ABSTRACT

Initial Mesozoic sedimentation in western Chiapas, represented by the Upper Jurassic-lowermost Cretaceous Todos Santos and San Ricardo Formations, occurred over a horst-and-graben terrane formed during rifting in the ancestral Gulf of Mexico Basin. The Todos Santos consists of fluviolacustrine sequences deposited over block-faulted granodiorite. The San Ricardo overlies and interfingers with the Todos Santos, and consists of siliciclastics, carbonates, and evaporites deposited in arid tidal flat, beach, restricted back-barrier lagoon, barrier, shoreface, and shelf environments. San Ricardo facies distribution outlines three transgressive and regressive or still-stand sequences of Oxfordian, Kimmeridgian, and Tithonian-early Hauterivian age. The correspondence between the ages of these sequences and the age of eustatic cycles suggests that their development was primarily influenced by global sea-level changes. Superimposed on this eustatic pattern are stratigraphic features attributed to variable tectonic subsidence of basement blocks, including abrupt lateral thickening or thinning of facies, the localization of ooid grainstone deposition on horsts, and carbonate-platform drowning. Locally consistent patterns of thickening or thinning of both continental and marine facies indicates rift-related block faulting was active throughout Todos Santos and San Ricardo deposition.

This style of rift-basin sedimentation contrasts with the Atlantic-margin type by including normal-salinity marine facies in addition to continental and restricted-marine facies. The identification of this style in the Gulf of Mexico is important for reconstructing and differentiating rift and postrift stages of basin development and for predicting the subsurface pattern of Upper Jurassic-Neocomian facies.