Sedimentary Textures

Sediment Texture ► Grain size ► Sorting ► Grain rounding & grain shape ► Grain Fabric

What are terrigenous clastic sediments? ► 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 Grus, Arizona

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=-log 2 S ► 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

Sediment/Rock Classification Gravel Sand Silt Clay 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.

Graphical Representation of Grain Size Data Good grain size exercise here Using the graph, determine the approximate particle size at 15.78, 50, and percent probability. Determine the mass median particle diameter. Determine the geometric standard deviation of the particle mass distribution.

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.

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?

Grain Morphology ► Shape, sphericity and roundness ► Descriptive shapes  oblate  equant  bladed  prolate ►

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

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

Grain contacts ► point, concavo convex, sutured contacts, free floating

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

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

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