GEOLOGY OF THE NORTHERN EDWARDS AQUIFER SEGMENT ALONG THE INTERSTATE 35 CORRIDOR OF ROUND ROCK, GEORGETOWN, AND SALADO, CENTRAL TEXAS

Maps, cross sections, and geologic data of the northern Edwards aquifer display the aquifer's geologic framework within a 16-quadrangle region undergoing rapid population growth, and illustrate geologic similarities and differences with other Edwards aquifer segments. Important aspects of the northern aquifer's framework include faulting of aquifer strata and lateral thickness and lithostratigraphic changes in the Cretaceous shelf margin limestone, dolomitic limestone, dolomite, and argillaceous limestone composing the aquifer. The most porous and prolific part of the aquifer, the Edwards Formation, thins northward from ~350 to ~90 ft and is 2 to 4 times thinner than within other aquifer segments. Younger Georgetown rocks, absent to minor in other aquifer segments, thicken northward from ~50 to ~100 ft. The older Comanche Peak Formation is between ~30 and ~80 ft thick, and it interfingers with Edwards rocks southward, where it pinches out. Normal faults control the structural position of the aquifer strata although fault intensity in the study area is less than in other aquifer segments. In the study area, fault zone structural relief decreases northward from ~1,600 ft to about ~600 ft. Faults mostly strike north-northeastward and have maximum displacements between 40 and 150 ft. Some minor, smaller scale faults (<3 ft throw) strike westward. Apertures of minor faults and joints in the Comanche Peak and Georgetown rocks are generally less than 0.05 inch and appear to be common only near major faults or gentle flexures. Apertures in Edwards rocks can be several inches wide. Faults and joints can serve as conduits for groundwater flow although regionally they may have a somewhat nonuniform distribution. Two artesian springs are adjacent to faults. More than half of the 60 springs studied discharge near the contact between the Edwards and underlying Comanche Peak Formations, evidence that porosity differences exist between these units. Only one spring discharges from Georgetown rocks, suggesting they are not as porous as Edwards rocks, which contain most of the springs. Major streambeds crossing the recharge zone lie mostly within the Comanche Peak Formation, which probably allows less recharge than the more porous Edwards. Minor drainages in Edwards strata likely are significant recharge areas.