BAYLOR - Geology of Texas Arlington Tx. 76019US A SPRING 1978 Bulletin No. 34 Structural Evolution of the CHRISTOPHER T.HAYWAR D thinking is more important than elaborate FEANK PH.D. PROFESSOR OF GEOLOGY BAYLOR UNIVERSITY 1929-1934 Objectives of Geological Training at Baylor The training of a geologist in a university covers but a few years; his education continues throughout his active life. The purposes of train­ ing geologists at Baylor University are to provide a sound basis of understanding and to foster a truly geological point of view, both of which are essential for continued professional growth. The staff considers geology to be unique among sciences since it is primarily a field science. All geologic research in­ cluding that done in laboratories must be firmly supported by field observations. The student is encouraged to develop an inquiring ob­ jective attitude and to examine critically all geological concepts and principles. The development of a mature and professional attitude toward geology and geological research is a principal concern of the department. THE BAYLOR UNIVERSITY PRESS WACO, TEXAS BAYLOR GEOLOGICAL STUDIES BULLETIN NO. 34 Structural Evolution of the Waco Region Christopher T. Hayward BAYLOR UNIVERSITY Department of Geology Waco, Texas Spring, 1978 Baylor Geological Studies EDITORIAL STAFF Jean M. Spencer, M.S., Editor environmental and medical geology O. T. Hayward, Ph.D., Adivsor, Cartographic Editor urban geology and what have you R. L. Bronaugh, M.A., Business Manager archaeology, geomorphology, vertebrate paleontology James W. Dixon, Jr., Ph.D. stratigraphy, paleontology, structure Gustavo A. Morales, Ph.D. invertebrate paleontology, micropaleontology, stratigraphy, oceanography Jerry N. Namy, Ph.D. mineralogy, petrology Robert G. Font, Ph.D. engineering geology, geomechanics, tectonophysics STUDENT EDITORIAL STAFF Richard D. Bone, Associate Editor Robert Corwin, B.S., Associate Editor Tom French, Cartographer The Baylor Geological Studies Bulletin is published Spring and Fall, by the Department of Geology at Baylor University. The Bulletin is cally dedicated to the dissemination of geologic knowl­ edge for the of the people of Texas. The publication is designed to present the results of both pure and applied research which will ultimately be im­ portant in the economic and cultural growth of the State. ISSN 0005-7266 Additional copies of this bulletin can be obtained from the Department of Geology, Baylor University, Waco, Texas 76703. $1.05 postpaid. CONTENTS Page Abstract 5 Introduction 5 Purpese Location 7 Methods 7 Previous Works 7 Acknowledgments 7 Structural Evolution 9 Basic Assumptions Pre-Cretaceous surface 9 Hosston deposition 9 deposition 13 deposition 23 Glen Rose deposition 27 Fredericksburg deposition .27 Post-Fredericksburg deposition 28 Model of Cretac'eous Deposition, Waco Region .... 28 Relationship Between Structural Evolution of the Waco Region and Balcones Faulting 32 Conclusions 34 Recommendations for Further Study 34 Appendix I . 35 Appendix II 35 Appendix III 36 References .......... 39 Index 39 ILLUSTRATIONS FIGURE Page 1. Index map showing area of study 6 2. Stratigraphic section and type of geologic formation of central Texas 8 3. Altitude of the base of the Cretaceous section 10 4. Isopach of the Hosston Formation 11 5. Residuals of the Hosston Isopach 12 6. Regional cross section 13 7. Diagrammatic west-to-east cross section of the Hosston Formation 13 8. Adjusted residuals of the Hosston isopach 14 9. Isopach of the Pearsall Formation 15 10. Isopach of the Hensel Sand 16 Isopach from the top of the Hensel Formation to the top of the Hosston Formation 17 12. Residual isopach from the top of the Hensel Formation to the top of the Hosston Formation 18 13. Isopach of the Glen Rose Formation 19 14. Residual isopach of the Glen Rose Formation 22 15. Isopach of the Fredericksburg Group 23 16. Isopach from the top of the Georgetown Formation to the top of the Edwards Formation 24 17. Isopach from the base of the Austin Formation to the top of the Georgetown Formation 25 18. Isopach of the Austin Formation 26 19. Summary type sections 27 20. Diagrammatic west-to-east cross section of the data explained by the intraformation model 28 21. Generalized section of the study area 20, 21 22. Sketch of intraformation model run one results 29 23. Sketch of isopach model results 31 24. Stress trajectories and possible fault planes 32 Faulting in the Austin Chalk along the Bosque River at Lovers' Leap 33 26. Growth faulting in the Austin Chalk along the Bosque River at Waco 27. Erosional features in the cliffs along the Bosque River at Waco ... 34 Structural Evolution of the Waco Region Christopher T. Hayward ABSTRACT The eastward thickening wedge of Cretaceous sedi­ intervals, shows very few anomalies, perhaps due to ments dominates the regional structure of the Waco the masking effect of the large total thickness. Tectonic area. Isopachs of eight units (the Formation, activity during Glen Rose deposition probably continued the Formation, the Hensel Formation, the to areas east of the Balcones fault zone. The Fredericks­ Glen Rose Formation, the Fredericksburg Group, the burg isopach is the most constant of all mapped in­ Georgetown Formation, the Del Rio Formation-Pepper tervals. The consistent lithology and isopach constancy Formation-Eagle Ford Group, and the Austin Forma­ of the Fredericksburg Group suggest that little tectonic tion) can lead to interpretations of stress patterns activity occurred during deposition of this unit. related to faulting along the fault. Analyses Post-Fredericksburg rocks are exposed only over the assume that isopach features are controlled by active eastern half of the area. The Georgetown Limestone structural disturbance during deposition. directly overlies rocks of the Fredericksburg Group, The pre-Cretaceous surface, which was of low relief, and southward thinning of its shale units suggests a extended far to the east of Waco. Initial fluvial Cre­ northward source, hence uplift in the north. The shales taceous deposition began with subsidence in the East of the Del Rio and Pepper Formations and Eagle Ford Texas basin and deposition of the Hosston Sand. Group overlie the Georgetown Formation and also Streams incised valleys through the Ouachita fold belt suggest uplift in the north and subsidence in the south, and deposited sands east of Waco. By the end of perhaps in conjunction with renewed activity along Hosston time, the surface of deposition was essentially faults. The Austin Chalk, a remarkably uniform section, flat. The presence of three Pearsall isopach features overlies the Eagle Ford Group and thickens both north parallel with known basal Cretaceous faults give evi­ and south of Waco, suggesting gentle uplift near Waco. dence of local readjustments and uplift along faults Surface fitting of the isopach data reveals that regional during Pearsall deposition. The overlying Hensel Sand­ activity was greatest during Hosston deposition, and stone exhibits characteristics of strandline deposits, and least during Fredericksburg deposition, increasing from isopach thicks of this unit are probably depositional Fredericksburg to Austin time. Field evidence of fault­ rather than tectonic. The isopach of the overlying ing during Austin time fits theoretical models of growth Glen Rose Formation, the thickest of all mapped faults and faulting due to increased vertical stresses. PURPOSE Previous studies of regional structure in central little indication of stress origins or fault timing. The Texas have been largely descriptive and emphasized evolution of regional structure as indicated by deposi­ Balcones faulting (Goodson, 1965; Hudson, 1972). tional patterns can provide such information, including However, Balcones faulting was simply a final episode the location of major structural warps during sedimen­ in the evolution of regional structure and in itself gives tation and some indication of time of greatest tectonic activity. These in turn can be related to stress patterns which may ultimately lead to major faulting. Thus, thesis submitted in partial fulfillment of the requirements this initial regional study may define areas of interest for the B.S. degree in Geology, Baylor University, 1976. where detailed study might follow. 6 BAYLOR GEOLOGICAL STUDIES Fig. 1. Index map showing area of study. STRUCTURAL EVOLUTION OF THE WACO REGION 7 The purposes of the present study are 1) to present thickness were related to probable stress locations and a generalized idea of the nature, age, and origin of the time associated with the evolution of Balcones faulting. tectonic history of the Waco region; 2) to define the In order to gain a better understanding of Balcones regional structure peculiar to the craton-basin faulting in the Austin Chalk, selected sections along the and 3) to identify areas for further, more detailed Brazos and Bosque Rivers were examined and photo­ studies. graphed. Also a model of the faulting using Hafner's (1951) approach was developed to suggest possible LOCATION configuration of subsurface fault planes. Later investi­ The study area is a one-degree quadrangle centered gations may utilize more extensive surface and sub­ in Waco, Texas, and bounded by latitudes and surface control, more detailed air photo interpretation north and by longitudes 45' and 45' west (Fig. and may concentrate in greater detail on a few selected 1). This area includes all of McLennan County and cross sections across the area. Each of these procedures