The marine fossils and paleoecological significance of the Southern edge of South Sumatra Basin in Linggapura Lampung, Indonesia

The South Sumatra Basin is identified as a Paleogene-Neogene sedimentary basin with Mesozoic basements. It is located in the eastern Barisan Mountains, covering Jambi, South Sumatra, and Linggapura (Lampung), Indonesia. Linggapura represents the southernmost part of the South Sumatra Basin. The basin consists of two well-known rock formations, namely the Talangakar and Baturaja Formations. In this study, we investigated these formations through fossil records and paleoecological interpretations. Our investigation was based on measured stratigraphy and fossil observations along the Way Penandingan River in Linggapura, Lampung. We discovered that the bottom of this area consists of Cretaceous granitoid, which forms a nonconformity with the sedimentary rocks in this area. The Talangakar Formation in this region comprises quartzy-grained sandstone and conglomerates containing fossilized remains of terrestrial plants/vegetation. These plant fossils have undergone coalification, with some being silicified. The Batujara Formation in Linggapura is dominated by bioclasts, primarily consisting of Miocene shoreface-lagoonal larger benthic foraminifera, along with other marine and ichnofossils. Based on our findings, we suggest that the paleoecological condition of this area is associated with shoreline transgression during the Oligo-Miocene period. The hotter Miocene climate contributed to a rise in sea levels, resulting in shoreline transgression. Gradually, the area transformed from a terrestrial environment to a highly bioproductive marine environment, at least from the Late Oligocene to the Early Miocene. This study represents an important milestone in better understanding the paleoenvironmental conditions at the edge of the South Sumatra Basin throughout its geological history.

IOP Publishing doi:10.1088/1755-1315/1245/1/012001 2 lower to middle Miocene, followed by subsequent tectonic compressional inversion [1,5].The presence of hydrocarbons and coal in the basin indicates the deposition and accumulation of organic carbon influenced by paleoecological conditions and ecosystem productivity during that time [6,7].These organic materials eventually underwent fossilization along with subsequent geological processes.
Referring to the evolutionary phases of the South Sumatra Basin, the early-to mid-Miocene period is a critical phase related to the optimum development of basin dimensions [2,8].The size of the basin dimensions is one of the factors determining the potential volume capacity of sediment accumulation [9].In this study, we focused on assessing the period at the southernmost edge of the South Sumatra Basin by interpreting sedimentation environments and ecological conditions based on litostratigraphical records and fossil findings.The primary objective of this study was to trace the fossil record and interpret the paleoecological conditions at the southernmost margin of the South Sumatra Basin (Figure 1), particularly during the peak period of basin formation.The results of this study will contribute to further research aimed at understanding paleoenvironmental changes, including paleobathymetry and paleogeography, that occurred during the evolution of the South Sumatra Basin.Linggapura, Lampung, is the southernmost boundary of the South Sumatra Basin [11,12].The region comprises several geological formations, including Cretaceous granitoids, Oligo-Miocene Talangakar Formation, Miocene Baturaja Formation, Plio-Pleistocene Kasai Formation, and Quaternary volcanic deposits.The Talangakar Formation is classified as a syn-rift deposit within the South Sumatra Basin, while the Baturaja Formation is considered a post-rift deposit [13,14].Both formations have been reported to contain a variety of fossils [15].In contrast, in the Talangakar Formation, the fossil composition tended to consist of terrestrial organisms, whereas in the Baturaja Formation, it tended to consist of marine organisms.Unfortunately, changes in the fossil composition and paleoecological conditions during formation deposition and basin evolution have rarely been studied.

Method
The study was conducted by stratigraphic measurements along the Penandingan River, Linggapura, Lampung (Figure 1).These measurements were presented in lithostratigraphic profiles that included information on thickness, texture, sedimentary structure, fossils, and other relevant features (Figure 2).
)LJXUH /LWRVWUDWLJUDSK\ SURILOH LQ WKH VWXGLHG DUHD DORQJ :D\ 3HQDQGLQJ 5LYHU /LQJJDSXUD FDSWXUHG E\ DXWKRUV 4 Representative samples were collected for thin-section preparation, and these samples were subsequently observed under a light microscope (Figure 3-5).The observations were carefully documented to interpret the depositional environment and paleoecological conditions based on fossil findings and rock texture.In thin-section observations, the sample (LG-01) from the bioclast grainstone facies was found to be composed of calcareous-siliciclastic angular grains and skeletal remains of marine organisms, including gastropod spiky shells and larger benthic foraminifera (Figure 3).These observations suggest that the sedimentation environment of this facies is associated with a marine environment, influenced by dominant terrestrial sediment sources and high-to mid-current depositional environments.Ecologically, gastropods with spiky shells have adapted to cope with tidal current dynamics, particularly during the transition from In such environments, the substrate tends to be finer and rich in suspended nutrients.Organisms such as mollusks, arthropods/crustaceans, and other epifaunal species often graze on the substrate, feeding on attached algae and fragments of broken corals.On thicker substrates, some infaunal organisms burrow as a life strategy for sheltering, grazing, and hunting.Crustaceans, in particular, employ this strategy to camouflage themselves before capturing prey.The presence of tubular crustacean skeletal remains on the rocks also indicates the presence of crustaceans.These organisms often feed on planktonic algae, foraminifera, or other smaller nektonic organisms, such as small fish.We interpret that this paleoenvironment is also associated with the presence of coraline algae, as indicated by traces of encrusted coralline algae preserved in the samples.Coralline algae thrive in submerged conditions and require exposure to sunlight.Algae play a significant ecological role as autotrophic producers in the reef environment, contributing to the food pyramid and material cycle.)LJXUH Proposed schematic representation of the geological process and paleoecological changes during the deposition of the Talangakar to Baturaja Formation in the Linggapura region (not scaled).
Starting in the Miocene, the Earth experienced relatively high temperatures, resulting in the melting of ice at the poles [22,23].This ice melt led to a significant global rise in sea levels known as the Miocene Climate Optimum (MCO).The rising sea levels caused transgression in coastal areas worldwide, resulting in changes to ecosystems and depositional environments.In the study area, there was a simultaneous shift from terrestrial to marine environments during this period.This transition gradually led to the formation of a carbonate environment in the seaward region.As the shoreline reached this area, a mixture of terrestrial and marine sediments occurred.Siliciclastic materials, primarily transported by fluvial activity, carried granitoid-siliciclastic and other terrestrial debris from the landward side.The angular shape of these siliciclastic materials indicates a relatively short transportation distance from their source outcrop.The marine currents in this environment were also relatively strong.In the shallow environment characterized by tides and the warm Miocene climate, the deposition (bed) environment experienced high evaporation [22,24].This resulted in increased salinity in the depositional environment, similar to a deltaic to lagoonallittoral environment.Such ecological conditions were ideal for the growth of foraminifera, specifically Miliolida or Rotalida/Numulites [25].Large foraminifera are typically abundant in shallow, warm, highly saline, and well-oxygenated marine environments.They thrived in the Miocene tropical and subtropical seas, particularly in areas with high organic matter and nutrient inputs.These conditions are represent in the bioclastic grainstone LG-01, that found in Linggapura.
During the Middle Miocene, there was a significant rise in sea levels, leading to the flooding of shores and landward expansion.The warmer Miocene climate, known as the Miocene Climate Optimum (MCO), played a role in the development and vertical growth of reef ecosystems in response to the rising sea levels.
As the bathymetry of certain depositional environments deepened due to the sea level rise, the water column conditions became calmer, resulting in the easier sedimentation of finer carbonate particles.The presence of nutrient-rich carbonate suspensions in the water column attracted substrate-grazing organisms, particularly mollusks.These shelled mollusks adapted to this environment by filtering the suspended particles from the water column.They often inhabited areas around coral formations.Additionally, some benthic foraminifera were present in this environment.These ecological characteristics were observed in the bioclastic packstone LG-02 of the Baturaja Formation in Linggapura.
Compared to the other samples, LG-03 exhibits shallower conditions and a relatively lower abundance of clay-sized material (calcite micro-sparite) compared to LG-02.Although not as prominent as in LG-01, interclastic siliciclastic material can be observed in LG-03.In terms of ichnofossils, organisms with substrate grazing behavior were commonly found in LG-03.Some of these organisms were encrusted with red algae and skeletal remains of crustaceans.This group of organisms thrives in reef-associated environments, although with calmer currents and a more seaward location compared to LG-01.Based on these findings, we propose that LG-03 represents a back-reef ecological environment, specifically a muddy sand apron or foreshore.
Referring to previous research, the depositional environment of the Baturaja Formation is generally interpreted as being associated with back-reef, reef-mound, and/or fore-reef environments [26][27][28].In the study area of Linggapura, which is located at the edge of the South Sumatra Basin, a back-reef environment is likely to be present.Additionally, other observations from thin-section analysis of cores drilled in several wells in the South Sumatra Basin suggest that this facies exhibits varied porosity.The larger porosity could be attributed to the intergranular spaces between clastic grains.However, the variations in porosity, which typically manifest as intracrystalline, intergranular, or vuggy porosity, are primarily caused by the interaction between meteoric water and marine water.This interaction leads to the recrystallization of carbonate minerals (dolomitization), resulting in smaller porosities.Based on a previous review, it is likely that the carbonate facies forming the Baturaja Formation in Linggapura represent a rimmed carbonate environment dominated by bioclastics.This is in contrast to the limestone facies of the Baturaja Formation in South Sumatra, located 50 km north of Linggapura, which predominantly consists of boundstone [31].The presence of boundstone suggests the existence of a massive barrier reef environment.The higher clastic content observed in the limestone at Linggapura is attributed to the stronger water currents and its relatively closer proximity to the shoreline.Terrestrial clastic materials also make a significant contribution to the sediment supply during this period.However, this environment is supported by favorable paleoecological conditions that facilitate high bioproductivity within the ecosystem.Various marine fossils were discovered in the samples, indicating the presence of a relatively complex food chain ecosystem.This ecological condition also contributes to the accumulation of organic carbon and carbonates in this type of environment.

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The limestone found in Linggapura primarily comprises marine fossils associated with shore-to-shallow carbonate environments.These fossils include larger benthic foraminifera, massive corals, intertidalshallow marine mollusks, crustaceans, encrusted remains of red algae, and various shore-related ichnofossils.The carbonate rocks exhibit clastic textures, mainly consisting of bioclastic grainstone, packstone, and wackestone, which indicate the formation of rimmed carbonate environments.The deposition of the Baturaja Formation in Linggapura was influenced by a hotter climate and sea-level rise during the Miocene.These factors played a role in the transgressive nature of the depositional environment.
To gain a more comprehensive understanding of the paleobathymetric changes that have occurred, we recommend conducting a detailed geochemical study of the stratigraphy in this area, supported by a thorough paleontological review.

Figure 1 .
Figure 1.Geological map of Linggapura, Lampung (left), and its position in the southern South Sumatra Basin (right).The geological cross-section and the position of the Limestone of Baturaja Formation in the studied area (below), after [10].
Other findings indicate that the most appropriate carbonate facies model for the Baturaja Formation is an isolated platform following the Rimmed Shelf Accretionary model proposed byTucker and Wright [29].When carbonate rocks are exposed to the atmosphere, the intensified interaction between meteoric water and isolated seawater triggers extensive recrystallization, thereby transforming primary porosity into secondary porosity.The reef depositional environment has contributed to the Baturaja Formation becoming a hydrocarbon reservoir.In contrast, if the depositional environment is a lagoon, it is typically dominated by mudstone lithology.Mudstone, being very fine-grained, exhibits minimal porosity, making it an unfavorable environment for hydrocarbon retention.The depositional model proposed bySusilowati and Suyoto [30]illustrates that a