Abstract

A set of well-developed, bed-bound natural fractures of the Eagle Ford in the Del Rio (Texas) area formed in response to far-field compressive stresses. These NE-SW oriented fractures are best developed in, but not restricted to, tabular limestone beds at the base of the Upper Eagle Ford where the formation consists of cm- to dm-scale interbeds of limestone and marlstone. Where definable, mean fracture spacing is less than bed thickness; in some cases, fracture spacing shows a log-normal distribution. They are observable in outcrops that stretch ∼120 km (75 mi) from the NW to SE (Sycamore Creek to Antonio Canyon) and at other locations in between.

Their NE-SW orientation has led to confusion about the mechanisms responsible for their formation. That orientation is approximately parallel to the strike of extension fractures developed in Mesozoic and Tertiary strata of the southern Texas Coastal Plain. Fractures with that orientation are observable in outcrops of the Austin Chalk in the San Antonio area. On the other hand, the fractures are perpendicular to fold hinge lines of the Sierra Madre Occidentale to the southwest of the outcrop belt in Mexico and the Chittum Anticline in the subsurface of Texas to the southeast of the outcrop belt. Plumose structures on fracture faces imply fracture growth parallel to bedding, indicating that they formed in response to compressive stresses (σ₁ horizontal).