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Abstract: The Characteristics of Neogene Sediments and Structures in the Siberuang Area (Central Sumatra, Indonesia) Based on Gravity Data

Djoko Santoso

Neogene sediments in the Siberuang area consist of the Sihapas, Telisa, and Petani formations. The Sihapas Formation is a friable, white to brown sandstone with intercalated claystones, siltstones, and some coal layers. It was deposited in channels or bars of a braided-stream system during early Miocene N4-N8 times. The Telisa Formation, which unconformably overlies the Sihapas Formation, is a calcareous claystone with intercalated glauconitic sands and limestone lenses deposited in a middle to outer neritic environment during middle Miocene N8-N10 times. The Petani Formation is a claystone containing clasts of terrestrial material with some coal lenses and tuffs in the upper part of the section. It is a regressive sequence deposited in an outer neritic environment. These Neogene sedi ents lie unconformably above the Paleogene Pematang Formation, which is mainly composed of brown conglomerate and conglomeratic sandstones. The Pematang Formation lies unconformably above pre-Tertiary rocks. A Bouguer gravity map processed using a density value of l.9 gm/cc reveals anomalies with wavelengths of 10 and 15 km. Moving average filtering has been applied to make a regional-residual separation. Map interpretation and modeling along a profile run northeast across the structural trend of the region and indicate a graben-type structure with Neogene sediment thickness increasing from 600 m in the northeast to 2800 m in the southwest. Greatest sediment thickness is attained in the center of the graben, which is delineated by northeast-dipping faults southwest of the area and southw st-dipping faults northeast of the area.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994