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USGS1994

Simulations of flow in the Edwards-Trinity aquifer system and contiguous hydraulically connected units, west-central Texas

The Edwards-Trinity aquifer system is currently (1993) being studied as part of the Regional Aquifer-Systems Analysis program. A major goal of the project is to understand and describe the regional ground-water flow system. A finite-element model for simulating two-dimensional steady-state ground-water flow was applied to the major aquifers of the Edwards-Trinity aquifer system and contiguous hydraulically connected units for the winter of 1974-75 and for the predevelopment conditions. The major aquifers are the Edwards-Trinity in the western semiarid part of the study area, the Trinity in the eastern subhumid part of the study area, and the Edwards in the southeastern part of the study area. The Edwards-Trinity, upper part of the Trinity, and part of the Edwards aquifers are shallow and unconfined over most of the study area. Regional ground-water flow is toward the perennial streams for the Edwards-Trinity and Trinity aquifers, as indicated by the observed and simulated potentiometric surfaces. The transmissivity values used in the simulations were within estimated ranges and generally are: 1,000 to 10,000 ft 2 /d (feet squared per day) for the Edwards-Trinity and Trinity aquifers; 100,000 to greater than 1 million ft 2 /d for the Edwards aquifer; and less than 500 to 10,000 ft 2 /d in contiguous hydraulically connected units. Simulated flow through the Edwards-Trinity aquifer system and contiguous hydraulically connected units is about 3 million acre-feet per year. Estimates of areally distributed recharge from the simulations range from 0.1 to 1 inch per year for the Edwards-Trinity aquifer and increase to 4 inches per year for the Trinity aquifer. Recharge to the Edwards aquifer occurs along streambeds that cross outcropped high-permeability rocks of the Edwards Group through joints and faults. Many of the streams are diverted completely underground during periods of no precipitation. The movement of a substantial quantity of water (about 400 cubic feet per second) from the Trinity and Edwards-Trinity aquifers into the Edwards aquifer was simulated. Results of the simulations indicate that anisotropy strongly influences flow in the Edwards aquifer. In the San Antonio and Austin areas, the Edwards aquifer is the most active part of the ground-water flow system with one-third of ground-water discharge occurring in 5 percent of the modeled area for both simulations.

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USGS1994

Analysis of data from test-well sites along the downdip limit of freshwater in the Edwards Aquifer, San Antonio, Texas, 1985-87

Many researchers have studied the downdip limit of freshwater in the Edwards aquifer or various aspects of the saline-water zone and its relation to the freshwater zone. These studies were summarized and used to synthesize a consistent hydrologic and geochemical framework from which to interpret data from field studies. The concept derived from the previous work on the downdip limit of the freshwater zone is that fresh recharge water entered the aquifer and developed a vast flow system controlled by barrier faults. Some recharge water flows into the saline-water zone rather than toward major freshwater discharge points. The water that enters the salinewater zone continues to dissolve gypsum and dolomite, and calcite precipitates out of the water. This process of dedolomitization has helped to develop the large secondary porosity of the freshwater zone as the downdip limit of the freshwater zone progressively moved downdip in recent geologic time. The drilling of test holes and installation of monitoring wells began in 1985 and helped to define the downdip limit of the freshwater zone at one location. Dolomite was found in greater amounts in rocks from the saline-water-zone test hole than in rocks from the freshwater-zone test hole. Other mineralogic and lithologic contrasts between the saline-water-zone test hole and the freshwater-zone test hole support the conceptual model of dedolomitization. Geophysical logs and test-hole survey logs helped to define the stratification of freshwater and associated altered rock textures, secondary porosity development, and water chemistry in the freshwater-, saline-water-, and transition-zone test holes. The differences in actual measured head among the seven completed monitor wells varied over time, especially during periods of substantial water use. The water levels in two monitor wells completed at the freshwater zone (site D; wells Dl and D2) responded quickly and strongly to withdrawals. In the transition zone (site C; wells Cl and C2), the water level in the shallow monitor well (Cl) completed in a cavern responded quickly, but because it was farther from the water-supply wells near site D, it did not respond as strongly to changes in withdrawals at the nearby well field. The water levels in the three salinewater wells at site A (Al, A2, and A3) and the deep site C well (C2) were less responsive to stresses relative to the water-level changes in the freshwater wells (Dl, D2, and Cl). Large amounts of freshwater were produced from the upper 300 to 350 feet of the aquifer in the freshwater zone (site D). Water produced from below this interval was as saline as that from the saline-water zone (site A). The cavern near the top of the aquifer in the transition zone (site C) produced large amounts of freshwater. The freshwater produced from the cavern was of a different geochemical type than the water from the other wells. The saline-water-zone test hole (site A) produced small amounts of water having specific conductance generally about 5,800 to 6,200 microsiemens per centimeter at 25 degrees Celsius. A consistent trend in the water quality was not detected in the monitor-well data for July 1986-April 1987. This was caused, in part, by the average to above-average rainfall and by the lack of large withdrawals during the period. The water quality of samples from several of the wells was similar to the water quality determined by a previous study of the area. Geochemistry of the oil- or gas-well brines from downdip in the saline-water zone had slight resemblance to the geochemistry of the water at the downdip limit of freshwater; updip flow of saline water toward the freshwater zone was not indicated.

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USGS1994

Dissolved-solids concentrations and hydrochemical facies in water of the Edwards-Trinity aquifer system, west-central Texas

Much of the Edwards-Trinity aquifer system contains freshwater, but sizable parts contain marginally fresh or slightly saline water. The predominant water type in the aquifer system is calcium bicarbonate; however, one of seven other hydrochemical facies characterizes the water in places. The median dissolved-solids concentration of water samples from the Edwards aquifer in the Balcones fault zone is 297 mg/L (milligrams per liter); the interquartile range is 93 mg/L. In the freshwater zone of the Edwards aquifer updip of a freshwater/saline-water transition zone, the water is almost exclusively calcium bicarbonate. The median dissolved-solids concentration of water samples from the Trinity aquifer in the Hill Country is 537 mg/L and the interquartile range is 573 mg/L. Four bicarbonate and sulfate facies, spread vertically throughout the saturated section, characterize most of the Hill Country analyses; calcium bicarbonate predominates. The median concentration of dissolved solids in water samples from the Edwards-Trinity aquifer in the Edwards Plateau is 379 mg/L and the interquartile range is 547 mg/L. Freshwater is nearly everywhere in the southern and northeastern parts of the aquifer, and mostly slightly saline water is in the northwestern part. The distribution of hydrochemical facies shows a similar pattern to dissolved-solids concentration, with bicarbonate water nearly everywhere in the southern and northeastern parts of the aquifer. Sulfate and chloride facies characterize the northwestern part of the Edwards Plateau. The median concentration of dissolved solids among water samples from the Edwards-Trinity aquifer in the Trans-Pecos is 929 mg/L and the interquartile range is 1,626 mg/L. Fresh, calcium bicarbonate water predominates in the southern part, and more saline mixed and sulfate waters are the most common in the northwestern part.

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USGS1994

Estimated freshwater withdrawals in Texas, 1990

This report presents 1990 freshwater withdrawal estimates for Texas by source and category. Withdrawal source is either ground water or surface water. Withdrawal categories include: self-supplied irrigation, thermoelectric-power generation, water supply, industrial and mining, and other (domestic, commercial, livestock). Withdrawal data are aggregated by county, major aquifer, and principal river basin. Only the four major categories of irrigation, thermoelectric-power generation, water supply, and industrial and mining are illustrated in this report, although all data are tabulated.

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USGS1994

Organic compounds downstream from a treated-wastewater discharge near Dallas, Texas, March 1987

Water and streambed-sediment samples were collected on March 9 and 10,1987 from one site upstream and three sites downstream of the discharge from a municipal wastewater-treatment plant on Rowlett Creek near Dallas, Texas. To extract and separate organic compounds, purgeand-trap, closed-loop stripping, and pH-adjusted solvent extraction methods were used for water samples; and a Soxhlet-solvent extraction method was used for streambed sediment. These methods were combined with gas chromatography/mass spectrometry to identify organic compounds in Rowlett Creek. Results from this study confirm the persistence of many organic compounds in water as far as 13.5 kilometers downstream of the wastewater discharge. These include: (1) the volatile organic compounds chloroform, 1,2-dichlorobenzene, 1,4-dichlorobenzene, tetrachloroethene, and trichloroethene; (2) several linear alkylbenzene compounds, octyl phenol, and a tetramethylbutyl phenol isomer that are related to detergent use; (3) 9-phenyl-9H-carbazole, a compound related to coal tars and coal combustion residues; and (4) caffeine. The only compound present in water in concentrations greater than U.S. Environmental Protection Agency maximum contaminant levels for drinking water was tetrachloroethene (6.0 micrograms per liter) in a sample collected 13.5 kilometers downstream from the waste water discharge. Compounds identified from the streambed-sediment samples include a xylene isomer at 7.7 kilometers downstream and chrysene, fluoranthene, pyrene, and a xylene isomer at 13.5 kilometers downstream from the wastewater discharge. Comparison of instantaneous flux values of selected organic compounds in water from downstream sites indicates: (1) the formation of chloroform in the stream following the discharge of the treated effluent, and that (2) instream biodegradation may be decreasing concentrations of linear alkylbenzene compounds in water. The relative persistence of many of the selected organic compounds in Rowlett Creek downstream from the municipal wastewater-treatment plant indicates that they could be transported into Lake Ray Hubbard, a source of municipal water supply.

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USGS1994

Use of landsat thematic mapper data to identify crop types and estimate irrigated acreage, Uvalde and Medina counties, Texas, 1991

Landsat Thematic Mapper (TM) data were used to estimate that about 51,000 acres of crops were irrigated with water pumped from the Edwards aquifer in Uvalde and Medina Counties, Texas in 1991. Bands 2, 3,4, and 5 from a TM image for August 10,1991, were classified using the maximum-likelihood, unsupervised-classification procedure to identify the areas of crops irrigated in the two counties. Detailed vegetation distribution maps of two calibration sites in the study area, and boundaries of the areas probably irrigated in 1991, were used to interpret the results and to separate probable irrigated areas by county from the rest of the image. Areas calculated for irrigated crops were 31,000 acres for Uvalde County, about 35 percent less than the area calculated using Landsat multispectral scanner (MSS) data in 1989, and 20,000 acres for Medina County, about 13 percent less than in 1989, a total decrease of about 28 percent for the two counties. Quantities of water pumped from the Edwards aquifer to irrigate crops in 1991 were estimated as 65,000 acre-feet for Uvalde County and 18,000 acre-feet for Medina County, a total decrease of about 56 percent from the value calculated using crop acreages from MSS data for 1989. Differences were attributed primarily to greater precipitation in 1991 than in 1989, resulting in smaller irrigation water requirements and less supplemental irrigation in 1991. Differences between results from 1989 and 1991, and between results from Uvalde and Medina Counties in 1991, were attributed primarily to greater precipitation in 1991, particularly in Medina County. The total number of acres of irrigated crops estimated using Landsat TM data was about 9 percent lower in Uvalde County and about 13 percent lower in Medina County than the number of acres calculated from data reported by the U.S. Department of Agriculture, Agricultural Stabilization and Conservation Service (ASCS). The total quantity of water pumped from the Edwards aquifer for irrigation in the two counties in 1991, about 83,000 acre-feet, was about 5 percent greater than the quantity calculated from data reported by the ASCS.

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USGS1994

Characteristics of streams and aquifers and processes affecting the salinity of water in the upper Colorado River basin, Texas

The upper Colorado River and some of its tributaries between Lake J.B. Thomas and O.H. Ivie Reservoir contain saline water (defined as water having dissolved-solids concentrations greater than 1,000 milligrams per liter). Dissolved-solids loads at nine streamflow water-quality stations increased from 1986 to 1988. The largest increases were in Beals Creek and in the Colorado River downstream from Beals Creek as a result of outflow of saline water from Natural Dam Salt Lake. The outflow contained 654,000 tons of dissolved solids and had a mean dissolved-solids concentration of 7,900 milligrams per liter. This amount represents about 51 percent of the dissolved-solids load to E.V. Spence Reservoir during 1986-88. The concentration of dissolved solids in streamflow in the upper Colorado River, however, generally decreased in the downstream direction as flows increased in all reaches of the river and its tributaries. Diversion of low flows at a site on the Colorado River upstream from the E.V. Spence and O.H. Ivie Reservoirs decreased the dissolved-solids concentrations of inflow to the reservoirs by about 7 percent while removing only about 3 percent of the total flow. The estimated dissolved-solids concentration exceeded 1,000 milligrams per liter for water from about 233 of 386 wells and springs for which specific conductance was measured in 1986. The mean value of the estimated dissolved-solids concentration for water from the 386 wells and springs was about 1,750 milligrams per liter. Some of the dissolved-solids content of streamflow and shallow-aquifer water was attributed to evapotranspiration in areas where water was close to land surface, in irrigated areas, and along streams. The evapotranspiration was largely from phreatophytes and was comparable to the mean streamflow from the study area during 1969- 86. However, evapotranspiration was not considered to have an effect on the salinity of water in shallow aquifers on a regional scale. Dissolution of sulfur-bearing minerals such as gypsum and pyrite in shallow aquifers contributed to salinity because of their presence in soil and shallowaquifer formations. Mixing with brine associated with oil and gas production is a major process affecting the salinity of streamflow and shallow-aquifer water. Brines can move upward through the more than 20,000 abandoned oil- and gas-related boreholes, most of which were abandoned prior to current requirements for casing, cementing, and plugging. Leakage from pits, disposal wells, and secondary-recovery wells, which have been used to dispose of brine, a by-product of oil and gas production, also contributed to dissolved solids in streamflow and shallow-aquifer water. The ratio of brine production to oil production in the seven major oil-producing counties increased from 0.60 to 6.5 between 1957 and 1983. About 359 million barrels of brine were injected into oil-producing formations through secondary-recovery wells in 1983, compared to 11 million barrels through disposal wells. The chemical characteristics of the saline water in streams and shallow aquifers in the study area were compared to characteristics of water that would result from the probable processes affecting the salinity of water, such as evapotranspiration, mineral dissolution, and mixing of water from streams and shallow-aquifer water with brines from deep aquifers. Dissolution of halite or mixing with deep-aquifer water was the most common cause of increased salinity in 48.0 percent of 77 water samples from shallow aquifers, as classified using salt-norm analysis; the second most common cause was the weathering and dissolution of sulfur-bearing minerals. Mixing with water from soil-mineral dissolution was classified as the principal source of chloride in 28.4 percent of 67 water samples from shallow aquifers with nitrate determinations. Trace-species/chloride ratios indicated that mixing with water from deep aquifers in rocks of the Pennsylvanian System was the principal source of chloride in 24.4 percent of 45 shallow-aquifer samples lacking nitrate determinations.

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USGS1994

Geologic history and hydrogeologic setting of the Edwards-Trinity aquifer system, west-central Texas

The Edwards-Trinity aquifer system underlies about 42,000 square miles of west-central Texas. Nearly flat-lying, mostly Comanche (Lower Cretaceous) strata of the aquifer system thin northwestward atop massive pre-Cretaceous rocks that are comparatively impermeable and structurally complex. From predominately terrigenous clastic sediments in the east and fluvialdeltaic (terrestrial) deposits in the west, the rocks of early Trinitian age grade upward into supratidal evaporitic and dolomitic strata, intertidal limestone and dolostone, and shallow-marine, openshelf, and reefal strata of late Trinitian, Fredericksburgian, and Washitan age. A thick, downfaulted remnant of mostly open-marine strata of Eaglefordian through Navarroan age composes a small, southeastern part of the aquifer system. The Trinity Group was deposited atop a rolling peneplain of pre-Cretaceous rocks during three predominately transgressive cycles of sedimentation that encroached upon the Llano uplift. The Fredericksburg and Washita Groups were deposited above the Trinity Group mostly in the lee of the Stuart City reef trend, a shelf margin ridge that sheltered depositional environments in the study area. The Washita Group subsequently was covered with thick, mostly fine-grained Gulf strata. During late Oligocene through early Miocene time, large-scale normal faulting formed the Balcones fault zone, where the Cretaceous strata were downfaulted, intensively fractured, and differentially rotated within a series of northeasttrending fault blocks. In addition to fracturing the rocks in the fault zone and extending the depth of freshwater diagenesis, the faulting vertically displaced the terrain, which steepened hydraulic gradients and maintained relatively high flow velocities near the surface. A shallow regime of dynamic ground-water flow evolved that promoted dissolution and enhanced the transmissivity of the Edwards Group in the Balcones fault zone. Cementation, recrystallization, and mineral replacement caused by deeper, comparatively sluggish ground-water circulation combined to diminish the transmissivity of the underlying Trinity Group, as well as most Cretaceous strata in the Hill Country, Edwards-Plateau, and Trans-Pecos. The Trinity, Fredericksburg, and Washita strata compose a regional aquifer system of three aquifers, whose water-transmitting characteristics generally are continuous in the lateral direction, and two hydraulically tight confining units. The aquifers are the Edwards aquifer in the Balcones fault zone, the Trinity aquifer in the Balcones fault zone and Hill Country, and the Edwards-Trinity aquifer in the Edwards Plateau and Trans-Pecos. The Navarro-Del Rio confining unit overlies the subcrop of the Edwards aquifer, and the Hammett confining unit lies within the updip, basal part of the Trinity aquifer and a small southeastern fringe of the Edwards-Trinity aquifer. The confining units are mostly calcareous mudstone, siltstone, and shale of low-energy terrigenous and openshelf marine depositional environments. The aquifers mainly result from fractures, joint cavities, and porosity caused by the dissolution of evaporites and unstable carbonate constituents. Because the diagenetic effects of cementation, recrystallization, and mineral replacement diminish the hydraulic conductivity of most rocks composing the Trinity and Edwards-Trinity aquifers, transmissivity values average less than 10,000 feet squared per day over more than 90 percent of the study area. However, the effects of tectonic fractures and dissolution in the Balcones fault zone cause transmissivity values to average about 750,000 feet squared per day in the Edwards aquifer, which occupies less than 10 percent of the study area.

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USGS1994

Hydrogeology and selected water-quality aspects of the Hueco Bolson Aquifer at the Hueco Bolson Recharge Project area, El Paso, Texas

Samples of ground water and tertiary-treated water were evaluated to determine the distribution of injected water and trihalomethane compounds in the Hueco bolson aquifer near El Paso, Texas. Chloride and nitrate concentrations and oxygen-18/oxygen-16 (δ 18 O) values were used with end-member mixing analysis to estimate the fractional contributions of injected water, irrigation-affected water, saline ground water, and freshwater to ground-water chemistry. Several laterally continuous sand and gravel layers are the principal water-yielding lithologies in the aquifer. The potentiometric surface sloped toward the south and southwest during January 1990. Average linear ground-water velocities were as large as 1.4 feet per day near the zone of largest hydraulic conductivity and were substantially smaller farther south and east. Ground-water samples from as far as 2,900 feet from an injection well contained injectedwater tracers and trihalomethane compounds. Ground-water samples with injected-water tracers and no trihalomethane compounds may indicate infiltration from septic systems or oxidation ponds. Apparent breakthrough velocities of injected water in the aquifer ranged from 0.13 to 1.3 feet per day. Irrigation-affected water and saline water also were identified in water from several wells. Ground-water transport of bromoform and dibromochloromethane is attenuated relative to injected water, chloroform, and dichlorobromomethane. Microbial transformation of bromoform and dibromochloromethane probably was responsible for their disappearance from ground water. Chloroform and dichlorobromomethane concentrations were affected principally by advective and dispersive transport of injected water.

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USGS1994

Geologic framework and hydrogeologic characteristics of the Edwards Aquifer outcrop, Comal County, Texas

All of the hydrogeologic subdivisions within the Edwards aquifer outcrop in Comal County have some porosity and permeability. The most porous and permeable appear to be hydrogeologic subdivision VI, the Kirschberg evaporite member of the Kainer Formation; hydrogeologic subdivision III, the leached and collapsed members, undivided; and hydrogeologic subdivision II, the cyclic and marine members, undivided, of the Person Formation. The two types of porosity in the Edwards aquifer outcrop are fabric selective, which is related to depositional or diagenetic elements and typically exists in specific stratigraphic horizons; and not fabric selective, which can exist in any litho strati graphic horizon. Permeability, the capacity of porous rock to transmit water, depends on the physical properties of the rock such as pore size, shape, distribution, fissuring, and dissolution. Two faults, Comal Springs and Hueco Springs, completely, or almost completely, offset the Edwards aquifer along much of their respective traces across Comal County. Porous and permeable Edwards aquifer limestone is juxtaposed against impermeable upper confining units in these areas. These faults completely, or almost completely, offset the Edwards aquifer and are thought to be barriers or partial barriers to groundwater flow where the beds are juxtaposed. In Comal County, the Edwards aquifer is probably most vulnerable to surface contamination in the rapidly urbanizing areas on the Edwards aquifer outcrop. Possible contamination can result from spills, leakage of hazardous materials, or runoff onto the intensely faulted and fractured, karstic limestone outcrops characteristic of the recharge zone.

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USGS1994

Sources of trends in water-quality data for selected streams in Texas, 1975-89 water years

Sources of trends in water-quality data for selected streams in Texas for the 1975-89 water years were investigated in this study. The investigation of sources was confined to distinct geographic patterns in the trend indicators for one constituent or for a group of related constituents. The probable source of trend patterns in nutrients and measures of oxygen in the Trinity River Basin was changes in the wastewater treatment facilities in the Dallas-Fort Worth metropolitan area. A pattern of increased concentrations of inorganic constituents in the upper Colorado River Basin resulted from emergency releases of water from the Natural Darn Lake, a salinity control structure. Trend patterns in inorganic constituents in the Rio Grande Basin were a result of increasing concentrations in the Pecos River and, to a lesser extent, the Rio Grande above the Amistad Reservoir, combined with the effects of reservoir regulation. A pattern of increasing concentrations of organic plus ammonia nitrogen and ammonia nitrogen was detected for the 1975-86 water years for stations with low concentrations (generally less than 5 milligrams per liter) of these nitrogen species. The trends were no longer evident when the period of trend analysis was extended to the 1989 water year. A positive bias in the data caused by the addition of mercuric chloride tablets to preserve nutrient samples during 1980-86 was the probable source of this trend pattern. A pattern of increasing concentrations in dissolved sulfate in the eastern part of the State was a result of a positive bias in the analytical results of a turbidimetric method of sulfate analysis. The source of a statewide pattern of increased pH in streams could not be identified.

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USGS1994

Tonsteins and clay-rich layers in coal-bearing intervals of the Eocene Manning formation, east-central Texas

Six samples from clay-rich intervals in the coal-bearing upper part of the Eocene Manning Formation were analyzed by scanning-electron microscopy and energy-dispersive X-ray fluorescence to determine the origin of minerals in the samples. Two samples were from surface-mine exposures of the 3500 coal bed near Bryan, Texas, and the remaining samples were from an exposure of a correlative interval at the Lake Somerville spillway about 60 km (37 mi) southwest of Bryan. Preliminary data suggest that both a 2-cm-thick (0.75-in) claystone from the upper part of the 3500 bed and the upper part of an 11-cm-thick (4.25-in) mudstone from the floor of the lower coal bed at the spillway were derived from volcanic ash falls. Both clay layers identified as possible tonsteins are composed of kaolinite and accessory quartz, euhedral to subhedral zircon, feldspars, and Ca-Al phosphates (crandallite?). Both alkali and plagioclase feldspars are observed in the two samples, but K-feldspar predominates in the upper clay layer of the 3500 bed, and plagioclase, with accessory Ti-bearing biotite, predominates in the sample from the floor of the spillway. These compositional differences suggest two separate volcanic ash falls. The other sampled clay layers contain rounded to subrounded zircons and feldspars in a mixed-layer clay groundmass, which suggests detrital rather than ash-fall origins.

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USGS1993

Trace elements and organochlorines in the shoalgrass community of the lower Laguna Madre Texas

Our objectives were to measure concentrations of seven trace elements and 14 organochlorine compounds in sediment and biota of the shoalgrass ( Halodule wrightii ) community of the lower Laguna Madre of south Texas and to determine whether chemicals associated with agriculture (e.g. mercury, arsenic, selenium, organochlorine pesticides) were highest near agricultural drainages. Arsenic, mercury, selenium, lead, cadmium, and organochlorines were generally at background concentrations throughout the lower Laguna Madre. Nickel and chromium concentrations were exceptionally high in shrimp and pinfish ( Lagodon rhomboides ), which is difficult to explain because of no known anthropogenic sources for these trace elements. For sediment and blue crabs ( Callinectes sapidus ), mercury was highest near agricultural drainages. Also, DDE was more frequently detected in blue crabs near agricultural drainages than farther away. In contrast, selenium concentrations did not differ among collecting sites and arsenic concentrations were lowest in shoalgrass, blue crabs, and brown shrimp ( Penaeus aztecus ) near agricultural drainages.

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USGS1993

Attributes and origins of ancient submarine slides and filled embayments: Examples from the Gulf Coast basin

Large submarine slides and associated shelf-margin embayments represent an intermediate member in the continuum of unstable shelf-margin features. On seismic profiles, they may resemble submarine canyons, but are different in their size, morphology, origin, and hydrocarbon exploration potential. Two large Neogene submarine slides, located in the northwestern Gulf Coast Basin, formed on the upper slope and flanks of prominent shelf-margin deltas. The basal detachment surface of each slide is a structural discontinuity that may be misinterpreted as an erosional unconformity and, therefore, misidentified as a stratigraphic boundary separating depositional sequences. Regional stratigraphic correlations indicate that both slides were initiated after the continental platform wa flooded. The condensed sections deposited during the rise in relative sea level contain the basal detachment surfaces. Beyond these observations, the relationships between the slides and sea level fluctuations are uncertain. The shelf-margin embayments created by the slides apparently were partly excavated during periods of lowered relative sea level and were filled during the subsequent relative sea level rise and highstand. Eventually the preslide morphology of the shelf margin was restored by coalesced prograding deltas. Submarine slides exhibit landward-dipping, wavy, mounded, and chaotic seismic reflections that are manifestations of slump blocks and other mass transport material. Composition of these internally derived slide deposits depends on the composition of the preexisting shelf margin. Embayment fill above the slide consists mostly of externally derived mudstones and sandstones deposited by various disorganized slope processes, as well as more organized submarine channel-levee systems. Thickest slope sandstones, which are potential hydrocarbon reservoirs, commonly occur above the basal slide mudstones where seismic reflections change from chaotic patterns to overlying wavy or subhorizontal reflections.

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USGS1993

Laguna madre: Seagrass changes continue decades after salinity reduction

Vegetation maps of the lower Laguna Madre prepared from surveys conducted in 1965–1967, 1974–1976, and 1988 document a >330 km 2 decrease in cover by Halodule wrightii , an increase of almost 190 km 2 in other seagrass species, and an increase of 140 km 2 in bare bottom. Loss in seagrass cover is confined to deeper parts of the laguna; turbidity caused by maintenance dredging is the suspected cause. The species shifts are consistent with observed reductions in salinity maxima. Although the hydrological alterations and climatic shift responsible for moderating the salinity regime occurred between 1948 and 1965, the biological changes continue. Establishment of patches away from source meadows appears to be the process for displacing species that limits their rate of expansion into suitable habitat in this elongate embayment.

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USGS1993

Quaternary geologic map of the Austin 4° x 6° quadrangle, United States

This map is part of the Quaternary Geologic Atlas of the United States (I-1420). It was first published as a printed edition in 1993. The geologic data have now been captured digitally and are presented here along with images of the printed map sheet and component parts as PDF files. The Quaternary Geologic Map of the Austin 4° x 6° Quadrangle was mapped as part of the Quaternary Geologic Atlas of the United States. The atlas was begun as an effort to depict the areal distribution of surficial geologic deposits and other materials that accumulated or formed during the past 2+ million years, the period that includes all activities of the human species. These materials are at the surface of the Earth. They make up the ground on which we walk, the dirt in which we dig foundations, and the soil in which we grow crops. Most of our human activity is related in one way or another to these surface materials that are referred to collectively by many geologists as regolith, the mantle of fragmental and generally unconsolidated material that overlies the bedrock foundation of the continent. The maps were compiled at 1:1,000,000 scale. In recent years, surficial deposits and materials have become the focus of much interest by scientists, environmentalists, governmental agencies, and the general public. They are the foundations of ecosystems, the materials that support plant growth and animal habitat, and the materials through which travels much of the water required for our agriculture, our industry, and our general well being. They also are materials that easily can become contaminated by pesticides, fertilizers, and toxic wastes. In this context, the value of the surficial geologic map is evident.

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