Structural Geology of the Late Cretaceous Boquillas Formation, Eastern Big Bend National Park (Solis to Persimmon Gap), Trans-Pecos Texas

Detailed geologic mapping (1:24,000) of the Boquillas Formation in the eastern part of Big Bend National Park from Solis to Persimmon Gap (a distance of 80 km) identified several structural attributes.

First, there is a change in the predominant fault directions/trends, from NNE and NNW to strongly NW, affecting the Boquillas Formation from south to north. The southern half is dominated by faults trending N10°E–N20°W while the northern half is dominated by faults trending N20°-60°W. Almost all faults are high angle (dips of 65° to 90° with most greater than 75°).

Second, the Boquillas Fm adjacent to some major faults was rotated by drag such that bedding is vertical to locally overturned. Most of this deformation is more consistent with dip-slip movement rather than strike-slip movement.

Third, the Boquillas Fm consists of intervals of more competent limestone units/beds (7.6-27.5 m thick) that alternate with incompetent mud and shale units (15-27.5 m thick). The more competent intervals are more commonly folded while less competent intervals are not folded. The alternation and vertical spacing of competent and incompetent intervals resulted in local disharmonic folding.

Fourth, splay faults are associated with and related to primary faults trending N0°–15°W±90°. The splay faults generally trend N40°–50°W±90° or they dip steeply (75°–85°) to the northeast. The amount of displacement on these secondary faults decreases as the distance from their intersections with the primary or master faults increases. In some instances these faults disappear within 1–2 miles from the master fault and no displacement or offset occurs. In certain areas, 'third-order' and 'fourth-order' splay faults were also identified. These were primarily mapped in narrow (±1km wide) grabens where space problems developed as the central down-dropped block was internally fractured and faulted, producing numerous narrow triangular-shaped fault wedges with both normal and reverse dip-slip displacement.