Download - Dolomitization (AAPG Course Notes 24) [Lynton Stuart Land]

8/11/2019 Dolomitization (AAPG Course Notes 24) [Lynton Stuart Land]

1/21

Dolomitization

Presented at the 1982 AAPG Fall Education Conference

in Denver, Colorado.

Education Course Note Series 24

Lynton S. Land

University of Texas at Austin

fflP


2/21

Dolomite

It is probably safe to s tat e t ha t in 1982 no single model of dolomitization unequivocally

accoun ts for all aspects of any m assively dolomitized ancient limestone. All models have

significant flaws, and our unde rstan ding of the dolomitization process and its relation to other

diagenetic processes (silicification, stylolitization, organic maturation, etc.)

is

imperfect.

Ra ther th an advoca te one solution over another, Iwilltry to summarize some of the strengths

and w eaknesses of several of the models which have been proposed.

As a s tar tin g point I will review several imp ortan t aspects of dolomite mineralogy and

chem istry th at place cons train ts on all models and th at are sometimes overlooked.

Mineralogy

Dolomite is a rhombohedral carbon ate w ith the ideal formula

CaMg[C0

3

2

in which calcium

and m agnesium occupy preferred

sites.

In th e ideal

m ineral,

planes of

C0

3

anions alterna te

with planes of cations w ith the c-axisof the crysta l perpendicular to the altern ating stacked

anion and cation planes . Ordering occurs by the additional alternation of cation planes

containing only calcium w ith cation planes con taining only magnesium (Fig.1).Itispossible

to conceive of a mineral having th e same com position as ideal dolomite ((Cao.

6

Mg

06

)C0

3

)in

which all cation p lanes are alike, containing equal numb ers of calcium and magnesium

ions.

Such a mineral is not dolomite. Such a disordered arrangem ent ofionsoccupiesmorevolume

than tha t of the ideal dolomite structure and is unstable with respect to an ordered phase.

Perh aps surprisingly, the two com pounds jus t described, ideal dolomite and a disordered

1-to-lratio Ca-Mg carbo nate, are both rare in sedimen tary rocks. Ideal dolomite rarely

comprises ancient dolomitic sediments and never modern sedim ents, and the completely

disordered polymorph does not occur at

all.

T he dolomite which does occur in sedim entary

rocks is commonly Ca-rich, having compositions which rang e from about Ca(Cao.

16

Mgo

g

4)(C0

3

)

2

to ideality, and/or exhib its weak, diffuse, X -ray diffraction, suggesting considerably less

structu ral order than its composition should dictate. With respect to ideal

dolomite,all

such

natu rally occurring dolomite is metas table, and the capacity ex ists for reactions to occur

toward a m ore stable (more stoichiometric or better ordered) phase.

The term p rotodolomite was defined by Graf and Goldsmith(1956)as "single-phase

rhombo hedral carb onate s which deviate from the composition of the dolomite tha t is stable in

a given environm ent, or are imperfectly ordered, or both, but which would transform to

dolomite if equilibrium w ere establish ed." Gaines(1977)modified the definition to include only

ordered phases. I recommended(1980)th at the te rm be dropped altogether, since almost all

sedim entary dolomite is really protodolomite by G aines' definition. Wh atisimportantisnot

what

we

call these natu ral m aterials, bu t w hat they really are.

1


3/21

J ^

o

CARBONATE MAGNESIUM CALCIUM

Figure

1

Schem atic representation of the crystal stru ctu re of dolomite showing the

alternatio n of cation an d anion (carbonate) planes, and th e altern ation of calcium and

magnesium planes.

2


4/21

H ydr oth erm al experim ents (Graf and G oldsmith, 1956; Goldsm ith and Heard, 1961),

extrap olated to low tem pera ture, d em onstr ate t ha t calcite and dolomite are essentially ideal in

compo sition at 25 C (Fig. 2). In o ther w ords, any double carbon ate crystal of Ca and M g a t

25 C which is no t essentially pu re dolomite is either m etastab le or unstab le with respect to a

mix ture of pure calcite plus pu re dolomite. The same thi ng is true with respect to ideal

dolomite plus mag nesite. The composition of phases which we observe at E ar th 's surface

define th e range of metastabili ty. U nstable p hase s are only observed as transien t s ta tes in the

laboratory. In the case of dolomite, few phases containing more than about 8% excess calcium

(on a molar basis) hav e been reporte d to da te, although the da ta are adm ittedly sparce.

Reeder (1981) has shown that the structure of various kinds of dolomite revealed by

tran sm issio n electron micro scopy an d electron diffraction can be classified into at least th ree

typ es. All str uc tur es are ordered, alth oug h the degree of order is variab le and difficult to

quantify. Th e first, char acte ristic only of Holocene dolom ite, con sists of irregular " m osa ics" on

a scale of tens or hund reds of An gstro m s. The crystals are characterized by extremely high

densities of crystallograph ic faults and dislocations, and can be tho ugh t of as an aggre gate of

"m icro-c rystals" who se compositions m ay vary, forming a very discontinuous latt ice. This

leads to man y unsatisfied or s trained chemical bon ds and to X-ray diffraction pat tern s w ith

bro ad, gene rally weak reflections. This kind of dolom ite is also characterized b y large trac e

element s ub stitutio ns, especially s tr ont ium (Behrens and Land, 1972), and sodium (Land and

Ho ops, 1973). Qu alitative da ta su ggest th at th is material is extremely soluble compared to

be tter ordered forms of dolomite. M y a tte m pt s to beneficiate samples composed of mixtures of

this kind of dolomite and arago nite (for example, supratidal cru sts from Florida and th e

Ba ham as) by slow leaching in acetic acid resulte d in only slight concentratio n of the dolomite

by selective solution of arago nite. C0

2

for isotopic ana lyses of H olocene dolomite is evolved

much faster tha n from finely groun d ancient dolomite. All evidence suggests tha t Holocene

dolom ite is a uniq ue, highly soluble materia l. It is clearly a m etas tab le phase, unkno wn (in an

unmodified form) in ancient rocks.

The second and m ost comm on kind of sedimen tary dolomite exhibits a lamellar or "tw eed"

stru ctu re w hen examined b y transm ission electron microscopy and electron diffraction, which

Reeder (1981) ha s interp rete d as a stru ctu ral and /or compositional mod ulation on a scale of

several hundred A ngstro ms (Fig.3).At p resen t this kind of dolomite is thou ght to consist of

two intim ately intergro wn lamellar dom ains parallel to the rhom b face with slightly different

stru ctu res and/or compo sitions. The te xtu re resembles spinoidal decomposition, or solid s tat e

unm ixing on a scale of a few h undre d a ng strom s from a single homogeneous precursor. The

exact s tr uct ure and comp osition of the two doma ins or lamellae is not known, althoug h one

m ust be more stable (and presum ably more mag nesium rich) than the other. This type of

dolomite is clearly me tastab le, bu t continued stabilization can not proceed spontaneo usly

becau se it is limited b y solid sta te diffusion. C ontinue d stabilization can occur as a result of

solution-reprecipitation p rocesses however, and it has been de m onstrated th at bulk Ca-rich

dolomites dissolve more rapidly than ideal dolomite (Busenberg and Plummer, 1982).

Continued stabilization tow ard a more stoichiometric dolomite would presum ably be

pro m ote d if pore fluids in th e rock chan ged t o enable dissolving out of th e less stable, Ca-rich

dom ain. Poro sity could easily increase under the se conditions.

3


5/21

1000 -

800

TEMP.

(C)

600

400

Ordered Dolomite

Dolomite + Magnesite

Lower limit

of

experimental data

Ranges of metastable p hases

observed in natu re

10

20 30 40 50

60

MOLE Mg C0

3

Figure

2

Stability relations in the system C aC0

3

MgC0

3


6/21

Figure 3Dark field transmission electron micrograph of a calcian dolomite

(Caj

i

2

Mg

088

(C0

2

)

2

)of Eocene age. The prominent mod ulated struc ture is typical of sedimen tary

dolomite, and such cry stals are meta stable with respect to ideal stoichiometric dolomite.

Photograph by Richard Reeder.

A th ird k ind of dolomite is nearly ideal in composition, and when examined by transmission

electron microscopy and e lectron diffraction is observed to

be

homogeneous, consisting of

large single domains . This kind of dolomite

is

presently known mostly from ancient, deeply

buried sequences and from metam orphic rocks.

The philosophy tha t, like limestone, the diagenesis of dolomite is dominated by the

stabilization of meta stab le dolomitic phases, is relatively new. T hereisno question tha t

calcium-rich dolomite has the capacity to react t o form crysta ls with a more stoichiometric

composition, but many important questions remain. What kinds of diagenetic environments

promote th e reaction? Does stabilization to ideal dolomite take place all at once orinseveral

stage s? How far from ideality mu st a phase be before it is prevented from further reaction for

kinetic reasons? Many of these questions mu st be answered both by laboratory work and by

careful mineralogical analysis of particular dolomites under investigation before thinking can

advance much further.

5


7/21

Aqu eous solution equilibria

Seve ral lines of evidence hav e been used to deter min e the solubility of dolomite a t

sedim entary and early burial conditions. The da ta a re complicated by the m ineralogical

variatio ns in dolomite already discussed. All m etastab le phases m ust be more soluble than

ideal dolomite, and variations in the degree of metastability can obviously occur.

D at a have been derived from tw o sources,(1)high temperature experiments and(2)natura l

dolomite aquifers. Of interest is the equilibrium constant, K, for reactions between the ideal

solids,

2 C a C 0

3

+ M g

++

^ CaMg(Co

3

)

2

+ C a

+ +

K = (Ca

+ +

) / (Mg

+ +

)

or th e calcium-to-m agnesium activ ity ratio of a solution at equilibrium wit h calcite + dolom ite

(as a function of tem peratu re). Solutions m ore magnesium-rich tha n th e equilibrium solution

should cau se dolom itization of calcite, while solutions more calcium-rich should cau se

dedolomitization.

Dolom ite is easily synthesized hyd rotherm ally at ab out 300C ,with reaction time s of only a

few da ys . Progressive ly slower reaction is observed a t lower tem pera tures and below a bout

100C very long expe rime nts are required. Nobody h as yet synthesized dolomite at

Earth-surface conditions (although a Da lma tian has , Mansfield, 1980). Expe rimen tal data are

in reasonable agr eem ent around 300C, and the m olar Ca/Mg ratio of a solution in equilibrium

with calcite and do lomite is about 15. In oth er words, as temp eratu re increases, dolomite

becom es increasingly less soluble tha n calcite. A ny solution w ith a mo lar Ca/Mg ratio of less

th an 15 is capable of dolom itizing at 300 C (Fig. 4)

A t lower tem pera ture s, experim ental da ta become more conflicting, the reason being, I

suspect, th at m etasta ble Ca-rich phases are mu ch more easily formed. Kinetic exp eriments

(Land, 1967) have shown that the formation of a Ca-rich (metastable) dolomite rather than the

ideal ph ase is favored (within the s tability field of dolomite) by(1)higher Ca/M g ratio of the

solution,(2)lower solution concentration, an d(3)lower temp eratur e. Metasta ble Ca-rich

ph ase s are more soluble and therefore w ill coexist with more magn esium-rich fluids (Helgeson

et al, 1978).

H su (1963), Ho lland e t al, (1964), Ba rne s and Back (1964) and Lan gm uir (1971) all studied

the C a/Mg ratio of natur al dolomite aquifers , reasoning tha t equilibrium w ith dolomite would

eventually be reached as water rech arged a dolomite aquifer and moved dow ndip at ra tes

typical for groun dw ater flow. Langm uir 's compilation is plotted on Figure

4 .

The

extrapolation of Ro senbu rg and H olland's (1964) da ta to intercept Lan gm uir 's low

tem per atur e dat a is no t too unreason able if one accepts that th e lower tem pera ture

hydrothermal data points of Rosenburg and Holland may be displaced toward

mag nesium -rich co mp osition s becau se of formation of a non-ideal (more soluble) pha se. The

reasonable agreem ent between low tem per atur e and high tem pera ture da ta ignores non-ideal

solution behavior, which is significant in the saline solutions Rosenburg and Holland used. But

at 30 0C, experim ents a t 2M, 1M an d 0.5M solutions all yield similar results , suggestin g the

effects are not large. Fu rth er su ppo rt for extrapolation between the two type s of da ta wa s

6


8/21

3.5 -

3.0

2.5

1000

T(K)

2.0

Langmuir, 1971

DOLOMITE

1.5

Temp. (C)

10

25

CALCITE

50

100

2 CaC0

3

+ M g

++

^

CaMg(C03)

2

+ Ca

++

150

t

200

Rosenberg and

Holland, 1964

Gaines

(pers.

comm.)

Land.

Rosenberg, Bu rt and Holland

250

300

0.2 0.0 0.2 0.4

0.6 0.8 1.0 1.2

I I

5 10

1.4 logCa/Mg

I

25 Ca/Mg

Figure

4

Aqueous solution compositions presumed to

be

in equilibrium with calcite plus

dolomite.


9/21

obtained by Pakhomov and Kisson(1973)(reproduced in Carpenter,1980) whoplotted the

Ca/Mg ratio of saline formation w ater from th e Russian platform versus tem peratu re. Despite

the fact th at they totally ignored rock composition (calcite plus dolomite may no t both hav e

been presen t to control th e solution composition), and obtained considerable scatter, their

regression line essentially connects Rosenburg and Holland's and Lang mu ir's data Until

further experimental work is conducted (which mus t include characterization of the dolomite

phase) the dat a presented in Figure4arealltha t are available. They are consistent b oth with a

gross overs atura tion of seawater with dolomite, and the Mg-depleted natu re of most saline

formation water.

The reason for th e gross oversaturation of seawater w ith respect to dolomite ultimately lies

in the kine tic problem of nucleating and growing the ordered crystal (Goldsmith, 1953). The

molar Ca/Mg ratio of seawater(0.19)is apparen tly incapable of causing dolomitization at

observable

rates.

By either decreasing the molar Ca/Mg ratio of seawater (say by gypsum

precipitation) or decreasing th e activ ity Ca/Mg ratio th e kinetic con strain ts can be overcome,

at least t o th e poin t of being able to nuclea te and grow a poorly crystalline Ca-rich phase. The

activity Ca/Mg ratio of seaw ateris0.18 (Berner, 1971), and canbedecreased by dehydra ting

the Mg

+ +

ion (Usdowski, 1968) or by removing components which form stron g ion pairs w ith

Mg

+ +

(for example, S0

4

=

, Baker and Kastner, 1981). These factorsdonot alter the equilibrium

relations (Fig.4)and only provide the kinetic " pus h" to form th e initial phase. The

early-formed p hase can then stabilize by further reaction.

An other va riable in th e dolomitization process which needs additional confirmationisthe

role of organic m aterial, particu larly dissolved organic acids. Dissolved organic acids are

known to control the kind of calcium carbon ate which precipitate s from solution. Increased

organic acid content favors M g-calcite over aragonite precipitation (Kitano and Kanamori,

1966).A lthough algal processes h ave been invoked as being able to cause dolomitization

(Gebelein, 1973), the "organic grem lin" is neither proven nor disproven.

Stable Isotopic Geochemistry

M ost current evidence supports the contention th at sedimentary dolomiteisenriched in

18

0

about3to4pp t w ith respec t to a co-existing calcite in the range of sedimentary and burial

diagenetic temp eratures of normal interest(Land,1980). Little evidence exists for dolomite

replacement of calcite with out change of isotopic composition (Katz and Matthew s, 1977). The

fact th at m any ancient dolomites are significantly depleted in

18

0

is bes t explained by

stabilization of an earlier-formed phase during burial (Fig.

5).

The isotopic composition of the

dolomite comprising sedimentary rocks is controlled both by the chem istry of the la test

recrystallization (stabilization) event and by the chem istry of the precursor (aragonite,

Mg-calcite, calcite and/or dolomite). Dolomite rarely recrystallizes homogeneously in an open

aqueous chemical system , accurately recording the conditions of recrystallization, jus t as it

almost never accurately retains the chemistry of the precursor.Recrystallization may be

incomplete, leaving an inhomogeneous rock, and the composition of the replaced phase may

"co nta mina te" th e replacing phase (Land, 1980). The practical problem of analyzing intima te

mixture s of dolomite of slightly different compositions

is

not ye t solved.

8


10/21

200

1 6 0 -

Temp.

1 2

o

(C)

80

4 0


11/21

Trace Element Geochemistry

Trace element p artitio ning is complicated by kinetic factors. The ratio of the concentration

of a trace element in a crysta l to th e concentration of the element for which it subs titute s (say

Sr/Ca) is dependent on the con centration ra tios of the elements in the solution from which the

crys tal forms, on tempera ture , on pressure (usually ignored), and on other variables such as

the r ate of crys tal grow th. The distribu tion coefficient

"D "

in th e following equ ationisthus a

function of variables w hich are not always easy to define:

(Sr/Ca)

crygtal

= Dx(Sr/Ca)

solution

Modern marine and hy persaline dolomite has an Sr conten t of about 600 ppm (Behrens and

Land, 1972), yet few ancient dolomites contain more than200ppmSr,even when presumed to

be initially of hypersaline origin. Although it was once assumed th at rem oval of the trac e

elements by flushing with a low S r (meteoric) water w as required (Land, 1973), thisis nolonger

acceptable for all ancient dolomite.

As an exam ple of this problem, Bein and Land

(1982)

studied Perm ian San Andres dolomite

from th e subsurface in no rth

Texas,

where dolomite beds are intimately interbedded w ith

bedded halite and anhydrite. Bo th halite and anhydrite display sedimentary structures

indicating a primary subaqueous origin, and both contain trace elements (Br in halite and Sr in

anhydrite) indicative of primary prec ipitation. I t seems clear th at th in dolomite beds

intimately interbedded with and "entombed" by primary evaporites could never have formed

from or been modified by low Sr (meteoric) water. Yet the dolom itesallcontainlessthan 200

ppm Sr.Bein and Land suggest tha t although the original dolomite may have resembled

Holocene analogs (about

600

pp m

Sr),

during burial it stabilized to a more ordered s tructu re,

expelling Sr to form

celestite.

In othe r words, at least two distribu tion coefficients apply to

this situatio n, one for the formation of the original phase, and a second(lower)for the

stabilization reaction to a more ordered, stoichiometric phase.

Because of these kinetic problems which plague other sedimentary p hases a s

well

anh ydr ite (Kushnir, 1980), halite (Holser, 1979), trace element analyses of dolomite are

of limited p ractical value today. Hopefully, more experimental workwillrectify this situation.

Mechanisms of Dolomitization

Clearcut petro graph ic evidence indicates t ha t m ost dolomite initially forms by replacing a

precursor carbonate. Th atis,a fluid simultaneously imports M g

++

, dissolves the precursor

phase, precipitates dolomite, and exports C a

++

. Of course, the situation is actually more

complex due to the import and export of other components such as other trace elements and

their isotopes (for example,

87

Sr/

86

Sr),

carbon an d oxygen isotopes, C0

2

,etc.Becauseof

considerable com positional differences between dolomite an d any presumed precursor (calcite,

aragon ite, or Mg-calcite), considerab le fluid tran sp or t is required. Advection (fluid flow) must

accomplish most of the tra nspo rt, altho ugh diffusion may play an impo rtant p art on a local

scale. Models for dolomitization are therefore basically hydrologic models. Before discussing

10


12/21


13/21

upper p art of the B ay sequence, from the sea floor to about7m below, are extremely well

laminated, docum enting a ltern atin g periods of aragonite (and rarely Mg-calcite) precipitation

and terrigen ous deposition, which occurred du ring and after storm s (often hurricanes). The

bay is normally hypersaline except after hurricanes, and so the deposition of chemical

precip itates dur ing hypersaline periods and th e deposition of terrigenous m aterial

accompanying runoff accounts for the laminations, and the hypersalinity for their

preserva tion. The middle pa rt of the sequence, from about

7

to

13 m

below sea floor, formed in

about

5 m

of water a bout

3500

-1000 years ago (Behrens, 1974), and

is

texturally similar

except for th e presence of dolomitebeds.Very little terrigenous materialispresent w ithin the

dolomitebeds,ruling out any kind of a mixing model since fresh water would have con tributed

terrigeneous mud.Inter stiti al water analyses of the very impermeable sedimen ts, obtained by

hydraulic squeezer, have a relatively uniform chlorinity(36ppt),molar Ca/Mg ratio(0.15)and


14/21

Therefore, 807 pore volum es of seawa ter are required to comp letely dolomitize1cu m of

sedim ent. If se aw ater dilu ted 10 tim es with m eteoric water (say in a mix ing zone) is utilized,

the n 8.1 x 10

3

pore volumes are needed. If seaw ater havin g an M g content of about 8 x10"

1

and

a Ca conten t of abou t 8 x

10

"moles/Kg is utilized (a typic al brine which h as precip itated

gy psu m and ev apor ated to th e point of halite saturation), then only 44 pore volumes are

neede d. If the vario us solutio ns do no t reach equilibrium with calcite + dolom ite (the Mg/Ca

rat io does no t fall to

1),

or the b rine has not reached halite saturation, then proportionately

m ore pore volum es of fluid are required. No po rosity red uction h as been achieved, and if

dolomite cementation occurs, additional fluid flow is required.

Reflux

Reflux, as defined by Adams and Rhodes (1960) occurs when "hypersaline brines eventually

become heavy enough t o displace the con nate waters an d seep slowly downward thro ugh the

slightly permeable carb onates at th e lagoon floor." Ex am ination of Holocene sabkh as h as

sug geste d th at dow nward m oving water driven solely by potential energy resulting from

increased density of the fluid at co nsta nt head is probably no t as importa nt a s the increased

head caused by elevation of wa ter onto the sabk ha surface by storm s.

H su and Siegen thaler (1964) sum ma rized va riou s ideas of sab kh a hydrology. Basically,

considering a s abk ha w hich extend s relatively far along strike relative to i ts width (a two

dimensional system), the directions of wa ter m ovem ent are quite l imited. A t any point in the

sabk ha, w ater can either mo ve up or down, seaward or landward (Fig.6).It is assumed th at an

infinite reservoir of m agn esiu m (seawater) is available at som e con stan t level at th e margin of

the sabkh a. Only two processes can m ove seawater (the source of magnesium) landw ard in the

absence of interaction w ith an independent und erlying aquifer system , namely storm recharge

and evap orative draw down . Evapo rative d rawdown, or the lowering of the water table by

evapo ration, can only occur if the landw ard p ar t of the sabk ha is depressed below sea level by

subsidence, compaction and/or wind deflation. Uneva porated seawater mu st be kep t from

flooding th e depre ssion b y som e sort of sill, either a physical barrier or a long distan ce. In a ny

case, landw ard flow of seaw ater into a depression will resu lt in rapid evap oration and

con sequ ent filling of the basin b y evap orite min erals, effectively haltin g flow by eliminating

the h ead difference. Th e am oun t of water required to produce1cu m of gyp sum is about

sufficient to completely dolomitize 1 cu m of carbonate sediment. Eva porativ e draw down

(possibly aided by cap illary withdraw al) is, at be st, a tran sien t condition an d is self-limiting.

Sto rm r ech arge , however, can contin uou sly (geologically speaking) drive wate r up onto th e

sabk ha, wh ere i t evap orates and flows seaward, driven by elevation head, and aided by its

increased density. Such a mechanism dom inates modern sabk has (McKenzie, Hsu, and

Schneider, 1980; A m du rer a nd La nd, 1982). In th e case of the Truc ial Co ast of the Persia n

Gulf,dolomitization tak es place only in the sto rm recharge zone, and the am ount of dolomite

correlates with th e frequency of recharge (Patterson and Kinsm an, 1982).

Considerable am oun ts of gy psu m m ay be precipitated as the result of brine evolution. For

examp le, usin g the figures previously discussed, 44 pore volumes of halite-saturated b rine

were required t o dolomitize1cu m of sediment. Ab out1cu m of gyp sum w ould hav e

13


15/21

DOLOMITE

200

SUPERSATURATED

UNDERSATURATED

ZONE OF DOLOM ITIZATION

0

20

1 j

40 60

percent seawater

80

100

50

100

Figure7 Percent s atura tion for mixture s of seawater and a typical meteoric groundwater

having aP = 10

2

atmospheres (after Plummer, 1975).

precipita ted from the volume of seawater required to generate th at much brine, leading to a

gypsum-to-dolomite volume ratio of

one.

Ad van tage s of the reflux mechanism are the rapidity w ith which dolomite canbeformed as

docum ented by Holocene studie s, and th e relatively smaller volumes of water required due to

its magnesium-rich n ature (Sears and

Lucia,

1980). This m echanism clearly dominates in

evaporitic set ting s. In th e absence of evaporites th e model is more constrained, barring

fluctuations of th e Ca/Mg ratio and/or th e sulfate content of seawater. The efficient removal

of

calcium by the formation of surficial algal micrite prior to evapo rative concentration can also

suppressCaSO


16/21

pote ntials caused solely by density differences ap parently c annot move water very far thro ugh

sed im ents of relatively low permeability. Elev ation hea d is required, and in addition to stor m

rech arg e it mig ht easily be accomp lished by periodic lowering of the reservoir of sea water

eithe r by a local m echa nism (say evapo ration of a restric ted sea) or on a larger scale

(eustatic/tectonic), draining of the sabk has periodically in the same w ay m odern coastal plains

were drained d uring Pleistocene glacial even ts .

Meteoric Mixing

In o rder to acco unt for evaporite-free dolom ite sequences, the mixin g of meteoric w ater

(providing the driving force th rou gh elevation head) with seaw ater (providing the m agnesium)

has been advocated (Hanshaw , Back, and Deike,

1971;

Land, 1973). Geochemical

consideration s (Fig. 7) (Plumm er, 1975) sugge st th at the m echanism is plausible even thoug h

much longer times are required for dolomitization (Sears and Lucia, 1980). Although examples

of Holocene mixing-zone dolom ite (mostly as cements ) continue to be found (Magaritz et al,

1980), a major prob lem w ith th e model is explaining w hy dolom ite is not m ore comm on, since

mix ing of seaw ater an d m eteoric water is a ubiqu itous worldwide process. The model appar

ently requires a relatively stable hydrologic setting to establish sufficient continuous recharge

for establi shm ent of a mixing cell with seaw ater over a long period of time to drive the dolomit

ization reaction. Kinetic problems are overcome by reducing th e Ca/Mg activity ratio of the

m ixtur e thro ug h lowering of the ionic s trengt h. This may n ot be too much of a problem in a

subtropical setting as, say, tidal flats prograde across a shelf leaving behind vast areas for

rech arge . Bu t in an arid climate th e model is difficult to apply unless large adjacent coastal

plains prov ide the rec harg e zone and ev apo rites are sealed off from the actively circu lating

water.

The eva porativ e concen tration of continental water accounts for playa-type dolomite includ

ing the C ooron g exam ples (von der Borch , Lock and Schwebel, 1975).

Burial D iagenesis

Do lom ite can clearly form as a directly precip itated la te cement, as exemplified by stu dies of

sand ston e bu rial diagenesis (Boles, 1978; Lan d an d D utton , 1978). The dolomite is comm only

ferroan, and can approach ankerite in composition, reflecting the large amount of ferrous iron

comm only presen t in the terrigenou s system . Althoug h it is true th at shales in a sedim entary

bas in are possible sources for nearly ev ery conceivable com ponen t required for any conceivable

kin d of diagenesis, it is no t clear th at th ey a re sources for magn esium . In fact, t he

prec ipitatio n of chlorite within th e shales may be a local sink for mag nesium . Saline form ation

wa ters are typically very magnesium-poor, and on the whole comm only app roach

calcite-dolomite equilibrium (Pakhomov and Kissin, 1973). Supplying large amounts of

ma gnesium from a water nearly in equilibrium with calcite plus dolomite requires vast

am ou nts of water, a definite problem , especially in relatively imperm eable rocks. In add ition,

Fig ure 4 indicate s th at if a wa ter initially in equilibrium w ith calcite plus dolom ite moves

15


17/21

up dip (and cools), it becom es un de rsa tur ate d w ith dolomite and will either dissolve dolom ite or

dedolomitize. This exac t subsurface reaction has been observed by Land and Prezbindowski

(1981) an d B ud ai (1981).

Therefore, at the pre sent t ime, the formation of large am oun ts of new replacement dolom ite

is difficult by th is me chan ism . No large-scale source for mag nesiu m h as been identified.

Mo ving magn esium aroun d within a basin without prod ucing any net new dolomite appears to

be quite possible, bu t in this case the "n ew " replacement dolomite or cement m ust be balanced

by either "n ew " dedolomite or by second ary porosity somew here within the basin. The

mobili ty of calcium, magne sium and dissolved carbonate after burial m ust n ot be disregarded.

Sha les are rapidly "decalcified" du rin g burial (Hower et al, 1976) and prov ide a large-scale

source for new carbo nate p hase s. Bu t s ince calcium loss exceeds mag nesium loss by a t least a

factor of 6, mu ch m ore calcite th an dolomite is involved in the process. Sand stone diagenesis

can involve immen se qu antit ie s of carbon ate which is bo th precipitated and removed (to form

secondary porosity). Sandsto nes can be carbonate-cemented, decemented and then recemented

(Milliken et al, 1982), and c arb on ate s proba bly un derg o similar complex histories. La te

secondary po rosity dev elopm ent in carbonates is known (Moore and Dru ckman , 1981), and

som e text ur es in deeply burie d carb on ates m ay be th e result of selective dissolution of calcite,

leaving th e dolomitic com pon ent of the rocks as an "inso luble residu e" (Wanless, 1979).

It is im po rtan t to "deco uple" th e process of dolomitization/dedolomitization (controlled by

the Ca/Mg of the solution) from cementation/secondary porosity generation (controlled by the

acidity of th e solution). The dolom itization proce ss is rarely C 0

3

=

-conservative (Weyl, 1960;

De gen s and E pst ein , 1964). A solu tion with a low Ca/M g ratio and capable of dolom itizing can

either cause net cem entation or net solution, depending on changes occurring in the tota l

dissolved carbon ate co ntent of the solution as i t mov es throug h th e rocks. Addition of C 0

2

by

organic m atu ratio n can cau se net solution, whereas loss of C0

2

to adjacen t str at a of lower

carbo nate co nten t can cause net precipitation. Thu s dolomitization can either result in

porosity d ecrease (by cem entation and/or by compaction accom panying recrystall ization), or

porosity increase (secondary poro sity formation). The sam e is true of the dedolomitization

reaction.

Classic dolomite reservoirs containing intercrystall ine porosity m ay possibly result from

recrystall ization of a meta stab le Ca-rich precursor phase induced by a C0

2

-rich (corrosive)

solution . Some or all of th e more Ca-rich (more soluble) dom ains of the m etas tab le pha se m ay

be lost to the solution, and add itional dolom ite m ay even be dissolved. The less soluble

com ponen t m us t recrystall ize, and intercrystall ine porosity results from the volume loss of the

Ca-rich dom ains. I t is possible tha t su ch situatio ns m ay even be "self-reservoiring" in the

sense tha t C0

2

evolved during early m atur ation ma y be responsible for creating the reservoir

by dolomite recrystall ization

Oth er P ossibilit ies

We should be careful abo ut bein g too actualistic in our appro ach to dolomitization. Only 25

year s ago, we thou gh t t ha t essentially no H olocene dolomite existed (Fairbridge, 1957). Each

case of Holocene dolom itization h as resulte d in considerable over-reaction and

16


18/21

"bandw agon-jum ping" soon after the discovery.

One intrigu ing possibility, which is gaining considerable sup port recently, is tha t "th e

present is a lousy key to the pa st because seawater ha s changed." The observation th at the

percen tage of dolomite in carbo nate rocks increases as

we go

back in geologic time w as

originally att rib ute d to m ore time available for dolomitization (the source of magnesium w as

no t specified) (Chilingar, 1956). Chang es in the composition of seawater resu lting

in

times

of

"easier" dolomitization in the pas t cannot be discounted. Tucker(1982)recently sugg ested a

prim ary origin for a Pre-Cam brian o osparite (oolitic grainstone) composed entirely of dolomite

(including the"spar" ).Changes in salinity, in Ca/Mg ratio,SC%

=

concentration andP

C02

have

all been invoked (Sand burg,1975;B aker and Kastner,1981;Mackenzie and Pig ott, 1981), and

sym path etic varia tion of several comp onents may be particularly effective, and ultimately

related to crustal cycles.

Conclusions

No panaceas exist for dolomitization. Each case mu st be studied on its own merits, and

many scenarios ex ist. Modern scenarios begin to break down if seawa ter and/or sediment

compositions have evolved with

time.

Reflux can account for th e initial formation of many

evaporite-related do lomites but since the poorly ordered phases formed in hypersaline

environm ents are not found in ancient rocks, recrystallization m ust occur.Mixing zone

dolomitization is capable of upgrad ing early hypersaline phases to amorestable phase, but is

not necessary as "isochemical" recrystallization can occur in saline brines. Mixing zones are

capable of producing dolomite cemen ts and new replacement phases , given enough time and

with sufficient recharg e zones. Burial diagenesis can g enerate dolomite cements (commonly

ferroan), induce recrystallization of previously formed, m etastab le phases, and move

previously formed do lomite from place to place. Recrystallization can take

place

in essentially

closed chemical sys tems , or in partly open s ystems resulting in gross changes in the chem istry

of th e dolom ite and in the selective removal of either calcite or dolomite from the sequence.

Few (ifany)carbonate rock s, dolomitized or not, exist as theywereoriginally deposited.

Most have resulted fromoneor more processes of formation, and a t least one stabilization

(recrystallization) event.

Acknowledgements

Several stu dents and colleagues critiqued earlier versions of the manuscript and offered

valuable corrections, including Jam es A nderson, David Budd, Bob Folk, Donald Miser,and

Richard Reeder. Richard Reeder kindly provided Figure 3.Supp ort of the Geology Foundation

of the Un iversity of Texas a t A ustinisgratefully acknowledged.

17


19/21

REFEREN ES

A dam s, J . E . and M. L. Rhodes, 1960, Dolom itization by seepage refluxion: AA PG Bull. , v. 44,

p .

1912-1920.

Am durer, M. an d L. S. Land , 1982, Geochemistry, hydrology and m ineralogy of the sand bu lge

area, Lagu na M adre flats , S outh T exas: Jour. Sed. Petrology, v. p.

Baker,P.A. and M . Kastner, 1981,Con strain ts on the formation of sedimentary dolomite:

Science, v. 213 , p. 214-216.

Barn es, I . and W.Back , 1964, Do lom ite solubility in grou ndw ater: U. S. Geol. Surv eyProf.

Pap er 475-D, p. 179-180.

Be hren s, E. W., 1974, Holocene sea level rise effect o n the de velopm ent of an estu arine

carbonate depositional environment: Memoires de 1 Justitatde Geologie du Ba ssin

d'Aq uitaine, No. 7, p.337-341.

and L . S. Lan d, 1972, Subtid al Holocene dolom ite, Baffin Bay, Tex as: Jour. Sed.

Petrology, v. 42, p .155-161.

Bein, A. and L. S. Lan d, 1982, San A ndre s carbon ates in the Tex as panhandle; sedimentation

and diagenesis associated with magnesium-calcium-chloride brines: Austin, Texas, Univ. of

Texa s, Bureau of Econ . Geology, Rept. of Invest. , No . 121,48 p.

Berner, R. A., 1971, Prin ciples of chemical sedime ntology: McG raw-Hill, 240 p.

Boles, J . R., 1978, Activ e anke rite cemen tation in the subsurface Eocene of southwest T exas:

Co ntrib . M ineralog y a nd P etrology, v. 68, p. 13-22.

Bud ai, J . M., 1981 , Subsurface dedolomitization of the M adison limestone, Wyom ing: Geol.

Soc. Am erica Ab s. with Prog ram s, p. 419.

Bu sen ber g, E . and L. N. Plum mer, 1982, Th e kinetics of dissolution of dolomite in CO

2

-H

2

0

syste m s at 1.5 to 65C and 0 to

1

a tm Pco

2

: Am . Jou r. Sci., v. 282, p. 45-78.

Carp enter, A. B., 1980, Th e chem istry of dolom ite form ation I; the stab ility of dolomite,inD .

H. Zenger, J . B . Du nha m, and R. L. Ething ton, eds. , Concepts and models of dolom itization:

SE PM Spec. Pub . No. 28, p .111-121.

Chilingar, G. V., 1956, Re lationsh ip betw een Ca/M g ratio an d geologic age: AA PG Bull., v. 40,

p.2256-2266.

Deg ens, E. T , a nd S. Epste in, 1964, Oxyg en an d carbon isotope ratios in coexisting calcites

and dolomites from recen t an d ancient sed iments: Geochim. et Cosmochim. Acta, v. 28, p.

23-44.

Fairbridg e, R. W ., 1957, The dolomite question,inR. J. LeB lanc and J. G. Breeding, eds.,

Regional asp ects of carbon ate deposition: SE PM Spec. Pub. No. 5, p. 125-178.

Ga ines, A. M., 1977, Proto dolo m ite redefined: Jour . Sed. Petrology, v.47,p. 543-546.

, 1978, Reply, pro todo lom ite redefined: Jour . Sed. Petrology, v. 48, p. 1009-1011.

Gebelein, C. D., 1973, Alg al origin of dolomite lam inatio ns in strom atolitic limeston e: Jour.

Sed. Petrology, v.

4 3 ,

p. 603-613.

Goldsm ith, Jr. , 1953, A "simp lexity principle" and its relation to "ea se" of crystall ization:

Jou r. Geology, v. 62, p.

439-451.

and H . C. Heard, 1961,Subsolidus phase relations in the syste m C aC0

3

-MgC0

3

: Jour.

Geology, v. 69 , p . 45-74.

18


20/21


21/21

the sa bkh a, Ab u D habi, U. A. E., and its relationship to evaporative dolomite genesis ,inD .

H. Zenger, J . B . Du nha m , and R. L. Eth ing ton , eds. , Concepts an d models of dolomitization:

SE PM Spec. Pub . No. 28, pp . 11-30.

M aga ritz, M., et al, 1980, Do lom ite form ation in th e seawater-freshwater interface: N atu re, v.

287,

p . 622-624.

Milliken, K. L., L. S. Lan d, an d R. G. Louck s,1981,His tory of burial diagenesis determined

from isotopic geochemistry, Frio F orm ation, B razoria County, Texas: AA PG Bull. , v.65,p .

1397-1413.

Moore , C. H. and K. Druckm an,

1981,

B urial diagenesis and porosity evolution, Up per

Jur ass ic Sm ackover, Ark an sas and Louisiana: AAP G Bull., v. 65, p. 597-628.

Pakh om ov, S. I . , an d I. G. Kissin, 1973, Hyd rogeoch emistry of magnesium in deep aquifer

zones: A kad . Na uk SSS R D oklady, v. 209, p. 205-208. (English translatio n, 1974, Am erican

Geological Insti t ute) .

Patt erso n, R. J . a nd D. J . J . K insman , 1982, Form ation of diagenetic dolomite in coastal

sabk ha along the Arab ian (Persian)Gulf:AA PG Bull., v. 66, p.28-43.

Plumm er, L. N., 1975, Mixing of seawater with calcium ca rbonate ground water: Geol. Soc. of

Am erica M em . 142, p . 219-236.

Reeder, R. J., 1981,Electro n optical investigation of sedime ntary dolom ites: Contr.

Min eralogy Petrology, v. 76. p. 148-157.

Ro senb erg, P. E., D. M . B urt , and H . D. Holland , 1967, Calcite-dolomite-magnesite stabilit y

relatio ns in solutions; the effect of ionic stre ng th : Geochim. et Cosmochim. Ac ta, v.3 1,p .

391-396.

Rosenburg,P.E., and H . D. Holland, 1964, Calcite-dolomite-mag nesite stability rela tions in

solutions at elevated tem per atur es: Science, v.145,p.700-701.

San dbu rg, E A., 1975, New inter pretatio ns of Great Salt Lake ooids and of ancient

non-sk eletal carb on ate m ineralogy : Sedimentology, v. 22, p. 497-537.

Sea rs, S. 0., and F.J . Lucia, 1980, Dolom itization of north ern Michigan Niag ara reefs by brine

refluxion an d freshw ater/seawater mixing, in D. H. Zenger, J . D . Dunham , and R. L.

Eth ing ton , eds. , Con cepts and models of dolomitization: SEPM Spec. Pub . No. 28, p.

215-235.

Tucker, M. E., 1982, Precam brian dolomites; petrograp hic and isotopic evidence that they

differ from Phan erozoic d olom ites: Geology, v.10 ,p . 7-12.

Usd ow ski, H . E., 19 68, Th e formation of dolom ite in sedim ents, in G. Muller, and G. M.

Fried ma nn, eds. , Recent develop men ts in carbonate sedimentology in central Europe:

Springer-Verlag, p . 21-32.

von der B orch, C. C , D . E. Lock, and D . Schwebel, 1975, Grou ndw ater formation of dolomite

in the C ooron g region of Sout h A ustra lia: Geology, v. 3, p. 283-285.

Wanless, H. R., 1979, Lim estone respo nse to s tress; pressure solution and dolom itization: Jour.

Sed. Petrology, v . 49, p . 437-462.

Weyl, K., 1960, Porosity through dolomitization; conservation-of-mass requirements: Jour.

Sed. Petrology, v. 30 , p. 85-90.

Download - Dolomitization (AAPG Course Notes 24) [Lynton Stuart Land]

Top Related