The formations of the Delaware Basin shown above will be referred to throughout this post.
Roadcut 1 was located west of the Guadalupe mountains in a salt flat that is between two normal faults (Fig. 1). The salt flat region sits in the graben, which is of Tertiary age, and the Guadalupe mountains is the horst. Further west passed the salt flat there are gypsum and alluvial domes. The salt flat is mostly halite that precipitated during heavy rains from calcium and deposited at the base of the Guadalupe Mountains. Mudcracks are abundant on the salt flat and are evidence of wetting and drying conditions (Fig. 2). Clay-rich rocks will expand when wet and will contract when dry, this results in cracks on the surface of the rock.
Figure 1. a diagram of a horst and graben system between two normal faults. (USGS, 2011).
Figure 2. Mudcracks at the salt flat indicative of wetting and drying conditions. Pencil is 14cm for scale
Roadcut 2 featured the Cutoff formation, which contains organic material dating back to the permian, had alternating layers of black to grey to light grey micritic limestone. According to Harris (1987) the Cutoff formation formed during alternating layers of erosion and deposition over a drowned shelf margin. A sample of micritic black limestone with calcite veins (Fig. 3) was taken from the site. The calcite veins are secondary because the mineral precipitated in fractures of the rock after it was already deposited. The color of the limestone is attributed to the percent of organic material (oil) in its composition. Thin sections were made of a sample from the outcrop to help describe the composition and sedimentology of the black limestone (Fig.4). Because the sample is very dark in color and fine grained without fossils, it formed in an anaerobic environment without oxygen, benthic fauna, or shelled fossils and had a high organic content (Harris, 1987).
Figure 3. Micritic black limestone with calcite veins toward the bottom of the image.
Figure 4. Image was taken at 40x. Matrix is very-fine grained and micritic. Calcite composition is present in the veins.
Roadcut 3 featured Brushy Canyon formation, the lowest unit in the Delaware Mountain Group, is a fine-grained sandstone. The outcrop (Fig.5) was cut to form a submarine channel that is perpendicular to the shelf margin (Onac, 2019). Brushy Canyon sandstone sits at the top of the outcrop then there's a layer of black overbank siltstone above a layer of folded sandstone and at the bottom is finely laminated shale (Fig.6). The east end of the outcrop is plunging down and there is deformation caused by sand injection.
Figure 5. shows the outcrop plunging down and the variations in the layers. Alternating units of grading is present.
Figure 6. displays a field sketch of the layers at the outcrop.
Roadcut 4 is an enigmatic contact between Brushy Canyon formation and Cherry Canyon formation and is a direct result of the Tertiary normal faulting system described at roadcut 1 (Fig.7). Cherry Canyon formation is mainly fine-grained sandstone but it also has three named limestone members, which are from youngest to oldest, the Getaway, South Wells, and the Manzanita (Hayes, 1964). At the contact which is shale, the Cherry Canyon formation overlies the Brushy Canyon formation.
Figure 7. shows the Cherry Canyon at the top of the contact and Brushy Canyon below. Classmates can be used as scales.
Roadcut 5 depicted a submarine slide on the northwest side of the outcrop, by which fragments of limestone detached from the slope and rolled off and into the Bell Canyon formation. This created embedded inclusions of conglomerates (limestone clasts) inside of the Rader Member of Bell Canyon formation (Fig.8). The Rader Member of the Bell Canyon formation is the sandstone matrix of the outcrop. The clasts are very poorly sorted and have a large range in size. On the southwest of the outcrop, there were alternating lamination layers of shale, limestone, and siltstone that are indicative of a fluxing sea level.
Figure 8. shows the limestone clasts inside the Rader Member of the Bell Canyon formation.
Roadcut 6 displayed the Castile Formation which is within the Ochoa series. It is the oldest unit of Ochoan sediment in the region and overlies the Bell Canyon Formation (Onac, 2019). It is also the youngest part of the Capitan and Tansill Formations and thus covers the basin area. The Castile Formation is alternating layers of anhydrite and halite, which could be from climatic changes during the end of the Permian age (Fig.9).
Figure 9. The Castile Formation and it's layers. Pencil is 14cm.
References
Hayes, P.T (1964). Geology of the Guadalupe Mountains New Mexico. Retrieved from https://pubs.usgs.gov/pp/0446/report.pdf
Harris, M.T (1987). Sedimentology of the Cutoff Formation (Permian), western Guadalupe Mountains, west Texas. Retrieved from https://geoinfo.nmt.edu/publications/periodicals/nmg/9/n4/nmg_v9_n4_p74.pdf
Onac, Bogdon. (2019, September 20-25). Sedimentary Petrology class (GLY 3552) Field trip guide.
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