Depositional Environmental Changes of Cimanceuri Formation Based on Mollusk Fossil Assemblages in Bayah, Banten Province

Bayah is located in Lebak Regency, Banten Province. This location is chosen due to its abundant mollusk fossils which exposed along the outcrops. The aim of this research is to determine depositional environmental changes using mollusk fossil assemblages. Data obtained from a measured stratigraphic section of Cimanceuri Formation. It is dominated by very fine-fine sandstones with claystone intercalation. A total thickness of measured stratigraphic section is 4.2 meters. There are at least seventeen mollusk associations (bottom-top) consisting of 1) Ringicula arctatoides - Olivella tomlini were obtained. 2) Ringicula arctatoides - Marginella (Cryptospira) ventricosa sangiranensis. 3) Olivella tomlini, 4) Ringicula arctatoides - Olivella tomlini, 5) Ringicula arctatoides, 6) Turritella (Turritella) bantamensis - Scapharca (Scapharca) gedinganensis, 7) Polinices aurantius - Marginella (Cryptospira) ventricosa sangiranensis, 8) Scapharca (Scapharca) gedinganensis, 9) Scapharca (Scapharca) multiformis - Timoclea bataviana, 10) Turritella (Turritella) bantamensis tjicumpaiensis - Ringicula arctatoides, 11) Turritella (Turritella) bantamensis - Ringicula arctatoides, 12) Turritella (Turritella) bantamensis tjicumpaiensis - Turritella (Turritella) bantamensis, 13) Turritella (Turritella) bantamensis tjicumpaiensis - Ringicula arctatoides, 14) Turritella (Turritella) bantamensis - Architectonica sp., 15) Turritella (Turritella) bantamensis tjicumpaiensis, 16) Turritella (Turritella) bantamensis Turritella (Turritella) bantamensis tjicumpaiensis, and 17) Turritella (Turritella) bantamensis. The condition with the most stable ecosystem is the association of Turritella (Turritella) bantamensis tjicumpaiensis - Turritella (Turritella) bantamensis (Association 12). At least there are seven depositional environmental changes that occur in this research area with two shallowing deepening cycles : 1) open shallow marine, 2) subtidal open shallow marine, 3) open shallow marine, 4) open shallow marine subtidal, 5) subtidal, 6) subtidal open shallow marine, and 7) open shallow marine.


Introduction
Mollusks are one of the phylum with a very abundant number of organisms, so it is potential to be used as an indicator to interpret the depositional environment.In addition, the environmental distribution of mollusks is wide ranging from marine to terrestrial.Bayah region has many marine sediment outcrops.Bayah region is chosen for studying due to its abundant mollusk fossils which exposed along the outcrops.In this location, foraminifera fossils that commonly used for interpretation of depositional environments is rarely found.Based on this condition we propose the mollusk fossils to be used as indicators of depositional environment interpretation.

Material and Method
Twenty-two rock samples were obtained from 3 measured stratigraphic sections in Cikumpay and Cikuya Rivers (Fig. 3.).Systematically rocks sampling are carried out with an interval of ±20 cm at each layer.
Description and measurement of sediment thickness were carried out in the field.Afterwards, the samples then prepared.The preparation process aims to clean up the remaining sediment until the fossils ready to be identified.A binocular microscope is used to identify microsized mollusks.The references to identification and determination refer to Martin (1879Martin ( ,1880)), Oostingh (1933), Leloux (2009), andSufiati (2012), while for the ecological determination of each species refers to Abbott (1991), Okutani (2000), Aswan (2006), and Prasetyo et al. (2012).Species that have been identified are then used for determine mollusk associations of each layer.
The mollusk association zone is used to indicate mollusk fossil assemblages that live in similar habitats associated with certain lithologies.Classification of depositional environment refers to Fan (2012) (Fig. 4.).The name of mollusk fossils association is based on insitu and abundant mollusk fossils, whether one or more species appear (Prasetyo et al., 2012).The result of these associations are used for interpretation of depositional environmental changes in Cimanceuri Formation.The mollusk fossils obtained from Bayah are stored at Bandung Geological Museum.

Shannon-Wiener
diversity index is used to identified the condition of ecosystem in research area, (Bakus, 2007): Low diversity (H'<1) indicates that ecosystem conditions are unstable or disturbed, moderate diversity (1<H'<3) indicates a fairly stable ecosystem, and high diversity (H'>3) indicates a stable ecosystem condition (Jurnaliah, 2011).The Shannondiversity index is calculated using Microsoft Excel.

Result and Discussion
Based on this research, 2373 mollusk fossils were obtained from 22 samples which is taken from Cikumpay and Cikuya rivers.Its consist of 222 species (56 bivalves, 164 gastropods, and 2 scaphopods).The data of dominant mollusk fossil species in CKY 1, CKY 2, and CKY 3 showed by graphic (Fig. 5,6, and 7).The dominant mollusk fossils are used to determine depositional environmental.
Lithology and mollusk fossils association in CKY 1 and CKY 2 shows similarities so they are interpreted as the same layer.Based on the strike / dip measurements, CKY 3 is interpreted to be younger than CKY 1 and CKY 2. Total thickness of measured stratigraphic section is 4.2 meters.The lithology is dominated by very fine-fine grained calcareous sandstones with non-calcareous claystone intercalation.At the top, there is a coquina sandstone with abundant mollusk fossils (Fig. 8.).
The results of determination represents 17 mollusk associations with 10 species variations.From these associations, it can used to analyze the depositional environmental changes from old to young layers which are correlated with local sea level changes.Classification of the depositional environmental is divided into 3, namely: intertidal, subtidal, and open shallow marine.At least there are seven depositional environmental changes that occurs in this research area, there are: open shallow marine environments that change slightly to open shallow marine subtidal environment and then back to open shallow marine environment.Furthermore, the environment will revert gradually to subtidal, then slowly deeper into open shallow marine environment.
The diversity index value of the samples which is taken from this study ranged from 1,095 -3,182 (Table 1.).The highest diversity index value is at CKY3 L5a, while the lowest diversity index value is at CKY3 L6.This can be interpreted that the research area is a medium high productivity area and has a fairly stable stable ecosystem.
of individuals of species i N = the total number of individuals of all species

Figure 8 .
Figure 8. Composite stratigraphic coloumn and local sea levelchanges in research area (getting to the right → deeper).

Fig. 10 .
Fig. 10.The Examples of micro-mollusk fossils in research area (a: Ventral view, b:Dorsal view

Table 1 .
Table Diversity index in this study