Sierra Madre Formation
Sierra Madre Formation, Chiapas, Mexico
The Sierra Madre Formation is a thick layer of rocks in southern Mexico, about 2,590 meters (8,500 feet) thick. It formed during the Middle Cretaceous period, roughly 120 to 94 million years ago (Aptian to Cenomanian). The rocks are mainly marine dolomites and limestones, deposited in a shallow, brackish coastal setting such as a lagoon or estuary, with occasional stronger waves or currents.
This formation sits on top of the older Santiago and San Ricardo Formations and is itself overlain by the Ocozocoautla Formation (Campanian to Maastrichtian). The environment was rich in oxygen and productivity, and the rocks preserve many fossils, including fish, plants, and reef-builders called rudists.
In the El Espinal quarry area, the rocks show dolomite and dolomitic breccia, interbedded with orange clay and cream limestone, with features like ripple marks and desiccation cracks that reflect a dynamic shallow-water setting.
The Sierra Madre Formation has been studied since 1956, when it was formally described, and researchers have subdivided it into many subunits and lithofacies, totaling about 21 in some schemes. The thickness and fossil content make it a key reference for Cretaceous rocks in the region, similar in interest to the nearby Tlayúa Formation in Puebla.
Fossils include ancient fishes such as Archaeochiapasa mardoqueoi and Pepemkay (a Cenomanian ray-finned fish), along with other fish groups (Macrosemiids, Clupeomorphs, and Enchodus) and reef-building rudists. The top of the formation ends with an unconformity before the Ocozocoautla Formation.
This page was last edited on 2 February 2026, at 06:06 (CET).