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Permian Reef Complex Virtual Field Trip
Stop III-2: Tansill Near-Back-Reef Facies

— Peter Scholle
Geographic location and stratigraphic position of this stop.
Geographic location and stratigraphic position of this stop.

This locality (equivalent to part of Dunham's (1972) Stop I-1) exposes dolomites and calcitic dolomites of the near-back-reef Tansill Formation. A wide variety of sediment types are present here, typical of the complex, small-scale microfacies patterns in this paleogeographic zone. We can see pisolitic packstones, birdseye dolomites, cross-bedded to massive green-algal grainstones (some of which look deceptively like mudstones), fenestral dolomites with tepee-structures, and other lithologies intimately intermingled at this site. Fusulinid Foraminifera, belerophontid gastropods, pelecypods, green algae, and probable blue-green algae are particularly abundant in these strata.

Low angle, seaward-dipping cross-beds in the upper Tansill Formation.
Low angle, seaward-dipping cross-beds in the upper Tansill Formation. These skeletal grainstones of probable beach origin overlie subtidal and lower intertidal grainstones and packstones (base of photograph) and, in turn, are overlain by pisolitic strata — thus, they are part of one of the many shallowing-upward sequences in this part of the section, probably associated with small island barriers in the very near-back-reef zone. These barriers are thought to have restricted the flow of marine water from the basin onto the shelf. Outcrop on south side of Dark Canyon, less than 1 km west of the canyon mouth, Eddy Co., NM.
© Peter A. Scholle, 1999
Peloidal and skeletal grainstones with fenestral fabric
Peloidal and skeletal grainstones with fenestral fabric in the near-back-reef Tansill Formation. These strata were probably deposited on a relatively high-energy tidal flat. Outcrop on south side of Dark Canyon, less than 1 km west of the canyon mouth, Eddy Co., NM. Coin is 2.4 cm in diameter.
© Peter A. Scholle, 1999

These beds apparently represent a series of migrating islands or banks with shallowing upward sequences (Mazzullo et al., 1989; Parsley and Warren, 1989; Scholle, 1980). The cross-bedded grainstones (see photo) represent basinward beach deposits; the fenestral (birdseye) dolomites (see photo) represent leeward subaerial and intertidal flats, the pisolitic packstones to wackestones likely reflect subtidal restricted or sheltered sediment accumulation sites; the massive fossiliferous packstones and grainstones may have formed in intervening tidal channels and unrestricted subtidal areas; and the tepee dolomites found near the top of the section probably represent exposure surfaces which cap the sequence. These facies patterns are quite similar to ones found in the Bahamas in regions such as Joulter's Cay, or in the barrier-lagoon complexes of the Persian Gulf.

Diagenetically, these Permian microfacies are equally complex. Aphanocrystalline dolomitization is quite extensive, especially in the peritidal and supratidal facies. Associated lobate and partitioned vugs clearly represent leached and/or calcitized early-diagenetic nodular evaporites. Note the close association between the areas of such evaporite vugs and dolomitization — both probably result from early reflux of evaporitic brines from more shelfward hypersaline lagoons.

photomicrograph of a near-back-reef grainstone in the Tansill Formation
Thin-section photomicrograph (plane-polarized light) of a near-back-reef grainstone in the Tansill Formation. The predominant grains again are Mizzia green algae but in this example, the primary porosity was partially filled with an early diagenetic cement produced in the vadose zone — note slight corrosion of tops of grains and pendants or microstalactites of calcite cement below the grains. Subaerial exposure may have occurred on small islands during sedimentation or in association with a major (early Ochoan) hiatus that follows Tansill deposition. Sample from Walnut Canyon (New Mexico Highway 7), less than 1 km west of the canyon mouth, Carlsbad Caverns National Park, Eddy Co., NM. Long axis of photo = 6 mm..
© Peter A. Scholle, 1999
photomicrograph of a near-back-reef grainstone in the Tansill Formation
Thin-section photomicrograph (cross-polarized light with gypsum plate) of a near-back-reef grainstone in the Tansill Formation. The predominant grains are gastropods and bivalves that were encrusted penecontemporaneously with inorganic marine cement (cloudy, fibrous to bladed crusts). The sample subsequently was subjected to leaching of the originally aragonitic molluscan grains, presumably as a result of local meteoric exposure. Sample from Walnut Canyon (New Mexico Highway 7), less than 1 km west of the canyon mouth, Carlsbad Caverns National Park, Eddy Co., NM. Long axis of photo = 6 mm.
© Peter A. Scholle, 1999

In addition, one can find (most easily using a microscope) examples of aragonitic submarine cement as well as vadose and phreatic meteoric calcite cements (see photo above and photo nearby). Selective leaching of bioclasts is common in this setting, producing moldic fabrics (see photo). Porosities in this zone are variable, but include some of the highest values found anywhere in the Guadalupian facies spectrum. Subsurface equivalents of this facies suite host large volumes of oil (see photo).

Peloidal and skeletal grainstones with fenestral fabric
Core photograph of dolomitized shelfal packstone/wackestone with leached fossil fragments. Unplugged fossil molds and intercrystalline porosity are main contributors to the hydrocarbon productivity of this reservoir rock. Gulf Oil Co. N14N well, 4413 ft depth, Central Basin Platform northwest of Odessa, west Texas. Large scale divisions are centimeters.
© Peter A. Scholle, 1999

More modern diagenesis is also apparent at this outcrop. The cave system in the center of the outcrop, and the solution-enlarged, calcite-filled fracture systems associated with it, were probably produced by the same Tertiary hydrologic system which formed Carlsbad Caverns. This late-stage meteoric water flow was also responsible for leaching or calcitization of precursor evaporites which can be seen with increasing frequency from this locality landward.

Turn around and return to the main road.

Return to Dark Canyon-Sitting Bull Falls-Rocky Arroyo Roadlog