High-resolution stratigraphic and structural characterization of the fault-partitioned Hickory Sandstone aquifer system, Mason County, central Texas

Abstract

The Hickory Sandstone is an important aquifer in central Texas and is partitioned by faults that impede cross-fault fluid flow. This study provides a detailed stratigraphic and structural model in the vicinity of a normal, oblique-slip fault with 60' (18.3 m) of stratigraphic throw. The model is developed using 3500' (1050 m) of continuous core and geophysical logs from eleven closely spaced boreholes. The local stratigraphy is studied in detail and environments of deposition inferred. A model of fault evolution is inferred using the observed fault structure and throw distributions. Locally, the Hickory Sandstone consists of 450' (137 m) of Cambrian-aged, quartzose and arkosic sandstone with localized mudstone and siltstone interbeds and overlies Precambrian Town Mountain Granite. Within the study area, the Hickory Sandstone is subdivided into four facies: the cross-bedded facies, the mudstone facies, the interbedded sandstone facies and the hematite facies. These facies form a stacked sequence representing an initial braided-stream fluvial environment that grades into a high energy, open marine environment that closely matches the tide-dominated, high microtidal estuarine model of Reinson (1992). Lateral correlation of strata packages in the cross-bedded facies was very difficult and complicated development of the fault model. The study fault is a linked fault system consisting of several major segments. Two segments overlap and locally hard link along both strike and dip. The major fault segments also consist of several linked subsegments. Net stratigraphic throw decreases slightly upward from a maximum of 60' (18.3 m) near the granite basement. Where the major segments overlap, the throw exhibits systematic variations consistent with displacement transfer between the neighboring segments. The linked fault system is inferred to have formed by interaction and linkage of two, early, en echelon basement faults. Ultimately these faults propagated upward and laterally into the overlying Hickory Sandstone, interacted and partially hard-linked, producing a large linkage structure that affected subsequent, neighboring hanging wall deformation. Subsidiary small faults are common but do not exhibit simple spatial relations with the large fault segments. There is only a weak correlation between a fault's shear zone thickness and stratigraphic throw for faults with 1' to 60' (0.3 to 18.3 m) of throw.