Geometry and mechanisms of folding related to growth faulting, Wilcox formation, De Witt County, Texas

Abstract

Growth faults and related folds are geologically and economically the most important structural features in the Gulf Coast province. Most existing theories do not adequately explain the observed fold geometries, and consequently fold geometry is difficult to predict. Predictability will improve where the relative contribution of various fold mechanisms can be determined. For example, tectonic folding, or folding due to faulting, causes successive fold crests to shift basinward with depth. Drape compaction folding causes fold crests to align vertically with depth, and differential compaction puts a fold crest in the area of highest sandstone percentage. Calculations from electric logs of interval thicknesses both before and after compaction provide a method for separating the various fold mechanisms. Previous theories have shown tectonic folding to be accomplished by beds on the downthrown side of the fault bending down toward the fault. This research recognizes an additional component of tectonic folding, called upfolding, whereby fold crests have moved up relative to regional dip. Overlying intervals are thin over the fold crests and thicken in all directions off the crest. Three field areas in De Witt County, Texas, provide examples for folding analysis. The Nordheim area is dominated by upfolding. Structural closure at the deepest horizon, the Migura, is 250 ft (76 m), and upfolding accounts for 200 ft (60 m). Geometry of two lower relief folds at lower Wilcox level is dominated by drape compaction in one and differential compaction in the other. Fold crests in Cook field are stacked almost vertically with depth. Upfolding and drape compaction both contribute to fold geometry. Steeper original dip on the lower Wilcox caused the lack of fold crest shift with depth. In South Cuero field, the lower Wilcox fold has 400 ft (120 m) of relief. The geometry of overlying horizons have a component of upfolding, but drape compaction over the steep lower Wilcox fold dominates and causes vertically stacked fold crests. The proposed mechanism for upfolding is the overcrowding caused by material moving down a concave-shaped fault surface. Volume calculations at South Cuero on the lower Wilcox indicate that about 7.5% of the material which moved down the fault surface was upfolded.