Fluvial geomorphic features of the Lower Mississippi alluvial valley

The Lower Mississippi Valley (LMV) has been one of the most intensively studied alluvial valleys in the world in terms of it's geological and geomorphic framework and history. A brief outline of the history of the major geological and geomorphological investigations of the LMV is provided. The results of these investigations are discussed in terms of the fluvial geomorphic framework of the valley and the apparent significant changes in the regime of the Mississippi River during the Late Wisconsinan and Holocene stages.

The LMV occupies the broad deep synclinal trough of the Mississippi Embayment which extends from Cairo, Illinois, to the Gulf of Mexico in a slightly sinuous north-south trend. The embayment is filled with a north to south thickening wedge of non-marine and marine sediment ranging in age from Jurassic to Holocene. The major landscapes of the LMV may be considered in four regions: (1) a narrow active meander belt in a broad valley of Late Pleistocene valley train in the northern third; (2) a wide mosaic of interwoven Holocene meander belts in the middle third; (3) a relatively narrow valley of the Atchafalaya Basin bounded on each side by narrow meander belts in the upper part of the lower third; and (4) the broad distributary wedge of the deltaic plain in the southernmost region of the valley. The valley trains vary in age and landform with the oldest occurring as slightly dissected low ridges and the youngest as broad flats separated by shallow interwoven former braided channels. Meander belts formed throughout the Holocene are comprised of low natural levee ridges flanking abandoned courses and bordered by crescent-shaped oxbow lakes and ridge and swale topography. In the middle third of the valley, meander belts are separated by expansive backswamps of very little relief. The deltaic plain is also exceptionally flat, interrupted by the low natural levee ridges of the abandoned deltaic distributaries.

The floodplain of the LMV is a complex mosaic of fluvial features and landscapes within the four landscape regions. Included in this mosaic are abandoned channels and courses, lateral accretion topography of ridges and swales, natural levees, crevasses and crevasse channels, distributary channels, backswamps and rimswamps, alluvial fans and aprons, valley trains (braided stream terraces), lakes and lacustrine deltas, terraces, and the alluvial valley bluff.

Changes in the hydraulic regime of the Lower Mississippi River (LMR) since the Late Pleistocene have played a major role in the development of the landscape of the valley. The most important regime change was the diminishment of the influence of Wisconsinan glaciation in the upper Midwest and the resultant evolution of the Mississippi River from a broad braided outwash channel to a more narrow but sinuous meandering channel at the end of the Pleistocene. During the Holocene, the Mississippi River undoubtedly responded to major climatic changes, rising sea level, tributary stream influence, and possibly tectonism, diapirism, and subsidence through the growth and evolution, and abandonment of it's meander belts and deltas.     

Late Quaternary Upper Mississippi River alluvial episodes and their significance to the Lower Mississippi River system

The period in the Upper Mississippi Valley (UMV) from about 25 000 years B.P. until the time of strong human influence on the landscape beginning about 150–200 years ago can be characterized by three distinctly different alluvial episodes. The first episode is dominated by the direct and indirect effects of Late Wisconsin glacial ice in the basin headwaters. This period, which lasted until about 14 000 years B.P., was generally a time of progressive valley aggradation by a braided river system transporting large quantities of bedload sediment. An island braided system evolved during the second episode, which extended from about 14 000 to 9000 years B.P. The second episode is associated with major environmental changes of deglaciation when occurrences of major floods and sustained flows of low sediment concentration from drainage of proglacial lakes produced major downcutting. By the time of the beginning of the third episode about 9000 years B.P., most vegetation communities had established their approximate average Holocene locations. The change of climate and establishment of good vegetation cover caused upland landscapes of the UMV to become relatively stable during the Holocene in comparison to their relative instability during the Late Wisconsin. However, Holocene remobilization of Late Wisconsin age sediment stored in tributary valleys resulted in a return to long-term upper Mississippi River aggradation. The dominance of Holocene deposition over transportation reflects the abundance of sandy bedload sediment introduced from tributaries and the situation that energy conditions for floods and the hydraulic gradient of the upper Mississippi River are much less for the Holocene than they were for the Late Wisconsin and deglaciation periods.Outburst floods from glacial lakes appear to have been common in the UMV during the Late Wisconsin and especially during deglaciation. Magnitudes for the Late Wisconsin floods are generally poorly understood, but an estimate of 10 000–15 000 m³ s⁻¹ was determined for one of the largest events in the northern UMV based on heights of paleo-foreset beds in a flood unit deposited in the Savanna Terrace. For comparison, the great flood of 1993 on the upper Mississippi River was about 12 000 m³ s⁻¹ at Keokuk, Iowa, near the Des Moines River confluence where it represented the 500-year event in relation to modem flood series. Exceptionally large outburst floods derived from the rapid drainage of pro-glacial Lake Michigan and adjacent smaller proglacial lakes between about 16 000 and 15 500 years B.P. are a likely cause of the final diversion of the Mississippi River through the Bell City-Oran Gap at the upstream end of the Lower Mississippi Valley (LMV). The largest outburst flood from northern extremities of the UMV appears to have occurred between about 11700 and 10 800 years B.P. when the southern outlet of Lake Agassiz was incised. Based on the probable maximum capacity of the Agassiz flood channel 600 km downstream near the junction of the Wisconsin and Mississippi Rivers, the Agassiz flood discharge apparently did not exceed 30 000 m³ s⁻¹. However, if the Agassiz flood channel here is expanded to include an incised component, then the flood discharge maximum could have been as large as 100,000 to 125 000 m³ s⁻¹. The larger flood is presently viewed as unlikely, however, because field evidence suggests that the incised component of the cross-section probably developed after the main Agassiz flood event. Nevertheless, the large Agassiz flood between about 11 700 and 10 800 years B.P. produced major erosional downcutting and removal of Late Wisconsin sediment in the UMV. This flood also appears to be mainly responsible for the final diversion of the Mississippi River through Thebes Gap in extreme southwestern Illinois and the formation of the Charleston alluvial fan at the head of the LMV.After about 9000 years B.P. prairie-forest ecotones with associated steep seasonal climatic boundaries were established across the northern and southern regions of the UMV. The general presence of these steep climatically sensitive boundaries throughout the Holocene, in concert with the natural tendency for grasslands to be especially sensitive to climatic change, may partially explain why widespread synchroneity of Holocene alluvial episodes is recognized across the upper Mississippi River and Missouri River drainage systems. Comparison of estimated beginning ages of Holocene flood episodes and alluvial chronologies for upper Mississippi River and Missouri River systems with beginning ages for LMV meander belts and delta lobes shows a relatively strong correlation. At present, dating controls are not sufficiently adequate and confidence intervals associated with the identified ages representing system changes are too large to establish firm causal connections. Although the limitations of the existing data are numerous, the implicit causal connections suggested from existing information suggest that further exploration would be beneficial to improving the understanding of how upper valley hydrological and geomorphic events are influencing hydrological and geomorphic activity in the LMV. Since nearly 80% of the Mississippi River drainage system lies upstream of the confluence of the Mississippi and Ohio Rivers, there is a strong basis for supporting the idea that UMV fluvial activity should be having a strong influence on LMV fluvial activity. If this assertion is correct, then the traditional assignment of strong to dominant control by eustatic sea level variations for explaining channel avulsions, delta lobes, and meander belts in the LMV needs re-examination. A stronger role for upper valley fluvial activity as a factor influencing lower valley fluvial activity does not disregard the role of eustatic sea level, tectonic processes or other factors. Rather, upper valley fluvial episodes or specific events such as extreme floods may commonly serve as a “triggering mechanism” that causes a threshold of instability to be exceeded in a system that was poised for change due to sea level rise, tectonic uplift, or other environmental factors. In other situations, the upper valley fluvial activity may exert a more dominant control over many LMV fluvial processes and landforms as frequently was the case during times of glacial climatic conditions.     

Prehistoric settlement and landscape change on alluvial fans in the upper Mississippi river valley

Geoarchaeological studies of alluvial fans in the northern Sny Bottom (Upper Mississippi River valley) focused on distal fan lobe processes, the effects of small-scale landscape change on prehistoric locational decisions, and the resulting structure of the archaeological record. Aerial imagery together with coring and trenching of paleochannels on distal lobes found that frequent channel avulsion (1) produced abrupt landform/habitat changes and (2) shifted the primary loci of sediment deposition on fans through time. Analysis of wood charcoal from prehistoric occupations indicates that vegetation succession during post-avulsion overbank aggradation along a new channel was characterized by a shift from Fraxinus-dominated to mesophytic forest. Results also indicate that use of fans by Early Woodland (ca. 2550–2100 B.P.) and other foragers centered on portions along active channels. Shifting depositional loci on fans led to surfaces and depositional units of varying age, and hence a complex stratigraphic record of Holocene occupations.     

The decline in the suspended load of the Lower Mississippi River and its influence on adjacent wetlands

Since 1850, there has been an overall decrease in excess of 70 percent in the suspended load transported by the Lower Mississippi River. A decrease of 25 percent between the earliest measurements and 1950 may be partly the result of a decline in discharge and partly the result of a change in land use practices. The largest decrease occurred in 1952–53 following construction of major main-stem reservoirs on the Missouri River. Similar construction on the Arkansas River has resulted in a further decrease in 1962–63. The decrease in suspended load, combined with the artificial levee construction program and the overall enhancement of the river channel for navigation has been accompanied by an accelerating decline in land area of the Louisiana coastal zone from 17 km²/yr in 1913 to 102 km²/yr in 1980.     

Environmental constraints of human settlement in an evolving holocene alluvial system,the lower Mississippi valley

The lower Mississippi River has aggraded throughout the Holocene, resulting in stratification of cultural materials. Determination of this landscape evolution adds a temporal dimension to a model of human settlement. Sites most likely to be settled are high, well-drained surfaces near a stream. In the middle Holocene, conditions began to be suitable for settlement near Big Lake, Arkansas. A stream developed on the flood plain adjacent to a braided stream terrace. Flooding of the terrace formed a natural levee composed of well-drained, fertile, loam that was better suited for agriculture, if it existed at this time, and a living substrate than terrace sand to the west and backswamp clay to the east. Water and aquatic food sources were nearby. Open swamp conditions, present during middle Holocene, diminished by Late Holocene as bottomland arboreal habitats were becoming reestablished. This ideal environment has been intensely exploited for the past 3000 years, despite the temporary inconvenience of flooding.     

The response of the Lower Mississippi River to river engineering

An examination of the response of the Lower Mississippi River (LMR) to a variety of engineering activities is presented through the discussion of: (a) a brief history of engineering investigations and activities on the LMR; (b) the impact of artificial cutoffs on the channel geometry and water surface profiles of adjacent reaches; (c) the impact of channel alignment activities on channel morphology; and (d) the apparent impact of all of the LMR engineering activities on sediment dynamics in the channel.

Investigations by many agencies reflect over 150 years of study of the hydraulics and hydrology of the LMR, which have contributed significantly to our understanding of large alluvial rivers. In an effort to provide for flood control and navigation on the largest river in North America, private landowners and the US Army Corps of Engineers have performed a wide range of river engineering activities, including construction of levees, floodways, artificial cutoffs, bank revetment, training dikes, dredging, channel alignment, and reservoirs on the major tributaries. This unprecedented program of river engineering activities on the river during the last 100 years has resulted in the evolution of a freely meandering alluvial river to a highly trained and confined meandering channel. The LMR has increased its overall gradient and average top-bank width and generally increased its channel depth. The immediate response of the river to increased gradient as a result of the construction of artificial cutoffs was dampened in some locations by local geological controls.

Examination of the trends in sediment dynamics of the LMR reveals that the suspended load of the river has decreased during the 20th century. Conversely, a trend in the bed load transport in the channel for the years 1930 and 1989 cannot be determined with confidence because of the difficulty in acquiring representative samples. The highly trained river now responds to channel forming flows by attempting to build mid-channel bars rather than natural cutoffs of meanders.

The LMR should maintain a relatively stable plan form in the intermediate future, barring a very large and unprecedented flood. The river will continue to adjust its channel geometry and its local gradients as a response to variations in significant discharges. Continued channel maintenance and occasional dredging will insure the present state of sediment and water transport efficiency.     

The lowermost Mississippi River: a mixed bedrock‐alluvial channel

In this study, the distribution of channel‐bed sediment facies in the lowermost Mississippi River is analysed using multibeam data, complemented by sidescan sonar and compressed high‐intensity radar pulse seismic data, as well as grab and core samples of bed material. The channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls. The consolidated substratum is made up of latest Pleistocene and Early Holocene fluvio‐deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends. The exposed substratum commonly displays a suite of erosional features, including flutes that are quantitatively similar in form to those produced under known laboratory conditions. A total of five bed facies are mapped, three of which include modern alluvial deposits and two facies that are associated with the relict substratum. A radius of curvature analysis applied to the Mississippi River centreline demonstrates that the reach‐scale distribution of channel‐bed facies is related to river planform. From a broader perspective, the distribution of channel‐bed facies is related to channel sinuosity — higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment. For example, the ratio of alluvial cover to substratum is ca 1·5:1 for a 45 km segment of the river that has a sinuosity of 1·76 and this ratio increases to ca 3:1 for a 120 km segment of the river that has a sinuosity of 1·21. The exposed substratum is interpreted as bedrock and, given the relative coverage of alluvial sediment in the channel, the lowermost Mississippi River can be classified as a mixed bedrock‐alluvial channel. The analyses demonstrate that a mixed bedrock‐alluvial channel boundary can be associated with low‐gradient and sand‐bed rivers near their marine outlet.     

Geological structure of the Ubinka River valley (Northwestern Caucasus)

A thick olistostrome strata of late Paleocene-early Eocene age is outlined in the northern flank of the Northwestern Caucasus folded structure in the Ubinka river valley, which shows the significant role of earlier Cenozoic tectonic movements in forming the alpine structure of the region. The largest part of the strata is composed of dark weakly calcareous clays, which were earlier recognized as Lower Cretaceous deposits. Olistoliths and large olistoplaques are mostly of light calcareous rocks in which microfauna of Cenomanian and Maastrichtian ages were discovered in dark clays. A poor series of foraminifers was recognized in the dark clays hosting these olistoliths; this series do not enable one to determine with certainty the age of the strata (of approximately the late Paleocene-early Eocene). Small structural forms were recognized in olistoliths and olistoplaques, which are not traced in the matrix, this indicates that a series of folded and fissured structures were formed before these olistoliths and olistoplaques appeared in the olistostrome strata.     

Sediment accumulation and its effects on a Mississippi River oxbow lake

Recent sediment accumulation rates were measured in Moon Lake, a large (10.1 km²) Mississippi River oxbow lake in northwestern Mississippi. Moon Lake, which receives channeled inflow from an intensively cultivated soybean, rice, and cotton watershed (166 km²) and limited overland flow from surrounding lands, exhibited depositional patterns that were associated with (1) points of inflow, (2) flow patterns, and (3) lake morphology. From 1954 to 1965, 70 percent of the lake bottom experienced accumulation rates greater than 2 cm/yr. Accumulation rates exceeded 4 cm/yr in areas of delta formation. Changes in cropping systems during the 1960s, from cotton to soybeans and rice which require less cultivation, resulted in significantly (a = 0.01) less sediment accumulation during the period 1965- 1982 when 86 percent of the lake averaged less than 2 cm/yr sediment deposition. If current sediment accumulation rates continue, open water habitat in the lake will be reduced by only 3 to 7 percent during the next 50 years.Contribution of the Sedimentation Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Oxford, MS 38655     

Picophytoplankton and bacterioplankton in the Mississippi River plume and its adjacent waters

Picoplankton abundance and distribution in the Mississippi River plume and its adjacent waters were studied during two cruises in April (high discharge) and October (low discharge) 2000 using flow cytometry. Concentrations of photosynthetic picoplankton,Synechococcus and picoeukaryotes were low in the turbid plume water but high in the coastal waters—i.e., the green waters resulting from mixing of river and oceanic waters. In this region, three types ofSynechococcus, characterized by their phycoerythrin chromophore composition, were found:Synechococcus cells with a low phycourobilin to phycoerythrobilin ratio (PUB:PEB) occurred throughout the region and dominated the totalSymechococcus abundance during both seasons; high PUB:PEB cells, which are the dominant strains in the open or blue ocean, occurred only at the outer shelf stations; and PEB-onlySynechococcus were abundant in most of the surveyed area during april, but were not observed during October.Prochlorococcus cyanobacteria only occurred at the oceanic stations, but extended farther inshore in October compared to April. This was a consequence of the reduced discharge and plume size during October. Picophytoplankton were a less important component of total phytoplankton biomass in the turbid river water and more important in the oligotrophic Gulf water. Seasonally, the contribution of picophytoplankton to total phytoplankton biomass in the surveyed area was higher during low discharge in October than during high discharge in April, even though the spring 2000 river discharge was unusually low and might not present a typical high discharge scenario. The abundance of heterotrophic bacteria was weakly correlated to chlorophylla (chla) concentration, but better correlated to picophytoplankton biomass. A higher proportion of High DNA bacteria occurred in the river-impacted regions during both seasons, with the ratio of High DNA bacteria to Low DNA bacteria significantly higher in April.     

Perspectives on the geoarchaeology of the Lower Mississippi Valley

The impact of Harold N. Fisk's work on the archaeology and geoarchaeology of the Lower Mississippi Valley (LMV) has been monumental. As a result of his landmark publications on the geology of the alluvial valley of the Mississippi River our comprehension of the interplay between geological, geomorphic, and human actions is relatively well developed. However, geologists and archaeologists still need to work together more closely to appreciate and understand the contributions each field has to offer. Examination of the interplay between geologists and archaeologists in the realm of dating and landscape evolution provides the basis for an investigation of the state of geoarchaeology in the LMV today. Integration of research demands an appreciation of scale which must be approached from an historical perspective. Humans have, both in the past and the present, impacted the natural environment of the Mississippi River and its floodplain. Only when this fact is fully appreciated by archaeologists and geologists alike will it be possible to forge a new synthesis of the relationship between the dynamic alluvial valley and its human occupants.     

Vertical vulnerability evaluation of middle-low Esino River valley alluvial aquifer (Marche, Italy)

 The methodology applied in this research work allows the evaluation of the vertical vulnerability of the Esino River alluvial aquifer (Marche). Main factors determining the ease with which possible pollutant may come from the soil surface to reach the groundwater beneath, through a prevalently vertical path, have been evaluated. Following a "raster" logic, the study area has been discretized into a set of elemental areas. For each of these areas, dimensionless indices able to characterize the actual possibility for the groundwater to be reached by polluting substances have been estimated. Received: 25 February 1997 · Accepted: 13 November 1997     

Factors influencing the distribution of heavy metals in the alluvial soils of the Velika Morava River valley,Serbia

This investigation studies the distribution of heavy metals (Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Zn and As) in alluvial soils (Fluvisols and Humofluvisols) from the Velika (Greater) Morava river valley, over an area of approximately 110 000 ha used for intensive agricultural and food production. The sampling was carried out as composite (bulk) samples from the plough-layer (0–25 cm), and was based on a regular square grid with intervals set at 5 km. The results obtained are compatible with the average contents of heavy metals in soils, although higher contents of Ni, Pb and As occurred in several samples, particularly the Humofluvisols. Analysis of plants grown in the area under investigation have shown normal contents of heavy metals except for a slightly higher Ni content in beans and sweet-clover. Evidence is presented to show that the Humofluvisols may have received anthropogenic inputs of Pb and Ni but further work is required to confirm this.     

Quaternary paleoecology of the Lower Mississippi Valley

In 1938, Clair A. Brown published his classic paleobotanical discoveries from the Tunica Hills of southeastern Louisiana, indicating ice-age plant migrations of more than 1100 km. Brown collected fossils of both boreal trees such as white spruce (Picea glauca) and southern coastal plain plants from deposits mapped as the Port Hickey (Prairie) river terrace by Harold N. Fisk. Subsequent revisions of terrace mapping, radiocarbon dating, and paleoecological analysis reconciled Brown's conceptual and stratigraphic “mixing” of these two ecologically incompatible fossil plant groups. An older Terrace 2 (of Sangamonian to Altonian age) contains the warm-temperate assemblage. A younger Terrace 1 (of Farmdalian, Woodfordian, and Holocene age) includes full-glacial and late-glacial remains of both boreal and cool-temperate plants; and a warm-temperate suite of plants dates from the Holocene interglacial. New plant fossil localities with radiocarbon chronologies are now available from within the Lower Mississippi Valley of Missouri and Arkansas as well as from the adjacent Ozark Plateaus, the Interior Low Plateaus of Kentucky and Tennessee, and the bordering Blufflands of Tennessee, Mississippi, and Louisiana. These studies demonstrate that glacial and interglacial patterns of vegetation have been influenced by regional changes in climate, glacial runoff, and regime of the Mississippi River.     

Urbanization and agricultural intensification in the Lower Fraser River valley,: Impacts on water use and quality

The Fraser River drains a quarter of the province of British Columbia and is renown as one of the greatest salmon rivers in the world. The Fraser watershed is under threat from rapid urbanization in its lower reaches as pollutants generated from domestic and industrial wastes, and stormwater runoff threaten the health of the aquatic ecosystem. Plans for secondary sewage treatment in the metropolitan area will reduce waste loadings of some contaminants, but rapid growth will soon offset these benefits. Intensive urbanization, spreading up the Lower Fraser valley, with a network of impervious roads and increased traffic volumes contribute pollutants in surface runoff that are difficult to control. Rapid growth in agrobusiness to provide fresh produce to the nearby markets has resulted in very high densities of farm animals and excessive use of fertilizers and pesticides on the land. These pollutants threaten the groundwater and many of the remaining salmonid streams in the lower valley. The Fraser Basin Management Program, the Fraser River Estuary Management Program, and a variety of government agencies are coordinating studies and analyzing issues and conflicts to develop a strategy for sustainable development in the region.     

Loess stratigraphy of the Lower Mississippi Valley

Loesses of the Lower Mississippi Valley (LMV) are world-famous. Sir Charles Lyell (1847), Hilgard (1860), Stafford (1869), Call (1891) and Mabry (1898), thought the LMV loess was a single water deposit although “double submergence” was noted by Call (1891) and Salisbury (1891). Shimek (1902) and Emerson (1918) recognized LMV loess as a wind deposit which came from the valley. Although wind-deposited loess gained wide acceptance, Russell (1944a) published his controversial theory of “loessification” which entailed weathering of backswamp deposits, downslope movement and recharge by carbonates to form loess. Wascher et al. (1947) identified three LMV loesses, mapped distributions and strongly supported eolian deposition. Leighton and Willman (1950), identified four loesses and supported eolian deposition as did Krinitzsky and Turnbull (1967) and Snowden and Priddy (1968), but Krinitzky and Turnbull questioned the deepest loess. Daniels and Young (1968) and Touchet and Daniels (1970) studied the distribution of loesses in south-central Louisiana. West et al. (1980) and Rutledge et al. (1985) studied the source areas and wind directions which deposited the loesses on and adjoining Crowley's Ridge. B.J. Miller and co-workers (Miller et al., 1985, 1986, Miller and Alford, 1985) proposed that the Loveland Silt was Early Wisconsin rather than Illinoian age and advanced the name Sicily Island loess. They proposed the underlying loess was Illinoian and advanced the name Crowley's Ridge. We termed the loesses, from the surface downward, Peoria Loess, Roxana Silt, Loveland/Sicily Island loess, Crowley's Ridge Loess and Marianna loess. Researchers agree that the surfical Peoria Loess is Late Wisconsin and the Roxana Silt is Late to Middle Wisconsin, but little agreement exists on the age of the older loesses. Pye and Johnson (1988) proposed Early Wisconsin for the Loveland/Sicily Island. McKay and Follmer (1985) suggested this loess correlated with a loess under Illinoian till. Clark et al. (1989) agreed on Crowley's Ridge, but suggested the Loveland/Sicily Island loess on Sicily Island was older. Mirecki and Miller (1994) and Millard and Maat (1994) suggested an Illinoian age for the Loveland/Sicily Island loess. Miller and co-workers suggested, as did Pye and Johnson (1988), an Illinoian age for the Crowley's Ridge loess. McKay and Follmer (1985) suggested it correlated with a loess under “Kansan” till. Stratigraphy indicates the Marianna is the older of the five loesses.

Researchers identified loess on both the east and west side of the LMV as well as on higher terraces within the valley. Many researchers assumed unaltered loesses were commonly yellowish brown, and silts or silt loams (West et al., 1980; Miller et al., 1986). The nonclay fraction of unweathered LMV loesses was dominated by quartz followed * Corresponding author. by carbonates, mainly dolomites, followed by feldspars, and micas. Clays were dominated by montmorillonite followed by micaceous minerals, kaolinite and vermiculite (Miller et al., 1986). Soils in the Crowley's Ridge loess are most developed, followed by the soils in the Loveland/Sicily Island which are more developed than the modern soils in the Peoria Loess. Soils in the Roxana and Marianna loesses are least developed and the Farmdale Soil of the Roxana is the weaker of the two (Miller et al., 1986). There is certainly overlapping range in the degree of soil development in the various loesses.     

A re-assessment of the Upper Mississippi valley lead isotope data

Corrections for systematic bias, and a better regression method, are tested on the lead isotope data of Heyl, Delevaux, Zartman and Brock (1966). The new calculations suggest the time of mineralization in the Upper Mississippi zinc-lead district is more likely to lie in the range Mid Devonian (370 my) to Upper Pennsylvanian (280 my) on any reasonable assumption for the age of the basement. It is argued from the isotopic trends that the ore lead in this district was simply derived by radiogenic addition to lead originally derived from rocks of basement age, although the data do not exclude residence in the Lower Paleozoic sediments as an intermediate step. An isotopically different magmatic component is not required. Correlation with the geology suggests that genesis of the deposits probably was related to regional tectonic deformation in Middle to Late Paleozoic time.

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Zusammenfassung Korrekturen systematischer Fehler und eine bessere Regressionsmethode wurden anhand der Blei-Isotopendaten von Heyl, Delevaux, Zartman und Brock (1966) getestet. Diese neuen Berechnungen unter Annahme eines vernünftigen Alters des Grundgebirges zeigen, daß die Zeit der Mineralisation im Oberen Mississippi Zink-Blei Distrikt eher zwischen Mitteldevon (370 Millionen Jahre) und Oberkarbon (280 Millionen Jahre) zu setzen ist. Auf Grund der Verteilung der Isotopenverhältnisse wird auf eine einfache Formierung (durch Addition radioaktiver Zerfallsprodukte zum ursprünglichen Blei im Gestein mit demselben Alter wie das Grundgebirge) des Bleies im Bleierz geschlossen. Die Meßresultate schließen jedoch eine Ablagerung im Unterpaläozoikum als Übergangsstufe nicht aus. Magma mit andern Isotopenverhältnissen wird nicht angenommen. Vergleiche mit der Geologie deuten darauf hin, daß die Bildung der Ablagerungen im Zusammenhang mit tektonischen Deformationen im Mittleren bis Oberen Paläozoikum erfolgte.

     

Geological and anthropogenic influences on sediment metal composition in the upper Paracatu River Basin, Brazil

This work reports a geochemical study of sediments from the upper Paracatu River Basin. The objective is to define the influences of Au, Zn, and Pb mineral deposits and mining activities on the sediment metal sources, distribution, and accretion. The samples were analyzed using ICP/OES, AAS, and XRD techniques and were treated with principal components analysis and the geo-accumulation index. The main geochemical processes that control the sediment composition are pyrite oxidation, muscovite weathering, carbonate dissolution, and the erosion of oxisols enriched with Zn and Pb. The upper Rico Stream has high Al, Fe, Cu, Cr, Co, and Mn concentrations due erosion of oxisols and pyrite oxidation and muscovite alteration present in the parental rock. The artisanal alluvial gold mining increased the primary rock-minerals?? weathering and Hg sediment concentration. The lower Escuro River and Santa Catarina Stream are enriched with Zn and Pb due the erosion of metal-rich soils formed over galena, sphalerite, calamine, and willemite mineral deposits located upstream. Elements such as Ca, Mg, and Ba have low concentrations throughout the sampled area due the high solubility of these metals-bearing minerals. The dispersion of metals is limited by the basin geomorphology and their affinity to silt-clayey minerals and Fe and Mn oxides and hydroxides in circumneutral pH waters.