Progress report on geology of the Edwards aquifer, San Antonio area, Texas, and preliminary interpretation of borehole geophysical and laboratory data on carbonate rocks

Maclay, R.W.; Small, T.A.

doi:10.3133/ofr76627

Cited by 34 publications

(71 citation statements)

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“…Within the study area, the Edwards Group can be subdivided into several hydrostratigraphic units with different structural and hydrologic characteristics ( Fig. 2) (Maclay and Small, 1976;Small et al, 1996). The Natural Bridge caves are located within the interbedded limestone and dolomitic units of the upper Glen Rose and lower Walnut formations, which are also Early Cretaceous in age.…”

Section: Study Areamentioning

confidence: 99%

Temporal Variability of Cave-Air CO2 in Central Texas

Cowan¹,

Osborne²,

Banner³

2013

JCKS

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The growth rate and composition of cave calcite deposits (speleothems) are often used as proxies for past environmental change. There is, however, the potential for bias in the speleothem record due to seasonal fluctuations in calcite growth and dripwater chemistry. It has been proposed that the growth rate of speleothem calcite in Texas caves varies seasonally in response to density-driven fluctuations in cave-air CO 2 , with lower growth rates in the warmer months when cave-air CO 2 is highest. We monitored CO 2 in three undeveloped caves and three tourist caves spread over 130 km in central Texas to determine whether seasonal CO 2 fluctuations are confined to tourist caves, which have been modified from their natural states, and the extent to which cave-air CO 2 is controlled by variations in cave geometry, host rocks, cave volume, and soils. Nearly 150 lateral transects into six caves over three years show that CO 2 concentrations vary seasonally in five of the caves monitored, with peak concentrations in the warmer months and lower concentrations in the cooler months. The caves occur in six stratigraphic units of lower Cretaceous marine platform carbonate rocks and vary in volume (from 100 to .100,000 m 3 ) and geometry. Seasonal CO 2 fluctuations are regional in extent and unlikely due to human activity. Seasonal fluctuations are independent of cave geometry, volume, depth, soil thickness, and the hosting stratigraphic unit. Our findings indicate that seasonal variations in calcite deposition may introduce bias in the speleothem record, and should be considered when reconstructing paleoclimate using speleothem proxies.

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Section: Study Areamentioning

confidence: 99%

Temporal Variability of Cave-Air CO2 in Central Texas

Cowan¹,

Osborne²,

Banner³

2013

JCKS

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Section: Introductionmentioning

confidence: 99%

“…Host rock lithology in this zone contains relatively high concentrations of unoxidized organic material, pyrite, gypsum, and celestite (Maclay and Small 1976;Longman and Mench 1978). Generally, this zone has lower 314 A. S. ENGEL AND K. W. RANDALL permeability and porosity, making the extent of mixing between fresh and saline water limited (Maclay and Small 1976;Land and Prezbindowski 1981;Oetting et al 1996;Sharp and Banner 1997). The saline water has high dissolved hydrogen sulfide (H 2 S) concentrations, which have been attributed to microbial sulfate reduction associated with petroleum fields along the eastern edge of the aquifer (Rye et al 1981; Clement and Sharp 1988;Oetting et al 1996).…”

Section: Introductionmentioning

confidence: 99%

“…1), the aquifer also has a saline water zone that is distinct from the fresh water in the vicinity of the Balcones Fault Zone. Most of the fresh water movement is southwest to northeast, generally parallel to the fault zone, and strongly influenced by cavernous porosity (Maclay and Small 1976;Oetting et al 1996). Fresh water, with values for total dissolved solid (TDS) of ∼250-300 mg L −1 (Clement and Sharp 1988), discharges to springs along the fault zone.…”

Section: Introductionmentioning

confidence: 99%

“…Fresh water, with values for total dissolved solid (TDS) of ∼250-300 mg L −1 (Clement and Sharp 1988), discharges to springs along the fault zone. The host rock in the fresh water zone is predominately calcitic limestone (Maclay and Small 1976).…”

Section: Introductionmentioning

confidence: 99%

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Experimental Evidence for Microbially Mediated Carbonate Dissolution from the Saline Water Zone of the Edwards Aquifer, Central Texas

Engel

Randall

2011

Geomicrobiology Journal

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Microbially induced carbonate dissolution was evaluated from sulfidic wells in the Edwards Aquifer, Texas. Filamentous biomass covers rock surfaces, with Gammaproteobacteria and Epsilonproteobacteria dominating the attached community, but novel Alphaproteobacteria dominating the planktonic community. Despite fluids being saturated with respect to calcite, experimental calcite from in situ microcosms had significantly greater mass loss when colonized. Moreover, neoformed gypsum crystals were observed on colonized surfaces where fluids were undersaturated with respect to gypsum. The results are similar to findings from shallow cave and karst environments, and highlight the underappreciated role of microbes in the modification of carbonate aquifers and reservoirs.

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Bioturbation-influenced fluid pathways within a carbonate platform system: The Lower Cretaceous (Aptian–Albian) Glen Rose Limestone

Golab

Smith

Clark

et al. 2017

Palaeogeography, Palaeoclimatology, Palaeoecology

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The Aptian-AlbianGlen Rose Limestone (GRL) is an argillaceous shallow-marine carbonate deposit on the Central Texas Platform and contains the upper Trinity aquifer and the upper part of the middle Trinity aquifer. The GRL is divided into Upper and Lower GRL members, which have beenfurther subdivided into hydrostratigraphic units (HSUs). This study uses an integrated ichnological and sedimentological approach to record changes in ichnofabric index (ii) as a proxy for bioturbation within the GRL and relates these changes to fluid flow. Fluid pathways within HSUs are controlled by the complex interaction of faults and fractures, karst development, and large-scale bioturbation-influencedporosity and permeability. The effect of bioturbation-influencedporosity as an aquifer characteristic is the least studied of these factors. Postdepositional solution enhancement of ichnofossils is also common and has increased lateral and vertical fluid connectivity in some HSUs. Most GRL strata are dominated by Thalassinoides networks, but also contain Palaeophycus, Planolites, Ophiomorpha, Serpulid worm tubes, rhizoliths, andCruziana.Thalassinoides are commonly filled with coarser sediment than the surrounding matrix and act as fluid conduits within an otherwise low permeability matrix. Beds with ii3-4 and burrows with permeable fill transmit water readily. Beds with ii5-6 are commonly muddy and heavily homogenized, restricting fluid flow. Grainstone beds commonly have ii1-2 and are well cemented, restricting fluid flow to low intergranular flow. Pore systems dominated by Thalassinoides ichnofabrics, such as the GRL, are difficult to characterize on a large scale using many laboratory methods because they create heterogeneous flow paths depending on difference in permeability between the matrix and burrow fill. Understanding the effects of bioturbation-influenced porosity and permeability on subsurface fluid pathways is vital for creating a geologic and hydrostratigraphic framework for the Trinity aquifer.

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Progress report on geology of the Edwards aquifer, San Antonio area, Texas, and preliminary interpretation of borehole geophysical and laboratory data on carbonate rocks

Cited by 34 publications

References 0 publications

Temporal Variability of Cave-Air CO2 in Central Texas

Temporal Variability of Cave-Air CO2 in Central Texas

Experimental Evidence for Microbially Mediated Carbonate Dissolution from the Saline Water Zone of the Edwards Aquifer, Central Texas

Bioturbation-influenced fluid pathways within a carbonate platform system: The Lower Cretaceous (Aptian–Albian) Glen Rose Limestone

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