Earliest Mesozoic sediments in southeastern Mexico and western Guatemala consist of clastics of the Late Jurassic Todos Santos and Late Jurassic-Early Cretaceous San Ricardo formations. The Todos Santos is a red bed sequence which averages about 850 m in thickness, but varies significantly from area to area. The San Ricardo, formally defined here for the first time, is about 400 m thick and is composed of limestone and clastics of marginal marine origin. Stratigraphic sections and detailed lithologic descriptions are presented to define these 2 formations.

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