SPRING 2007GEOLOGY 285:INTRO. PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversitySandstones and Conglomerates• Detrital or clastic sedimentary rocks – made of solid products (detritus) from weathering of pre-existing rocks• They make up 20-25% of the stratigraphic record, but receive much more attention from sedimentary petrologists than 25%What geologists want to learn from Sandstones• Source area – rock type– direction– weathering environment• Transport– medium, energy– distance• Depositional environment– marine or non-marine– physical environment (beach, river, delta, etc.)What clues are present in Sandstones?•Grain size•Grain shape• Grain sorting• Grain mineralogy• Sedimentary structuresGrain size• Detrital or clastic rocks have a huge range in grain size• We need a log scale to represent this wide size range•The Phi (φ) Scale: φ = -log2(mm)mm = 2- φ(memorize)• Each φ step represents a doubling (smaller φ# or more neg.) or halving (larger φ#) in sizeFor example• size in mm = 2- φ•-6 φ = 26mm = 64 mm•0 φ = 20mm = 1 mm•2 φ = 2-2mm = 1/4mm = 0.25 mm•4 φ = 2-4mm = 1/16mm = 0.0625 mmSize ranges are given namesGravel > -1 φ (>2mm)Sand: 4 φ to -1 φ(0.0625mm to 2mm)Mud < 4 φ<0.0625mm<62.5 μmClay < 8 φ< 1/256<0.004mm<4 μmLoose sediments can be separated by sieving•-2φ (4mm)•-1φ (2mm)•1φ (0.5mm)•2φ (0.25mm)•3φ (0.125mm)•4φ (0.062mm)•closedGrain size comparator for lithified SandstonesSorting = range in grain size• Usually the size range (in φ) that includes 2/3of the grains% of grainsφ size% of grainsφ sizepoorly-sorted well-sortedSorting by comparisonGrain Shape• Sphericity - relative equidimensionality of three mutually perpendicular axes• Roundness - lack of sharp corners; larger grains round faster because of more impactsHigh sphericityLow sphericityIncreasing roundnessMudrocks• Composed mostly of detrital material smaller than 4φ, i.e., smaller than 0.062 mm or 62.5 μm (mud=silt+clay)• Non-fissile (-stone) or fissile (-shale)• Named by proportion of Silt and Clay> 2/3 silt Siltstone Silt-shale1/3 to 2/3 silt Mudstone Mud-shale>2/3 clay Claystone Clay-shaleConglomerates• No agreement about the % of gravel sized material required to make a sediment a GRAVEL or a sedimentary rock a CONGLOMERATE• We’ll say >30% gravel size material for a conglomerate• Small amount of pebbles or cobbles is very noticeable, so bewareClassification of Sandstones is based on Detrital Minerals•Quartzis the most abundant sand-size grain– very stable in sedimentary environment•Feldsparmay be abundant– may indicate rapid burial, dry climate, granite in source area– K-feldspar vs. Plagioclase? • Unstable Lithic Fragments are least abundant (least stable), but most informative– Chert, a stable lithic fragment, goes with QuartzAccessory Minerals (other minerals)• Micas - may float in water; muscovite, especially is very stable• Heavy minerals - higher density than quartz and feldspar, some are very stable: zircon, tourmaline, rutile, hornblende, garnet, ilmenite, magnetite, apatite, pyroxene, etc. If loose sand or disaggregated sandstone is put in a heavy liquid (s.g. 2.8-3.0) “heavies” sink to the bottom• These minerals can be quite informativeDetrital Grain Types: QuartzChert is a stable lithic fragment, grouped with QuartzFeldspars distinguished from quartz by alteration, twinning and perthiteWhat’s the large grain?Lithic FragmentsVolcanic and Plutonic Lithic Fragments in a
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