Stephen F. Austin State University SFA ScholarWorks Faculty Publications Department of Geology 3-6-2016 Geology of the Northern Llano Uplift, Junction to Llano, Texas R. LaRell Nielson Stephen F Austin State University, [email protected] Chris Barker Stephen F Austin State University, [email protected] Follow this and additional works at: https://scholarworks.sfasu.edu/geology Part of the Geology Commons Tell us how this article helped you. Repository Citation Nielson, R. LaRell and Barker, Chris, "Geology of the Northern Llano Uplift, Junction to Llano, Texas" (2016). Faculty Publications. 15. https://scholarworks.sfasu.edu/geology/15 This Article is brought to you for free and open access by the Department of Geology at SFA ScholarWorks. It has been accepted for inclusion in Faculty Publications by an authorized administrator of SFA ScholarWorks. For more information, please contact [email protected]. Northern Llano Uplift TAS 2016 Field Trip, Page | 1 Texas Academy of Science, 2016 Field Trip: Geology of the Northern Llano Uplift, Junction to Llano, Texas March 6, 2016 Boudin Structure Llano River Bridge, Llano, Texas Field Trip Leaders: R. LaRell Nielson and Chris A. Barker Department of Geology Stephen F. Austin State University Nacogdoches, Texas Northern Llano Uplift TAS 2016 Field Trip, Page | 2 This page intentionally left blank Northern Llano Uplift TAS 2016 Field Trip, Page | 3 Texas Academy of Science, 2016, Field Trip: Geology of the Northern Llano Uplift, Junction to Llano, Texas March 6, 2016 Stromatolitic bioherms at the picnic area where U.S. Highway 377 crosses the San Saba River, Texas. Note the large mound structure that contains a number of smaller elongate build-ups. Northern Llano Uplift TAS 2016 Field Trip, Page | 4 Field Trip Leaders: R. LaRell Nielson and Chris A. Barker Department of Geology Stephen F. Austin State University Nacogdoches, Texas Acknowledgements The writers are grateful to the Geology Department at Stephen F. Austin State University for providing the opportunity to conduct geological research and to lead field trips to the Hill Country for many years. Thanks goes to the Department of Geology for partially funding these field trips and research projects and to our students who have helped us learn more about the area through their questions. A great deal of thanks and appreciation is given to our spouses (Sylvia and Anne) for their support and encouragement to complete this project. Stromatolitic bioherms in San Saba River where it crosses U.S. Highway 377. A number of the larger buildups are present in the river bed and have a northeast- southwest orientation. Northern Llano Uplift TAS 2016 Field Trip, Page | 5 Table of Contents Acknowledgements ...............................................................................................4 Introduction ...........................................................................................................9 STOP 1: Fort Terrett Formation - Junction, Texas ........................................ 19 STOP 2: Fort Terrett Formation - Junction, Texas ........................................ 20 STOP 3: Trinity Group - Junction, Texas .......................................................22 STOP A: Contact between Trinity Group and Fort Terrett Formation ......... 23 STOP B: Trinity Group on U.S. Highway 2.3 miles south of Junction, TX .. 24 STOP 4: Spheroidally Weathered Boulders - Streeter, Texas ..................... 25 STOP 5: Bioherms - Saba River Crossing, Texas ......................................... 28 STOP 6: Hickory Sandstone - Pontotoc, Texas ............................................. 32 STOP 7: Llanite Dike - North of Llano - Texas ............................................... 35 STOP 8: Llano River Bridge – Packsaddle Schist, Llano, Texas .................. 38 STOP 9: Cooper’s Bar-B-Que - Llano, Texas ................................................. 52 References ..........................................................................................................52 Northern Llano Uplift TAS 2016 Field Trip, Page | 6 Table of Figures Figure 1 – Index map to TAS 2016 stops ..............................................................9 Figure 2 – Sloss cycles .......................................................................................10 Figure 3 – Generalized stratigraphic column for Junction, Texas ........................ 10 Figure 4 – Generalized stratigraphic section for the western Llano Uplift ............ 11 Figure 5 – Geologic map for Stops 6, 7, 8, and 9 ................................................ 12 Figure 6 – Geologic map for Stops 1, 2, 3, A, and B ........................................... 14 Figure 7 – Generalized column for the Cretaceous rocks, Llano area ................ 15 Figure 8 – Stratigraphic section for Junction, Texas ........................................... 16 Figure 9 – Physiographic features of the Early Cretaceous, Period, Texas ........ 17 Figure 10 – Generalized block diagram for the Stuart City Reef, Texas.............. 18 Figure 11 – Photo of Fort Terrett Formation at Stop 1......................................... 19 Figure 12 – Photo of Fort Terrett Formation at Stop 2......................................... 20 Figure 13 – Bioherm in the Fort Terrett Formation ..............................................21 Figure 14 – White claystones and pink marls of the Trinity Group at Stop 3 ....... 22 Figure 15 – Fort Terrett Formation and Trinity Group at Stop A .......................... 23 Figure 16 – Trinity Group south of Junction, Texas at Stop B ............................. 24 Figure 17 – Spheroidial boulders near Streeter, Texas at Stop 4 ........................ 25 Figure 18 – Diagram explaining the development of spheroidal boulders ........... 26 Figure 19 – Geologic map of the Streeter, Texas area at Stop 4 ........................ 27 Figure 20 – Geologic map of the area north of Camp San Saba at Stop 5 ......... 28 Figure 21 – Model for Bioherm development ......................................................29 Figure 22 – Bioherms in the San Saba River at Stop 5 ....................................... 30 Figure 23 – Internal structure of the bioherms at Stop 5 ..................................... 30 Figure 24 – Internal structure of the bioherms at Stop 5 ..................................... 31 Figure 25 – Diagram illustrating the formation of bioherms at Stop 5 .................. 31 Figure 26 – Outcrop of the Hickory Sandstone near Ponototoc Texas, Stop 6 ... 32 Figure 27 – Geologic map of the Ponototoc, Texas area at Stop 6 ..................... 33 Figure 28 – Lingula (brachiopod) in the Hickory Sandstone ................................ 33 Figure 29 – Diagram of Lingula brachiopod in Hickory Sandstone ...................... 34 Figure 30 – Quarry in the Hickory Sandstone near Stop 6 .................................. 34 Figure 31 – Geologic map of the Baby Head, Texas area, Stop 7 ...................... 36 Figure 32 – Llanite dike near Baby Head, Texas, Stop 7 .................................... 37 Figure 33 – Blue quartz (Llanite) at Stop 7 .......................................................... 37 Figure 34 – Overview of Pack Saddle Schist at Llano, Texas, at Stop 8 ............. 38 Figure 35 – Geologic map of Llano, Texas showing Stop 8 ................................ 39 Figure 36 – Strongly foliated Packsaddle Schist, Llano Texas, Stop 8 ............... 40 Figure 37 – Sigma-winged porphyroclast, Llano, Texas ...................................... 41 Figure 38 – Packsaddle Schist with felsic porphyroclast, Llano, Texas, Stop 8 .. 42 Figure 39 – Block diagram of foliated and lineated mylonite, Llano, Texas ......... 42 Figure 40 – Packsaddle Schist Llano River bridge, Llano, Texas, Stop 8 ........... 43 Figure 41 – Geologic map of Llano uplift showing Stop 8 ................................... 44 Figure 42 – Geologic and topographic map of Llano Uplift .................................. 45 Northern Llano Uplift TAS 2016 Field Trip, Page | 7 Figure 43 – Felsic boudins at the Llano River Bridge, Llano, Texas, Stop 8 ....... 46 Figure 44 – Asymmetric boudins at the Llano River Bridge, Stop 8 .................... 47 Figure 45 – Synform at the Llano River Bridge, Stop 8 ....................................... 48 Figure 46 – Granite outcrop at the Llano River Bridge, Stop 8 ............................ 49 Figure 47 – Pegmatite dike at the Llano River Bridge, Stop 8 ............................. 50 Figure 48 – Left lateral strike-slip fault at the Llano River Bridge, Stop 8 ............ 51 Northern Llano Uplift TAS 2016 Field Trip, Page | 8 This page intentionally left blank Northern Llano Uplift TAS 2016 Field Trip, Page | 9 Introduction This year's Texas Academy of Science Geology Field Trip will visit nine interesting locations in two different areas in the Texas Hill Country (Figure 1). In the first area we will look at the Cretaceous stratigraphy around Junction, Texas (Figures 1, 2, 3, and 4). The expedition will then travel to the northern part of the Llano uplift and study Cambrian and Precambrian stratigraphy and structure of the northern Llano Uplift (Figures 4 and 5). Stop 9 will be at Cooper’s BBQ for lunch. 5 7 6 8 9 4 1-3 Figure1. Index map to the 2016 Texas Academy of Science field trip stops. Northern Llano Uplift TAS 2016 Field Trip, Page | 10 Figure 2. Sloss (1963) cycles - after Donovan (2010 p. 24). Geologic Lithostratigraphic Units Time Units Buda Limestone Del Rio Clay Edwards Group Segovia Formation Fort Terrett