Bedrock Geology of Round Rock and Surrounding Areas, Williamson and Travis Counties, Texas

The purpose of this study was to produce a map of the bedrock geology of the city of Round Rock, Texas and its environs and to evaluate the geologic structure of the area. Most of the City of Round Rock lies within the Round Rock 7.5 minute quadrangle, Williamson County, Texas, although parts of the city also lie within the Pflugerville West 7.5 minute quadrangle, Travis County,Texas and the Hutto 7.5 minute quadrangle, Williamson County, Texas. Part of the Pflugerville East 7.5 minute quadrangle, Travis County, Texas was included in this study as well. The full extent of the area mapped in this study is bounded by latitudes 30° 27' 30" and 30° 37' 30", and longitudes 97° 32' 30" and 97° 45'. Round Rock sits astride the break between the Grand and Black prairies (Hill, 1901). The Black Prairie owes its name to the thick black calcareous soils which cover the Late Cretaceous shales, marls and chalks that underlie the prairie. The Grand Prairie, on the other hand, is a northern extension of the Edwards Plateau, and is characterized by thin, rocky soils overlying Lower Cretaceous limestone, dolomitic limestone, marl and chert. The Round Rock area is bisected by the Balcones Fault Zone, a series of generally down-to-the-east normal faults which juxtapose Lower Cretaceous rocks to the west against Upper Cretaceous Rocks to the east. This juxtaposition of two very different sequences of rocks is ultimately responsible for the development of the two very different geographic provinces upon them. Prior to the urbanization of the Round Rock area, the Black Prairie was a region of extensive row agriculture of crops such as cotton, corn, oats and grain sorghums. The carbonate rocks of the Grand Prairie supported a dense growth of live oak, post oak, hackberry, pecan, short grasses, mesquite and cedar. This thick vegetation and the thin soil generally favored the use of this area for grazing as opposed to row crops. Outcrop within the Round Rock area is, in general, poor, especially in the eastern half of the map area where extensive alluvial deposits, formed on a number of different terrace levels, and thick soils effectively obscure much of the bedrock geology. As a result, much geologic information was gained from the inspection of trenches and other excavations associated with construction and utility projects between 1997 and 2006. These "outcrops" are ephemeral by their very nature. Outcrop is generally better in the western half of the quadrangle, the footwall side of the Balcones Fault Zone, where Lower Cretaceous carbonate formations are covered by only a thin soil. For the most part the Round Rock area lies within the drainages of the San Gabriel River and Brushy Creek, which joins the San Gabriel River farther downstream, and eventually flows into the Brazos River. Terrace and alluvial deposits are extensive and thick within the valleys of both of these rivers and their tributaries; the terrace deposits may even be found at high elevations on the divides in the region where they blanket the landscape. The terrace deposits coupled with the thick soils of the eastern, or Black Prairie, region combines to effectively obscure much of the bedrock strata and geologic relationships. Inspection of the existing maps of the Round Rock area and comparison of what can be seen and inferred regarding the relative elevations of strata versus geologic structures as shown indicates that the current structural understanding of the area is deficient. The purpose of this study is to delineate the bedrock geology of the Round Rock area in order to better elucidate the structural geology of the region. To that end, Tertiary and Quaternary terrace and alluvial deposits have been treated as something to "see through" in the production of this map. These deposits are already dealt with in existing maps of the area, and the interested reader is referred to them for further information (Marks, 1950; Walls, 1950; Gordon, 1951; Tydlaska, 1951). A structure contour map and perspective block diagram of the elevation of the top of the Edwards Formation was prepared from logs from water wells drilled in the area. The structure contour map further constrains the faulting in the map area and the degree of offset along these faults.