The Lower Cretaceous (Barremian-lower Aptian) Cupido Formation in NE Mexico is part of the extensive carbonate platform system that extends almost continuously from NE Mexico to the Florida Peninsula. Spectacular Cupido exposures in NE Mexico provide an important outcrop analog for this largely subsurface platform system, which includes the Sligo Formation of onshore Texas and Louisiana.

The lower part of the Cupido Formation, the focus of this preliminary study, consists of a complex array of deep- to shallow-water carbonate lithofacies in outcrops near Monterrey and Saltillo. These lithofacies were deposited in a broad platform-to-basin transition zone between wave-agitated shoal environments and deeper water, sub-wave base settings. Several, five- to ten-km-long outcrops of relatively flat-lying Cupido strata (e.g., Bustamante Canyon) indicate that the Early Cretaceous platform in NE Mexico had a low-relief, ramp-like profile, with evidence for only local steepening.

Three third-order depositional sequences are recognized within the lower Cupido Formation. The first depositional sequence (100-180 m thick) consists of transgressive deeper water mudstone/wackestone facies that grade upward into either wackestone/packstone facies with sponge-algal mud mounds or thick-bedded to massive skeletal-peloidal packstone facies. The second sequence (40-45 m thick) consists of transgressive deeper water mudstone/wackestone facies which grade upward into skeletal-oncolite packstone/grainstone facies that were deposited in wave-agitated environments. The upper grainy interval contains strong fabric evidence for meteoric alteration. The third sequence (120-140 m thick) also begins with deeper water mudstone/wackestone facies (± local sponge-algal-coral-stromatoporoid mud mounds) that grade upward into thick skeletal-oncolite packstone/grainstone facies. Each of the Lower Cupido depositional sequences consists of a lower transgressive interval (lime mudstone/wackestone facies) that is overlain by shallower water facies. Strata that might be placed into lowstand systems tracts for each sequence are not obvious, which supports our preliminary contention that the lower Cupido platform had a ramp-like profile. The systematic increase of shallow-water lithofacies within each successive depositional sequence indicates the larger-scale progradational character of the Cupido Formation, which has been interpreted previously as the highstand systems tract of a second-order supersequence.