weathering & formation of sediment.doc

8/17/2019 weathering & formation of sediment.doc

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Laboratory 3

Weathering of Rocksand

Formation of SedimentPamela J. W. Gore

Georgia Perimeter College

Clarkston, GA 3002

Copyright © 1998-2004 Pamela J. W. Gore

!his la" introd#ces the $rod#cts of rock %eathering. Weathering is im$ortant

"eca#se it is the $rocess thro#gh %hich rocks are "roken do%n and sediment isformed. Sediment is loose $artic#late material %hich "ecomes cemented and

com$acted to form sedimentar& rocks.

!'P(S )F W(A!*(R+G

!here are three ma-or t&$es of %eathering

1. physial2. hemial3. biologial

Physial !eatheri"g "reaks rocks do%n into smaller $ieces. !&$es of$h&sical %eathering incl#de frost %edging, e/foliation, and thermale/$ansion.

Chemial !eatheri"g "reaks rocks do%n chemicall& adding or remoingchemical elements, and changes them into other materials. Chemical

%eathering consists of chemical reactions, most of %hich inole %ater.!&$es of chemical %eathering incl#de

• #issol$tio"

• hy#rolysis

• o%i#atio"

&iologial !eatheri"g is the "reakdo%n of rock ca#sed "& the action ofliing organisms, incl#ding $lants, "#rro%ing animals, and lihe" 1acr#st&, r#""er&, light green organic material that gro%s in $atches onrocks as %ell as on %ood. ichen is a com"ination of f#ng#s and algae,liing together in a s&m"iotic relationshi$. ichens can lie on "are rock,

and the& "reak do%n rocks "& secreting acids and other chemicals. !he

f#ngal $art of the association secretes the acids, %hich react to dissolethe minerals, %hich are then #sed "& the algae. ater, %ater see$s into the


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creices etched "& the acid, and assists in the "reakdo%n thro#gh free4ing1frost5%edging and chemical %eathering.

'(P)* +, P(*C/L W)/')G

. ,rost !e#gi"g 5 %ater e/$ands %hen it free4es, "reakingrocks into ang#lar fragments

Talusslope, Lost River, West Virginia

!he a"oe $hotogra$h ill#strates a sediment so#rce

area. !he "edrock is "eing "roken do%n intosediment of a ariet& of si4es, $rimaril& "& $h&sical

%eathering $rocesses.

2. )%oliatio" 5 the "edrock "reaks into flat sheets along -oints %hich $arallel the gro#nd s#rface. !his $henomenon isca#sed "& the e/$ansion of rock %hen the $ress#re ofoerl&ing rock is remoed "& erosion. +t is someti#mescalled #nloading.

(/foliation of granite at Stone 6o#ntain has $rod#ced aro#nded mo#ntain. At the time the granite "od& cooled, it is

calc#lated that the land in this area stood a"o#t 0,000 fthigher than at $resent. )er the $ast 327 million &ears, this0,000 ft of rock has "een eroded a%a&.


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Stone 6o#ntain, Georgia. Stone 6o#ntain is a granite "od&%hich is a sediment so#rce area. !he second image sho%sactie e/foliation.

3. 'hermal e%pa"sio" 5 heat ca#ses e/$ansion8 coolingca#ses contraction. 9ifferent minerals e/$and and contract

at different rates ca#sing stresses along mineral "o#ndaries.Re$eated dail& heating and cooling of rock ca#ses the rock

to "reak do%n.

'(P)* +, C)C/L W)/')G

. issol$tio" alters rocks "& remoing sol#"le minerals.6inerals s#ch as halite, g&$s#m, and calcite are sol#"le1dissole in %ater 1es$eciall& %ater that is slightl& acidic.When the minerals react %ith %ater, ions 1s#ch as Ca and

a are released. !he ions are carried as :dissoled load: "&riers flo%ing to lakes or to the sea. As lake or sea %ater

ea$orates, the dissoled minerals $reci$itate or cr&stalli4eo#t as solid minerals. 1(/am$les halite, g&$s#m, or calcite.!hese t&$es of minerals that form from the ea$oration ofsea %ater are called e5aporites. 6inerals ma& also$reci$itate or cr&stalli4e from gro#nd %ater in and aro#nd

s$rings 1$artic#larl& hot s$rings, and in caes 1e/am$le 5traertine.


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2. y#rolysis is the $rocess "& %hich felds$ar 1and someother al#min#m5"earing silicate minerals are %eathered toform cla&. For e/am$le, $otassi#m felds$ar %eathers to formthe mineral kaolinite.

2;AlSi3) Al2Si2)71)*> + 2;=

+ 2*C)35

+ >Si)2

$otassi#mfelds$ar

car"onicacid

%ater

kaolinite

1cla& formedthro#gh

%eathering

$otassi#m

ion1dissoled

in %ater

"icar"onate

ion1dissoled

in %ater

silica1dissoled

in %ater

+n h#mid climates 1s#ch as the so#theastern ?nited States, mostof the felds$ar in rocks s#ch as granite %ill %eather to form cla&.

earl& all of the minerals in the common rocks of the (arth@s cr#st%ill %eather to form cla& 1%ith the e/ce$tion of #art4. Beca#se of this, cla&s make #$ nearl& half of the sedimentar& rocks on (arth.

+%i#atio" is the $rocess "& %hich iron5"earing minerals%eather to $rod#ce iron o/ides 1or :r#st:. +ron5"earing silicateminerals %hich also contain al#min#m 1s#ch as $&ro/ene,

am$hi"ole, and "iotite #ndergo "oth o/idation and h&drol&sis,forming "oth iron o/ides and cla&s. +ron5"earing al#mino5silicate

minerals %eather to form the red cla&e& soils, s#ch as are fo#nd inGeorgia, as %ell as lateritic soils formed in more tro$ical areas.

6+(RA S!AB++!' + !*( W(A!*(R+G (+R)6(!

Some minerals %eather more #ickl& than others. A fe% minerals arereadil& sol#"le in slightl& acidic %ater, %hereas others %eather to $rod#cecla&, and still others are er& resistant to %eathering, and $ersist for along time %itho#t alteration. )ne of the controls on the %eathering ofminerals is the tem$erat#re at %hich the minerals originall& formed %hen

the& cr&stalli4ed from magma or laa.

i"erals !hih orme# at high temperat$res a"# press$res are

least stable i" the !eatheri"g e"5iro"me"t6 a"# !eather most7$ily. !his is "eca#se the& are farther from their :4one of sta"ilit&:, orthe conditions #nder %hich the& formed. )n the other hand, mi"erals

!hih orme# at lo!er temperat$res a"# press$res are moststable $"#er !eatheri"g o"#itio"s.

!he order in %hich minerals tend to %eather is related to the tem$erat#re

at %hich the& cr&stalli4ed. 'o# ma& remem"er Bo%en@s Reaction Series,%hich descri"ed the order in %hich minerals cr&stalli4e from magma. !here

is a similar ordering of minerals as related to their %eathering rates, and itis called the Gol#ih *tability *eries.

!he order of mineral sta"ilit& in the %eathering enironment is the sameorder as &o!e"s eatio" *eries.

Least stable (high temperature minerals)


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Olivine Ca plagioclase feldspar

Pyroxene

Amphibole

Biotite Na plagioclase feldspar

Potassium feldspar

Muscovite uart!

Most stable (lo" temperature minerals)

What ha$$ens %hen granite is %eatheredD

?n%eathered granite contains these minerals

. a $lagioclase felds$ar 1%hite2. ; felds$ar 1$ink, "#t ma& "e %hite in

other granites3. E#art4 1gra&>. Small amo#nts of "iotite andor am$hi"ole

1"lack

7. and sometimes m#scoite 1not sho%n

*ere is %hat %ill ha$$en to each of the mineral constit#ents in agranite #nder %arm, h#mid %eathering conditions

. !he el#spars %ill #ndergo hy#rolysis to form kaolinite1lay and a and ; ions

2. !he so#i$m a"# potassi$m io"s %ill "e remoed thro#gh

leaching and %ill "e carried i" sol$tio" in r#nning %ater3. !he biotite a"#:or amphibole %ill #ndergo hy#rolysis to

form cla&, and o%i#atio" to form iro" o%i#es.>. !he 7$art; 1and m$so5ite, if $resent %ill remain as

resi#$al mi"erals "eca#se the& are er& resista"t to!eatheri"g.

?nder %arm, h#mid conditions, the granite "edrock %ill %eather in

$lace #ntil the felds$ars alter to soft cla&. !he %eathered rock iscalled saprolite, a term meaning :rotten rock:. +n areas of the

so#theastern ?.S. %hich are #nderlain "& granite 1and otherigneo#s and metamor$hic rocks, a thick soil 4one of %eathered

rock or sa$rolite has deelo$ed. Where the "edock contained iron5"earing minerals 1s#ch as "iotite, am$hi"ole, or $&ro/ene %hich%eathered to iron o/ides, the sa$rolite has "een stained a dee$ redcolor. 1!his is the same $rinci$le "& %hich one red sock in a load ofla#ndr& can stain all of the clothes red. !he red5stained cla& in thesa$rolite has gien rise to the famo#s Georgia e# Clay.

What ha$$ens after the rock has "een %eathered to sa$roliteD

. !he lays %ill "e eroded and trans$orted "& r#nning %aterto the sea. Cla& is fine5grained and remains s#s$ended inthe %ater col#mn. !he cla& ma& #ltimatel& "e de$osited in

dee$ 7$iet !ater far from shore.


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2. As the soft cla& is remoed, the #n%eathered, resid#al#art4 grains %ill "e released from the sa$rolite "& erosion.!he #art4 in granite is sand5si4ed, and it "ecomes 7$art;sa"#. !he #art4 sand is #ltimatel& trans$orted to the sea,%here it acc#m#lates to form beahes.

3. !he #issol5e# io"s 1sodi#m and $otassi#m %ill "e

trans$orted "& riers to the sea, and %ill "ecome $art of thesalts in the sea.

!*( C*ARAC!(R+S!+CS )F S(9+6(!

'errige"o$s se#ime"t is deried from the %eathering of $re5e/isting

rocks. 1Sometimes it is also called lasti or *LCCL/*'C )R #etritalsediment. !he grain si4e of sediment de$ends on the t&$es of rocks in theso#rce area from %hich the sediment %as deried. !he te/t#res and

mineralog& of the rocks in the so#rce area control the grain si4e andcom$osition of the res#lting sediment.

)*C&G ') ')


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Very inesand

0.0625 - 0.125 mm

Silt

1!256 - 1!16 mm"or 0.004 - 0.0625 mm#

Clay $ 1!256 mm"or $ 0.004 mm#

Gra5el forms thro#gh $h&sical %eathering of rock. A $iece of grael is #s#all& a :rockfragment: com$osed of more than one mineral. Sometimes a $iece of grael is a singlemineral, most commonl& #art4. !his is "eca#se #art4 is sometimes $resent as eins,%hich ma& "e seeral inches %ide 1or more, th#s $rod#cing grael5si4ed clasts.

*a"# forms thro#gh the "reakdo%n and disintegration of rocks %hich hae sand5si4ed1 5 2mm grains, s#ch as granite and gneiss.

+n h#mid climates, #art4 sand grains are released from granite after the felds$ar grainsalter to cla& "& chemical %eathering 1h&drol&sis. +n more arid areas, granite "reaks do%n"& $h&sical %eathering 1s#ch as frost %edging, releasing "oth felds$ar and #art4 grains.

*ilt originates from the chi$$ing of coarser grains d#ring sediment trans$ort, or from thedisintegration of fine5grained cr&stalline rocks 1s#ch as slates, $h&llites, and schists.

Clay originates $rimaril& thro#gh chemical %eathering of felds$ars and other al#mino5silicate minerals 1those %hich contain al#min#m and silicon. !he term :cla&: refers to a$artic#lar si4e of sediment $article, %hich co#ld "e a #art4 grain or a cla& mineral flake,or some other er& small mineral fragment. !he term :cla&: is also #sed to refer to agro#$ of minerals. !here are a n#m"er of cla& minerals, incl#ding aoli"ite 1the %hitecla& mined in central Georgia and #sed for shin& coatings on $a$er, and addities tor#""er, illite 1%hich contains $otassi#m, and mo"tmorillo"ite or smetite 1a gro#$of cla&s %hich can take in large amo#nts of %ater, and as a res#lt these cla&s arecommonl& referred to as :s%elling cla&s:.

*+'G

*orti"g refers to the range in grain si4es in a sediment or sedimentar& rock.Sediment 1or rock %hich is !ell sorte# %ill hae most of the grains ro#ghl& thesame si4e. A poorly sorte# sediment or rock has a %ide range of grain si4es.Sorting can "e estimated #sing a is#al com$arison chart.

+=)**

Ro#ndness is a meas#re of the shar$ness or ro#ndness of the corners of asedimentar& $article. Ro#ndness is determined "& com$aring the sand grains%ith a is#al com$arison chart.

As sediment is trans$orted, it #ndergoes abrasio" "& coming into contact %iththe stream "ottom, sea5floor, or other grains of sediment. !he a"rasion tends to:ro#nd5off: the shar$ edges or corners. Ro#nding is also related to the si4e ofthe grains. Bo#lders tend to ro#nd m#ch more #ickl& than sand grains "eca#sethe& strike each other %ith m#ch greater force.

*P)C'(

Grains of sediment are three dimensional. S$hericit& refers to :e#aldimensions:. +s the sediment $article elo"gate# 1one dimension longer than theother t%o, latte"e# or sheet-lie 1one dimension m#ch smaller than theother t%o dimensions, or is it s$herical 1its three dimensions ro#ghl& the samelengthD S$hericit& can "e descri"ed as high or lo%. According to this definition,a "all %o#ld hae highl& s$hericit&, "#t so %o#ld a c#"e 1high s$hericit&, "#t lo%ro#ndness. +n contrast, a s#"marine sand%ich %o#ld hae lo% s$hericit&, "#thigh ro#ndness. A shoe"o/ %o#ld hae "oth lo% s$hericit& and lo% ro#ndness.Sand grains ma& hae high or lo% s$hericit&. Some minerals ma& $rod#ce

http://www.eos.ubc.ca/courses/eosc221/sed/sili/siligsize.htmlhttp://www.eos.ubc.ca/courses/eosc221/sed/sili/siligsize.htmlhttp://www.eos.ubc.ca/courses/eosc221/sed/sili/siligrshap.htmlhttp://www.eos.ubc.ca/courses/eosc221/sed/sili/siligrshap.htmlhttp://www.eos.ubc.ca/courses/eosc221/sed/sili/siligsize.htmlhttp://www.eos.ubc.ca/courses/eosc221/sed/sili/siligrshap.html


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elongated or flattened grains, de$ending $rimaril& on original cr&stal sha$e andcleaage.

Be caref#l not to conf#se ro#nding %ith s$hericit&. A %ell5ro#nded grain ma& orma& not resem"le a s$here. And a s$herical grain ma& or ma& not "e %ellro#nded.

')P)'G ') ')


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2. *tage 2 - *$bmat$re 5 Poorl& sorted sediment %ith no m#d3. *tage 3- at$re 5 Well sorted sediment %ith no m#d>. *tage 4 - *$permat$re 5 Well sorted and ro#nded sediment %ith no m#d

!hree ste$s are inoled

. Wi""o!i"g or %ashing o#t of fines 5 makes an immat#re sediment "ecome

s#"mat#re2. *orti"g of grain si4es 5 makes a s#"mat#re sediment "ecome mat#re3. o$"#i"g 5 makes a mat#re sediment "ecome s#$ermat#re

)*C&G ') )/L+GC C+P+*'+ +, */*

!he minerals in sands 1and in sandstones can "e identified #sing a microsco$e 1or ahandlens if a microsco$e is not aaila"le. +dentif&ing the minerals $resent is im$ortant"eca#se sandstones are classified "ased on the com$osition of their grains.

!hree com$onents are considered %hen naming sandstones

. ?$art; grains2. ,el#spar grains3. Fine5grained ro ragme"t grains. Possi"ilities incl#de shale, slate, $h&llite,

"asalt, rh&olite, andesite, chert, and $ossi"l& schist. imestones %o#ld not "eincl#ded #s#all& "eca#se the& dissole so readil&.

!he three ma-or t&$es of sandstone are

. ?$art; sa"#sto"e 1also called 7$art; are"ite 5 %hich is dominated "&7$art;

2. /rose 5 %hich is dominated "& el#spar 3. Lithare"ite or lithi sa"#sto"e 1commonl& "#t im$recisel& called gray!ae

5 %hich is dominated "& ro ragme"t grai"s.

)ther minerals ma& also "e $resent in sands and sandstones. +n fact, in some areas,sands ma& "e com$osed almost entirel& of minerals other than #art4 and felds$ar. Fore/am$le, at White Sands ational 6on#ment in e% 6e/ico, the sands are com$osed ofg&$s#m grains. !here is a "each on the so#thern end of the Big +sland of *a%aii that hasgreen sand com$osed of oliine grains. !here are "eaches in tro$ical areas in man& $artsof the %orld that are com$osed almost entirel& of the sand5si4ed shells and shellfragments of marine organisms 1made of calci#m car"onate 5 calcite or aragonite.

+t is im$ortant to kee$ in mind that :sand: is a texture term, not a composition term. Asand can "e com$osed of an& t&$es of sand5si4ed mineral or rock5fragment grains.

+n addition to the ma-or constit#ents in sand, there is often a s#ite of hea5y mi"erals 1those %ith high s$ecific grait& 5 greater than 2.


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I?$date 5 +n A#g#st 2003, 9#Pont donated a ,000 acre tract of land ad-acent to the)kefenokee S%am$ to the Conseration F#nd, a non5$rofit land $reseration gro#$. !hisdonation $ermanentl& $rotects the acreage from deelo$ment and mining.

I?$date +n A$ril 200>, another com$an& +l#ka Reso#rces !( Consolidated 1see $. oflinked article, o$ened a ne% stri$ mining for titani#m -#st east of ah#nta on high%a&


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colorless

Am$hi"ole (longated to fi"ro#s, greenish

Biotite Bro%n, flat sheets, shin&

*ematite Red

P&rite Brass& gold, metallic,

')P)'G ') )/L+GC C+P+*'+ +, */*

(ach t&$e of sandstone im$lies something a"o#t de$ositional histor&

. ?$art; sa"#sto"e im$lies a long time in the de$ositional "asin.2. /rose im$lies a short time in the de$ositional "asin 1"eca#se felds$ar t&$icall&

%eathers #ickl& to cla&. Arkose also im$lies ra$id erosion, arid climate,tectonic actiit&, stee$ slo$es.

3. Lithare"ite im$lies ra$id erosion, tem$erate or arid 1not h#mid climate

As noted a"oe, the $artic#lar s#ite of hea& minerals $resent in sand also can tell a lota"o#t the so#rce area from %hich the sediment is deried.

R(A9+G !*( R(C)R9 + !*( R)C;SA SA9S!)( +!(RPR(!A!+) G?+9(

)ne of the goals in *istorical Geolog& is to tr& to inter$ret the de$ositional conditions of thesedimentar& rocks that make #$ the geologic record.

Sandstone te/t#res and com$ositions ma& "e #sed to inter$ret man& things a"o#t the histor& ofthe sand, incl#ding so#rce area litholog&, $aleoclimate, tectonic actiit&, $rocesses acting in the

de$ositional "asin, and time d#ration in the "asin. Remem"er that the so#rce area is the land %hichis %eathering and eroding to s#$$l& terrigeno#s de"ris to the de$ositional "asin.

*+=C) /)/ L'+L+G(

Com$osition gies the ke& information 1minerals or rock fragments $resent. Remem"erthat #art4 sandstone or #art4 arenite is dominated "& 7$art; grains8 arkose isdominated "& el#spar grains 1#s#all& $otassi#m felds$ar8 and gra&%acke is dominated"& ro ragme"t grains.

• Sand5si4ed 7$art; grains co#ld come from the %eathering of so#rce area rocks

s#ch as granite, gneiss, or other sandstones %hich contain #art4 1rec&cledsandstones.

• Sand5si4ed el#spar grains co#ld come from the %eathering of so#rce area

rocks s#ch as granite or gneiss.

•

Sand5si4ed ro ragme"t grains come from the %eathering of fine5grainedso#rce rocks. Possi"ilities incl#de shale, slate, $h&llite, "asalt, rh&olite, andesite,chert, and $ossi"l& schist.

P/L)+CL/')

Paleoclimate refers to the climate %hich e/isted in the so#rce area. We are $artic#larl&concerned %ith %eathering rates here. Remem"er that in h$mi# climates, felds$ar%eathers to lay "& hy#rolysis. )ther minerals also %eather to cla& 1%ith associated irono/ides, s#ch as oliine, $&ro/ene, and am$hi"ole.

Also remem"er the difference "et%een !eatheri"g 1BR(A;9)W of rock "& h&drol&sis,dissol#tion, o/idation, e/foliation, frost %edging, or free4e tha%, and erosion1!RASP)R!A!+) of $articles.


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• +f el#spar is $resent in &o#r sand, it indicates that the climate %as $ro"a"l&

arid. 1)r that erosion rates %ere er& ra$id, and that tectonic actiit& %ase/tremel& high 5 lots of #$lift,and stee$ slo$es.

• +f 7$art; is the dominant mineral in the sand, the climate %as $ro"a"l& h$mi#

1all of the felds$ars %eathered a%a& to cla&.

• +f ro ragme"ts are $resent in &o#r sand, it hel$s to kno% %hat litholog&

the& are. +f the& are rock t&$es %hich %o#ld %eather ra$idl& 1s#ch as "asalt orlimestone fragments, the climate %as $ro"a"l& arid. +f the& are rock t&$es%hich %o#ld "e relatiel& sta"le 1shale, slate, or chert, the climate ma& hae"een tem$erate to h#mid. 1remem"er Bo%en@s Reaction Series and the GoldichSta"ilit& Series to determine %hat is sta"le or #nsta"le. +f rock fragments are$resent and no rock t&$es are gien, a good com$romise ans%er %o#ld "etem$erate climate.

')C'+C /C'A'( ') *+=C) /)/

We are "asicall& classif&ing tectonic actiit& as Bati5eB or Bpassi5eB. For a good model,consider the !est oast o the =* as teto"ially ati5e 5 stee$ slo$es, mo#ntainsclose to the sea, lots of earth#akes, tectonic #$lift, and olcanic actiit&. )n the otherhand, consider the east oast o the =* as teto"ially passi5e 5 "road, flat coastal$lain, fe% or no earth#akes, no #$lift, and no olcanic actiit&.

• +f a sand has a lot of el#spar or ro ragme"ts, it $ro"a"l& indicates high

teto"i ati5ity.

• +f a sand has a lot of 7$art;6 it $ro"a"l& indicates lo% tectonic actiit& 5 a

passi5e setti"g.

!ectonic actiit& also infl#ences sorting, time d#ration in the de$ositional enironment1and to some e/tent, com$ositional mat#rit&. *igh tectonic actiit& might $rod#ce ra$idd#m$ing of sediments into the "asin %ith little or no time for sorting. o% tectonic actiit&means little #$lift, lo% erosion rates, and therefore little sediment s#$$lied to the "asin8%hat sediment that is there is likel& to %ash aro#nd for a long time and "ecome %ellsorted and ro#nded, and grains other than #art4 are likel& to "e destro&ed 1"& a"rasionor chemical %eathering.

P+C)**)* /C'G ') )P+*'+/L &/* !his refers to e"ergy le5els 1:high: s. :lo%: and o"siste"y of energ&. !e/t#re giesthe ke& information.

Grai" si;e

• Coarse sediments generall& indicate high energ&, and fine sediments indicate lo%

energ&.

*orti"g

• Well sorte# se#ime"ts indicate o"siste"t6 airly high e"ergy le5els.

1Winno%ing and %ashing.

• Poorly sorte# se#ime"ts indicate i"o"siste"t e"ergy le5els 5 ra$id

d#m$ing 1%hich might inole short e$isodes of high energ&, follo%ed "& lo%

energ& conditions.

') =/'+ ') )P+*'+/L )A+)'

Both mineralog& and te/t#re can "e #sed to determine time in the de$ositionalenironment.

A sand %ith a"#ndant #art4 grains s#ggests a long time in the de$ositional enironment.E#art4 is more resistant to a"rasion than felds$ar or rock fragments.

A sand %ith a"#ndant felds$ar or rock fragment grains s#ggests a short time in thede$ositional enironment.

!e/t#ral mat#rit& is also #sef#l in inter$reting time in the de$ositional enironment.

+mmat#re or s#"mat#re sediments $ro"a"l& s$ent onl& a short time in the "asin "efore"#rial. 6at#re or s#$ermat#re sediments %ere $ro"a"l& rolling aro#nd in the "asin for along time "efore "#rial. o$"#"ess is a good cl#e to a long time in the de$ositional


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enironment. Ro#nding of grains takes a long time8 it is more likel& in a tectonicall&$assie sit#ation. 9esert sands are often %ell ro#nded "eca#se of the :sand"lasting:$rocess of %ind trans$ort. *ence, in an arid desert, it is $ossi"le to get a %ell5ro#nded1s#$ermat#re arkose.

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