Big Bend Region Research Articles

The dung beetle fauna of the Big Bend region of Texas (Coleoptera: Scarabaeidae: Scarabaeinae)

This paper reports the results of a 2001-2009 field study of the scarabaeine dung beetle fauna (Coleoptera: Scarabaeidae) of the Big Bend region of Texas, a three-county area of the Trans-Pecos portion of the Chihuahuan Desert. The observed fauna comprises 10 native species, Canthon blumei Halffter and Halffter, C. imitator Brown, C. praticola LeConte, and C. mixtus Robinson; Onthophagus browni Howden and Cartwright, O. knausi Brown, O. velutinus Horn and O. brevifrons Horn; Copris arizonensis Schaeffer and Phanaeus texensis Edmonds; as well as two exotic species introduced in the 1970s, Digitonthophagus gazella (Fabricius) and Euoniticellus intermedius (Reiche). The existing native fauna antedates the completion of desertification approximately 9,000 yrs BP and is similar ecologically and taxonomically to those in southeastern Arizona and northern Mexico. Ecological distribution follows three broad, overlapping habitat zones: desert montane forest, desert grassland and desert scrub. Species accounts include diagnoses, geographic distribution data, and information on collection method, habitat distribution and daily activity.

Dynamics of microphytobenthos photosynthetic activity along a depth transect in the sandy northeastern Gulf of Mexico shelf

The temporal and spatial dynamics of microphytobenthos activity in sandy shelf sediments are poorly understood, which limits assessments of the role of the benthic production in the coastal carbon cycle. The goal of this two-year time series study in the northeastern Gulf of Mexico therefore was to determine how benthic primary production rates in the West Florida Shelf change over the seasons and water depth. The study took place in the Big Bend region of Florida and used three stations along a transect starting at 5 m water depth and ending 29 km offshore, at 18 m water depth. Multisensor YSI 6600 probes installed at each station recorded key environmental parameters at the seafloor. Sediment cores, collected by SCUBA divers from each of the 3 stations at near-monthly intervals over a two-year time period were incubated in the laboratory for production and consumption rate measurements. Incubation of the cores at 200 μE m−2 s−1 allowed comparison of potential productivity between stations and seasons. Community light response curves, produced from sediment incubations at different light intensities, were used to calculate benthic productivity for measured in-situ light levels. The benthic production rates calculated from the core incubations and those calculated using the community light response curves agreed within a factor of three. Average sediment gross primary production rates based on in-situ light intensities ranged from 48 ± 60 mg C m−2 d−1 at 5 m and 81 ± 104 mg C m−2 d−1 at 10 m, to 13 ± 18 mg C m−2 d−1 at 18 m water depth. The corresponding average sediment consumption rates were −62 ± 36, −50 ± 55, and −29 ± 46 mg C m−2 d−1 at 5, 10 and 18 m water depth respectively. On average, production and consumption in the benthic system were found to be balanced, and the nearshore Stations A and B may temporarily turn net autotrophic during periods of high light intensity at the bottom. Our results suggest that the benthic production in this region may contribute up to 50% to the shelf primary production.

Seasonal and interannual cross-shelf transport over the Texas and Louisiana continental shelf

Numerical drifters are tracked in a hydrodynamic simulation of circulation over the Texas-Louisiana shelf to analyze patterns in cross-shelf transport of materials. While the important forcing mechanisms in the region (wind, river, and deep eddies) and associated flow patterns are known, the resultant material transport is less well understood. The primary metric used in the calculations is the percent of drifters released within a region that cross the 100 m isobath. Results of the analysis indicate that, averaged over the eleven years of the simulation, there are two regions on the shelf – over the Texas shelf during winter, and over the Louisiana shelf in summer – with increased seasonal probability for offshore transport. Among the two other distinct regions, the big bend region in Texas has increased probability for onshore transport, and the Mississippi Delta region has an increase in offshore transport, for both seasons. Some of these regions of offshore transport have marked interannual variability. This interannual variability is correlated to interannual changes in forcing conditions. Winter transport off of the Texas shelf is correlated with winter mean wind direction, with more northerly winds enhancing offshore transport; summer transport off the Louisiana shelf is correlated with Mississippi River discharge.

The Boquillas Formation of the Big Bend National Park, Texas, USA, a reference Cenomanian through Santonian (Upper Cretaceous) carbonate succession at the southern end of the Western Interior Seaway

Abstract The upper lower Cenomanian through middle Santonian (Upper Cretaceous) of the Boquillas Formation in the Big Bend Region of Trans-Pecos Texas consists of a marine carbonate succession deposited at the southern end of the Western Interior Seaway. The Boquillas Formation, subdivided into the lower, c. 78 m thick limestone-shale Ernst Member, and the upper, c. 132 m thick limestone/chalk/marl San Vicente Member, was deposited in a shallow shelf open marine environment at the junction between the Western Interior Seaway and the western margins of the Tethys Basin. Biogeographically, the area was closely tied with the southern Western Interior Seaway. The richly fossiliferous upper Turonian, Coniacian and lower Santonian parts of the Boquillas Formation are particularly promising for multistratigraphic studies.

Phytogeographical relationships and analysis of the flora of South Texas Plains

The southernmost tip of Texas is a part of the Tamaulipan Province of northeastern Mexico which some authors consider in the Madrean Subkingdom of the Holarctic Kingdom, while others the “Xerofitica Mexicana” of the Neotropical Kingdom. To shed more light on this question, a natural flora of fourteen counties in the S TX Plains ecoregion, South Texas Plains flora (S TX), was compiled, studied from a biogeographical perspective, and analyzed in this work. The analysis was based primarily on the species and genera distribution outlines. Phylogenetic literature was searched to find relationships, patterns of migrations, and geographical connections of the species of major clades. Taxonometric and geographic spectra of the S TX flora were obtained and compared with those of two other floras in southern Texas: SC Texas (EP, or Edwards Plateau) and Big Bend Region (BB). There are 1250 native species in 553 genera and 117 families in the S TX flora. These species were classified into 25 geographic (floristic) elements. Herein is presented a checklist of S TX accompanied by the geoelement descriptions, a comparison of geographical spectra of the species and genera in all three floras, and the biogeographical analysis of the S TX flora. South TX, having a flat topography and being on the crossroads of migration routes, has multiple connections with adjacent floristic centers. The Tamaulipan endemism is not particularly high (8% in S TX) in comparison with much higher Chihuahuan endemism in BB (23% in BB; 10% in S TX). Proportions of taxa of the largest families in the three southern Texas floras show much higher numbers of tropical-subtropical, Tamaulipan, Gulf Coast, and Mesoamerican (coastal) species in the S TX flora in comparison with those of EP and BB. North temperate, E North American, and Prairie species are best represented in EP (47%), followed by S TX (36%), with only 17% in BB. The majority of the BB differential species are in the Chihuahuan, SW N American, Sonoran/Apachian, and the Madrean geoelements. The genera of the largest family, Asteraceae, that are differential among the three floras show importance of the Mexican (Madrean) centers of diversity as well as migrations from the Tethyan Subkingdom in the assembly of the S TX flora. Tethyan connections are also evident among the Boraginaceae s. str. and some other groups found in the flora. The spectra of generic and specific geographical elements of S TX illustrate transitional position of its flora. It represents an ecotone between the Western (Madrean) and the Eastern (Atlantic and Gulf Coast) N American Regions, with a very high influence of the various Neotropical elements and a higher affinity to the EP flora than to that of BB.

Weatherscapes: Big Bend National Park — Frontier Desert Solitude

Located under skies of predominantly subtropical high pressure, the landscape of the Big Bend region is very much representative of the arid environment in which it exists: weathered and eroded geo...

Basement control of alkalic flood rhyolite magmatism of the Davis Mountains volcanic field, Trans-Pecos Texas, U.S.A.

Voluminous silicic lava flows, erupted ~37.4Ma from widespread centers within the Davis Mountains Volcanic Field (DMVF), covered approximately 10,000km2 with an initial volume as great as 1000km3. Lava flows form three major stratigraphic units: the Star Mountain Rhyolite (minimum 220km3) of the eastern Davis Mountains and adjacent Barilla Mountains, the Crossen Formation (~75km3) of the southern Davis Mountains, and the Bracks Rhyolite (~75km3) of the Rim Rock region west of the Davis Mountains proper. Similar extensive rhyolite lava also occurs in slightly younger units (Adobe Canyon Rhyolite, ~125km3, 37.1Ma), Sheep Pasture Formation (~125km3, ~36Ma) and, less voluminously, in the Paisano central volcano (~36.9Ma) and younger units in the Davis Mountains. Individual lava flows from these units formed fields as extensive as 55km and 300-m-thick.Flood rhyolite lavas of the Davis Mountains are marginally peralkaline quartz trachyte to low-silica rhyolite. Phenocrysts include alkali feldspar, clinopyroxene, FeTi oxides, and apatite, and, rarely, fayalite, as well as zircon in less peralkaline units. Many Star Mountain flows may be assigned to one of four geochemical groupings. Temperatures were moderately high, ranging from ~911 to 860°C in quartz trachyte and low silica rhyolite. We suggest that flood rhyolite magma evolved from trachyte magma by filter pressing processes, and trachyte from mafic magma in deeper seated plutons.The Davis Mountains segment of Trans-Pecos Texas overlies Grenville basement and is separated from the older Southern Granite and Rhyolite Province to the north by the Grenville Front, and from the younger Coahuila terrane to the south by the Ouachita Front. We suggest that basement structure strongly influenced the timing and nature of Trans-Pecos magmatism, probably in varying degrees of impeding the ascent of mantle-derived mafic magmas, which were produced by upwelling of asthenospheric mantle above the foundered Farallon slab. Basalt was able to penetrate Coahuila crust in the Big Bend region. Thicker Grenville crust under the Davis Mountains retarded ascent of mafic magmas, allowing mafic plutons to differentiate into silicic magma that was eventually erupted as flood lava. North of the Grenville Front, magmatism was further delayed and thicker, older crust there may have helped concentrate magmatism under the Davis Mountain region. Only after the onset of Basin and Range faulting was true basalt erupted over much of the Trans Pecos.

Estimating sea surface salinity in the northern Gulf of Mexico from satellite ocean color measurements

Sea surface salinity (SSS) is an important parameter to characterize physical and biogeochemical processes, yet its remote estimation in coastal waters has been difficult because satellite sensors designed to “measure” SSS lack sufficient resolution and coverage, and higher-resolution ocean color measurements suffer from optical and biogeochemical complexity when used to estimate SSS. In the northern Gulf of Mexico (GOM), this challenge is addressed through modeling, validation, and extensive tests in contrasting environments. Specifically, using extensive SSS datasets collected by many groups spanning >10years and MODIS (Moderate Resolution Imaging Spectroradiometer) and SeaWiFS (Sea-Viewing Wide Field-of-View Sensor) estimated remote sensing reflectance (Rrs) at 412, 443, 488 (490), 555, and 667 (670) nm and sea surface temperature (SST), a multilayer perceptron neural network-based (MPNN) SSS model has been developed and validated with a spatial resolution of ~1km. The MPNN was selected over many other empirical approaches such as principle component analysis (PCA), multi-nonlinear regression (MNR), decision tree, random forest, and supporting vector machines (SVMs) after extensive evaluations. The MPNN was trained by a back-propagation learning technique with Levenberg-Marquardt optimization and Bayesian regularization. The model showed an overall performance of root mean square error (RMSE)=1.2, with coefficient of determination (R2)=0.86, mean bias (MB)=0.0, and mean ratio (MR)=1.0 for SSS ranging between ~1 and ~37 (N=3640). Validation using an independent dataset showed a RMSE of 1.1, MB of 0.0, and MR of 1.0 for SSS ranging between ~27 and ~37 (N=412). The model with its original parameterization has been tested in the Mississippi-Atchafalaya coastal region, Florida's Big Bend region, and in the offshore Mississippi River plume, with satisfactory performance obtained in each case. Comparison with concurrent Aquarius-derived SSS maps (110-km resolution) showed similar agreement in offshore waters as indicated above, but the new 1-km resolution SSS maps revealed more finer-scale features as well as salinity gradients in coastal waters. The sensitivity of the model to realistic model input errors in satellite-derived SST and Rrs was also thoroughly examined, with uncertainties in the model-derived SSS being always <1 for SSS>30. The extensive validation, evaluation, and sensitivity test all indicated the robustness of the MPNN model in estimating SSS in most, if not all, coastal waters and offshore plumes in the northern GOM. Thus, the model provided a basis for generating near real-time 1-km resolution SSS maps from satellite measurements. However, the model showed limitations when applied to regions with known algal blooms or upwelling as they both led to low Rrs in the blue bands that may be falsely recognized as caused by low SSS.

Ecosystem services of the Big Bend region of the Chihuahuan Desert

Ecosystem services estimates have not been published for some biomes, notably desert ecosystems. The Chihuahuan bioregion, which is the largest desert in North America, exhibits high biodiversity and important cultural significance for parts of Mexico and the United States. With low levels of development, the Big Bend region is a relatively unmodified ecosystem, which makes it a good representative landscape to study desert ecosystem services. We found that this region has $504 (2015 USD) of annual value per hectare ($1.61 billion for entire study area), with raw materials, climate regulation, ornamental services, and cultural services contributing the most value. This estimate is markedly lower than published values for other terrestrial biomes, which is not surprising considering deserts are low productivity environments. However, given the size of the Chihuahuan Desert, the ecosystem services value for the entire bioregion is likely considerable. The Chihuahuan Desert is facing numerous threats, including energy development and overuse of natural resources. Projected growth in oil, gas, and wind energy could further degrade the services provided by this region. The relatively low ecosystem services values for this desert environment also indicate that the widespread desertification occurring globally is causing large decreases in ecosystem services across many landscapes.

A casualty of climate change? Loss of freshwater forest islands on Florida's Gulf Coast.

Sea level rise elicits short- and long-term changes in coastal plant communities by altering the physical conditions that affect ecosystem processes and species distributions. While the effects of sea level rise on salt marshes and mangroves are well studied, we focus on its effects on coastal islands of freshwater forest in Florida's Big Bend region, extending a dataset initiated in 1992. In 2014-2015, we evaluated tree survival, regeneration, and understory composition in 13 previously established plots located along a tidal creek; 10 plots are on forest islands surrounded by salt marsh, and three are in continuous forest. Earlier studies found that salt stress from increased tidal flooding prevented tree regeneration in frequently flooded forest islands. Between 1992 and 2014, tidal flooding of forest islands increased by 22%-117%, corresponding with declines in tree species richness, regeneration, and survival of the dominant tree species, Sabal palmetto (cabbage palm) and Juniperus virginiana (southern red cedar). Rates of S. palmetto and J. virginiana mortality increased nonlinearly over time on the six most frequently flooded islands, while salt marsh herbs and shrubs replaced forest understory vegetation along a tidal flooding gradient. Frequencies of tidal flooding, rates of tree mortality, and understory composition in continuous forest stands remained relatively stable, but tree regeneration substantially declined. Long-term trends identified in this study demonstrate the effect of sea level rise on spatial and temporal community reassembly trajectories that are dynamically re-shaping the unique coastal landscape of the Big Bend.

An Investigation of Effects of the Deepwater Horizon Oil Spill on Coastal Fishes in the Florida Big Bend Using Fishery-Independent Surveys and Stable Isotope Analysis

Abstract We employed catch data from 2 fishery-independent shark surveys conducted from 2009–2012, as well as stable isotope analysis, to investigate potential effects of the 2010 Deepwater Horizon oil spill on large coastal fishes in Florida's Big Bend region. The catch-per-unit-effort of 5 indicator species (3 sharks and 2 teleosts) varied significantly among years in only 2 cases. The stable isotope profiles were significantly different among years in 2 of 5 indicator species, but the relative differences were small, and patterns were not consistent among taxa analyzed. Our results provide no evidence that the Deepwater Horizon oil spill had a significant effect on the relative abundances and food-web structure among large coastal fishes in Florida's Big Bend.

Sediment supply versus local hydraulic controls on sediment transport and storage in a river with large sediment loads

AbstractThe Rio Grande in the Big Bend region of Texas, USA, and Chihuahua and Coahuila, Mexico, undergoes rapid geomorphic changes as a result of its large sediment supply and variable hydrology; thus, it is a useful natural laboratory to investigate the relative importance of flow strength and sediment supply in controlling alluvial channel change. We analyzed a suite of sediment transport and geomorphic data to determine the cumulative influence of different flood types on changing channel form. In this study, physically based analyses suggest that channel change in the Rio Grande is controlled by both changes in flow strength and sediment supply over different spatial and temporal scales. Channel narrowing is primarily caused by substantial deposition of sediment supplied to the Rio Grande during tributary‐sourced flash floods. Tributary floods have large suspended‐sediment concentrations, occur for short durations, and attenuate rapidly downstream in the Rio Grande, depositing much of their sediment in downstream reaches. Long‐duration floods on the mainstem have the capacity to enlarge the Rio Grande, and these floods, released from upstream dams, can either erode or deposit sediment in the Rio Grande depending upon the antecedent in‐channel sediment supply and the magnitude and duration of the flood. Geomorphic and sediment transport analyses show that the locations and rates of sand erosion and deposition during long‐duration floods are most strongly controlled by spatial changes in flow strength, largely through changes in channel slope. However, spatial differences in the in‐channel sediment supply regulate sediment evacuation or accumulation over time in long reaches (greater than a kilometer).

Kinematic Evolution of the Eastern Chalk Draw Fault during Basin and Range Extension: Evaluating the Role Played by Preexisting Structural Fabrics in Along-Strike Displacement Patterns

Basin and Range extensional structures in Trans-Pecos Texas have been interpreted to be strongly influenced by preexisting structures, including igneous centers in the Big Bend region. These igneous centers are interpreted to have deflected propagating faults, as they are rheologically strong relative to the surrounding rocks. We conducted detailed field mapping and analysis of fracture and fault data along the eastern segment of the Chalk Draw Fault in Big Bend National Park to evaluate its kinematic evolution and interaction with Paleogene plutons. Our primary results document the following key relationships: (1) the Rosillos Laccolith and other Paleogene plutons in the study area are pervasively fractured, dominantly by northwest-striking mode I fractures, which generally do not extend into adjacent sedimentary rock sequences; (2) throw on the eastern Chalk Draw Fault is strongly asymmetric, decreasing rapidly as it enters the Rosillos Laccolith; and (3) analysis of fault-slip data in the region documents dominantly N 52° E–directed regional extension along the northwest-striking segment of the Chalk Draw Fault and secondary structures. We interpret these relationships to indicate that the Rosillos Laccolith developed pervasive mode I fractures with a dominant northwest-southeast orientation during cooling under Basin and Range extensional stresses but that this occurred prior to extensional faulting within the region. We suggest that the highly fractured nature of the Rosillos Laccolith resulted in it being rheologically weaker than the surrounding Cretaceous sedimentary rocks, which resulted in (1) hindering southward propagation of the Chalk Draw Fault by dissipating stress at the fault tip within the highly fractured laccolith and (2) the strongly asymmetric distribution of slip due to a strong rheological contrast during fault propagation.

Integrating Environmental Flows into Multi-Objective Reservoir Management for a Transboundary, Water-Scarce River Basin: Rio Grande/Bravo

Integrated water management seeks to balance the interests of multiple stakeholders who desire many end-uses for water within the context of institutions and regulations. This problem is particularly complex in transboundary and water-scarce basins. In the Big Bend region of the Rio Grande/Bravo, an arid, monsoonal climate combines with multiple human and environmental water demands and established treaty requirements to stress available water resources. We analyzed reservoir operation strategies in the basin to integrate environmental flow (EF) considerations into existing management objectives using a linear programming model to assess reservoir operation policies. Five potential EF regimes are evaluated for improving aquatic and riparian habitat in the Big Bend region. The model uses the historical hydrologic record of river inflows, data for flood control and bi-national water allocation requirements, and parameters for human demands and infrastructure; to compare current and optimized operations of Luis L. Leon reservoir for multiple objectives. Results indicate that alternative operational policies for monthly reservoir storage (compared to historic values) can increase EF allocations without affecting water deliveries or treaty allocations. Some tradeoffs may exist, however, in managing reservoirs for both EFs and flood control. Our approach informs management strategies for the water-stressed basin that seek to incorporate environmental goals into existing infrastructure and operations.

A harmful algal bloom of Karenia brevis in the northeastern Gulf of Mexico as revealed by MODIS and VIIRS: a comparison.

The most recent Visible Infrared Imager Radiometer Suite (VIIRS) is not equipped with a spectral band to detect solar-stimulated phytoplankton fluorescence. The lack of such a band may affect the ability of VIIRS to detect and quantify harmful algal blooms (HABs) in coastal waters rich in colored dissolved organic matter (CDOM) because of the overlap of CDOM and chlorophyll absorption within the blue-green spectrum. A recent HAB dominated by the toxin-producing dinoflagellate Karenia brevis in the northeastern Gulf of Mexico, offshore of Florida's Big Bend region, allowed for comparison of the capacities of VIIRS and Moderate Resolution Imaging Spectroradiometer (MODIS) to detect blooms in CDOM-rich waters. Both VIIRS and MODIS showed general consistency in mapping the CDOM-rich dark water, which measured a maximum area of 8900 km2 by mid-July 2014. However, within the dark water, only MODIS allowed detection of bloom patches—as indicated by high normalized fluorescence line height (nFLH). Field surveys between late July and mid-September confirmed Karenia brevis at bloom abundances up to 20 million cells·L−1 within these patches. The bloom patches were well captured by the MODIS nFLH images, but not by the default chlorophyll a concentration (Chla) images from either MODIS or VIIRS. Spectral analysis showed that VIIRS could not discriminate these high-phytoplankton water patches within the dark water due to its lack of fluorescence band. Such a deficiency may be overcome with new algorithms or future satellite missions such as the U.S. NASA's Pre-Aerosol-Clouds-Ecology mission and the European Space Agency's Sentinel-3 mission.

Faunal communities and habitat characteristics of the Big Bend seagrass meadows, 2009–2010.

Seagrass meadows are important habitats that serve as nursery, feeding, and sheltering grounds for many marine species. In addition to the ecosystem functions and services they provide, seagrass habitats and associated fauna are commonly observed to have naturally high levels of heterogeneity, making them ideal for the study of ecological patterns and processes across multiple spatial scales. However, seagrass systems worldwide have undergone sharp declines in coverage and increased levels of fragmentation at both local and regional spatial scales, thus compromising their ecological functions and services and reducing their value as unaltered marine systems in which to conduct ecological studies. Covering nearly 3000 km2, the seagrass meadows of the Big Bend region in the eastern Gulf of Mexico represents one of the largest in the world, and given its separation from human population centers and coastal development, is also considered to be one of the most intact and least disturbed. The objective of our study was to provide the first region‐wide characterization of the habitats and faunal communities in seagrass meadows of the Big Bend. This two‐year study occurred in 2009 and 2010 during the summers when peak productivity in seagrass systems is highest. Sites were selected using a spatially balanced approach and sampling was conducted with beam trawls. A total of 170 sites were sampled, and all animals were identified to lowest taxonomic level possible, counted, and their sizes measured. Habitat characteristics were concurrently measured at both local (e.g., seagrass areal coverage and composition, volume of drift algae) and regional scales (e.g., latitude, type of adjacent coastal habitat).

Mercury concentrations and distribution in soil, water, mine waste leachates, and air in and around mercury mines in the Big Bend region, Texas, USA.

Samples of soil, water, mine waste leachates, soil gas, and air were collected from areas mined for mercury (Hg) and baseline sites in the Big Bend area, Texas, to evaluate potential Hg contamination in the region. Soil samples collected within 300 m of an inactive Hg mine contained elevated Hg concentrations (3.8–11 µg/g), which were considerably higher than Hg in soil collected from baseline sites (0.03–0.05 µg/g) distal (as much as 24 km) from mines. Only three soil samples collected within 300 m of the mine exceeded the probable effect concentration for Hg of 1.06 µg/g, above which harmful effects are likely to be observed in sediment-dwelling organisms. Concentrations of Hg in mine water runoff (7.9–14 ng/L) were generally higher than those found in springs and wells (0.05–3.1 ng/L), baseline streams (1.1–9.7 ng/L), and sources of drinking water (0.63–9.1 ng/L) collected in the Big Bend region. Concentrations of Hg in all water samples collected in this study were considerably below the 2,000 ng/L drinking water Hg guideline and the 770 ng/L guideline recommended by the U.S. Environmental Protection Agency (USEPA) to protect aquatic wildlife from chronic effects of Hg. Concentrations of Hg in water leachates obtained from leaching of mine wastes varied widely from <0.001 to 760 µg of Hg in leachate/g of sample leached, but only one leachate exceeded the USEPA Hg industrial soil screening level of 31 µg/g. Concentrations of Hg in soil gas collected at mined sites (690–82,000 ng/m3) were highly elevated compared to soil gas collected from baseline sites (1.2–77 ng/m3). However, air collected from mined areas at a height of 2 m above the ground surface contained concentrations of Hg (4.9–64 ng/m3) that were considerably lower than Hg in soil gas from the mined areas. Although concentrations of Hg emitted from mine-contaminated soils and mine wastes were elevated, persistent wind in southwest Texas disperses Hg in the air within a few meters of the ground surface.

A Multidisciplinary Analysis of the Unique Iron-Rich Turonian–Coniacian Boundary Interval (~3.16M Thick) within the Boquillas Formation, Big Bend Region, TX, That Includes the Allocrioceras Hazzardi Zone

A Multidisciplinary Analysis of the Unique Iron-Rich Turonian–Coniacian Boundary Interval (~3.16M Thick) within the Boquillas Formation, Big Bend Region, TX, That Includes the <i>Allocrioceras Hazzardi</i> Zone

Seagrass-Associated Molluscan Death Assemblages in the Big Bend Region of Florida, Gulf of Mexico

Seagrass-Associated Molluscan Death Assemblages in the Big Bend Region of Florida, Gulf of Mexico

Polyphase Laramide Structures and Possible Folded Tertiary(?) Sills at Dagger mountain, Big Bend National Park, Texas

Dagger Mountain, in Sierra del Carmen within Big Bend National Park, Texas, is a 5 km-long, doubly-plunging, southwest-vergent anticline adjacent to a doubly plunging syncline. Dagger Mountain lies near the eastern margins of the Cordilleran orogen and the Basin and Range province. Mapping at 1:12,000 scale reveals details about three phases of Laramide and Basin and Range structures. Mapping and descriptive structural analysis complement previous mapping at 1:12,000 – 1:75,000 scales (Poth, 1979; Moustafa, 1988; Cooper and others, 2011; Turner and others, 2011; Maxwell and others, 1967). Four distinctive formations of Cretaceous age crop out on Dagger Mountain: Santa Elena Limestone, Del Rio Clay, Buda Limestone, and Boquillas Formation. At least two phaneritic, mafic, feldpathoid-rich sills intrude the Boquillas Formation. A similar, possibly correlative sill south of Dagger Mountain is dated at 32.47 ± 0.41 Ma ( 40Ar/39 Ar on groundmass; Morgan and Shanks, 2008). One well-exposed Dagger Mountain sill can be traced from one map-scale fold limb, through the hinge, and into the other limb. The Dagger Mountain anticline is a first-phase (D1) fold. D1 map- and outcrop-scale folds contain subvertical NNW-striking axial planes and subhorizontal fold axes. Second-phase (D2) folds produced NNW and SSE plunges of the DM anticline. D2 map- and outcrop-scale folds display subvertical NE-striking axial planes and subhorizontal fold axes. Third-phase (D3) high-angle faults strike NNW and NW and cross-cut D1 folds and Tertiary sills. Drag during D3 faulting produced D3 folds. Dagger Mountain structures are significant because: a) few polyphase folds have been documented in the Big Bend region, b) the west-verging Dagger Mountain anticline and other D1 folds show fault-propagation fold characteristics, c) the apparently folded Tertiary(?) sill suggests that Laramide deformation at Dagger Mountain is post-32 Ma and unusually recent. Alternatively, the sill could be as old as Cretaceous, or the sill could have intruded both limbs and hinge of an existing fold.