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 (φ) Scale: φ = -log2 (mm) • mm = 2-φ (memorize) • Each φ step represents a doubling (smaller # or more neg.) or halving (larger #) in size For example • size in mm = 2- φ • -6 φ = 26 mm = 64 mm • 0 φ = 20 mm = 1 mm • 2 φ = 2-2 mm = 1/4 mm = 0.25 mm• 4 φ = 2-4 mm = 1/16 mm = 0.0625 mm Size ranges are given names (See handout of Udden-Wentworth scale) Gravel larger than -1φ > 2 mm Sand 4φ to -1φ 0.0625 mm to 2 mm Mud smaller than 4φ < 0.0625 mm Clay smaller than 8φ < 0.004 mm < 4 μm Loose sediments can be separated by sieving • -2φ (4mm) • -1φ (2mm) • 1φ (0.5mm) • 2φ (0.25mm) • 3φ (0.125mm) • 4φ (0.062mm) • closed Grain size comparator for lithified SandstoneSorting = range in grain size • Usually the size range (in φ) 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 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-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 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 • 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
View Full Document
Unlocking...