Let’s start with the oldest layer: the Bone Spring Limestone. We covered this in the first post, but here’s a recap. It formed in a deep marine basin, composed of organic-rich limestone and turbidites. Think of it as the deep-sea foundation of the region, laid down long before reefs began to grow.
Next are the Victorio Peak and Cutoff formations, which sit in the transition zone between the slope and the basin. These thin-bedded limestone, chert, and siltstone layers mark the onset of slope deposition. This is where the ocean floor began to rise, setting the stage for reef-building carbonates.
Following these are the Cherry Canyon, Brushy Canyon, and Bell Canyon formations. These sandstone-rich layers, interbedded with siltstones and limestones, were deposited by submarine fans, which are essentially underwater river deltas spreading sediments farther into the basin. They record the gradual expansion of the depositional environment as the basin filled.
Probably the most famous formation comes next: the Capitan formation. This massive, horseshoe-shaped barrier reef is composed of thick limestone and dolomite, dotted with fossils. It acted like a natural seawall, separating the open ocean from the restricted Delaware Basin. Before it, the Goat Seep Dolomite formed as the early reef precursor. This dolomitized limestone captures the first hints of reef-building activity.
Above the reef, calmer, shallow waters allowed back-reef carbonates to accumulate, forming the Artesia Group (shown in pink in the figure above). These layers include evaporites, dolostones, and sandstones. Think of these as Permian lagoons and tidal flats forming atop and behind the reef.
Now that the background information is complete, let’s begin day two with a roadside stop on the way from Carlsbad to the Permian Trail. Our first stop offers an excellent exposure of the Castile Formation, characterized by alternating, thinly laminated layers of anhydrite and halite. These layers were deposited in the deep waters of a restricted basin, where subtle chemical fluctuations allowed thin interbeds of calcite and organic matter to form alongside the evaporites. Such layers represent varves, which are annual deposits formed by seasonal changes in water chemistry. Darker layers correspond to periods of higher biological activity and organic accumulation during wetter intervals, while lighter layers reflect more arid, evaporitic conditions within the restricted basin. By counting and analyzing these varves, geologists have been able to reconstruct hundreds of thousands of years of paleoclimate history recorded in the formation’s roughly 90 meters (300 feet) of deposits. (Anderson & Dean, 1995)
Castile formation showing thin layers of alternating anhydrite and halite laminae with layers of calcite and organic matter.
The restricted basin was situated near the western margin of Pangea, approximately 10–15° north of the paleoequator. Marine water initially entered through the Hovey Channel, but circulation was eventually restricted, likely due to the growth of the Capitan Reef complex. Once marine inflow was cut off, salinity in the basin increased, leading to the deposition of the thick evaporite sequences seen today. (Anderson & Dean, 1995)
In some outcrops, you might also notice a distinctive “chicken-wire” texture within the anhydrite layers. This pattern formed when gypsum, buried under increasing pressure and temperature, dehydrated and recrystallized into anhydrite. As the anhydrite nodules grew, they pushed against one another, forming a web-like, polygonal network. (Dean et al 2000)
After examining the evaporitic deposits of the Castile Formation, we continued toward the next major highlight of the trip: the Permian Reef Trail in Guadalupe Mountains National Park. This trail offers a rare opportunity to observe the transition from basin evaporites into the ancient reef and back-reef facies that record the dynamic evolution of the Permian Basin.
The Permian Reef Trail is every geologist’s dream. It provides an exceptional opportunity to walk through the depositional facies and diagenetic features of a rimmed carbonate platform. Interestingly, the trail’s development was encouraged by professional geologists, petroleum experts, and the U.S. Geological Survey to facilitate detailed study of these world-class exposures. Understanding carbonate systems and the environments in which they form is crucial not only for academic research but also for petroleum and resource exploration.
The trail encompasses a portion of the Capitan Reef complex within McKittrick Canyon. Because our group was large and time was limited, we only explored the lower section of the trail. I’ll walk you through what we observed there and briefly describe what you would encounter along the full route. The hike covers approximately 9.9 miles round trip with an elevation gain of about 2,380 feet. There is little shade and plenty of cactus along the way, so proper footwear, water, and snacks are essential for a safe and enjoyable journey. (Bebout & Kerans, 1993 p. 1-4)
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| Trail head of the Permian Trail |
The walk begins at the toe of the slope, passing through exposures of the Lamar Limestone facies of the Bell Canyon Formation. Along the way, we crossed a riverbed containing poorly sorted, subrounded to well-rounded clasts, indicating that the sediments had traveled some distance from their source. These are Quaternary alluvial fan deposits, formed during flash-flood events along the slope. Observing these modern deposits serves as a reminder that landscapes are continually evolving, reshaping the terrain long after the ancient reef and basin systems were established. (Bebout & Kerans, 1993 p. 5)
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| Poorly sorted, rounded to sub-rounded alluvial fan Quaternary deposits |
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| Brecciated limestone near the base of the trail, marking slope collapse deposits |
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| Silicified fossils exposed in cherty limestone after weathering |
As you continue upward, you begin walking along the thin bedding planes of the Lamar Limestone, which can easily peel apart. Occasional shaded alcoves, or grottoes, appear along the trail. One outcrop on the right side displays well-bedded Lamar wackestones and mudstones, which are deep-marine deposits that formed at depths of approximately 300–1000 meters.
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| Walking along the thin bedding planes of the Lamar Limestone, where layers easily split and peel apart |
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| Deep marine lamar wackestone and mudstone bedding |
Beyond this point, the limestone shows increased siliciclastic content, marking the transition to a shallower marine environment. Here, bioturbation becomes more common due to the activity of burrowing invertebrates, such as worms and small crustaceans, that churned the sediment in search of food. A striking outcrop on the left side of the trail exposes four distinct stratigraphic layers. The upper two-thirds of the outcrop is a heavily bioturbated wackestone, where the original bedding has been completely disrupted. Beneath it lies a thinly bedded wackestone, followed by a microporous wackestone and packstone, and finally, thinly bedded wackestones interlayered with packstone. (Bebout & Kerans, 1993 p. 8)
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| Bioturbated wackestone in the upper layers of a four-part outcrop along the Permian Reef Trail |
Just beyond this outcrop, karrens can be seen across the limestone surface. These features form through dissolution by slightly acidic water, often along fractures or bedding planes, and are a classic indicator of karstic weathering. A short distance farther along the trail, another limestone outcrop appears, characterized by larger clasts embedded within a muddy matrix. This deposit represents a mudflow or debris flow on the slope, likely triggered by turbidity currents moving downslope.
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| Limestone etched with karrens, grooves and pits formed by rainwater dissolving the rock |
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| Outcrop showing mud-supported clasts within a wackestone matrix, indicative of deposition from turbidity currents along the slope of the Permian basin. |
As the trail continues, additional turbidite features become visible. You can observe bedded packstone interlayered with thin horizons containing larger clasts and fossil fragments that were briefly suspended and then deposited from the waning energy of a turbidite current. These features mark the transition from relatively stable slope deposits to more energetically reworked sedimentary layers. This section coincides with the start of the switchbacks, which provide an expansive and beautiful view of the valley below.
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| Panoramic view of McKittrick Canyon from the Permian Reef Trail, with the steep limestone cliffs of the Capitan Reef framing the valley below and the distant peaks of the Guadalupe Mountains in the background |
This is the point in the trail where you'll reach the switchbacks. It is worth continuing the hike if you have time. The route continues upward through the Capitan Reef complex, where the slope deposits transition into massive reefal limestones composed of sponges, algae, and other reef-building organisms. Higher still, the trail enters the back-reef facies, characterized by grainstones, dolostones, and evaporitic layers that formed in shallow, restricted lagoonal environments. Together, these exposures record a complete cross section of a Permian carbonate platform system, from deep basin to reef crest and back-reef lagoon. (Bebout & Kerans, 1993 p. 22-24)
Third Stop: Rattlesnake Springs
Rattlesnake Springs is a desert karst spring where geology, hydrology, and human stewardship converge. The Capitan limestone underlying the spring was deposited in the Permian sea. Between four to six million years ago, the area was uplifted and hydrogen sulfide rich waters traveled along the fractures of the capitan limestone to create a vast network of cave systems. This network of conduits ultimately feeds the spring, allowing water enriched in calcium and other minerals to emerge at the surface. (National Park Service, 2025)
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| Rattlesnake Springs, a lush riparian oasis in the Chihuahuan Desert, fed by a karst aquifer connected to Carlsbad Caverns |
At the discharge pond, algae are coated in calcium carbonate minerals, providing a vivid example of biologically mediated mineral precipitation. Green algae precipitate calcium carbonate on their stems and branches through this process, where photosynthesis uses the carbon dioxide ( CO₂) in the water and supersaturates the water surrounding the algae with bicarbonate. This combines with the calcium-enriched water to precipitate the minerals.
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| Up-close view of the green algae present at Rattlesnake Springs. If you look closely, you can see calcium carbonate precipitating on the stems and branches |
The pond and surrounding irrigation features, ditches, and historic pump houses reflect human modifications designed to manage the spring’s flow while preserving its riparian habitat. This constant water source supports a diverse ecological community, including numerous migratory and resident birds. The air carries the faint scent of mint, a reminder of the rich plant life sustained by this oasis (Swanson, 1986).
After leaving Rattlesnake Springs, make a quick stop along Washington Ranch Road across the Black River to see a fascinating geological feature: a tufa dam. This large, porous rock is riddled with holes where vegetation has grown and decayed over time. Tufa forms from calcium carbonate deposited as spring water degasses CO₂. When the water flows over vegetation or other structures, CO₂ escapes, causing calcium carbonate to precipitate. Over long periods, this process can build massive, porous blocks of tufa, creating striking, natural sculptures along the river.
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| A tufa deposit along the Black River, formed as spring water degasses CO₂ and calcium carbonate precipitates over vegetation, slowly building porous, sculpted blocks over time Ready to go underground? Day 3 of our trip dives into Carlsbad Cavern, which contains one of the largest individual cave chambers in North America. From towering stalagmites to delicate mineral formations, we’ll uncover how water, limestone, and millions of years of time have shaped a subterranean wonderland you won’t want to miss. |
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