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The Lower Cretaceous stratigraphic section in the northern Gulf of Mexico (GOM) comprises several well-documented carbonate systems that have prominent shelf-margin buildups, including the Berriasian Knowles Limestone ramp and shelf system, Barremian Sligo Formation shelf system, Aptian Pearsall Formation ramp system, and shelf systems of the Albian Glen Rose, Edwards, and Stuart City Formations. Two lesser-known but large-scale Lower Cretaceous Valanginian carbonate shelf to shelf-margin systems are documented in this investigation using core, wireline-log, and regional two-dimensional seismic data. The platform-margin succession of the older Calvin section comprises a shelf-margin buildup (back reef, reef, and fore reef facies) that may be as thick as 2000 ft (610 m). The seaward margin of the shelf, as displayed by seismic data, suggests shelf-to-basin relief of as much as 1000 ft (305 m) and margin-to-lagoon relief of 200 to 500 ft (60 to 140 m) (relief estimates are not decompacted). Major reef-building organisms are Lithocodium, stromatoporoids, and corals. The Winn limestone is younger than the Calvin limestone, and at the shelf margin, it may have been 600 to 800 ft (180 to 245 m) thick. It is composed of organisms similar to those of the Calvin reef complex. Considering the vertical position of the Calvin platform, succeeding the major bypass of Calvin lowstand siliciclastic sands, the platform probably records a transgression, which is consistent with its highly aggradational depositional style. The Winn shelf margin was initiated landward of the Calvin shelf margin and did not prograde as far seaward as the Calvin shelf margin. The recognition of these two Lower Cretaceous carbonate systems is important because they further detail the stratigraphic history and architecture of this deeply buried, lowermost Lower Cretaceous section in the GOM. These formations are also potential deep-gas, tight-carbonate (low-porosity) plays.