Search

This Nile Delta case study provides quantitative information on a process that we must understand and consider in full before attempting provenance interpretation of ancient clastic wedges. Petrographic and heavy-mineral data on partly lithified sand, silt, and mud samples cored from the up to 8.5 km-thick post-Eocene succession of the offshore Nile Delta document systematic unidirectional trends. With increasing age and burial depth, quartz increases at the expense of feldspars and especially of mafic volcanic rock fragments. Heavy-mineral concentration decreases drastically, transparent heavy minerals represent progressively lower percentages of the heavy fraction, and zircon, tourmaline, rutile, apatite, monazite, and Cr-spinel relatively increase at the expense mainly of amphibole in Pliocene sediments and of epidote in Miocene sediments. Recent studies have shown that the entire succession of the Nile Delta was deposited by a long drainage system connected with the Ethiopian volcanic highlands similar to the modern Nile since the lower Oligocene. The original mineralogy should thus have resembled that of modern Delta sand much more closely than the present quartzose residue containing only chemically durable heavy minerals. Stratigraphic compositional trends, although controlled by a complex interplay of different factors, document a selective exponential decay of non-durable species through the cored succession that explains up to 95% of the observed mineralogical variability. Our calculations suggest that heavy minerals may not represent >20% of the original assemblage in sediments buried less than ~1.5 km, >5% in sediments buried between 1.5 and 2.5 km, and >1% for sediments buried >4.5 km. No remarkable difference is detected in the intensity of mineral dissolution in mud, silt, and sand samples, which argues against the widely held idea that unstable minerals are prone to be preserved better in finer-grained and therefore presumably less permeable layers. Intrastratal dissolution, acting through long periods of time at the progressively higher temperatures reached during burial, can modify very drastically the relative abundance of detrital components in sedimentary rocks. Failure to recognize such a fundamental diagenetic bias leads to grossly mistaken paleogeographic reconstructions, as documented paradigmatically by previous provenance studies of ancient Nile sediments.

The idea that prominent geological phenomena observed in the crust and at the surface may be caused by the detachment of subducting dense lithosphere has been anticipated by tectonic models in the Sixties and Seventies, and imaged with improved geophysical techniques in the Eighties. In the mid-Nineties, the model of slab breakoff was defined formally by Davies and von Blanckenburg. Initially proposed as a thought-provoking working hypothesis, the theory rapidly received wide a priori acceptance and was applied to virtually every mountain range around the world, and even in orogens as old as Paleoproterozoic and Archean, to explain a range of different phenomena. These include magmatic flare-ups, rapid topographic uplift, increasing sediment supply, fast exhumation of metamorphic rocks, ore mineralizations, anomalous distribution of seismicity, changes in stress regimes, and inversion of subduction polarity. Even multiple breakoff events were assumed to have occurred at different times in the same orogenic belt or subduction zone. In the last 20 years, slab breakoff has been invoked in so many settings and time frames that it could have hardly taken place in each and every case in which it was called upon. This article does not critically examine the theory, but critically examines its use. The extensive review of the vast literature presented here on the subject reveals how the model has been often applied to provide ad hoc explanations for a range of poorly understood observations based on incomplete evidence of deep-seated processes. Our aim is to illustrate a paradigmatic example of how earth scientists, in the face of evidence that challenges our capacity of understanding, often recur to hypotheses based on other hypotheses. Such an approach may induce researchers to look for confirmation in the absence of compelling constraints, or even in the face of conflicting evidence. The faith in models should not lead us to confuse speculative theories with axiomatic truths, and to build upon them theoretical edifices that are vulnerable and exposed to the risk of circular reasoning.

We here present the first comprehensive provenance study of the Sahara Desert using a combination of multiple provenance proxies and state-of-the-art statistical analysis. Our dataset comprises 44 aeolian-dune samples, collected across the region from 12°N (Nigeria) to 34°N (Tunisia) and from 33°E (Egypt) to 16°W (Mauritania) and characterized by bulk-petrography, heavy-mineral, and detrital-zircon U Pb geochronology analyses. A set of statistical tools including Multidimensional Scaling, Correspondence Analysis, Individual Difference Scaling, and General Procrustes Analysis was applied to discriminate among sample groups with the purpose to reveal meaningful compositional patterns and infer sediment transport pathways on a geological scale. The overall homogenity across sand samples, however, precluded a detailed narrative. Saharan dune fields are, with a few local exceptions, composed of pure quartzose sand with very poor heavy-mineral suites dominated by durable zircon, tourmaline, and rutile. Some feldspars, amphibole, epidote, garnet, or staurolite occur closer to basement exposures, and carbonate grains, clinopyroxene and olivine near a basaltic field in Libya. Relatively varied compositions also characterize sand along the Nile Valley and the southern front of the Anti-Atlas fold belt in Morocco. Otherwise, from the Sahel to the Mediterranean Sea and from the Nile River to the Atlantic Ocean, sand consists nearly exclusively of quartz and durable minerals. These have been concentrated through multiple cycles of erosion, deposition, and diagenesis of Phanerozoic siliciclastic rocks during the long period of relative tectonic quiescence that followed the Neoproterozoic Pan-African orogeny, the last episode of major crustal growth in the region. The principal ultimate source of recycled sand is held to be represented by the thick blanket of quartz-rich sandstones that were deposited in the Cambro-Ordovician from the newly formed Arabian-Nubian Shield in the east to Mauritania in the west. Durability of zircon grains and their likelihood to be recycled from older sedimentary rocks argues against the assumption, too often implicitly taken for granted in provenance studies based on detrital-zircon ages, that their age distribution reflects transport pathways existing at the time of deposition rather than inheritance from multiple and remote landscapes of the past.

As a latest Pleistocene repository of Indus River sand at the entry point to the Himalayan foreland basin, the Thal dune field in northern Pakistan stores crucial information that can be used to reconstruct the erosional evolution of the Himalayan-Karakorum orogen and the changes in the foreland-basin landscape that took place between the Last Glacial Maximum and the early Holocene. This comprehensive provenance study of Thal Desert sand integrates previously existing petrographic, heavy-mineral, mineral-chemical, isotopic, and geochronological databases with original bulk-sediment geochemistry, zircon-age, and Nd-isotope data. Dune sand is low in quartz and rich in feldspars, volcanic, metavolcanic and metabasite grains, contains a very rich transparent heavy-mineral suite including hypersthene and common zircon grains dated as Late Cretaceous to early Paleogene, and is characterized by high Mg, Sc, V, Co, Ni, Cu concentrations and by eNd values as high as −3.5. Together, these data indicate that ~40% of Thal dune sand was supplied by erosion of the Kohistan arc, a proportion that far exceeds the one assessed for modern Upper Indus sand. Greater detrital supply from the Kohistan arc indicates notably different conditions of sediment generation, during a period in which the sediment-transport capacity of the Upper Indus in the dry lowlands was reduced and volumes of sand were extensively reworked by wind and accumulated in dune fields across the foreland basin. In the early Holocene, the renewed strength of the South Asian monsoon and consequently markedly increased water and sediment discharge led to incision of the Thal and Thar dune fields by the Indus River and its Punjab tributaries draining the Himalayan front directly hit by heavy monsoonal rains.

Cosmogenic nuclide analysis in sediment from the Earth's largest rivers yields mean denudation rates of the sediment-producing areas that average out the local variations commonly found in small rivers. Using this approach, we measured in situ cosmogenic 26Al and 10Be in sand of >50 large rivers over a range of climatic and tectonic regimes covering 32% of the Earth's terrestrial surface. In 35% of the analyzed rivers, we find 26Al/10Be ratios significantly lower than these nuclides´ surface-production-rate ratio of 6.75 in quartz, indicating radioactive decay over periods exceeding 0.5 Myr. We invoke a combination of slow erosion, shielding in the source area, and sediment storage and burial during long-distance transport to explain these low ratios. In the other 65% of studied rivers we find 26Al/10Be ratios within uncertainty of their surface production-rate ratio, indicating cosmogenic steady state. For these rivers, we obtain a global source area denudation rate of 141 t/km2×yr (54 mm/kyr of rock-equivalent) that translates to a flux of 3.07 ± 0.56 Gt/yr. By assuming that this sub-dataset is representative of the global land surface, we upscale this value to the total surface area for exorheic basins, thereby obtaining a global denudation flux of 15.2 ± 2.8 Gt/yr that integrates over the past 11 kyr. This value is slightly lower than published values from cosmogenic nuclides from small river basins (23 (+53/−16)) Gt/yr) upscaled using a global slope model, and also lower than modern sediment and dissolved loads exported to the oceans (24.0 Gt/yr). Our new approach confirms an estimate of global dissolved and solid matter transfer that converges to an encouragingly narrow range of within 35%; whereas the use of paired nuclides in large rivers provides estimates of the buffering timescales of sediment transport.

The modern Pearl River originates from SE Tibet and debouches into the South China Sea. The development of the Pearl River is closely related to the evolving topography following the tectonic evolution of the southern China continental margin and uplift of Tibet caused by the India-Eurasia collision. How topographic changes affected the development of the Pearl River, however, is still unclear. Here we use original and literature data on detrital zircon ages from both modern Pearl River sands and ancient strata drilled in offshore basins to reconstruct the evolution of the paleo-Pearl River catchment through time. Six phases are identified: 1) Early Cretaceous: the paleo-Pacific plate was subducting beneath the South China block and topography in South China was tilted to the west. The paleo-Dong River began to develop with limited length. 2) Late Cretaceous: back-arc extension in the South China Sea contributed further to the west-tilted topography. The paleo-Bei River started to develop and the paleo-Dong River continued to expand across southeasternmost China. 3) Paleocene to Eocene: active rifting in the South China Sea induced a major topographic change. The paleo-Dong and paleo-Bei joined, forming the paleo-Lower Xi River. 4) Early Oligocene: active uplift of Tibet and onset of sea-floor spreading in the South China Sea led to subsidence in the Cathaysia block. The paleo-Dong, paleo-Bei, and paleo-Lower Xi rivers remained limited to eastern Cathaysia. 5) Late Oligocene: accelerated uplift of eastern Tibet and post-rift subsidence of the northern South China Sea margin induced a radical change in the landscape of southern China, and transition from west-tilting to east-tilting topography. The paleo-Pearl River started to incorporate also its present western branches. 6) Early to middle Miocene: the east-tilting topography was enhanced during rapid uplift of Tibet and progressive closure of the proto-South China Sea, while the Pearl River evolved to its present configuration.

Despite the influence of other geological and geomorphological factors, chemical weathering at the Earth's surface is strongly controlled by climate. Thus, a measure of weathering intensity determined from soils or sediments should provide information about the climatic conditions associated with their formation. Available geochemical and mineralogical data on modern fluvial and marine muds from different regions of southern Africa and its Atlantic continental margin are used to review the links between sediment composition and climatic properties together with the possible causes of variance. Although river muds may not be generated exclusively in a single sedimentary cycle and erosion and weathering processes do not necessarily take place in a spatially homogeneous way, significant relationships between mineralogical and geochemical signatures of river mud and rainfall in the corresponding catchment area were recognised. Our study shows that the composition of clay is strongly influenced by climatically-driven weathering, whilst coarser mud fractions tend to be more affected by provenance, grain size, hydraulic sorting, and recycling. In the marine environment the climatic signal may be lost even in clay, because of hydraulic fractionation, authigenic mineral growth and mixing with foreign particles. Given the ubiquitous character of fluvial muds, and the easy and non-expensive methods available for separating and analysing clay fractions, their geochemical fingerprints represent a most precious source of information concerning climate. Any geochemical parameter used as a regional proxy of climate, however, still requires that the diversity of geological, geomorphological, and biological factors that affect its value are cautiously considered.

Subduction complexes large enough to be exposed subaerially and become significant sources of terrigenous detritus are formed by tectonic accretion above trenches choked with thick sections of remnant-ocean turbidites. They thus need to be connected along strike to a major collision zone, where huge volumes of orogenic detritus are produced and conveyed via a major fluvio-deltaic system to the deep sea. In this article we investigate sediment generation and recycling in the archetype of such settings, the eastern prolongation of the Himalayan collisional system. We illustrate the petrographic and heavy-mineral suites of modern sands produced all along the Indo-Burman-Andaman-Nicobar subduction complex, which includes accreted abyssal-plain sediments overthrust by ophiolites and unconformably overlain by volcaniclastic forearc strata. "Subduction Complex Provenance" is thus composite, and overwhelmingly consists of detritus recycled from largely turbiditic parent rocks (Recycled Clastic Provenance), with local supply from obducted ultramafic and mafic rocks of forearc lithosphere (Ophiolite Provenance) or recycled paleovolcanic to neovolcanic sources (Volcanic Arc Provenance). In order to specifically investigate the effect of recycling, we characterize the diverse detrital signatures of Cenozoic sandstones originally deposited during subsequent stages of "soft" and "hard" Himalayan collision and presently exposed from Bangladesh to the Andaman Islands, and discuss the reasons for compositional discrepancies between parent sandstones and their recycled daughter sands. Long-distance, multistep and multicyclic sediment transfer along and across convergent plate boundaries follows complex trajectories in space and time, which must be resolved whenever we want to obtain a reasonably faithful paleogeographic reconstruction for the recent and less recent geological past. © 2013 Elsevier B.V.

This article reconstructs the evolution of the passive northern margin of peninsular India facing the Neotethys, from the earliest rifting stages in the Carboniferous to final collision with the active southern margin of Asia in the Early Eocene. Classical techniques of basin analysis were applied to an extensive and coherent stratigraphic data base. Facies analysis, biostratigraphic dating, and palaeoenvironmental interpretation of an over 10 km-thick sedimentary succession from eight main composite sections allowed us to draw accurate sedimentation and backstripped subsidence curves, and to place quantitative constraints on the palaeotectonic scenario inferred from the sedimentary record. In each of the investigated composite sections, three major stratigraphic gaps are consistently recognised in the rift sequence: the “rift unconformity” (Mississippian), the “Carboniferous/Permian hiatus” (largely Kasimovian to Asselian), and the “break-up unconformity” (mid-Sakmarian). Two or even all three gaps may merge into a single hiatus, spanning up to 70 Ma overall. Gaps are associated with rift-shoulder uplift, which after calculation of backstripped subsidence sums up to 600 m at least in three of the investigated sections. Thermal subsidence and low sedimentation rates are documented by the mid-Permian to Middle Triassic drift sequence, deposited while the Neotethys was expanding between northern Gondwana and the detached Peri-Gondwanan blocks. A marked increase in accumulation rates is recorded in the Carnian–Norian, but calculations of uniform lithospheric stretching suggest that extension was minor and associated with tectonic processes affecting distant parts of Gondwana. The Tethys Himalayan margin was uplifted around the Jurassic/Cretaceous boundary, at the onset of a major volcanic episode. Finally, uplift exceeding in magnitude all previous events is documented by the uppermost units of the stratigraphic column, deposited during the earliest stages of collision between India and Asia in the latest Paleocene to Early Eocene.

The Po Basin, a Pliocene marine gulf between the Alps and the Apennines, was filled progressively from west to east and finally capped by fluvial sediments during the Pleistocene. By similar to 1.25 Ma, a meandering trunk river (paleoDora Baltea) reached westernmost Lombardia (Cilavegna); at similar to 1 Ma, the paleoAdda, entering the plain through the Como Valley, joined the paleoTicino in the Milano area. The coastline was directed NNE/SSW, and the open sea persisted east of similar to 10 degrees E. Metamorphiclastic prodelta sediments of the Alpine trunk river reached central Lombardia (Pianengo) by MIS 36 and eastern Lombardia (Palosco) at MIS 31. Trunk-river delta foresets accumulated rapidly in easternmost Lombardia (Ghedi) during the Jaramillo. Fluvial sedimentation, continuous at Pianengo since MIS 28, eventually reached Ghedi at MIS 22. In this time interval, the embayment secluded between the prograding trunk-river delta and the Southalpine front was progressively filled by Southalpine fan deltas.With the onset of major Alpine glaciations in the late Matuyama, detrital supply increased markedly and fluvial deposits spread all over Lombardia. Because of outward growth of Alpine fans, the paleoDora Baltea was replaced at Cilavegna by the paleoSesia and paleoToce. Milano lay in the paleoAdda braidplain. Southalpine alluvial fans reached as far south as Pianengo. The paleoOglio glacier exited Lake Iseo at Cremignane.In the early Brunhes, accumulation rates markedly dropped in Lombardia; carbonaticlastic paleoPiave turbidites accumulated rapidly at Venezia, capped by the prograding paleoPo delta and finally by fluvial paleoBrenta-Bacchiglione deposits. Subsequently, accumulation rates decreased further, and paleosols developed during stages of prolonged exposure. The paleoTicino continued to flow southwest of Milano, where detritus from the paleoOlona is documented locally. PaleoAdda sediments were deposited at Milano and sedimentaclastic detritus at Trezzo up to a few meters from ground surface, indicating that the final shift of River Adda to the Lecco branch of Lake Como was a very recent event. The paleoOglio exited Lake Iseo in its present position. At Ghedi the paleoChiese was replaced by the paleoAdige, and finally re-established before development of a paleosol overlain by topmost Pleistocene marsh deposits (MIS 2).This study indicates new guidelines for studies of drainage evolution, and more in general of depositional architecture in foreland basins. (C) 2010 Elsevier B.V. All rights reserved.