Geology and ground-water resources of the Balmorhea area, western Texas

Abstract

Balmorhea is the center of a thriving farming community, the lands of which are irrigated with water derived chiefly from large springs but partly from the storm flow of Toyah Creek. The storm flow of the creek and a part of the winter flow of the springs is stored in a reservoir near Balmorhea and used later to supplement the flow of the springs. The present investigation was made to determine the geologic and hydrologic relations of the springs, whether additional water can be obtained from wells, and what effect the withdrawal of large amounts of water from wells would have upon the discharge of the springs. Balmorhea is situated near the foot of the Davis and Barrilla Mountains and along the southwestern margin of the Toyah Basin. The mountains and adjacent basin are drained by Toyah and Limpia Creeks. The group of springs around Balmorhea occur in the floor of the valley of Toyah Creek. They have been divided into artesian springs--Phantom Lake, Giffin and San Solomon Springs; and gravity springs--Toyah Creek, Saragosa, East Sandia and West Sandia Springs. The combined discharge of the springs during dry years is about 23,000 gallons a minute, of which amount the artesian springs supply more than 90 percent. The underground reservoir which supplies the artesian springs is the fractured and cavernous Lower Cretaceous limestone. This limestone, about 500 feet thick, is underlain by impermeable rocks, probably of Permian age, and is overlain by impermeable Upper Cretaceous strata that have a maximum thickness of about 500 feet. These are in turn overlain in the mountains by Tertiary lava and on the plains by gravel and other surficial deposits. The Lower Cretaceous limestone is at the surface or covered by a thin layer of gravel in a belt that lies athwart the stream channels and extends from Gomez Peak southeastward along the foothills of the Davis Mountains. In this belt all the streams suffer heavy seepage losses. From this belt the limestone dips gently northeastward to the axis of a northwestward-trending syncline and then rises to the surface in the vicinity of Phantom Lake, where a part of the water is discharged. About 1,000 feet northeast of this lake is a northwestward-trending fault of small displacement, on the northeast side of which the limestone is downthrown, northeastward from this fault the limestone rises gently and appears at the surface about a mile to the northeast, where it is again downfaulted, but the throw is not sufficient to affect the movement of the water. For several miles to the north the water-bearing Lower Cretaceous limestone is covered by 400 to 500 feet of impermeable Upper Cretaceous strata. It is believed that the Lower Cretaceous rocks are again near the surface and covered by only a thin mantle of gravel and other surficial deposits at San Solomon and Giffin Springs and that just northeast of the springs a fault crosses the valley along which the impermeable Upper Cretaceous rocks are faulted into a position opposite the Lower Cretaceous rocks, thus obstructing further northward movement of the water in the Lower Cretaceous limestone and forcing it to issue as large springs. Between this fault and Brogada the Lower Cretaceous rocks are believed to lie at a depth of about 500 feet and are overlain by Upper Cretaceous strata and a blanket of gravel and other surficial deposits, which are the source of the water of Toyah Creek, Sandia, and Saragosa Springs. Northeast of Brogada the Lower Cretaceous lies at a greater depth, and the mantle of gravel is much thicker. Wells put down to the limestone in the vicinity of San Solomon and Giffin Springs would decrease the flow of the springs. The effect of wells in limestone between the fault near these springs and the Brogado Hills on the flow of the springs would depend on the completeness with which the fault cuts off northward movement of ground water in the limestone. If the movement of water across the fault has been prev