The Talacasto Formation is a geological formation in Argentina, exposed primarily across San Juan Province, with an additional outcrop in La Rioja Province.

It preserves fossils dating back to the Devonian period.

The Talacasto Formation is exposed in the Precordillera Basin of Argentina, with outcrops most widespread in San Juan Province, though an isolated exposure also exists at Sierra de las Minitas in La Rioja Province.

[1][2] Along with the Punta Negra Formation, it makes up the Gualilán Group, which represent Devonian-aged deposits in the Argentine Precordillera.

[3] The thickness of the Talacasto Formation varies across its extent; at the type locality of Quebrada de Talacasto in the south it is 300 m (980 ft) thick, while at the Loma de los Piojos section near the Jáchal River in the north it exceeds 1,000 m (3,300 ft) in thickness.