Brian Waldron

Reelfoot rift and its impact on Quaternary deformation in the central Mississippi River valley

Geophysical and drill-hole data within the Reelfoot rift of Arkansas, Tennessee, Missouri, and Kentucky, USA, were integrated to create a structure contour map and threedimensional computer model of the top of the Precambrian crystalline basement. The basement map and model clearly defi ne the northeast-trending Cambrian Reelfoot rift, which is crosscut by southeast-trending basement faults. The Reelfoot rift consists of two major basins, separated by an intrarift uplift, that are further subdivided into eight subbasins bound by northeast- and southeast-striking rift faults. The rift is bound to the south by the White River fault zone and to the north by the Reelfoot normal fault. The modern Reelfoot thrust fault, responsible for most of the New Madrid seismic zone earthquakes, is interpreted as an inverted basement normal fault. Geologic interpretation of 5077 shallow borings in the central Mississippi River valley enabled the construction of a structure contour map of the Pliocene‐Pleistocene unconformity (top of the Eocene‐base of Mississippi River alluvium) that overlies most of the Reelfoot rift. This map reveals both river erosion and tectonic deformation. Deformation of the Pliocene‐Pleistocene unconformity appears to be controlled by the northeast- and southeast-trending basement faults. The northeast-trending rift faults have undergone and continue to undergo Quaternary dextral transpression. This has resulted in displacement of two major rift blocks and formation of the Lake County uplift, Joiner ridge, and the southern half of Crowley’s Ridge as compressional stepover zones that appear to have originated above basement fault intersections. The Lake County uplift has been tectonically active over the past ~2400 yr and corresponds with a major segment of the New Madrid seismic zone. The aseismic Joiner ridge and the southern portion of Crowley’s Ridge may refl ect earlier uplift, thus indicating Quaternary strain migration within the Reelfoot rift.

Quaternary deformation and fault structure in the Northern Mississippi Embayment as imaged by near‐surface seismic reflection data

Seismicity in the New Madrid seismic zone (NMSZ) in the central United States constrains the location of present deformation at depth along four main distinct arms, while the surface expression of the ongoing deformation is still unclear. To better constrain the surface deformation in the NMSZ, we integrate existing seismic reflection data with a new ~300 km-long high-resolution seismic reflection profile acquired along the Mississippi River from Cape Girardeau, MO, to Caruthersville, MO. Based on the data, we interpret the Reelfoot Thrust and the New Markham Fault as upward splays of a blind master fault defined by the seismicity and extending at depth farther north. To the south, two faults, the Axial Fault and the Cottonwood Grove Fault, are imaged above the southern arm of the NMSZ. Both fault display deformation of the Paleozoic through the Tertiary sediments, and a relief of ~20–25 m at the base of the Quaternary alluvium, which we interpret as the result of strike-slip motion along a complex fault plane geometry. We propose two alternative interpretations for the relationship between the shallow faults and the seismicity in this area: (1) the faults merge at depth and are presently both active and (2) the faults are distinct at depth and were active during the Quaternary and only the Axial Fault is presently deforming. Geological structures mapped at the surface as part of this study show that Quaternary deformation is accommodated along a fault network that is more complex than the simple four-arm system illuminated by the seismicity, a behavior predicted by analog and computer models.

Urban effects on flood plain natural hazards: Wolf River, Tennessee, USA

Abstract The Wolf River and its flood plain in the City of Memphis and Shelby County, TN, have been extensively altered since the flood plain underwent liquefaction during the earthquakes of 1811–1812. Flood plain denudation and aggradation (primarily cut and fill) are documented by subtracting flood plain surface elevations obtained in the 1940s, 1965, 1988, and 2001. The resultant maps, cross-sections, and histograms illustrate the complex history and extreme changes that the flood plain has undergone. As a consequence of these post-1940s changes, liquefaction potential of the flood plain is now very complex; the flood plains area has locally been diminished by approximately 50%, thereby requiring remapping of the areas that will be inundated by 100- and 500-year floods; and entrenchment of the Wolf River and its flood plain may have increased connectivity between Wolf River water and Shelby Countys aquifer.

Imaging a shallow aquitard with seismic reflection data in Memphis, Tennessee, USA. Part I: source comparison, walk-away tests and the plus-minus method

Two walk-away tests were conducted at two sites in Memphis, Tennessee, USA. One site is representative of an urban setting (road median) while the other represents a rural setting (metropolitan park). Three P-wave sources, a 7.5 kg sledgehammer, a 20 kg weight-drop and a 12-gauge shotgun, were tested. Analysis of the data collected indicates that the seismic data recorded from the shotgun source possess the strongest energy, the highest dominant frequency, the broadest frequency band and the least amount of ground roll energy. The source repeatability was also studied by observing the first cycle of each seismic source, showing that the shotgun can generate the most repeatable source wavelets. None of the data recorded from the three sources show significant seismic energy above 100 Hz due to seismic wave attenuation. The loess in the rural site exacerbated the attenuation and resulted in a much lower peak frequency (43.7 Hz), which is nearly half of the peak frequency recorded at the urban site (85.3 Hz). Since attenuation can be a big factor in shallow reflection surveying, we recommend that site attenuation be considered before a reflection survey is performed in the Memphis area and a reflection survey be conducted outside of the loess blanketed area when possible. Since the final goal of the survey is to search for aquitard breaches, Hagedoorns’ plus-minus method was applied to the walk-away data set to map the first refractor, the top of the aquitard. One depression from the obtained structure was interpreted as a paleochannel, indicating that river channel erosion may be one of the causes for the formation of aquitard breaches.

LITE Flood: Simple GIS-Based Mapping Approach for Real-Time Redelineation of Multifrequency Floods

AbstractFlood zones with 1 and 0.02% of annual flooding chance are projected in FEMA’s digital flood insurance rate maps (DFIRMs) and are suited for identifying flood risk at the largest impacts. H...

GIS-Based Gold Potential Mapping in the Muteh Deposit Area, Iran, with Respect to a New Mineralization Concept

The Muteh deposit is a major Iranian gold mine that is located in the Sanadaj-Sirjan metallogenic zone. Gold deposition in Muteh has been previously interpreted to be Precambrian in age, but new studies propose the deposits may be younger and of late Eocene age. Therefore, geologic unit ages of and older than late Eocene are likely exploration targets for gold deposits. The focus of this study is to build a predictive model for mapping gold mineralization in the Muteh region. Based on the new gold mineralization concept in the Muteh deposits, the following conditions appear to be favorable for gold occurrence: (1) proximity to granite/leucogranite intrusive bodies as heat source, (2) presence of hydroxyl alteration minerals, (3) proximity to northeast oriented lineaments/faults, (4) presence of geochemical gold anomalies, (5) co-occurrence of gold pathfinder elements, and (6) presence of heavy mineral signatures. For this study, the potential heat sources were delineated from regional geological maps. The hydroxyl alteration minerals were extracted by applying the Crosta method on ETM+ satellite data, and the fusion of the ETM+ and DEM data was used to interpret structural features. The factor analysis on stream sediment data were utilized to reveal mineralization related geochemical anomalies. Mapping was carried out using a knowledge-based fuzzy logic overlay of evidential maps. Fuzzy scoring was assigned to different classes of evidential maps based on their favor ability in gold potential mapping. The final gold deposition potential map generated from fuzzy integration of geo-exploration dataset suggests 4.4% of the study area is favorable for gold mineralization. Known gold depositional environments were matched using the fuzzy logic approach, and one new potential gold prospect was identified.

Imaging a shallow aquitard with seismic reflection data in Memphis, Tennessee, USA. Part II: data analysis, interpretation and traveltime tomography

We collected P-wave seismic reflection data to image suspected breaches in the confining unit (aquitard) above the Memphis aquifer in Memphis, Tennessee, USA, where previous studies of water quality have suggested potential contamination. A 1-km-long reflection line was acquired across depressions of the water table that have been interpreted to reflect a breach in the aquitard. Although raw data are dominated by surface waves, after frequency filtering, shot static correction and f-k filtering, consistent reflections can be observed. Integration of constant velocity analysis (CVS) and super gather semblance analysis was used to determine the RMS velocity field. The stack section shows a set of shallow reflectors interpreted to correspond to the top and bottom of the confining unit. The aquitard thins by about half from the north end to the south end along the reflection line. The continuity of the top and bottom of the aquitard was disrupted by a few faults interpreted along the line. None of these faults juxtapose the surficial aquifer against the Memphis aquifer but it is possible that the faults themselves may act as a hydraulic connection between the surficial aquifer and the Memphis aquifer and thus serve as pathways for a potential leakage. The refraction tomography technique was applied to the first arrival data and it revealed three depressions that are interpreted as paleochannels on the upper part of the confining unit, which is consistent with the background geology of the area. These inferred paleochannels may suggest that ancient river channel erosion may contribute to the process responsible for the formation of aquitard breaches in this area.