FACIES ARCHITECTURE AND SEDIMENTARY PROVENANCE OF THE MIOCENE OMBILIN FORMATION: INTERPLAY OF TECTONICS AND EUSTASY IN A NEOGENE INTERMONTANE BASIN, CENTRAL SUMATRA, INDONESIA

The Early–Middle Miocene Ombilin Formation in the intermontane Ombilin Basin, Central Sumatra, records a critical interval of paleoenvironmental change shaped by regional tectonics and global sea-level fluctuations. Despite its significance, the formation’s sedimentological architecture, provenance, and sea-level history have been poorly constrained. This study integrates detailed outcrop-based sedimentary logging and petrographic analysis to address these gaps. Ten lithofacies are identified and grouped into three facies associations: open marine, tidal-influenced channels, and intertidal mixed tidal flats deposits. Their vertical distributions show normal and inverse transitions. The normal transition indicates a shift from sublittoral to tidally dominated environments, interpreted as a result of seaward progradation driven by increased sediment supply from the uplifting Barisan Mountains and short-term global sea-level falls (e.g. SEA34–SEA46). The inverse transition, however, indicates a transgression occurred in the Ombilin Basin. Petrographic analysis in the Ombilin Formation show arkosic sandstones sourced primarily from a magmatic arc, consistent with contemporaneous volcanism west of the basin, with minor reworking of marine units and older clastic materials. Stratigraphic evidence suggests an initial regression, likely driven by the uplifting of Barisan Mountain and global sea-level fall, which occurred in the Ombilin Basin during Early Miocene. This event was followed by deposition aligned with global transgressive trends. These findings provide new insight into the sedimentary evolution, provenance, and tectono-eustatic interplay controlling Ombilin Basin development during the Early–Middle Miocene.