The coastal to marginally marine sandstone, siltstone, claystone, tuff and conglomerate formation dates to the latest Serravallian to Tortonian (Clarendonian to Hemphillian in the NALMA classification), from 12 to 8.5 Ma.

[4] In the Canal Zone the contact between the formation with the underlying Caimito Formation is covered by the waters of Lake Gatún and even before the flooding of the lake perhaps all of the contact was concealed by swamps.

[5] Massive medium- to very fine-grained sandstones and siltstones are the chief constituents of the Gatún Formation.

The sandstone contains numerous grains of black and greenish volcanic rocks and is practically a subgraywacke.

Conglomerates and hard brittle very fine-grained tuff make up a small part of the formation.