Biostratigraphic Interpretation of Lutut Beds, Kerek Formation, Based on Foraminifera Fossils

Lutut Beds are the sandstone beds exposed in the northwest margin of Kendeng Basin, which contain abundant metamorphic-quartz grains, frequent recycled sedimentary quartz, and reworked bioclasts. Lutut Beds are not shown on the regional geological map and are often neglected in determining the geological history of Kendeng Basin. However, they have significant roles because it was deposited on the basin edge, which may carry important keys. This study aims to discover the larger and smaller foraminifera fossils contained in the Lutut Beds. Outcrop samples were collected in Kali Lutut and its surrounding area, including Lutut Beds and its overlying layers. Larger foraminifera was identified within 3 of 11 thin sections, mostly of Order Rotaliida, which occur as reworked bioclasts. The smaller foraminifera was barely found in most samples except for the samples from the overlying layers of Lutut Beds. The study reveals that Lutut Beds were deposited during Early to Middle Miocene, marked by the occurrence of Miogypsina sp. and Miogypsinoides sp. and also contains reworked Eocene – Early Oligocene larger foraminifera such as Nummulites sp., Discocyclina sp., and Dictyoconus sp. Besides, the overlying layer of Lutut Beds is identified to be deposited in the Middle Miocene to Pliocene based on the occurence of smaller planktonic foraminifera, Sphaeroidinella subdehiscens. The bathymetric interpretation based on smaller benthic foraminifera showed that Lutut Beds were deposited in the upper-middle bathyal zone. We also believe that the larger benthic foraminifera fossils in Lutut Beds were transported along the slope from its original life position. Accordingly, it is considered as allochthonous fossils.