Day One: From El Paso to Carlsbad — First Encounters with the Capitan Reef

Map of the stops for day one. Basemap sources: National Geographic Society and i-cubed (2013); Esri, TomTom, Garmin, FAO, NOAA, USGS, OpenStreetMap contributors, and the GIS User Community. Additional data: PermianBasin_Boundary_Structural_Tectonic. Map created by Brittney A. Hawley in ArcGIS Pro 

Have you ever driven past a van pulled over on the side of the highway, and noticed a group of people gathered around a rock outcrop pointing and taking notes? You were probably looking at geologists studying the roadcuts. When we study rocks, it’s not enough to observe the weathered surface; we have to look deeper. Breaking open a rock reveals its mineralogy, textures, and clues about its history. We notice bedding, stratigraphic relationships, and structural features like folds, faults, joints, and intrusions. What kinds of rocks are present? Are there clasts, and if so, are they angular or rounded? Each of these details tells part of the story of how the rocks formed and were later altered.

Our first day in the field took us from the El Paso airport toward Carlsbad, New Mexico, with three roadside stops along the way. Each provided an opportunity to stretch our legs, shake off the travel fatigue, and begin exploring the region’s remarkable geology. The first stop was at the Salt Flat Bolson, where we had an excellent view of the Delaware and Guadalupe Mountains. This salt basin represents a graben, a down-dropped block of crust bounded by normal faults. Faulting played an integral role in the formation of the basin’s distinctive white dunes, creating a closed depression into which nearby rivers drained. With no outlet for the water, evaporation increased salinity, leading to the deposition of thick beds of gypsum and halite.

View encapsulating how the flats meet the mountains

During the Pleistocene Epoch, cooler and wetter climatic conditions led to the formation of a shallow, ephemeral lake within the basin. Seasonal precipitation and runoff from the surrounding highlands periodically filled the depression, further concentrating the salinity as the lake water evaporated. Over time, these repeated cycles of flooding and desiccation increased the accumulation of evaporite minerals. The resulting landscape preserves a record of both tectonic activity from the Miocene and climatic oscillations from the Pleistocene. This illustrates the dynamic interplay between tectonics, hydrology, and climate in shaping desert basins of the American Southwest. Standing there, with the wind sweeping across the broad expanse of the Salt Flats and the Guadalupe Mountains rising in the distance, it was easy to imagine the shifting lakes and ancient shorelines that once occupied this now arid landscape. (National Park Service, 2022)

The second stop along the way was a roadcut exposing the Bone Spring Limestone, the oldest formation exposed in the Delaware and Guadalupe mountains. This dark, organic-rich limestone was deposited as thin, successive beds of bituminous or cherty limestone interbedded with calcareous shales. In the photo below, you’ll notice that some bedding layers protrude more prominently than others. These more resistant layers are composed of cherty limestone, which weathers more slowly due to its siliceous mineral composition. The outcrop is littered with rock fragments, and once you find a piece of bituminous limestone and strike it to expose a fresh surface, you may notice a strong, tar-like odor. This distinctive smell is caused by the high organic content and indicates deposition in a quiet, anoxic marine setting, conditions ideal for preserving organic matter.

Roadcut exposure of the Bone Spring Limestone showing thin-bedded, organic-rich carbonate layers interbedded with calcareous shale. Note the differential erosion; cherty limestone beds protrude due to their resistance to weathering, while softer shale layers have receded.

At this same outcrop, one rock fragment revealed a fracture filled with white calcite which is a common diagenetic feature in carbonate systems. These calcite veins form when calcium-rich fluids migrate through cracks in the rock, and precipitate minerals as the fluids cool or chemically interact with the surrounding material. Their presence suggests post-depositional tectonic activity and fluid movement within the formation. Importantly, this feature contributes to secondary porosity, which plays a key role in reservoir capacity. Limestone is an excellent host rock for hydrocarbon deposits, and oil can become trapped along fractures and within the pore spaces created by these fracture-filling crystals. In formations like the Bone Spring Limestone, such diagenetic features can significantly influence the rock’s ability to store and transmit fluids, making them critical in oil and gas exploration. (King, 1948)

A hand sample collected from the Bone Spring Limestone outcrop shows a fracture filled with white calcite.

The final stop before reaching our hotel in Carlsbad was the El Capitan viewpoint, offering a breathtaking look at the ancient, buried reef system that defines the Guadalupe Mountains. From this vantage point, the scale and geological significance of the Capitan Reef become strikingly clear as its towering limestone cliffs are remnants of a thriving Permian marine ecosystem. This marks the end of day one, but the journey continues. In the next post, I’ll explore the formations of the Guadalupe Mountains in more detail, including highlights from day two of the trip along the Permian Trail, where the story of deep time and reef evolution unfolds even further.

Picture from El Capitan viewpoint