Abstract

Recognition of the presence of wrench faulting in the Permian Basin, Val Verde Basin and other basins in West Texas began at a slow pace. Moody & Hill (1956) described wrench faulting, but did not mention wrench faulting within the Permian Basin even though they were in Midland with Gulf Oil at the time. The author believes that they were not allowed to reveal what they knew. John Hills (1970) was among the first authors to apply wrench faulting to the Permian Basin in his paper “Late Paleozoic structural directions in the southern Permian Basin, west Texas and southeastern New Mexico.”

The preponderance of evidence now reveals that the underlying structural pattern of West Texas is that of a left-lateral wrench fault system that has controlled the orientation of the structure of oil and gas fields in the various basins. Other deformation models, such as the well-known strain ellipsoid model cannot be effectively applied to the tectonics of the basins of West Texas. However, the Riedel Model can be applied directly to West Texas, and especially to the oil and gas fields of the Permian Basin.

In his laboratory experiments Riedel (1929) placed a clay layer on top of two wooden boards. One of these boards was then slid past the other. The shear fractures observed in the overlying clay layer were named, and the angles between the shears were measured. This simple laboratory experiment simulated a strike-slip, or wrench fault as viewed on the Earth’s surface. Various workers, notably Tchalenko (1970), and Webster (1980) have expanded upon Riedel’s experiment and developed the terminology that can be directly applied to the wrench fault tectonics of the Permian Basin.

The angular directions presented in this paper are measured clockwise from True North (T), and not relative to the primary direction of movement of the primary wrench fault system. Four basic shear directions of movement are common in West Texas. The conjugate Riedel shear (R’) is right-lateral, and at ≅ 54°T. The Riedel shear (R) is left-lateral, and at ≅ 90°T. The primary shear (P) is left-lateral, and at ≅ 320°T. The fold shear (F) is right-lateral, and at ≅ 355° T. The primary shear fault is a combination of R and R’. Primary shears are usually observed as connected en echelon faults. Fold shears and rotated blocks were not described by Riedel but can be seen on his original models.

The P and R’ are perpendicular to each other and are left-lateral and right-lateral respectively. Therefore, the rocks will collide south of the intersection forming a positive structure. Between parallel pairs of either P shears or R’ shears there can be rotated blocks that are positive flower, or pop-up structures.

Flower structures from 700’ to nearly 90 miles in diameter have been documented in the field by the author. Some locations are as follows: Allan pop-up block (Figure 1), Blackstone Slaughter Ranches, Brown Bassett Field (Brown Mobley #1), Terrell County, Anacacho Mountains, Kinney County, Brazos River rotations (Figure 5), and curved faults in a rotated block (Figure 6), and the Horseshoe Atoll.

Shear stress is caused by shrinking of the Earth’s Crust and other compressional forces within the Earth’s Mantle. As the shear stress is translated upward, various shear fractures form in the sedimentary rocks, as outlined above. As the walls of rocks slide past each other, some blocks of rocks detach and may rotate. These rotated blocks of rocks are documented both in map view, and on the outcrop. The six figures show examples of these rotated blocks, and other wrench fault features. natural gas.