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Sandstones.  Framework fraction –Silicate grains 1/16 to 2 mm  Matrix –Cement and very fine-size material <~0.03 mm.

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Presentation on theme: "Sandstones.  Framework fraction –Silicate grains 1/16 to 2 mm  Matrix –Cement and very fine-size material <~0.03 mm."— Presentation transcript:

1 Sandstones

2  Framework fraction –Silicate grains 1/16 to 2 mm  Matrix –Cement and very fine-size material <~0.03 mm

3 Framework Mineralogy  Major Minerals –Quartz –Feldpars –Clay minerals & fine micas  Accessory minerals  Rock fragments  Chemical cements

4 Quartz  Most Stable: –Greatest resistance to chemical decomposition –Multiple recycling  50-60% of framework fraction  Monocrystalline  Polycrystalline  Undulatory extinction  Origin: felsic plutonic rocks-granites, metamorphic rocks, and older sandstones

5 Feldspars  10-20% of framework grains  Alkali feldspars (K-feldspars) –orthoclase, microcline, sanidine, anorthoclase –More abundant in sandstones –Plutonic or Metomorphic origin  Plagioclase feldspars –Albite (Na), oligoclase, andesine, labradorite, bytownite, anorthite(Ca) –More abundant in sandstones derived from volcanic rocks –Felsic or continental crust origin

6 Clay minerals & fine micas  clay minerals –kaolinite group – illite group –smectite group –chlorite group  fine micas –muscovite – biotite

7 Accessory Minerals  <~1-2%  Include: muscovite, biotite, & heavy minerals (specific gravity > 2.9)  Muscovite more stable than biotite, more abundant  Easy to concentrate  Stable nonopaque-zircon, tourmaline, rutile  Metastable nonopaque-amphiboles, pyroxenes, garnet, apatite, epidote, topaz monazite  Stable opaque-hematite, limonite  Metastable opaque-magnetite, ilmentite, leucoxene.

8 Rock Fragments  10-15% of framework mineral grains  Range from 0-95%  Igneous: cystalline colvanic rock and volcanic glass are most common in sandstones  Metamorphic: metaquartzie, schist, phyllite, slate, argillite, and less commonly gneiss clasts  Sedimentary: Chert-microcrystalline quartz  All preserved as sand-size fragments

9 Mineral Cements  Silicate Minerals: –Quartz –Microquartz (Chert) –Opal –Feldspars –Zeolites

10 Mineral Cements  Most common: –quartz, calcite, clay minerals, and hematite  Also pyrite, gypsum, and barite can also form cements under special geologic conditions

11 Quartz Cements  form in environments of high energy currents, such as beach deposits, marine bars, desert dunes, and some fluvial sandbars  most of the quartz cements are derived from the sands themselves or quartz sands

12 Quartz Cements  Overgrowths- rim of cement where the quartz cement is chemically attached to the crystal lattice of existing quartz grains  Syntaxial- when overgrowth retains crystallographic continuity of the grain

13 Mineral Cements  Carbonate Minerals: –Calcite –Aragonite –Dolomite –Siderite

14 Calcite Cements  patchy cement soluble in surface waters  Often partially dissolved cements  secondary porosity

15 Mineral Cements  Iron Oxide Minerals: –Hematite –Limonite –Goethite

16 Hematite Cements  hematite cement indicates an oxidizing environment during diagenesis  most common oxidation state is Fe +2  Fe +2 is brought near the surface where the iron oxidizes to Fe +3 and can be carried away by hydrous fluids  Precipitation of Fe +3 forms hematite (Fe 2 O 3 )

17 Mineral Cements  Sulfate Minerals: –Anhydrite –Gypsum –Barite

18 Sulfate Mineral Cements  Barite (BaSO 4 ) can form if the fluids are rich in Ba  Gypsum (CaSO 4. H 2 O) can from if the fluids are oxidizing and rich in sulfur

19 Sulfate Mineral Cements  Sand Crystals- crystallographically continuous crystals in the cement when the cements form near the surface

20 Sands of the Gulf of Mexico

21 Sands in the Gulf of Mexico I. Eastern Gulf of Mexico - Kyanite + Staurolite (32%) derived from metamorphic rock in the Appalachian Mountains. II. Mississippi River Province - Augite (23%), Hornblende (40%), Epidote (16%), and Garnet (3%) derived from glacial deposits in upper Mississippi River drainage. III. Central Texas Province - Hornblende (58%), Epidote (17%), and Garnet (7%) but no Augite. Mostly from Colorado River of Texas.

22 Sands in the Gulf of Mexico VI. Rio Grande Province - Epidote (15%), Hornblende (23%), Augite (24%), and brown hornblende from volcanic rocks (7%). VII. Mexican Province - There are few studies of these sands, but they are expected to be similar to Rio Grande Province, reflecting a volcanic source.

23 QFL Classification

24 Arenites

25 Wackes

26 Characteristics of Sandstones tell us:  Source area –rock type –current directions –weathering environment  Transport –medium, energy –distance  Depositional environment –marine or non-marine –physical environment (beach, river, delta, etc.)

27 Increased Textural Maturity:  clay removal  increased sorting  increased rounding  breakdown (absence) of unstable fragments  breakdown (absence) of unstable minerals

28 Super-mature Sandstones:  Clean (no mud matrix)  well-sorted  well-rounded grains  mostly quartz grains  quartz arenites  Cratonic, typically recycled, formed in beach or other high energy environment

29 References  h8_sands.html h8_sands.html  ndst&cong.htm ndst&cong.htm  15-99SsClassn/sld001.htm 15-99SsClassn/sld001.htm

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