Interpretation of paleoenvironment using benthic foraminifera from Tertiary rocks, northern Sarawak

. In this work, we collected and examined three samples from an exposed 50 cm thick fossiliferous bed and prepared thin and polished sections for petrographic analysis. The benthic foraminifera assemblages and other marine fossils are studied to infer the paleo depositional environment. Based on the benthic foraminifera and their common ecological preferences, it is inferred that this layer has deposited between the intertidal and neritic zones, and within the photic zone, they are under oxygenated conditions. In addition, the presence of the glauconite mineral also supported the neritic environment interpretation, suggesting a water depth of less than 300 metres during the deposition, denoting shallow water conditions. Even though the results presented in this paper are at an elementary level, this could serve as additional information to develop a better understanding of the geology on a much larger scale.

that have highlighted the foraminifera distribution (e.g., [17], [18], [19]) within the Tertiary rocks in Sarawak. However, based on the existing literature, their distribution across the local onshore outcrop is still incomplete. Here, we use petrographic analysis to identify and classify the fossiliferous bed. The identification of the mineralogical and fossil composition of the fossiliferous bed allows us to constrain the depositional environment of the studied layer.

Geological Background
The investigated samples were collected from an outcrop exposed near Bakun Bridge Junction, Bintulu (Figure 1). The measured thickness of the bed was around 50 cm. Based on the geological map of Sarawak, this outcrop is located within the Setap Shale Formation. The age of the formation ranges from Upper Oligocene-Miocene [20], [21], [22]. Generally, the formation is described as "thick, monotonous succession of dark-clay shales" with the presence of limestone at certain localities [19]. The depositional environment is mostly marine, extending from the outer neritic to the littoral environment [19].

Materials and Methods
Three (3) samples were collected and brought back to the lab for further analysis. A total of twenty thin sections were prepared at the sedimentology lab and petrographic observations were conducted at the mineralogy lab, Curtin Malaysia, under plane polarized light (PPL) and cross polarized light (XPL) views using Nikon Eclipse LV100N while microphotographs were taken using NIS (DS-Fi2). The identification of the foraminifera and other marine fossils was based on the standard published works (i.e., [23], [24], [25]).

Results and Discussions
Overall, the collected hand samples (i.e., calcareous siltstone) that displayed a light grey colour in appearance were highly fossiliferous and reacted mildly to acid. Samples also showed packstone texture with abundant numbers of large benthic foraminifera and other marine fossils scattered throughout each section. From the observation under the thin section, the grain size was classified as ranging from coarse silt to very fine sand.

Mineralogy
In general, the observed samples are poorly-sorted and mineral grains are mostly showing angular to subrounded. Mineralogically, the samples contain quartz, calcite, and glauconite minerals (Figure 2). The observed thin-section samples contain marine fossils such as foraminifera, shell fragments, coralline algae, bryozoan, and echinoderms. This confirms the field observation which showed that the bed contains numerous marine fossils such as coral (Figure 2i) and shell fragments. Foraminifera is the most abundant biogenic component observed under the thin sections, with benthic foraminifera occurring as the dominant group (Figure 2a-f). In addition, planktic foraminifers (Figure 2g-h) were observed as well. Other marine fossils such as coralline algae, coral, and echinoderm spine and plate (Figure 2l) were also identified but their abundance were relatively low across all the samples studied.

Possible depositional environment
The interpretation of the paleoenvironment was attempted based on the presence of the fossil foraminifera, coralline algae, and coral. The combination of typical open-marine skeletal fauna (i.e., large benthic forams, bryozoans, echinoids) and the micritic mud suggests that the past depositional environment was an open marine environment with low-medium energy [26]. The presence of benthic foraminifera and coralline algae in these samples indicates that the deposition took place within the photic zone which could indicate a shallow water environment. Foraminifera such as Amphistegina sp. and Elphidium sp. are associated with the highly oxygenated zone and in water depths ranging from 40 meters to 80 meters. Meanwhile, Heterostegina sp. is more commonly linked to deeper depth [27]. Discocyclina sp. is commonly associated with the marine outer shelf environment [28]. The robust ovate shape of the observed foraminifera indicates shallower settings, which is in contrast with deeper water foraminifera which displays flat and elongated test shapes [29]. The environment with low oxygen is represented by the presence of taxa that exhibits thin-walled tests, weak ornamentation, elongated and flat shape, and tapered and cylindrical morphotypes [30], [31]. Furthermore, normal salinity seawater condition is indicated by the benthic foraminifers, which mostly has a hyaline wall. The presence of solitary coral, although their abundances are low across all observed samples, indicates that ocean water with normal salinity existed. The development of coral favours an environment with normal salinity water, with the salinity ranging between 30 -40% [32], [33], [34].

Echinoid spine
The low abundance of planktonic foraminifera also suggests that the environment stretches from the inner to the middle shelf. The distribution of the planktonic foraminifera is low or very rare within the inner and middle shelf, with increasing abundance towards the outer shelf and upper slope [8]. Planktonic foraminifers occupy up to 80-95% of the modern ocean outer neritic to mid bathyal foraminifera sediment assemblages [8].
The coralline algae, which can be found worldwide, is able to tolerate various ecology and plays vital roles in the tropical and temperate carbonates system, where it is more commonly to be present at "depths of up to 100-120 m" with low terrigenous supply [35], [36]. Coralline algae are common in marine shallow-water carbonate and siliciclastic rocks of the Oligocene and Miocene ages. The algae can also be found as part of the deep-water re-deposited sediments, in which they have particles removed from the platform [35].
The shallow water environment is further confirmed by the occurrence of bryozoan and coral in the samples, though not in abundance. The bryozoan is commonly found in the intertidal zone with a depth of about 80 m [37]. Delicate forms of bryozoans are more common in calm water where the robust encrusting type is associated with a high-energy environment [38]. Generally, bryozoans avoid turbid and muddy environments as these areas have a high influx of sediments which is capable of disrupting the feeding activity and damaging their lophophores [39]. This could suggest that the paleoenvironment could be located further away from the river mouth areas, which supply most of the sediments to the ocean.
In addition, the interpretation of the paleoenvironment is also assisted by the presence of the glauconite mineral. In general, glauconite is an iron-rich clay granule and can be found in two formsglaucony and verdine. The glauconite in our samples ranges from pale green to dark green, with some showing brown colour, which is a typical glaucony feature. Glauconite is commonly found within neritic areas, occupying moderate to shallow depths. Glaucony prefers to be formed at latitudes as high as 50° and in water depths up to 1000 m. However, a shelf-slope transition with water depths ranging between 150 m to 300 m is preferable in the equatorial region [40]. Glauconite is commonly formed between in depth of approximately 18 to 700 m, with tropical glauconite favouring cooler waters below 50 meters, and mainly below 250 meters [41].

Conclusions
The petrographic study of the fossiliferous layer reveals the presence of numerous fossils consisting of foraminifera, coralline algae, shell fragments, echinoderms, and bryozoan. The foraminifera assemblages suggest that the age of this layer is between Upper Oligocene and Middle Miocene. Based on the identified macrofossils and microfossils, the paleoenvironment for this area is suggested to have sufficient oxygen supply within the photic zone and approximate depth of < 200 meters, suggesting a shallow water environment, most probably between intertidal to neritic zone.