AN ANOMALOUSLY LARGE LIQUEFACTION STRUCTURE, OLIGOCENE, OMBILIN BASIN, WEST SUMATRA, INDONESIA

Soft sediment deformation features are common in the Tertiary sediment ary succession of the Ombilin Basin, West Sumatra. A spectacularly lar ge scale example of one of these features from fluvial sandstones and shales of the Sawahlunto Formation of the Ombilin Basin is illustrated and described. The structure is interpreted to be a sandstone load ba ll formed by syn-sedimentary liquefaction, the size of which merits de scription, since such large structures are rare in fluvial sequences a nd are more typically observed on a much smaller centimetre or millime tre scale or within marine, deep water sequences. The cause of the liq uefaction is suggested to be from syn-sedimentary tectonic activity wi thin the basin and/or along its margins, possibly related to movements on a precursor of the presently active Sumatran Fault System. Implica tions for palaeo-seismicity along the Sumatran Fault System and geolog y of the Ombilin Basin are discussed. Copyright (C) 1996 Elsevier Scie nce Ltd