Geology of the McMillan Ranch in Mason, Texas: An Assessment of the Nature of Normal Faults in the Mason Area



Journal Title

Journal ISSN

Volume Title



Mason, Texas and the surrounding areas have been previously studied and mapped at small scales, showing the large normal faults that cut through the area. Many secondary faults exist close to the large faults, and are not mapped in previous studies because of the small scale of the maps. The large number of faults, when the smaller secondary faults are considered, makes Mason a good place for studying the nature of normal faults in this region and making generalizations about their nature. This thesis examines one of these faults, the McMillan Fault, and the secondary faults in its hanging wall at a large scale, in order to assess the nature of normal faults in the Mason area.

The McMillan Ranch in Mason, Texas, was mapped at a scale of 1:7,000 using both traditional and digital mapping methods, to determine the lengths and displacements of each fault, and attempt to determine a length/displacement ratio which can be applied to all normal faults in this area. A single length/displacement ratio was not determined, just as in previous studies. This study determined that the normal faults in the area are planar, high angle normal faults with varying displacement amounts. As a result, observations determined that deformation in the hanging wall of normal faults exceeded the deformation in the footwalls of the same faults. The main fault on the McMillan Ranch is the McMillan Fault, and its shape is determined based upon the orientation of the subsidiary normal faults in its hanging wall.

A detailed study of the geology of the McMillan Ranch and the surrounding area, including a geologic history of the area, geologic map and cross section, and stratigraphic descriptions including bed-by-bed descriptions, stratigraphic column, and thin sections of each unit was carried out as a preliminary step to perform analyses of the faults on the ranch. The presence of the McMillan Fault was already known, and the pasture that was chosen for this study was best represented at a scale of 1:7,000. At such a large scale, it was necessary to recognize precisely where in the stratigraphic section the mapper was located, as some subsidiary faults were recognized by beds missing, rather than entire units.

The structural data gathered from the field convey the varying natures of faults, even within the same area, and support the conclusion that length alone is not sufficient to predict displacement value on a fault.