Abstract

The Eagle Ford and equivalent Boquillas formations in outcrop across southwestern Texas (Fig. 1) contain small-scale folds formed by (i) slumping during deposition and (ii) Laramide contraction after burial and lithification. Slump folds are present in natural outcrop and roadcut exposures of both the lower and upper Eagle Ford (and Boquillas) near the northwestern margin of the Maverick Basin (Lock and Peschier, 2006; Lock et al., 2010; Donovan et al., 2012; Loucks, 2018; Minisini et al., 2018; Lehrmann et al., 2019). The folds typically occur in 1–2 m thick zones, with observed outcrop extents of 10s to 100s of meters to >1 km—actual lateral extents are likely much greater. Curvilinear fold hinges changing trend by >90° and lens or eye-shaped outcrop patterns suggest sheath folding. Bed thickness changes reflect flowage of unlithified sediment during slumping, and subsequent compaction. Tectonic folds in the Eagle Ford and Boquillas formations are common in the subsurface and outcrop west of the Laramide macrostructural front (Ferrill et al., 2022a, 2022b). Distinguishing the origin of synsedimentary folds formed by slumping during deposition versus tectonic folds formed after burial, compaction, and lithification can be challenging— differentiation relies on a variety of geological observations and relationships.

Slump folds in the Eagle Ford and Boquillas formations are characterized by (i) ductile deformation of chalk and limestone beds, (ii) discontinuous bedding and thickness changes of chalk and limestone beds, (iii) low-angle or recumbent axial surfaces, (iv) horizontal compactional fabric at high angle to bedding in recumbent fold hinges, and (v) erosion and/or onlap of later beds on top of folded beds. Figure 2 shows an outcrop example of a slump fold in the pelagic chalk and mudrock facies of the upper Eagle Ford Formation at the Sycamore Bluffs exposure. This fold is within the slump folded interval from the stratigraphic height of 1.4 to 2.8 m in the Sycamore Bluffs measured section of Ferrill et al. (2017). The chalk bed laterally changes thickness without associated brittle fabrics, indicating ductile flowage. The recumbent isoclinal fold hinge exhibits a subhorizontal compactional fabric in mudstone at high angles to bedding around the inner arc of the folded chalk bed. The bed-perpendicular thickness of the chalk bed is greatest in the hinge, due in part to vertical compaction after folding. Vertical compaction reduced bed-perpendicular thickness where bedding is subhorizontal but had no effect on bed-perpendicular thickness in the recumbent fold hinge where bedding is vertical. The ductile behavior of both chalk and mudrock, and evidence of significant compaction after folding, are indicative of soft sedimentary folding. A small brittle normal fault with calcite slickensides nucleated on the dipping chalk-mudrock interface around the fold hinge, but otherwise there is little evidence of brittle fracturing associated with the fold.

In contrast to the largely ductile deformation associated with slump folds in the Eagle Ford Formation, tectonic folds are distinguished by (i) fold-related fracturing, (ii) constant bed-normal thickness of chalk and limestone beds, and (iii) calcite or gypsum veins formed during folding and/or deformed by folding and associated flexural slip (Ferrill et al., 2022a, 2022b). Figure 3 shows an example of a tectonic fold in upper Eagle Ford equivalent Boquillas Formation along U.S. Highway 90 approximately 5 km east of Langtry, Texas. The fold exhibits evidence of brittle deformation including bed-perpendicular extension fractures that fan around the fold, as well as folded bed-parallel gypsum beef veins with vein fibers perpendicular or oblique to bedding. Gypsum beef veins in the region formed in a thrust faulting stress regime and were actively forming during contractional folding (Ferrill et al., 2022a).

Slump folds in the Eagle Ford Formation have implications for understanding paleobathymetry, facies distribution, lateral changes in thickness, and fracture prediction. Slump folds can form on extremely gentle slopes (<1°) triggered by earthquakes, sea level fluctuations, and storms. Slump folds may represent significant shortening (e.g., 50%) and thickening (e.g., 100%) of the slumped interval, and absence of the same interval up-dip represented by missing section, boudinage, bed terminations, and/or extensional faulting. Slumping may be an important contributor to lateral thickness changes on gentle depositional slopes (e.g., along the Edwards and Sligo reef margins or on the flanks of the San Marcos Arch or Maverick Basin). Criteria for recognizing slumping in the subsurface include thickness changes; discontinuous and contorted beds, fold hinges, and dip changes; and inverted or repeated section. Fracture networks in slump intervals are less regular, and at least qualitatively appear to have lower fracture intensities than similar but flat-lying beds where systematic fracture networks tend to be well developed (e.g., Ferrill et al., 2014). Laramide tectonic folds are likely in the western part of the Eagle Ford play in the vicinity and forelandward of macroscale folding (Chittim Anticline, Zavala Syncline; Ferrill et al., 2022a, 2022b). Areas that have experienced contractional tectonic folding tend to have complex brittle tectonic fabrics associated with experiencing thrust faulting stress regimes and transitional strike-slip regimes prior to and following thrust faulting stress regime conditions (Ferrill et al., 2022a, 2022b). By comparison, Eagle Ford strata in areas further east that have not experienced contractional deformation are likely to have less complex deformation fabrics, due to the less complex stress and deformation history. For unconventional reservoir development, slump intervals and tectonic folds can result in heterogeneity that in different ways may influence reservoir quality, drilling and associated hazard, and optimal stimulation during completion.