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The Tansill Formation (Permian, Guadalupian) comprises the youngest back-reef platform facies of the Capitan Reef in the Guadalupe Mountains of New Mexico and Texas (Fig. 1). Here the Tansill is divided into "outer-shelf" and "shelf-crest" facies which pass landward into coeval bedded evaporites and sandstones (Pray, 1977; Mazzullo and Hedrick, 1985). The shelf-crest facies, consisting of peritidal dolomites with conspicuous pisolites, tepees, and some sandstones, is interpreted as a linear, low-lying island complex situated 1.6 km (1 mile) landward of the Capitan Reef (Pray, 1977; Mazzullo and Hedrick, 1985). It is therefore analogous to other island complexes of the Tansill in the subsurface (Ordonez, 1984) as well as to modern outer-shelf islands in Belize (Mazzullo and Reid, 1986). The outer-shelf facies consists mainly of biograins tones and, locally, patch-reefs deposited in shallow, high-energy environments (Mazzullo and Hedrick, 1985). Outcrops in an abandoned quarry near the mouth of Dark Canyon include spectacular exposures of stacked island deposits in the outer-shelf facies tract of the middle Tansill platform (Fig. 2A and B). These exposures lie within 0.8 km (0.5 miles) of the subsurface Capitan Reef shelf margin and are physically separated from contiguous shelf-crest facies (Fig. 2D). The occurrence of these islands is of particular significance in interpreting the depositional history of the Tansill platform, and their study provides critical information on sedimentation and diagenesis as they are related to relative sea-level fluctuations.

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