Sediments can be considered geological archives as they record and preserve the signatures of ancient geological events affecting source areas. This primitive provenance signal is modified by physical and chemical processes during transport and deposition. The present thesis is devoted to highlight the importance of heavy minerals analysis on modern sediments to refine classical provenance models, unraveling sediment sources, transport conditions, diagenetic processes and recycling phenomena. In the first part of the thesis a classification of surface textures observed on detrital heavy mineral grains in sands and sandstones is proposed in order to enhance data reproducibility among operators and to implement the use of high-resolution heavy-mineral data in studies on sediment-generation, provenance and diagenesis. Different stages of progressive weathering are collected in numerous color tables for visual comparison. They are specifically devised to systematically collect valuable information for paleoclimatic or diagenetic interpretation during routine grain-counting operations under the microscope. This catalog (Andò et al., 2012) represents a useful subsidiary tool to reveal the different degrees of weathering for diverse minerals in modern sands of equatorial Africa, and to identify post-depositional modifications of detrital assemblages in buried orogenic sediments of the Bengal Basin. These data, integrated with the concentration of heavy minerals in each sample, provide the fundamental clue to quantify the degree of heavy-mineral depletion caused by either pre-depositional or post-depositional processes (useful to understand the development of secondary porosity and to assess the potential of water and hydrocarbon reservoirs). The second part of the thesis focuses on two regional studies on compositional variability, provenance and long-distance trasport of terrigenous sediments from Barbados Island (Limonta et al., in prep.) and Indo-Burma-Andaman-Nicobar Ridge (Garzanti et al., 2013a). Subduction complexes large enough to be exposed subaerially and to 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 large Alpine-type or Andean-type orogen, where huge volumes of orogenic detritus are produced and conveyed via a major fluvio-deltaic system to the deep sea (Ingersoll et al., 2003). We investigated sediment generation and recycling in the Indo-Burman-Andaman-Nicobar subduction complex, representing the archetype of such settings in the eastern prolongation of the Himalayan collisional system. “Subduction Complex Provenance” is composite, and chiefly consists of detritus recycled from largely turbiditic parent rocks (Recycled Clastic Provenance), with local supply from ultramafic and mafic rocks of forearc lithosphere (Ophiolite Provenance) or recycled paleovolcanic to neovolcanic sources (Volcanic Arc Provenance; Garzanti et al., 2007). In order to specifically investigate the effect of recycling, we characterized the diverse detrital signatures of Cenozoic sandstones deposited during subsequent stages of “soft” and “hard” Himalayan collision and exposed from Bangladesh to the Andaman Islands, and discuss the reasons for compositional discrepancies between parent sandstones and their recycled daughter sands. A companion study was carried out with the same methodologies, rationale and goals on Barbados Island. Also modern Barbados sands are largely multicyclic, reflecting mixing in various proportions of detritus from the basal Scotland Formation (sandstones and mudrocks), their stratigraphic and tectonic cover, the Oceanic Formation (quartzose turbidites and deep-water biogenic oozes including radiolarites), and from the Pleistocene calcarenite and reef caps, as well as from volcanic layers ultimately derived from the Lesser Antilles. Mixing of detritus recycled from orogen-derived turbidites transported long distance with detritus from oceanic mèlange, pelagic sediments and younger calcareous cap rocks and in addition volcaniclastic products thus redefines the diagnostic mark of Subduction Complex Provenance as quite distinct from the original definition by Dickinson and Suczek (1979).

(2014). Heavy minerals: a key to unravel orogenic processes . Sediment generation and recycling at convergent plate boundaries (indo-burman-andaman-nicobar and barbados ridges). (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2014).

Heavy minerals: a key to unravel orogenic processes . Sediment generation and recycling at convergent plate boundaries (indo-burman-andaman-nicobar and barbados ridges)

LIMONTA, MARA
2014

Abstract

Sediments can be considered geological archives as they record and preserve the signatures of ancient geological events affecting source areas. This primitive provenance signal is modified by physical and chemical processes during transport and deposition. The present thesis is devoted to highlight the importance of heavy minerals analysis on modern sediments to refine classical provenance models, unraveling sediment sources, transport conditions, diagenetic processes and recycling phenomena. In the first part of the thesis a classification of surface textures observed on detrital heavy mineral grains in sands and sandstones is proposed in order to enhance data reproducibility among operators and to implement the use of high-resolution heavy-mineral data in studies on sediment-generation, provenance and diagenesis. Different stages of progressive weathering are collected in numerous color tables for visual comparison. They are specifically devised to systematically collect valuable information for paleoclimatic or diagenetic interpretation during routine grain-counting operations under the microscope. This catalog (Andò et al., 2012) represents a useful subsidiary tool to reveal the different degrees of weathering for diverse minerals in modern sands of equatorial Africa, and to identify post-depositional modifications of detrital assemblages in buried orogenic sediments of the Bengal Basin. These data, integrated with the concentration of heavy minerals in each sample, provide the fundamental clue to quantify the degree of heavy-mineral depletion caused by either pre-depositional or post-depositional processes (useful to understand the development of secondary porosity and to assess the potential of water and hydrocarbon reservoirs). The second part of the thesis focuses on two regional studies on compositional variability, provenance and long-distance trasport of terrigenous sediments from Barbados Island (Limonta et al., in prep.) and Indo-Burma-Andaman-Nicobar Ridge (Garzanti et al., 2013a). Subduction complexes large enough to be exposed subaerially and to 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 large Alpine-type or Andean-type orogen, where huge volumes of orogenic detritus are produced and conveyed via a major fluvio-deltaic system to the deep sea (Ingersoll et al., 2003). We investigated sediment generation and recycling in the Indo-Burman-Andaman-Nicobar subduction complex, representing the archetype of such settings in the eastern prolongation of the Himalayan collisional system. “Subduction Complex Provenance” is composite, and chiefly consists of detritus recycled from largely turbiditic parent rocks (Recycled Clastic Provenance), with local supply from ultramafic and mafic rocks of forearc lithosphere (Ophiolite Provenance) or recycled paleovolcanic to neovolcanic sources (Volcanic Arc Provenance; Garzanti et al., 2007). In order to specifically investigate the effect of recycling, we characterized the diverse detrital signatures of Cenozoic sandstones deposited during subsequent stages of “soft” and “hard” Himalayan collision and exposed from Bangladesh to the Andaman Islands, and discuss the reasons for compositional discrepancies between parent sandstones and their recycled daughter sands. A companion study was carried out with the same methodologies, rationale and goals on Barbados Island. Also modern Barbados sands are largely multicyclic, reflecting mixing in various proportions of detritus from the basal Scotland Formation (sandstones and mudrocks), their stratigraphic and tectonic cover, the Oceanic Formation (quartzose turbidites and deep-water biogenic oozes including radiolarites), and from the Pleistocene calcarenite and reef caps, as well as from volcanic layers ultimately derived from the Lesser Antilles. Mixing of detritus recycled from orogen-derived turbidites transported long distance with detritus from oceanic mèlange, pelagic sediments and younger calcareous cap rocks and in addition volcaniclastic products thus redefines the diagnostic mark of Subduction Complex Provenance as quite distinct from the original definition by Dickinson and Suczek (1979).
GARZANTI, EDUARDO
ANDO' SERGIO
heavy minerals, provenance studies, Himalaya, Barbados, andaman and nicobar
GEO/02 - GEOLOGIA STRATIGRAFICA E SEDIMENTOLOGICA
English
Scuola di dottorato di Scienze
SCIENZE DELLA TERRA - 61R
26
2012/2013
(2014). Heavy minerals: a key to unravel orogenic processes . Sediment generation and recycling at convergent plate boundaries (indo-burman-andaman-nicobar and barbados ridges). (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2014).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/50093
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