Day 2 : Stop 2

DAY 2 (9/27/2025) - STOP 2 

"Permian Reef Trail - McKittrick Canyon"

Location: Permian Reef Trail, Guadalupe Mountains National Park

Fig. 10 Wackestone - Lamar Formation
                                 (Stop 2)                                  

    The Permian Reef Geology Trail in McKittrick Canyon provides a view into the Capitan Reef system, a limestone structure that marked the edge of the Delaware Basin during the Permian Period. The first portion of the trail traverses the toe-of-slope environment, where deep-water sediments were deposited from the reef above. The geology encountered in Stops 1 through 8 reveals the transition from modern deposits to the deep-water Lamar Limestone Member of the Bell Canyon Formation, a unit that records gravity-driven sediment transport into the basin. More specifically, stops 1-2 highlight the canyon floor, stops 3-4 depict thicker, debris-rich beds with an increasing slope angle, stops 5-6 depict the canyon walls with the presence of turbidites, while stops 7-8 are characterized by thick packstone beds as well as transitioning to the slope. 

Stop 1: Quaternary-Age Alluvial-Fan Conglomerate

This first stop is geologically the youngest, a thin depositional unit with low elevation lying over the Permian bedrock. The rock is a calcite-cemented conglomerate, a coarse gravel deposit composed of well-rounded limestone clasts that accumulated in a stream or alluvial fan setting long after the Permian sea disappeared. It was laid down on an eroded surface of the much older rocks beneath, and is important as a record of recent land-surface processes.

 Fig. 11 Chert (Stop 3)

Stop 2: Distal Toe-of-Slope Wackestone

The second stop introduces the oldest rocks on the trail, part of the Lamar Limestone. The dominant rock type is a dark, fine-grained, mud-rich limestone that’s categorized as skeletal-peloidal wackestone using the Dunham scheme (fig. 10). The depositional conditions were generally low-energy and characterized by the slow settling of carbonate mud, tiny shell fragments, and peloidal grains from the water column. The lack of coarse material or high-energy features suggests this spot is the farthest out part of the toe-of-slope environment, where only the finest sediment from the reef reached.

Fig. 12 Well Laminated Mudstone / Wackestone - 
Lamar Formation (Stop 4)

Stop 3: Distal Toe-of-Slope Debris-Flow Deposits

The third stop, however, brings many geomorphological differences from the previous stops as the layers here are thicker and the rock is a much coarser-grained limestone, categorized as a skeletal and intraclast packstone by the Dunham scheme. The depositional environment was highly energetic with debris flows and high-density turbidity currents down the reef slope. We also observed the presence of chert at this stop (fig. 11). Key rock types observed include large intraclasts and fragments of sponge-algal reef boundstone, which shows that these flows originated high up on the Capitan reef.

Stop 4: Vertical and Lateral Changes in Toe-of-Slope Sedimentation

The fourth stop shows a major change in the ancient ocean environment, rather than just depositional energy. It’s characterized by the presence of fine-grained wackestones and mudstones (fig. 12), along with skeletal packstone interbeds. Typically, dark deep-sea muds lack signs of life, indicating the water was anoxic, however, the presence of marine burrows, like Planolites, in this layer depicts a major, temporary incursion of oxygenated water and bioturbidity into the deep basin.
 

Stop 5: Stratigraphic Relations on the South Side of the Canyon

The fifth stop offers a view into the relationship between the deep-basin sediments, Lamar, and the shallow-water reef, Capitan, confirming the source of all the transported sediment. This stop is characterized by the presence of wackestone and packstone, as classified by the Dunham scheme. 

Stop 6: Turbidites and Debris-Flow Deposits

As we moved towards the sixth spot, it was clear that the depositional conditions here were highly active with high-energy gravity flows. The rock is a thin/medium-bedded wackestone and packstone, as classified by the Dunham scheme, deposited by both debris flows and high-density turbidity currents (fig. 13). We observed cross-bedding within the layers, formed by strong, moving currents, which confirmed that this site was in a zone of active sediment transport. The rocks are noticeably coarser and more resistant to erosion compared to the fine-grained wackestones farther down the trail.

                       Fig. 13 Turbidites                              

Stop 7: Transition from Toe of Slope to Slope Environment

This stop marks the beginning of the geomorphological transition to the main, steeper reef slope. The layers are thicker, and the sediments are coarser and richer in skeletal grains than those observed at previous stops. The rock consists of skeletal-peloidal packstones and wackestones, which are highly resistant to erosion, and show evidence of channelization, or sediment flows, a typical feature of a major slope environment.

Stop 8: Top of the Lamar Limestone Member

This final stop designates the highest stratigraphic level of the Lamar Limestone Member. It marks the boundary where the deep-water Lamar unit ends and the rock units of the main, massive Capitan Reef slope system begin. Geologically, it is the highest point of the toe-of-slope section before entering the truly steep, massive deposits of the Permian reef front.