Sedimentology and Diagenesis of a Rift Basin Lacustrine Sandstone: Pematang Group, Central Sumatra, Indonesia

J. S. Janks, P. A. Kelley, H. H. Williams

The Central Sumatra basin is a back-arc basin that formed during the Paleocene as a series of half-graben structures. These early formed half-graben structures were filled with nonmarine clastics and lacustrine sediments of the Pematang Group, sourced from local highland areas. The Pematang Group consists of the Lower Red Beds, Brown Shale, Coal Zone Member, and Lake Fill Formation (in ascending order). The Pematang Group sedimentology is intimately related to the regional and basinal tectonic development and history. At least three events relate to trough development: (1) pregraben stage (Lower Red Beds deposition), (2) graben stage (Brown Shale and/or Coal Zone deposition), and (3) postgraben stage (Lake Fill deposition). In the pregraben stage alluvial fans prograded t ansversely along bounding faults, and a complex distributary plain/swamp developed in the interior of the troughs. In the graben stage extensive organic-rich deep lacustrine facies developed, as well as minor platform and delta-front facies. Stable isotope data from limestones in the platform facies suggest significant climatic changes occurred during lacustrine deposition. The volumetrically limited platform facies is a mixture of gastropod-rich lime and terrigenous muds. Shallow lacustrine facies with occasional thin coal stringers and minor sandstones suggesting distributary plain environments comprise the Coal Zone Member. Final deposits belong to the Lake Fill Formation and are characterized by lacustrine, fluvial, distributary plain/swamp, delta-front, and alluvial facies. Facies a sociations are often complex and show cyclic repetition.

Sandstones of the Pematang Group are predominantly sublitharenites and litharenites; feldspars are rare. Sandstone diagenesis is relatively uniform regardless of the depositional environment. Diagenetic modifications include compaction, early calcite and dolomite cementation, quartz overgrowth formation, unstable rock-fragment dissolution, kaolinite precipitation, siderite formation, and local illite formation. Secondary porosity accounts for up to 50% of the effective porosity and was created by rock-fragment dissolution. This dissolution is probably caused by the organic acids released during kerogen maturation. Stable isotope data from diagenetic siderite are presented.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.