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Sedimentary Textures

Sedimentary Textures. Sediment Texture. Grain size Sorting Grain rounding & grain shape Grain Fabric. What are terrigenous clastic sediments?. Grus, Arizona http://alliance.la.asu.edu/slides/stop03/3a/3a.html. Mdstn , s.s ., cong., breccia

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Sedimentary Textures

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  1. Sedimentary Textures

  2. Sediment Texture • Grain size • Sorting • Grain rounding & grain shape • Grain Fabric

  3. What are terrigenous clastic sediments? Grus, Arizona http://alliance.la.asu.edu/slides/stop03/3a/3a.html • Mdstn, s.s., cong., breccia • Clastic sediments formed from fragments or clasts derived from pre-existing rk • detrital grains = mostly quartz & feldspar - abraded • composition depends on: • distance of transport • source area composition • climate • diagenetic processes

  4. What is Grain Size? • Grain size scales used • Udden scale combined with Wentworth classes • gravel = sed > 2mm = loose sed - don’t mix apple and oranges • Kubein introduced phi scale • Phi=-log2S • Easier to plot data • gravel = sed > 2mm = loose sed - don’t mix apple and oranges • The greater the distance of transport the smaller the grain size. • Caution. Grain size may reflect what is available to be transported

  5. Gravel Sand Silt Clay Sediment/Rock Classification Calculating Ǿ Ǿ=particle size in phi units d= diameter of particle in mm Negative sign in formula is so that phi values come out positive Calculate phi if grain size= ½ mm.

  6. Graphical Representation of Grain Size Datahttp://www.epa.gov/apti/bces/module3/distribu/distribu.htmGood grain size exercise here • Using the graph, determine the approximate particle size at 15.78, 50, and 84.13 percent probability. • Determine the mass median particle diameter.                         • Determine the geometric standard deviation of the particle mass distribution.      • Determine the geometric standard deviation of the particle mass distribution.

  7. Grain Sorting • Measure of grains size distribution • effectiveness of dep. med in separating grains of dif sizes • Sorting determined by • sed source (granite vs s.s.) • grains size - coarse sed more poorly sorted • depositional mechanism • Poor sorting usually means limited transport • Better sorting usually means greater distance of transport • Rivers are good examples • Used to distinguish seds of dif. env.

  8. Interpretation of Grain Size • Distinguish seds of dif env;.info on dep processes & flow conditions • Use w/ sed structures • Problems to consider • sandstone reworking or sand supplied by pre-existing env. • f. gr. matrix—depositional/ diagenetic • Dif processes operate in 1 environment and similar process occur in dif env Can we distinguish this braided river environment using grain size alone?

  9. Grain Morphology • Shape, sphericity and roundness • Descriptive shapes • oblate • equant • bladed • prolate

  10. Grain Rounding • Roundess = curvature of corners of grains • angular to well rounded • sphericity = grains approach to spheroidal shape • For env. interp, roundness more significant than sphericity • need 3d view for spherically • Grain morphology depend on • mineral • nature of source rk • degree of weathering • degree of abrasion • corrosion or solution during diagenesis • generally -- roundness increases w/length of transport • beach & desert generally better rounded than river or glacial outwash • Caution of rounding • may be inherited • intense abrasion may lead to fracturing an angular grains

  11. Grain Fabric--Orientation, packing & boundaries • orientation • Grain alignment • prolate pebble parallel (sliding) perpendicular (roller) • glacial sed--long axis parallel to flow also turbidites • commonly have imbrication • sand grains commonly parallel to flow • grain orientation can be used as paleocurrent http://www.umt.edu/geosciences/faculty/hendrix/g100/L6B.html

  12. Grain contacts • point, concavo convex, sutured contacts, free floating

  13. Usefulness of Fabric • Get info of dep processes • matrix-supported cong--mudflow, glacial deposits, debris flow • river congl. grain supported, little matrix • use fabric w/ sed structures

  14. Textural Maturity • Immature = lots of matrix, poor sorting, angular grains • Mature = little matrix, mod-good sorting, well rounded • Supermature = no matrix, v. good sorting, well rounded • Provides clue to depositional process • persistent currents = more mature seds • immature seds = fluvial and glacial • more mature = desert, beach, shallow marine

  15. Grain Rounding • Roundness = curvature of corners of grains • Have angular to well-rounded • The rounder the grain, the greater the distance of transport • Rounded quartz grains imply many cycles of uplift, erosion, deposition • Beach and desert better rounded than river/glacial seds • Large grains round fastest

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