Stratigraphic architecture, platform evolution, and mud-mound development in the Lower Cupido Formation (Lower Cretaceous), northeastern Mexic

The Lower Cretaceous (Upper Hauterivian-Lower Aptian) Cupido Formation is part of a carbonate-platform system that extended from northeastern Mexico and across the U.S. Gulf Coast area from southern Texas to Louisiana (Sligo and Pettet formations) and west Florida (Pumpkin Bay Formation). In this study, the stratigraphic architecture of the lower part of the Cupido, Formation was analyzed in detail in the Huasteca-Monterrey-Bustamante corridor, where the lower Cupido platform-margin-to-basin transition is located. Several kilometer-scale outcrops of relatively undeformed Cupido strata indicate that the early Cupido platform had a low-relief, ramp-like profile. At least three regionally correlatable, shallowing- and coarsening-upward stratigraphic packages were identified in the lower Cupido Formation. These 25 to 170 m-thick packages are interpreted as third-order ( ∼ 1--3 Myr duration) "high-frequency sequences" (HFSs) and consist of basal, lime-mudstone-dominated, transgressive strata that are overlain by more grain-rich regressive facies. Overall, these HFSs have a larger-scale, progradational stacking pattern that reflects the long-term progradation of the lower Cupido platform.;Four types of sponge-microbial mud mounds are found within the lower Cupido Formation: Type 1, siliceous sponge-microbial mud mounds are the oldest and are associated with adjacent deep ramp/basinal lime mudstone facies; Type 2, sponge-microbial mud mounds, with adjacent outer-ramp packstone/wackestone facies; Type 3, sponge-microbial coral mud mounds, with adjacent outer-ramp packstone facies; and Type 4, skeletal-rich calcisponge-microbial-coral biodetrital mud mounds, with adjacent outer-ramp packstone and ramp-crest rudist-rich packstone facies. Mound-core facies (Type 1, 2, and 3 mounds) consists of massive sponge-microbial boundstone with abundant microbial fabrics. Both the length and height of individual mounds range from a few to tens of meters. Only the biodetrital mud mounds reach several hundred meters in width and are up to 90 m thick. Type 1 and 2 mounds (deepest water) contain siliceous hexactinellid and lithistid sponges. Type 3 and 4 mounds contain mostly calcareous sponges and corals, with fewer siliceous sponges. The systematic changes in the attributes (i.e., fossil assemblage, morphologies, dimensions, abundance, distribution, and associated facies) of the lower Cupido mounds were largely controlled by the ramp-like depositional profile and the long-term accommodation trends that influenced the Cupido platform during Early Cretaceous time.