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POST DEPOSITIONAL PROCESSES
DIAGENESIS: physical, chemical and biological changes
ha sedimen is s!b"eced o a#e$ g$ains a$e deposied%hich $es!l in
a ROC&Diagenesis $es!ls #$om a sligh inc$ease in empe$a!$e 'd!e
o geohe$mal
g$adien( %hich s!pplies he aci)aion ene$gy needed o ma*e he
mine$als mo$e sol!bleand mo$e d!cile+
LITI-ICATIONThe p$ocess by %hich loose de$ial g$ains a$e
con)e$ed o a solid mass+
This !s!ally enails boh compacion and some *ind o#
cemenaion+
.y CO/PACTION/!ds/ode$n m!ds consis o# abo! 012 %ae$+ The %ae$
m!s be s3!ee4ed
o! and compaced o ma*e a m!dsone+ This %ill #o$m a $oc* i# 51 o
612
compacion occ!$s+
SandsI# yo! %al* on %e sand he %ae$ doesn7 s3!ee4e o!+ 8!a$4
g$ains a$e
!s!ally %ell $o!nded and sphe$ical o o)al in shape+ So %hen hey
pac* he$e %illal%ays be )oid space, h!s p$od!cing po$osiy+
Depending on he so$ing and $o!ndness, sand %ill ha)e 95
52 po$espace+ In lihi#ied sandsone he pe$cenage o# po$e space
'#illed %ih cemen o$
)oid( is 152+ The$e is a di##e$ence o# ;52 d!e o compacion
alone+ -o$
sands o lihi#y mo$e han compacion is needed+
lihic sandsI# he sand g$ains a$e $oc* #$agmens, $ahe$ han 3!a$4
o$ #eldspa$
g$ains, compacion can be almos oal and po$osiy %ill be abo! 12+
In s!ch
cases he o$iginal hic*ness o# he lihic sands, once lihi#ied, a$e
$ed!ced by 512+
This occ!$s beca!se he lihic class ac as d!cile #$agmens and
$ec$ysalli4e
.y RECRTION AND REPLACE/ENTP$ess!$e sol!ion may a*e place,
%he$eby g$ains 'especially 3!a$4( ha
a$e !nde$ p$ess!$e %ill dissol)e a he high p$ess!$e poins and
$ep$ecipiae a he
lo%?p$ess!$e a$eas '*no%n as p$ess!$e shado%s(+
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.y CE/ENTATIONCemenaion is he g$o%h o# ne% mine$al mae$ ino po$e
space ha
c$eaes an @inimae@ conac+ The deg$ee o# lihi#icaion depends on
he amo!no# cemen o g$ain conac+ A small amo!n o# cemen $es!ls in
friable$oc*s+
Occ!$$ence and ype o# cemen
;+ 3!a$4Cemenaion by 3!a$4 is con#ined mosly o sandsones %hose
de$ialcomposiion is 1?;112 3!a$4+ The $e)e$se ho%e)e$, is no $!e+
Be
#ind 3!a$4 cemen in $oc*s #o$med #$om ancien beaches, ma$ine
sandba$s, dese$ d!nes and some #l!)ial sandba$s+ 8!a$4 cemen
is$a$e in all!)ial #ans and !$bidie se3!ences+ 8!a$4 cemened
3!a$4
a$enies a$e )e$y $esisan o %eahe$ing and ypically sand o!
in$elie#+
+ ca$bonaeCa$bonae commonly occ!$s in sandsones o# all
deposiionalen)i$onmens ando$ mine$al composiion+ Ca$bonae
cemened
sandsone can g$ade lae$ally ino sandy limesone and hen o
p!$e
limesone+
+ hemaieemaie impa$s a $ed colo$ o sandsones+ I is p$esen in
amo!ns o#;2 o$ less+ In some se3!ences, he di##e$en inensiies o#
$edcolo$aion can be seen, $e#lecing he #ac ha mos hemaie #o$ms
a#e$ sedimen is deposied+ -e is o#en leached #$om -e?/g
mine$als
in he sedimen and hen $edeposied as cemen+
9+ claysClay mine$als canno be seen e)en %ih a hand lens,
ho%e)e$, i# i isp$esen in amo!ns g$eae$ han ;12, i %ill coa he sand
g$ains, and i
can be seen+
Permeability versus PorosityThis is impo$an d!$ing cemenaion
beca!se he$e can be a$eas in he sedimens
ha a$e locally mo$e pe$meable and $es!l in $oc*s %ih locally
cemened a$eas %ihin a
mo$e #$iable laye$ o# sandsone o$ silsone+ These a$eas a$e *no%n
as conc$eions and
can ha)e a %ide )a$iey o# shapes and si4es+
