Day 2 Road Log: Carrizo Arroyo and San Jose Gorge
— B.A. Frey, K.E. Karlstrom, S.G. Lucas, J. Ricketts, M. Channer, P.L. Miller, N. Dunbar, S. Williams, and D. McCraw

Today’s trip takes us primarily to Carrizo Arroyo in the Lucero uplift, which is located along the edge of the Colorado Plateau and the Rio Grande rift. The visit to Carrizo Arroyo allows us to examine the remarkably well-exposed geology of this important structural boundary in central New Mexico and to discuss and contrast Cretaceous-Laramide compressional versus Miocene-Recent rift extensional tectonic styles.  Carrizo Arroyo also exposes a world famous stratigraphic section through the Pennsylvanian-Permian boundary in the Bursum Formation, strata of mixed marine-nonmarine origin that yield a diverse and well-studied fossil record of everything from cockroaches to conodonts.

We begin by leaving Belen, driving north on I-25 to exit at Los Lunas and then drive west on NM Highway 6 to a point where we leave the paved highway and proceed south on an all-weather, unpaved road. In less than 10 miles we will leave this road for a dirt road that requires high-clearance vehicles and four-wheel driving.

At the first stop (Fig. 1), just east of the rift margin, we examine late-Cenozoic strata of the Popotosa Formation of the Santa Fe Group that are folded into a broad fault-related syncline and overlain to form an angular unconformity by a travertine-cemented, stream-terrace gravel that reflects the landscape position of Carrizo Arroyo 179 thousand years ago. We will also discuss Bobo Butte, a similar travertine-capped butte gravel that is about 1 million years old. Continuing west, we soon arrive at

Figure 1. Day 2 stops. Day 2 stops near Carrizo Arroyo: Stop 1- Santa Fe Group and angular unconformity, Stop 2- Park and Introduction, Stop 3- Lunch, Optional Stop 4- Structure hike, Optional Stop 5- Stratigraphy hike. Participants will decide at Stop 3 whether they will proceed to Stop 4 with Karl Karlstrom or Stop 5 with Spencer Lucas.

Stop 2 at Carrizo Spring. Here, we will park; introduce the structural geology of the Colorado Plateau margin, the carbonic waters and the travertines of the modern arroyo; and discuss a highly unusual Upper-Triassic limestone exposed at this location. The group will walk from here up Carrizo Arroyo examining small scale faults in the Abo and Yeso formations and have lunch at Stop 3 (where the lunch truck can meet us). We then divide into two hiking groups. One group will hike north to stop 4 and examine the structural margin of the Colorado Plateau and intrusive igneous rocks that have been newly dated as 27.5 Ma. The other group will work toward stop 5 and examine the Pennsylvanian and Permian stratigraphic section in Carrizo Arroyo with focus on the Pennsylvanian-Permian transition as recorded in the Bursum Formation.

We leave Carrizo Arroyo by retracing our route to Highway 6 and continue west, where we stop to view the gorge where the Rio San Jose is incising through the 184 ka Suwanee basalt flow at the edge of the Rio Grande rift. We conclude the day at this stop with a greeting from the Pueblo of Laguna, discussion of the geomorphology and incision history of the Rio San Jose, and summary of the young basaltic volcanism of the Lucero uplift region.