Interpreting Natural Fracture Patterns in Outcrop in the Raton Basin

of Colorado and New Mexico

By

Scott P. Cooper, John C. Lorenz, and Russell G. Keefe

Sandia National Laboratories, Albuquerque, NM

The most common fractures in Cretaceous and Tertiary strata cropping out in the Raton Basin are related to Laramide-age eastward thrusting of the Sangre de Cristo Mountains at the western margin of the basin. This is suggested by extension fracture orientations that strike approximately east-west, nearly normal to the thrust front, and by conjugate shear fractures with an acute-angle bisector having the same trend. These fractures and east-west face cleats in coals occur throughout the basin, including the eastern margins of the basin over 75 miles from the thrust front. Bedding-parallel shear surfaces and meter-scale thrusts show that small-scale thrusting and strain accommodation are pervasive within the basin-filling strata. Fracture orientations and surface features at even distal locations indicate horizontal shearing, suggesting fracture formation under conditions of significant horizontal loading related to thrusting. Other fractures within shales and coals are related to syn-depositional compaction.

Uncertainties complicate interpretations of the fracture-forming, in situ stress systems. Igneous dikes that post-date the Laramide orogeny also strike east-west, suggesting remnant Laramide stresses at crustal levels. However, stress orientations changed about the time of intrusive activity, probably related to east-west extension and Rio Grande rifting. The present day maximum horizontal compressive stress trends north-south. Extension fractures in post-Laramide sills strike east-west, parallel to fractures that presumably formed significantly earlier in the associated sandstones, rather than north-south parallel to the contemporaneous rift-related stresses. Moreover, penecontemporaneous intrusion of dikes and sills suggests a complicated interplay of pore pressure, regional stresses, and local stresses during intrusion.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming