ABSTRACT

The nonmarine Upper Jurassic-lowermost Cretaceous Todos Santos Formation of central Chiapas, Mexico, was deposited in half-graben basins formed during the early rifting stage of the southern Gulf of Mexico Basin. This formation consists of alluvial-fan, fluvial, and lacustrine sequences that were deposited under arid climatic conditions. Alluvial fans were built outwardly from the elongate basin margins by runoff that flowed transverse to the basin axis, whereas fluvial units were deposited by rivers that flowed parallel to the basin axis. Lacustrine deposition took place in a topographic depression adjacent to the basin margin. The presence of alluvial-fan and fluvial or lacustrine deposits in vertically stacked, cyclic megasequences hundreds of meters thick indicates that the basin opography changed through time. This cyclicity denotes that periodically fluvial or lacustrine environments occupied the basin-margin position and at other times alluvial-fan sedimentation occurred there. Cyclicity was caused by periodic changes from high to low rates of basin subsidene. The response of these three depositional environments to basin subsidence differed due to the unique hydrologic controls on sediment transport and deposition in each environment. Lakes were maintained by springs that emanated from the fractured fault margin and by fluvial discharge. Fluvial discharge and deposition resulted from precipitation or annual snowmelt anywhere in the river's expansive drainage basin, whereas alluvial-fan sedimentation occurred only when there was infrequent, significant precipi ation in the small drainage basin from which fan sediment was derived. As a result of these hydrologic controls, the lacustrine or fluvial environments responded more quickly to periods of active tectonic subsidence and migrated over the fans to occupy the basin-margin depression. Aided by a decrease in basin subsidence, alluvial fans eventually prograded and displaced the fluvial or lacustrine environments away from the basin margin.