Controls on the evolution of carbonate mud mounds in the Lower Cretaceous Cupido Formation, northeastern Mexico

The Lower Cretaceous Cupido Formation in northeastern Mexico represents part of an extensive post-rift carbonate platform system that stretched across much of the Gulf of Mexico passive margin. Numerous sponge-microbial mud mounds are found within the lower part of the Cupido Formation; four mound types were identified at Bustamante Canyon and other localities in the State of Nuevo Leon, Mexico. Type 1 siliceous sponge-microbial mounds contain boundstone core facies, are the oldest mounds, and are associated with adjacent deep-ramp to basinal lime mudstone facies. Type 2 sponge-microbial mounds also have boundstone core facies, are associated with outer-ramp wackestone-packstone facies, and overlie strata that contain Type 1 mounds. Type 3 sponge-microbial-coral mounds contain only 40-50% boundstone core facies and overlie or are locally interstratified with Type 2 mound-bearing strata, although Type 3 mounds are associated with outer-ramp packstone facies. Type 4 calcisponge-microbial-coral biodetrital (dominantly skeletal packstone) mounds are the youngest mounds and are associated with outer-ramp packstone and ramp-crest rudist packstone facies. Both the length and height (i.e., synoptic relief) of individual Types 1, 2, and 3 mounds range from a few meters to tens of meters. Only Type 4 mounds reach several hundred meters in length and are up to 90 m thick, although synoptic relief on Type 4 mounds was minor. Systematic changes in the biota, morphologies, dimensions, abundance, stratigraphic distribution, and facies associations of the lower Cupido mounds were largely controlled by the initial ramp-like depositional profile of the lower Cupido platform and the long-term (second-order) accommodation trends that affected ramp evolution during Early Cretaceous time. The ramp-like profile was not conducive to transport of large volumes of allochthonous sediment from updip source areas to mound locations on the outer ramp. This allowed slowly accumulating Types 1, 2, and 3 mounds to build synoptic relief because adjacent outer-ramp facies accumulated at even slower rates. In contrast, Type 4 mound cores contain abundant coarse-grained skeletal sediment and also developed the least synoptic relief of all lower Cupido mounds because of higher accumulation rates associated with adjacent grain-rich ramp-crest lithofacies. We suggest that these systematic changes in mound attributes largely reflect long-term (second-order) accommodation loss and regional progradation of the lower Cupido ramp during Early Cretaceous time. Accordingly, the upward transition from Type 1 to Types 2 and 3 mounds, and ultimately to Type 4 buildups, can be explained by the progressive shallowing of outer-ramp environments as more updip facies tracts prograded eastward from late Hauterivian to early Aptian time. There are few stratigraphic successions in the rock record that provide better evidence for the links between large-scale stratigraphic relationships and mound evolution along low-gradient carbonate ramps.