Virtual Geologic Field Trips to the Permian Reef Complex, Guadalupe and Delaware Mountains, New Mexico & West Texas
— Geologic Road Log: Dark Canyon – Sitting Bull Falls – Rocky Arroyo

Head west on paved road to Dark Canyon at mileage 151.3 (reverse mileage 8.1) on El Paso-Carlsbad roadlog. (Alternatively, you can take a short-cut by heading west and southwest on a road which takes off on the right, just before the Texaco gas station on the south side of Carlsbad).

The Hanson and Yates, King No. 1 well on the right is the entire Dark Canyon oil field. Numerous offset wells were dry holes.

Rustler Formation dolomite, sandstone, and gypsum make up the low Frontier Hills ahead.

Rustler Formation outcrops in hills on both sides of road.

Mouth of Dark Canyon. We have left the Delaware basin and are now on the Northwest Shelf in the northeastern part of the Guadalupe Mountains. Bedded Capitan Limestone outcrops are present on both sides of the canyon.

Road intersection; take sharp right.

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STOP III-1. Capitan reef-backreef at mouth of Dark Canyon. When finished with this stop, turn around and return to the main road.

Intersection with main Dark Canyon road; turn right.

STOP III-2. Tansill near-back-reef strata. Turn at junction with small dirt road on left. Park and walk down road to cliff outcrop on south side of canyon.

STOP III-3. Tansill-Yates contact. Cross stream wash and examine two outcrops on north side of valley.

OPTIONAL STOP III-3a. Pisolitic dolomites with "tepee" structures well exposed (see photo) on south side of valley. Note the upward propagation of tepees through a thick stack of carbonate layers as well as the laminated sandstone-siltstone unit interrupting and truncating some "tepees." The sandstone-siltstone is the uppermost part of the Yates Fm.; overlying dolomites are in the Tansill Fm.

Yates outcrop on left contains pisolitic dolomite and sandstone.

Yates Fm.(?) pisolitic dolomite with evaporite crystal casts on right.

Yates Fm.(?) outcrop of thick-bedded, dolomitic mudstones with sparse evaporite crystal casts.

Dolomitic mudstones, peloidal grainstones, and pisolitic beds of Yates Fm.(?) on right.

Yates Fm.(?) dolomitized peloidal grainstones on left.

Road junction; continue straight ahead on Eddy Co. 408 toward Sitting Bull Falls.

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STOP III-4. Yates-Seven Rivers lagoonal deposits.

Road junction; bear right.

Start former gravel road (now paved); continue straight ahead.

W. G. Smith ranch road on right; continue straight ahead. Road is on Seven Rivers Fm.

Road intersection on right; continue straight ahead on main road.

OPTIONAL STOP III-5. Interbedded thin-bedded, dolomitized mudstones and red, far-back-reef siltstones of the Seven Rivers Fm. on left. The Seven Rivers is the oldest Capitan-equivalent unit in the Artesia Group. These outcrops have been mapped as basal Yates Fm. by Motts (1962b) but have been considered to belong to the Seven Rivers by most other workers. Note the uniformity of the aphanocrystalline to very-finely-crystalline replacement dolomite. The depositional environment presumably represents a shallow lagoonal or lower sabkha environment. Such finely-crystalline dolomitization is thought to represent the same processes seen on modern sabkhas (e.g. Patterson and Kinsman, 1982). In this model, the precipitation of calcium sulfates (gypsum or anhydrite) from sabkha brines leads to depletion of Ca⁺² and relative enrichment of Mg⁺² in pore waters. With calcium to magnesium ratios in excess of 60, the pore fluids react with very small aragonite needles to produce penecontemporaneous replacement dolomite.

The redbeds in this section show few sedimentary structures. Indistinct horizontal lamination, minor adhesion ripples, evaporite crystal casts, and the general absence of trace fossils all point toward deposition in an arid, dominantly non-marine setting. Although no dune structures are found, most recent workers consider that these are eolian sediments deposited mainly in interdune flats.

Medium-scale contortions visible in Seven Rivers Fm. These were probably caused by near-surface dissolution of interbedded gypsum and anhydrite, although the Seven Rivers consists mainly of dolomites and siltstones in this area.

Road intersection on left; continue straight ahead.

Road intersection on left; bear right on main road.

Varicolored sediments on right are interbedded massive gypsum, dolomite, and red beds of Seven Rivers Fm.

Ranch road on left; continue straight ahead.

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STOP III-6. Borrow pit in gypsum of Seven Rivers Formation.

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Road junction to right; continue straight ahead.

Road on left; continue straight ahead on main road.

Ranch house on right. Well drilled to left (Humble's Bandanna Point Unit No. 1 gas well) was completed as a gas producer from Morrowan (Pennsylvanian) sandstone. It encountered the following units: San Andres Fm. (top at 229m; 750 ft depth); Bone Spring Ls. (858 m; 2,815 ft); Wolfcamp limestone (2,179 m; 7,150 ft); Pennsylvanian (2,301 m; 7,550 ft); Mississippian (Chester) (3,119 m; 10,234 ft); Woodford Shale (3,313 m; 10,868 ft); Devonian (3,332 m; 10,932 ft); Montoya Group (3,542 m; 11,622 ft); Simpson Group (3,656 m; 11,995 ft); and Ellenburger Group (3,673 m; 12,050 ft) (data from Anonymous, 1962, p. 18).

Azotea Mesa on right is composed of Seven Rivers Fm. gypsum capped by a prominent dolomitic ledge. The ridge ahead in the distance consists of Queen and Grayburg beds downwarped along the Huapache Monocline. We are now entering the main part of the Seven Rivers Embayment with the road on thin alluvium over Queen Fm.

Intersection with New Mexico Highway 137 (a paved road from El Paso Gap to Carlsbad). Turn left toward El Paso Gap. Guadalupe Mountains ahead in distance; road traverses the Seven Rivers Embayment atop the Queen Fm.

Junction with road to Sitting Bull Falls on right; turn right. Ahead lies the main western prong of the Guadalupe Mountains underlain by lower Guadalupian strata (San Andres, Queen, and Grayburg formations) uplifted across the Huapache monocline.

Entering Lincoln National Forest.

Road crosses first wash and traverses Huapache monocline ahead. Although the feature had precursors in the Pennsylvanian, the present flexure is a Tertiary feature (Hayes, 1964).

Road crosses onto San Andres Limestone.

Iron gate (key can be obtained from Forest Service office in Carlsbad. About 30 m (100 ft) east of the gate is the site of the Humble Huapache Unit No. 1, a 3,850 m (12,631 ft) dry hole drilled in 1955. The well encountered a repeated section of Siluro-Devonian at about 3 km (10,000 ft) depth; a thrust fault, presumably associated with the Huapache flexure has been inferred for this structure. Throw on the fault exceeds 1,200 m (4,000 ft) in this area and 1,920 m (6,300 ft) farther northeast (Reid et al, 1988).

Lenticular, partly silicified, skeletal grainstones of the San Andres, probably filling channels, on right at base of slope near stream crossing. Brown, thin-bedded Cherry Canyon sandstone beds can be seen in cliffs on right. This tongue of the generally basinal Cherry Canyon Fm. extends many miles into the shelf environment in this area. The Cherry Canyon sandstone tongue is 50 m (164 ft) thick near the mouth of Sitting Bull Canyon.

A massive upper San Andres buildup, overlain by Grayburg Fm. is visible ahead on right. Note lenticular bedding to the left of the "bioherm" (originally described as such but now though of as a skeltal bank).

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STOP III-7. Junction of Sitting Bull Canyon and Last Chance Canyon.

The Cherry Canyon-San Andres unconformity is well exposed on both sides of the road.

Cherry Canyon sandstone outcrops with well developed cross-bedding are visible on right.

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STOP III-8. Sitting Bull Falls. When finished at this stop, return down-canyon to the El Paso Gap-Carlsbad road.

Junction with New Mexico Highway 137 (El Paso Gap-Carlsbad road). Note West Hess Hills at 2:00 to 3:00 o'clock and Azotea Mesa at 10:00 to 11:00 o'clock - both are composed of gypsum, red siltstones, and dolomites of the Seven Rivers Fm. Turn left toward Carlsbad.

Unpaved Dark Canyon road on right; continue straight ahead. Road is on Queen Fm.

Road now crossing the approximate contact between the Queen and Seven Rivers Fms. The road is located almost directly on this contact for the next few miles, with Queen sediments on the left and Seven Rivers on the right.

Hills ahead and to the right are composed of Seven Rivers evaporites and red siltstones. The strongly developed vertical gullying is characteristic of the evaporite facies of the Guadalupian far-back-reef units and contrasts sharply with the horizontal bedding which dominates in areas of carbonate facies within these same units (Photo Fig. 32). Most of these hills are capped by the resistant "Azotea Tongue" (usage of Sarg, 1976), a dolomite unit in the Seven Rivers Fm.

Road passes from Seven Rivers Embayment into Rocky Arroyo.

Road is at the level of the contact of the Queen Fm. (Shattuck Sandstone unit of Sarg, 1976) and Seven Rivers Fm. We are nearly 1.6 km (1 mi) shelfward (northwest) of the carbonate-evaporite transition of the Seven Rivers Fm. (Sarg, 1976; Sarg, 1981). This transition is remarkably abrupt (within about 150m/500 ft) and remains in approximately the same location for nearly 200 ft of section (Bates, 1942; Pray and Esteban, 1977). This Seven Rivers facies transition has been shown by Sarg (1976; 1981) to be related to a depositional ridge in the underlying Shattuck Sandstone of the Queen Fm.

The gullied hillside on the southeast side of the road has good exposures of the Seven Rivers evaporite facies (see Pray and Esteban, 1977, Stop VII). The section is dominated by bedded, nodular, mosaic gypsum with thin, pelletal or grapestone-bearing dolomites and red, gypsum-cemented, sandy siltstones

Note invigorating, heady aroma of hydrocarbons in the air; it emanates from the Indian basin gas field about 0.3 miles ahead. The main production is from extremely permeable Upper Pennsylvanian (Cisco and Canyon) dolomites at 2,130-2,440 m (7,000-8,000 ft); further production comes from Morrowan sandstones at 2,740-3,000 m (9,000-10,000 ft) (Frenzel, 1988).

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Low road cuts on right are Queen Fm. (Shattuck Sandstone). Conical hill visible to the north of the road is "The Tepee" and is capped by the resistant "Azotea Tongue" a massive dolomite of the Seven Rivers Fm. Underlying Seven Rivers evaporites, the Shattuck Sandstone and dolomites of the Queen Fm. are also exposed.

Intersection with road on left leading to Marathon Oil Co. Indian basin gas field and plant. Production here is from Upper Pennsylvanian and Lower Permian reservoirs. Continue on main road.

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OPTIONAL VIEW STOP III-8a. Excellent view of the carbonate-evaporite facies transition in the Seven Rivers Fm. (see description of Stop VI in Pray and Esteban, 1977) on north wall of Rocky Arroyo. This extremely rapid transition can be seen in a narrow, nearly vertical, band in the upper half to two-thirds of the far wall of the valley. The transition is visible because of the radically different weathering patterns of the evaporite (vertical gullying) and carbonate (horizontal bedding) facies. The transition was first described by Bates (1942) and has been recently examined by Sarg (1976; 1981)

Also exposed in the lower part of the cliff is the upper dolomite and the overlying Shattuck Sandstone unit of the Queen Fm. The Shattuck, generally about 27 m (90 ft) thick in this region, thickens to about 43 m (140 ft) beneath the carbonate-evaporite transition and may have been partly responsible for the generation of restricted, evaporitic conditions shelfward of this point during Seven Rivers deposition (Sarg, in Pray and Esteban, 1977).

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STOP III-9. Shattuck Sandstone (Queen Fm.) and Seven Rivers Fm. collapse breccia.

Cyclic deposits of dolomite and red, silty shales. Generally grouped in the Seven Rivers Fm., these sediments were included in the uppermost part of the Shattuck Sandstone (Queen Fm.) by Sarg (1976) and Pray and Esteban (1977; see description for Stop VIII). These strata have been interpreted as "dolocalcrete cycles" with evidence of repeated deposition, exposure, weathering, and calichification (Pray and Esteban, 1977). Note also the abundant crystal- and nodule-casts of former gypsum (some voids now partly filled with calcite; see photo).

A thick section of Seven Rivers dolomitic packstones and mudstones of probable lagoonal origin exposed on left. Note absence of evaporites and red beds.

Strata exposed behind Shafer Ranch are fossiliferous, dolomitic, pelletal packstones.

Small cemetery on right. Cliffs to southeast expose Seven Rivers Fm. fossiliferous, dolomitic, pelletal packstones containing ostracods, calcispheres, and some small foraminifers. Queen Fm. (Shattuck Sandstone) is exposed at the cliff base.

On the south side of the arroyo are cliffs exposing thin-bedded dolomite of the Seven Rivers Fm. dominated by pelletal wackestone and mudstone. Stromatolitic(?) units have been described from this locality (Sarg, 1976). Other, similar outcrops are along road on left.

OPTIONAL STOP III-9a. Road cuts to left are Seven Rivers dolomite (Dunham's (1972) Stop I-6). This section, 18.8 km (11.7 mi) shelfward of the Capitan escarpment, consists of thin-bedded, stromatolitic(?), dolomitic wackestone with pellets, ostracods, and calcispheres. In and along the stream valley, travertine is abundant along with travertine-cemented gravels. The deposits are characterized by repeated meter-scale upward-shallowing cycles. These progress from subtidal wackestones and mudstones to intertidal stromatolitic sediments, with occasional supratidal caps.

The dolomite in these sediments, as in most of the back-reef areas, is very finely crystalline to aphanocrystalline and appears to be of very early diagenetic origin. Formation of early dolomite is probably related to restricted circulation in these back-reef lagoonal areas, precipitation of CaSO_>4 minerals, and consequent increase in Mg/Ca ratios of lagoonal and interstitial fluids. Contact of Mg-rich surface and interstitial waters with unstable aragonitic muds has led to partial dolomitization of modern carbonate muds in the Persian Gulf and this most likely took place in the Permian back-reef areas as well. The exact mechanisms of this dolomitization (reflux brine movement, evaporative pumping, and other models) are debated even in modern setting and are even more disputed for the Permian examples.

Leaving Rocky Arroyo. Outcrops to the south are upper Seven Rivers Fm. Yates Fm. is present on the crest and eastern side of the hills to the south.

Junction with U. S. Highway 285; turn right toward Carlsbad. The road here is on Quaternary alluvium overlying Yates Fm. The Seven Rivers Hills, to the northwest, are the type section of the Seven Rivers Fm. (Meinzer et al, 1926).

Bridge crossing over Rocky Arroyo.

Interbedded dolomites and red-brown sandstone of the Yates Fm. on both left and right.

Yates Fm. exposed in road cut.

Yates Fm. exposed in road cut.

Intersection of U. S. Highway 285 with U. S. 285 (Truck Rte.) on right. Continue left on main U. S. Highway 285.

Basal Tansill Fm. is exposed in low cuts on both sides of road. Thin-bedded to laminated dolomitic mudstones with evaporite (anhydrite?) crystal casts can be found just below road level to the west. All back-reef units dip gently basinward in this area.

The Ocotillo Hills at 1:00 o'clock and Avalon Hills at 11:00 to 12:00 o'clock are composed of back-reef Tansill and Yates evaporitic dolomites. The hills are the topographic expression of an anticlinal structure over the buried Capitan reef, one of many such structures in the shelf area to the north and west of Carlsbad.

Thin-bedded Tansill dolomites with evaporite crystal casts present in roadcut. The type locality of the Tansill Fm., as described by DeFord and Riggs (1941), is located nearby in the Ocotillo Hills.

Gently dipping Tansill and Yates sediments in canyon to right are on eastern flank of the Tracy Dome.

Tansill dolomite overlying Yates sandstone is exposed in canyon on right.

Pecos River on left. Tansill Fm. in road cuts on right.

Living Desert State Park turn-off on right.

Carlsbad city center (junction of U. S. Highways 62-180 (from north) and 285; La Caverna Hotel). Continue straight ahead.

Intersection of U. S. Highways 62-180 (from south) and 285 (from southeast).

Roadlog ends.