ABSTRACT

Limestones of the Lower Cretaceous Cupido Formation (late Neocomian to Early Aptian) were deposited during the general Mesozoic transgression of a warm, clear-water, epeiric seaway over northeastern Mexico.

The development of the massive Cupido carbonate platform, with the Coahuila peninsula acting as a nucleus, was primarily due to the sudden proliferation of rudistid bivalves in the Lower Cretaceous. Rudists, mostly requinids and caprinids, along with dendroid corals, red algae, and stromatoporids constructed ecologic reefs in the higher energy zone as waves touched bottom along a shallow carbonate ramp (formed by the underlying Taraises Formation). The reefs trapped bioclastic debris and baffled the wave energy to produce lower energy back reef environments. The accumulation of carbonate material in the reef complex kept up with basinal subsidence and sea level rise, and in approximately nine million years formed a platform with a shelf margin of considerable submarine topographic relief.

The vertical sequence of lithologies found at several localities indicate localized regression of the sea as the reef and its laterally coexisting environmental facies prograded over basinal lime muds. Six laterally coexisting facies are listed below in the order in which they are commonly found in a vertical measured section:

6. Lagoon, restricted lagoon, and sabkha facies

5. Near-reef tidal flat facies

4. Near-reef shoal facies

3. Organic reef facies

2. Fore-reef slope talus facies

1. Basinal facies.

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