Sangkarewang Formation

The Sankarewang Formation is an ?Early or Middle 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]

TypeGeological formation

Unit ofOmbilin Basin

UnderliesSawahlunto Formation

OverliesBrani Formation

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Sangkarewang Formation
Stratigraphic range: ?early-mid Eocene PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Unit of	Ombilin Basin
Underlies	Sawahlunto Formation
Overlies	Brani Formation
Lithology
Primary	Shale, sandstone
Other	Oil shales
Location
Coordinates	0.6°S 100.8°E / -0.6; 100.8
Approximate paleocoordinates	0.5°S 110.3°E / -0.5; 110.3
Region	West Sumatra
Country	Indonesia
Sangkarewang Formation (Indonesia)

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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, 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][3]

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.^[4][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.^[4] The formation has been explored for its oil shales.^[5]