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 (Fe 2 O 3 )

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

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

Sulfate Mineral Cements  Sand Crystals- crystallographically 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  h8_sands.html h8_sands.html  ndst&cong.htm ndst&cong.htm  SsClassn/sld001.htm SsClassn/sld001.htm