Characterization of the structure of faults in the Eocene Carrizo Formation near Gause, Milam County, Texas

Abstract

This study characterizes the structure and evolution of a series of excellently exposed, normal faults in a small quarry in the Eocene Carrizo Formation near the town of Gause, Milam County, Texas. The faults strike NE-SW and comprise a small-scale graben accommodating NW-SE extension. The Carrizo Fm. consists primarily of friable to weakly lithified quartz sandstones with several horizons with interbedded siltstones and shales. Five faults and fault zones occur in the quarry with displacements ranging from 0.3 m to between 6-15 m. Smaller displacement faults occur only in direct association with the larger faults except for a system of small faults that forms locally between two of the larger faults. In the initial stages of development, a fault consists of multiple fault segments arranged as "en echelon" and localized in a narrow zone. Neighboring fault segments usually overlap and link either directly by one segment curving to abut the other or the two segments are linked in the overlapping region by a system of secondary faults that form between and abut the earlier formed larger fault segments. Three different systems of secondary faults are observed and are related to sense of fault stepover and shear as well as the magnitude of overlap. With increasing displacement, fault structure becomes more complex. A fault zone evolves as more fault segments develop in proximity to earlier formed segments. As in the earlier stage, systems of secondary faults form in the overlap regions of larger faults. Wall rock deformation adjacent a fault occurs but is of a limited nature. Locally, drag folding occurs, but this appears to be more common where the adjacent fault segment exhibits a nonplanar local geometry. Enhanced development of antithetic small faults also occurs near these geometric irregularities. Clay smears develop along fault segments where they offset interbedded sandstone and clay of one of the subunits (unit A) of the Carrizo. In contrast, clay smears do not develop along faults that offset interbedded sandstone and clay in another subunit (unit B). The cause for this difference is unresolved. Cataclasis within the shear zones of faults in the sandstone consists of some limited grain breakage together with rigid body grain rotation. The limited grain breakage together with other sedimentological data suggest the depth of burial during faulting was between 100 and 900 m.