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Geol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Spring 2007 Sandstones and Conglomerates – Clastic Sedimentary Rocks 20-25% of 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 structures Grain 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 () Scale: = -log2 (mm) mm = 2- Each step represents a doubling (smaller # or more neg.) or halving (larger #) (memorize) in size For example size in mm = 2- -6 26 mm = 64 mm 0 = 20 mm = 1 mm 2 = 2-2 mm = 1/4 mm = 0.25 mm 4 = 2-4 mm = 1/16 mm = 0.0625 mm Size ranges are given names (See handout of Udden-Wentworth scale) Gravel larger than -1 > 2 mm Sand 4 to -1 0.0625 mm to 2 mm Mud smaller than 4 mm Clay smaller than 8 mm m Loose sediments can be separated by sieving -2 (4mm) -1 (2mm) 1 (0.5mm) 2 (0.25mm) 3 (0.125mm) 4 (0.062mm) closed Grain size comparator for lithified Sandstone Sorting = range in grain size Usually the size range (in ) that includes 2/3 of the grains Sorting by comparison Grain Shape Sphericity - relative equidimensionality of three mutually perpendicular axes Roundness - lack of sharp corners; larger grains round faster because of more impacts Mudrocks Composed mostly of detrital material smaller than 4, i.e., smaller than 0.062 mm or 62.5 m (mud=silt+clay) Non-fissile (-stone) or fissile (-shale) Named by proportion of Silt and Clay > 2/3 silt Siltstone Silt-shale 1/3 to 2/3 silt Mudstone Mud-shale >2/3 clay Claystone Clay-shale Conglomerates No agreement about the % of gravel sized material required to make a sediment a GRAVEL or a sedimentary rock a CONGLOMERATE Let’s say >30% gravel size material for a conglomerate Small amount of pebbles or cobbles is very noticeable Classification of Sandstones is based on Detrital Minerals Quartz is most abundant sand-size grain very stable in sedimentary environment Feldspar may be abundant may indicate rapid burial, dry climate, granite in source area K-feldspar vs. Plagioclase? Unstable Lithic Fragments are least abundant, but most informative Chert, a stable lithic fragment, goes with Quartz Accessory 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 Can be quite informative Detrital Grain Types: Quartz Chert is a stable lithic fragment, grouped with Quartz Feldspars distinguished from quartz by alteration, twinning and perthite What’s the large grain? Lithic Fragments Volcanic and Plutonic Lithic Fragments in a Sandstone