Plio-Pleistocene foraminiferal biostratigraphy of the eastern Kendeng Zone (Java, Indonesia): The Marmoyo and Sumberingin Sections

Abstract

The Plio-Pleistocene of the eastern Kendeng Zone (Java, Indonesia) consists of a >1250 m thick series, passing upwards from diatomaceous laminated mudstones to blue clays and deltaic sandstones. The fine-grained marine beds have received little attention from researchers. Age assumptions are based on correlations with better-studied sections in the western Kendeng Zone. In contrast, the deltaic sandstones at the top of the series have received ample attention. These beds are renowned for their fossil vertebrate fauna, including Homo erectus. Lately, doubts have arisen concerning the age and stratigraphic position of the fossiliferous sandstones. The aim of this study is to establish a detailed biostratigraphy of the fine-grained marine beds, which may also provide new age indications for the overlying fossil-bearing sandstones.We selected the Marmoyo and Sumberingin Sections as suitable representatives for the local marine stratigraphy. Both sections were logged and sampled and the samples were analysed for their foraminifera content. The sections provide sufficient biostratigraphic markers to allow for a reasonable age control. We found that the diatomaceous facies is older than previously thought: it dates from the Early Pliocene and corresponds in age with bathyal, foraminifera-rich mudstones of the western Kendeng Zone. It implies the existence of two bio-geographic zones in the Early Pliocene basin, with eutrophic, oxygen-depleted conditions in the east and normal oxygen conditions in the west. The diatomaceous mudstones change upwards into foraminifera-rich mudstones, marking a return to normal oxygen conditions later in the Early Pliocene, and eventually into blue clays, representing a shift from bathyal to outer shelf conditions in the Late Pliocene. Higher up in the blue clays, changes in the planktonic foraminifera assemblage indicate cooler water conditions, which we relate to the beginning of the Pleistocene.Although generally regarded as a continuous regressive series, our work revealed the existence of an unconformity within the blue clays. The blue clays above this unconformity represent a shallow marine, deltaic setting. It is this second blue clay series that passes conformably into the overlying deltaic sandstones. The presence of reworked planktonic foraminifera in the upper clay-series hampers adequate biostratigraphic dating, but the stratigraphic break within the clays suggests that this second sequence is younger than previously assumed.