Biostratigraphy and strontium isotope dating of Oligocene-Miocene strata, East Java, Indonesia

Abstract

ligocene-Miocene carbonate and siliciclastic strata exposed in the East Java basin can be grouped into three stratigraphic intervals or formations: from oldest to the youngest, they are the Kujung, Tuban and Ngrayong Formations. The Bulu Member of Wonocolo Formation caps the studied stratigraphic succession. Each of these stratigraphic intervals is defined by its fossil assemblages and lithologic characteristics. However, the most dominant fossils in all formations are warm water, shallow marine groups including larger benthic foraminifera, hermatypic corals and coralline algae. Larger foraminifera and planktonic foraminifera overlap in occurrence in many localities allowing direct comparison of larger foraminifera “letter stages" biozones with oceanic plankton scales. The biostratigraphic ranges of the identified larger benthic and planktonic foraminifera have been tied to the ages constrained from the strontium isotope dating of some of the most abundant large benthic foraminifera. Foraminiferal assemblages and strontium data have provided precise age ranges of the different stratigraphic units such as the ages of Lower Kujung (late Early Oligocene, Rupelian P20-Late Oligocene, Te1-4, 28.78 - 28.27 Ma respectively), Upper Kujung (Late Oligocene, Chattian, P22, Te1-4- Early Miocene, Aquitanian, -N4, -Te5, 24.31- 23.44 Ma respectively), Tuban (late Early Miocene, Burdigalian- Middle Miocene, Langhian, N5-N9, Te5-Tf1, 20.80 – 15.25 Ma respectively ) and Ngrayong (late Middle Miocene, Serravallian, N10-N12, Tf2,~5.00-~13.00 Ma) Formations. The age boundaries between the lithostratigraphic units have also been determined such as Upper Kujung and Tuban (22 Ma), Tuban-Ngrayong (15.25 Ma) Formations and Ngrayong Formation-Bulu Member (12.98 Ma).