SEDIMENTS AND SEDIMENTARY ROCKS: SANDSTONES READING • • • • Prothero & Schwab, Ch. 5 p. 76-81 Mineralogy of Sandstones p. 90-98 Classification of Sandstones (and review information on grain size, shape and roundness, p. 81-90) Sandstone • • • A major siliciclastic rock type 20-25% of all sedimentary rocks Composed of: framework grains (1/16 - 2 mm) ave. 58% quartz, 22% feldspar, 20% lithics clay matrix mineral cement Quartz • Hard • Weak cleavage • Chemically "inert“ • Monocrystalline or polycrystalline (polycrystalline grain) Quartz M P P Polycrystalline (P) Monocrystalline (M) M with overgrowth (formed during diagenesis) P&S, Fig. 5.8 Feldspar • Fsp. ~60% of most rocks • Fsp / qz ratio: ~3:1 in igneous + metamorphic rocks ~1:5 in sandstones • Less hard than qz; cleavage, chemically reactive • Alters to: clays (kaolinite, sericite) carbonates Feldspars Feldspar crystal Blue = pore space (crystal largely dissolved during deep burial) Feldspar Calcite crystals growing within feldspar Dissolution along cleavage planes Volcanics Lithic Grains Many are fine-grained rocks: volcanics slates carbonates Many are resistant rocks: chert quartzite Proportion of lithics decreases as grain size decreases Chert (silica) Plutonic Igneous Metamorphic Shale P&S, Fig. 5.11 Micas • derived from granitic and metamorphic rocks • mainly muscovite, also biotite (less stable) • platy form -- settle slowly, with fines Heavy Minerals • • • • • • • • • Tourmaline Apatite Garnet Zircon Rutile Magnetite Cassiterite Monazite rarely gold • rarely >1% of a sandstone • density 3.0 - 5.2 -concentrated with coarse quartz • Form placer deposits and paleoplacers Heavy Minerals Extracting heavy minerals from old beach sands, Australia Heavy Minerals Heavy minerals (titano-magnetite), form black spreads in the Shubenacadie river, NS Glauconite • yellow-green mineral ("greensands") • K-Ferric silicate • an Fe-rich clay related to illite • forms on marine shelves, under mildly reducing conditions • forms within shells and fecal pellets, and coats exposed sediment surfaces Matrix • Clay + silt deposited at same time as framework grains • clay minerals, silt-sized qz and fsp, carbonaceous material, Fe oxides • Note: some clays in sandstones are: authigenic (formed within the rocks) Cement Most common cements: • Quartz • Calcite • Dolomite • Siderite • Hematite • Limonite • Mn oxides • Pyrite • Gypsum • Barite • Precipitate form groundwaters during burial • precipitation reflects local chemistry and nucleation factors Black = Pyrite grains (cement) Grain Types and Grain Size in gravel and coarse sand in sand + silt in fine silt + clay % of sediment Grain Size SANDSTONE CLASSIFICATION • Provides information about: a) provenance (source rocks from which components derived) b) transport processes • Concept of maturity: Physically mature -all grains well rounded all grains same size no matrix all grains spherical Chemically mature -all grains are quartz Stable Grains 100% = HIGHLY MATURE Matrix 100% Unstable Grains 100% SANDSTONE CLASSIFICATION • Pettijohn’s classification • • • • • • • Note: based on QFL triangles -- qz, fsp & lithics at corners quartzite and chert grouped with qz fsp and lithics cover most of the triangle area Pettijohn also uses matrix % no simple scheme for physical maturity needs thin section -- rarely possible in hand specimen SANDSTONE CLASSIFICATION Wackes Arenites Quartz Arenite Arkosic Arenite Q F Lithic Arenite L 1) Quartz Arenites >95% quartz Grains well sorted and rounded, little matrix Quartz concentrated during prolonged, deep weathering (breaks down feldspars) Common where weathered cratons drowned by sealevel rise Quartz Arenites 2) Arkosic Arenites Abundant feldspar + micas + heavies – low maturity Poorly sorted, angular grains Red colour common Calcite cement common Rapid exhumation, erosion and burial of sediment near to the source 3) Lithic Arenites Abundant -- 56% of 718 samples: van Andel, 1958 Broad drainage basins with many rock types available Himalaya Mountains Lithic Arenites (Volcanic) Volcanic source areas, Iceland: black volcanic sands Ice cap above volcano Outburst flows Braided river plain 4) Wackes (greywackes) • Matrix-rich (>15%), with sericite, chlorite, siltsized quartz • May be rich in feldspars, lithics (commonly volcanic), angular grains, poorly sorted, calcite cement • mainly turbidity current deposits near active tectonic belts? • Note: not all turbidity currents are matrix-rich Tectonic Classification Major Tectonic Settings TOTAL QUARTZ Qt Link between grain type and tectonic setting: Dickinson, 1979 F FELDSPAR L LITHICS Phalen Colliery: Gas Outbursts from Sandstones Adit on Phalen Coal, New Waterford “Disking” of core: gas release Phalen Colliery: Gas Outburst Alteration of K-feldspar Single Feldspar grain – Totally altered Phalen Colliery: Gas Outburst Model for Feldspar Diagenesis During Deep Burial