H.S. Nance

Interfingering of evaporites and red beds: An example from the Queen/Grayburg formation, Texas☆

Abstract Interbedded red sandstone, siltstone, mudstone, anhydrite and halite comprise the Queen/Grayburg formation (Guadalupian) of the Palo Duro Basin. These deposits accumulated in a 28,000 km2 epicontinental basin that was characterized by regional aridity and episodic, nearly basinwide influx of marine-derived hypersaline waters. In each of several recurrent progradational cycles a variety of environments developed in this basin including eolian dunes, interdune areas, sand sheets, saline mud flats, and broad, shallow expanses of marine-derived brine (salinas). Siliciclastic sources were in the west and northwest whereas brines were derived from the south and southwest. Eolian dune deposits are represented by medium- to high-angle (15–35°) cross-stratified, fine-grained sandstone. Interdune areas accumulated flat- to low-angle bedded, wind-rippled strata, which are moderately to well sorted, fine-grained sand with frosted grains and illuviation structures. Sand sheets are characterized by deposits of horizontally bedded, rippled to massive, well-sorted, very fine-grained sandstone with illuviation structures. Sheetwash deposition on sand sheets and mud flats produced sandstones and siltstones with graded bedding, ripples with mud drapes, and intraclasts, and also flaser-bedded siltstones and mudstones deformed internally probably by haloturbation. Saline mud flats produced admixtures of mud and coarsely crystalline halite (chaotic halite-mudstone) and were located on the margins of regionally extensive hypersaline environments (salinas) in which bedded anhydrite and halite were precipitated subaqueously. Queen/Grayburg facies are cyclic and occur in characteristic vertical sequences that comprise, from base to top, bedded anhydrite, bedded halite, chaotic halite-mudstone, fine-grained clastics with displacive halite, siltstone and mudstone with deformed ripples, and eolian clastics. The sharp contacts between these shallowing-upward sequences suggest that transgressions were rapid.

Dynamic Depositional and Early Diagenetic Processes in a Deep-Water Shelf Setting, Upper Cretaceous Austin Chalk, North Texas

ABSTRACT The Austin Chalk of north Texas was deposited on a deep-water shelf north of the San Marcos Platform during a worldwide Coniacian and Santonian sea-level highstand. Transgressive (lowermost lower Austin Chalk), highstand (uppermost lower Austin Chalk), and regressive (middle and upper Austin Chalk) phases of cyclic chalk and marl sedimentation are recognized in core and outcrop and on log cross sections. Sedimentary structures in excellent exposures in excavations and tunnels created in Ellis County for the Superconducting Super Collider provide new evidence of sediment transport during Austin Chalk deposition. During transgression, bottom currents syndepositionally reworked nannoplankton oozes, incising channels as much as 120 ft across and 8 ft deep. Weakly burrowed channel fills having preservation of fine lamination document rapid infilling. Channel fills are composed of pyritized and carbonized wood and Inoceramus lag deposits, pellets, echinoderm fragments, and globigerinid grainstones, and coccolith ooze. During maximum highstand, bottom reworking was suppressed. Detrital content of highstand marls is low (<20 percent); organic content is high (1.4 to 3.5 percent). Coccolith preservation is excellent because of minimal diagenetic alteration. Regression is marked by resumed channel cutting and storm-bed winnowing in the middle and upper Austin Chalk. Suppressed resistivity log response and recessive weathering characteristics of the middle Austin Chalk are not primarily related to depositional environment but rather to increased input of volcanic ash during the accumulation of this interval. Early stabilization of ash produced clay-coated microfabrics in sediments that are otherwise similar to the transgressive deposits.

Combining airborne electromagnetic induction and hydrochemistry to quantify salinity contributions to a large basin stream, Colorado River, Texas, USA

We combined multifrequency airborne electromagnetic induction (EM) measurements of apparent ground conductivity with chemical analyses of surface water to delineate natural and oilfield salinity sources that degrade surface water quality by elevating total dissolved solids, chloride and sulphate concentrations along several hundred kilometres of the Colorado River (western Texas, USA). To reduce the cost of airborne geophysical surveying over such large areas, we used a helicopter to tow an EM instrument at low altitude along the stream-axis and measure the apparent electrical conductivity of the ground at multiple frequencies, examined results in the field to identify salinized stream segments and optimal water sampling locations and then flew more detailed surveys over these limited areas rather than over the entire basin as is typical in salinization studies. Minimally processed stream-axis EM data (including apparent conductivities measured at single frequencies and multifrequency ‘spectrograms’ along the stream-axis) helped identify salinized streambed segments, discriminate between surface and subsurface sources of salinity and determine water sampling locations upstream and downstream from each segment. We integrated EM, streamflow and hydrochemical data to calculate salinity loads, identify specific natural and oilfield salinity sources and guide and implement remediation efforts. Stream-axis flight lines offer the advantage of rapidly acquiring high-resolution subsurface conductivity data along long stream segments where traditional gridded flight-line surveys and waterborne measurements are impractical or prohibitively expensive. They also overcome difficulties associated with topographic effects when surveying deeply incised streams. Such surveys provide valuable information on location, extent and type of salinization and can guide water sampling and more intensive ground or airborne measurements.

STREAMBED INDUCTION LOGS: AN AIRBORNE APPROACH TO IDENTIFYING SALINITY SOURCES AND QUANTIFYING SALINITY LOADS

We delineated natural and oil-field salinity sources that degrade water quality in the upper Colorado River (west Texas) and Petronila Creek (Texas coast) by combining multifrequency airborne EM measurements of apparent ground conductivity with chemical analyses of surface water at key stream locations. To reduce the cost of high-resolution airborne surveying over such large areas, we first flew along the stream axes and then examined preliminary results in the field to identify likely salinized stream segments. We then flew more detailed surveys over these areas rather than over the entire basin. Stream-axis EM data also helped identify water-sampling locations upstream and downstream from each salinized segment. We used these data to calculate salinity loads, discriminate among possible natural and oil-field salinity sources, and more effectively implement best-management practices to optimize remediation efforts. We acquired stream-axis airborne EM data along the upper Colorado River using a Geophex GEM-2A instrument operating at five frequencies between 450 Hz and 39 kHz. Increases in chloride, sulfate, and total dissolved solids loading in the upper Colorado River basin occur along eleven segments of elevated apparent conductivity identified from airborne EM data. Each segment encompasses areas of baseflow salinity contributions to the stream from natural dissolution of evaporite minerals in the Permian basin, from oil-field produced water, or both. Analyses of surface water confirm increases salinity loading associated with each segment. Airborne EM data acquired on the coast along Petronila Creek revealed three stream segments with elevated ground conductivity. Increases in chloride, sulfate, and total dissolved solids loading are attributed to shallow baseflow contributions. Using airborne EM and hydrochemistry data, we interpret the dominant salinization mechanism to be historic discharge of produced water into unlined drainage ditches and pits, infiltration into sandy Pleistocene channel deposits, lateral migration as far as several kilometers, and discharge into the stream.

Fault and Joint Measurements in Austin Chalk, Superconducting Super Collider Site, Texas

ABSTRACT Structure maps of 9.4 mi of nearly continuous tunnel excavations and more than 10 mi of other exposures and excavations in Austin Chalk at the Superconducting Super Collider (SSC) site in Ellis County, Texas, record normal-fault and joint populations in the subsurface within the northern segment of the Balcones Fault Zone with unmatched resolution for such a long traverse. Small faults (<10 ft throw) occur in clusters or swarms that have as many as 24 faults. Fault swarms are as much as 2,000 ft wide, and spacing between swarms ranges from 800 to 2,000 ft, averaging about 1,000 ft. Predominantly northeast-trending joints are in swarms spaced 500 to more than 21,000 ft apart.

Stream-Axis EM from a helicopter: identifying salinity sources in a large river basin

Stream-Axis EM from a Helicopter: Identifying Salinity Sources in a Large River Basin Jeffrey G. Paine (Bureau of Economic Geology Jackson School of Geosciences The University of Texas at Austin) Edward W. Collins (Bureau of Economic Geology Jackson School of Geosciences The University of Texas at Austin) and H.S. Nance (Bureau of Economic Geology Jackson School of Geosciences The University of Texas at Austin) SUMMARY____________________________________________________________ We combined multifrequency airborne EM measurements of apparent ground conductivity with chemical analyses of surface water to delineate natural and oil-field salinity sources that degrade water quality in the upper Colorado River (western Texas U.S.A).