The basin has an approximate area of 8,000 square kilometres (3,100 sq mi) and at its deeper parts the surface of its sedimentary fill reaches 200 metres (660 ft) below sea-level.

[4] Given a high density of geological faults that have displaced the coal beds and the thin nature of these (less than one metre) mining activity in Arauco Basin has proven difficult to mechanize.

At present it reaches 100 metres (330 ft) above sea level in some locations and is dissected by a number of small valleys.

[6] The base of the Ranquil Formation is the so-called "main unconformity" which is thought to have formed by erosion during a period of tectonic inversion.

Then a basin inversion lasting from the Middle Eocene to the Miocene causing the uplift and erosion that created the "main unconformity" and finally a post-inversion phase of strike-slip faulting in the Pliocene and Pleistocene.