Modern Alluvium Research Articles

Quartz luminescence sensitivity enhanced by residence time in the critical zone

The emerging use of quartz luminescence properties to characterize Earth-surface processes shows promise, with optically stimulated luminescence (OSL) sensitivity proposed as a valuable tool for provenance or sediment history tracing. However, the geologic processes that lead to quartz sensitization remain unclear. Here we study the impact of source rock and surface processes on the luminescence properties of quartz sand from bedrock and modern and Late Pleistocene alluvium generated from a mountainous catchment in northern Utah, USA. Continuous wave and linear modulated OSL are used to characterize the luminescence sensitivity and intensity of the fast-decay component. We compare the OSL sensitivity with sand-grain provenance and with proxies for surface processes such as topographic metrics, cosmogenic 10Be-derived erosion rates, chemical weathering indices, and magnetic susceptibility. Late Pleistocene sediment has low OSL sensitivity and a weak fast-decay component, similar to bedrock samples from the source area. In contrast, modern alluvium is dominated by the fast-decay component and has higher and more variable OSL sensitivity, with no clear relationship to upstream bedrock source. There is, however, an inverse relationship between OSL sensitivity and catchment-averaged erosion rates and a positive relationship with chemical weathering indices and magnetic susceptibility. These metrics suggest that the modern alluvium has experienced increased residence time in the shallow critical zone compared to the Late Pleistocene sediments. We suggest that changes in hillslope processes between the effectively wetter, cooler Pleistocene and the dryer, warmer conditions of the Holocene enhanced the luminescence properties. The results suggest that climatic controls on rates and processes of chemical and mechanical weathering and sediment transport and residence within the critical zone are encoded in the luminescence properties of quartz sand.

New geophysical and zircon data support an impact hypothesis for the Velingara structure (Senegal)

On Earth, impact structures are rare in intertropical zones. Here we evaluate the 35–40 km diameter Velingara depression in Senegal as a candidate impact structure. The depression has a topographic relief of only 50 m and is essentially buried under modern sediments. For the first time, potential-field ground observations and a microstructural survey of detrital zircons for shock features were conducted at Velingara. A 15 mGal gravity low in a 10 km diameter central anomaly was found, while significant (100 nT) amplitude magnetic field anomalies were observed in the depression. Forward modeling was used to explain the crustal sources of the anomalies, arguing in favor of the presence of a buried impact structure. An electron backscatter diffraction survey of circa 5,000 detrital zircons from a sample of modern alluvium identified a single granular zircon; orientation data for the grain does not record the former presence of reidite, but does closely resemble grains with highly dispersed neoblast orientations that have only been reported from impact settings. Age determination by LA-ICP-MS indicates the unusual granular zircon recorded an age of ca. 550 Ma. Results of the geophysical data and zircon microstructural survey reported here are consistent with, but do not yet prove, an impact origin hypothesis for the Velingara structure. The difference in amplitude between the diameter of the depression, and that of the gravity anomaly remains puzzling. Several impact scenarios are considered, which include the subsequent evolution via surface processes of a 10-km crater into a larger depression, or a large-scale (d = 40 km) complex impact crater with a peculiar geophysical anomaly limited to its central region.

Commercial products of mining and metallurgical companies in river sediments of industrial regions

Modern alluvium is a complex multicomponent system that includes both natural and man-made material. In the process of research on the pollution of river sediments with industrial waste, the authors established the presence of commercial products of mining and processing enterprises in the sediments. It includes crushed granite, metallurgical slags, ores and concentrates, coal and other products. Significant volumes of inputs of these materials into the ecosystem motivate the development of special technologies aimed at additional production of mineral products through the complex processing of modern river alluvium. In addition to economic benefits, the implementation of these projects will improve the state of the environment in regions with significant technogenic load.

How old are the Jack Hills metasediments really?: The case for contamination of bedrock by zircon grains in transported regolith

Abstract The enigmatic occurrence of Neoarchean- and Proterozoic-aged zircon grains in some samples of metasedimentary rocks from the Jack Hills, Western Australia, that otherwise appear to have been deposited before 3.0 Ga is explained by contamination with embedded zircon grains sourced from transported regolith. Zircon U-Pb age spectra obtained by laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of modern alluvium in creek beds draining the metasedimentary belts of Mount Narryer and Jack Hills are dominated by these exotically sourced younger components, as is the red soil that partly covers and infiltrates fractures in the exposed Hadean zircon-bearing bedrock at Jack Hills. The consistency of the age patterns found in superficial deposits sampled over some 100 km implies a dominance of colluvium transported from the wider Yilgarn craton, adjacent Capricorn orogen, and more distal sources rather than material produced from the local (Mesoarchean and older) bedrocks. The potential for similar cases of “environmental contamination” of surface rocks by exogenous zircons needs to be considered more widely, particularly in situations where interpreted maximum deposition ages are based on only a small number of outlying youngest analyzed grains.

The Internal Structure of Placer Gold of the North-East of the Siberian Platform as a Reliable Search Sign of Primary Sources

Typomorphic features of placer gold are carriers of the most important information necessary for the reconstruction of the history of the formation of placer and ore occurrences of gold and can be used as mineralogical criteria in the search for primary sources of placers. The study of these features is relevant for placer areas with unidentified gold sources, which include the territory of the north-east of the Siberian Platform. The internal structure of native gold is one of the most informative typomorphic features is. We studied the internal gold structures of modern placers and conglomerates of the Early Permian age in the north-east of the Siberian Platform. The purpose of the work: to identify the features of the internal structure of placer gold, depending on the chemical composition and the degree of its transformations, as well as the possibility of their use as a prospecting indicator. Structural etching of native gold was performed using the reagent: HCl + HNO3 + FeCl3 × 6H2O + CrO3 + thiurea + water. As a result of the research, it was found that the well-rounded high-grade gold of modern placers has undergone repeated redeposition through intermediate sources. The discovery of slightly rounded gold with an internal structure without signs of exogenous transformation indicates that it entered the modern alluvium from a nearby primary source. This was a prerequisite for setting up ore gold exploration, resulting in identification of apocarbonate hydrothermal-metasomatic formations with disseminated gold-sulfide mineralization were identified. Thus, the features of the internal structures of placer gold in combination with other typomorphic features are additional indications of determining the sources of formation of gold-bearing placers.

Mineralogical features of gold of the Permian conglomerates of the Sololiy uplift of the Olenek high (Northeastern Siberian platform)

Typomorphic features of placer gold of conglomerates of the Ortokinskaya formation of the Early Permian age (P1or) of the Sololiy Uplift of the Olenek High and modern channel alluvium of the Ortoku Eekit river, which drains these deposits, are studied. It is found that extremely heterogeneous composition and complex inner structure are typical features of the vast majority of gold of the Permian conglomerates. There are several stages of different fineness (from 251‰ to 999‰) within a single gold particle, as a result, gold has a mosaic-block inner structure. It is proposed that such heterogeneity is formed due to epigenetic transformations of primary medium-grade gold in an intermediate reservoir of the Permian age, and also under influence of superposed hydrothermal mineralization with low-grade gold, related to the processes of the Mesozoic tectonomagmatic activation. The latter is indicated by the discovery of impregnated gold-sulfide mineralization in the Permian conglomerates. Ore mineralization is represented by gold, silver, pyrite, sphalerite, chalcopyrite, galena, argentite, native tin, and barite. Association mineralization to thin tectonic zones of conglomerate fragmentation, suggests that superposed gold-sulfide hydrothermal mineralization of impregnated type was formed as a result of manifestations of the Mesozoic tectonomagmatic activation, when gold-bearing hydrothermal solutions migrated in faults. It is determined that auriferous conglomerates of the Ortokinskaya formation of the Permian age were major sources of placer gold of modern alluvium in the studied part of the Ortoku Eekit.

Фациально-генетическое расчленение современного аллювия в нижнем течении р.Куи на северо-западе Большеземельской тундры

Фациально-генетическое расчленение современного аллювия в нижнем течении р.Куи на северо-западе Большеземельской тундры

Landscape dynamics revealed by luminescence signals of feldspars from fluvial terraces

Luminescence signals of quartz and feldspar minerals are widely used to determine the burial age of Quaternary sediments. Although luminescence signals bleach rapidly with sunlight exposure, incomplete bleaching may affect luminescence ages, in particular in fluvial settings where an unbleached remnant signal is commonly encountered in modern alluvium. Here, we use feldspar single-grain post-infrared IR stimulation (pIRIR) dating to show that recent (<11 ka) fluvial terraces of the Rangitikei River (New Zealand) were formed in a context of non-linear incision rate. We relate this pattern to the rapid reinstatement of steady-state incision following the formation of a major, climate-driven, aggradation terrace, causing a phase of accelerated incision. In addition, we show systematic variations in the proportion of unbleached grains in the fluvial sediments over time, mirroring incision rate at the time of deposition. Deposits formed during rapid incision contain fewer bleached grains, which we attribute to large input of unbleached material and limited bleaching opportunities during fluvial transport. This finding demonstrates that the luminescence signals recorded in fluvial terraces not only yield age information, but also inform us on past fluvial transport and ultimately, landscape dynamics.

Fluvial transport of impact evidence from cratonic interior to passive margin: Vredefort-derived shocked zircon on the Atlantic coast of South Africak

Meteorite impacts produce shocked minerals in target rocks that record diagnostic high-pressure deformation microstructures unique to hypervelocity processes. When impact craters erode, detrital shocked minerals can be transported by fluvial processes, as has been demonstrated through studies of modern alluvium at some of the largest known impact structures. However, the ultimate fate of distally transported detrital shocked minerals in fluvial systems is not well understood and is an important parameter for constraining the location of a source crater. In South Africa, detrital shocked minerals from the 2020 Ma Vredefort impact structure have been documented in the Vaal River basin, downriver from the structure. Here, we report results of an extensive microstructural survey of detrital zircon from the Orange River basin and the Atlantic coast of South Africa to search for the presence of far-traveled Vredefort-derived detrital shocked zircon grains in different modern sedimentary environments. Three shocked grains were found out of 11 168 grains surveyed (0.03%) by scanning electron microscopy, including two in beach sand on the Atlantic coast and one from a sandbar 15 km upstream from the mouth of the Orange River. Shock-produced {112} twins documented by electron backscatter diffraction in each of the three grains confirm their impact provenance, and U-Pb ages from 3130 to 3040 Ma are consistent with derivation from bedrock at the Vredefort impact structure. These results demonstrate the transport of Vredefort-derived shocked zircon to the coast via the Vaal-Orange river system, which requires 1940 km of fluvial transport from their point source on the Kaapvaal craton to the Atlantic coast passive margin. These results further demonstrate that shocked zircon grains can be detected in detrital populations at abundances

Surficial geology of the Woodstock, IL 7.5 minute Quadrangle, Mchenry County, Illinois, scale 1:24,000

ABSTRACTThe surficial geology of the Woodstock 7.5-minute Quadrangle, which is located in the northwest part of the Chicago-Metropolitan Area, was mapped to better address natural resource issues and better understand local geologic history. In the study area, the population is dependent completely upon groundwater for water supplies, and groundwater withdrawals are expected to increase dramatically in coming decades. The landscape and surficial geologic materials in the Woodstock Quadrangle are a result of numerous glaciations during the Quaternary. Surficial sand and gravel deposits, along with buried outwash deposits from older glacial events, constitute important aquifer and aggregate resources for local residents and industry. The landscape within the Woodstock Quadrangle includes glacial moraines, ice-walled lake plains, kettles, incised valleys, and glacial lacustrine deposits. These landforms, coupled with the thick glacial sediments, record a complicated geologic history associated with glacial processes and paleoclimate change. The surficial geologic map of the Woodstock Quadrangle consists of seven lithostratigraphic units associated with the most recent Wisconsinan glaciations in North America. These geologic units include multiple till deposits, outwash sediments, lacustrine sediments, and modern alluvium and peat deposits. This map provides the base geologic knowledge for planners, decision-makers, and other scientists.

Distribution and provenance of modern pollen and spores in the surface sediments of Liaodong Bay, China

To elucidate modern pollen and spore distribution and provenance based on their dispersal patterns, we analyzed 72 surface sediment samples from Liaodong Bay, China and 39 analogous samples from its five inflowing rivers. Our results reveal that most of the pollen and spores in the nearshore marine sediments (water depth <8m) originated from riverine input and that their assemblages corresponded well to the watershed vegetation. Variation in the pollen assemblages in different parts of the marine area could reflect differences in local vegetation. The content of herbaceous pollen decreased with increasing distance offshore, while arboreal pollen and spores increased. Due to the differences in the sedimentary environments and the source areas of pollen and spores, pollen concentrations in the marine area were higher than those in analog sediments in the modern alluvium of the inflowing rivers. The highest pollen concentrations occurred in the northwestern area of Liaodong Bay, where pollen was derived from multiple inflowing rivers. According to the distribution characteristics of pollen and spores from alluvium to marine sediments, it can be inferred that most pollen grains such as Artemisia, Chenopodiaceae, Gramineae, Cyperaceae, Typha, deciduous Quercus, Betula, and Corylus pollen were primarily transported through the inflowing rivers, whereas Pinus and Carpinus pollen were likely to have been transported by wind. The results of DCA [detrended correspondence analysis] analysis and analog dissimilarity analysis suggested that the pollen-spore distribution in the nearshore area of Liaodong Bay mainly reflects fluvial and marine hydrodynamics or water sorting effects, as well as the vegetation distribution onshore. These findings are of critical importance to the interpretation of Quaternary marine pollen data from Liaodong Bay, China.

Preservation of detrital shocked minerals derived from the 1.85 Ga Sudbury impact structure in modern alluvium and Holocene glacial deposits

Detrital shocked minerals can provide valuable residual records of eroded impact structures. Recent studies have reported shocked minerals in modern alluvium in a subtropical climate from the deeply eroded 2.02 Ga Vredefort Dome impact basin in South Africa. To evaluate the detrital shocked mineral record at a large impact structure in a temperate setting with a Holocene glacial erosional history, we investigated ∼4000 detrital zircons and ∼20,000 quartz grains at the less-eroded 1.85 Ga Sudbury Basin in Ontario, Canada, for the presence of shocked sand grains. Modern alluvium from rivers within and outside the basin, and Holocene glaciofluvial sands (eskers and outwash deltas) across the basin were investigated for shocked minerals. Shocked zircon and/or quartz were found in all modern rivers and most Holocene glacial deposits within, but not outside, the basin. Petrography and scanning electron microscopy (SEM; backscattered electron [BSE]; cathodoluminescence [CL]) imaging and analysis (energy-dispersive X-ray spectroscopy [EDS], electron backscatter diffraction [EBSD]) were used to document shock microstructures. Of the total detrital zircons surveyed, ∼3% (118/3978) were identified as shocked; Holocene samples contained higher average percentages of shocked zircon (63/1361, or 4.6%, with a high of 29%) compared to modern alluvium (55/2617, or 2.1%, with a high of 6%). EBSD analysis revealed a range of shock microstructures, including planar fractures, deformation microtwins, and crystal plastic deformation. At Sudbury, detrital shocked quartz is rare compared to zircon; only 15 grains (∼0.08%) were identified, all with decorated planar deformation features (PDFs). These results demonstrate that a detrital shocked mineral record exists at a large impact basin that is in a youthful stage of erosion, despite its age. In addition to modern alluvium, our results also identify glaciofluvial eskers and deltas as reservoirs for detrital shocked minerals; glacial episodes thus enhance the dispersal and preservation of shocked detritus in sedimentary systems. Despite physical differences, the observation that the two largest Precambrian impact basins continue to contribute detrital shocked minerals 2 b.y. after impact suggests that a shocked mineral record of impacts on early Earth should reside in Precambrian siliciclastic rocks.

Insights into southern Appalachian tectonics from ages of detrital monazite and zircon in modern alluvium

Ages of detrital monazite and zircon from alluvium collected from the French Broad River drainage basin, an orogen-crossing main trunk river, and alluvium in first-order tributary streams, provide an unconventional perspective for examining the regional tectonic and metamorphic history of the southern Appalachian orogen (eastern United States). The French Broad River system samples migmatitic Ashe–Tallulah Falls suite paragneisses (with inferred Neoproterozoic clastic protoliths) of the Eastern Blue Ridge and western Inner Piedmont, Mesoproterozoic basement orthogneisses, numerous Paleozoic metaplutonic gneisses, and tectonite equivalents of these lithologies in the Brevard fault zone. Middle Ordovician ages dominate the monazite age spectrum. Monazite from tributaries has a dominant 208 Pb- 232 Th age peak ca. 463 Ma. Monazite from the French Broad River alluvium suite yields a dominant 208 Pb- 232 Th age mode ca. 450 Ma, but differs from the tributaries in having scattered Mesoproterozoic, Silurian–Devonian, and Carboniferous ages. Electron microprobe total Th-U-Pb chemical ages for selected tributary monazite grains also analyzed by ion microprobe reveal additional monazite growth events (i.e., metamorphic reaction) at 480–475 Ma and 445–440 Ma. Tributary and French Broad River zircon age spectra are dominated by Mesoproterozoic and Ordovician grains. Most Ordovician zircon from the French Broad River has Th/U > 0.1 and is most likely derived from the Henderson orthogneiss (447.6 ± 5.4 Ma), the largest pluton in the French Broad River headwaters region. A minor zircon age population at 450 Ma, represented primarily by metamorphic zircon rims with Th/U Ordovician monazite ages and zircon rim ages correspond to Taconian metamorphism in the Eastern Blue Ridge province. The dominant monazite age mode (463 Ma) from tributaries is slightly older than the Ordovician metamorphic zircon age mode (450 Ma), which is accounted for by monazite growth primarily via prograde metamorphic reactions, and zircon growth by melt-forming reactions in migmatites at the thermal peak. The scattered middle to late Paleozoic zircon and monazite ages attest to the lack of significant thermotectonic and magmatic events of that age in the Southern Blue Ridge providing sediment to the French Broad River drainage system. This dearth of ages is consistent with the pattern of nonpenetrative late Paleozoic deformation, retrograde metamorphism, and scattered plutonism northwest of the Brevard fault zone. A reasonable source of Neoproterozoic zircon in alluvium or in Ashe–Tallulah Falls paragneisses is Neoproterozoic rift-related magmatic rocks. The rarity of Mesoproterozoic monazite compared to zircon contrasts with the marked abundance of monazite in Ashe–Tallulah Falls paragneisses, and emphasizes the responsiveness of monazite compared to zircon in regional metamorphism. The abundant Mesoproterozoic zircon was inherited from Mesoproterozoic basement lithologies by Neoproterozoic sediments, preserved through regional metamorphism and three phases of orogenesis, and persists in modern alluvium being shed by the orogen.

Kimberlites of the Manchary pipe: a new kimberlite field in Central Yakutia

This paper reports new petrographic and mineralogical data on the Manchary kimberlite pipe, which was discovered south of Yakutsk (Central Yakutia) in 2007–2008, 100 km. The pipe breaks through the Upper Cambrian carbonate deposits and is overlain by Jurassic terrigenous rock masses about 100 m thick. It is composed of greenish-gray kimberlite breccia with a serpentine-micaceous cement of massive structure. The porphyry texture of kimberlite is due to the presence of olivine, phlogopite, and picroilmenite phenocrysts. The SiO 2 and Al 2O 3 contents of the groundmass are indicative of typical noncontaminated kimberlites. The groundmass has a significant content of ore minerals: Fe- and Cr-spinels, perovskite, magnetite, and, less commonly, magnesian Cr-magnetite. Pyropes occur in kimberlites as sharp-edged fragments and show uneven distribution. Chemically, they belong to lherzolite, wehrlite, or nondiamondiferous dunite–harzburgite parageneses. Garnets corresponding to lherzolites of anomalous composition make up 8%; this is close to the garnet content of Middle Paleozoic kimberlites from the Yakutian kimberlite province. The pyropes from the new pipe are compositionally similar to those from diamond-poor Middle Paleozoic kimberlites in the north of the Yakutian diamondiferous province. Chemically, pyropes from the Manchary pipe and those from the modern alluvium of the Kengkeme and Chakyya Rivers differ substantially. Consequently, the rocks of the pipe could not be a source of pyropes for this alluvium. They probably occured from other sources. This fact along with numerous “pipelike” geophysical anomalies, suggest the existence of a new kimberlite field in Central Yakutia.

The oxygen isotopic composition as an indicator of the genesis of “Basaltic” corundum

This paper reports an oxygen isotopic study of corundums and associated minerals from the Yogo lamprophyres (Montana, US), plagioclase-corundum inclusions in alkali basalt (Tunkin depression, Russia), and from modern alluvium of Podgelbanochnyi alkaline basaltic volcano (Primorye, Russia). It is shown that all sapphires genetically related to mafic magmatic rocks have a similar oxygen isotopic composition (the variations of δ18O are within 2.5‰) with most values plotting between +4.5 and +7.0‰ SMOW. The oxygen isotopic ratios in the associated minerals (olivines, pyroxenes, mica, and others) and host rocks are plotted in the same interval. This indicates that the sapphire crystallized during evolution of the parental mafic magma. However, there are xenogenic corundums, which were only transported by basaltic magma to the Earth’s surface. They have a sharply distinct oxygen isotopic composition, which suggests their disequilibrium with the host lavas, and, correspondingly, a different genetic nature.

Ultrahigh pressure macro diamonds from Copeton (New South Wales, Australia), based on Raman spectroscopy of inclusions

Mining of Cenozoic alluvial deposits at Copeton and Bingara (Eastern Australia) has produced two million macrodiamonds (0.25 ct median size). Raman spectroscopy is used to identify included minerals within uncut Copeton diamonds, with sealed chamber remnant pressures of 31.7 to 35.6 kbar for coesite, 13.6 and 22.7 kbar for clinopyroxene, and 7.6 kbar for grossular garnet. Assuming elastic behaviour, these values generate inclusion entrapment PT loci which intersect, restricting diamond formation conditions: from 250 °C, 43 kbar to 800 °C, 52 kbar. Larger than error (± 100 °C and ± 4 kbar), this range shows a systematic variation in inclusion composition with diamond zoning and N properties. Published research shows 1) Copeton and Bingara diamonds are unique, and 2) modern alluvium in the Bingara district carries mantle-formed garnet, captured by post-tectonic alkali basalt from an extensive diamondiferous ultrahigh pressure (UHP) terrane that stalled at depth because it is dominated by mafic eclogite. The combined Raman and geological results indicate two sets of subduction UHP diamond formation conditions/protolith are required, firstly cooler oceanic slab and secondly including higher temperature continental crust. The Copeton and Bingara stones are UHP macrodiamonds, and Carboniferous 40Ar/ 39Ar age dates on clinopyroxene inclusions should be interpreted as ages of crystallisation, representing the termination of subduction. The characteristic features of ruptured inclusions and etched percussion marks on Copeton and Bingara diamond indicate volcanic delivery to the earth's surface. Alluvial deposits elsewhere in Eastern Australia may carry similar diamond along with diamond of different origin.

Clay coatings from a modern soil chronosequence: A tool for estimating the relative age of well-drained paleosols

Blue light optically stimulated luminescence (OSL) dates obtained from upland and terrace soil profiles in southeastern Mississippi have been used to generate a model for illuvial clay accumulation in deeply weathered, subtropical soils (e.g. Ultisols). The illuvial clays developed in upland vs. alluvial terrace soils of southeastern Mississippi are distinct in terms of thickness and total volume of translocated clay. The coatings in the Mississippi soils are also an analogue for clay coatings observed in mid-Cretaceous paleosols of North America. Many of the Cretaceous paleosols had a polygenetic history. The relative age of the well-drained phase of pedogenic development, characterized by illuvial clay accumulations, may be estimated using the model generated from the Mississippi soils. As many of the mid-Cretaceous paleosols mark unconformities, it would be beneficial to estimate the duration of the hiatuses. Three geomorphic surfaces characterize the landscape of southeastern Mississippi: uplands, stream-cut terraces, and modern alluvium. The present floodplain, has poorly developed soils, and lacks illuvial accumulations. Soils developing in the uplands have abundant and prominent illuvial clay coatings up to 2 mm thick, while the terraces have minor clay coatings less than 1 mm thick.

Origin, distribution and age of high-level chert gravels (Plio-Pleistocene) in eastern Kansas

It is probable that the rounded flint has partly originated from these [Flint] hills. G.C. Broadhead (1880).High-level (upland) chert [flint] gravels occur in almost all of the counties in the Osage Cuesta country of eastern Kansas. The locations of the high-level chert gravel deposits now are well known as a result of recent remapping of the state on a scale of 1:24:000, but the origin and age of the chert gravels remain as debatable today as they were 125 years ago. The gravels are composed of brownish, smooth, angular to subangular chert pebbles and cobbles ranging up to 6 inches (15 cm) in size.The chert clasts have been derived from Lower Permian limestones that contain abundant chert [flint] and crop out in the topographically high Flint Hills in east-central Kansas. The chert weathers from the limestones to create lag gravels, which in turn are reworked and transported eastward and southeastward by streams.In the glaciated region in northeastern Kansas, remnants of high-level chert gravel deposits occur in association with glacial tills, outwash, and erratics. In places the chert gravels occur beneath glacial deposits. South of the glaciated region, however, the remnants of the high-level gravels occur on bedrock surfaces whose elevations range from 20 to 200 feet (6 to 60 m) above the floodplains of major streams.The chert itself that forms the gravels is known to be Early Permian in age, based on fossils preserved in the pebbles and cobbles. Deposition of the high-level gravels, however, cannot be dated directly, but information bearing on their relative ages is provided by the degree of soil development over them and their elevations with respect to modern floodplains. Deposits at higher elevations may be Pliocene, whereas deposits at lower elevations may be early to middle Pleistocene. Chert gravels that occur on low-lying floodplains and in modern alluvium and are judged to be late Pleistocene to Recent in age.

The micromammal fauna of the Dnieper modern channel alluvium: taphonomic and biostratigraphic implications [ La faune des petits mammifères des alluvions du lit fluvial actuel du Dniepr : implications taphonomiques et biostratigraphiques.

Cet article présente une étude des restes des petits mammifères des alluvions du lit fluvial actuel du Dniepr. L'obtention d'une information pertinente à partir de cette taphocénose mixte est rendue possible grâce à l'existence d'une relation entre le degré de fossilisation des os et leur âge géologique relatif. II existe ainsi trois stades de fossilisation selon l'âge géologique des restes. Les âges des groupes de fossiles déterminés sur la base des indices du niveau d'évolution des Arvicolidés sont rapportés à l'Eémien et au Weichselien ancien, au Weichselien moyen et récent et à l'Holocène. Quoique les fossiles holocènes soient les plus proches de la période de formation des alluvions, les fossiles les plus anciens (première moitié du Pléistocène supérieur) dominent dans la structure de cette taphocénose. Il est évident qu'ils ont été redéposés dans l'actuelle plaine d'inondation à partir de terrasses du Pléistocène récent (Ie, IIe, et peut-être IIIe). Cela permet d'espérer trouver de petits mammifères dans les dépôts de ces terrasses qui jusqu'à présent n'ont pratiquement pas fourni de caractérisation microthériologique. II est à supposer que la composition des associations et les mécanismes de leur formation sont communs à la plupart des taphocénoses mixtes. Ainsi dans leur étude il faut attacher une grande attention à la partie la plus jeune de l'ensemble général des fossiles, la plus avancée du point de vue d'évolution. C'est elle qui sera synchrone au moment de la formation de la séquence alluviale , les fossiles plus anciens seront considérés comme redéposés même s'ils dominent quantitativement. En outre, l'analyse morphotypique des fossiles de Arvicola terrestris de la taphocénose des alluvions actuelles du Dniepr et l'étude des Arvicola récents des diverses zones climatiques de l'Europe orientale démontrent la possibilité de certaines fluctuations des indices du niveau d'évolution chez les Arvicolidae sous l'influence des changements de l'environnement. Ce phénomène doit ainsi influer sérieusement sur la précision de la date des sédiments, sur la base des indices du niveau d'évolution des Arvicolidae.

Types, abundances and distribution of kimberlite indicator minerals in alluvial sediments, Wawa–Kinniwabi Lake area, Northeastern Ontario: implications for the presence of diamond-bearing kimberlite

In response to the discovery of diamonds within modern alluvium in the glaciated area of Wawa, Ontario, Canada, the Ontario Geological Survey undertook a regional program of surficial mapping and modern alluvial sediment sampling to assess the potential of the area for diamond-bearing kimberlite. Five varieties of kimberlite-derived indicator minerals were recovered and the composition of three varieties was evaluated, resulting in the identification of G10 Cr-pyrope garnet, inclusion field chromite and Mg-ilmenite. The distribution of indicator minerals was examined in the context of the glacial and bedrock geology. Glacial dispersal from non-kimberlitic marker units is restricted (commonly less than 200 m) and many kimberlite indicator minerals were recovered from samples collected close to cross-cutting NE–SW and NW–SE faults and a strong NE–SW trend in the bedrock associated with the Kapuskasing Structural Zone. From this, several potential exploration targets for diamond-bearing kimberlite are defined.