MIOCENE STRATIGRAPHY AND PALEOGEOGRAPHY OF LOWER KUTEI BASIN, EAST KALIMANTAN - A SYNTHESIS

摘要

A geological section, 3500 m thick, early to late Miocene in age (late Burdigalian (nonoplancton zone NN4, ~ 17 Ma) to early Tortonian (~10 Ma)), is exposed in the Samarinda area, East Kalimantan in the Lower Kutei basin. New outcrops have been open in this area these last 15 years, revealing a continuous section from the core of Separi anticline up to the center of the next syncline (fig. B, C). We have published papers about different parts of this section. This paper represents a synthesis of all the previous studies carried out during many years. The section has been observed in four different locations, all of them well correlated laterally, showing the vertical continuity of the Lower to Middle Miocene of the Lower Kutei basin. Several lithostratigraphic units are distinguished. A synthesis of all lithological units shows the vertical stacking pattern of the sediments in the section and the sedimentary environment evolution through time: 1. ? The Hutunan village is situated at the very core of the Separi anticline and shows the oldest rocks cropping out in the Samarinda anticlinorium in and around Samarinda city. The Hutunan section is constituted by slope and basin gravity deposits (channel-levees slope turbidites Cibaj 2011). It terminates at the top with shelf break deposits represented by the Batu Putih carbonate 2.? Deposits observed in the Air Putih area are interpreted as shallow marine (Cibaj & Wiweko 2008, Cibaj 2009). They are constituted by dark shales followed by fine-grained sandstones, the whole covered by transgressive carbonate buildups, organized in 10-50 m thick prograding-retrograding parasequences. 3.? southward from Batu Putih carbonate, just opposite to the bridge across Mahakam river connecting Samarinda city with Loa Janan and Palaran. The section is constituted at the lower half by shallow marine parasequences and at the upper half by 20-40m thick prograding- retrograding deltaic parasequences (Cibaj 2009). 4.? The Harapan Baru village is situated in the Loa Janan area, just across the river from the Sungai Kunjang section. The geological section goes through the Pesona Mahakam and Tamansari housing complexes. The section has been opened and greatly benefited by the building of these two housing complexes. It is constituted mainly by thick sand bodies; very continuous laterally, shale and coal organized in 20-60 m thick cyclothemes. The section is interpreted as representing fluvial deposits (Cibaj, 2009) 5.? The new road in Palaran has been open to give access to the new Stadion Utama. It is situated 15 km south of Samarinda city. The lower one third of the section is constituted by 20-40 m thick prograding-retrograding deltaic parasequences (Cibaj et al. 2007). The upper two thirds of the section is constituted by thick sand bodies, shale and coal, organized in 20-60 m thick cyclothemes, interpreted as fluvial deposits (Cibaj, 2010) The interpretation of sedimentary environments and their spatial-temporal evolution provides a good picture of the geological history of this part of the basin during Miocene time, very useful for better understanding the occurrence of hydrocarbon fields of the Mahakam area.