John-Paul Zonneveld, Nabilah Adani, Aswan, Jonathan I. Bloch, Antonino Briguglio, Russell L. Ciochon, Laura J. Cotton, Agus T. Hascaryo, Jason Head, Javier Luque, Yan Rizal, Nadia Santodomingo, Thierry Smith, Jonathan Todd, Peter Wilf, Yahdi Zaim
Marine sedimentary rocks of the late Eocene Pagat Member of the Tanjung Formation in the Asem Asem Basin near Satui, Kalimantan, provide an important geological archive for understanding the paleontological evolution of southern Kalimantan (Indonesian Borneo) in the interval leading up the development of the Central Indo-Pacific marine biodiversity hotspot. In this paper, we describe a moderately diverse assemblage of marine invertebrates within a sedimentological and stratigraphical context. In the studied section, the Pagat Member of the Tanjung Formation records an interval of overall marine transgression and chronicles a transition from the marginal marine and continental siliciclastic succession in the underlying Tambak Member to the carbonate platform succession in the overlying Berai Formation. The lower part of the Pagat Member contains heterolithic interbedded siliciclastic sandstone and glauconitic shale, with thin bioclastic floatstone and bioclastic rudstone beds. This segues into a calcareous shale succession with common foraminiferal packstone/rudstone lenses interpreted as low-relief biostromes. A diverse trace fossil assemblage occurs primarily in a muddy/glauconitic sandstone, sandy mudstone, and bioclastic packstone/rudstone succession, constraining the depositional setting to a mid-ramp/mid to distal continental shelf setting below fair-weather wave base but above storm wave base. Each biostrome rests upon a storm-generated ravinement surface characterized by a low-diversity Glossifungites or Trypanites trace fossil assemblage. The erosional surfaces were colonized by organisms that preferred stable substrates, including larger benthic foraminifera, solitary corals, oysters, and serpulid annelid worms. The biostromes comprised islands of high marine biodiversity on the mud-dominated Pagat coastline. Together, the biostromes analyzed in this study contained 13 genera of symbiont-bearing larger benthic foraminifera, ∼40 mollusk taxa, at least 5 brachyuran decapod genera, and 6 coral genera (Anthemiphyllia, Balanophyllia, Caryophyllia, Cycloseris, Trachyphyllia, and Trochocyathus), as well as a variety of bryozoans, serpulids, echinoids, and asterozoans. High foraminiferal and molluscan diversity, coupled with modest coral diversity, supports the hypothesis that the origin of the diverse tropical invertebrate faunas that characterize the modern Indo-Australian region may have occurred in the latest Eocene/earliest Oligocene. Copyright © The Author(s), 2025.
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada; Faculty of Earth Science and Technology, Bandung Institute of Technology, Indonesia; Florida Museum of Natural History, University of Florida, Gainesville, FL, United States; Dipto. di Sci. della Terra Dell'Ambiente e della Vita Universita Degli Studi di Genova Corso Europa, Genova, 26 - 16132, Italy; Department of Anthropology, University of Iowa, Iowa City, IA, United States; Natural History Museum of Denmark, Oster Voldgade 5, 7, Kobenhavn K, 1350, Denmark; Department of Zoology and University Museum of Zoology, University of Cambridge, Cambridge, United Kingdom; Natural History Museum, London, United Kingdom; Institute of Earth Sciences (ISTE), University of Lausanne, Switzerland; Royal Belgian Institute of Natural Sciences, Brussels, Belgium; Department of Geosciences, Pennsylvania State University, University Park, PA, United States; Department of Geology, Institut Teknologi Sumatera - ITERA, Indonesia