ABSTRACT: The areal control of building blocks of the third order depositional sequences in the Mahakam delta

Lambert, Bernard¹, Stefano Mora², and Agung Wiweko²
(1) TotalFina, Paris la Defense, France 
(2) Total Indonesie, Balikpapan, Indonesia

In the Kutei basin (East Kalimantan) a continuous deltaic sedimentation took place during the Neogene. Several kilometers of sediments filled the space created by the huge subsidence in an overall regressive sequence.

Starting during the Middle Miocene, a compression phase induced the uplift of the present day onshore area (a large amount of sediments was eroded) and the tilting of the offshore area where the subsidence remains high (50m/100000y) during the Upper Miocene and Plio-Pleistocene.

During the Upper Miocene, significant differences in the seismic patterns (Progradation/Aggradation of deltaic cycles) are observed for a same stratigraphical interval between North and South part of the Mahakam area. This phenomena is not related to local changes in the subsidence ratios or third order sea level variations but clearly associated to changes in the sedimentary influx direction. This case study shows that in a deltaic environment characterised by a high rate of subsidence and a strong sediment influx, the seismic stacking patterns are mainly controlled by large-scale autocyclic processes.

During the Plio-Pleistocene, the strong rate of subsidence is counterbalanced by the ice ages high frequency sea level variations, allowing a brief shelf exposure. But the duration of this exundation is not sufficient enough to create a lowstand prograding wedge. The sediments are directly deposited on the slope through a fluvial channels network (on the shelf) and incised valleys (on the shelf edge). When high frequency sea level changes appear, allocyclic processes become an important factor controlling the cyclic stacking patterns.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia