49 Introduction to the Geology and Hydrogeology of the Hill Country Trinity Group Briarcliff ISOPACH and STRUCTURE MAPS LLANO UPLIFT Lakeway Alex S. Broun, P.G. 42 “Geologic knowledge of aquifers…... (is) mandatory for intelligent groundwater resource Wimberley. Here it is down-dropped to the southeast through a series of normal faults related to 202 K N utilization.” F.G. Driscoll, 1986. Early Miocene tectonic movement (Ewing, 1991). During late Lower Glen Rose time, a low-lying A B swamp with ferns covered the shoreline and dinosaurs and crocodiles waded in the shallow, R 100 E E During the past 120 years, considerable work has been done on the geology and hydrogeology of muddy waters (Figure 3-4 and Plate 17, Photo 15). V L A O the Texas Hill Country Trinity. From the pioneer work of R. T. Hill (circa 1890’s) to current university H & S 183 graduate studies, the rocks of the Trinity Group have been hammered, sampled, sliced, photo- Schematic Geologic Cross Section: The schematic cross section, Figure 4-4, is drawn roughly M & A graphed and studied. Critical analyses by many outstanding geoscientists have established a clear northwest to southeast, from the Llano Uplift to the Edwards recharge zone. The Paleozoic units E E 0 N 97 R 4 Plate structural and lithostratigraphic framework for the area (see references). Building on that work, we that subcrop beneath the Lower Cretaceous sedimentary wedge are diagrammatically shown after O T Johnson 0 0 T S S D 300 hope that through the addition of acquired subsurface data and hydrological analyses that this Flawn (1961). The overall transgressive Trinity Group sedimentary rocks are shown onlapping and City T L E 153 337 I 200 ID Atlas can provide a practical tool that today’s earth scientists and policy makers can use to better covering the Paleozoic foreland facies of the Llano Uplift. The overall thickness of the Trinity Group S A N t R 195 210 R u B 245 B o understand and manage Trinity Group Aquifers. is about 1000 feet and consists of three clastic-carbonate cycles or couplets. The section dips - h ALLUVIAL FAN D c gently to the southeast until intersected by Early Miocene-age faulting of the Balcones Fault Zone. E in R , p Sycamore Onlap 400 Isopach and Structure Contour Maps Structural History: The structural grain of the Central Texas Hill Country follows the Paleozoic The section suggests that a structural hinge-line exists at the frontal edge of the Ouachita Thrust. E o 186 Austin Lower Trinity ( 15 17 N O ig CHANNEL SANDS 17 T l 148 287 tectonic template defined by the Llano (Massif) Uplift and the Ouachita Orogenic Belt. Late Paleo- In addition, that differential subsidence along this northeast-southwest trending front affected the S S D 115 zoic tectonic plate movement to the northwest resulted in the thrusting of a thick basinal facies, overlying Trinity sedimentary section as evidenced by: Sligo pinch out, Hensel-Lower Glen Rose N BEACH/BAR A Hays Co. Dripping S 259 sedimentary prism against the Llano Uplift. The frontal thrust of the Ouachita facies deformation facies change, northeast trend of Trinity shoreline facies and, may have influenced the location of , Springs MARINE HIGH ENERGY E Blanco Co. 100 T 107 A t Travis Co. belt, which was mapped by Flawn (1961), transects the study area (see Figure 4-3). Llano, Pre- Lower Miocene-age faulting. 77102 R n E o Hays Co. r 300 110 cambrian and Cambrian igneous and metamorphic rocks and surrounding Paleozoic foreland M f LO 250 n G io facies were uplifted at the end of the Paleozoic. The San Marcos Arch appears as the plunging t 305 N a CO nose of the uplift. E rm L DELTA PLAIN fo 4-4 Schematic Geologic Cross Section BB e E d P a With the opening of the Gulf of Mexico, the peneplained Paleozoic surface tilted to the southeast it 300 400 BLANCO CO. HAYS CO. ch NW SE a and was flooded by early Cretaceous onlapping sediments. Basinward, towards the Gulf, the Blanco u O Ked f Kgru o 400 Lower Cretaceous rests on Jurassic salt. In the Hill Country the Cretaceous rests unconformably Facies p Kgrl Onlap transition o Khe cr Hosston and Sligo) over the Paleozoic. Aptian age structural movement is evidenced on Cross Section C-C’ in Blanco Khe-Kgrl 155 b 200 u S Co. The Hensel Sand facies of the Trinity Group rests unconformably over the Cow Creek forma- Buda Upper Glen 176 tion. Late Cretaceous tectonic movement, possibly Laramide, folded the Cretaceous section, as Rose (Kgru) Jacob’s Edwards 294 shown on the structure maps. The Llano Uplift was positive during parts of the Lower Cretaceous Hensel (Khe) Well Lower Glen Rose (Kgrl) Edwards 280 (Ked) WITH DOLOMITE Woodcreek until covered by Hensel and Glen Rose sediments. During Early Miocene time a number of factors locally confining unit Kgru Cow Creek (Kcc) THICK DELTAIC SANDSTONE, combined to drop the section to the southeast. The Balcones Fault Zone (BFZ) was formed during Hammett (Kha) SILTSTONE,PRO SHALE DELTA Kyle regional confining unit Kgru RED-BROWN SILTSTONE & SHALE & THIN SANDSTONE this time, resulting in a series of en-echelon normal faults, down-to-the southeast. Fracturing asso- Sligo (Ksl) Kgrl Geologic Explanation ciated with structural stress during each of these events shattered the brittle carbonate section, Khe 140 Hosston (Kho) Kgrl 186 Kcc providing pathways for solution conduits and groundwater movement. Kha Precambrian Igneous Ksl Kcc 4-1 Lower Trinity and Foreland Facies Lower Paleozoic Ksl Metamorphic Lower Paleozoic Upper Paleozoic Ouachita Facies Kho Ksl Isopach rocks weakly Metamorphosed sediments Wimberley Kho (Hosston and Sligo) Sligo Formation (subsurface formation; does not crop out) LLANO UPLIFT Paleozoic Llano OUACHITA TECTONIC FRONT BALCONES FAULT SYSTEM Structural dome Fault Zone Structural Hinge Line Miocene-age (Balcones) normal faults Limestone and dolomite with basal siltstone and shale. 4-3 Tectonic and Depositional Maps block and transmit groundwater flow Figure modified from Broun and Wierman (2009); Stricklin and Amsbury (1974); Flawn (1961). 200 Thickness in feet Dolomite contains relict coarse skeletal fragments and rudists. 280 Selected Tectonic Elements of Central Lower Cretaceous Well penetrates Paleozoic (n=15), thickness expressed in feet Texas modified from Ewing (1991) Depositional Setting, Central Texas Hays Co. Comal Co. Ksy 140 Well with partial penetration of Lower Trinity (n=20), partial thickness expressed in feet San Cross Sections: Five pairs of cross sections were constructed to describe the report area and to Marcos Sycamore (Hosston) Formation present another dimension to the mapping program. Stratigraphic and Structural cross sections Note: Lower Trinity Isopach Map includes the Sligo, Hosston, and were drawn along each line. There are three dip sections and two strike sections (Plates 7-11). Sycamore Formations Basal conglomerate and fluvial sands, shoreline sands and siltstones with Only the environment of deposition and lithofacies Hosston/Sycamore are indicated. silty shale overbank deposits. Commonly red-brown. Hosston Fm (Kho) is ault Ouachita Front Stratigraphic Cross Sections: The primary sources of data for the stratigraphic cross-sections subsurface equivalent with stacked channel sands and one Z Llano F Carbonate are geophysical logs (wireline logs) from 150 area water wells. The log suites normally include Llano Islands Shelf high-energy “beach” deposits to east. natural gamma, SP, and single point resistivity curves. If the drill hole was not cased the suite of Llano Uplift AUSTIN d AUSTIN N n e San Marcos tr logs might also include a resistivity log, and when required, a caliper. A sonic log was run by the f Arch e Pz e USGS at the Sabino Ranch well. The natural gamma curve is present on all well logs and was used San Marcos R 924 ity Balcones o Platform Briarcliff t C ine to pick formation tops and to correlate between wells. Cuttings samples were described from 45 xic r Fault Zone e a ar tu M o Paleozoic, undifferentiated S al Gulf wells with a binocular microscope. Completion logs were constructed for each well that had both xic e 0 110 220 mi estr Open Gulf of M nc geophysical logs and described cuttings. Lithologic strip logs were constructed from wells with only A of M LLANO UPLIFT Lakeway samples. Where possible the cross sections also include outcrop data, measured sections and described core. Where no samples were available, the gamma curve was used to interpret lithol- PC Prominent structural features include: Prominent depositional features include: 700 ogy. The sections are datumed at the top of the Cow Creek formation. Colors and patterns are 820 725 Lower Miocene-age Balcones Fault Zone (en-echelon Llano Islands: Clastic sediment provenance. used to describe lithology. The sections were correlated at full log scale (1 inch = 20 feet), drafted Precambrian, undifferentiated normal faulting). at half scale and redrafted at a reduced scale for publication. San Marcos Platform: stable, low-relief shelf. Llano Uplift and subsurface San Marcos Arch 840 Depositional environment: wide, shallow water, Structural Cross Sections: The topography for the cross sections was drawn from USGS topo- Paleozoic-age Ouachita Structural Belt (compression carbonate-shelf facies w/shoreline, tidal and graphic map sheets. Geophysical well logs were used to correlate subsurface Trinity rock forma- 793 692 650 400 and wrench faulting) deltaic environments; inner shelf rudistid tions. Faulting was taken from mapped surface faults (Geological Map Plate 1). The faults are N mound buildups. 785 300 200 drawn to show Paleozoic displacement but there is no specific data to support this model. Paleo- 665 Johnson zoic involvement however, seems reasonable given the geologic history of the basin.