LITHOSTRATIGRAPHY AND ALGAL-FORAMINIFERAL BIOSTRATIGRAPHY OF THE CUPIDO FORMATION, LOWER CRETACEOUS, NORTHEAST MEXICO
Authors: SELVIUS DOUGLAS BRIAN , WILSON JAMES LEE 
Published: October 2019

ABSTRACT
The Cupido Formation in northeastern Mexico is part of a Lower Cretaceous sedimentary sequence deposited in and adjacent to the Sabinas Basin. The Sligo Formation in the subsurface of South Texas is both an age and facies equivalent of the Cupido Formation. Both represent seaward progradation of a major carbonate platform system whose distribution and thickness was controlled in part by faulted remnants of the Late Paleozoic Ouachita-Marathon Orogeny. The platform system observed in Cupido Formation sections of northeast Mexico prograded southeastward across the Sabinas Basin and later built rapidly upward in response to a rise of relative sea level.
Six major lithofacies, each genetically related to a depositional environment, exist in the Cupido/Sligo platform system. These are basinal, shelf slope, reefal, backreef shoal, lagoonal, and tidal flat facies. Basinal facies consist of black, laminated, homogeneous mudstones deposited in a euxinic basinal environment overlain by black to gray bioclastic wackestones. Shelf slope facies are lithociastic, biociastic packstones and wackestones containing fragments of coral, rudist, stromato-poroid, and sponge. The massive reef facies is composed of stromatoporoid-sponge boundstones overlain by coral, algal, and stromatoporoid bound-stones and packstones. The reef is often pervasively dolomitized in Mexico and the same may be true in South Texas. Backreef shoal facies are composed of carbonate sand grainstones and packstones. Sand bodies are well sorted and cross-bedded and are occasionally interbedded w ith muddy, quiet-water sediments. Lagoonal facies range from thick-bedded mudstones to peloidal-algal-bioclastic wackestones, packstones, and grainstones. Tidal flat facies alternate cyclically from poorly winnowed wackestones and packstones to well laminated algal mudstones. Collapse breccias and evaporites are present but not abundant.
In addition to lithofacies, four biofacies were also developed in the Cupido/Sligo Formation. The biofacies were defined by two dasycladacean alga genera, Cylindroporella and Salpingoporella, and two foraminiferal genera, Choffatella and Orbitolina. Cylindroporella was found restricted to high energy shoal environments associated with backreef and tidal bar deposits, while Salpingoporella occurred in all shelf environments, but primarily quiet water lagoon deposits. Choffatella was observed across a broad time and stratigraphic range, but only in beds stratigraphically above the reef. It was apparently limited to backreef lagoonal environments, but not to any particular niche within the lagoon. Orbitolina existed over a much narrower time and stratigraphic range at the top of the Cupido Formation section, and stratigraphically above the Choffatella biofacies. In all samples, Orbitolina was closely associated with tidal flat deposits.
Oil and gas production from the Cupido Formation in Mexico is far better than production from the Sligo Formation in South Texas. Reasons for the difference are not entirely clear, but are undoubtedly related to the greater amount of dolomitization and fracturing in Mexican reservoirs. Recently, however, wildcat drilling activity has increased for the Sligo Formation. During the period from late 1982 through early 1984, eight wells were active along the trend, and that activity has continued into 1985. Depositional environments tested by the wildcats include forereef talus deposits, biohermal reef buildups, and backreef grain shoal deposits. Hydrocarbon discoveries were made in reefal and grain shoal reservoirs.