Stratigraphy and geochronology of the Ola Kile Formation (central Flores, Indonesia): Implication for the depositional history of the So’a Basin

The So’a Basin is situated in central region of Flores, which forms part of the inner volcanic arc of the Banda Arc subduction system in Indonesia. This basin is a small sedimentary basin surrounded by volcanic morphology and folded older rocks. It is incised by the Ae Sesa river system, exposing the basement rocks in the deeper river valleys. The stratigraphic framework of the basin comprises the Ola Kile Formation (OKF), serving as the basement rocks and unconformably overlain by the Ola Bula Formation (OBF), followed by younger volcanic and alluvial deposits. Despite extensive studies on the OBF, including its stratigraphic subdivisions, lateral and vertical distribution, geochemical fingerprinting of marker tuff and ignimbrite layers, and age determination using various dating methods, the OKF remains understudied.This study aims to address the knowledge gap in the OKF by collecting detailed stratigraphic data from 16 key sections distributed across the western, central, eastern, and northeastern regions of the So’a Basin. Lithofacies analyses and depositional environment interpretations formed the basis for subdividing the OKF into distinct members. To validate these subdivisions, selected tuff, ignimbrite, and lava flow samples were analyzed for whole-rock major elements and glass shards trace elements compositions. Depositional ages for each stratigraphic member were determined using 40Ar/39Ar dating techniques applied to selected samples.The OKF is divided into four stratigraphic members: the Limestone Member, Lower Member, Middle Member, and Upper Member. The formation generally dips southwest to south-southwest at angles ranging from 6° to 35°, except for the Upper Member, which dips eastward at 6°-14°. The dips in the OKF are presumably interpreted as representing the original angles of repose formed during deposition, with minimal influence from post-depositional processes.The Limestone Member, exposed only in the Pakitoro area in the west, contains limestone layers with planktonic and benthic foraminifera fossils, as well as, brackish-water ostracods. These limestones are interbedded with debris-hyperconcentrated flow deposits and pyroclastic layers derived from multiple sources. The pyroclastic deposits are predominantly associated with the Welas volcanic eruptions in the northwest, while additional contributions may have originated from volcanic activity in the northeast, likely from the Keli Lambo Volcanic Center (KLVC), or from the Raja Volcanic center to the east. Based on facies association analysis, this member is interpreted to have been deposited in a lagoonal environment influenced by both volcanic activities and an influx of marine materials.The Lower Member, confined in the northeastern part of the basin, consists of black pumice ignimbrites associated with a caldera-forming eruption of the KLVC or Raja. These deposits reach a significant thickness of 265 meters. Around 2.43 ± 0.02 Ma, the KLVC underwent multiple effusive eruptions, forming andesitic lava flows up to 138 meters thick in the Dhadowawo area, as determined through 40Ar/39Ar dating of andesite groundmass.The Middle Member is dominated by the Lowo Meli Ignimbrite, a sequence of pyroclastic surge and flow deposits associated with a caldera-forming eruption likely originating from Raja or the KLVC. A published zircon fission-track age of ~1.86 ± 0.12 Ma from a tuff layer at Kopowatu, ~2 km to the west of Lowo Meli, is thought to represent the timing of this eruption.The Upper Member, spanning the western, central, northern, and southern regions of the basin, unconformably overlies older stratigraphic units of the OKF. It consists of a 70-meter-thick sequence of debris-hyperconcentrated flow deposits and black pumice ignimbrites derived from a caldera-forming eruption of Welas, with thicknesses of at least 100 meters in the central basin and 135 meters in the north.The proposed stratigraphic units of the OKF reveals valuable insights into the depositional and tectonic evolution of the So’a Basin. The transitions from shallow marine limestone to volcaniclastic-dominated sequences suggest shifts in sedimentation likely driven by uplift, subsidence, and volcanic activity preceding the emergence of Flores above sea level. These changes reflect the basin's response to regional compressional regimes and magmatic activity, possibly linked to the broader tectonic framework of the Flores region. Collectively, these findings contribute to advancing and potentially refining the established stratigraphic framework of the basin, while highlighting the OKF’s role as a geologic archive for reconstructing the tectonostratigraphic development of central Flores.