Transcript Sandstones
Sandstones Sandstones Framework fraction – Silicate grains • 1/16 to 2 mm Matrix – Cement and very fine-size material • <~0.03 mm Framework Mineralogy Major Minerals – Quartz – Feldpars – Clay minerals & fine micas Accessory minerals Rock fragments Chemical cements 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 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 Clay minerals & fine micas clay minerals – – – – kaolinite group illite group smectite group chlorite group fine micas – muscovite – biotite 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. 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 Mineral Cements Silicate Minerals: – – – – – Quartz Microquartz (Chert) Opal Feldspars Zeolites Mineral Cements Most common: – quartz, calcite, clay minerals, and hematite Also pyrite, gypsum, and barite can also form cements under special geologic conditions 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 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 Mineral Cements Carbonate Minerals: – – – – Calcite Aragonite Dolomite Siderite Calcite Cements patchy cement soluble in surface waters Often partially dissolved cements secondary porosity Mineral Cements Iron Oxide Minerals: – Hematite – Limonite – Goethite 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 (Fe2O3) Mineral Cements Sulfate Minerals: – Anhydrite – Gypsum – Barite Sulfate Mineral Cements Barite (BaSO4) can form if the fluids are rich in Ba Gypsum (CaSO4.H2O) can from if the fluids are oxidizing and rich in sulfur Sulfate Mineral Cements Sand Crystalscrystallographically continuous crystals in the cement when the cements form near the surface Sands of the Gulf of Mexico 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. 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. QFL Classification Arenites Wackes 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.) Increased Textural Maturity: clay removal increased sorting increased rounding breakdown (absence) of unstable fragments breakdown (absence) of unstable minerals 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 References http://www.science.ubc.ca/~geol256/notes/c h8_sands.html http://www.tulane.edu/~sanelson/geol212/sa ndst&cong.htm http://www.geo.wvu.edu/~lang/Geol185/Pet 15-99SsClassn/sld001.htm