Structural geology of the Terlingua quicksilver district, Texas

Geologic mapping, supplemented by detailed investigations at the mines, has revealed new information about the geologic structure of the Terlingua quicksilver district, Texas. Attention is focused on the nature and origin of domes, grabens, and breccia pipes, structures that are unusually well developed in the area. The sedimentary rocks, of Cretaceous age, are overlain and intruded by igneous rocks of Tertiary age. Much of the structural deformation was an accompaniment of igneous intrusion.

The Terlingua uplift, an irregular dome with a structural relief of several thousand feet, covers about half of the district. The southern boundary of the uplift is a steep monocline. Smaller domes, found throughout the district, were formed by igneous uplift. Evidence for this origin includes exposures of the igneous rock, the occurrence of baked rock on the domes, and theoretical consideration of the forces necessary to form a dome.

Almost all the major faults are normal faults bounding depressed tracts, or grabens. Most of the major faults trend northwest; the strike varies markedly, with both sinuous curves and angular turns. The vertical displacement of the grabens below adjacent blocks varies, up to approximately 2,000 feet. Minor faults belong to two sets; one set trends northwest, parallel to the major faults, and the other northeast.

Masses of breccia with a pipelike or chimneylike shape have been designated descriptively as breccia-filled pipes, or simply breccia pipes. A total of 76 breccia pipes was mapped, and four typical breccia pipes are described in detail. They range from 75 feet to 500 feet in diameter, and one has been explored vertically for more than 800 feet. The top of one breccia pipe has been found, in the Chisos mine, but the bottom of none has been found. The displacement of the fragments is downward, commonly several hundred feet, and in one instance more than 1,000 feet. The nature of the breccia and its walls indicates collapse due to undermining as the origin of the Terlingua pipes. Features, common to all the pipes, that point to this origin are the downward displacement of the fragments, the absence of drag or thrust structures, the presence of slump structures along the walls, and the vertical cylindrical form.

The general structural problems of the district include the age relations, the origin of the uplift, the origin of the domes, and the origin of the grabens. The earliest structural movement involving the Cretaceous rocks was the beginning of rise of the Terlingua uplift. Before this rise ceased, the graben faulting probably began. Volcanism and widespread hypabyssal intrusion, which was accompanied by doming, was partly contemporaneous with graben faulting. The breccia pipes were formed after the beginning, and before the end, of igneous activity. Minor faulting, accompanied by mineralization, continued later than other structural movements, The Terlingua uplift was probably caused primarily by laccolithic igneous intrusion but was influenced by horizontal compression. A basic intrusion, rather than a silicic intrusion, is required to explain the exposed igneous rocks. The geometry of the large grabens indicates that they are relatively superficial accompaniments of horizontal extension. It is suggested that horizontal extension of the upper part of the earth's crust occurred during the formation of the Terlingua uplift and was accompanied by graben faulting.