Allogenic River in the Hydrogeological System of Gremeng Cave, Gunungsewu Karst Area, Java Island, Indonesia

Allogenic rivers play a pivotal role in the development of the karst area. Their open system creates aggressive conditions that cause more intensive rock dissolution and, therefore, possibly form a primary underground river system in a hydrogeological regime. This study aimed to inventory allogenic rivers in the hydrogeological system of Gremeng Cave. This study also investigates Gremeng Cave typology based on underlying geological conditions by analyzing Indonesian topographic and geologic maps and high-resolution remote sensing imagery. In this study, a detailed geological survey was conducted to determine factors triggering the formation of allogenic rivers at the research site based on geological and geomorphological characteristics. The results showed at least five allogenic river subsystems uniting into one primary system. Allogenic from underlying beds exposed updip was the type of the allogenic rivers emerging exactly at the contact between tertiary volcanic rocks and limestones at the Gunungsewu karst area. © 2020 IOP Publishing Ltd.