Abstract
K. MASATANI (1967) presented the oil geology of East Kalimantan on this journal. This short note is supplement to his paper. From the oil geologic point of view, the geologic development of the Meratus Range and the eastern rim of the Barito Basin is discussed herein.
Newly revised stratigraphy of the pre-Tertiary rocks encountered in the region drained by the Riam Kanan and Riam Kiwa Rivers is introduced and also geologic age of Koolhoven's Manunggul Formation is emended. The discovery of Esteriids from the base of the Rantaulajung Formation occupied at the top of the Manunggul Group which begins with the basal conglomerate and is overlain by the Turonian fossil-bearing formation and others strongly suggest that the necessity of reconsideration of Senonian age of the fauna described by K. MARTIN. The Cenomanian species of Orbitolina, namely O. concava reported by L.H. Krol is considered to be other species, such as Orbitolina lenticularis. Judging from the foraminiferal fauna of the Hantakan Limestone, geologic age of the Orbitolina-bearing formation exposed as the longitudinal strip along the west side of the Meratus Range is considered to be Aptian or so.
The Tertiary stratigraphy of the lower reaches of the Riam Kiwa River and the Tandjung oilfield is described. The boundary between the stages Tc and Tab is settled at the base of the C Sandstone which contains reworked Tab foraminifers in the northern part of the Tandjung anticline. The C Sandstone forming the Tandjung anticline unconformably overlies the Tab Formation which is already suffered slightly from folding occurred prior to the deposition of the C Sandstone. However, the Tc formation in the Pengaron area, more than 100km south of Tand-jung, overlies the Tab formation with an apparent conformity. The thickness of the Tc formation in the Tandjung area is 118m, while it is 20-90m in the Pengaron area. An unconformable relation between the Td and Tc formations is presumed in the Tandjung area, but MOHLER stated that "the B.P.M. field geologist took Tertiary c and d togcther, calling "Tcd"… the Tertiary every ofted begins with a slight unconformity". On the contrary, the Te formation in the Tand-jung area conformably overlies the Td formation. On the other hand, HASHIMOTO and KOIKE did not recognize any of the marine Td formation below the marine Te limestone, the Mataraman Formation, in their reconnaissance route along the Riam Kiwa River. A probable nonmarine formation, the Bumirata, is underlain conformably below the Mataraman.
It was recorded in the Kahajan test well drilled at Kahajan, about 120km southwest of Pengaron, that the Tcd limestone directly rests on the Tab formation. Thus, the "Te" reduces its thickness toward south and becomes absent, but it increases its thickness toward north and becomes 1, 800m thick in the Upper Mahakam region.
