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The Guadalupian shelf crest provides a unique setting for the study of changing carbonate-shelf environments. Previous works by King, Newell, Dunham, Smith, Pray, and others have provided models and interpretations to define a generalized morphology of the carbonate facies for the Guadalupian shelf, behind the Capitan Limestone. Detailed field studies of about 50 m of carbonate units within the upper Yates and lower Tansill Formations reveal significantly different facies patterns and suggest changing shelf profiles and environments through time.
Alternating carbonate and sandstone units are apparent as the product of cyclic shelf sedimentation. Major carbonate units within the stratigraphic interval studied are informally named (Hairpin dolomite, Triplet dolomite, and basal Tansill dolomite). Recognition of five major carbonate lithofacies within the three identified carbonate units at the shelf crest (the paleotopographic high of Dunham's marginal mound) indicates variance in deposition owing to highly restricted water conditions as well as more "normal" marine water circulation.
The three carbonate units differ in the following respects: the Hairpin dolomite is dominated by fenestral peloid grain-supported, pisolite-rich facies, and is associated with erosion surfaces and abundant tepee structures. Carbonate facies in the Hairpin dolomite repeatedly grew to "fill level," and the shelf crest extended at least 3 km perpendicular to the Capitan Limestone. The Triplet dolomite is composed primarily of peloidal grainstones with abundant admixed siliciclastic grains. Shelf-crest features, that is, pisolites, tepees, and fenestral strata, are uncommon, suggesting an absence of a nearly emergent paleotopographic high and a more continuous basinward sloping shelf. The basal Tansill dolomite contains abundant skeletal grains, that is, dasycladacean algae, ostracods, ga tropods, and foraminifers, suggesting a deeper shelfward penetration of more open marine water than for the other two units.
Erosion surfaces common at the upper boundaries of the identified major carbonate units are most common and traceable horizontally 0.5 to 3 km behind the Capitan front, locally 50 m behind the Capitan Limestone. Simple progradation of the shelf-crest facies with the Capitan Limestone through time does not explain the change of facies within the three major carbonate units.
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