This is also late Oligocene in age and represents a basinal depositional environment. It is considered to be an offshore, outer shelf or basinal, deep-water taxon. Early Namurian basinal mudstones are the source rocks for these hydrocarbons. Examples of such basinal environments include backarc, forearc, passive margin, epicontinental, and extensional basins. The genesis of the basinal lakes, however, S.V. Parnachov admitted so far as ice-dammed one. The basinal sequence starts with the Imbituba Formation, a unit consisting of basaltic volcanic rocks.