Los Bastos Formation comprises mudstones and sandstones deposited in a fluvial environment.

The formation was named by Garrido in 2010 as shaly unit conformably and transitionally overlying the Portezuelo Formation, to which the layers were formerly defined and in the same manner underlying the Sierra Barrosa Formation, all belonging to the Río Neuquén Subgroup of the Neuquén Group in the Neuquén Basin.

The formation at its type section reaches a thickness of 35 metres (115 ft).

The formation comprises red mudstones intercalated by thin levels of siltstones and fine well-sorted yellowish to greenish grey sandstones.

Based on the stratigraphic relations with the overlying and underlying units, the age has been estimated to be early to middle Coniacian.