Study Area Hydrostratigraphy Section 3 Detailed geologic (lithostratigraphic) descriptions of the study area units are provided on the geologic map produced by Woodruff and Hydrostratigraphy Collins (2018). The hydrostratigraphy of the region is well-defi ned by George et al. (2011), and for the study area by Wierman et al. (2010) Study Area and references therein. A brief description of the study area stratigraphy Introduction is outlined below from stratigraphically lowest to highest (Figure 3.3). Hydrostratigraphy This section describes the study area stratigraphy and depositional Paleozoic Units environments that underpin the hydrogeology of the region. The geologic units of the study area consist of gently dipping Lower The Paleozoic basement rocks are exposed along the margins of the Cretaceous clastic and carbonate strata (Figures 3.1 and 3.2). Figure Llano Uplift and radiate from the Uplift into the subsurface. These 3.3 illustrates the stratigraphy of the study area with depositional rocks are composed of fractured, faulted, and dipping limestones, environments and hydrogeologic designations and was modifi ed dolomites, and shales. These units occur in the northwestern portion of from Wierman et al. (2010). Figure 3.4 illustrates the lateral and the map area (predominantly Blanco and Burnet Counties) and provide vertical relationship of the geologic units of the study area. Figure the substrate (basement) upon which the Cretaceous sediments were 3.5 illustrates the relationship of the geology, depositional setting and deposited (Figure 3.4). The Paleozoic units are considered a minor facies changes to the aquifer units of the study area in the Hamilton aquifer of Texas and include (from oldest to youngest) the Hickory, Pool #1 well (modifi ed from Kerans et al., 2019). Figure 3.6 illustrates Ellenburger-San Saba, and Marble Falls Aquifers (Standen and map and cross-sectional views of the depositional model for some of Ruggiero, 2007; George et al., 2011; Shi et al., 2016). the relevant geologic units. The fi gure highlights that the geologic units are time-equivalent and represent diff erent depositional environments Paleozoic aquifer units potentially occur in the subsurface of the and lateral facies. westernmost portions of SWTC, west of the Ouachita Thrust Front. Those aquifers include the Marble Falls, Ellenburger-San Saba, and Hickory Aquifers, depending on depth. The Marble Falls Aquifer is a limestone (karstic) aquifer exposed near the Burnet-Travis County line along the Colorado River. The unit is overlain by the Smithwick Shale, which is generally considered an aquitard. Although the TWDB maps the Hickory Aquifer into the westernmost area of SWTC, the depth of the Hickory Aquifer would likely be several thousand feet below the surface (Shi et al., 2016), preventing it from being economically viable for production. Further work is needed to evaluate the groundwater or storage potential of the Paleozoic units in the region. Lower Trinity Units The Sycamore Sand (Hosston equivalent) consists of terrigenous, clastic, fi ne- to coarse-grained feldspathic sandstone and cobble conglomerate (Figure 3.2) and unconformably overlays the Paleozoic basement (Figure 3.4). The clastic deposits of the Lower Trinity are deltaic and fl uvial in nature and are sourced from the Llano Uplift and other portions of the North American Craton. These units would correspond to the “Sycamore River” system postulated by Tom Ewing within SWTC (Figure 2.6; Ewing, 2016). The Sligo Formation occurs only in the subsurface and is a shallow-water (high-energy) carbonate that pinches out in the middle of the study area and thickens to the east (Figure 3.4). The Lower Trinity Aquifer is composed of the Sycamore (Hosston) and Sligo Formations. The Hammett Shale represents off shore shelf deposits overlying the Lower Trinity units (Figures 3.5 and 3.6). The Hammett is a ubiquitous shale that behaves as a regional aquitard between the overlying Middle Trinity and underlying Lower Trinity Aquifers. The Hammett Shale transitions upward into the Cow Creek. Middle Trinity Units Figure 3.2 Sycamore Outcrop. The Cow Creek transitions from a muddy, clastic, oyster-rich (near- An outcrop of the Sycamore Sand near Shaff er Bend on the Colorado shore) carbonate unit upward to a more massive nearshore beach Figure 3.1 Cow Creek Outcrop. River in western Travis County. The Sycamore (Hosston equivalent) and complex containing carbonate and clastic sands (Figure 3.1). The An outcrop of the Cow Creek at Milton Reimers Ranch Park. The base of the Cow Creek is composed of thin- Sligo Formation (subsurface only) compose the Lower Trinity Aquifer. Cow Creek is a major aquifer unit within the Middle Trinity Aquifer. bedded, off shore, muddy, oyster limestone beds. The unit becomes increasingly thicker-bedded up section and The Hensel unconformably overlies the Cow Creek and represents transitions into a sandy calcarenite beach deposit. The Cow Creek is an important Middle Trinity Aquifer unit. 10 alluvial-plain setting with abundant near-shore clastics (Figures 3.5 and 3.6). In Formation and is present in the northern (Jollyville Plateau) and eastern portions the western portion of the study area, the Hensel is a relatively thick clastic unit of the regional study area. However, within the SWTC only small erosional Generalized with sediment sources derived from the Llano Uplift. In this area, the Hensel is an remnants of the Edwards Group are locally present on hilltops. Group or Stratigraphic Column Depositional Hydro- Formation & Geologic Features Environment stratigraphy important aquifer and recharge unit. However, the Hensel transitions into a thin, Period Age Litho- stratigraphic silty carbonate (dolomite) to the east and can behave locally as an aquitard (Figure 3.4). The Lower Glen Rose is composed of relatively thick-bedded fossiliferous Conclusions Chert, dolomitic Shallow Shelf limestone units. Reef deposits, where present, within the Lower Glen Rose can be limestone Edwards Group very productive and important aquifer units of the Middle Trinity Aquifer. The stratigraphy of the study area is generally consistent with the stratigraphy Aquifer described by Wierman et al. (2010). However, there are notable diff erences Edwards Grp. Fredericksburg “Walnut Fm” 0 Kainer Fm. (Ked) Upper Trinity Units and Edwards Group between the stratigraphy of southwest Travis County and the stratigraphy of Hays Semi- County that appear to infl uence primary porosity and permeability, and therefore confining The Upper Glen Rose covers much of the study area and is composed of thin- groundwater availability. These diff erences include increased thicknesses of the Upper Trinity bedded alternating limestone and dolomite, which create the typical stair-step “perched aquifer” Lower Trinity (Hosston) in SWTC. In addition, there appear to be increased clastic 100 topography of the Hill Country region. The Upper Glen Rose is equivalent to the content and facies at the expense of carbonate facies within the Middle Trinity Shallow Shelf Upper Trinity Aquifer, which is characterized as a local, perched groundwater units in SWTC. Specifi cally, there appear to be poorly developed patch reefs system sustaining seeps and springs that provide basefl ows to creeks. Generally, (bioherms) within the Lower Glen Rose in the western and central portions of Member units within the Upper Trinity can act as a barrier to vertical fl ow (and recharge) SWTC when compared to Hays County (Soto-Kerans et al., 2018). The increase 200 Abundant Orbitolina “Unit 3” to the underlying hydrogeologic units, except where eroded and fractured. in clastics and potential decrease in carbonate reef development could be related Upper Glen Rose (Kgru) to a persistent infl ux of sediment along the axis of the “Sycamore River.” #### # Evaporite A The Edwards Aquifer is composed of the Edwards Group and Georgetown Supratidal Evaporite B Aquitard 300 # ### # Corbula interval NW SE Llano Uplift Balcones Fault Zone Upper reef/stacked mounds Glen Rose Formation Outcrop Subsurface (caprinids; Hays Co.) 400 Member Edwards Group (includingEdwards Group Walnut Fm.) Shallow Shelf Middle Trinity Aquifer Thickness (feet) 500 Lower Glen Rose (Kgrl) Lower Lower biostrome (corals & rudists; Narrows equivalent in Hays Co.) Biostrome Hensel Upper Glen Rose (Khe) 600 T R I N I T Y G R O U P T T R I N I Beach/Shoreline Semi- confining Evaporites Cow (Kcc) Creek Shallow Shelf Hensel Sand Upper Aptian Lower Albian Oysters Corbula 700 T A C E O U S W E R C R E L O Hammett (Kha) Aquitard Lower Glen Rose Bioherm (Patch Reef) Bay (Ksl) Sligo 800 Berremian Lower Aptian “Clastic” Lagoon Lower Trinity Sycamore Sand Aquifer Hensel 900 CowCow CreekCreek Lmst “Silty Carbonate” Stacked channels Fluvial/Deltaic Sycamore/ Paleozoic 1000 Hosston (Ksy/Kho) Hammett Shale Basal conglomerate Foreland Strata Valenginian-Hautervian Arkosic Alluvial fan Major Unconformity Sligo Ouachita major unconformity Undifferentiated UNDIFFERENTIATED Marble Falls Facies Sligo Fm. 1100 PALEOZOIC Smithwick Paleozoic Hosston (Pz) Aquifers Limestone (micritic) Siliciclastic (silt, clay) Generalized Dolomite Limestone (skeletal, grains) Siliciclastic (sand, silt) Paleozoic Rock Types Shale/marl Limestone (reef) Gypsum/anhydrite Conglomerate Not to scale Ouachita Facies Figure 3.3 Stratigraphic and Hydrostratigraphic Column. Figure 3.4 Generalized Stratigraphic Diagram. Figure shows the stratigraphy and hydrostratigraphy of the study area, including Hays and Figure shows the vertical and lateral