## 2 Study Area and Geological Settings
### Tectonic Evolution  
Borneo Island, which hosts the Niah-Subis Limestone, originated from a group of micro-continental fragments [37; 38; 39]. The Sarawak Basin is a fore-deep basin located at the eastern end of the Sundaland shelf, commonly known as northwest of Borneo Island. Although Borneo Island is currently geologically stable, it has, historically, been subjected to intense tectonic activity that altered the architecture of the carbonate build-ups [36; 40; 41].  
The western interior zone of Borneo's basement comprises Palaeozoic and Mesozoic igneous, sedimentary, and metamorphic rocks that acted as a craton during the Middle to Late Tertiary [37]. During the Paleocene, the Luconia Block drifted away from China and towards Borneo [39; 42], and the floor between Borneo and the Luconia Block was subducted beneath Borneo. Subduction occurred along the Lupar Line, which extended into central Kalimantan, resulting in an accretionary complex comprised of the Rajang Group rocks, which formed the mountain range in central Borneo. The subduction aided the transition from a deep marine setting in the Sarawak Basin to a shallower environment to the southwest, allowing coral reefs to thrive [43]. During the Middle Miocene, the Luconia Block collided with the crust of central Borneo. This impact further elevated the Rajang Mountain Range in central Borneo [33]. Borneo experienced regional uplift after the Middle Miocene, resulting in massive deltaic systems that overlie the older marine deposits in Sarawak, thereby terminating carbonate sedimentation [44]. During the Pliocene, Sarawak underwent compressional deformation, which was most likely caused by (1) the subduction of the Proto-South China Sea (PSCs) through the North Borneo Trough or (2) by transmission along strike-slip faults generated by the extrusion of the Indo-China Block across the South China Sea up to Sarawak [45].  
The main tectonic domains of the study area are the Miri, Sibu, and Kuching Zones, which represent the North, Central, and Southwest of Sarawak, respectively [25; 33; 46; 47]. The Miri Zone, limited to the south by the Bukit Mersing Line (BML) and to the north by the Jerudong Fault [48], is where the Niah-Subis Limestone complex was formed. The Miri Zone comprises less deformed shallow marine and deltaic siliciclastic rocks that overlie the Oligocene-Micene deltaic to marine sediments (Tatau, Nivalau, Setap Miri, and Lambir and Tukau formations). These sediments are predominantly clastics with some isolated carbonate build-ups (Melinau, Subis, Suai, and their equivalent limestones).  
Large swathes of the Sarawak terrain are characterised by an arcuate fold and thrust belt, also known as the northwest Borneo Rajang Group fold-thrust belt, which extends over 1000 km along the strike and has a width of more than 300 km [33; 34]. Meanwhile, the structural trend lines run in broad curves in central and north Sarawak. Tight, overturned folds are common within these zones and are interrupted by a series of thrust faults. Following the fold-thrust event in central Sarawak, strike-slip reactivation of pre-existing faults occurred. Northwest-southeast compression or left-lateral strike-slip motion along North-northwest-trending faults has resulted in folds as well as normal, strike-slip, and thrust faults [34]. The Tinjar Line is a northeast to north-northeast-trending dextral transform fault that accommodated the spreading and subduction of the Proto-South China Sea beneath northwest Borneo from the Eocene to Early Miocene [49; 50; 51].