## 2 Geological Setting  
The Limboto Lake--Pentadio area is part of the North Sulawesi Arc on Sulawesi Island, Indonesia, which was formed by the collision of three large plates: the Eurasian, Indo-Australian, and Pacific plates, during the Late Mesozoic to Cenozoic periods [25]. During this period, Sulawesi underwent three tectonic phases: the subduction system around Makassar during the Early Cretaceous period (110 Ma), the subduction of Central Sulawesi during the Middle Oligocene (28-31 Ma), and the Sula Microcontinent Collision in the Early Miocene [26; 27; 28]. Our study area was related to the tectonic process of the Sula Platform collision in the Middle Miocene. As a result of this collision, the tectonic evolution of the North Sulawesi arc can be divided into two main stages. The first stage was the Early Miocene westward subduction system that formed in the southern part of North Sulawesi [26; 27; 28; 25], which ended after being cut off by the Sula Platform collision. The second stage consisted of the post-collision rifting and uplifting of the arc and subduction along the North Sulawesi Trench during the Late Miocene to Quaternary periods. The rifting system in the North Sulawesi Arc can be traced from Kotamubagu to Gorontalo and west of Tolitoli. This W-E-directed rifting structure is known as the Limboto-Bone Rift [29]. This rifting tectonic process resulted in younger volcanism, with the formation of the Pilomba and Lompotoo volcanoes around Gorontalo. This young volcanic system formed a dyke system that extends parallel to the Gorontalo graben zone below the surface. The tectonic activity in the North Sulawesi Arc has affected magmatism and volcanism [30]. The North Sulawesi arc in the Sulawesi lithotectonic structure [31] is a magmatic and volcanic pathway at the eastern end of the Sunda Shelf, extending from the northern arm to the southern arm of Sulawesi Island. This arc is composed mainly of sedimentary rocks from the Mesozoic-Tertiary periods and volcanic-plutonic rocks from the Paleogene-Quaternary periods. The volcanic rocks in the old arc were associated with the Central Sulawesi subduction system during the Eocene-Central Oligocene period [28] prior to the orientation caused by the Sula Platform collision. These volcanic rocks have a basaltic to andesitic composition and belong to the Timombo Formation [32]. Following the Sula Platform collision, magmatism and volcanism in the North Sulawesi Arc were more likely to have been impacted by the rifting process than by the North Sulawesi subduction system [29]. Based on rock age dating, magmatism in the North Sulawesi Arc can be divided into two phases: Early Miocene (22-16 Ma) and Late Miocene-Quaternary (<9 Ma) magmatism [33; 34]. The volcanism in the Early Miocene was characterized by the Bilungala Volcano products, which consist of tuff, volcanic breccias, and lava with a basaltic to andesitic composition, interspersed with the shallow marine sedimentary rocks of the Dolokapa and Tapadaka formations [32]. Volcanism and sedimentation in a shallow environment led to the formation of limestone carbonate sediments. Co-magmatic granitidis in the composition range of diorite to granite [29] associated with diorite bone breakthrough rock [32] were formed in the Late Miocene. Further volcanism in the North Sulawesi Arc is estimated to have occurred in the Pliocene-Pleistocene periods, forming the Pinogu Volcano group [32] whose eruption products are tuff, volcanic breccias, and lava with an andesitic-to-basaltic composition. During the Pleistocene, uplifting caused the North Sulawesi Arc to undergo erosion and surface sedimentation in the form of lake and river alluvial deposits. Thus, erosion caused co-magmatic granitoid rocks to emerge on the surface as intrusive rocks. The study area had five groups of rocks, from old to young: old volcanic rocks, intrusive rocks, sedimentary rocks, young volcanic rocks, and alluvium deposits (Figure 1). The old volcanic rock was composed of three rock units: basaltic lava, andesitic lava, and pyroclastic deposits.