenda Cave, Located at coordinates 07O44'47.5 "LS, 110O08'26.4" BT ( Figure 6). At this location stratigraphic measurements are carried out. Thickness reaches almost 25 meters. Based on microscopic analysis showed bioclastic limestones, which are composed of large foraminifera fossils, benthic small foraminifera and shell fragments of molluscs.  The rock conditions generally have experienced a strong crystallinity process, so that the shells of mollusks cannot be separated from rocks. Currently this location is also a tourist spot in Mount Sibolong. This is an obstacle for the preparation of sampling mollusk fossils. Based on the description of the Sepolong location carried out by Martin (1919), it is likely that Mount Sibolong is a Spolong type location. Martin (1919) explained the location of the Spolong located west of Jonggrangan Village.

Paleontology Data
From the three observation locations, paleontological analysis has been carried out both macrofossil and microfossil. Some of the fossils identified are mollusks, ostracoda, foraminifera, and coral. The complete results of the fossils identified from the three locations can be seen in table 2. Specific sampling locations for KS05 were not possible due to the prohibition of taking rocks at that location.

Martin
(1919) describes two locations representing the levels of West Progo beds, namely Sokkoh and Spolong Flowers. Kembang Sokkoh is described as being on the eastern slope of Mount Kelir located in the flow of the waterfall. Meanwhile the described Spatial area is to the west of Jonggrangan hamlet. Based on the second description of the location of Martin (1919) and the findings in the field, the KS01 and KS02 locations are most likely the Kembang Sokkoh location. Whereas the KS05 location which is to the west of Jonggrangan village is the location of Spolong, this is also from the resemblance of spolong and sibolong names.
Based on the fossil content found in the field, age analysis can be carried out from both types of locations. Based on the content of mollusk fossils of Haustator subulata and Conus spolongensis it is thought to have Early Miocene age (Martin, 1919). Some fossils of benthic foraminifera have a very long life span, such as Gyroidina soldanii (Oligocene to Recent), Elphidium sp (Miocene to Recent) and Quinqueloculina sp (Miocene to Recent). Fossils Ostracoda that appear also range in longevity such as Laxoconcha (Oligocene to Recent) and Dolerocypris sinensis (Miocene-Pleistocene). Based on these fossils, it is estimated that rocks exposed in Kembang Soka and Spolong have Early Miocene age.

Discussion
Referring to the purpose of the study to identify the location of the West Progo type, the location of the Kembang Soka and the Spolong hill are two locations that were once studied by Martin (1919). The West Progo type location is arranged by Martin (1919), lithostratigraphy is part of the Jonggrangan Formation. The name Jonggrangan Formation itself was first introduced by van Bemmellen (1949) and placed its age in the Early Miocene.

A B
The results of this study indicate that the age of the two locations, Kembang Soka and Sibolong showed an estimate in the Early Miocene. This condition is the same as that proposed by Martin (1919) and also van Bemmelen (1949). However, the determination of the age of the two outcrop locations still needs to be strengthened by another age analysis. This is because the paleontological data used for analysis still has a long range, namely Miocene to Resen. Two methods that need to be used are radiometric dating and Nannoplankton.

Conclusion
Overall data from the results of the study succeeded in re-identifying the location of the type of mollusk fossil discovery from the West Progo level compiled by Martin (1919). Both locations are located in Kembang Soka and Spolong Hill. There is an addition of paleontological data in the form of ostracoda, although it needs to be examined in more detail. The stratigraphic position can also be determined namely in the Early Miocene with its lithography as part of the Jonggrangan Formation.