SEDIMENTARY TEXTURES:

Transcription

1 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 mm mm 4 Major Categories clay silt sand gravel 4 subcategorizes (very fine, fine, medium, coarse) 1

2 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 Grain Size Data diam. (mm) 2

3 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

4 Graphical/Statistical Representation of Grain Size histogram and frequency curves- weight % of each size 14 Grain Size Data weight 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 well sorted poorly sorted arithmetic scale coarse phi size fine 4

5 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 well sorted poorly sorted 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 ø ø ø ø ø >4.00ø sorting very well sorted well sorted moderate well sorted moderate sorted poorly sorted very poorly sorted extremely poorly sorted 5

6 weight Size Distribution Skewness - natural sediment populations tend to show asymmetry (either fine or coarse) Normal Distribution 16% Grain Size Data X 1! 1! 68.27% 84% phi size weight Grain Size Data Median Mean Kurtosis - degree of peakness (narrow vs. broad) Negatively Skewed Mode phi size coarse fine Textural/Mineralogical Maturity Example of composition of detrital constituents in sediments of varying grain size 6

7 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

8 Photomicrograph-subrounded quartz grains which are single crystal Quartz grains - subangular to subrounded Sources: 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

9 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

10 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