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 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.
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.)
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