Plate Isopach and Structure Contour Maps 4 Lower Trinity (Hosston and Sligo) 10 mi Lower Trinity Isopach h t i mation front s w s. Ouachita defor st one Sligo . st e t l N 5 mi hal pinchout Inferred fault lines Fault (down to the east, unless noted otherwise) s and rudi and s nt Hosston d e Contour Interval = 50 ft on U D i e d t one e a at sands and si i st t at t m i l ragm n t ne i f 0 e l n d-brown. Hosston Fm (Kho) is si r al e t Elevations are in feet above mean sea level e r f re e f e l Paleozoic Sycamore i y f e f d i y n z d onl C sl s h basal sk t n P i P m K K w , u om e c t (Hosston) For oarse i sands, shore oi e al z i onlap om or s. C Alex S. Broun, P.G. S. Broun, Alex o t uv e l f am al osi P p Geologic Explanation ns relict c Precambrian, u Austin Syc on (subsurface formation; does not crop out) and ai and dol de i e high-energy “beach” deposits to east. structure map surface at ont rat one m e st rbank or e e om m i subsurface equivalent with stacked channel sands and subsurface equivalent with stacked channel sands ov Austin L ISOPACH and STRUCTURE MAPS and STRUCTURE ISOPACH go F

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600 Thickness in feet Thickness penetrates Paleozoic (n=15), thickness expressed in feet Well (n=20), partial Trinity with partial penetration of Lower Well thickness expressed in feet Map includes the Sligo, Hosston, and Isopach Trinity Note: Lower Sycamore Formations are indicated. Only the environment of deposition and lithofacies Hosston/Sycamore 562 970

200 280 140 - - SE well

Ksl Kcc Kgrl Kho Kgru (Ked) Edwards Kha Ksl Kcc Kgrl Kho Khe Kgru Edwards BALCONES FAULT SYSTEM BALCONES FAULT Miocene-age (Balcones) normal faults block and transmit groundwater flow Well Jacob’s Sligo (Ksl) Ouachita Facies Ouachita Facies Hosston (Kho) Lower Paleozoic Lower Paleozoic

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r Lower Glen Rose (Kgrl) OUACHITA TECTONIC FRONT TECTONIC OUACHITA Figure modified from Broun and Wierman (2009); Stricklin and Amsbury (1974); Flawn (1961). Figure modified from Broun and Wierman (2009); Stricklin and Cow Creek (Kcc) HAYS CO. HAYS 4-4 Schematic Geologic Cross Section 4-4 Schematic Geologic Foreland Facies Foreland Facies Onlap

Hensel (Khe) Lower Paleozoic Upper Paleozoic Lower Paleozoic Upper Paleozoic Paleozoic Llano Fault Zone BLANCO CO. and and rocks rocks Ked Kgru Kgrl Igneous Igneous Metamorphic Metamorphic LLANO UPLIFT Precambrian Precambrian Structural dome Khe to and area report the describe to constructed were sections cross of pairs Five Sections: Cross present another dimension to the mapping program. Stratigraphic and Structural cross sections strike sections (Plates 7-11). There are three dip sections and two were drawn along each line. Stratigraphic Cross Sections: The primary sources of data for the stratigraphic cross-sections are geophysical logs (wireline logs) from 150 area water wells. The log natural and gamma, single suites SP, point resistivity normally curves. If include the drill hole was not cased the suite of logs might also include a resistivity log, used and was when and required, logs sonic A a well log caliper. all was on run present by is the curve gamma natural The well. Ranch Sabino the at USGS to pick formation tops and to correlate between wells. Cuttings samples were described from 45 wells with a binocular microscope. Completion logs were constructed for each well that had both only with wells from constructed were logs strip Lithologic cuttings. described and logs geophysical samples. Where possible the cross sections also include outcrop data, measured described sections core. Where no samples and were available, the gamma curve was used to interpret lithol Wimberley. Here Wimberley. it is down-dropped to the southeast through a series of normal faults related to Early Miocene tectonic movement (Ewing, 1991). During late Lower Glen Rose time, a low-lying swamp with ferns covered the shoreline and 15). (Figure 3-4 and Plate 17, Photo muddy waters dinosaurs and crocodiles waded in the shallow, Schematic Geologic Cross Section: The schematic cross section, Figure 4-4, is drawn roughly northwest to southeast, from the Llano Uplift to the Edwards recharge zone. The Paleozoic units after shown diagrammatically are wedge sedimentary Cretaceous Lower the beneath subcrop that and onlapping shown are rocks sedimentary Group Trinity transgressive overall The (1961). Flawn Group Trinity the of thickness overall The Uplift. Llano the of facies foreland Paleozoic the covering is about 1000 feet and consists of three Zone. Fault clastic-carbonate Balcones the of faulting Miocene-age Early by intersected cycles until southeast the to gently or couplets. The section dips The section suggests that a structural hinge-line exists at the frontal edge of the Thrust. Ouachita In addition, that differential subsidence along this northeast-southwest trending front affected the overlying Trinity sedimentary section as evidenced by: Sligo pinch out, Hensel-Lower Glen Rose facies change, shoreline northeast facies trend Trinity and, of may have influenced the location of Lower Miocene-age faulting. ogy. The sections are datumed at the top of the Cow Creek formation. Colors and patterns The sections were correlated at are full log scale (1 used inch to describe = 20 lithology. feet), drafted at half scale and redrafted at a reduced scale for publication. Structural Cross Sections: The topography for the cross sections was drawn from USGS topo- graphic map sheets. Geophysical well logs were used to correlate subsurface Trinity rock forma- tions. Faulting was taken from mapped surface faults (Geological Map drawn to show Plate Paleozoic displacement but there 1). is no specific data The to support this model. Paleo faults are to depth The basin. the of history geologic the given reasonable seems however, involvement zoic geo- represent colors section cross The data. regional from projected was Paleozoic the of top the logic formations, not lithology. 4-1 Lower Trinity Isopach Map: Hosston/Sycamore The sedimentary map rocks. consists Lower of sandstones and conglomerates deltaplain and fan Alluvial Trinity surface. Paleozoic a peneplained eroded, clastics combined were thickness deposited of Sligo directly were deposited and on along the south flank the of the Llano Uplift. Surface outcrops are referred and mapped is onlap formation to The equivalent. subsurface the is Hosston Sycamore; as literature in the can be seen on outcrop near Pedernales State Park. To the east pebble are conglomerates thick and deltaic red-brown sandstones, siltstone and shale. A braided stream facies and channel stacked sands can be interpreted in geophysical logs from wells in eastern Hays Co. The sand- stones are water high-energy bearing. A beach or bar sequence was seen at the Signal Hill with Orbitolina texana sand buildups throughout the Hosston interval. A prodelta facies may present to the southeast. be Due to its depth, the Hosston is poorly explored over much of the study challenge ongoing an is unit facies clastic thick this of potential water-bearing Understanding area. to evaluating the future water supply of the tri-county area. The Sligo pinchout is shown on the Figure 4-1 isopach The map. formation thickens to the south- predominantly clastic facies A east and may reach 100 feet in eastern and southeastern Hays Co. to the west, the Sligo becomes carbonate to the east. The upper beds are normally seen as tight porosity rhombic well-developed with dolomite crystalline fine Underlying limestone. grain-skeletal produces water in the area. pres - where formation Sligo the of top on contoured is map The Map: Structure Trinity Lower 4-2 - Trin absent. is Sligo the where area the of portion western the over Hosston the of top the and ent, Glen Lower through Trinity Lower from form structural similar very a retain maps structure Group ity Rose indicating post-Trinity structural movement. Both the truncated plunging Shattered nose atlas. this in and presented maps structure uniform the of all to common are east-northeast the to dip structural “ramps” southeast of the BFZ are also common to the maps. The thrust-front of the Paleozoic Ouachita facies deformation belt Trinity subcrops beneath Lower sediments (Flawn, 1961). This northeast trending front can be seen on the map in western Hays Co. It is sub-parallel and proximal to the Sligo pinchout, the upper Hensel-lower Lower Glen Rose facies change, the Cow Creek shoreline and Early Miocene-age faulting. On cross sections, this sec- Paleozoic thick the of subsidence Differential line. hinge a as report this in identified is feature tion along this suggested hinge line may have Com- had section. some carbonate influence Trinity on Trinity the sedimentation. The of trend shoreline the defined west-northwest the to Uplift Llano stratigraphy. Trinity bined with the hinge line, these features had an important impact on NW

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A u t S en eous , C Platform etac San Marcos r Llano Islands etting er C w o L epositional S Prominent depositional features include: Llano Islands: Clastic sediment provenance. San Marcos Platform: stable, low-relief shelf. Depositional environment: wide, shallow water, carbonate-shelf facies w/shoreline, tidal and deltaic environments; inner shelf rudistid mound buildups. D 220 mi 10 N 1

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F Llano 4-3 Tectonic and Depositional Maps 4-3 Tectonic Balcones Fault Zone t onic Elemen t Introduction to the Geology and Hydrogeology of the Hill Country Trinity Group Group Trinity Hill Country of the Hydrogeology and Geology to the Introduction Llano Uplif ec T F.G. Driscoll, 1986. Driscoll, F.G. ed t xas modified from Ewing (1991) e T elec Prominent structural features include: Lower Miocene-age Balcones Fault Zone (en-echelon normal faulting). Llano Uplift and subsurface San Marcos Paleozoic-age Ouachita Structural Belt (compression and wrench faulting) S “Geologic knowledge of aquifers…... (is) mandatory for intelligent groundwater resource mandatory for intelligent groundwater of aquifers…... (is) “Geologic knowledge formable surface may have been subaerially exposed. The Hammett is the base of the Trinity second Group clastic-carbonate couplet. The contact with the overlying sea- Cow prograde sand/calcarenite carbonate sheet water shallow Creek and beach Creek Cow transitional. formation is the at boundary sequence The 1977). Loucks, and (Bebout facies shoreface clastic fine over ward top of the Cow Creek is critical to understanding groundwater movement and the sustainability of flow from major springs in the study There area. is evidence for subaerial and shallow tidal expo- sure at the top of the Cow Creek. The overlying Hensel formation is composed of marine, nodular limestone, mostly red-brown siltstone, shale and non- conglomerate lenses. The presence of paleosol layers suggest a warm arid climate (Hopkins, 1982). The Cow Creek subjected sediments to were meteoric water infiltration during this period. Early diagenesis vuggy The porosity. section resulted was buried to in a depth moldic of 3,000 to 5,000 feet; however, porosity was the for base the is Hensel The logs. geophysical and outcrop samples, core, in seen as preserved third depositional couplet and is transitional with the overlying Lower Glen Rose formation. In the subsurface, the Lower Glen Rose can be subdivided, with the use of geophysical logs, into four distinct lithofacies: A basal biostrome sequence equated to the western Hays Co. (Blanco River, “Narrows” outcrop section Depositional History: The broad, truncated, the transgressed seas Cretaceous shallow Early break. shelf City Stuart water, the to southeast Massif Cretaceous shelf stretched from the Llano eroded, peneplained Paleozoic surface. The exposed Llano Islands with outcropping granite and meta-sediments were the provenance for onlapped and Lower shelf subsiding Trinity gradually the on clastic deposited were formation facies. Hosston the of Basal sandstone conglomerate and the Llano Islands to Alluvial the fan northwest. and delta plain terrigenous deposits built seaward towards the proto-Gulf of Mexico, becoming increasingly more marine Hosston to the facies east. was A shoreline noted in several wells in onlapped the Hosston northeast clastics and Hays pinches out in Co. western Hays The Co. (Figure 4-2). marine The Sligo is Sligo a formation clastic facies to the west becoming predominantly more carbonate to the southeast. It forms the top of the first depositional couplet, or cycle, with a clastic, terrigenous base (Hosston formation) and an upward shoaling, prograding carbonate top (Stricklin et al., 1971). The mid-Aptian sequence boundary is a worldwide, sea level event. Within the study area, Ham- To Sligo. the overlap and sediments Trinity Lower the onlap shale silty and clay transgressive mett the northwest, the Hammett “shale” onlaps Paleozoic rocks that rim the Llano Uplift. The uncon utilization.” of hydrogeology and geology the on done been has work considerable years, 120 past the During university current to 1890’s) (circa Hill T. R. of work pioneer the From Trinity. Country Hill Texas the graduate studies, the rocks of the clear Trinity a Group established have have geoscientists outstanding been many by hammered, analyses sampled, Critical studied. sliced, and graphed photo- we work, that on Building references). (see area the for framework lithostratigraphic and structural hope that through the addition of acquired subsurface data and hydrological Atlas can provide analyses a practical tool earth that scientists today’s and policy makers can use to better that this Aquifers. Group Trinity understand and manage Structural History: The structural grain of the Central Hill Texas Country follows the Paleozoic Paleo Late Belt. Orogenic Ouachita the and Uplift (Massif) Llano the by defined template tectonic zoic tectonic plate movement to the northwest resulted in the thrusting of a thick basinal facies, sedimentary prism against the Llano Uplift. The frontal thrust of the Ouachita facies deformation belt, which was mapped by Flawn (1961), transects the study area (see Figure 4-3). Llano, Pre- cambrian and Cambrian igneous and metamorphic rocks facies and were uplifted at surrounding the end Paleozoic of the Paleozoic. foreland The San Marcos Arch appears as the plunging nose of the uplift. With the opening of the Gulf of Mexico, the peneplained Paleozoic surface tilted to the southeast and was flooded by early Cretaceous onlapping sediments. Lower Cretaceous rests on Jurassic salt. Basinward, In the Hill Country the Cretaceous towards rests unconformably the Gulf, the Blanco in C-C’ Section Cross on evidenced is movement structural age Aptian Paleozoic. the over The Co. Hensel Group Sand rests unconformably facies of Trinity over the the Cow Creek forma- tion. Late Cretaceous tectonic movement, possibly Laramide, folded the Cretaceous section, as shown on The the Llano structure Uplift maps. was positive during parts of the Lower Cretaceous factors of number a time Miocene Early During sediments. Rose Glen and Hensel by covered until during formed was (BFZ) Zone Fault Balcones The southeast. the to section the drop to combined asso- Fracturing southeast. down-to-the faults, normal en-echelon of series a in resulting time, this ciated with structural stress during each of these events shattered the brittle carbonate section, movement. providing pathways for solution conduits and groundwater ous shale breaks representing flood events; an upper rudist bioherm/stacked mound series that is that series mound bioherm/stacked rudist upper an events; flood representing breaks shale ous transgressive, a and 1977); (Petta, Co. Bandera in bioherm Creek Pipe the with equivalent roughly Corbula the by capped section dolomite and marl, limestone, argillaceous thin-bedded alternating to east Co. Blanco from watershed River Blanco the along exposed is Rose Glen Lower The bed.