The Sankarewang Formation is an ?Eocene-aged geological formation in Sumatra, Indonesia near Padang. It is among the very few Paleogene fossil deposits from Southeast Asia that preserves a freshwater ecosystem, and contains many of the earliest records of freshwater fish taxa that now predominate the region. Many of the fishes from this formation are well-preserved as articulated skeletons. The fossils of the formation have been known since the 1870s, although they only received significant attention during the 1930s and again starting from the mid-2010s.[1]
The age for this formation has long been disputed, with suggested ages ranging from the Cretaceous to the Miocene. Although the exact age still remains uncertain, most recent studies have settled on a tentative Paleogene age, with estimates ranging from the Paleocene[2] to the Oligocene. More recently, unpublished palynological data suggests that the overlying Sawahlunto Formation is of middle-late Eocene in age, which would most likely place the Sangkarewang Formation in the early-mid Eocene.[1]
It was deposited within the Ombilin Basin, a small Paleogene-aged rift basin that formed from deformation along the Great Sumatran fault. The Sangkarewang Formation was deposited within a freshwater rift lake that formed in this basin early on, with anoxic bottom waters allowing for the fossilization of the fish skeletons.[3][1] The Sawahlunto Formation, long exploited for its coal seams, was later deposited in the lake on top of the Sangkarewang Formation. During the Oligocene, this gave way to a river delta (the Sawahtambang Formation), and was later flooded by the sea by the Miocene (the Ombilin Formation), before a tectonic uplift raised it above the sea.[3] The formation has been explored for its oil shales.[4]
Paleobiota
Bony fish
Partially based on Woodward (1901) & Sanders (1934).[5][6] Sanders (1934) published a comprehensive monograph about the fishes of the formation using a large number of specimens. However, the vast majority of these specimens are now lost, and may have potentially been destroyed during World War II,[7] preventing any further study of her specimens.
A gourami initially assigned to the modern giant gourami, and likely in the genus Osphronemus; however, the specimens have been lost, hindering further analysis.[8]
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