Geol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Spring 2007 Sandstones and Conglomerates – Clastic Sedimentary Rocks20-25% of stratigraphic record, but receive much more attention fromsedimentary 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- (memorize) - Each - step represents a doubling (smaller # or more neg.) or halving (larger #) 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 mmSize ranges are given names (See handout of Udden-Wentworth scale)Gravel larger than -1- > 2 mmSand 4- to -1- 0.0625 mm to 2 mmMud 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 SandstoneSorting = 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 mmor 62.5 -m (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-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
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