Diagenetic control on mineralogical suites in sand, silt, and mud (Cenozoic Nile Delta): Implications for provenance reconstructions

Garzanti, Eduardo; Andó, Sergio; Limonta, Mara; Fielding, Laura; Najman, Yani

doi:10.1016/j.earscirev.2018.05.010

Cited by 61 publications

(30 citation statements)

References 73 publications

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“…Researchers working with ancient sandstones indeed find zircon, tourmaline, and rutile far more commonly than other minerals in their heavy mineral mounts, but this is just because zircon, tourmaline, and rutile are the durable ones that stand the best chance to survive chemical attack through multiple sedimentary cycles. Figure 2 in [12], [19], and [118,119]. Heavy minerals are progressively dissolved during burial diagenesis and finally leached out at depths varying notably from basin to basin depending on original mineral abundance, pore-fluid composition, and geothermal gradient (18-26.5 °C/Km for the Nile Delta, 20-30 °C/Km for the Gulf of Mexico, 30-40 °C/Km for the North Sea) [120][121][122].…”

Section: Diagenetic Bias: What You See Is Not All There Wasmentioning

confidence: 99%

Heavy Minerals for Junior Woodchucks

2019

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In the last two centuries, since the dawn of modern geology, heavy minerals have been used to investigate sediment provenance and for many other scientific or practical applications. Not always, however, with the correct approach. Difficulties are diverse, not just technical and related to the identification of tiny grains, but also procedural and conceptual. Even the definition of “heavy minerals” is elusive, and possibly impossible. Sampling is critical. In many environments (e.g., beaches), both absolute and relative heavy mineral abundances invariably increase or decrease locally to different degrees owing to hydraulic-sorting processes, so that samples close to "neutral composition" are hard to obtain. Several widely shared opinions are misleading. Choosing a narrow size-window for analysis leads to increased bias, not to increased accuracy or precision. Only point-counting provides real volume percentages, whereas grain-counting distorts results in favor of smaller minerals. This paper also briefly reviews the heavy mineral associations typically found in diverse plate-tectonic settings. A mineralogical assemblage, however, only reproduces the mineralogy of source rocks, which does not correlate univocally with the geodynamic setting in which those source rocks were formed and assembled. Moreover, it is affected by environmental bias, and by diagenetic bias on top in the case of ancient sandstones. One fruitful way to extract information on both provenance and sedimentological processes is to look for anomalies in mineralogical–textural relationships (e.g., denser minerals bigger than lower-density minerals; harder minerals better rounded than softer minerals; less durable minerals increasing with stratal age and stratigraphic depth). To minimize mistakes, it is necessary to invariably combine heavy mineral investigations with the petrographic analysis of bulk sand. Analysis of thin sections allows us to see also those source rocks that do not shed significant amounts of heavy minerals, such as limestone or granite, and helps us to assess heavy mineral concentration, the “outer” message carrying the key to decipher the “inner message” contained in the heavy mineral suite. The task becomes thorny indeed when dealing with samples with strong diagenetic overprint, which is, unfortunately, the case of most ancient sandstones. Diagenesis is the Moloch that devours all grains that are not chemically resistant, leaving a meager residue difficult or even impossible to interpret when diagenetic effects accumulate through multiple sedimentary cycles. We have conceived this friendly little handbook to help the student facing these problems, hoping that it may serve the purpose.

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Section: Diagenetic Bias: What You See Is Not All There Wasmentioning

confidence: 99%

Heavy Minerals for Junior Woodchucks

2019

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“…A robust quantitative comparison among mineralogical assemblages, however, can be drawn only for sediments buried less than a few hundreds of meters, because the tHM suites of ancient strata are exposed to selective intrastratal dissolution of less durable minerals progressing with increasing age and burial depth ( [148][149][150]). Heavy-mineral studies of Bengal Fan turbidites have documented the common occurrence of unstable pyroxene and olivine only in the upper part of the studied cores (i.e., Zone I of [22], dated as~0.5 Ma, burial depth ≤ 160 m b.s.f.).…”

Section: From the Bengal Shelf To The Bengal-nicobar Fanmentioning

confidence: 99%

Provenance of Bengal Shelf Sediments: 2. Petrology and Geochemistry of Sand

et al. 2019

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The Bangladesh lowlands are traversed by the largest sediment flux on the planet. Detritus generated mostly in Himalayan highlands and conveyed through the Ganga–Brahmaputra rivers and Meghna estuary reaches the Bay of Bengal, where it forms a composite deltaic system. This study integrates the vast existing database on Ganga–Brahmaputra sediments of all grain sizes from clay to sand with new petrographic, mineralogical, and geochemical data on estuarine and shallow-marine sands. A large spectrum of compositional signatures was used to: (i) assess the relative supply of the Ganga and Brahmaputra rivers to estuarine and shelfal sediments; (ii) define the compositional variability of estuarine sediments and the impact exerted by hydraulic sorting and climate-related chemical weathering on provenance signals; (iii) define the compositional variability of shelf sediments and the potential hydrodynamic segregation of fast-settling heavy minerals in coastal environments and of slow-settling platy micas on low-energy outer-shelf floors; (iv) consider the potential additional mud supply from the western subaerial part of the delta formerly built by the Ganga River; and (v) draw a preliminary mineralogical comparison between fluvio-deltaic sediments and turbidites of the Bengal–Nicobar deep-sea fan, thus tracing sediment dispersal across the huge sedimentary system extending from Tibet to the equatorial Indian Ocean. All investigated mineralogical and geochemical parameters, as well as Sr and Nd isotope ratios and clay–mineral assemblages, showed a clear prevalence in sediment supply from the Brahmaputra (60–70%) over the Ganga (30–40%). Heavy-mineral suites and Sr and Nd isotope fingerprints of Bengal shelf sediments are nearly identical to those of the Brahmaputra River and Meghna estuary, also because the Brahmaputra carries almost twice as many Ca-plagioclase grains and heavy minerals including epidote than the Ganga, and these minerals control the large majority of the Sr and Nd budgets. The experience gained in modern settings can be directly extrapolated only to the recent past, because sediments older than the late Pleistocene and buried more than a few hundred meters begin to lose less durable ferromagnesian minerals by selective chemical dissolution, which makes quantitative estimates progressively less robust in more deeply buried older strata.

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“…Garnet, common in orogenic sediments derived from metasedimentary rocks, is a particularly valuable provenance tracer because it displays a wide range of major-element compositions and resists diagenetic dissolution better than epidote, amphibole, and pyroxene [116][117][118].…”

Section: Garnet Chemistrymentioning

confidence: 99%

“…The denser a rock is, the greater amount of dense minerals it contains and therefore can shed. Heavy-mineral concentration in sediments, however, can be modified even by an order of magnitude or more by hydraulic sorting during erosion, transport and sedimentation [106], or by chemical processes including weathering in soils and intrastratal dissolution during burial diagenesis [118,130]. Only in the absence of such environmental and diagenetic bias can terrigenous detritus be considered as produced purely by physical comminution and the mineralogy of daughter sand held to faithfully reflect the mineralogy of parent rocks.…”

Section: Heavy Mineral Concentration and Provenance Estimatesmentioning

confidence: 99%

Multimineral Fingerprinting of Transhimalayan and Himalayan Sources of Indus-Derived Thal Desert Sand (Central Pakistan)

et al. 2019

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As a Quaternary repository of wind-reworked Indus River sand at the entry point in the Himalayan foreland basin, the Thal Desert in northern Pakistan stores mineralogical information useful to trace erosion patterns across the western Himalayan syntaxis and the adjacent orogenic segments that fed detritus into the Indus delta and huge deep-sea fan throughout the Neogene. Provenance analysis of Thal Desert sand was carried out by applying optical and semi-automated Raman spectroscopy on heavy-mineral suites of four eolian and 11 fluvial sand samples collected in selected tributaries draining one specific tectonic domain each in the upper Indus catchment. In each sample, the different types of amphibole, garnet, epidote and pyroxene grains—the four dominant heavy-mineral species in orogenic sediment worldwide—were characterized by SEM-EDS spectroscopy. The chemical composition of 4249 grains was thus determined. Heavy-mineral concentration, the relative proportion of heavy-mineral species, and their minerochemical fingerprints indicate that the Kohistan arc has played the principal role as a source, especially of pyroxene and epidote. Within the western Himalayan syntaxis undergoing rapid exhumation, the Southern Karakorum belt drained by the Hispar River and the Nanga Parbat massif were revealed as important sources of garnet, amphibole, and possibly epidote. Sediment supply from the Greater Himalaya, Lesser Himalaya, and Subhimalaya is dominant only for Punjab tributaries that join the Indus River downstream and do not contribute sand to the Thal Desert. The detailed compositional fingerprint of Thal Desert sand, if contrasted with that of lower course tributaries exclusively draining the Himalaya, provides a semi-actualistic key to be used, in conjunction with complementary provenance datasets and geological information, to reconstruct changes in paleodrainage and unravel the relationship between climatic and tectonic forces that controlled the erosional evolution of the western Himalayan-Karakorum orogen in space and time.

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Diagenetic control on mineralogical suites in sand, silt, and mud (Cenozoic Nile Delta): Implications for provenance reconstructions

Cited by 61 publications

References 73 publications

Heavy Minerals for Junior Woodchucks

Heavy Minerals for Junior Woodchucks

Provenance of Bengal Shelf Sediments: 2. Petrology and Geochemistry of Sand

Multimineral Fingerprinting of Transhimalayan and Himalayan Sources of Indus-Derived Thal Desert Sand (Central Pakistan)

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