Abstract

The Royal Gorge arch is a highly-jointed, anticlinal structure, well exposed over a 60 km² area with vertical relief up to 360 m. Joint patterns were studied within the arch to determine possible relationships between joint orientation, joint frequency, lithology, and basement fabric. Outcrops were examined in detail in both the Precambrian crystalline core of the uplift, and in the overlying Phanerozoic sedimentary cover. Joint spacing data, joint orientation data, and the attitudes of the foliation surfaces or bedding surfaces were recorded at over 550 uniformly spaced locations.

The Precambrian core of the arch is composed of two general lithologies. A well-foliated, quartzofeldspathic gneiss makes up most of the southern two-thirds of the basement core. A weakly-foliated granodiorite makes up most of the basement to the north. Phanerozoic sediments dip gently off the western side and steeply off the eastern side of the arch. Major faults are located on the northwestern and southern sectors, and a major east-striking fault penetrates the central sector.

In most locations, joints tend to be parallel and perpendicular to foliation and bedding. Because of differing Precambrian foliation versus Phanerozoic bedding orientations, joint patterns in the basement and cover rocks do not match. Joint frequency is greatest in the quartzofeldspathic gneiss (greater than one per meter) and least in the weakly-foliated granodiorite and flanking sediments (less than one per meter). Joint frequencies increase in strongly-foliated basement regions and in the vicinity of faults.