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SEDIMENTARY ROCK COMPOSITION

SEDIMENTARY ROCK COMPOSITION. Sedimentology 340. The story behind sedimentary rock composition is in the Rock Cycle. Mineral (and rock) composition of sedimentary rocks is a fundamental property that is used for: interpretation of the history of the deposit

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SEDIMENTARY ROCK COMPOSITION

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  1. SEDIMENTARY ROCK COMPOSITION Sedimentology 340

  2. The story behind sedimentary rock composition is in the Rock Cycle

  3. Mineral (and rock) composition of sedimentary rocks is a fundamental property that is used for: • interpretation of the history of the deposit • determining source rocks (provenance) • classifying the rock Composition is analyzed using a petrographic microscope and thin sections Only crude description and preliminary identification can be done in the field or from hand specimens with eye or hand lens

  4. Source Area (provenance) • Rock type • Weathering environment • Transport direction Provenance is the original source rock……often mountain ranges that no longer exist

  5. Weathering varies with • - mineralogy • climate • amount and type of • cement • Source Area (provenance) • Rock type • Weathering environment • Clast transport

  6. energy & agent • Clast Transport • Energy • Agent • Distance • Depositional Environment • Marine vs. non-marine • Physical environment • (beach, river, delta, desert, etc.) distance

  7. Rock exposed to weathering is the lithosphere (continental and oceanic crust) Crustal rocks vary: igneous rocks, sedimentary rocks and metamorphic rocks

  8. The Earth's crust is dominated by seven mineral or groups of minerals. In decreasing order of abundance, these are: feldspar 51% with two common varieties: plagioclase [(CaAl2Si2O8 or NaAlSi3O8] 39% orthoclase [KAlSi3O8] 12% quartz [SiO2] 12% pyroxene [(Ca,Na)(Mg,Fe,Al)(Si,Al)2O6] 11% mica (muscovite, biotite) 5% amphibole 5% clay 4.6% olivine [(Mg,Fe)2SiO4] 3% TOTAL 91.6%

  9. Physical and chemical weathering break down rocks producing rock fragments and minerals Stablity is related to the mineral structure; usually silicate structure Bowen’s Reaction Series

  10. Source Rocks granite basalt Sedimentary rock (conglomerate)

  11. Grain composition mineralogy proportions are different from source (bedrock) to “sink” (site of deposition) Igneous & Metamorphic Weathering & Transport process Average in sed rocks shalesandstone feldspar 60% 5% 10-15% quartz 15-20% 30% 65% rock fragments 0% 10-15% clay minerals 0% 60% 5% fine micas heavy minerals 20% 3% 8%

  12. framework grains Framework Grains major minerals quartz feldspar clay minerals accessory minerals mafic, heavy minerals stable, non opaque, Z,R,Ts rock fragments igneous (IRF & VRF) sedimentary (SRF) metamorphic (MRF) chemical cements silicate, carbonate, iron oxide, sulfate matrix cement See TABLE 5.1

  13. Major minerals quartz Quartz structure varies with origin: Monocrystalline quartz is igneous Polycrystalline quartz is metamorphic Si O2 framework silicate Single-crystal grains = monocrystalline ( in igneous rocks)

  14. Major minerals quartz Quartz structure varies with origin: Monocrystalline quartz is igneous Polycrystalline quartz is metamorphic Si O2 framework silicate Multiple interlocking quartz grains = polycrystalline (in metamorphic rocks)

  15. Orthoclase KAlSi3O8 Plagioclase NaAlSi3O8 - CaAl2Si2O8 Orthoclase Plagioclase Major minerals feldspar plagioclase framework silicate

  16. Petrography – thin sections Quartz, orthoclase feldspar and plagioclase feldspar are main minerals. Feldspar shows the alteration. Mica and calcite are common as accessory minerals. Grain boundaries and fractures on quartz and feldspar reveals evidence of intense weathering. Q, Quartz, F, Feldspar, C, Calcite, M, Mica, B, Biotite.

  17. Petrography – thin sections Quartz, orthoclase feldspar and plagioclase feldspar are main minerals. Feldspar shows the alteration. Mica and calcite are common as accessory minerals. Grain boundaries and fractures on quartz and feldspar reveals evidence of intense weathering. Q, Quartz, F, Feldspar, C, Calcite, M, Mica, B, Biotite.

  18. Sheet silicate Major minerals Mica and Clay minerals Most clay minerals are too small to identify without x-ray difraction mica Examples: kaolinite, illite, chlorite

  19. Accessory minerals (1-2%) mafic, heavy minerals stable, non opaque, Z,R,Ts Mafic = magnesium (mg2+) and iron (fe3+) = dark black/green in color examples = hornblende, amphibole, magnetite Heavy minerals = mineral density > 2.9 gm/cm3 garnet stable, non opaque, Z,R,Ts Z = zircon R = rutile T = tourmaline Last to chemically weather quartz hornblende

  20. rock fragments igneous (IRF & VRF)) sedimentary (SRF) metamorphic (MRF) Pieces of ancient source rocks that were NOT physically or chemically weathered Rock fragments make up 15-20% of framework grains in sandstones; are most common in gravel-sized particles of conglomerates; are rare in shales.

  21. framework grains chemical cements silicate, carbonate, iron oxide, sulfate matrix Cements are precipitated from solutes (cations, anions, and complex molecules that are produced during chemical weathering. cement silicate cement – chert, opal, feldspar, zeolites carbonate – calcite, dolomite (mg), siderite (fe) iron oxide – hematite, limonite, geothite sulfate – anhydrite, gypsum, barite Table 5.1

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