Silt-sized Material Research Articles

Characterisation of aeolian sediments from the Strzelecki and Tirari Deserts, Australia: Implications for reconstructing palaeoenvironmental conditions

In arid environments dominated by desert dunes, aeolian sediments represent a major source of potential palaeoenvironmental information. However, the extent to which dune sedimentology can be used for past environmental reconstruction is unclear. This paper assesses the value of dune sediments as palaeoenvironmental indicators by investigating the sedimentological characteristics of linear dunes in the Strzelecki and Tirari Deserts of central Australia. We use thin section microscopy, particle size analysis and X-ray Diffractometry (XRD) mineralogy to characterise the sediments occurring within the dunefields of the region, and examine the stratigraphic record in the context of dune preservation and sediment reworking. We find that it is rarely possible to reconstruct past environmental change within desert dunefields purely on the basis of sedimentological evidence. However, limited palaeoenvironmental information is preserved within dune sediments which can be combined with additional arid zone proxies and geochronological techniques to enable the most reliable reconstruction of past conditions. Paleosols provide the most informative and unambiguous palaeoenvironmental indicators within dune sediments, acting as markers of relative environmental stability. Paleosols are characterised by pedogenic carbonates and gypsum, and illuviated clays in the form of intact grain coatings. The potential for incomplete stratigraphic preservation within dunes is a critical consideration for reconstructing palaeoenvironments, however the preservation of paleosols indicates that the dunes in this region have not been completely reworked. Grain coatings provide important evidence both for pedogenesis when they are intact, and for aeolian reworking when they are partially abraded. Abraded cutans are ubiquitous across the region studied. Linear dune mineralogy of the Strzelecki and Tirari Deserts is dominated by quartz, with minor feldspar and clay minerals, the latter in the form of grain coatings and aggregates. The occurrence of intact sand-sized aggregates composed of clay and silt particles at several sites indicates incipient rather than peak aridity, and may, within this geomorphic context, indicate reduced stream discharges. Grain size shows a regional bias, although silt-sized material is most likely to have been deposited as long-travelled dust. The regional grain size characteristics, combined with local occurrences of rock fragments and high feldspar content, suggest that sediments are mostly derived from local sources.

Reply to the discussion by Au and Yeung on “Forms and sand transport in shallow hydraulic fractures in residual soil”Appears in Canadian Geotechnical Journal,44: 1474–1475.

Au and Young have studied our paper carefully, and we appreciate both their interest in our work and the opportunity to address some of the questions that arose from their study. We would like to thank them for pointing out their papers and dissertations, and describing results that resemble those from our work. A significant body of research over the last 50 years describes studies of hydraulic fractures in the laboratory. Hubbert and Willis (1957) were among the first, and the papers by Au and Yeung and their colleagues are solid contributions to this effort. However, our paper focused on a field study and we decided to restrict the discussion of laboratory studies to maintain focus on the field problem. The effects of external boundaries must be carefully considered when evaluating the form of hydraulic fractures created in the laboratory. For example, our results, along with those of many other investigators, suggest that hydraulic fractures curve upward when they interact mechanically with an overlying, stress-free surface. The apparatus used by Au et al. (e.g., Fig. 2, Au et al. 2003) shows rigid plates confining the top and bottom of the sample, along with rigid walls around it—no stress-free surface appears to be present. All of the papers cited by Au and Yeung seem to use this same apparatus. Their apparatus is nicely designed and certainly well suited for studying some aspects of hydraulic fractures, and we do not dispute the importance of the findings described in their papers. However, the boundary conditions imposed by the upper and lower plates were different than the stress-free conditions assumed to influence fracture form at shallow depths in the field. The difference in boundary conditions made it difficult for us to assess the relevance of the forms of the fractures they created in the laboratory to the ones we created in the field. We avoided this comparison in the paper as a result. It is worth noting that experimental techniques were described by Galland et al. (2003) for evaluating interactions between hydraulic fractures and free surfaces in silt-sized material with high lateral stresses. We only recently learned about this work and regrettably did not cite it in our paper. Au and Yeung raised several other good points. Some of their questions can be answered by referring to the theses that form the basis of our paper. We are aware that it can be difficult to access the theses and dissertations through normal channels, so to address this issue we have posted the theses written by Tan (2003), Richardson (2003), and Malin (2005) at http://www.ces.clemson.edu/hydro/ murdoch/Student%20Project/index.htm. That site also has photographs and other supporting material. For the purposes of clarity, the comments of Au and Yueng will be summarized into several questions before presenting a reply. Why did we create hydraulic fractures in the B horizon of the soil? The hydraulic fractures were created in the B horizon primarily because this material is relatively homogeneous and isotropic compared with the underlying saprolite, which has a banded fabric as a weathered relic from the original gneiss. The gneiss is strongly folded and the orientation of the mineral banding in the saprolite can be spatially variable. We wanted to conduct experiments in material with relatively uniform properties because we were interested in evaluating the forms of the fractures independent from the influence of major heterogeneities and complex structures, which could possibly dominate the fracture form. This

An overview of the role of weathering in the production of quartz silt

Quartz silt is a widespread detrital sediment with large aeolian (loess) and alluvial silt deposits forming important components of many contemporary landscapes. Despite research findings which indicate that a range of opportunities exist for the comminution of quartz into silt particles within a wide variety of geomorphic environments (e.g. glacial grinding, fluvial comminution, aeolian abrasion, frost weathering, salt weathering, insolation weathering and deep weathering), the ‘glacial–aeolian’ hypothesis has traditionally been favoured as the most likely explanation for loess formation and loess is seen as primarily a Quaternary phenomenon. As a consequence there has been a tendency to underestimate sediment inputs into loess systems by geomorphological processes operating within pre-Quaternary environments. In particular, earth scientists may have considerably underestimated the role of weathering in global silt generation, as there are now many references to the existence of a patchy but widespread distribution of pre-Quaternary weathering profiles across the glacial and periglacial landscapes of the Northern Hemisphere (e.g. North America, British Isles, north and central Europe), landscapes within which many of the classical loess deposits are located. These observations suggest that weathering profiles may have covered large tracts of the Northern Hemisphere land surface prior to the Quaternary glaciations. This, in turn, may have important implications for quartz silt generation as: (1) experimental studies indicate that many weathering processes are capable of generating significant quantities of silt-sized debris, and (2) particle characteristics displayed by saprolitic material that has developed on quartz rich crystalline source rocks often include significant quantities of either silt-sized material, or quartz grains that are weakened by weathering derived microfractures. Thus, weathering profiles may represent ‘mines’ of actual and potential quartz silt, available for release into sedimentary systems when these profiles are subsequently reworked by geomorphological processes such as glacial, fluvial and aeolian erosion.

Luminescence dating of proglacial sediments from the Eastern Alps

The potential of optically stimulated luminescence (OSL) for dating proglacial deposits is tested here at three last glacial key sites in the Eastern Alps. This was undertaken using both sand-sized feldspar and quartz grains as well as fine silt-sized material. The samples reveal rather poor luminescence intensities of the investigated quartz grains and it is also clearly demonstrated that several of the deposits were incompletely bleached prior to deposition. Bleaching experiments were carried out and different approaches for extracting the aliquots where the OSL was completely bleached prior to burial were tested. Applying different OSL measurement techniques and analytical procedures, we investigated which approach was most appropriate for dating known age proglacial sediments. Best results were achieved using Single-Aliquot Regenerative-Dose methodology on coarse grain quartz and feldspar samples, in combination with the approach of Preusser et al. [2007. Luminescence dating of proglacial sediments from Switzerland. Boreas, in press] for ED calculation.

Formation of the Aude, Orb and Herault prodeltas and their characterisation using physicochemical and sedimentological parameters

The formation of Mediterranean prodeltas depends on microtidal estuarine dynamics. The study of the Aude estuary and others shows the usual presence of a salt wedge which induces the formation of two turbidity maxima created by two different processes [Pauc, H. 1997. Fluvio–marine interface in a microtidal mediterranean estuarine environment. The nepheloïd layer supply problem. Journ. Rech. Océanogr. 22 (3), 83–90]. The first process is a loss of charge due to the annulment of the current momentum, driving sedimentation of the silt-size material. The second process is flocculation, which results in deposition of fine sediment. The end results of the combination of these two processes is a sedimentary sequence with two distinct sediment units: silt below and clay minerals above. Granulometrical and mineralogical analysis of the sediment body enables the zone of sedimentation of each type of material to be determined, and hence the style of sedimentation to be inferred. Illite, quartz and dolomite are of detrital origin and accumulate close to the river mouths; while smectite is deposited by both dynamical and flocculation processes, and accumulating over the surface of the prodelta and also seaward by some 30–40 m depth. Calcite is of biogenic origin, and is found further offshore, associated with muddy sediments only.

Site formation processes at Zhoukoudian, China

Zhoukoudian is often cited for its human remains and the early evidence of fire. Yet, since its first excavations over 70 years ago, detailed studies of processes responsible for the accumulation of anthropogenic and geogenic sediments in the site have been sparse. This paper provides some details of site formation processes mainly through field observations of the extant section at Locality 1, and the use of soil micromorphology and Fourier Transform Infrared Spectrometry (FTIR) analyses of the sediments. Samples from Layers 10 through 3 show extensive water deposition of fine silt-sized material (reworked loess), including fine-grained organic matter. The dark organic-rich unit in Layer 10—often cited as one of the earliest evidence of fire—is a water-laid accumulation. Much of the fine-grained sediment was derived from outside Locality 1, implying that the site was open to varying extents throughout most of its depositional history. The 4–6m accumulation of “ashes” in Layer 4 represents subaerial water-laid silt deposits derived from the loess-covered hillslopes surrounding the site. They presumably accumulated in an open depression that formed after the collapse of the brecciated roof deposits represented by Layer 6. Diagenesis is present in many of the Layers, and is exemplified by calcite precipitation and dissolution, and localized apatite (dahllite) replacement of calcite. In Layer 4 diagenesis is more advanced, including calcite/dahllite precipitation, subaerial weathering of the loess and associated precipitation of hematite, alteration of clay and the neoformation of quartz. Many of our conclusions concur with those of Teilhard de Chardin & Young published over 70 years ago.

Making loess-sized quartz silt: data from laboratory simulations and implications for sediment transport pathways and the formation of ‘desert’ loess deposits associated with the Sahara

Abstract Results from a series of laboratory simulations designed to investigate quartz silt generation by a number of geomorphic mechanisms are reviewed. These findings are then used to discuss potential sediment pathways involved in the formation of ‘desert’ loess deposits and a pathway is presented to show a proposed sequence of events involved in the formation of the loess deposits associated with the Sahara. The results indicate that both fluvial and aeolian systems can be very effective at producing quartz silt. It is suggested that fluvial systems within hot desert environments may play a central role in both the production and distribution of loessic silt. First, the highly energetic but periodic fluvial regimes associated with arid and semi-arid environments may be responsible for considerable quantities of particle comminution. Second, the intermittent nature of these systems will help maintain geomorphologically active surfaces from which silt-sized material can be easily deflated. As well as fluvial comminution, a range of weathering mechanisms and aeolian abrasion may also be responsible for the generation of silts in ‘desert’ loess. Consequently, silt sources and mechanisms of silt generation are likely to be much more diverse for ‘desert’ loess than those associated with glacial loess.

Bioturbation and Holocene sediment accumulation fluxes in the north-east Atlantic Ocean (Benthic Boundary Layer experiment sites)

Bioturbation and Holocene sediment accumulation are quantified in the three experimental areas of the Benthic Boundary Layer (BENBO) programme by means of the natural radionuclides 14C and 210Pb and the artificial radionuclides 137Cs and 241Am. The Holocene accumulation rates, determined by the radiocarbon method, are 4.4 and 6.5 cm kyr−1 at sites B (Rockall Plateau, 1100 m water depth) and C (Feni Drift, 1925 m water depth), respectively. Accumulation at site A, situated between Feni Drift and Porcupine Bank at 3570 m water depth, was interrupted by an erosional event in the mid-Holocene, which removed 0.25 m or more of the uppermost sediment present at that time. The estimated accumulation rate since that event is 2.1 cm kyr−1. Different estimates of surficial bioturbation mixing depths at site B are returned by the 210Pbexcess and 14C methods, with the former indicating <10 cm and the latter unusually deep at 17 cm. At site C, 210Pbexcess and the fallout radionuclides, 137Cs and 241Am, are present in two distinctly-separated depth zones, with the deepest mixing down to ∼15 cm, similar to the 14C mixed layer depth. This is ascribed to deep burrowing by sipunculid or echiuran worms at site C, and similar deep mixing is inferred to be necessary at site B to produce the differences in mixed layer depths derived from the longer- and shorter-lived radionuclide profiles, although the deep burrowing episodes must be rare (<1 event per 102 yr). The greater accumulation rate at site C compared with site B is produced by an enhanced flux of current-driven clay- and silt-sized material. This fine material both dilutes the CaCO3 content of the site C sediment and is responsible for the higher Corg content observed at site C compared with site B. Unlike the site B and C sediments, which are fine-grained, the coarse, 63–125 μm size fraction is the most abundant size class in the late Holocene sediments at site A, suggesting that the sediments at this location are winnowed.

The utility of aggregate processing fines in the rehabilitation of dolomite quarries

It is economically and environmentally desirable to use aggregate processing fines (pond fines), a by-product of dolomite crushing/washing operations, in the rehabilitation plans of the aggregate quarry where they are produced. A study was undertaken to determine if the physical and mechanical properties of these dominantly silt-sized materials also made them suitable as embankment material from a geotechnical perspective. The after-use proposed for the dolomite quarry investigated in this study (Ontario, Canada) was to be a naturalized area surrounding a large water reservoir, and pond fines were being used to construct sloped embankments at the base of the vertical perimeter quarry walls for this purpose. The specific objectives were (1) to measure and analyse the physical and mechanical properties of the pond fines that are relevant to their usage in site rehabilitation (earthen embankment), and (2) to predict the long-term stability of the embankments given current emplacement practices and a scenario of partial submergence by a reservoir. Shear strength, measured by direct shear and drop cone penetrometer methods, increased with higher clay-sized particle content and higher dry bulk density, with lower sand content, and it peaked at soil water contents (4–9 per cent kg kg−1) well below the measured field capacity of about 16 per cent kg kg−1. Factors of safety calculated for slope stability (modified Bishop method of analysis) decreased with decreasing shear strength (cohesion, angle of shearing resistance), with increasing slope gradient, and with the introduction of a water table. Nevertheless, the factors of safety were acceptably high for all scenarios tested (≥1·8), including scenarios using the maximum measured slope gradient of 37 per cent. Increasing the dry bulk density by 0·1 g cm−3 over the mean density measured in situ (i.e. to about 87 per cent of the Standard Proctor maximum dry density) did not improve the calculated factors of safety for slope stability enough to warrant increased mechanical compaction of the pond fines during embankment construction. Copyright © 2000 John Wiley & Sons, Ltd.

Biochemical distributions (amino acids, neutral sugars, and lignin phenols) among size-classes of modern marine sediments from the Washington coast

In order to examine relationships of organic matter source, composition, and diagenesis with particle size and mineralogy in modern marine depositional regimes, sediments from the continental shelf and slope along the Northwest Pacific rim (Washington coast, USA) were sorted into hydrodynamic size fractions (sand: >250, 63–250 μm; silt: 35–63, 17–35, 8–17, 3–8 μm; and clay-sized: 1–3, 0.5–1, <0.5 μm). The size fractions were then density fractionated to separate distinct organic debris from mineral-associated organic matter, and the various separates were analyzed for their amino acid, aldose, and lignin compositions. The composition of organic matter in the separates changes markedly as a function of particle size and density. Large compositional differences were observed between the clay-sized fractions (dominated mineralogically by smectites), the sand-sized mineral-associated isolates (quartz-rich), and floated coarse organic matter (dominated by vascular plant debris). Organic matter intimately associated with the clay-sized fractions shows the most extensive diagenetic alteration, as reflected in high abundances of nonprotein amino acids (especially β-alanine), elevated lignin phenol acid/aldehyde ratios, and high relative concentrations of the deoxyhexoses fucose and rhamnose. Organic matter in the silt fractions, though degraded, is not as diagenetically altered as in the clay fractions. Enrichment of pollen grains in the silt-size material is reflected by high cinnamic acid to ferulic acid lignin phenol ratios. The highest pollen biochemical signal is observed in the silt fractions of the deepest station (1835 m), where pollen abundances are also highest. Organic matter tightly bound in the silt and sand-sized fractions are enriched in aldoses and show indications of enhanced microbial biomass as reflected by high weight percentages of ribose. Distinct organic debris was composed of relatively unaltered vascular plant remains as reflected by high lignin phenol yields and low acid/aldehyde ratios. Clay-size fractions are enriched in nitrogenous components, as reflected by elevated yields of total and basic amino acids (especially lysine). Silt- and sand-size fractions rich in quartz and albite show slightly higher yields of neutral amino acids. Consistent trends across all size classes and among the different depositional settings illustrates that only a small portion of the organic matter is present as distinct organic debris (e.g. pollen, vascular plant tissues, etc.), but that this debris can be isolated in specific size classes. The data for surface-associated organic matter are consistent with, but not conclusive of, selective partitioning of some organic matter to specific mineral surfaces. The dominant size class-specific trends in organic matter composition are due to changes in both source and diagenetic alteration.

Silt Aggregates in Laminated Marine Sediment Produced by Agglutinated Foraminifera

Fabric studies of Miocene to Holocene hemipelagic and pelagic marine sediments by means of backscattered electron imagery reveal the common presence of exotic aggregates of well to poorly sorted silt-size particles. New observations of silt aggregates associated with readily identifiable multi-chambered agglutinated foraminifera, when combined with recent experimental insights into the chamber-building activities of these organisms, provide a new model for the formation of these aggregates. Some silt aggregates of well-sorted grains appear to represent the collapsed or compacted tests of agglutinated foraminifera. Aggregates of poorly sorted, silt-size material probably represent the discarded that agglutinated foraminifera erect around themselves while constructing a new chamber. These remnant tests and detritic covers are likely to be preserved in settings of low bottom-water oxygenation where bioturbation is limited or absent. The long stratigraphic range (Cambrian to Recent) and diverse habitats (hyposaline lagoons to abyssal environments) occupied by agglutinated foraminifera suggest that they may be an important, but hitherto unrecognized, contributor to the production of early fabrics in fine-grained marine sediments. Their presence may provide unique evidence on the concentration of dissolved oxygen in the bottom waters.

Glacial comminution of quartz sand grains and the production of loessic silt: A simulation study

Traditional views have attributed the formation of loess-sized quartz silt particles to glacial processes. Glacial grinding was considered to be the only mechanism capable of producing loessic silt in the quantities necessary for the formation of loess deposits. This idea has been strengthened by the apparent close association between present or former glacierised regions and major global loess deposits. However, some doubts exist as to whether or not glacial grinding is capable of producing sufficient quantities of quartz silt. Because of the difficulties entailed in the observations and direct measurement of subglacial abrasion and crushing, this paper describes and discusses the results of a simulation study designed to examine abrasion and size reduction of quartz grains under “grinding” conditions analogous to those experienced in a subglacial environment. The findings from these experimental runs suggest that, although particle breakage and comminution does occur, little silt-sized material is actually produced. The amount of silt produced ranges from approximately 9% of the original sample in one experimental run to less than 1% for the other seven runs. It is suggested that the results obtained may show considerably less abrasion and loessic silt production than may be expected to occur within a natural subglacial environment because: (1) subglacial grains experience much greater overburden pressures for much longer periods of time; (2) the use of sand-sized grains may represent a stable component resistant to further size reduction through crushing or abrasion; and (3) the use of a monomineralic sample of relatively sound quartz grains. Although care must be taken when relating results obtained from these experimental runs to silt production within a glacial environment, findings from this work do question whether large amounts of quartz silts are produced within subglacial environments. The importance of glacial processes in the formation of loess deposits may lie in the reworking of pre-existing sediments by ice rather than the actual production of vast quantities of silt. This in turn suggests that care should be taken in the interpretation of the provenance of particles comprising loess deposits.

SIZE FRACTIONATION OF SOIL ORGANO-MINERAL COMPLEXES USING ULTRASONIC DISPERSION

Potential artifacts created by soil fractionation procedures are frequently ignored when studying the interaction of natural and synthetic organic compounds with clay-sized materials. Ultrasonic dispersion treatments of 0–7.66 kJ applied to soil in water were used in combination with centrifugation and sedimentation techniques to obtain sand, silt, coarse clay, and medium clay fractions from soils with 19.2 to 44.2 g organic C kg−1 soil. No more than 1.37 kJ of sonication energy was needed for maximum yields of silt-sized materials, whereas sand yields decreased monotonically and clay yields increased at a decreasing rate with energy inputs up to 7.66 kJ. Coulter counter analysis of silt-sized fractions showed that no redistribution in particle-size occurred within this fraction with increased sonication energy. Changes in total C and N concentrations in the silt-sized fractions showed no consistent trends with increased sonication energy. Total C and N concentrations increased in the coarse clay-sized fractions and decreased in the medium clay-sized fractions with a greater input of sonication energy. Diffuse reflectance Fourier transform infrared analysis did not reveal any qualitative changes in the coarse clay fractions, although changes in the medium clay fractions of the two soils highest in organic C occurred when energy was increased from 0.84 to 2.94 kJ. We conclude that excessive sonication energies may cause undesirable artifacts, especially in the medium clay-sized fraction. It is suggested that a standard sonication procedure to obtain adequate particle size yields and limit artifacts in the fractionated organomineral complexes include the application of no more than 3–5 kJ of energy to 10 g soil in 50 mL of water.

Cultivated and Adjacent Virgin Soils in Northcentral South Dakota: II. Mineralogical and Micromorphological Comparisons

AbstractCultivated and adjacent virgin soils [Williams (fine‐loamy, mixed, Typic Argiboroll) and variants of Williams] from northcentral South Dakota were compared, by horizon, to examine the effects of cultivation on mineralogical and micromorphological properties of the soils. Two major mineralogical differences were noted. Silt‐sized material from the Btk, Bk, and C horizons of a cultivated pedon had smaller calcite‐to‐dolomite peak‐height ratios, as determined by x‐ray diffraction, than similar horizons from a virgin pedon. These findings, combined with the results of additional experimentation, suggest calcite is being leached from Btk horizons and is accumulating in Bk horizons of cultivated pedons. The A, Bt, and Btk horizons of a virgin pedon had more subdued x‐ray peak intensities for the 001 spacing of smectite than similar horizons from a cultivated pedon. When similar horizons of a cultivated and a virgin pedon were compared, micromorphological differences were as follows: (i) the Ap horizon displayed less macroporosity, more diffuse and lighter‐colored organic glaebules, fewer and less pronounced microaggregates, and fewer surface coatings on sand‐sized particles; (ii) the Bt horizon from the virgin pedon had greater macroporosity; (iii) the Btk and Bk horizons of the virgin pedon contained more particles coated with and more composite particles bound with organo‐mineral material; (iv) a far greater proportion of voids were filled with black, opaque cutans in the Btk horizons of the cultivated pedons that, by electron microprobe analysis, was dominantly Si.

Deep-Water Density Current Deposits of Delaware Mountain Group (Permian), Delaware Basin, Texas and New Mexico

The Guadalupian Delaware Mountain Group is a 1,000-1,600-m (3,281-5,250-ft) thick section of siltstone and sandstone deposited in a deep-water density-stratified basin surrounded by carbonate banks or reefs and broad shallow evaporite-clastic shelves. The most prevalent style of basinal deposition was suspension settling of silt. Laminated siltstone beds are laterally extensive and cover basin-floor topographic irregularities and flat-floored channels as much as 30 m (99 ft) deep and 1 km or more wide. Channels can be observed in outcrop at the basin margin and can be inferred from closely spaced wells in the basin. The channels are straight to slightly sinuous, trend at high angles to the basin margin, and extend at least 70 km (43 mi) into the basin. Sandstone beds, con ined to channels, form numerous stratigraphic traps. Hydrocarbon sealing beds are provided by laminated organic siltstone, which laterally can form the erosional margin where channels are cut into siltstone beds. Thick beds of very fine-grained sandstones fill the channels. These sandstones contain abundant large and small-scale traction-current-produced stratification. These sandy channel deposits generally lack texturally graded sedimentation units and show no regular vertical sequence of stratification types or bed thickness. Outcrop and subsurface evidence indicates Delaware Mountain Group sediments were deposited by saline density currents. Dense saline water originated on evaporitic shelves and spilled across the carbonate rim, down steep marginal slopes, and into the basin. Basinal waters were density stratified. Denser flows moved along the basin floor cutting channels or depositing sand in existing channels; less-dense flows moved along density interfaces in the water column and carried silt-size material far into the basin where it settled to the floor as thin alternating layers of detrital silt and organic debris. Little proximal to distal change occurred in the size or nature of the channels. Exploration predictions based on submarine fan models formed by turbidity currents would anticipate very d fferent proximal-distal changes in sandstone geometry and facies.

Comparison of Morphological, Physical, and Chemical Properties of a Minesoil and a Natural Soil of Southeast Kansas

Morphological, physical and chemical properties of minesoil were determined and compared to natural soil in southeast Kansas. The morphological and physical properties of the minesoil were less favorable for plant growth than those of the natural soil and probably would be major limiting factors in the establishment and growth of plants on minesoil. The minesoil lacked well-developed and contrasting horizons and contained shale fragments ranging in amounts from less than 5 to 25 percent. The structure of the minesoil was mostly clods, varying in shape and size, with many open voids in the subsoil. The average coarse-fragment (> 2 mm) content of the minesoil was 15 percent compared to less than 2 percent for the natural soil. The minesoil contained more material in the sand-size ranges of the fine-earth (<2 mm) fraction. Averaged over depth, the amount of silt-size material in the minesoil was less than that of the natural soil, and the amount of claysize material was nearly equal. The surface 12 inches of the minesoil contained from 35.3 to 61.8 percent less plant-available water than the natural soil. The major differences between the chemical properties of the minesoil and the natural soil were essentially in their distributions with depth. In the natural soil elemental concentrations increased with depth, and in the minesoil they were more uniformly distributed. Averaged over depth the chemical properties of the two soils were quite similar indicating that from a chemical perspective the minesoil was adequate and maybe more favorable for plant growth than the natural soil. The slight differences that existed were attributed to different fertilizer management histories and weathering of primary minerals in freshly exposed shale fragments in the minesoil. Surface coal mining disturbs large areas of land and alters the natural soil in a mined area. Soil is a natural resource and over the long term its value This content downloaded from 157.55.39.118 on Mon, 06 Mar 2017 18:36:51 UTC All use subject to http://about.jstor.org/terms VOLUME 91, NUMBERS 3-4 109 may be greater than that of coal. Natural processes of soil formation are slow and can take decades to form new soil. To hasten soil formation processes and to achieve a soil productivity level equal to or greater than the premined condition, man implements land reclamation procedures and constructs a new soil. The characteristics of this new soil or minesoil depend on the kinds and sequences of reclamation procedures employed. Few data exist on properties of Kansas minesoils (Brady and Livingston, 1979). Comparisons of properties of minesoils and natural soils to assess the effects of mining and reclamation procedures on the properties of minesoil are limited. This study was initiated to provide some basic data on soil properties of minesoil on surface-mined land in southeast Kansas. The specific objectives were to determine the morphological, physical, and chemical properties of minesoils and compare these to the same properties of the predominant natural soil in the area to assess the potential of the minesoil for sustaining plants.

Silt production by weathering of a sandstone under hot arid conditions: an experimental study

Various mechanisms have been proposed for the formation of silt-sized material, of which loess deposits mainly consist. These range from glacial grinding to aeolian abrasion and, more recently, salt weathering of loose, sand-sized quartz grains which relies upon salt solutions penetrating and exploiting already existing cracks. If, however, the original grains are partly constrained within a rock mass it could be argued that the effectiveness of salt weathering would be enhanced, and the requirement for pre-existing cracks might possibly be negated. To test this contention, cubes of unbedded quartz sandstone were subjected, under laboratory conditions, to 60 diurnal cycles of heating and cooling between approximately 21 and 54°C directed through one exposed face and wetting with solutions of NaCl, Na 2 SO 4 and MgSO 4 . Debris produced by the simulation was examined by scanning electron microscopy and thin sections were made from the sandstone blocks. Micrographs and particle size analysis show that most debris consists of more or less intact sand-sized grains liberated from the parent rock. However, coarse and medium silt-sized material, often characterised by fresh fracture surfaces, was also observed. Examination of the parent rock suggests that this silt-sized material originates from two sources: breaking away of silica cement and coatings of secondary silica from around sand-sized grains, and the microfracturing of quartz sand grains. Microfracture patterns observed in thin sections appear to result from point-loading by adjacent grains, which results in compressive and/or shear stresses sufficient to fracture individual grains. No attempt is made to quantify the amounts of silt produced, but it is suggested that the mechanisms described could, under suitable conditions of rock type and salt availability, provide a viable source of loessic silt.

Laboratory simulation of ‘the wick effect’ in salt weathering of rock

Abstract‘The wick effect’ is the upward migration of saline solutions into rocks and their subsequent crystallization. Lower Carboniferous sandstone blocks of rectangular shape have been subjected to this process in the laboratory using a range of salt types, a range of salt concentrations, and various mixtures of salts. Some treatments produced severe disintegration, notably sodium carbonate and a mixture of sodium carbonate with magnesium sulphate, whereas other salts (including sodium chloride and gypsum) were much less effective. The debris produced by this experimental salt weathering included appreciable quantities of silt‐sized material, which were analysed with a laser granulometer. Such material could provide a source for desert loess.

Diagenetic fluorite in chalks from Stevns Klint and Møns Klint, Denmark

Small quantities of fluorite occur as sand- and silt-sized material in a Cretaceous/Tertiary chalk-marl-limestone sequence at Stevns Klint and in a Lower Maastrichtian Chalk sequence at Møns Klint, Denmark. The habit of the fluorite is either hexahedral or irregular. The fluorite grains enclose calcite particles that commonly have a parallel optical orientation. On the basis of morphology and inclusions, it is suggested that the fluorite is diagenetic, having formed by replacement of skeletal grains.

Textural properties of regoliths on vegetated steep slopes in upland regions, Scotland

ABSTRACTThe textural properties of many sediments provide a good indication of their provenance, but surprisingly little information is available on the transitional stages between the breakdown of a rock and the incorporation of the material into a fluvial sediment. These transitional stages are important as certain fractions (particularly the finer ones) may be selectively removed. Regoliths developed on steep slopes represent an early stage in the debris cascade and they are here examined in detail to assess the role of parent lithology on the textural properties of the regolith. There are substantial variations between lithologies, although the majority of regoliths are dominated by coarser fractions and are poorly sorted. Most particle size distributions show some degree of fit to both log-normal probability distributions and Rosin distributions. Differences from these can be ascribed to the processes operating on steep slopes, particularly the influx of sand- and silt-sized material by colluvial processes and the removal of clay-sized material by leaching. The regoliths form a distinct facies type which may be recognisable in the geological record.