Depositional Environment of Coral–Rudist Associations inthe Upper Cretaceous Cardenas Formation (Central Mexico)

Abstract

In the Cardenas Formation (central Mexico), a 175 m thick sedimentarysequence of Maastrichtian age was analyzed with respect to its palaeontology and sedimentology. A wide variety of lithological and palaeontological features characterize this sequence comprising unfossiliferous and fossil-bearing sand- and siltstones, and diverse rudist and coral–rudist associations in carbonate or mixed carbonate/clastic lithologies. A total of 24 rudist and coral–rudist associations are exposed in the investigated section, which are grouped into 5 limestone units. Radiolitid assemblages, coral–rudist reefs, coral-domi-nated reefs, and hippuritid-dominated reefs are present. The stacking pattern of these reef intervals indicates a general transgressive trend through the entire section. Smaller-scale facies trends could be distinguished within each limestone unit, comprising deepening-upward sequences, defined by a shoreface–calcareous algae–radiolitid–marl facies transition, and shallowing-upward sequences defined by a hippuritid–actaeonellid–coral/rudist facies transition. This cyclic sedimentation pattern is obscured by an episodic input of clastic sedimentsderived from the uplifting Sierra Madre Oriental, which in turn triggered either the development or decline of reefs.