An anomalously large liquefaction structure, oligocene, Ombilin Basin, West Sumatra, Indonesia

Soft sediment deformation features are common in the Tertiary sedimentary succession of the Ombilin Basin, West Sumatra. A spectacularly large 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 ball formed by syn-sedimentary liquefaction, the size of which merits description, since such large structures are rare in fluvial sequences and are more typically observed on a much smaller centimetre or millimetre scale or within marine, deep water sequences. The cause of the liquefaction is suggested to be from syn-sedimentary tectonic activity within the basin and/or along its margins, possibly related to movements on a precursor of the presently active Sumatran Fault System. Implications for palaeo-seismicity along the Sumatran Fault System and geology of the Ombilin Basin are discussed.