STRATIGRAPHIC ANALYSIS OF THE WATER-BEARING UNITS OF THE SUNNY GLEN DRAINAGE BASIN NEAR ALPINE, TEXAS

The aquifer system for the Alpine, Texas area resides in Tertiary age igneous rock. This system, unfortunately, has been the object of only a few general geologic studies. In an arid region totally dependent upon groundwater for municipal water supply (such as Alpine) it is important that we know as much as possible about our groundwater sources. The purpose of this research was to conduct a detailed geologic study of the Sunny Glen Drainage Basin (SGDB) area (which is one of the areas from which the Alpine municipal water supply comes). Many water wells have been drilled in the Alpine area, but most have not been accurately logged. This lack of basic subsurface data makes it difficult to assess the lithology of the water bearing units in the area. Most professionals believe that the primary water bearing units are the Cottonwood Spring basalt (Tcb), the Crossen trachyte (Tpct), and Quaternary Alluvium. For the SGDB, the results of this work challenge this widely believed tenet. Whole rock data (mineralogy and chemistry) were collected from previous works and new whole rock data were created from outcrop and subsurface samples of significant formations. Subsurface geologic exploration (geochemical and mineralogical) of the SGDB revealed that the Potato Hill andesite (Tpp) and Sheep Canyon basalt (Tps) members of the Pruett Formation are the groundwater bearing units. Attempts were also made in the research to correlate units between wells in the SGDB for which cuttings were available. This correlation was made using thin section observations, whole rock chemistry (as determined by XRF), and mineralogy (as determined by XRD). Nevertheless, the research suggests not only that the Tcb does not occur in the subsurface, but also that the Tpp and Tps do occur in the subsurface, that the Tpp and Tps can be correlated throughout the SGDB subsurface, and that there may be a marker bed (the Sunny Glen tuff) that separates them.