How long has sedimentary rock been around?

As of August 2014, the oldest sedimentary rocks known are banded iron formations from Greenland and Quebec that have been dated to 3.8 or 3.9 billion years old. The material composing these rocks may have been extracted from the Earth's mantle as much as 4.28 billion years ago.

Researchers from the University of Chicago and the University of Colorado state that these rocks probably precipitated from seawater under oxygen-starved conditions and that the iron came from hydrothermal vents along mid-ocean ridges. Banded iron formations are not being formed in modern times because seawater now contains a much higher proportion of dissolved oxygen that would cause iron to precipitate rapidly near the vents.