200
Chapter 5 Oceanic Sediments
a
SILICA DIAGENESIS
POROSITY
Si0 2 CONTENT
LlTHOLOGY
(PREDOMINANTLY
SILICEOUS)
o 20 40 60 80 %
O+-__ ~ __ - L __ -L __ ~-, __ ~
500
1000
1500
m
OPAL-A
I
/
/
/
/
(:!: INTACT
SKELETONS)
I //
-=- -...=--=t=\.
OPAL-CT
(+ SOLUTION
AND REPRECIPITATION.
+ MECHANICAL
COMPACTION)
QUARTZ
CHALCEDONY
I "
I
"I
"I
I
I
I
I /
1:../
I '
I \,
"- "I
'
I
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I
I
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~<
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DEPTH
°c ~ "
TEMPERATURE \,
SILICEOUS MJ\RLS AND LIMESTONES
b
LIMESTONE WITH
.l~" CHERT NODULES
DIRECTION ~. , ..
OF GROWTH :':
. ':,
.
- ,.'
..
IMPURE CHERT ... .,.,:'
Fig. 5.6. a Processes and results of silica diagenesis
in pelagic, siliceous oozes. (Based on several
sourees, see text). Note that banded cherts may be
partially generated by a kind of rhythrnic unmixing,
which accentuates primary minor differences in texSILICEOUS
OOZE
:!: BIOTURBATED
:!: LAMINATED
. PORCELLANITE
AND NEW CLAY
MINERALS
(ZEOLITES)
I-:-::..iiiiiil ...... CHERT
MIGRATiON OF Si02 BY DIFFUSION
(AND ADVECTlON)
c MARL WITH
~~. LAYERS AND
NODULES OF
CHERT AND
LIMESTONE
CALCAREOUS
TURBIDITE
CHERT
SILICEOUS
CLAYSTONE
CHERT
(10+30 %
IMPURITIESj
e VEINLETS
OF CHERT
1~~~f1~~
ture or composition of beds. b-d Diagenetically
formed bands and concretions of chert or carbonate
in limestones and marls. e Veinlets of chert or laminated marl deformed after silica (quartz) precipitation. (After Lancelot 1973)