Richard P. Langford

The Holocene history of the White Sands dune field and influences on eolian deflation and playa lakes

Abstract White Sands National Monument is the worlds largest gypsum dune field. The dunes are downwind of a 20-m-deep, 19-km-wide deflation basin containing large playa lakes. Today, the gypsum sand is derived from the edge of the deflation basin, next to the dune field, rather than the alkali flat and playa lakes, where gypsum crystals are forming. Three erosional shorelines mark wetter episodes, when playa lakes formed in the deflation basin. Successively, younger shorelines formed within older ones as the basin deepened, but was not widened by deflation. The modern shoreline is forming around Lake Lucero Playa. The oldest shoreline, termed L1, is degraded and formed near the Pleistocene–Holocene transition. Deflation from the L1 to the L2 shoreline cut through Pleistocene bedded evaporites and probably marks initiation of the dune field. This event was before 5840 yr BP , based on radiocarbon dates. This reinforces an evolving consensus that episodes of deflation have characterized desert basins in the southwestern United States. Regional deflation events have been dated at 7000 and 4000 yr BP . The shorelines imply the White Sands dune field was created in short episodes and the modern dune field may not represent conditions during expansion of the dune sea.

Computer-based data acquisition and visualization systems in field geology: Results from 12 years of experimentation and future potential

Paper-based geologic mapping is now archaic, and it is essential that geologists transition out of paper-based fi eld work and embrace new fi eld geographic information system (GIS) technology. Based on ~12 yr of experience with using handheld computers and a variety of fi eld GIS software, we have developed a working model for using fi eld GIS systems. Currently this system uses software products from ESRI (Environmental Systems Research Institute, Inc.) (ArcGIS and ArcPad), but the data model could be applied to any GIS system. This fi eld data model is aimed at simultaneously increasing the effi ciency of fi work while adding the attributing capability of GIS to develop fi eld data products that are more data rich than any paper map could ever achieve. We emphasize three basic rules in the development of this data structure. (1) A fi eld GIS map should emphasize line and point objects, avoiding polygons, objects that can easily be constructed outside of the fi eld environment. (2) Keep it simple stupid (KISS) is a critical rule for setting up data structures to avoid fi eld GIS systems that are less effi cient than paper. (3) Data structures need to develop a compromise between display and data entry, with display always trumping data entry because geologic insight is the primary goal. This paper contains two sample blank databases that illustrate these approaches for two applications: (1) generic bedrock geologic mapping, and (2) metamorphic geology mapping multiple generations of fabrics. Key features in our approach are to use display as a fi rst-order attribute, sorting point objects into four basic types (station, orientation, sample, photo) and lines into the four basic contact types (depositional contact, unconformity, intrusive contact, fault), plus other specialized data layers where needed. Individual GIS objects are further attributed, but attributing is limited to critical information with all objects carrying a special “note” fi eld for input of nonstandard information. We suggest that when fi eld GIS systems become the norm, fi eld geology should enjoy a revolution both in the attitude of the fi eld geologist toward his or her data and the ability to address problems using the fi eld information. However, fi eld geologists will need to adjust to the changing technology, and many longestablished fi eld paradigms should be reevaluated. One example is the rule that all linework on geologic maps needs to be perfected in the fi eld setting. Our experience suggests that with modern high-resolution imagery (aerial photography and topographic shaded reliefs) and digital elevation models, fi eld work should evolve into an iterative process where map linework is roughed out in the fi eld, refi ned during evening fi eld sessions, then potentially revisited if problems arise. This procedure is particularly effi cient when three-dimensional visualization is added to the system, a feature that will soon become the norm rather than the exception. We note that using these systems is particularly important for future developments in metamorphic geology, sedimentary geology, and astrogeology, but other applications are clearly also possible. For geoscience instructors who teach fi eld geology classes, we note that it is critical that these systems be incorporated into all geoscience fi eld programs, but research is needed on the best teaching approaches in the use of the technology.

Positional accuracy of the Google Earth terrain model derived from stratigraphic unconformities in the Big Bend region, Texas, USA

The Google Earth terrain model could prove beneficial for extraction of positional data in the future. At present, only an aging independent benchmark study (Potere, D., 2008. Horizontal position accuracy of Google Earths high-resolution imagery archive. Sensors, 8, 7973–7981) provides constraints on positional accuracy for Google Earth imagery. In this investigation, we compared virtually traced positions against high-precision (<1 m) field measurements along three stratigraphic unconformity sub-sections in the Big Bend region to determine current positional accuracy for the Google Earth terrain model. A horizontal position accuracy of 2.64 m RMSEr was determined for the Google Earth terrain model with mean offset distance being 6.95 m. A vertical position accuracy of 1.63 m RMSEz with mean offset distance of 2.66 m was also calculated for the terrain model. Results suggest data extracted from the Google Earth terrain model could plausibly be used in future studies. However, we urge caution in using Google Earth data due to limited information disclosures by developers.

The Miocene to Pleistocene filling of a mature extensional basin in Trans-Pecos Texas: geomorphic and hydrologic controls on deposition

Abstract Northwest Eagle Flat Basin, in Trans-Pecos (West) Texas, is a Late Tertiary–Quaternary extensional basin in the Basin and Range Province. The basin has largely filled with sediment, so that bedrock uplands only crop out around its rim. Movement on normal faults has effectively ceased, and the geomorphology and deposition differ from active extensional basins, where tectonism is the primary control on basin evolution. Eagle Flat differs from typical extensional basins because progressive changes in basin morphology caused by the aggrading basin-fill were the principal controls on sedimentation. A low-gradient alluvial basin floor expanded to cover the former, fault-defined basin margins. The playa lies on what was upland through the Miocene and Early Pliocene. Only the upper few meters of basin-fill crop out; however, a suite of 88 cores was drilled in the southern part of the basin. Nine of the cores and one trench were sampled for paleomagnetic reversal dating. Correlating the dated cores with interspersed cores allowed us to piece together the basin filling-history and explain how the mature features developed. The cores record the gradual burial of the southern half of the basin. The oldest basin-fill strata were cored in the deepest part of the basin, 219 m below the surface, at ∼12 Ma, in the Miocene. After deposition of 50 m of alluvial-fan gravel along the trend of an inferred normal fault, the basin floor aggraded and expanded. By the 780-ka Brunhes–Matuyama reversal, basin-floor drainage had reversed and was from the north. Sediment accumulation ended during the mid-Pleistocene. A fine-grained sediment supply that did not decrease, and outpaced subsidence was the primary control on basin deposition. This caused a progressive loss of relief and drainage-basin area as uplands were buried under the aggrading basin sediments. After the basin-margin faults were buried, the shape of the basin and its filling-history were little controlled by the original tectonically formed geomorphic elements.

Monitoring Yearly Changes and Their Influence on Electrical Properties of the Shallow Subsurface at Two Sites Near the Rio Grande, West Texas

We present the results of a case study that monitored changes in the electrical properties of soils on a bi-monthly basis between August 1999 and November 2000 at two sites located along the banks of the Rio Grande, west Texas. One site was located within an abandoned channel of the Rio Grande and showed relatively homogeneous layering, while the second site exhibited complex interfingerings of crevasse splay (sands and silts) and floodplain muds. Repeated EM-31 ground conductivity measurements at both sites showed that conductivities within the upper 3m of soil could change by up to a factor of 2 within a year’s time. DC resistivity studies suggested up to a factor of 5 change in resistivity values within the upper 1m of soil. These changes appear to be related to remobilization of salt in the soil following rainfall or irrigation events. These dramatic fluctuations make it difficult to image deeper (>5m depth) conductivity changes in the subsurface.

Seasonal Variations in Geophysical Surveys of Alluvial Sediments Near the Rio Grande, West Texas

We have used geophysical techniques (seismic refraction, DC resistivity, conductivity, spectral analysis of surface waves) to examine the ability of these surveys to determine grain size/stratigraphy variations in alluvial sediments along a two-dimensional profile near the Rio Grande in west Texas. We also examined the variation in survey results through time and found that there were large temporal variations in the geophysical properties of the sediments due to changes in water table depth, soil salinity, and moisture content. Geological information from boreholes has helped us to separate these seasonal effects from stratigraphic and grain size variations. Our goal was to determine which geophysical techniques would work best for determining geologic variations, regardless of the season of the investigation. Our results show that temporal changes in soil moisture and soil salinity caused up to 25% variations in shear/compressional velocities of seismic waves and up to 50% variations in conductivity/resistivity values. It was difficult to compare measurements from different geophysical surveys unless they were conducted at nearly the same time. A combination of resistivity with the spectral analysis of surface waves technique appears to be the fastest, most reliable, joint technique to determine geological, soil moisture and salinity variations.

Effects of Arroyo Sediment Influxes on the Rio Grande River Channel near El Paso, Texas

Arroyos that flow into the Rio Grande River channel along the U.S.–Mexico border provide intermittent influxes of sediment that may obstruct the channel and cause overflow as well as sedimentation problems downstream. These phenomena were studied using a recently developed, unique, in situ method for measuring the erosion properties of sediments with depth and at high shear stresses. Results of the investigation confirm that the arroyo sediments can affect the channel of the Rio Grande by introducing sediments that are more difficult to erode compared to those already present. Two sites were mapped and characterized in terms of vegetation and soil distribution. Sediment samples were collected, and erosion rates, mineralogy, and sediment grain-size distributions were determined. Results showed that large flows in both arroyos were capable of obstructing the Rio Grande channel by introducing sediments that were more difficult to erode than the existing channel sediments.

AIRSAR/TOPSAR Image Processing and Interpretation: An Application Example in Far West Texas, U.S.A.

Abstract JPL/NASAs AIRSAR/TOPSAR is a multipolarimetric, multifrequency, and interferometric airborne synthetic aperture radar capable of imaging in C‐, L‐, and P‐ bands (5.7, 24.5, and 68 cm) with 10 m spatial resolution. This paper introduces and explains 5 conversion processes that users need to apply to data products received from JPLs integrated AIRSAR processor. Numerical algorithms and approaches to remove both speckle and banding noises that are common in AIRSAR data have been investigated and evaluated. Among statistical adaptive speckle removal algorithms, G‐MAP (Gamma Maximum A Posteriori) had the best performance. For banding removal, a new method that we call combined principal components analysis (CPCA) has been developed. This method is proved very effective with our data. Radar signature differences were studied and compared at different wavelengths and between radar and optical (ETM+) images. Some subsurface objects (like water pipelines) were clearly visible in the radar images, especially from the P‐ band. Data fusion based on the color transform technique was employed to integrate Landsat 7 (30 m ETM+ data fused with the accompanying 15 m panchromatic data) and TOPSAR data after speckle and banding removal. The resulting fused image (10 m) brought out new features that were not evident in the original images and helped identify many features whose origin was not clear in the original images.

Sediment erosion and transport at the Rio Grande mouth : report for the National Border Technology Program and International Boundary and Water Commission.

The mouth of the Rio Grande has become silted up, obstructing its flow into the Gulf of Mexico. This is problematic in that it has created extensive flooding. The purpose of this study was to determine the erosion and transport potential of the sediments obstructing the flow of the Rio Grande by employing a unique Mobile High Shear Stress flume developed by Sandias Carlsbad Programs Group for the US Army Corps of Engineers. The flume measures in-situ sediment erosion properties at shear stresses ranging from normal flow to flood conditions for a variable depth sediment core. The flume is in a self-contained trailer that can be placed on site in the field. Erosion rates and sediment grain size distributions were determined from sediment samples collected in and around the obstruction and were subsequently used to characterize the erosion potential of the sediments under investigation.

Pliocene–Holocene deformation in the southern Rio Grande rift as inferred from topography and uplifted terraces of the Franklin Mountains, southern New Mexico and western Texas

In most extensional terrains such as the Rio Grande rift, alluvial fans and bajadas cover faults and terraces as extension progresses, thus limiting the faults and terraces as useful records of uplift. However, in the Franklin Mountains of western Texas and southern New Mexico (USA), rapid aggradation of basin floors by extensive playa lakes and floodplain deposits of the Rio Grande during the Pliocene buried the irregular mountain-front fans, thus creating a low-gradient surface. This originally planar surface was subsequently uplifted and deformed during faulting, providing a record of the Pliocene–Holocene extensional deformation in the southern Rio Grande rift. Deformation and uplift of the Franklin Mountains in the southern Rio Grande rift was estimated by measuring the elevation of late Pliocene terraces that are adjacent to range-bounding faults. The uplifted terraces are exposed along both sides of the Franklin Mountains, and lie as much as 130 m above their original elevation. Together the uplifted terraces form an anticlinal arch that mimics the profile of the range crest of the mountains. Three important conclusions can be drawn from the similarity of profiles among the terraces and mountain crest. First, the observation that the terraces mimic the range crest implies that the present-day topography of the mountains is likely tectonic in origin. Second, the east-side terraces are higher than the west-side terraces, suggesting rotation of the mountains during deformation. Estimated rotation since the Pliocene is ~5% of the total rotation. Third, fault throw rate calculations indicate differential slip along the length of the eastern boundary fault zone. The fault profile and throw rate calculations along the eastern margin of the range are skewed to the south, suggesting that the southern segment of the Franklin Mountains has accumulated a majority of the slip during this time frame. These observations, coupled with geophysical data highlighting buried faults beneath the El Paso (Texas)–Juárez (Mexico) metropolitan region, suggest that normal faults related to uplift of the Franklin Mountains have been growing in length toward the south over the last several million years. INTRODUCTION Normal fault systems develop through incremental slip events (earthquakes) that, over time, result in the final displacement profile of the fault. Extended terranes are also characterized by multiple normal fault systems that overlap to form relay ramps or intersect as extension progresses and individual faults grow in length (e.g., Peacock and Sanderson, 1991; Childs et al., 1995; Peacock, 2002; Nicol et al., 2010). These characteristics are true in cases of simple lithology and sandbox experiments (e.g., McClay and Ellis, 1987; McClay, 1990; Childs et al., 1995) but are also applicable to regions of diverse rock types and heterogeneous crustal features (e.g., Nicol et al., 2005). Although these relationships appear to be generally true for a wide range of extensional faults regard less of fault size, amount of offset, or lithology (e.g., Schlische et al., 1996), geologic evidence documenting sequential fault growth is commonly lacking. In models of idealized, isolated normal faults, displacement profiles should preserve maximum displacement at the center of the fault, and displacement should decrease to zero at the fault tip lines (Barnett et al., 1987; Walsh and Watterson, 1987). However, displacement patterns are almost always segmented and irregular, and are also commonly asymmetrical, where maximum fault throw is centered closer to one fault tip than the other (e.g., Walsh and Watterson, 1987; Childs et al., 1995; Mansfield and Cartwright, 1996; Schlagenhauf et al., 2008). As slip accumulates, fault tips typically propagate in two directions and fault length increases. This process results in a power-law relationship, where fault length scales linearly with fault displacement (e.g., Pickering et al., 1995; Schlische et al., 1996). As fault length increases, adjacent faults eventually overlap and link, which has been shown to be an important fault growth mechanism in extensional settings (e.g., Cartwright et al., 1995; Finch and Gawthorpe, 2017; Whipp et al., 2017). Alternatively, some fault systems develop through a constant-fault-length model, where they approach their maximum length early and fault tips become fixed in space as fault displacement continues to accumulate (e.g., Nicol et al., 2005; Amos et al., 2010; Mouslopoulou et al., 2012; Curry et al., 2016). GEOSPHERE GEOSPHERE; v. 14, no. 4 https://doi.org/10.1130/GES01572.1