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Sedimentological field study of the upper three sandstones of the upper Yates Formation (Permian, Guadalupian), Guadalupe Mountains, New Mexico, has shed considerable light on the shelf depositional environment, morphologic profile, and temporal relationships of alternating carbonate and siliciclastic deposition. Outcrops were examined in detail, and 31 stratigraphic sections were measured, described, and correlated within the region 5 km (3 mi) shelfward of the Capitan reef and encompassing an area of 180 km2 (69 mi2).
The three sandstones in the upper 15 to 30 m (49 to 98 ft) of the Yates are continuously traceable across the study area as thin, nonchanneled sheet sandstones 0.5 to 8 m (1.6 to 26 ft) thick. They exhibit no deepening or "shoaling-upward" sequences; no beach or tidal sedimentation features; and no vertical repetitive sedimentation patterns. Primary sedimentary structures are rare, obscure, small scale (<= 20 cm; 8 in., height), and discontinuous. All structures indicate subaqueous deposition. From the sandstones nearest the Capitan, the shelfward progression of sedimentary structures indicates shelfward diminution of hydraulic energy. Sandstones are largely abiotic suggesting maintenance of inhospitable marine conditions (mesohaline?) during siliciclastic deposition.
Evidence for a marginal mound shelf profile during sandstone deposition is inconclusive. A localized area of fenestral porosity in the lowermost sandstone unit, and a narrow region of probable fossil caliche along the Yates/Tansill formational contact of the upper sandstone, both suggest local emergence of a paleotopographic high located 1.5 to 3.0 km (.9 to 1.8 mi) and 1.5 to 3.25 km (.9 to 2 mi) respectively, shelfward of the Capitan. All three sandstones continuously overlie the area of the marginal mound as inferred from the underlying carbonate facies, hence a marginal mound was not present or had no affect on transport of siliciclastics across the outer 5 km (3 mi) of the shelf.
Each sandstone is characterized by a sharp, subplanar erosional base, and typically grades upward into peritidal carbonates, which exhibit along the shelf crest of the marginal mound one or more 1 to 2 m (3 to 6 ft) shoaling-upward hemicycles which commonly built up to depositional fill-level in response to episodic shelf subsidence of 1 to 2 m (3 to 6 ft). The eroded fill-level carbonates of the shelf crest underlying two of the three sandstone intervals were diagenetically micritized analogous to incipient pedogenesis prior to subsidence and burial by subaqueously deposited siliciclastics. Local emergence of 1 to 2 m (3 to 6 ft) relief could explain the inferred pedogenesis, with siliciclastic deposition being favored by subsidence and greater water depth which governed siliciclasti transport processes.
The following depositional sequence is repeated twice in the study interval: subaqueous deposition of siliciclastics across the eroded surface of the underlying carbonate unit; upward siliciclastic gradation into peritidal carbonates characterized by one or more 1 to 2 m (3 to 6 ft) shoaling-upward hemicyclic deposits which proceeded to fill-level; local emergence with concomitant subaerial erosion and micritization; subsidence and renewed subaqueous siliciclastic deposition. Reconnaissance observations from the middle Yates and lower Tansill reveal numerous analogous sandstone/carbonate relationships.
The sandstone intervals interbedded with shelf crest carbonate facies comprise sandstone/carbonate shoaling-upward hemicycles; contrary to current interpretation the sandstone represents the basal "deeper" water deposition, the carbonates represent the "shoal water" deposition.
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