Synsedimentary slump folding: Examples and consequences of an under-recognized process in epicratonic basins

Abstract

Small-scale synsedimentary folds in the Eagle Ford Formation in southwest Texas formed during deposition in a shallow epicratonic basin on the North American craton. These folds formed by slumping (i.e., gravity-driven deformation) and are characterized by (i) plastic deformation of limestone beds, (ii) discontinuous bedding and thickness changes of limestone beds, (iii) horizontal compactional fabric at high angle to folded bedding, (iv) low-angle or recumbent axial surfaces, and (v) erosion of slumped intervals and/or onlapping by resumed sedimentation. Exposures of the Eagle Ford near the northwest margin of the Maverick Basin reveal slump folds in 1–2 m thick zones with observed lateral extents of 10's to 100's of meters to >1 km – actual extents are likely much larger. Curvilinear fold hinges changing trend by >90° over short distances, indicating markedly non-cylindrical fold geometries. Bed thickness changes reflect flowage of unlithified sediment during slumping, and subsequent compaction. Previous descriptions of slump folds in the Eagle Ford Formation have focused on examples from the hydrodynamic facies of the basal portion, but slump folds are also present in pelagic facies of the upper Eagle Ford. Slump folding locally represents significant shortening (e.g., 50%) and thickening (e.g., 100%) of slumped intervals, with up-dip absence of the same interval represented by missing section, bed terminations, boudinage, and/or extensional faulting. Repeated slumping can result in significant cumulative updip thinning and downdip thickening along depositional slopes. Our experience working unconventional reservoirs in a variety of North American epicratonic basins indicates that slump folding is common and underrepresented in the literature. Criteria for recognizing slumping in the subsurface include a combination of discontinuous and contorted beds, thickening into fold hinges, compactional fabric postdating folds, and inverted or repeated section. Natural fracture networks may be less regular than in similar but flat-lying beds. Occurrence of slump folded intervals adds complexity and unpredictability to unconventional reservoirs, with implications for reservoir quality, drilling, and optimal stimulation during completion.