Geological Framework and Rock Occurrence History of the Long Loreh Region, South Malinau District, North Kalimantan, Indonesia

Abstract

Northern Kalimantan preserves a complex record of tectono-sedimentary evolution from the Eocene to the Late Miocene. This study applies systematic surface geological mapping supported by detailed field observations of lithology, sedimentary structures, and measurements of bedding and fault orientations, complemented by regional geomorphic analysis. The Bilabekayuk Sandstone, Bilabekayuk Siltstone, and Gongsolok Argillite, assigned to the Malinau Formation of the Rajang–Embaluh Group, were deposited during a Middle–Late Eocene marine transgression in a shallow-marine to tidal-flat setting. Oligocene–Miocene subduction of the Palawan oceanic plate resulted in regional compression, thrust faulting, uplift of the Kuching High, and development of prominent W–E-trending lineaments with steeply tilted strata. Associated Miocene magmatism produced the Jelai volcanic rocks and Sengayan Tuff. Subsequent basin isolation led to Late Miocene lacustrine deposition of the Langap Formation, including the Sengayan Conglomerate and Sengayan Sandstone with local swamp influence. These results document a regional transition from marine sedimentation to tectonic inversion, volcanism, and continental basin development in northern Kalimantan.