Mississippi Embayment Research Articles

Bulk Sediment Qp and Qs in the Mississippi Embayment, Central United States

We have estimated P-wave andS-wave anelastic attenuation coefficients for the thick, unconsolidated sediments of the Mississippi embayment, central United States, using the spectral distance decay of explosion P and Rayleigh waves. The sediment-trapped P wave, Psed, is observed to ranges of 80 km at 10 Hz, and 1-Hz Rayleigh waves are observed out to 130 km from a 5000-lb borehole explosion in the northern part of the embayment. Rayleigh waves of 4 Hz are seen to distances of 3 km from a smaller 50-lb explosion. Analysis of the group velocity and amplitude- distance decay of both waves yields an average Qs of 100 and Qp of 200 for embay- ment sediments that are independent of frequency. Scatter in the Q estimates comes from interference of multiple P-wave reverberations and Rayleigh-wave modes. The attenuation model is self-consistent in that it is the same as obtained by the analysis of synthetic seismograms using the inferred Q-values. Inferred Qp and Qs values are more than three times higher than previous estimates and imply that unconsolidated sediments of the embayment do not significantly attenuate small-strain earthquake ground motions. These estimates represent a lower bound to Q of the sediments since significant scattering is observed in the waveform data that contributes to the distance decay of wave amplitude. Higher Q values also imply that the unconsolidated sedi- ments of the embayment will form an efficient wave guide for surface waves radiated from shallow earthquakes or large earthquakes that rupture into the sediments, pro- ducing high-amplitude, long-duration wave trains that should be considered in earth- quake hazard assessments.

Seismic ground motions from a bolide shock wave

An intensely brilliant bolide accompanied by an audible sonic boom occurred on the night of 3 November 2003 (∼2150 LT or 4 November 2003, 0350 UT) in northeastern Arkansas. The sonic boom was well recorded by 22 three‐component stations of the Center for Earthquake Research and Information Cooperative Seismic Network. Arrival times and wave polarizations were used to reconstruct the acoustic wave front propagating across the network and to estimate trajectory parameters of the bolide. A grid search location technique constrains the trajectory and gives a reference height of 50–100 km above the southern edge of the network, an azimuth of propagation of 270°–290° toward the northwest and a plunge of 38°–48° downward. The trajectory and date of the bolide event are consistent with a Taurid shower meteor. Transient ground motions from the sonic boom were studied using plane wave, Cagniard‐deHoop, and propagator matrix theory for an incident pressure pulse impinging on a layered elastic Earth model. General characteristics of the vertical and radial polarization and wave frequency content require acoustic coupling within a thin (∼10 m) near‐surface layer with low P wave (∼0.25 km/s) and S wave (∼0.125 km/s) velocities. Ground motions are generally confined to this near‐surface layer, are not sensitive to deeper Earth structure within the Mississippi embayment unconsolidated sediments, and display both “leaky” mode PL (a type of dispersed, P‐type seismic wave) and “locked” mode Rayleigh wave propagation. Acoustic wave/ground motion coupling studies may be a useful way to analyze near‐surface site responses to determine velocity structure in earthquake shaking hazard studies.

Preliminary Assessment of Sand Blows in the Southern Mississippi Embayment

Trenching of sand blows in two liquefaction fields on late- to mid-Holocene alluvium within the southern Mississippi Embayment was conducted to determine the origin of the sand blows and their possible relationship to New Madrid seismic zone events. Trench stratigraphy and crosscutting relationships show evidence of multiple events of seismically induced sand venting. These two fields of moderate to intense liquefaction are 175 km and 240 km southwest of the southern limit of similarly intense sand blows within the New Madrid seismic zone (NMSZ) liquefaction field. The southernmost field (Ashley County, Arkansas) is ∼25 km in diameter, and sand blows are densely spaced. Sand blows are more sparsely spaced in the larger (∼30 km by 45 km) northern field (Desha County, Arkansas). Various correlations of liquefaction events between these two fields and between each of these fields and the NMSZ can be accommodated by our radiocarbon and infrared stimulated luminescence (IRSL) ages of sediments predating and postdating vented sand deposits in our trenches. These ages show a major liquefaction event at both the Ashley and Desha County fields between 4600 and 5500 years ago and a major event at the Ashley County field approximately 700 years ago. The early venting episode is older than the documented NMSZ chronology, and so a NMSZ seismic source cannot be assessed. The age of the later venting episode falls between two postulated NMSZ events, so it may reflect seismicity from a different source zone. Our trench logs show at least three other moderate to minor sand-venting episodes at the Ashley County field and two at the Desha County field, and one may be the same event at both fields about 2200 years ago. If these sand blows formed during large NMSZ events (≥175 km northeast), the area of strong ground motion for this seismic source has been underestimated. Alternatively, if these liquefaction fields formed due to local seismicity, then additional significant seismic source zones are present in the Mississippi Embayment outside the NMSZ. Each of the two fields sits astride seismicity alignments associated with the margins of a previously identified active fault block between the Arkansas River fault zone that projects through the Desha County field and the Saline River fault zone that projects through the Ashley County field.

Shear-wave Velocities of the Post-Paleozoic Sediments across the Upper Mississippi Embayment

A P - and S -wave velocity model of the post-Paleozoic sediments has been developed across the Upper Mississippi Embayment between the latitudes of 35¼°N and 35½°N. The model was constructed by P -wave soundings and reversed SH -wave refraction/reflection profiles acquired at 5-km intervals along the corridor. The results from these data were integrated with previously acquired P - and SH -wave velocity estimates, P -wave CDP reflection profiles, P -wave sonic logs, travel-time differences between earthquake-generated S and Sp waves, and top-of-bedrock elevation from nearby drillholes. A three-layered S -wave velocity model is proposed from this data set. The uppermost layer, which is not discussed in this paper (see Street et al. , 2001), varies from a few tens of meters thick near the edges of the embayment to as much as 190 m thick near the center of the study area; the S -wave velocities of these unlithified to poorly lithified sediments are highly variable (typically ranging between 150 and 600 m/s) and site-dependent. The second layer in the S -wave velocity model extends from the base of the near-surface layer to the acoustical top of the Cretaceous sediments, which is, ∼650 m below sea level near the center of the study area. The lateral S -wave velocity variance of this layer is defined in three segments: Near the western edge of the study area in northeastern Arkansas the velocity varies between 650 and 700 m/s; the central study area ranges between 795 and 840 m/s; and near the eastern edge of the study area in western Tennessee it ranges between 500 and 550 m/s. The S -wave velocities of the third layer, the Cretaceous section, vary between 725 and 775 m/s at the edges of the study area but between 1,010 and 1,060 m/s near the center.

Forest Communities of Natchez Trace State Forest, Western Tennessee Coastal Plain

In an effort to clarify vegetation gradients in a Mississippi Embayment Coastal Plain forest, we classified sixty-one forest stands in Natchez Trace State Forest (NTSF). Ten dominance types were delineated from cluster analysis that matched seven alliances of The Nature Conservancy's (TNC) National Vegetation Classification System. Ordination analysis clearly separated upland and lowland (floodplain and bottoms) dominance types, but differences among upland dominance types were more subtle and related to a topographic gradient involving slope position and aspect. NTSF is still recovering from severe anthropogenic disturbances (early in the 1900s), so a variety of early and mid-successional species are found throughout, e.g., Liquidambar styraciflua and Quercus alba. This successional component weakens vegetation-environment relationships and complicates interpretation of stand composition. Upland forests are succeeding to mesophytic dominance, especially by Fagus grandifolia. Vegetation was a mix between typical Coastal Plain forests, with Ilex opaca common, and typical Central Hardwood forests.

Evaluation of Deep Sediment Velocity Structure in the New Madrid Seismic Zone

Detailed knowledge of the physical properties of the sediments filling the Mississippi Embayment has proven critical to both unravel the tectonic frame-work operating in the region and assess the seismic hazards posed by the New Madrid Seismic Zone. In this article we show that independent geotechnical estimates for P - and S -wave velocities are compatible with a sedimentary model of K-feldspar clasts embeded in water, and we test its validity by modeling receiver functions at a number of broadband stations. By constraining the bulk sediment thicknesses beneath each station from independent reflection profiling estimates, we have been able to recover the depth to the top of the Cretaceous from the receiver function data at individual stations. Our receiver function modeling thus provides confidence in the velocity and density structures extrapolated from in situ geotechnical measurements in the Upper Mississippi Embayment.

Local Earthquake Wave Propagation through Mississippi Embayment Sediments, Part I: Body-Wave Phases and Local Site Responses

P and S body waves from microearthquakes in the New Madrid Seismic Zone (NMSZ) are investigated at selected sites in an effort to understand wave propagation from future large earthquakes. Earthquake body waveforms display distinctive features that constrain the nature of P - and S -wave local site responses and wave propagation within the unconsolidated Mississippi embayment sediments. Modeling of the waveforms demonstrates that a near-surface low-velocity zone is characteristic of structure within the upper 60 m of the sedimentary column and produces large P - and S -wave resonance effects that can be used to infer near-site conditions. Site resonance effects change because of velocity heterogeneity between individual receivers but imply that embayment sediments will substantially amplify ground motions at high frequencies. Site resonance affects P - and S -wave amplitude spectra and can bias estimates of source and anelastic attenuation parameters. Travel times of observed body-wave phases such as P, PpPhp (the first P -wave reverberation within the entire sedimentary column), Ps, Sp, S , and SsPhp can be used to estimate the average wave slownesses and Poisson's ratio within the embayment sediments; an average Poisson's ratio of 0.44 is obtained for the central NMSZ under station PEBM. Detailed S -wave velocities are derived for a Nafe-Drake sediment model using acoustic well logs and the travel-time constraints of observed seismic phases. V p/ V s ratios vary from 5.5 near the surface to approximately 2.4 at the base of the sediments. Use of the well-log data in wave calculations also explains much of the nature of P - and S -wave coda within the waveforms and shows that 1D heterogeneity is a first-order influence on seismic-wave propagation within the Mississippi embayment. Manuscript received 11 March 2003.

Local Earthquake Wave Propagation through Mississippi Embayment Sediments, Part II: Influence of Local Site Velocity Structure on Qp-Qs Determinations

Elastic, near-surface structure and receiver depth effects are seen to control body-wave amplitude spectra in the band from 2 to 50 Hz for local earthquake observations in the New Madrid Seismic Zone. P - and S -wave spectral peaks are created by resonance in shallow low-velocity zones, and spectral nulls are created by interference of direct and free-surface reflections due to receiver depth. These wave propagation effects dominate Sp/S spectral ratios and mimic anelastic attenuation effects by making it appear that S waves are deficient in higher frequencies relative to Sp waves. There is no evidence from Sp/S spectral ratios that unconsolidated sediments of the Mississippi embayment are highly attenuating. Dominance of P - and S -wave resonance suggests that the unconsolidated sediments amplify rather than attenuate seismic waves at high frequency. This has important implications for estimating hazards from strong ground motions in the area. In the absence of any identifiable anelastic attenuation effect in the spectral ratio, the only allowable Q p- Q s relationship for embayment sediments is that Q s = 3 Q p, based on consideration of wave travel times. Manuscript received 11 March 2003.

Comment on “Microtremor observations of deep sediment resonance in metropolitan Memphis, Tennessee” by Paul Bodin, Kevin Smith, Steve Horton and Howard Hwang

Bodin et al. [Eng. Geol. 62 (2001) 159] reported three sets of maxima in observed H/V particle-motion spectra, where the longest-period set of peaks provided clear mapping information on the thickness of post-Cretaceous sediments in the Mississippi embayment, but the remaining peaks remained largely unexplained. A review of microtremor array studies for similar sites and similar frequencies suggests that the microtremor field is best interpreted in terms of Rayleigh-wave energy. Use of this approach, with modeling of particle-motion H/V ratios for realistic models of the Mississippi embayment, shows that the [Eng. Geol. 62 (2001) 159] second set of spectral maxima can be associated with higher-mode Rayleigh-wave energy, and may contain information on a possible velocity inversion in the shear-velocity profile of the site. A third spectral maximum appears to be associated with soft soils (loess) and may be able to be used for mapping the near-surface thickness (order 15 m) of this unit.

Elevated lateral stress in unlithified sediment, Midcontinent, United States—geotechnical and geophysical indicators for a tectonic origin

Indirect and direct geotechnical measurements revealed the presence of high lateral earth pressure ( K o) in shallow, unlithified sediment at a site in the northernmost Mississippi embayment region of the central United States. Results from pile-load and pressuremeter tests showed maximum K o values greater than 10; however, the complex geologic environment of the Midcontinent made defining an origin for the anomalous K o based solely on these measurements equivocal. Although in situ sediment characteristics indicated that indirect tectonic or nontectonic geologic mechanisms that include transient overburden loads (e.g., fluvial deposition/erosion, glacial advance/retreat) and dynamic shear loads (e.g., earthquakes) were not the dominant cause, they were unable to provide indicators for a direct tectonic generation. Localized stresses induced anthropogenically by the geotechnical field tests were also considered, but ruled out as the primary origin. A high-resolution shear-wave (SH) reflection image of geologic structure in the immediate vicinity of the test site revealed compression-style neotectonism, and suggested that the elevated stress was a tectonic manifestation. Post-Paleozoic reflectors exhibit a Tertiary (?) structural inversion, as evidenced by post-Cretaceous fault displacement and pronounced positive folds in the hanging wall of the interpreted faults. The latest stratigraphic extent of the stress effects (i.e., all measurements were in the Late Cretaceous to Tertiary McNairy Formation), as well as the relationship of stress orientation with the orientation of local structure and regional stress, remain unknown. These are the subjects of ongoing studies.

Variable near-surface deformation along the Commerce segment of the Commerce geophysical lineament, southeast Missouri to southern Illinois, USA

Recent studies have demonstrated a plausible link between surface and near-surface tectonic features and the vertical projection of the Commerce geophysical lineament (CGL). The CGL is a 5- to 10-km-wide zone of basement magnetic and gravity anomalies traceable for more than 600 km, extending from Arkansas through southeast Missouri and southern Illinois and into Indiana. Twelve kilometers of high-resolution seismic reflection data, collected at four sites along a 175-km segment of the CGL projection, are interpreted to show varying amounts of deformation involving Tertiary and some Quaternary sediments. Some of the locally anomalous geomorphic features in the northern Mississippi embayment region (i.e., paleoliquefaction features, anomalous directional changes in stream channels, and areas of linear bluff escarpments) overlying the CGL can be correlated with specific faults and/or narrow zones of deformed (faulted and folded) strata that are imaged on high-resolution seismic reflection data. There is an observable change in near-surface deformation style and complexity progressing from the southwest to the northeast along the trace of the CGL. The seismic reflection data collaborate mapping evidence which suggests that this region has undergone a complex history of deformation, some of which is documented to be as young as Quaternary, during multiple episodes of reactivation under varying stress fields. This work, along with that of other studies presented in this volume, points to the existence of at least one major crustal feature outside the currently defined zone of seismic activity (New Madrid Seismic Zone) that should be considered as a significant potential source zone for seismogenic activity within the midcontinent region of the United States.

Variable post-Paleozoic deformation detected by seismic reflection profiling across the northwestern “prong” of New Madrid seismic zone

High-resolution shallow seismic reflection profiles across the northwesternmost part of the New Madrid seismic zone (NMSZ) and northwestern margin of the Reelfoot rift, near the confluence of the Ohio and Mississippi Rivers in the northern Mississippi embayment, reveal intense structural deformation that apparently took place during the late Paleozoic and/or Mesozoic up to near the end of the Cretaceous Period. The seismic profiles were sited on both sides of the northeast-trending Olmsted fault, defined by varying elevations of the top of Mississippian (locally base of Cretaceous) bedrock. The trend of this fault is close to and parallel with an unusually straight segment of the Ohio River and is approximately on trend with the westernmost of two groups of northeast-aligned epicenters (“prongs”) in the NMSZ. Initially suspected on the basis of pre-existing borehole data, the deformation along the fault has been confirmed by four seismic reflection profiles, combined with some new information from drilling. The new data reveal (1) many high-angle normal and reverse faults expressed as narrow grabens and anticlines (suggesting both extensional and compressional regimes) that involved the largest displacements during the late Cretaceous (McNairy); (2) a different style of deformation involving probably more horizontal displacements (i.e., thrusting) that occurred at the end of this phase near the end of McNairy deposition, with some fault offsets of Paleocene and younger units; (3) zones of steeply dipping faults that bound chaotic blocks similar to that observed previously from the nearby Commerce geophysical lineament (CGL); and (4) complex internal deformation stratigraphically restricted to the McNairy, suggestive of major sediment liquefaction or landsliding. Our results thus confirm the prevalence of complex Cretaceous deformations continuing up into Tertiary strata near the northern terminus of the NMSZ.

Experiment in New Madrid Zone to employ active source

An active source seismic experiment is scheduled for the end of October in the central United States' New Madrid Seismic Zone (NMSZ). Researchers from the center for Earthquake Research and Information (CERI) of the University of Memphis and the U.S. Geological Survey are planning 2600‐lb and 5000‐lb explosions at the southern and northern ends of the NMSZ, respectively.Broadband seismic instruments, other temporary broadband seismic stations, and an array of accelerographs near each source will record the large surface waves generated by the explosions in the unconsolidated sediments of the Mississippi embayment.

An equivalent linear algorithm with frequency- and pressure-dependent moduli and damping for the seismic analysis of deep sites

The seismic analysis of soil deposits is most often carried out with an iterative computational scheme, proposed by Seed and Idriss, in which inelastic effects are only approximately modeled through soil degradation curves. Laboratory experimental data indicate that for highly confined materials, the standardized reduction curves commonly used overestimate the capacity of soils to dissipate energy. This paper first presents the results obtained with a simple four-parameter constitutive soil model, which when used to simulate cyclic loading, produces results that agree well with available laboratory experiments for soils under arbitrarily large confining pressures. Thereafter, a frequency- and pressure-dependent iterative algorithm for seismic amplification is proposed, which provides time histories that match well the results obtained with a true non-linear model. Finally, the modified linear iterative analysis is successfully used for the seismic analysis of a 1 km deep model for the Mississippi embayment near Memphis, Tennessee, and a class-A prediction of the seismic amplification in Treasure Island during the Loma Prieta earthquake.

Quaternary Fault Reactivation in the Fluorspar Area Fault Complex of Western Kentucky: Evidence from Shallow SH-wave Reflection Profiles

Shallow shear-wave seismic-reflection profiles were collected over the southwestern projection of the Fluorspar Area fault complex into the northern Jackson Purchase region of western Kentucky. The area lies at the northern end of the sediment-filled Mississippi embayment, where the Paleozoic carbonate rocks are masked by a relatively thin, approximately 100 m sequence of nonlithified Cretaceous, Tertiary, and Quaternary sediments. The interpreted profiles imaged clear evidence of fault and apparent fold propagation into the near-surface Quaternary units. The profiles also showed evidence of various structural styles associated with episodic movement. The exact timing of the latest tectonic episode exhibited on the profiles is not known because of the lack of more accurate stratigraphic detail coincident with the lines. However, physical evidence of Quaternary deformation less than 10 m below the ground surface, along with the instrumentally recorded seismic events located in the immediate vicinity of the study area, emphasizes the problematic nature of these fault segments for the design of critical or high-hazard structures.

Tectonic Framework of the Southwestern Margin of the Illinois Basin and Its Influence on Neotectonism and Seismicity

Structures in Phanerozoic rocks along the southwestern margin of the intracratonic Illinois basin vary in deformational styles and orientations. These variations have been inherited from the Proterozoic basement. Based on them, a tectonic framework is developed that divides the margin into three tectonic domains separated by two northeast-trending zones of accommodation. Both zones of accommodation are interpreted as major Proterozoic crustal boundaries that have partitioned strain throughout the Phanerozoic. Each tectonic domain has a distinct neotectonic and seismic history, which is also attributable to strain partitioning along the zones of accommodation. The northern tectonic domain constitutes part of the Mississippi River arch, which lies north of St. Louis, Missouri. This arch separates the northern part of the Illinois basin from the Forest City basin to the west and contains faulted, asymmetrical anticlines and monoclines that have high structural relief, and broad anticlines and synclines that have low structural relief. Structural trends range from north-northwest to west-northwest. These structures underwent multiple Paleozoic deformations but have been mostly inactive since the late Paleozoic. There are few suggestions of any neotectonic activity in this domain and very little seismicity during the past 25 years. The northeast-trending St. Charles lineament (SCL) is a zone of accommodation that separates the northern and central tectonic domains. The SCL coincides with an extensive Proterozoic crustal boundary as defined by Nd geochemistry, across which basement rocks of differing lithologies and ages are juxtaposed. This crustal boundary is interpreted as a possible ancient transform fault. Locally, strike-slip faulting occurred along the SCL, probably in the late Paleozoic. Possible neotectonism occurred along the SCL, as well as some recent light to very minor seismicity. The central tectonic domain lies between the two zones of accommodation and contains complex fault systems that trend prominently northwest. Geologic mapping and structural analysis document a complex history of episodic left-lateral strike-slip and reverse faulting during the Paleozoic. Some of these faults may be branches of a deeply rooted flower structure in the crust and may have influenced the emplacement of ultramafic diatremes in the Devonian. Neotectonic features are localized in the southern part of this domain. Recorded seismicity is scattered throughout the central tectonic domain, but is more concentrated in the southeastern half. The Commerce geophysical lineament (CGL) is a northeast-trending feature that extends from northeastern Arkansas to at least Vincennes, Indiana. It is a zone of accommodation between the central and southern tectonic domains. This feature has been interpreted to consist of en-echelon faults and igneous intrusions in the basement that are related to the Neoproterozoic to early Paleozoic Reelfoot rift; however, it is postulated to have an even older ancestry. In the Thebes Gap area of Missouri and Illinois, a well developed system of northeast-to-north-northeast-trending, strike-slip faults occurs directly over the CGL. These faults cut Paleozoic, Mesozoic, and Cenozoic formations and have had a long-lived and episodic tectonic history, including Pleistocene and Holocene activity. In the past 25 years, several dozen light to minor earthquakes ( mb 2-4) have occurred on, or adjacent to, the CGL. The southern tectonic domain is roughly equivalent to the northern Mississippi embayment and contains many northeast-trending faults related to the Reelfoot rift. This domain also contains the New Madrid seismic zone, the most seismically active area east of the Rocky Mountains and the site of historical great earthquakes. During the late Paleozoic, uplift of the broad northwest-trending Pascola arch, which directly overlies the Mesoproterozoic Missouri batholith, closed the southwestern margin of the Illinois basin from its connection to the Arkoma basin. This uplifted arch extends across the central and southern tectonic domains and is partitioned also by the CGL. We propose a model in which the New Madrid seismic zone is a descendant of similar, intense, late Paleozoic tectonism that was concentrated in the southern domain at the intersection of the Pascola arch and the Reelfoot rift.

Integrated Geological and Geophysical Study of Neogene and Quaternary-age Deformation in the Northern Mississippi Embayment

A program of shallow drilling, trenching, outcrop mapping, and seismic-reflection acquisition in southern Illinois, just north of the New Madrid seismic zone, has produced the most comprehensive documentation to date of Quaternary-age faulting in the northern Mississippi embayment. Detailed structural cross-sections over five northeast-trending faults that continue from the Fluorspar Area fault complex southward into the embayment (Massac County, Illinois) indicate narrow grabens into which latest Cretaceous, Paleocene, and Eocene sediments were dropped and protected from erosion. The bounding faults were active during Neogene through middle Pleistocene time; in the deepest graben, the Late Miocene-Pleistocene-age Mounds Gravel is downthrown 150 m. Notably, definitive faulting of Wisconsinan loess or Holocene alluvium is not observed at any site, which would indicate that the faults have been inactive for at least 55 ka (basal loess ages) to 128 ka (youngest Illinoian age). Seismic profiles reveal that faults in Quaternary sediments penetrate Paleozoic bedrock, thus indicating that these are tectonic faults and not products of landsliding, solution collapse, or other merely near-surface processes. The faults exhibit vertical to steeply dipping normal and occasional reverse displacements that outline a variety of structures, including a series of narrow grabens. Tilting or folding of strata associated with faulting is also locally observed. At one site, observed faulting is superimposed over a prominent bedrock anticline, which may itself be a result of the Quaternary deformation that created the steep normal and reverse faults. Our results indicate that shallow sediment and bedrock faulting extends beyond the localized fault zones, implying that Quaternary deformation of the northern Mississippi embayment is more pervasive than formerly believed. The trend of Quaternary-age structures in southern Illinois is in line with the northeast-trending New Madrid seismic zone and is parallel to other geologically active features in the region, such as the Commerce geophysical lineament; the lack of observed Holocene movement, however, suggests that the faults in our study area are not currently active and thus may not contribute to seismic hazard. We propose a dynamic structural model that suggests a mechanism by which seismicity and active (Holocene) faulting have shifted within the central Mississippi Valley (away from the Fluorspar Area fault complex) over the last several 10,0009s of years.

Organochlorine chemical residues in fish from the Mississippi River basin, 1995.

Fish were collected in late 1995 from 34 National Contaminant Biomonitoring Program (NCBP) stations and 13 National Water Quality Assessment Program (NAWQA) stations in the Mississippi River basin (MRB) and in late 1996 from a reference site in West Virginia. Four composite samples, each comprising (nominally) 10 adult common carp (Cyprinus carpio) or black bass (Micropterus spp.) of the same sex, were collected from each site and analyzed for organochlorine chemical residues by gas chromatography with electron capture detection. At the NCBP stations, which are located on relatively large rivers, concentrations of organochlorine chemical residues were generally lower than when last sampled in the mid-1980s. Residues derived from DDT (primarily p,p'-DDE) were detected at all sites (including the reference site); however, only traces (<or= 0.02 microg/g) of the parent insecticide (p,p'-DDT) were present, which indicates continued weathering of residual DDT from past use. Nevertheless, concentrations of DDT (as p,p'-DDE) in fish from the cotton-farming regions of the lower MRB were great enough to constitute a hazard to fish-eating wildlife and were especially high at the NAWQA sites on the lower-order rivers and streams of the Mississippi embayment. Mirex was detected at only two sites, both in Louisiana, and toxaphene was found exclusively in the lower MRB. Most cyclodiene pesticides (dieldrin, chlordane, and heptachlor epoxide) were more widespread in their distributions, but concentrations were lower than in the 1980s except at a site on the Mississippi River near Memphis, TN. Concentrations were also somewhat elevated at sites in the Corn Belt. Endrin was detected exclusively at the Memphis site. PCB concentrations generally declined, and residues were detected (>or= 0.05 microg/g) at only 35% of the stations, mostly in the more industrialized parts of the MRB.

Unexpected Values of Qs in the Unconsolidated Sediments of the Mississippi Embayment

We studied the attenuation of shear waves at three sites in the Mississippi embayment using data recorded in boreholes drilled to depths of up to 60 m. The source was a highly repeatable compressed-air-driven hammer. To estimate attenuation we used a spectral ratio technique for fixed depth and variable frequency. The best-fit line for each depth z gives a measure of the cumulative attenuation, indicated by α( z ). Then we fit a straight line to α( z ) for a range of values of z . The slope of this line gives an estimate of the average attenuation per distance and was used to determine an average Q s. For one of the sites (Newport, northeastern Arkansas), Q s ranges between 34 (1.5 m ≤ z ≤ 44.2 m) and 44 (1.5 m ≤ z ≤ 51.8 m). These values are significantly higher than the more typical value of about 10 determined for unconsolidated sediments by other authors. In addition, these high values correspond to sediments with low average shear-wave velocity (about 300 m/sec). In contrast, average Q s and velocity for sediments in Shelby Forest (near Memphis, Tennessee) are 22 and 348 m/sec (22.6 m ≤ z ≤ 60.1 m), respectively. Therefore, these results go against the conventional wisdom that low velocity implies low Q . For the third site (Marked Tree, northeastern Arkansas), average Q s and velocity are 18 and 251 m/sec (9.8 m ≤ z ≤ 33.6 m), respectively. This site is about 75 km from Newport, and the differences in attenuation appear related to differences in lithology. Manuscript received 5 April 2001.

The Mississippi Embayment, North America: a first order continental structure generated by the Cretaceous superplume mantle event

The Mississippi Embayment of North America, a northward extension of the Gulf of Mexico coastal plain, is a southwestward-plunging trough containing ∼1.5 km of Cretaceous and Cenozoic sediments. The Embayment is underlain by the early Paleozoic Mississippi Valley graben basement fault complex. Previous authors have attributed Embayment subsidence to the opening of the Gulf of Mexico. However, the Embayment subsided 60 million years after cessation of the sea-floor spreading in the Gulf. We have previously argued that the Mississippi Embayment formed as a result of the westward passage of faulted crust (Mississippi Valley graben) over the Bermuda hotspot in mid-Cretaceous. More recently published age data clarify age progressive (northwest-to-southeast) mid-Cretaceous volcanism that crosses the Mississippi Embayment, beginning ∼115 Ma in eastern Kansas and ending ∼65 Ma in central Mississippi. This line of volcanism coincides with the predicted Bermuda hotspot path and has isotopic signatures consistent with a mantle hotspot source. We propose that during mid-Cretaceous, the weak crust of the Mississippi Valley graben complex was uplifted 1–3 km as it passed over the Bermuda plume, and this upland was eroded. As the Mississippi Valley graben complex moved west of the hotspot, it subsided, and the eroded region became a topographic low that filled with fluvio-marine sediments, the Mississippi Embayment. Supporting evidence for mid-Cretaceous uplift and erosion of the Embayment region includes: (1) an angular unconformity on pre-Late Cretaceous rocks with ∼2 km eroded at mid-Cretaceous along the hotspot path; (2) a broad anticline in the Embayment at mid-Cretaceous (revealed by unfolding the down-warped basal Late Cretaceous unconformity); (3) exhumation and weathering of mid-Cretaceous plutons before burial by Late Cretaceous sediments; and (4) a mid-Cretaceous change in the northern part of the Gulf of Mexico sedimentation from a continuous carbonate platform to a large influx of deltaic clastics. We now suggest that magmatic activity and pronounced uplift in the Mississippi Valley graben region may have been a result of increased hotspot flux of the typically weak Bermuda hotspot during the Cretaceous superplume mantle event (∼120–80 Ma).