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Raman spectroscopy is an innovative tool with tremendous potential, serving as a fundamental complement to a variety of provenance methods including heavy-mineral analysis and detrital geochronology. Because of its accuracy, efficiency and versatility, the results of the Raman technique are indispensable for fully reliable identification of heavy minerals in grain mounts or thin sections. Thorny long-standing problems that cannot be solved confidently with a polarizing microscope alone, such as the determination of opaque and altered heavy minerals, of detrital grains as small as a few microns, or of colourless crystals with uncertain orientation and rounded morphology, can finally be addressed. Although the method can be highly automatized, the full ability and experience of the operator is required to combine Raman data with the optical information obtained under the microscope on the same grains, which is essential for the efficient application of the method in provenance studies. This article provides exemplary Raman spectra useful for the comparison and determination of over 70 different opaque and transparent heavy-mineral species commonly found in sediments, conveying specific information on the genesis of their source rocks, and thus is particularly useful in provenance diagnoses and palaeotectonic reconstructions.

This article investigates landscape characteristics and sediment composition in the western Greater Caucasus by using multiple methods at different timescales. Our ultimate goal is to compare short-term versus long-term trends in erosional processes and to reconstruct spatio-temporal changes in sediment fluxes as controlled by partitioning of crustal shortening and rock uplift in the orogenic belt. Areas of active recent uplift are assessed by quantitative geomorphological techniques [digital elevation model (DEM) analysis of stream profiles and their deviation from equilibrium] and compared with regions of rapid exhumation over longer time intervals as previously determined by fission-track and cosmogenic-nuclide analyses. Complementary information from petrographic and heavy-mineral analyses of modern sands and ancient sandstones is used to evaluate erosion integrated throughout the history of the orogen. River catchments displaying the highest relief, as shown by channel-steepness indices, correspond with the areas of most rapid exhumation as outlined by thermochronological data. The region of high stream gradients is spatially associated with the highest topography around Mount Elbrus, where sedimentary cover strata have long been completely eroded and river sediments display the highest metamorphic indices and generally high heavy-mineral concentrations. This study reinforces the suggestion that the bedrock-channel network can reveal much of the evolution of tectonically active landscapes, and implies that the controls on channel gradient ultimately dictate the topography and the relief along the Greater Caucasus. Our integrated datasets, obtained during a decade of continuing research, display a general agreement and regularity of erosion patterns through time, and consistently indicate westward decreasing rates of erosional unroofing from the central part of the range to the Black Sea. 2014 John Wiley & Sons, Ltd.

Amphibole and garnet are among the most widespread heavy minerals in orogenic sediments. Their chemical composition and optical properties vary markedly and systematically with temperature and pressure conditions during growth, and thus provide important information on the metamorphic evolution of source areas that is crucial in palaeotectonic and palaeogeodynamic reconstructions. This study investigates the chemical composition of detrital amphiboles and garnets derived from parent rocks of progressively increasing metamorphic grade through a well-studied composite section across the Central and Southern Alps, including the granulite-facies core of the Late Palaeozoic orogen exposed in the Ivrea–Verbano Zone and the amphibolite-facies core of the Cenozoic orogen exposed in the Lepontine Dome. We specifically focus on metamorphic grade because it represents the best proxy for tectono-stratigraphic crustal level, and hence degree of unroofing of source areas. In river sands collected between metamorphic isograds corresponding to crystallization temperatures ranging from c. 500 °C to c. 850 °C, TiO2 gradually increases in detrital amphibole while its colour progressively changes from blue-green in the lower amphibolite-facies where actinolite, hornblende and tschermakite are most abundant, to brown in the granulite facies where pargasite is dominant. Detrital garnets display moderate gradual changes across the amphibolite-facies Lepontine Dome, where low-Mg ‘type B’ garnets predominate. Almandine-spessartine is spatially associated with abundance of pegmatites while entering the zone of anatexis (Southern Steep Belt), where grossular or grossular-andradite-spessartine are occasionally found. A sharp change occurs while reaching granulite-facies in the Ivrea–Verbano Zone, where high-Mn garnets disappear and ‘type A’ almandine-pyrope (from ‘stronalite’ metasediments) and ‘type C’ almandine-pyrope-grossular (from metagabbros of the Mafic Complex) predominate. Also redefined in this article are a series of numerical indices based on amphibole colour and relative abundances of diverse key minerals (chloritoid, staurolite, andalusite, kyanite, fibrolitic and prismatic sillimanite), useful to accurately assess the average metamorphic grade of meta-igneous and metasedimentary source rocks.

Compositional evolution of orogenic sediment can be viewed as resulting from downstream mixing of detritus shed by several end-member sources to constitute the polygenic product deposited in foreland basins. Such a model of simple mixing, which does not consider modifications caused by physical and chemical processes through the sedimentary cycle, is applied here to first-cycle detritus presently produced in the Alps to assess sediment budgets and erosion patterns across the orogen. Using the same actualistic end members, the model is applied next to the recent past to reconstruct paleodrainage changes during major Pleistocene glaciations, and finally to a more remote past to tentatively infer erosional evolution and dispersal paths in the earliest Oligocene stages of Alpine morphogeny. A decisive factor of success in such an approach is the use of large multiple datasets displaying sufficiently distinct values in diverse end members. Forward mixing models can be made for bulk petrography and heavy-mineral suites as well as for any physical (grain density), geochemical (element concentrations, stable-isotope ratios), or geochronological parameters (apatite fission tracks, zircon U-Pb age spectra), and separately on sediments of various sizes and transport modes (suspended load and bedload). By comparing and integrating several independent estimates thus obtained we progressively gain a more robust insight about diverse geological processes, and may converge eventually toward a truer vision of our past.

Heavy mineral concentration in sediments depends primarily on the chemistry and tectono-stratigraphic level of rocks eroded within continental-block, arc, or orogenic source terranes. Detritus derived from mantle peridotites and lower crustal gabbros, as well as from high-pressure (oceanic or continental eclogite) and high-temperature (amphibolite, granulite) metamorphic rocks contains one to two orders of magnitude more heavy minerals than detritus derived from upper crustal rocks including granites and sedimentary successions. Concentrated heavy mineral assemblages, however, may result from density-sorting during erosion, transport, or deposition by tractive currents, which can very effectively segregate minerals with even small differences in density within distinct grain-size fractions and sedimentary environments (e.g., fluvial channel versus overbank, beach versus shelf). Conversely, depleted heavy mineral assemblages may result from severe diagenetic dissolution in ancient sandstones, a process that in Alpine and Himalayan foreland basins very extensively affected clastic wedges older than the Pleistocene. Through a series of key modern examples from various geodynamic settings in arid to semi-arid climate, we stress the importance of giving full consideration to heavy mineral concentration while interpreting provenance of terrigenous sediments and sedimentary rocks. The concentration parameters introduced herein (Heavy Mineral Concentration index, Source Rock Density index), coupled with parameters based on heavy mineral species with either contrasting density (% opaque, % ultradense, % ZR) or chemical stability, allow us to reveal and quantify also the effects of hydraulic sorting in the depositional environment and of diagenetic dissolution in ancient terrigenous rocks. We document that in most modern sands, the relative abundance of chemically stable species (e.g., zircon, tourmaline, rutile, apatite, chrome spinel) is considerably less than the experience on ancient sandstones generally induces to believe, and show that the actualistic approach provides crucial insight for a correct interpretation of heavy mineral suites

This study discusses the laws which govern sediment deposition, and consequently determine sizedependent compositional variability. A theoretical approach is substantiated by robust datasets on major Alpine, Himalayan, and African sedimentary systems. Integrated (bulk-petrography, heavy-mineral, X-ray powder diffraction) multiple-window analyses at 0.25ϕ to 0.50ϕ sieve interval of eighty-five fluvial, beach, and eolian-dune samples, ranging from very fine silt to coarse sand, document homologous intrasample compositional trends, revealed by systematic concentration of denser grains in finer-grained fractions (“size– density sorting”). These trends are explained by the settling-equivalence principle, stating that detrital minerals are deposited together if their settling velocity is the same. Settling of silt is chiefly resisted by fluid viscosity, and Stokes' law predicts that size differences between detrital minerals in ϕ units (“size shifts”) are half the difference between the logarithms of their submerged densities. Settling of pebbles is chiefly resisted by turbulence effects, and the Impact law predicts double size shifts than Stokes' law. Settling of sand is resisted by both viscosity and turbulence, the settling-equivalence formula is complex, and size shifts increase – with increasing settling velocity and grain size – from those predicted by Stokes' law to those predicted by the Impact law. In wind-laid sands, size shifts match those predicted by the Impact law; size– density sorting is thus greater than in water-laid fine sands. New analytical, graphical, and statistical techniques for rigorous settling-equivalence analysis of terrigenous sediments are illustrated. Deviations associated with non-spherical shape, density anomalies, inheritance from source rocks, or mixing of detrital species with contrasting provenance and different size distribution are also tentatively assessed. Such integrated theoretical and experimental approach allows us to mathematically predict intrasample compositional variability of water-laid and wind-laid sediments, once the density of detrital components is known.