150
Chapter 3 Coastal and Shallow Sea Sediments
a SEA-WATEA
C MIXING ZONE
~
.j..
~
HUMID
CIACULATION
LOW PER·
, . ' ,,~VADOSE ZONE
\
ME ABILITY
, ' -_-_ '. ',' PREVAILING
SEA .LEVEL
__ ••• v. ••••.• _ •• _-- DISSOLUTION
- - - - - - --.:;.;:- - - - - .-;.
-- -:-.::':" IIII'~ ~~:-'II .=-=- ('j :>:~- PHAEATICZONE
. . : . _ .. . . : . .
~ . . . . ~ ~ . : . : :.?t , : ; ;#j· ,', ,:. ;,3;_, =- ~'I M~llli~I"I::'!\":';II':"'r:' -:-~~ ~ , " ::'_, g~S~~N\~~I,g~ AND
. . -
. ' , 11111I1I.'II"'I~ """, --' . ~ (' . - -=-.' (LOW-Mg CALCITE)
= -=- "11'"' ~ , \ . ~ . ......::;-. .... -
' , '
MARINE CEMENTS
MIXING OF FRESH OR BRACKISH
•
. ',' (ARAGONITE . Mg CALCITE)
WATER WITH MARINE PORE -WATER:
DEEP .BURIAL
" ,
LlTTLE OR NO CEMENTATION
DOLOMITIZATION
r-- DIAGENESIS
COMPACTION
..... ~ .... k
LOW- Mg CALCITE
_ 11 ~
SOME DOLOMITE
d :,Mrt~:l~'~ _ ~ ~':'t~
~::_~"'" REEF •• ~. ~::::;= ... ' :. COMPACTION FLOW
b EVAPORATIVE REFLUX
(ARID CLiMATE)
HYPERSAlINE
BRINE
EVAPORATION
:!: GYPSUM
MIXING OF HIGH ·Mg BRINE
WITH MARINE PORE WATER:
DOLOMITI ZATION !: CHERT
Fig. 3.30. Diagenesis of reefs and carbonate buildups
(overview). a Early marine cementation. b Dolomitization by evaporative reflux of lagoonal brine
(after precipitation of gypsum). c Dissolution and
cementation in the zones influenced by meteoric water as weIl as dolomitization in the mixing zone between freshlbrackish water and marine pore water.
d Compaction flow and deep-burial diagenesis (calcarbonate sediment becomes more solid and the total
structure undergoes significant compaction. Reef
cores are subject to less compaction than their surrounding finer grained sediments. Their relief is
therefore enhanced and, in addition, they tend to
drain the expelled pore-water from their neighboring
sediments (Fig. 3.30d). Neomorphism usually occurs
simultaneously with the filling of pore space. Skeletal aragonite and fine crystalline mosaics of other
carbonate minerals are replaced by coarser calcite.
Magnesium, released by dissolution of high-Mg-calcite, can form a limited amount of dolomite, but this
process cannot explain pervasive dolomitization of
large limestone bodies.
Dolomitization
Biota secrete calcareous skeletons but not dolomite.
Chemical precipitation of primary dolomite does not
occur in normal sea-water; it is known only from
some lakes under specific environmental and
e COORONG MODEL (SEMI-ARID)
OPEN L AGOON • . EPHEMERAL
SEA
MIXING:
DOLOMITE
cite and some dolomite). e Lacustrine prirnary
dolomitic muds on top of marine and lagoonal skeletal carbonates, caused by Mg-rich groundwater/seawater mixing (Coorong model). (Based on Blatt et aL
1980; Sellwood 1986; Schroeder and Purser 1987;
Von der Borch and Lock 1979; Warren 1990, and
others)
hydrogeochemical conditions (see Coorong model
below). On the other hand, dolomites and dolomitic
rocks (dolostones) are very common in the geological
record.
The transformation of limestones (mainly CaC03)
to dolomites (mainly CaMg(C0 3 )2) is referred to as
dolomitization. It is dear from several lines of evidence that dolomite has replaced calcareous material
and is therefore secondary in origin. This is indicated, e g., by incomplete dolomitization of ca1careous rocks with irregular boundaries, altemations of
dolomitic rocks with limestones, their (partly obliterated) fossil content, etc.
Significant dolomitization of ca1careous carbonate
buildups is only possible in open systems when large
quantities ofMg++ can be exchanged for Ca++ by circulating pore waters, particularly sea water (see Sect.
13.3). For reasons mentioned above, reefs are especially susceptible to early and late dolomitization.
Many artic\es and books have dealt with this complex
topic. It can be inferred from these studies that there is no
Chapter 3 Coastal and Shallow Sea Sediments
a SEA-WATEA
C MIXING ZONE
~
.j..
~
HUMID
CIACULATION
LOW PER·
, . ' ,,~VADOSE ZONE
\
ME ABILITY
, ' -_-_ '. ',' PREVAILING
SEA .LEVEL
__ ••• v. ••••.• _ •• _-- DISSOLUTION
- - - - - - --.:;.;:- - - - - .-;.
-- -:-.::':" IIII'~ ~~:-'II .=-=- ('j :>:~- PHAEATICZONE
. . : . _ .. . . : . .
~ . . . . ~ ~ . : . : :.?t , : ; ;#j· ,', ,:. ;,3;_, =- ~'I M~llli~I"I::'!\":';II':"'r:' -:-~~ ~ , " ::'_, g~S~~N\~~I,g~ AND
. . -
. ' , 11111I1I.'II"'I~ """, --' . ~ (' . - -=-.' (LOW-Mg CALCITE)
= -=- "11'"' ~ , \ . ~ . ......::;-. .... -
' , '
MARINE CEMENTS
MIXING OF FRESH OR BRACKISH
•
. ',' (ARAGONITE . Mg CALCITE)
WATER WITH MARINE PORE -WATER:
DEEP .BURIAL
" ,
LlTTLE OR NO CEMENTATION
DOLOMITIZATION
r-- DIAGENESIS
COMPACTION
..... ~ .... k
LOW- Mg CALCITE
_ 11 ~
SOME DOLOMITE
d :,Mrt~:l~'~ _ ~ ~':'t~
~::_~"'" REEF •• ~. ~::::;= ... ' :. COMPACTION FLOW
b EVAPORATIVE REFLUX
(ARID CLiMATE)
HYPERSAlINE
BRINE
EVAPORATION
:!: GYPSUM
MIXING OF HIGH ·Mg BRINE
WITH MARINE PORE WATER:
DOLOMITI ZATION !: CHERT
Fig. 3.30. Diagenesis of reefs and carbonate buildups
(overview). a Early marine cementation. b Dolomitization by evaporative reflux of lagoonal brine
(after precipitation of gypsum). c Dissolution and
cementation in the zones influenced by meteoric water as weIl as dolomitization in the mixing zone between freshlbrackish water and marine pore water.
d Compaction flow and deep-burial diagenesis (calcarbonate sediment becomes more solid and the total
structure undergoes significant compaction. Reef
cores are subject to less compaction than their surrounding finer grained sediments. Their relief is
therefore enhanced and, in addition, they tend to
drain the expelled pore-water from their neighboring
sediments (Fig. 3.30d). Neomorphism usually occurs
simultaneously with the filling of pore space. Skeletal aragonite and fine crystalline mosaics of other
carbonate minerals are replaced by coarser calcite.
Magnesium, released by dissolution of high-Mg-calcite, can form a limited amount of dolomite, but this
process cannot explain pervasive dolomitization of
large limestone bodies.
Dolomitization
Biota secrete calcareous skeletons but not dolomite.
Chemical precipitation of primary dolomite does not
occur in normal sea-water; it is known only from
some lakes under specific environmental and
e COORONG MODEL (SEMI-ARID)
OPEN L AGOON • . EPHEMERAL
SEA
MIXING:
DOLOMITE
cite and some dolomite). e Lacustrine prirnary
dolomitic muds on top of marine and lagoonal skeletal carbonates, caused by Mg-rich groundwater/seawater mixing (Coorong model). (Based on Blatt et aL
1980; Sellwood 1986; Schroeder and Purser 1987;
Von der Borch and Lock 1979; Warren 1990, and
others)
hydrogeochemical conditions (see Coorong model
below). On the other hand, dolomites and dolomitic
rocks (dolostones) are very common in the geological
record.
The transformation of limestones (mainly CaC03)
to dolomites (mainly CaMg(C0 3 )2) is referred to as
dolomitization. It is dear from several lines of evidence that dolomite has replaced calcareous material
and is therefore secondary in origin. This is indicated, e g., by incomplete dolomitization of ca1careous rocks with irregular boundaries, altemations of
dolomitic rocks with limestones, their (partly obliterated) fossil content, etc.
Significant dolomitization of ca1careous carbonate
buildups is only possible in open systems when large
quantities ofMg++ can be exchanged for Ca++ by circulating pore waters, particularly sea water (see Sect.
13.3). For reasons mentioned above, reefs are especially susceptible to early and late dolomitization.
Many artic\es and books have dealt with this complex
topic. It can be inferred from these studies that there is no