Abstract

The Pennsylvanian carbonate bank system of the Graford Formation crops out mostly in Jack, Palo Pinto, and Wise counties along a northeast-southwest trend in North Central Texas. These rocks were deposited in a delta system at the close of the Ouachita orogeny. Numerous structural features, related to the Bend arch, occur at depth with little or no surface expression. Extensive erosion and regional tilting produced a broad paleoplain which underlies the massive sedimentary wedge along the Gulf Coast.

The Pennsylvanian limestone units are systematically jointed throughout their outcrop area. The joints exhibit a wide variance in orientation, although three dominant joint sets are recognizable. The most widely occuring set is North 60 degrees East, with a minor complementary set at North 60 degrees West. The major set, related to the Balcones system and the Ouachita trend, resulted from regional tilting to the southeast. An intermediate set, ranging from North to North 25 degrees West, follows minor structural highs in the subsurface and provides a wider range of orientations than the major set. Scattered minor sets show no relationship to structural features, occur in localized areas, and are probably the result of variations in rock fabric and geometry.

Systematic jointing of structureless rocks in the area of investigation is the result of tensile stresses which were present during burial or were introduced during erosion of the overburden. The dominant joint set coincides with the regional strike of the Pennsylvanian strata and probably reflects trends of recently formed, significant slumps. As the overburden on the Graford carbonates was reduced by erosion, variations in differential loading related to local deep structures were reflected in joint development.