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

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Bedrock Geology of Round Rock and Surrounding Areas, Williamson and Travis Counties, Texas Bedrock Geology of Round Rock and Surrounding Areas, Williamson and Travis Counties, Texas Todd B. Housh Bedrock Geology of Round Rock and Surrounding Areas, Williamson and Travis Counties, Texas Todd B. Housh Copyright 2007 Todd B Housh, PhD, PG Round Rock, TX 78664 Cover photograph: The “Round Rock,” an erosional pedestal of Edward’s limestone that marked the low‐water crossing of Brushy Creek by the Chisholm Trail. 2 Table of Contents Introduction 5 Tectonic History 6 Previous Studies 8 Other Geologic Constraints 9 Stratigraphy 9 Comanche Series Fredericksburg Group Walnut Formation 10 Comanche Peak 10 Edwards 12 Kiamichi 13 Washita Group Georgetown 14 Del Rio 15 Buda 16 Gulf Series Woodbine Group Pepper 16 Eagle Ford Group 17 Austin Group 19 Taylor Group 21 Tertiary and Quaternary Systems Plio‐Pleistocene to Recent 22 Structure 23 Acknowledgements 27 Bibliography 28 Appendix 1. Compilation of sources of other geologic information. 34 Appendix 2. Localities of note to observe important geologic 40 features in the Round Rock Area. Appendix 3. Checklist of Cretaceous and Pleistocene fossils 45 3 4 Bedrock Geology of Round Rock and Surrounding Areas, Williamson and Travis Counties, Texas Todd B. Housh Introduction 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, Texas1, although parts of the city also lie within the Pflugerville West 7.5 minute quadrangle, Travis County, Texas2 and the Hutto 7.5 minute quadrangle, Williamson County, Texas3. Part of the Pflugerville East 7.5 minute quadrangle, Travis County, Texas4 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 1 Reston, Virginia: United States Geologic Survey, 1987, scale 1:24,000 2 Ibid. 3 Ibid. 4 Ibid. 5 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. Tectonic History Geophysical studies indicate that Williamson County lies near the outboard edge of Laurentian continental crust (Kruger and Keller, 1986; Keller, et al., 1989; Sims et al., 2005). The transition from extended continental crust to noncontinental crust is marked by an abrupt linear gravity maximum coincident with a series of magnetic highs (Keller, et al., 1989). This isostatic gravity anomaly maximum and a series of aeromagnetic highs pass through the western parts of Bastrop, Lee and Milam counties (Bankey, 2007). The Laurentian basement underlying the county is believed to be a part of the Llano Terrane, a Mid‐Proterozoic high‐grade orogenic belt that occupies the southeastern half of 6 the state of Texas; ages from the nearby Llano uplift range between 1098 and 1360 Ma (Mosher, 1998). In Cambrian time the southeastern margin of Laurentia rifted away (Arbenz, 1989) and a Cambro‐Ordivician carbonate platform developed on top of the extended Llano crust, with sporadic carbonate and fine‐ grained clastic sedimentation until the Late Devonian (Nicholas and Waddell, 1989). Williamson County lies along the suture between Laurentia and the tectonic plate with which it collided during the Ouachita Orogeny in the Late Paleozoic. All of Williamson County, except the southeast corner, is underlain by rocks of the Ouachita Frontal Zone; whereas the southeast corner is underlain by rocks of the Ouachita Interior Zone (Flawn et al., 1961; Ewing, 1991). The Frontal Zone comprises a belt of thin‐skinned, northwestward directed thrusting of weakly metamorphosed to unmetamorphosed Lower Paleozoic basinal strata over the cratonal margin of Laurentia. By contrast, the Interior Zone consists of strongly deformed and weakly metamorphosed schist, phyllite, metaquartzite and marble of unknown age and affiliation that have been thrust over the Ouachita facies rocks of the Frontal Zone (Flawn et al., 1961; Ewing, 1991). The depth to upper Paleozoic rocks of the Ouachita Orogenic Belt varies tremendously in Williamson County, from a depth of approximately 695 feet in the northwestern corner of the county to 2850 feet or more in the southeast corner of the county (Flawn et al., 1961). In contrast to the linear gravity maximum to the east of the Round Rock area that delimits the transition off Laurentian continental crust, the Round Rock area lies within a low in the isostatic gravity anomaly data (Bankey, 2007). This gravity low is often attributed to foreland basins that may, or may not, be overridden by Ouachita thrusts (Keller, et al., 1989). Although the isostatic gravity anomaly in the Round Rock area is less than that of the gravity high to the east, it likewise sits upon a local saddle between two profound gravity lows to the north and the south, which are parts of the Paleozoic Fort Worth Basin and the northern extension of the Kerr Basin, respectively (Meckel, et al., 1992). Extension along the southern margin of North America began in the Triassic, culminating in the Jurassic with rifting and the formation of the Gulf of Mexico basin. The hinge point of extension in east Texas corresponds to the now buried Stuart City Reef trend, marking the edge of the Lower Cretaceous shelf margin. Crust to the west of the shelf margin is only slightly to unextended, whereas crust to the east of the shelf margin is moderately to highly extended (Sawyer, et al., 1991). Central Texas was intermittently covered by shallow seas during the Cretaceous resulting in thick accumulations of carbonate rocks and associated marls and shales, particularly during the Lower and Upper 7 Cretaceous. These Cretaceous rocks constitute the bedrock geology that is the focus of this report. The tectonic development of the region during the Tertiary is dominated by down to the east normal faulting associated with the Balcones Fault Zone, an arcuate belt of normal faults extending from Del Rio towards Dallas (e.g. Ewing, 1990). Most of the movement along faults within the Balcones Fault Zone is believed to have occurred in the late Oligocene or early Miocene (Weeks, 1945), although some arguments have been made for both earlier, Late Cretaceous, and later, Pliocene, movements along the Balcones Fault Zone in addition.
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