Stratigraphy and sedimentology of Oligocene-Miocene mixed carbonate and siliciclastic strata, East Java Basin, Indonesia

Oligocene-Miocene strata of East Java are characterized by mixed carbonate and siliciclastic sediments. Factors such as local and regional tectonics, sea level variations, paleogeography and weathering of landmass had influenced the stratigraphic and sedimentologic evolution of the basin during this time. The stratigraphic framework of the Oligo-Miocene has been constructed by integrating outcrop data, ages constrained from the index foraminiferal assemblages and strontium isotopes dating, seismic lines and well logs. Key seismic reflectors have been identified in the field and age dated.; The Oligocene-Miocene strata of East Java are characterized by multiple stages of isolated carbonate mound growth surrounded by deeper marine off-mound sediments or by shallow-marine siliciclastics. Three stratigraphic intervals are recognized: Kujung (carbonate mound and off-mound), Tuban (mixed carbonate-siliciclastic) and Ngrayong (siliciclastic). Kujung exposures (∼28--22 Ma) are limited to few isolated outcrops. At the base, it is represented by high-energy, extensive, shallow marine carbonates that grade laterally into deep marine off-mound sediments of calcareous mudstone and chalk (lower Kujung). Lower Kujung shallow marine carbonates were covered by thick chalk and marl sediments during the middle-upper Kujung. In other locations, the middle and upper Kujung shallow-water carbonates were restricted to topographic-highs. The Tuban unit (Aquitanian-Langhian, ∼22--15 Ma) consists of widely exposed shallow marine mixed carbonates and siliciclastics and poorly exposed open marine shale and chalk facies. The Tuban at the subsurface is characterized by two main packages with lateral extension. The lower package (Aquitanian-Burdigalian) is rich in a series of low relief isolated mounds, highly fossiliferous carbonates and fine grained siliciclastics reflecting carbonate deposition with episodes of siliciclastic influx. The lower package buries the depositional topography created by the Kujung mounds. The upper package (Burdigalian-Langhian) is defined by rapidly alternating, bedded carbonates, mudstones and sandstones reflecting deposition under relatively planar sea floor. The Ngrayong unit (Serravallian) ∼15--12 Ma) represents a period of regional siliciclastic influx and progradion of tidally influenced deltas grading into turbidites, basinal shale, mudstone and chalk. Ngrayong beds are truncated by Bulu carbonates (Serravallian-Tortonian). This is consistent with the tectonic evolution of the region.