The Warukin Formation is the main formation in the Asam-Asam sub-basin where there is abundant coal resource, so it is interesting to study the rocks forming this formation. Not only coal deposit, but also clay and quartz, which are popular industrial raw materials, can be found in the formation. Claystone is very widespread and becomes one of the dominant lithology in the Warukin Formation. Claystone has many uses for various purposes, so knowing the details of claystone is very important as data requirement for industrial raw materials. Knowing claystone characteristics from the provenance, the physical and mechanical properties, and also the geochemical properties are very necessary. Information of geology – geomechanics and geochemistry (3G) will be the basic data for engineering needs. Geological information is related to the constituent mineral which is very useful in determining the provenance as well as the characteristics of claystone, so it can explain the prediction of certain unfavorable minerals presence in the Warukin Formation. Geomechanical property is related to rock strength and it provides a visualization of vertical and horizontal distribution of the rock strength, thus it can be used as a reference for demolition activity. Geochemical property will be very helpful in detailing claystone of the Warukin Formation and its relationship to the other minerals that need to be aware of. One of the clay minerals to be aware of is montmorillonite which is a clay mineral that has high swelling property. This 3G study (geology – geomechanics and geochemistry) is expected to provide detail explanation of potential presence of montmorillonite mineral in the Warukin Formation.
In geological study, especially in relation to provenance analysis, studying regional geological setting is very important. Different tectonic settings have different characteristics of rock [1]. For clastic sedimentary rock, the characteristic can be classified based on grain composition of the rock constituents. The grain size follows Wentworth rule (1992) [2]., while for fine-grained clastic sedimentary rock, it follows Tucker classification (1996). (Fig. 1). This grain size is the basis for naming fine-grained sedimentary rock. When 75% − 100% of the rock is composed by a certain grain size, the rock will be named the same as the name of the grain size. The other grain size with composition of no more than 25% will be attached behind the name of the rock. For example, “sandy claystone” is composed by 75% clay-sized grain and 25% sand-sized grain. The naming of mudstone is based on balanced grain composition between clay, sand, and silt [3]. According to the classification of [4]., clastic sedimentary rock is classified petrographically based on the percentage of quartz (Q), feldspar (F), and rock/lithic fragment (L) in the form of a triangle combined with the percentage of the matrix content (Fig. 2).
Pre-deposition history of sediment or sedimentary rock may be reconstructed through provenance analysis [5]. The provenance analysis studies the distance, direction, tectonic setting, climate, and relief of the origin area of sedimentary material [6]. The main assumption underlying provenance analysis is that different tectonic settings consist of different rock types with characteristic of producing a specific composition range of sandstone when eroded. The composition of sandstone reflects not only rock of the source area but also tectonic setting of the sandstone source area [7], [8]. used the QFL and QmFLt diagrams (Fig. 3) linking the composition of sandstone detritus with the main type of provenance consisting of continental block provenance (including sub-provenance craton interior, transitional, and uplifted basement), magmatic arc provenance (including sub-provenance undissected arc, transitional arc, and dissected arc), and recycled orogen provenance (including sub-provenance subdiction complex, collision orogen, and foreland uplift).
In the QFL diagram (Fig. 3, on the left), Qt = Qm + Qp, which is total detritus number of monocrystalline quartz (Qm) and polycrystalline quartz (Qp); F is number of feldspar detritus; and L = Lv + Ls + Lm, which is total detritus number of volcanic rock fragment (Lv), sedimentary rock fragment (Ls), and metamorphic rock fragment (Lm). In the QmFLt diagram (Fig. 3, on the right), Qm is number of monocrystalline quartz detritus and Lt = L + Qp, which is total detritus number of rock fragments added with number of polycrystalline quartz detritus. These two diagrams are used as a reference to determine provenance.
In general, the Warukin Formation is composed by sandstones, claystones, and coal. Sandstone dominates the rock outcrops, while claystone is only an insert. Study on sandstone of the Warukin Formation has been carried out by [9]. The result was the provenance of Warukin standstones is generally recycled orogen with subclassification of quartzose recycled.
1) Regional geology
Tectonic activity in Kalimantan has occurred since the Jurassic period. Ultramafic rocks and metamorphic rocks at that time were mixed and then intruded by granite and diorite in the Early Cretaceous or earlier. At the end of the Early Cretaceous, the Alino Group that was partly an olistostrome was formed, interspersed by volcanic activity of the Pitanak Group. The tectonic activity continued, until in the Early Cretaceous, it caused the ultramafic rocks and metamorphic rocks faulted over the Alino Group. In the Paleocene epoch, the tectonic activity caused uplift of Mesozoic rocks, accompanied by intrusion of porphyry andesite rock.
In the Late Cretaceous epoch, Kalimantan microcontinent and the Paternoster plate were collided, causing the southeastern part of Kalimantan to be uplifted. The area, later in the Early Tertiary, became an environment for the development of lacustrine deposit and alluvial fan of the Lower Tanjung Formation. A marine transgression in the beginning of the Middle Eocene resulted in domination of fluvio-deltaic sediments and eventually formed the Central Tanjung Formation, which was dominated by sea sediments. Then, the marine transgression gradually submerged the uplifted area, forming the upper part of the Tanjung Formation and the Berai Formation thereafter in the Late Eocene until the Early Oligocene. In the Miocene epoch, the deposits of the Warukin Formation were formed as a result of sea level decline due to the uplift of the Schwaner Mountains in the west and the Meratus Mountains in the middle. The uplift of the Meratus Mountains continued until the Pleistocene epoch, resulting in rock deposits of the Dahor Formation.
Rock formations in the study area are dominated by the Warukin Formation and, in some other parts, composed by the Berai Formation and the Tanjung Formation. These formations are in the Asam-Asam Basin, which is a part of the Barito Basin in South Kalimantan. The Barito Basin is divided into the Barito Basin and the Asam-Asam as a sub-basin (Fig. 4). These two basins are believed to be a depocentre in the Eocene epoch that were connected and separated due to uplift of the Meratus Mountains in the Late Miocene [10]. The general stratigraphy consists of Mesozoic bedrock and Cenozoic sedimentary rocks (Fig. 5). Cenozoic sedimentary deposits covering Mesozoic bedrock include the Tanjung Formation, the Berai Formation, the Warukin Formation, and the Dahor Formation [11].
The Tanjung Formation was formed during the Eocene epoch. This formation is dominated by fluvio-tidal sediments carrying coal seams to a marginal marine environment. The lithology is generally sandstone, carbonaceous claystone, and coal.
The Berai Formation was conformably deposited above the Tanjung Formation at the southern part of the basin. This formation was entirely influenced by marine environment. The Berai Formation is characterized as shallow-marine carbonate shelf rocks with lithology generally of claystone, marlstone, and limestone. The age of this formation is Early Oligocene to Middle Oligocene.
The Warukin Formation was conformably deposited above the Berai Formation. This formation showed deposition of a shallow marine that later became a fluvio-deltaic environment. The lithology is generally claystone, sandstone, and coal. Coal resources and reserves spread from the southwest to the northeast. The Warukin Formation is considered as a coal bearing formation [12]. The age of this formation is Middle Miocene - Late Miocene. The Warukin Formation is a major part of the rock unit revealed in the study area.
2) Geomechanics
Degradation of physical and mechanical properties of rocks is very likely to occur in rocks after exposure, especially in fine-grained sedimentary rocks such as claystone and sandstone. Weathering process occurs when rocks are exposed and gives impact on changes in the physical and mechanical properties of rocks. Degradation of mechanical properties in sedimentary rocks is influenced by 7 factors [14]. The factors are rock porosity, grain size distribution, quartz content, material density, average grain size, pore filler cement, and feldspar mineral content.
Stability of mineral forming the main rock (resistance to weathering) is expressed by Goldich series (Fig. 6). In this series, quartz is the most stable, followed by feldspar, mica, and other less stable minerals which are only present when weathering has occurred slightly. This Goldich series can explain mineral resistance to rock, so in analysis, it is combined with minerals obtained from mineralogical or petrographic tests, so that it will be able to strengthen the geological analysis (provenance).
Chemical process is characterized by entry of water and air into material to form a chemical reaction that can change the mechanical properties of rock [16]. Rock exposure that affects degradation of crystalline rock [17]. Weathering and degradation processes in mudstone and siltstone has a shorter time compared to weathering in crystalline rock [18]. Testing of mechanical properties on sedimentary rock in wet and dry conditions produces a significant difference between the two conditions [19]. Composition of clay mineral in rock influences the mechanical properties and the slope stability [20]. Composition and properties of clay mineral affects mechanical properties of rock [21]. The clay minerals are illite, montmorillonite, and kaolinite. Mineralogy of weak rocks is tended to be composed by clay mineral with limited silica mineral [22]. Clay mineral can increase the value of cohesion but reduce the value of internal friction angle. To improve the mechanical properties of clay mineral, a combination of maintaining physical properties and compaction to the material must be carried out [23].
3) Geochemistry
Petrographic analysis examines mineral composition and grain size. The results of petrographic analysis on sedimentary rocks will provide information about the composition of quartz, feldspar, and lithic. Based on this information, the rock samples can be classified. Understanding in the mineral composition can help in analyzing the provenance of sedimentary rock. It is done by identifying composition of the clastic sediment and its relationship to the genesis as well as the tectonic position. Provenance environment can be determined through composition analysis due to the fact that composition of fragments and minerals in a rock are influenced by the genesis of sedimentary rock formation at a certain tectonic position.
Clay minerals can be classified based on the mineral structure into 4 groups [24]., which are: Kaolin group (1:1), Hydrous mica group (2:1), Montmorillonite group (2:1), and Chlorite group (2:2)
One of weathering processes that commonly occurs in clastic sedimentary rocks is hydrolysis. Hydrolysis occurs due to replacement of cations in crystal structure by hydrogen, thus the crystal structure is damaged and destroyed. Hydrolysis is the most important chemical weathering because it can produce perfect destruction or drastic modification to easily-weathered minerals. The other common weathering process is feldspar weathering into clay minerals that can be kaolinite and illite. The Warukin Formations is composed by illite and kaolinite [25].