SEDIMENTARY TEXTURES: What are they? Grain size Grain shape Particle Surface texture Grain fabric Grain Size Depositional environment - subaerial, water depth, energy Sediment maturity - source, weathering and transport processes µm to meters Scales: logarithmic or geometric 1.Geometric scales: Wentworth Scale: each value 2x previous range 0.0039 mm - 256 mm 4 Major Categories clay silt sand gravel 4 subcategorizes (very fine, fine, medium, coarse) 1
Wentworth 2. Logarithmic Scales Phi Scale convenient for plotting phi ø = -log 2 d d=grain diameter (mm) For example: 1 mm = 0 ø Grain Size ø size 6.0 5.0 4.0 3.0 2.0 1.0 0.0-1.0 Grain Size Data -2.0 0 0.5 1 1.5 2 2.5 diam. (mm) 2
Measuring Grain Size caliper - large grains, consolidated rocks (gravel) sieving - weigh samples (all sizes) settling techniques: function of D & fluid viscosity (sand, clay) pipetting (clays) V = CD 2 D = V C V = settling velocity, C= constant, D = diameter spherical particles settle faster than non-spherical particles grain size counters - lasers, etc. Image analysis - digital Coulter Counter Electrical conductivity - resistance a function of number & size of particles - larger particles, greater change in resistance 3
Graphical/Statistical Representation of Grain Size histogram and frequency curves- weight % of each size 14 Grain Size Data 12 10 weight 8 6 4 2 0-2.0-1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 phi size Graphical/Statistical Representation of Grain Size cumulative curve- cumulative weight percent (frequency) vs. size. shape is independent of sieve interval linear or log probability scale cumulative weight percent 100 80 60 40 20 well sorted poorly sorted 0-2 -1 0 1 2 3 4 5 6 arithmetic scale coarse phi size fine 4
Grain Size Distribution mode - most frequently occurring grain size steepest point on the cumulative curve (inflection) median size - midpoint of grain size distribution (ie., 50th percentile) mean size - average approximation - averaging of selected percentiles from the cumulative curve (e.g.,16%, 50%, 84%) M z = ( ø 16 + ø 50 + ø 84 ) / 3 cumulative weight percent 100 80 60 40 20 well sorted poorly sorted 0-2 -1 0 1 2 3 4 5 6 arithmetic scale phi size Size Sorting measure of the range of grain sizes, and the magnitude and spread of grain sizes Standard deviation (s.d.) - mathematical expression of sorting 68% of the grain size values lies within plus or minus one s.d. of the mean size σ i = ( ø 84 - ø 16 ) / 4 + ( ø 95 - ø 5 ) / 6.6 s.d. <0.35ø 0.35-0.50ø 0.50-0.71ø 0.71-1.00ø 1.00-2.00ø 2.00-4.00ø >4.00ø sorting very well sorted well sorted moderate well sorted moderate sorted poorly sorted very poorly sorted extremely poorly sorted 5
weight Size Distribution Skewness - natural sediment populations tend to show asymmetry (either fine or coarse) 14 12 10 8 6 4 2 Normal Distribution 16% Grain Size Data X 1! 1! 68.27% 84% 0-2.0-1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 phi size weight 14 12 10 8 6 4 2 Grain Size Data Median Mean Kurtosis - degree of peakness (narrow vs. broad) Negatively Skewed Mode 0-2.0-1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 phi size coarse fine Textural/Mineralogical Maturity Example of composition of detrital constituents in sediments of varying grain size 6
Particle Shape Defined by three related attributes. form - refers to gross, overall configuration of particles roundness - measure of the sharpness of grain corners surface texture - small scale relief (pits, scratches, ridges) Form (sphericity) - relative lengths of the 3 major axes, long (L), intermediate (I), short (S). Affects settling V! = 3 Vofaparticle Vofcircumscribingsphere I/L! = D sd I 3 2 D L S/I Roundness ratio of the average radius of curvature of corners (r i ) to that of the largest inscribing circle (R) Function of Grain hardness (mineral type) Transport process Wind vs. Water Distance transported N= # of corners 7
Photomicrograph-subrounded quartz grains which are single crystal Quartz grains - subangular to subrounded Sources: http://plaza.snu.ac.kr/%7elee2602/atlas/atlas.html Surface Texture General features observable by eye or hand lens: - Polished - frosted Numerous microscale features (SEM) including; Pits, scratches, fractures, ridges Created by: mechanical abrasion chemical erosion (etching) chemical precipitation (authigenesis) General surface textures in 3 depositional environments: Eolian (wind) - smooth, round, silica solution and precipitation Littoral (waves) - v-shaped percussion marks, conchoidal breakage patterns Glacial (ice) - conchoidal fracture patterns, parallel to semi-parallel striations Conchoidal fracture on garnet sand grain (Cormier, 2000) Quartz overgrowths and dissolution 8
Framework Grains Matrix Cement Rock Fabric Grain Orientation - show some preferred orientation reflects on current direction & velocity (e.g. imbrication of pebbles) Orientation depends on shape - elongate or platy pebbles oriented w/ long axes normal (or parallel) to flow direction - imbrication to 10 to 15 imbricated pebbles within Quaternary alluvial fan deposits, Owens Valley, California 9
Rock Fabric Grain Packing - dense vs. porous Contacts; floating, tangential, etc. floating tangential The grain to grain alignment effects porosity Cubic packing (47.6% porosity) Rhombohedral packing (26.0% porosity) 10