New Mexico Geological Society Annual Spring Meeting — Abstracts

Many geologic mapping efforts in the San Juan basin (SJB), northwest New Mexico, reveal short (<10 km), often high-angle (dip angle >75°) normal faults. These faults generally are too small to be captured on statewide or basinwide geologic compilation maps, but are revealed in many areas within the basin by most field mapping efforts at the 1:100,000 scale or larger. Though the tectonic significance of basin-interior normal faults is almost certainly of lesser importance than the larger-scale compressional and/or transpressional tectonics more plainly manifest in San Juan basin deformation structures, it nonetheless deserves consideration. At present, however, it remains largely unaddressed.

Recent STATEMAP mapping on the Chaco Canyon 30’x60’ Quadrangle (Hobbs and Pearthree, 2021) and the Coyote Canyon 15’ Quadrangle (Hobbs and Krupnick, in preparation 2024), and geologic mapping related to hazards analysis on the southern Jicarilla Apache Nation (Hobbs and Pearthree, 2023) presented opportunities for detailed documentation of several San Juan basin-interior high-angle normal faults. In each of these locations throughout the central and southern SJB, normal faults are found in Cretaceous and Paleogene siliciclastic sedimentary units. Where fault planes can be measured, their dips range from 56° to 88°. Fault offset is up to 18 m and approaches 0 at fault tips, where faults merge into joints traceable for up to hundreds of m. Most observed fault planes do not preserve reliable kinematic indicators, but where striations are observed, they indicate dip-slip motion. Timing of fault motion is constrained only by cross-cutting relations, with maximum ages of deformation provided by the age of Late Cretaceous, Paleocene, or Eocene sedimentary rocks cut by faults, and minimum ages provided by undeformed Pleistocene or Holocene surficial sediments that overlie faults. SJB normal faults often are isolated; where they occur in proximity to one another, the sense of motion on each sub-parallel fault is the same (i.e., all faults in an area are down-to-the-south), suggesting a distributed fault zone as opposed to a series of grabens and horsts. Within individual quadrangles or similarly-sized areas of study, the faults reported here sometimes have systematic orientations, especially near the eastern and western basin margins. In general, however, the wide range of fault orientations does not indicate a basin-wide uniform stress orientation leading to SJB normal faulting.

Further work with more mapping and basinwide documentation and analysis is needed to understand more fully the causes, timing, and tectonic and/or basin evolution significance of the faults reported here. As SJB siliciclastic sedimentary units are further exploited as reservoirs for oil and gas, as aquifers, as repositories for produced water, and as targets for carbon sequestration, small-offset faults like these potentially serve as traps, leaks, or zones of increased permeability due to fault-zone fracturing, yet are unlikely to be recognized or documented in the subsurface. Characterization of these faults through geologic mapping is a first step to increasing understanding of the prevalence and significance, and can shed light on why there are widespread (though small) extensional features throughout the otherwise compressional SJB.

References:

1. Hobbs, K.M., and Krupnick, J.M., in preparation 2024, Geologic Map of the Coyote Canyon 15’ Quadrangle, Navajo Nation and McKinley County, New Mexico: New Mexico Bureau of Geology and Mineral Resources Open-file Geologic Map 314, scale 1:100,000, 1 sheet, 80 p. report, doi.org/10.58799/OF-GM-292.
2. Hobbs, K.M., and Pearthree, K.S., 2021, Geologic Map of the Chaco Canyon 30’x60’ Quadrangle, New Mexico: New Mexico Bureau of Geology and Mineral Resources Open-file Geologic Map 292, scale 1:100,000, 1 sheet, 24 p. report, doi.org/10.58799/OF-GM-292.
3. Hobbs, K.M., and Pearthree, K.S., 2023, Geologic Map of the Cañon Largo Watershed on the Jicarilla Apache Nation, Rio Arriba and Sandoval Counties, New Mexico: New Mexico Bureau of Geology and Mineral Resources Open-file Report 620A, scale 1:50,000, 1 sheet, 56 p. report, doi.org/10.58799/OFR-620A.