Sedimentary Sedimentary TexturesTextures

Sediment TextureSediment Texture►Grain sizeGrain size►SortingSorting►Grain rounding & grain shapeGrain rounding & grain shape►Grain FabricGrain Fabric

What are terrigenous clastic sediments?What are terrigenous clastic sediments?

► Mdstn, s.s., cong., Mdstn, s.s., cong., brecciabreccia

► Clastic sediments Clastic sediments formed from formed from fragments or clasts fragments or clasts derived from pre-derived from pre-existing rkexisting rk

► detrital grains = detrital grains = mostly quartz & mostly quartz & feldspar - abradedfeldspar - abraded composition composition

depends on:depends on: distance of distance of

transport transport source area source area

compositioncomposition climateclimate diagenetic diagenetic

processesprocessesGrus, Arizona http://alliance.la.asu.edu/slides/stop03/3a/3a.html

What is Grain What is Grain Size?Size?

► Grain size scales usedGrain size scales used Udden scale combined with Udden scale combined with

Wentworth classesWentworth classes gravel = sed > 2mm = gravel = sed > 2mm =

loose sed - don’t mix apple loose sed - don’t mix apple and orangesand oranges

Kubein introduced phi scaleKubein introduced phi scale► Phi=-logPhi=-log22SS► Easier to plot data Easier to plot data ► gravel = sed > 2mm = gravel = sed > 2mm =

loose sed - don’t mix loose sed - don’t mix apple and orangesapple and oranges

► The greater the distance of The greater the distance of transport the smaller the grain transport the smaller the grain size.size.

► Caution. Grain size may reflect Caution. Grain size may reflect what is available to be what is available to be transportedtransported

Sediment/Rock ClassificationSediment/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 DataGraphical Representation of Grain Size Datahttp://www.epa.gov/apti/bces/module3/distribu/distribu.htm

Good grain size exercise hereGood 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.

Grain Grain SortingSorting

► Measure of grains size distributionMeasure of grains size distribution effectiveness of dep. med in effectiveness of dep. med in

separating grains of dif sizesseparating grains of dif sizes► Sorting determined bySorting determined by

sed source (granite vs s.s.)sed source (granite vs s.s.) grains size - coarse sed more grains size - coarse sed more

poorly sortedpoorly sorted depositional mechanismdepositional mechanism

► Poor sorting usually means limited Poor sorting usually means limited transporttransport

► Better sorting usually means Better sorting usually means greater distance of transportgreater distance of transport

► Rivers are good examplesRivers are good examples► Used to distinguish seds of dif. env.Used to distinguish seds of dif. env.

Interpretation of Grain SizeInterpretation of Grain Size► Distinguish seds of Distinguish seds of

dif env;.info on dep dif env;.info on dep processes & flow processes & flow conditions conditions

► Use w/ sed Use w/ sed structuresstructures

► Problems to considerProblems to consider sandstone sandstone

reworking or sand reworking or sand supplied by pre-supplied by pre-existing env.existing env.

f. gr. matrix—f. gr. matrix—depositional/ depositional/ diagenetic diagenetic

► Dif processes Dif processes operate in 1 operate in 1 environment and environment and similar process similar process occur in dif envoccur in dif env

Can we distinguish this braided river environment using grain size alone?

Grain Morphology Grain Morphology ►Shape, sphericity and Shape, sphericity and roundnessroundness

►Descriptive shapesDescriptive shapesoblateoblate equantequantbladedbladedprolateprolate

►

Grain Grain RoundingRounding

► Roundess = curvature of corners of grainsRoundess = curvature of corners of grains angular to well roundedangular to well rounded sphericity = grains approach to spheroidal shapesphericity = grains approach to spheroidal shape For env. interp, roundness more significant than For env. interp, roundness more significant than

sphericitysphericity need 3d view for sphericallyneed 3d view for spherically

► Grain morphology depend onGrain morphology depend on mineralmineral nature of source rknature of source rk degree of weatheringdegree of weathering degree of abrasiondegree of abrasion corrosion or solution during diagenesiscorrosion or solution during diagenesis generally -- roundness increases w/length of transportgenerally -- roundness increases w/length of transport beach & desert generally better rounded than river or beach & desert generally better rounded than river or

glacial outwashglacial outwash► Caution of roundingCaution of rounding

may be inheritedmay be inherited intense abrasion may lead to fracturing an angular intense abrasion may lead to fracturing an angular

grainsgrains

Grain Fabric--Orientation, packing & Grain Fabric--Orientation, packing & boundariesboundaries

► orientationorientation Grain alignmentGrain alignment prolate pebble prolate pebble

parallel (sliding) parallel (sliding) perpendicular (roller)perpendicular (roller)

glacial sed--long axis glacial sed--long axis parallel to flow also parallel to flow also turbiditesturbidites

commonly have commonly have imbricationimbrication

sand grains sand grains commonly parallel to commonly parallel to flow flow

grain orientation can grain orientation can be used as be used as paleocurrentpaleocurrent

http://www.umt.edu/geosciences/faculty/hendrix/g100/L6B.html

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

Usefulness of FabricUsefulness of Fabric►Get info of dep processesGet info of dep processes

matrix-supported cong--mudflow, glacial matrix-supported cong--mudflow, glacial deposits, debris flowdeposits, debris flow

river congl. grain supported, little matrixriver congl. grain supported, little matrix use fabric w/ sed structuresuse fabric w/ sed structures

Textural MaturityTextural Maturity► Immature = lots of matrix, poor sorting, angular grainsImmature = lots of matrix, poor sorting, angular grains► Mature = little matrix, mod-good sorting, well roundedMature = little matrix, mod-good sorting, well rounded► Supermature = no matrix, v. good sorting, well roundedSupermature = no matrix, v. good sorting, well rounded► Provides clue to depositional processProvides clue to depositional process

► persistent currents = more mature sedspersistent currents = more mature seds► immature seds = fluvial and glacialimmature seds = fluvial and glacial► more mature = desert, beach, shallow marinemore mature = desert, beach, shallow marine

Grain RoundingGrain Rounding

► Roundness = curvature of Roundness = curvature of corners of grainscorners of grains

► Have angular to well-roundedHave angular to well-rounded► The rounder the grain, the The rounder the grain, the

greater the distance of greater the distance of transport transport

► Rounded quartz grains imply Rounded quartz grains imply many cycles of uplift, erosion, many cycles of uplift, erosion, depositiondeposition

► Beach and desert better Beach and desert better rounded than river/glacial seds rounded than river/glacial seds

► Large grains round fastestLarge grains round fastest