ABSTRACT The Edwards aquifers are typically faulted, karstified, and transmissive. Water quality is generally excellent; the hydrochemical facies is mostly a calcium bicarbonate water with total dissolved solids (TDS) <500–1000 mg/L. Exceptions to this result from both natural and anthropogenic factors. In the Edwards Plateau, mixing of the formation water with underlying water from the Trinity aquifers or Permian rocks increases salinity to the west. Along the Balcones fault zone, the southern and eastern borders of the Edwards (Balcones Fault Zone) Aquifer are demarcated by a bad-water line where salinity rises to over 1000 mg/L. Detailed studies show that this line is a band, because salinities in the aquifer are not uniform with depth. The bad-water (or saline-water) zone is relatively stable over time, and six hydrochemical facies were identified, which are created by different combinations of dissolution of evaporite and other minerals, mixing with basinal brines, dedolomitization, and cross-formational flow from underlying formations. Flow in this zone is restricted, the waters are reducing, and recent studies suggest that microbes play important chemical and physical roles. The bad-water zone has sufficient water in storage and sufficient permeability so that desalination could be a future water-source option.

ABSTRACT The Edwards (Balcones Fault Zone) Aquifer in central Texas is typically defined as having three segments: the San Antonio, the Barton Springs, and the Northern segment, which are separated by groundwater divides or points of discharge. The San Antonio segment of the Edwards Aquifer is defined as extending from east of Brackettville in the west to Hays County in the east. The San Antonio segment has been further delineated into two pools, the San Antonio Pool and the Uvalde Pool, for water management purposes. The San Antonio Pool is the larger of the two pools and is recharged by the Dry Frio, Frio, Sabinal, Medina, Cibolo, Guadalupe, and Blanco River watersheds, in addition to direct recharge and flow from the Uvalde Pool via the Knippa Gap. To a lesser extent, interformational flow between units stratigraphically above and below the Edwards Formation limestone also occurs. The most prominent points of discharge from the San Antonio Pool are Comal, San Marcos, and Hueco Springs. San Pedro and San Antonio Springs in Bexar County discharge during periods of high stage in the aquifer. There are limited numbers of additional springs in the Frio River watershed with limited discharge. Significant water is discharged from the Medina Lake and Diversion Lake (downstream from Medina Lake dam) system via conduits and surface flow to recharge paleo-alluvial deposits (Leona Gravel) in the Medina River floodplain. This discharge had previously been assumed to recharge the Edwards Aquifer, but it continues downgradient in the Leona Gravel and is lost to the aquifer.

ABSTRACT Tracer testing is established as one of the best techniques for determining groundwater velocities and identifying groundwater flow directions in karstic systems. It has been employed in the Edwards (Balcones Fault Zone) Aquifer since the mid-1980s. Nontoxic, fluorescent organic dyes are most commonly used because they are comparatively inexpensive, relatively easily accessible, detectable at low concentrations, and not harmful to organisms that use or dwell in the aquifer or its springs. Tracer tests provide empirical evidence that is difficult to obtain any other way. Tracer tests have shown rapid groundwater velocities in the contributing, recharge, and artesian zones. Groundwater velocities were found to range from 915 to 9150 m/d in the Barton Springs segment of the aquifer; 1–3600 m/d in the San Marcos Springs area; 300–640 m/d near Comal Springs; 13 to >5300 m/d in San Antonio/northern Bexar County; and 1–1367 m/d in Kinney County, Texas. Tracer testing has shown: (1) preferential flow paths are conduit-dominated; (2) in places, there is a hydraulic connection with the underlying Glen Rose Formation; (3) large offsets on faults are not barriers to flow; and (4) portions of the aquifer act as separate pools.

Abstract: Analyses of normal faults in mechanically layered strata reveal that material properties of rock layers strongly influence fault nucleation points, fault extent (trace length), failure mode (shear v. hybrid), fault geometry (e.g. refraction through mechanical layers), displacement gradient (and potential for fault tip folding), displacement partitioning (e.g. synthetic dip, synthetic faulting, fault core displacement), fault core and damage zone width, and fault zone deformation processes. These detailed investigations are progressively dispelling some common myths about normal faulting held by industry geologists, for example: (i) that faults tend to be linear in dip profile; (ii) that imbricate normal faults initiate due to sliding on low-angle detachments; (iii) that friction causes fault-related folds (so-called normal drag); (iv) that self-similar fault zone widening is a direct function of fault displacement; and (v) that faults are not dilational features and/or important sources of permeability.

Abstract A valid structural geologic interpretation should simultaneously honor available surface and subsurface data (e.g., well and seismic) to constrain structural geometry; ideally be restorable to an original unstrained condition – taking into account the possibility of three-dimensional (3-D) movement, volume loss, or volume gain; and incorporate structural styles known or expected for the mechanical stratigraphy and deformation conditions in the region. Incorporating what is known about the mechanical stratigraphy can provide crucial constraints on viable structural styles, for example, where faults are likely to cut across stratigraphy vs. where fault displacement is likely to be accommodated by alternative mechanisms (e.g., ductile flow or folding). Conversely, the structural style can often help to understand the mechanical stratigraphy, including the recognition of dominant competent or incompetent mechanical stratigraphic units. Using this approach provides the interpreter another set of constraints toward improving interpretations, testing hypotheses, and developing valid structural interpretations. Outcrop characterization provides insights into the influence of mechanical stratigraphy and structural position on seismic- and subseismic-scale deformation in the layers. Examples of extensional deformation in Cretaceous carbonate strata in central and west Texas illustrate the utility of considering how mechanical stratigraphy influences the development of different deformation styles, even where deformation conditions are otherwise similar.

ABSTRACT The purpose of this trip is to visit an internationally famous Quaternary vertebrate paleontology site, Friesenhahn Cave, on the eastern margin of the Edwards Plateau in the heart of the central Texas Hill Country. This site has a very long history of scientific investigations beginning in the early twentieth century and continuing today. The cave has produced the fossil remains of more than 50 vertebrate taxa, including amphibians, reptiles and mammals. However, the abundant remains of an extinct scimitar cat, Homotherium serum, including juvenile individuals along with hundreds of teeth, cranial, and postcranial elements of juvenile mammoths, Mammuthus cf. M. columbi, make it an especially unique site. Our visit to Friesenhahn Cave will focus on its physical setting, cave sediment stratigraphy, potential age and taphonomy as they relate to the adaptations of Homotherium in the late Pleistocene of central Texas and its relationship to its potential prey, juvenile mammoths. We will also discuss recent studies of the cave itself, and its protection for future investigations by Concordia College.

Silica-undersaturated magmas intruded stable continental crust in southern Texas throughout Late Cretaceous time, forming at least 200 small volcanic centers and intrusive bodies in a shallow, epicontinental sea. Rock types, in order of decreasing abundance in outcrop, are melilite-olivine nephelinite, olivine nephelinite, alkali basalt, phonolite, and nepheline basanite. Melilite-olivine nephelinite, olivine nephelinite, and nepheline basanite contain xenoliths of spinel lherzolite, dunite, and harzburgite. On the basis of Mg/(Mg + Fe) and Ni contents, some but not all analyzed samples of melilite-olivine nephelinite, olivine nephelinite, nepheline basanite, and alkali basalt could have crystallized from primitive mantle-derived liquids that underwent little fractionation. Preliminary trace-element data suggest that the four mafic types could all have formed from the same parent through extraction of varying amounts of liquid. Phonolites are strongly fractionated and peralkaline, and probably formed from olivine nephelinite magma, and possibly from the other mafic types, by removal of kaersutitic amphibole, olivine, and clinopyroxene at high pressure. The bimodal Balcones suite resembles others in which mafic rocks and phonolites are associated, but it may be unique in the absence of coeval silica-oversaturated magmas and in the subordinance of alkali basalt.