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Gregg, Jay Mason
TOE ORIGIN OF XENOTOPIC DOLOMITE TEXTURE
Michigan State University PH.D. 1982
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University Microfilms International
THE ORIGIN OF XENOTOPIC DOLOMITE TEXTURE
by
Jay Mason Gregg
A DISSERTATION
Submitted to Michigan State University in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
Department of Geology
1982 Frontplate:
"The Entrance to Marble Canyon, Culberson County, Texas."
B ackplate:
"Galena Dolomite Outcrop Locality 33, Grant County, Wisconsin.
THE ORIGIN OF XENOTOPIC DOLOMITE TEXTURE
Jay M* Gregg
Ph.D* D is s e rta tio n Michigan State University Department of Geology
1982
ERRATA
Abstracts line 3» omit "edral" line 9. omit "ing" p. i i i s ACKNOWLEDGMENTS line 9* Dr* K. C. Lohmann of the University of Michigan provided the oxygen and carbon isotope data and aided in it's interpretation• line 15, "Kable" should be "Kahle" p* 7: line 21, "The term is" should read "The terra a i s " eq« 1» "(i-N )"should read "(1-Na)" p* 11i line 3» should read "theoretically predicted a" p, 19s line 2, "10 m" should read "10 pm" p. 24: line 21, "idiotopic-A" should read "idiotopic-E" p. 25: line 12, "idiotopic-B" should read "idiotopic-S" p. 26: Table 2 caption, "idiotopic-B" should read "idiotopic-S" p« 5 1 J l i n e 31 should read (1011), (4051) p* 78s line 3* should read "the fossils are" p. 83* line 12, "JFl" should read "JF" p* 86s line 14, "xenotopic-C" should read "xenotopic-P" p. 89: line 19* "(table 4 and fig* 54)" should read "(table 6 and fig* 49)" p* 9 1: line 1, "table 6" should read "table 7" p. 95: line 20, read "C0^“" at end of the line p* 9 8: line 17, should read "begin to equally favor" p* 102i line 24, should read "was probably rock dominated" p* 103: line 8, " Table 6" should read "Table 7" p* 105: line 7» should read "is that it resulted" p« 107: line 17» should read " observation of xenotopic" ABSTRACT
The Origin of Xenotopic Dolomite Texture
by
Jay Mason Gregg
Xenotopic dolomite texture, commonly observed in ancient rocks, is defined as a mosaic of anhedra with irregular or curved intergrain boundaries and usually undulose extinction. Xenotopic dolomite edral texture is similar in appearance to neomorphic limestone textures.
Xenotopic texture contrasts with idiotopic dolomite texture (euhedral to subhedral crystals with straight intergrain boundaries) that is common in both Cenozoic and ancient dolomites.
Texture may be controlled by the temperature at which crystals grow. Crystal growth theory predicts that at low temperature, a ing smooth crystal surface is energetically favored, atoms are added to crystal faces layer by layer, with dislocations acting as nucleating sites. This results in faceted crystals and euhedral to subhedral crystal mosaics. Above a "critical roughening temperature" (CRT) a rough surface is energetically favored, surface nucleation does not require dislocations and atoms are randomly added to the crystal surface resulting in non-faceted growth and an anhedral crystal m osaic.
It is hypothesized that a "critical roughening temperature" exists for dolomite above 25°C. Xenotopic dolomites are produced by dolomitization of limestone and/or neomorphic recrystallization of dolomite at elevated temperature (above CRT) after burial. Idiotopic
dolomites are produced below CRT by near surface processes. Calcite
has a CRT below 25°C and, therefore, produces annhedral grain mosaics
(neomorphic texture) both at near surface and elevated temperature.
Synthetic xenotopic dolomite was produced in the laboratory by
dolomitization of aragonite and calcite skeletal fragments and by
recrystallization of nonstoichiometric Cenozoic dolomites at 250°C
and 300°C. Xenotopic dolomite resulted from the metamorphic
recrystallization of the idiotopic Hueco dolomite (Permian), Texas near the Marble Canyon intrusion, at temperature between 350°C and
600°C. Hydrothermal dolomitization of peridase-calcite marble near the intrusion also resulted in a xenotopic texture. Xenotopic dolomite in the Galena Group (Ordovician), Wisconsin, was produced by neomorphism of a pre-existing dolomite during the emplacment of lead-zinc sufides at temperatures between 80° and 120°C. In the
Trenton Formation (Ordovician), Michigan, xenotopic dolomite replaced limestone, during the migration of hot (>50°C) fluids along fracture systems. Xenotopic dolomite was not observed in Cenozoic dolomites which were subjected only to near surface temperatures. Dedicated to the
Memory o f
my F ather
Jay B. Gregg
ii ACKNOWLEDGEMENTS
I wish to express my deep appreciation to Dr. Duncan F. Sibley for suggesting this topic and providing guidance and encouragement throughout this project. I gratefully acknowledge the help and suggestions of Drs. David T. Long, James H. Fisher and especially Dr.
Thomas A. Vogel, who in itially suggested a relationship between dolomite texture and temperature.
Mr. David J. Delgado of Phillips Petroleum Co. provided invaluable assistance in providing stratigraphic information and o u tcro p lo c a tio n s fo r th e Galena Group. Mr. Stu McDonnald o f th e
University of Michigan provided valuable assistance in locating and sampling Trenton Formation core. Mr. Joe Williams of Texas
Architectural Aggregates, San Saba, Texas kindly granted me permission to collect samples in Marble Canyon.
1 also wish to thank Dr. Kenneth A. Jackson of the Bell
Laboratories, Murray H ill, N.J. and Dr. Charles F. Kable of Bowling
Green State University for valuable discussions during the course of th is work.
Dr. Lynton S. Land of the University of Texas, Dr. Carl M.
Cooper and Mr. Donald L. Childs of Michigan State University College of Engineering made valuable suggestions for the design of hydrothermal bombs used in this study, Drs. Stanley Flegler and Karen
Baker of the Michigan State University Center for Electron Optics for the use of and expert instruction on their scanning electron microscopes.
iii This study was partially supported by grants from the National
Science Foundation (no. EAR-8023736), The Geological Society of
America (no. 2837-81), Sigma Xi, and Shell Oil Company. 1 also wish to thank Sun Oil Company and Hunt Energy Corporation for contributing to my support during the first year of my graduate study at Michigan
State University.
Thanks are due to many friends in the Geology Department who helped me get through four years of study. Particularly Dr. Thomas R.
Taylor whose study of the Trenton Formation was concurrent with mine and porvided invaluable insights and information, and Mr. Alan D.
Trippel without whose assistance 1 doubt 1 could ever have obtained a single thin section. For their encouragement and friendship. I wish to thank Bud Ifoyer, Mary Jank, Melissa Wardlaw, Mick Hartzel, Abolfazl
Jameossanaie, Tom Fox, Dan Orr, Loretta Satchel and Carl Karlowski. 1 should not forget Kathy Caswell and Loretta Knutson for making sure th a t 1 got my m ail among o th e r th in g s .
A particular thanks goes to my sister Margaret Lynn Gregg
Haertling who contributed artwork used in some of the copies of this dissertation.
Finally, and most importantly of all, I wish to thank my wife,
Mickey and my children, Nicholas and Tricia, for providing me with support and encouragement and for their patience and understanding, without which 1 could have never completed this study.
iv TABLE OF CONTENTS
Page
LIST OF TABLES v l i
LIST OF FIGURES v i i i
INTRODUCTION 1
Dolomite Recrystallization - Previous Wbrk 3
Temperature Related Origin of Xenotopic Dolomite 5
EFFECTS OF TEMPERATURE ON CRYSTAL GROWTH - THEORY 7
SAMPLES USED IN STUDY 12
The Trenton and Galena Dolomites 12
The Hueco Dolomite and D olom itic Marble 16
Cenozoic Dolomites 16
METHODS' 16
RESULTS 20
Proposed Classification System for Dolomite Textures 20
Experimental Results 29
Aragonite to Dolomite and LMC 34
HMC to Dolomite and LMC 42
Recrystallization Experiments 46
Petrography, SEM and Distribution of the Hueco Dolomites 51
Petrography, SEM and Distribution of the Galena Dolomites 60
Petrography, SEM and Distribution of the Trenton Dolomites 74
Results of Stable Isotope Analyses 86
X-ray Diffraction of Hueco, Galena and Trenton Dolomites 92
v TABLE OF CONTENTS (continued)
Page
DISCUSSION 92
Application of Crystal Growth Theory to Carbonate Texture 92
Neomorphism of Dolomite 99
The Origin of Xenotopic Dolomite: Galena Group 102
Stable Isotope Studies 102
Other Evidence for Neomorphism of Galena Dolomites 103
The Origin of Xenotopic Dolomite: Trenton Formation 104
Significance and Further Study 107
CONCLUSIONS 108
APPENDICES 111
BIBLIOGRAPHY 146
vi LIST OF TABLES
Page
Table 1. Dolomite Textures and Fabrics 22
Table 2. Qualitative Differences Between Idiotopic-B and
Xenotopic-A Textures in Dolomites 26
Table 3. Criteria for Recognizing a Preserved Rhombic
Crystal Face Junction in A Compromise Boundary 30
Table 4„ Point Counting Results for Dolomite Classification 31
Table 5. Experimental Results 32
Table 6. Stable Isotope Values for Hueco Dolomites 87
Table 7. Stable Isotope Values for the Galena Group 90
vii LIST OF FIGURES Page Figure 1. Idiotopic dolomite texture. 2
F igure 2. Xenotopic dolomite texture. 2
F igure 3. Relative surface free energy as a function of the fraction of surface sites on a crystal face which are occupied. 9
F igure 4. Distribution of Trenton Formation dolomite facies in the Michigan Basin. 14
F igure 5. Upper Mississippi Valley lead-zinc district. 15
Figure 6. Marble Canyon, Culberson County, Texas 17
F igure 7. Proposed dolomite textural classification. 23
F igure 8. "Crystal face junctions" preserved by straight compromise boundaries. 27
F igure 9. How a "crystal face junction" forms. 28
F igure 10 . Unaltered skeletal aragonite of pelecypod Area, ponderosa. 35
F igure 11 Area ponderosa after alteration to dolomite X 24"hrs. 0 300°C). 37
F igure 12 . Area ponderosa altered to dolomite (24 hrs @ 300*C). 37
F igure 13 . Area ponderosa altered to dolomite (2 weeks @ 250®C). 39
F igure 14 . Area ponderosa altered to dolomite (23 hrs @ 250°C). 39
F igure 15 . Area ponderosa altered to calcite (2 weeks 0 250*0). 41
F igure 16 . Area ponderosa altered to calcite (1 week 0 W c ) . 41
F igure 17 . HMC skeletal material from the coralline algae Goniolithon sp. 43
F igure 18 . Goniolithon sp. (HMC) altered to dolomite (60 days 0 300°C). 45
F igure 19 . Goniolithon sp. (HMC) altered to LMC (2 weeks 0 300°C). 45
viii LIST OF FIGURES (continued)
Figure 20. Unaltered supratidal dolomite from Andros I s la n d , Bahamas.
Figure 21. Supratidal non-stoichiometric dolomite from Andros Island, Bahamas.
Figure 22. Unaltered dolomitized coralline algae from B onaire, N.A.
Figure 23. Dolomitized coralline algae from Bonaire, N.A.
F igure 24. Idiotopic-B dolomite of the Hueco Limestone, Marble Canyon, Texas.
F igure 25. Id io to p ic-B Hueco dolom ite.
Figure 26. Temperature gradient at Marble Canyon.
F igure 27. Dolomite crystal terminations (D) in a calcite filled vug (C).
F igure 28. Hueco dolom ite m arble.
Figure 29. Hueco dolom ite m arble.
F igure 30. Dolomite produced by hydrothermal alteration of a periclase marble.
F igure 31. Dolomite produced by hydrothermal alteration of a periclase marble.
F igure 32. Idiotoplc-D dolomite replacing limestone micrite of the Galena Group (locality 45).
F igure 33. Idiotopic-D dolomite (dark) replacing calcite micrite (light) of the Galena Group.
F igure 34. Idiotopic-B Galena dolomite from locality 20.
F igure 35. Idiotopic B Galena dolomite from locality 20.
F igure 36. Idiotopic-A Galena dolomite from locality 28.
F igure 37. Xenotopic-A Galena dolomite from locality 45.
F igure 38. Xenotopic-A Galena dolomite (dark) replacing calcite (light) from locality 45,
F igure 39. Xenotopic-A Galena dolomite from locality 19.
F igure 40. Xenotopic-A Galena dolomtie from locality 19. LIST OF FIGURES (continued) Page Figure 41. Idiotopic-B dolomite replacing micrite in the "cap" of the Trenton Formation (Br core). 76
Figure 42. Idiotopic-B dolomite from the "cap" of the Trenton Formation. 76
Figure 43. Xenotopic-A fracture related dolomite from the Trenton Formation. 77
Figure 44. Dolomitized biomicrite from the "cap" of the Trenton Formation. 80
Figure 45. Xenotopic-A dolomite replacing a fossil in the "cap" of the Trenton Formation. 80
Figure—46. Saddle shaped Xenotopic-C dolomite growing into fracture porosity of the Trenton Form ation. 82
F igure 47. Anhedra of xenotopic-C dolomite (light) partially replacing a calcite brachiopod fragment in the Trenton Formatin "cap". 85
F igure 48. Xenotopic-C dolomite replacing limestone micrite along a fracture in the Trenton Formation. 85
F igure 49. 1»0 values for Hueco dolomites plotted against temperature and distance from the in tru s io n . 88
x INTRODUCTION
Dolomite is usually recognized in thin section by the euhedral
rhombic crystal form, i.e. Friedman's (1965) idiotopic texture
(fig. 1). However, in ancient rocks, dolomites can often be found as
mosaics of anhedral crystals with irregular grain boundaries and
undulose extinction. This texture (fig. 2) falls into Friedman's
(1965) xenotopic textural category and is similar to neomorphic
textures observed in some limestones, particularly neospars and microspars (Folk, 1965 and Bathurst, 1976).
The hypothesis is investigated that xenotopic dolomite texture results from the replacement of limestone by dolomite or by neomorphic recrystallization of a pre-existing dolomite at elevated
temperature (above % 50°). This hypothesis is tested by: 1) petrographic analysis to determine timing of dolomitization and the distribution of textural types, 2) petrographic analysis of a contact metamorphic dolomite, 3) laboratory experiments using hydrothermal bombs to recrystallize dolomites and produce high temperature replacement dolomites, and 4) implication of crystal growth theory.
Recrystallization or Neomorphism of Dolomite
Neomorphic dolomite is defined here as any dolomite texture resulting from the recrystallization of a prior existing dolomite.
This vrould include the inversion process by which protodolomite
(Gaines, 1977) becomes stoichiometric, well ordered dolomite. This would not include the dolomitization of a calcite or aragonite sediment. The usage of Beals and Hardy (1976) to describe "...a dolostone in which the original limestone texture is entirely
1 2
Figure 1 Idiotopic dolomite texture? idealized representation.
Figure 2. Xenotopic dolomite texture; idealized representation. obliterated," Is disregarded here.
Folk (1965) and Bathurst (1976) recognized several types of neomorphism In limestones. These Include: 1) polymorphic transformation (Bathurst, 1976) or inversion (Folk, 1965) which involves the change of one polymorph into another such as aragonite into calcite, 2) aggrading neomorphism, whereby fine crystalline carbonate is replaced by a coarse crystalline mosaic of the same material. This process may begin in partly consolidated sediments and result in a sparry texture that is difficult and sometimes impossible to distinguish from a void filling cement. 3) Uet recrystallization involving the growth of large grains at the expense of smaller ones. This process may occur during the late stages of aggrading neomorphism. Neomorphic limestones are characterized by anhedral crystals with irregular intergrain boundaries.
Some ancient dolomites texturally resemble calcite neospars interpreted to have formed by aggrading neomorphism. This leads to the speculation that such dolomite textures are the result of aggrading neomorphism or recrystallization of a pre-existing finer grained dolomite.
Dolomite Recrystallization - Previous Work
Certain textures have been recognized as evidence of dolomite neomorphism, or recrystallization. The Sevey Dolomite (Devonian),
Nevada was described by Osmond (1954) as an interlocking mosaic of dolomite anhedra made up of irregular shaped, closely packed, clear crystals. These crystals ranged in size from 0.02mm to 0.09mm with most about 0.05mm. Scattered subhedral dolomite porphyrotopes were thought to represent recrystallizations of the smaller anhedral crystals. Unfortunately, Osmond did not provide a photomicrograph with his description.
A quantitative study of grain mosaic geometry was made by Karez
(1964) on several Cambrian dolomites from the Isle of Skye, Scotland.
By recording the frequency and shape of triple junctions between crystals and measuring the interfacial angles of these junctions it was determined that features attributable to recrystallization exist.
Grain growth was indicated by an interfacial angle frequency distribution with a maximum in the range of 100° to 140°, i.e ., greater than 90% of the triple junctions measured were obtuse angles.
Schmidt (1965) recognized three distinct kinds of recrystallization of dolomite in the Gigas beds of northwestern
Germany: 1) Recrystallized dolomite that is associated with iron silicate. Here dolomicrite was recrystallized into a texture made up of anhedral and subhedral crystals. 2) Doloarenites which were recrystallized to a highly porous medium with a fine grain idiotopic texture. In some areas, the original allochems can be observed as ghosts. The original intergranular porosity and allochem mold porosity were redistributed as intercrystalline and microvuggy porosity (Schmidt, 1965, fig. 20-2). 3) Nonporous dolomite having a marble-like texture made up of tight interlocking medium to coarse anhedral dolomite crystals was observed near faults. Strain during overthrust faulting was thought by Schmidt (1965, fig. 21-2) to be responsible for the texture.
In a discussion of trace element and stable isotopic geochemistry of dolomites Land (1980) points out that neomorphism of a dolomite or a protodolomite may affect the primary dolomite chemistry (see also Land et a l., 1975). Such a reaction would not necessarily be closed with respect to isotopes or trace elements.
Therefore, if recrystallization has taken place the isotope and trace element signature of an ancient rock may not reflect the composition of the original sediment.
Fine grained xenotopic dolomite from the Upper Knox carbonates
(Cambro-Ordovician) Tennessee are thought to be the result of aggrading neomorphism of a finer grained dolomite formed in an intratidal to supratidal environment (Churnet et a l., 1982). Low Na and Sr concentrations and near stoichiometry of these dolomites are attributed by the authors to neomorphism in the presence of fresh w ater.
Temperature Related Origin of Xenotopic Dolomite
Xenotopic texture in dolomite may be related to the temperature at which the pre-existing limestone was dolomitized. Some evidence exists in the literature to support this hypothesis.
Land (1967) dolomitized a fragment of the Pelecypod Argina sp. in a calcium and magnesium chloride solution within the dolomite stability range at 300°C in a hydrothermal bomb for 22 hours. The aragonite shell fragment became partially replaced by xenotopic dolom ite.
Dolomite, forming dikes and pods in the Wyman Formation, is interpreted as a replacement of limestone (Zenger, 1976). This replacement probably took place under low grade metamorphic conditions and seems to have resulted in a xenotopic texture (see 6
Zenger, 1976, fig. 5).
Radke and Mathis (1980) inferred a high temperature origin
(60°-150°C) for saddle dolomites. Associated with pore filling saddle cements one often observes undulose saddle dolomites
(xenotopic dolomites) as a replacement phase (Radke and Mathis, 1980, fig. 8). A high temperature (50°-130°C) origin for undulose ferroan dolomite cements found in Mississippian carbonates of the Illinois
Basin was also favored by Choquette (1970).
Zenger (1980) suggests that high temperature recrystallization and/or replacement may have produced coarse, undulose xenotopic dolomite in the Little Falls Dolostone (Upper Cambrian), east-central
New York. Several fluid inclusion analyses supported by 5^-®0 values indicate temperatures > 80°C during the formation of these dolomites. Fairchild (1980) described undulose dolomite from the
Precambrian of Scotland which he attributes to high temperature
(150°-200°C) recrystallization and replacement.
The difference between the idiotopic (euhedral rhombs) and xenotopic (anhedral crystals with irregular grain boundaries) replacement textures may be the temperature at which dolomitization took place; high temperatures producing xenotopic texture and low temperatures producing idiotopic texture. Dolomite from Bonaire,
N.A. replaces aragonite and high magnesium calcite (HMC) with an idiotopic (rhombic) habit. Other Quaternary dolomites from around th e hd rid also replace substrate calcium carbonate with euhedral to subhedral crystals (Schlanger, 1957; Uling et al., 1965; Buchbinder,
1979; McKenzie, 1981; and Sibley, in press, and many others). In all of these cases, it is doubtful that dolomitization took place at higher than near surface temperatures.
EFFECTS OF TEMPERATURE ON CRYSTAL GROWTH - THEORY
The temperature at which crystals grow can affect the mode of growth (Brice, 1973; Lewis, 1975; Kirkpatrick, 1981) and ultimately the texture of a mosaic of these crystals. A statistical mechanical model for the surface structure and kinetics of growth of crystals growing from a melt is given by Jackson (1958a and 1958b). Jackson's work is summarized as follows:
The change in free energy (AFS) of a growing crystal surface is divided into four components: 1) the energy gained by the addition of atoms on the crystal surface, 2) the energy gained because some of the atoms added to the surface will be adjacent to one another, 3) the entropy difference between the crystal face and the phase in which it is growing, 4) the entropy gained by the random arrangement of atoms on the crystal surface. If temperature is at the melting point (temperature at equilibrium so T = Tg) the above can be expressed as:
AF /NkT =aN (1-N )/N2 - ln[N/(N-N ) J - (N /N)ln[(N-N )/N J eq. 1 sea a a a a where N is the number of sites on the crystal face available for occupation, and Na is the number of single atoms (molecules or ions) added to the surface. The term is a roughness parameter derived from the relationship
a= (L/kTe)(N0/v) =AS/k(N0/v) eq. 2
L energy needed to take an atom from the crystal
and put it into the phase in which the crystal 8
is growing, [Latent heat or enthalpy of formation
of the vapor phase, liquid phase or latent heat of
dissolution (Lewis, 197)5).]
AS the change in entropy between the crystal and the
phase in which it is growing,
k Boltzman's constant
Te equilibrium temperature.
N0/v the fraction of the total number of nearest
neighbors in a plane parallel to the face under
consideration to the total number of possible
nearest neighbors. [The fraction of the total
binding energy per atom operating in the plane of
the surface (Kirkpatrick, 1981).] This will fall
between 0.5 and 1.
The roughness p aram eter, a, is directly proportional to L and inversely proportional to Te. As the temperature of the reaction increases, a becomes smaller. Figure 3 shows the relationship in equation 1 graphically. Curves are drawn for various values of a.
For any a le s s than 2 th e re is a minimum a t Na/N = 0.5 on th e horizontal scale (1/2 of the possible surface sites are filled).
F ree energy (th e v e r ti c a l s c a le ) th e re fo re has a minimum valu e when
1/2 of the sites are filled. As the probability of the formation of clusters of atoms on the half filled surface is large, new layers may start to form before the initial layer is filled. This will result in random addition of atoms on an atomically rough crystal surface and rapid continuous growth. This type of growth will produce nonfaceted crystals as opposed to crystals with well developed 9
1.0 -
111 uj ui
“ 0.3 “
c m .
- 0.5 OCCUPIED FRACTION OF SURFACE SITES
Figure 3. Relative surface free energy as a function of the fraction of surface sites on a crystal face which are occupied; a depends on the crystal face, th<£ type of crystal and the phase from which the crystal is growing (from Jackson, 1958a). 10 crystal faces (Jackson, 1958). A mosaic of nonfaceted crystals produces the interlocking anhedral grain mosacis such as observed in metal castings (all metals grow from their melts at a <2 (Jackson,
1958)). Mosaics of euhedra can only be produced if crystal faces develop during the growth of the crystals.
At any a>2 there are tw minima, one near Na = N and one near
Na 53 0. The maximum, or highest free energy configuration, is at
Na = 0.5. Thus new atoms must be added one layer at a time as partial filling of a new layer is energetically unfavorable. Growth is, therefore, difficult and proceeds by use of surface imperfections, such as screw dislocations, as described by Burton and
Cabrera (1949) and Frank (1949). Under these conditions crystal faces w ill develop during growth and mosaics of euhedral and subhedral crystals will form. This is the case when a material such as salol is solidified from a melt (Jackson, 1958a). At a= 2a transition between a rough and a smooth crystal surface is attained.
This occurs at a specific temperature for a specific crystal face.
Below this "critical roughening temperature" (CRT) a will be greater than 2 and the crystal surface will be smooth. Above CRT a w ill be less than 2 and the surface will be rough.
Temkin (1966) arrived at similar conclusions as Jackson (1958a and 1958b) using a multi-layer mean field model. This model does not give a CRT; rather, as temperature increases the crystal face becomes continuously rougher.
Experimental evidence indicates that the CRT for crystals growing from a melt are slightly higher than predicted (Jackson and
Gilmer, 1976). This may be because of low wettability (high 11 ' interfaclal tension) of the crystal face which has the effect of
Increasing the real a over the theoretically predicted a. The theoretically predicted assumes a high wettability of crystal faces and, therefore, the theoretical estimates of interfaclal free energy
(AFS) may be low (Zell and Mutaftschien, 1972). The CRT of crystals growing from the vapor phase has been found experimentally to be lower than predicted by a factor of two. This phenomenon is not well understood but may be due to the mobility of the atoms added to the surface of the crystal (Jackson and Gilmer, 1976 and Jackson and H iller, 1977).
Replacement of a calcium carbonate by dolomite involves a dissolution reprecipitation mechanism with an aqueous phase
(Bathurst, 1976 and Katz and Matthews, 1977). Therefore, any application of the above discussed principles of crystal growth to dolomitization must involve growth of dolomite crystals from a s o lu tio n .
Jackson's (1958a and 1958b) model can theoretically be applied to growth of crystals from solution (Bennema, 1974; Lewis, 1975).
Several studies apply Jackson's (1958a) model to crystal growth from solution for example: Veronkov and Chernov (1967) calculated that the critical roughening temperature is dependent on the concentration of the solution; the more concentrated the solution the lower the
CRT. A computer simulation model developed by Bennema and Van der
Eerden (1977) is used to calculate Jackson's (1958a) a for organic crystals growing in organic and aqueous solvents. Corrections are made for wettability and the problem that latent heat of dissolution is not accurately known when solid particles are dissolved in 12 saturated solutions.
Among the difficulties in calculating the CRT for a carbonate mineral is obtaining a latent heat value at saturation (equilibrium).
This i s not known fo r many su b sta n c e s, in c lu d in g dolom ite, and is complicated by the large hydration energies associated with the calcium and magnesium ions. Also, because dolomite does not form simple monoatomic crystals in the cubic system, it is difficult to quantitatively apply Jackson's (1958a) model, (tost theoretical work has been done on simple monatomic cubic crystals using computer simulation techniques. Although the simple model for crystal growth must apply to complex c r y s ta l s , many com plicating fa c to rs make an accurate prediction of the CRT difficult (K.A. Jackson, written communication, 1981). The model is, however, of considerable use in qualitatively understanding crystal growth and morphologies
(Kirkpatrick, 1981).
SAMPLES USED IN STUDY
The Trenton and Galena Dolomites
The Trenton Formation (Middle Ordovician), Michigan and Galena
Group (Middle Ordovician), Wisconsin, northern Illinois and eastern
Iowa were chosen for this study because: 1) they have a wide range of dolomite textures ranging from xenotopic to idiotopic, 2) their geologic history includes deep burial in the case of the Trenton
Formation and epigenetic hydrothermal activity in the case of both the Trenton Formation and the Galena Group, 3) the availability of enough sample control to conduct the study.
The limestone facies of the Trenton Formation are composed 13 primarily of uniform biomicrites which were deposited below wave base in a low energy open marine environment. There is no evidence of reef development in the Trenton and it is believed to be part of a shallow marine shelf complex (Ardrey, 1978).
Three basic facies of dolomite exist in the Trenton Formation of the Michigan Basin (fig. 4). These are 1) the cap dolomite involving the upper 50 feet of the formation, 2) epigenetic dolomitization associated with fracturing, including the northeast to southwest trending Albion Scipio and Northville oil fields, and 3) regional dolomitization in the southwestern part of Michigan (Steward, 1957;
Shaw, 1975; Newhart, 1976; Ardrey, 1978; Taylor, 1982).
The limestone facies of the Galena Group are composed of biomicrites and a few biosparites that were deposited on a stable, uniform, low energy sea bottom, in most cases below wave base
(Delgado, 1979 and written communication, 1980). Badiozamani (1972 and 1973) postulated a dorag or mixing zone model for regional dolomites during the uplift of the Wisconsin Arch. Dolomite is localized along the crest of the arch becoming limestone toward the flanks of the arch (Badiozomani, 1973).
The Galena Group was subjected to an epigenetic hydrothermal event resulting in the emplacement of the sulfide ores of the Upper
Mississippi Valley Zinc-lead D istrict (fig. 5) (Heyl et al., 1959).
The temperatures of emplacement of these ores are estimated, by fluid inclusion studies to have ranged from 80 to 120°C (Bailey and
Cameron, 1951), to 227°C (McLimans, 1977 and H. L. Barnes, personal communication, 1982). Hall and Friedman (1969) corroborate these temperatures with stable oxygen isotopes (5*®0 SHOW =' 20 for dolomites associated with ores). 14
HS
5' •i
Jbion-Scipio k Field Northville Field
W2
0 10 20mllea Regional dolomite
Extent of cap dolomite
• Location of core
Figure 4. Distribution of Trenton Formation facies in the Michigan Basin. Based on map by Taylor (1982) . IOWA
Extent of sulfide mineralization
•3 0 S
20
33
•16 •is WISCONSIN ILLINOIS
• Collecting Locality 20 mi,
Figure 5. Upper Mississippi Valley Zinc-lead District. 16
The Hueco Dolomite and D olom ltic Marble
A dolomitized facies of the Hueco Limestone (Wolfcamp), was intruded by a Tertiary syenite and gabbro stock (Kuehn, 1969) in
Marble Canyon, Culbertson County, Texas. The dolomite was recrystallized in and near a contact metamorphic aureole ranging in width from 150 to 300 ft. (fig. 6) (King, 1965; Bridge, 1966). Near the intrusion the dolomite altered to a bleached brucite-calcite marble which is currently being quarried for decorative stone.
Sixty-five samples were collected from massive dolomite and limestones on the east side of the Marble Canyon intrusion (fig. 6).
The samples were collected at distances ranging from immediately adjacent to the intrusion to 1200 ft. away.
Cenozoic Dolomites
Dolomites of the Seroe Do mi Formation (Pliocene), Bonaire and
Aruba, Netherlands Antilles (see Sibley, 1980) were used for comparison with ancient and synthetic dolomites and use in hydrothermal bomb experiments. The Pliocene dolomites were never subjected to other than surface temperatures. Samples of recent supratidal crusts containing as much as 60% dolomite (as determined with x-ray analysis and SEM) were obtained from Andros Island. These samples were also used for comparison with ancient and synthetic dolomites and in hydrothermal bomb recrystallization experiments.
METHODS
The samples collected from the Trenton Formation (130 samples shown on fig. 4), Galena Group (130 samples shown on fig. 5) and 17
^ r n m ir n Pb / i f f p p g L
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m fe '' ' v ' ipt Tortiary Intruaivas ' v f e l i l Iasi s' • '* * ,Ni i n Matamorphics f- . .'V;' ■ -' A / s. / * * t s I fefe ' ' ' N ' V ' 0 Bona Springs Formation Vv-'.-V/SvAV■*■ ■ •.''* A . ' / v. ' / / ' . .‘‘..•'•I
Huaeo Formation ^ • 1 4 ^ v U J p 0 Sections Collected A Sam plas 1 -5 B 6-15 C 16-46 D 47-51 TEXAS E 52 °Marbla Canyon F 53-61
Figure 6. Marble Canyon, Culberson County, Texas. Geologic map based on King (1965) and
Bridge (1966). 18
Hueco Limestone (65 samples shown on f i g . 6) were made in to stan d ard petrographic thin sections. Some of the synthetic dolomites were made into thin sections that were polished to less than 10 pm thickness. Samples were stained for calcite with alizerine red S and, in some cases, stained for iron with potassium ferrocyanide.
Selected thin sections were polished and examined for cathodoluminescence using a Nuclide Corp. model ELM-2A Luminoscope,
12 to 14 kV (DC), 40 to 50 ma, 0.5 to 1 cm diameter beam, cold cathode gun, helium gas, 150 m illitorr.
Samples including the Cenozoic dolomites and synthetic dolomites were also examined using IS1 Super III and JEOL 35C scanning electron microscopes equipped with tungsten filament guns at 15 to 35 KV. The
JEOL 35C was also equipped with a dual anular photolithographic disc backscattered electron detector. Some of the synthetic dolomites were examined using an ARL electron microprobe. Samples were gold or carbon coated for examination with SEM and carbon coated for microprobe. Only fresh fracture or polished surfaces were examined.
Samples collected from the Galena Group and the dolomitized
Hueco Limestone were analyzed for stable carbon and oxygen isotopes
(at the University of Michigan Stable Isotope Laboratory). The samples were crushed to a fine powder and cleaned in an acetone sonic bath. Some of the samples were boiled in a solution of disodium EDTA to remove calcite. All of the samples were examined with x-ray diffraction to make sure that dolomite was the only carbonate p re s e n t.
Stoichiometry and ordering of synthetic and natural carbonates were determined using a General Electric x-ray diffractometer, CuK 19 radiation, Ni filter, 1° exit slit and 0.1° and 0.05° scatter slits.
Samples were ground to less than 10 m and mixed with a fluorite or halite standard for analysis. Stoichiometery was determined from the position of the (104) dolomite reflection of the sample relative to the standard (see Lumsden and Chimahusky, 1980). Ordering in dolomites was qualitatively determined by observing the shape and height of the (006), (015) and (012) reflections relative to the
(110) reflection (see Graf and Goldsmith, 1956).
The hydrothermal experiments were carried out using two stainless steel bombs of 18.5 and 6.6 ml. capacity closed with cold copper gaskets (Appendix 1). Noble metal liners were not used in the experiments as it seemed unlikely that stainless steel would contaminate the experiments at the modest temperatures and pressures used. Microprobe analysis of some bomb products showed iron present only in trace amounts (less than 0.1%).
Reactant fluids were prepared using stock aqueous solutions of 1 and 2 molar CaCl 2 and MgC^. Skeletal and sedimentary carbonate samples were broken into fragments weighing less than 0.02 g in order to effect a more complete reaction with the surrounding solution.
The hydrothermal experiments were carried out in a Lindberg
Hevi-duty muffle furnace and a Sybran Thermolyne 2000 furnace.
Temperatures used were 250°C and 300°C at pressures of 39 and 85 atm., respectively. Pressures were calculated from standard steam ta b le s . Optimum Mg/Ca r a ti o s and co n ce n tratio n s were found empirically using published phase diagrams (Rosenberg and Holland,
1964) for maximum dolomite production. In the experiments involving aragonite moHusk fragments dolomite usually formed as a reaction rim 20
around the fragment with a center of inverted calcite.* Therefore, a
balance had to be established between fragments that were too large
(producing a small amount of dolomite in proportion to calcite) and
those too small for convenient light microscope and SEM examination.
This fragment size was found to be between 1 and 3 mm in diameter.
No difficulty was encountered in dolomitizing high magnesium
calcite (HMC) coralline algal or echinoid fragments. In all of these
experiments 100% dolomite was produced without a low magnesium
calcite (LMC) phase. Individual dolomite crystals produced in these
experiments were too small to be observed with light microscope.
RESULTS
Proposed Classification System for Dolomite Textures
During the course of this study it became obvious that a
textural classification of dolomites would have to be developed in
in order to consistently distinguish between the dolomites being
studied. The texture of a rock is a derived property determined by
the interrelationship of four of the fundamental properties of the
grains: 1) size, 2) shape, 3) orientation, and 4) packing (see Blatt,
et a l., 1980). Texture in dolomite is here defined as the
relationships between individual dolomite crystals (i.e., grains).
The aspects of the origin of dolomite crystals that are of most
*In further hydrothermal experiments of this type, carried out by Sue Bullen (personal communication, 1982), it was found that if the aragonite shell fragment was first oxidized in hydrogen peroxide or chlorine bleach, to remove organic matter, complete dolomitization occurred when th e sample was re a c te d in a bomb. 21 importance in discussing dolomite textural classifications are density of nucleation and growth mechanisms of the crystals. These aspects control the fundamental properties of grain size and grain shape respectively.
Friedman (1965) proposed a textural classification for crystalline sedimentary rocks (Table 1) which was specifically applied to dolomites by Friedman and Sanders (1967). This classification system recognizes the importance of crystal shape
(crystal texture) and relative size and orientation of grains
(crystal fabric) as contributing factors to the overall texture of the rock. Friedman's (1965) textural classification system was found to be inadequate for this study because it does not differentiate between xenotopic textures with smooth, straight compromise boundaries and those with curved or irregular intergrain boundaries.
This distinction may have considerable genetic importance.
The proposed classification system which was used in this study
(fig. 7) was designed to distinguish dolomites on the basis of crystal morphology and grain boundaries as opposed to crystal size.
Therefore, the overriding factor in how a dolomite will be classified is the mechanism of crystal growth rather than density of nucleation.
A binomial system is used to classify dolomite. The dolomite is initially categorized as idiotopic if the grains have straight, compromise boundaries and tend to be rhombic in shape; or xenotopic texture if the grains have mostly irregular intergrain boundaries and tend to not be rhombic in shape. Hypidiotopic texture of Friedman
(1965) is considered to be idiotopic under this system. Table 1. Dolomite Textures and Fabrics (from Friedman, 1965 and Friedman and Sanders, 1967).
Crystal Texture
1) Individual Crystal 2) Majority of crystals in rock as a whole (a) euhedral (a) idiotopic (b) subhedral (b) hypidiotopic (c) anhedral (c) xenotopic
Crystal Fabrics
1) Equigranular 2) Inequigranular
(a) Porphyrotopic-large dolomite crystals in a matrix of small crystals (b) Poikilotopic-large dolomite crystals in a matrix of small crystals of a different mineral Idiotopic Dolomite- Rhombic shaped euhedral to subhedral Xenotopic Dolomite- Non-rhombic usually anhedral grains grains (idiotopic and hypidiotopic of Friedman (1965)) (xenotopic of Friedman (1965) and saddle dolomites).
Idiotopic-E (Euhedral), almost all Xenotopic-A (Anhedral), tightly packed grains are euhedral rhombs, grain anhedral grains with mostly curved, supported with intergranular area lobate, serrated, indistinct, or filled by another mineral or otherwise irregular intergrain comprised of porosity (sucrose boundaries. Preserved crystal texture). face junctions are rare and grains often have undulose extinction under cross polarized light.
Idiotopic-S (Subhedral), subhedral to anhedral grains with low porosity or intergranular matrix, straight intergrain boundaries, and grains Xenotopic-C(Cement), pore lining saddle commonly have preserved crystal shaped or baroque dolomite crystals face junctions. characterized by scimitar-like terminations and sweeping extinction when observed in thin section. Idiotopic-C (Cement), dolomite euhedra lining large pores, vugs, or patches of another mineral such as gypsum or calcite.
Xenotopic-P (Poikilotopic), single anhedral grains or patches of anhedral grains of dolomite, usually having undulose extinction Idiotopic-P (Poikilotopic), Floating under cross polarized light, euhedra of dolomite in a limestone floating in a limestone matrix. matrix. Matrix supported rather than grain supported.
Figure 7. Proposed dolomite textural classification. 24
F u rth er refinem ent In c la s s if ic a tio n i s made by adding subcategories within the two dolomite types. Idiotopic-G (euhedral) contains those dolomites made up of loosely packed, well-formed dolomite rhombs. The rock must be grain supported. The intergranular area may be filled with another mineral such as calcite or porosity as in the case of sucrose dolomite. Idiotopic-S (subhedral) dolomites are those referred to as hypidiotopic by Friedman (1965) and may include some of Friedman's (1965) xenotopic dolomites. These low porosity dolomites are characterized by straight compromise boundaries and a large number of preserved crystal face junctions.
Dolomites that line pore space with terminations projecting into the open pore or vug are referred to as idiotopic-C (cement) dolomites. Such dolomites may be produced by growth of dolomite cement around the edges of the pore. Alternately, partial replacement of a calcium carbonate allochem, such as a fossil fragment, by impinging dolomite rhombs around its edge; followed by the dissolution of the allochem would result in a pore lined by dolomite rhombs. In many cases it might be difficult to distinguish whether the idiotopic-C dolomite is of cementation or replacement origin.
Free floating rhombs in a limestone matrix are classified as idiotopic-P (poikilotopic). These are distinguished from idiotopic-A dolomite in that they are matrix supported. (The terms "matrix" or
"grain" supported are not meant to imply a clastic origin for these dolomites.) Friedman (1965) uses the "crystal fabric" term poikilotopic to describe idiotopic-P texture.
Dolomites with low porosity, anhedral grains and mostly curved, lobate, serrated, indistinct, or otherwise irregular intergrain 25 boundaries are classified as xenotopic-A (anhedral) dolomite. These dolomites are similar in appearance to a calcite pseudospar and dolomitic marble. In addition to the irregular intergrain boundaries xenotopic-A dolomite often has a large number of inclusions (observed with SEM as "Swiss cheese" structure (Katz and Matthews, 1977)) which gives them a dirty appearance in thin section. Xenotopic-A dolomite usually displays undulose extinction under cross polars. Replacement saddle dolomite grains (Radke and Mathis, 1980) are classified as xenotopic-A as well as the low grade metamorphic dolomites described by Zenger (1971). Xenotoplc-A texture is also classified as xenotopic by Friedman (1965) although he did not differentiated from some dense, nonporous dolomites here classified as idiotopic-B.
Xenotopic-C (cement) dolomites are non-rhombic pore and vug lining dolomites. These can be saddle dolomites or in rare instances the distorted octahedral crystal form, [0001] and [4041], of dolomite. Saddle dolomites are best recognized in thin section by their sweeping extinction and by their long curved edges leading to pointed terminations, reminiscent of the Persian scimitar.
The last category, xenotopic-P (poikilotopic) texture, are
"poikilotopic" fabrics made up of dolomite anhedra replacing precursor carbonates (see figure, Zenger, 1982). Because, without staining, xenotopic-P dolomite is nearly impossible to distinguish from scattered patches of pseudospar or microspar in a limestone, it is possible that this dolomite texture is quite common but has not often been recognized.
A difficult and genetically important distinction that often must be made is between dense idiotopic-S dolomite (hypidiotopic and 26
sometimes xenotopic o£ Friedman (1965)) and xenotopic-A dolomite.
Table 2 lists some of the qualitative differences between these
textures in thin section.
The differentiation between idiotopic-S and xenotopic-A texture
can be made quantitatively by point counting the numbr of grains
TABLE 2 - Qualitative differences between idiotopic-B
and xenotopic-A textures in dolomite.
Idiotopic-S
1. Most intergrain boundaries are smooth and straight.
2. Crystals tend to maintain a rhombic shape. Angles formed
by the junction of crystal faces are often preserved as
compromise boundaries with neighboring crystals.
3. Crystals are rarely undulose under crossed polars
Xenotopic-A
1. Intergrain boundaries are usually irregular.
2. Crystals do not tend to have rhombic shape; crystal
face junctions are not often preserved by compromise
boundaries with neighboring crystals.
3. Crystals usually have undulose extinction. 27
Figure 8. "Crystal face junctions" (arrows) preserved by straight
compromise boundaries. This sample of dolomtie is from
the Pliocene of Bonaire, N.A. Partially crossed polars,
scale bar =* 0.05 mm. 28
Figure 9 How a "crystal face junction" forms. Crystals A and B meet at point b at time 1 (T^) as shown, and continue to grow at constant rates,
represented by T2 and Tg• At Tg straight compromise boundaries exist forming angle abc and preserving the crystal face junction of
crystal B at point b. 29 where compromise boundaries preserve a junction between two crystal faces of the same grain (crystal face junction). Figure 8 shows such compromise boundaries and figure 9 illustrates how these boundaries are formed. Table 3 lists the criteria for recognizing this relationship which were used in point counting.
Counts of 100 to 300 grains each were made for a number of idiotopic and xenotopic dolomites used in this study; the results of these counts are shown in table 4. Samples which were qualitatively recognized as idiotopic-S dolomites consistently have a significantly higher number of crystal face junctions preserved than xenotopic-A dolomite. Therefore, this appears to be a useful quantitative method of distinguishing between idiotopic-S and xenotopic-A dolomite te x tu r e s . The d iv id in g lin e between th e two te x tu r a l types i s probably where 30% of the grains have crystal face junctions preserved. Above this number the dolomite is idiotopic and below, it is xenotopic.
The majority of the petrographic descriptions for this study were done using the qualitative system of classification because most of the samples very obviously fell into one category or another.
Experimental Results
The'results of selected hydrothermal experiments are shown in
Table 5 and Appendix 2. The amount of ordering and the stoichiometry of the dolomite produced was a function of both the temperature of the reaction and the amount of time the reaction was allowed. At
250°C a short reaction (23 hrs.) produced slightly disordered calcium enriched dolomite (or protodolomite). If the reaction was longer 30
Table 3 - C riteria for recognizing a preserved rhombic
crystal face junction in a compromise boundary.
1. The rhombic crystal face junction must be in contact with a
neighboring dolomite grain and appears to make a "bite” into that
grain. This includes grains where the rhombic crystal face
junction overlaps the neighboring grain.
2. There is no observable porosity or other non-dolomite minerals
neighboring the grain being counted.
3. Angles larger than 160° should not be counted and care should be
taken not to count arcuate boundaries.
4. The tv» straight edges forming the interface must cover a
perimeter of at least 1/2 the longest diameter of the grain being
counted. The edges may be in contact with more than one grain as
long as they contact a single grain where they join, i.e. triple
boundaries cannot be counted. Table 4. Point counting results for dolomite classification.
Sample Textural Type Grain size A B C
M arble Canyon, Hueco dolom ite Id io to p ic -S 0.05 - 0 .2 mm 55 45 200
Marble Canyon, dolomite marble Xenotopic-A 0.1 - 0 .4 mm 11 89 200
Galena dolomite locality 20 Idiotopic-S 0.01 - 0.1 mm 55 45 300
Galena dolomite locality 19 Xenotopic-A 0.1 - 0.5 mm 24 76 300
Trenton Dolomite Core £1 Xenotopic-A 0.3 - 0.1 mm 18 82 100
Trenton Dolomite Core L12 Xenotopic-A 0.1 - 0.7 mm 27 73 150
Pleiocene dolomite, Bonaire, N.A. Idiotopic-S 0.01 - 0.1 mm 43 57 100
Pleiocene dolomite, Bonaire, N.A. Idiotopic-S 0.01 - 0.1 mm 45 55 100
Key: A - Percent of grains with crystal face junction. B - Percent of grains without crystal face junction. C - Total number of grains counted. Table 5. Experimental Results
No. Solid reactants Solid Products Time Temp.°C F lu id Mg/Mg+Ca* (h rs ) (Mg/CaCl2) 1 Mg C alc. ordered dolomite 98 300 2 molar .25 Goniollthon
2 839
3 *• 120 250
4 Mg C alc. ordered dolomite 168 300 .24 Echlnold
5 Mg Calc. c a l c it e 335 1 m olar .04 Goniolithon 00 N3 6 •• 1516 300 .05
7 aragonite c a l c it e 136 300 .15 Strombus trace dolomite
8 23 ia n .16
9 43% ordered 239 250 2 molar .30 dolom ite 57% calcite
10 aragonite 73% ordered 334 .30 Area dolom ite 27% calcite Table 5. Experimental Results (continued)
No. Solid reactants Solid Products Time Temp.°C F lu id Mg/Mg+Ca* (h rs ) (Mg/CaCl2) 11 aragonite 44% disordered 23 250 2 molar .30 Area dolom ite 56% calcite 12 aragonite 44% ordered 979 300 2 molar .23 Area dolom ite 56% calcite 13 43% ordered 24 .24 dolom ite 57% calcite
14 aragonite c a l c it e 120 " 1 molar 0 Strombus
15 aragonite c a l c it e 167 250 M 0 Area
16 disordered dolo ordered dolomite 311 300 2 molar .25 mite and aragonite
17 ordered dolomite 120
18 nonstoichiometric recrystallized 2184 dolomitized coral dolom ite line algae Bonaire
19 120
20 dolomitized mud no change 720 B onaire 34 than 1 week the dolomite attained stoichiometry and ordering.
Ordering and stoichiometry were achieved within 24 hours at 300°C during replacement of aragonite. There is no evidence from these experiments that HMC tended to produce more ordered or stoichiometric dolomite than aragonite; it was just dolomitized more thoroughly.
Aragonite to Dolomite and LMC:
Fragments of the pelecypod Area ponderosa and the gastropod
Strombus sp . were dolom itized a t 250°C, 39 Atm. and 300°C, 85 Atm.
In all cases an interlocking xenotopic-A dolomite texture replaced the aragonite shell fragment (figs. 10. 11 and 12). In thin section the dolomite had undulose extinction and very "dirty" appearance.
SEM examination showed very irregular intergrain boundaries and a rough surface texture with a "Swiss cheese" like appearance caused by the development of inclusions (fig. 12). Crystal edges and cleavage faces often appeared curved (fig. 13) and in the pore space of several samples, saddle dolomite cements developed (fig. 14).
The texture of the calcite produced by inversion of aragonite was comprised of interlocking anhedral crystals, as in dolomite textures. The calcites had sweeping extinction (fig. 15) and sometimes displayed fascicular texture characteristic of neomorphic spars. SEM revealed rough surface texture and very irregular intergrain boundaries (fig. 16). In some cases the c-axis orientation of the original aragonite in the shell fragments appeared to be maintained by the calcite replacing it. 35
Figure 10. Unaltered skeletal aragonite of pelecypod Area ponderosa
SEM, scale bar = 1 jim. 36
Figure 11 . Area ponderosa after alteration to dolomite (24 hrs. @
300°C). Note undulose extinction in some of the grains
and the irregular intergrain boundaries. Partially
crossed polars, scale bar = 10 pm.
Figure 12 • Area ponderosa altered to dolomite (24 hrs @ 300°C).
This is the same sample as in fig. 11, note the
irretular intergrain boundaries and "Swiss cheese"
structure. SEM, scale bar = 10 pm. 37
F igure 11
Figure 12 38
Figure 13. Area ponderosa altered to dolomite (2 weeks @ 250°C).
Note the curved edges of crystals (arrows) in center of
f ie l d .
SEM, scale bar = 10 pm.
Figure 14. Area ponderosa altered to dolomite (23 hrs. @ 250°G),
Note the development of saddle shaped dolomite crystals
(xenotopic-B texture) in the pore space (center of
field). SEM, scale bar =0.1 pm. 39
Figure 14 40
Figure 15. Area ponderosa altered to calcite (2 weeks @ 250°C).
Partially crossed polars, scale bar = 0.1 jum.
Figure 16. Area ponderosa altered to calcite (1 week @ 250°C).
SEM, s c a le bar - 10 jutm. 41
Figure 15
■>. . '“d i
Figure 16 42
HMC to Dolomite and LMC:
Fragments of the coralline algae Goniolithon sp. and the echinoid Caenopedina sp. comprised of HMC (25% and 13% MgC 0 3 , respectively, as determined by x-ray diffraction) were dolomitized at
300°C with reaction times from four days to nine weeks. The HMC was converted to 100% well ordered dolomite with 50% MgC 03 to 49%
MgC03 as determined by x-ray diffraction.
The in itia l HMC algal and echinoid fragments were composed of crystals too small to be resolved with the SEM instruments available
(fig. 17). The individual crystals produced by dolomitization of the echinoid fragments were also not resolvable using SEM. The dolomite did maintain the C-axis orientation of the precursor HMC crystals in the echlnoids because the dolomitized fragments have unit extinction.
Crystals of dolomite produced from HMC algal fragments were ovoid shaped with indistinct, poorly developed crystal faces (fig.
18). Their average size was from 0.25 to 1.0 pm. . Pore filling dolomites which sometimes grew in the algal concepticals were comprised of the rhombic (1011) crystal form with the steep rhombic
[4041] and basal pinacoid (0001) forms superimposed over it (fig.
18).
Experiments were run to convert HMC algal fragments to LMC. LMC crystals that replaced HMC algal fragments texturally resembled the synthetic dolomite. The crystals were ovoid shaped with poorly developed faces ranging in size from about 1/2 to 1 pm. Conceptical filling calcite crystals had the (2131) and (1011) forms (fig. 19). 43
Figure 17. HMC skeletal material from the coralline algae Goniollthon
sp. SEM, scale bar = 1 jtm. 44
Figure 18. Goniolithon sp. (HMC) altered to dolomtie (60 days
0 300°C). Note the poorly developed (rounded) crystal
faces, the development of [4041] and [0001] forms and
curved intergrain boundaries. These larger euhedra
appear to have grown as cements into the agal
concepticals smaller crystals replacing the cell walls
appear to be anhedral. SEM, scale bar = 1 fim.
Figure 19. Goniolithon sp. (HMC) altered to LMC (2 weeks 0 300°C).
Note the textural sim ilarity to the dolomite in Fig. 18.
The large pore lining calcite crystals have the (2131)
and (1011) forms. SEM, scale bar = 1 m.
46
Recrystallization Experiments:
Samples o f s u p ra tid a l sedim ent from Andros Is la n d , Bahamas
containing about 60% poorly ordered non-stoichiometric dolomite (40
mole % MgCOj) and about 40% aragonite were reacted in dolomitizing
solutions at 300°C for 5 and 13 days. The unaltered supratidal
sediment contained rhombic (1011) dolomite crystals less than 10 jim across (fig. 20). and aragonite needles less than 1 jim x 5 jjm. The
sediment was unconsolidated and very porous.
The product of these experiments was 100% ordered stoichiometric dolomite. The texture produced by the reaction was a porous mosaic of dolomite crystals exhibiting (1011), (4041) and (0001) forms as in the HMC to dolomite experiments. The crystals were slightly larger
(on the whole) than the initial protodolomite starting material and tended to be anhedral (fig. 21). The aragonite which was present in the initial sediment was entirely dolomitized.
In two experiments, fragments of dolomitized coralline algae from Bonaire, N.A. (ordered with 46 mole'% MgC 03) were reacted in dolomitizing solutions at 300°C for 5 days and three months. The starting material was comprised of dolomite rhombs (1011) mostly less th a n 2 fim across filling algal concepticals and finer grained dolomite replacing the conceptical walls (fig. 22). This dolomite is characterized by straight intergrain compromise boundaries and well formed rhombic crystals.
The products of these reactions were ordered stoichiometric dolomite in the case of the 3 month experiment and ordered dolomite with stoichiometry ranging from 50 mole % MgCOg to less than 48 mole % MgCOj in the case of the 5 day experiment. The 47
Figure 20. Unaltered supratidal dolomite from Andros Island,
Bahamas. This dolomite (or pro todolomite) is poorly
ordered and contains 40 mole % MgCOg). SEM, scale
b ar » 1 jim.
Figure 21. Supratidal non-stoichiometric dolomite (protodolomite)
from Andros Island, Bahamas recrystallized to ordered,
stoichiometric dolomite (2 weeks @ 300dC). Note that
most of the grains appear to be anhedral (arrows) and
note the development of the (4041) and (0001) forms.
SGM, s c a le b ar = 1/im. 48
Figure 21 49
Figure 22. Unaltered dolomitized corralline algae from Bonaire, N.A.
Note the well formed dolomite rhombs and straight inter
grain boundaries (arrows). Mast of the larger crystals
are probably conceptical filling cements. However, the
cell wall replacing crystals, which are at the limit of
the resolution of the SEM, also appear to be euhedral.
SEM, sc a le b ar = 1 fim.
Figure 23. Dolomitized coralline algae from Bonaire, N.A. after
r e c r y s ta lli z a ti o n (3 months @ 300°C). Arrows show some
curved intergrain boundaries. SEN, scale bar = 1 Jim. F igure 22
Figure 23 51 recrystallization textures of these dolomites were similar to those of the supratidal sediment exposed to the same treatment. Crystals with the (1011), (4041) and (0001) forms present were produced. The algal fabric of the starting texture was entirely obliterated by recrystallization. Individual crystals were slightly larger on the average than in the starting material. They also appeared slightly rounded, crystal faces poorly developed, and intergrain boundaries tended to be curved (fig. 23).
Attempts to recrystallize natural coarser grain (about 20 jam) non-stoichiometric (46 mole % MgCOj) , ordered dolomite with a reaction time of two months showed no evidence of grain growth or other textural change. Synthetic stoichiometric dolomite (grain size less than 1 jam) produced from coralline algae (fig. 18) showed no evidence of continuing grain growth even with reaction times of longer than 2 months. The final dolomite texture apparently develops within the first 100 hours of reaction and no further recrystallization (aggrading neomorphism or wet recrystallization) seems to occur.
Attempts to recrystallize fine grain LMC, which was synthetically produced replacing HMC goniolithon (fig. 19), also met with failure. Reaction times of more than two months at 300°C showed no textural difference from the sample produced in two weeks at the same temperature.
P etrography, SEM and D is trib u tio n of th e Hueco Dolomites
The unmetamorphosed dolom ite fa c ie s o f th e Hueco Limestone a t
Marble Canyon, Texas consists of a fine grained (0.01 to 1.5 mm, 52 mostly above 0.05 mm) idiotopic-E and S dolomite (figs. 24 and 25).
No significant compositional or cathodoluminescent zoning was observed in this dolomite. The Hueco dolomite has considerable
intercrystalline and fossil mold porosity. Near hydrothermal veins this porosity is sometimes filled by coarse grained calcite.
Occasionally fossil fusulinids are replaced, with their internal structure intact, by chert. Echinoids are sometimes replaced by single dolomite crystals preserving their unit extinction, elsewhere they are dissolved leaving fossil molds. Examination of nearby and stratigraphically equivalent limestones indicated that the dolomite probably replaced a fusilinid, echinoid biomicrite.
A thermal gradient in relation to horizontal distance from the
Marble Canyon intrusion was established by Bridge (1966). This gradient was based on a study of mineral assemblages in metamorphosed siliceous limestone and dolomite facies of the Hueco Limestone and overlying Bone Spring Formation (Molfcamp) (fig. 26). The gradient is based on four reactions recognized by Bridge (1966) and probably fall along a linear portion of an exponential curve described by H.
Jackson (1973b). A background temperature was not established but is assumed to be near 100°C. II. Jackson (1973b) determined that the dominant mode of heat transfer at Marble Canyon was fluid flow. This, resulted in rapid heating followed by slow cooling of the country rock.
Petrographic evidence of recrystallization of dolomite was observed to begin about 400 ft from the intrusion. The maximum temperature here was estimated to be between 250° and 300°C. The partially recrystallized dolomite is characterized by fine grain 53
Figure 24 . Id io to p ic -S dolom ite o f th e Hueco Lim estone, Marble
Canyon, Texas. Partially crossed polars, scale
bar *0.1 mm.
F igure 25 . Idiotopic-S Hueco dolomite (same sample as in fig. 24).
The flat surfaces of the grains parallel to the plane of
the photograph are compromise boundaries formed with
grains removed when the sample was fractured. SEM,
s c a le bar = 10 jjm. Figure 25 55
T°C 100
200
3 0 0
4 0 0 •/C a l+ Q ■'Wo+COj
500
Dol=Par+ Cal+CO2 6 0 0
7 0 0 Di+Fo+Cal=3Mo+2C02
8 0 0
• Sp+2Mo-2Ma+Cal
9 0 0 0 100 200 ' 3 0 04 0 0 5 0 0 6 0 0
distance in feet
Figure 26. Temperature gradient at Marble Canyon, based on the data of Bridge (1966). 56
(0.05 to 0.1mm) id io to p ic -E and S dolom ite w ith patches o f
xenotopic-A dolomite (0.1 to 0.2 mm). Some of the dolomite anhedra
have undulose extinction. The amount of coarse grained xenotopic-A
dolomite increases closer to the intrusion. Vugs up to 3 mm in
diameter are often filled by coarse grained calcite; sometimes one
calcite crystal completely filling the vug. The vugs are often lined
by idiotopic-C dolomite (0.1 to 0.2 mm) which has smooth rhombic
terminations in contact with the vug filling calcite but irregular grain boundaries with neighboring dolomite grains (fig. 27).
At a distance of about 300 ft. from the intrusion recrystallization of the dolomite is complete (fig. 28). At this
point the dolomite texture is entirely xenotopic-A (0.1 to 0.4 mm) except for idiotopic-C dolomite around calcite filled vugs. The dolomite anhedra often have undulose extinction. SEM reveals anhedral crystals and irregular intergrain boundaries (fig. 29).
Many of the grains in this and other samples have flat surfaces subparallel to the plane of the SEM micrographs. This is due to cleaving when the sample is fractured for mounting.
Nearer than 250 ft. from the intrusion the dolo-marble has been bleached by metamorphism and is light grey in hand specimen as opposed to dark grey in the unaltered dolomite. From about 140 ft. from the intrusion the dolomite grades into a white brucite-calcite marble. This marble was produced by intense thermal metamorphism which altered the dolomite to calcite and periclase. The periclase later altered to brucite (Bridge, 1966). Some of the periclase m arble ad jacen t to th e in tru s io n to about '60 f t . away was redolomitized by late hydrothermal activity (Bridge, 1966). This 57
Figure 27. Dolomite crystal terminations (D) in a calcite filled
vug (C) (idiotopic-C dolomite). The dolomite crystals
share irregular boundaries with neighboring dolomite
grains. This sample is from the recrystallized
metamorphic fa c ie s o f th e Hueco dolom ite. P a r tia lly
crossed polars, scale bar =0.1 /jm. 58
Figure 28. Hueco dolomite marble, collected 120 ft. from the Marble
Canyon intrusion. This sample exhibits xenotopic-A
texture. Partially crossed polars, scale bar =0.1 mm.
Figure 29. Hueco dolomite marble (same sample as in fig. 28). Mote
the irregular intergrain boundaries. The flat surface of
the large crystal in the center-right of the micrograph
i s probably a cleavage face formed when th e sample was
fractured for mounting. SEM, scale bar - 10 /tun. I
59
Figure 29 60 dolomite Is also characterized by a xenotopic-A texture made up of dirty appearing and sometimes undulose anhedra (0.05 to 0.2mm).
Aside from its proximity to the intrusion this dolomite is indistin guishable from the recrystallized dolomites (figs. 30 and 31).
Petrography, SEM and Distribution of the Galena Dolomites
Generalized petrographic descriptions of Galena Group collecting localities are given in Appendix 3. The undolomitized portions of the Galena Group in southwestern Wisconsin and eastern Iowa are characterized by biomicrites and some biosparites. Fossil content of these limestones include fragments of echinoids, bryozoa, brachiopods and trilobites. The limestones contain chert nodules and some sllicified fossils as well as a small amount (estimated to be less than 1%) well rounded quartz and possibly some feldspar sand. This sand content is common in the dolomitized as well as the limestone facies of the Galena Group. Dolomitization completely obliterated the initial limestone fabrics in most places. Occasional silicified fossils can be found in the dolomite and some of the vuggy porosity appears to be fossil molds.
Idiotopic dolomite texture was observed at all of the outcrops of the Galena Group that were sampled. These dolomites were found as: 1) individual rhombs of idiotopic-P dolomite (0.01 to 0.4 mm) replacing limestone micrite (figs. 32 and 33), 2) uniform fine grained idiotopic-E and S dolomite (0.05 to 0.2mm) (figs. 34 and 35),
3) coarse grained (up to 0.5 mm) idiotopic-E, S and C dolomites. The most abundant of these were the coarse grained dolomites. Some of this coarse grained dolomite has pronounced zoning and in the more 61
F igure 30 . Dolomite produced by hydrothermal alteration of a
periclase marble; collected 30 ft. from the Marble Canyon
intrusion. Partially crossed polars, scale bar “ 0.1 mm.
Figure 31. Dolomite produced by hydrothermal alteration of a
periclase marble (same sample as in fig. 30). SEM,
scale bar = 0.1 mm.
63
Figure 32. Idiotopic-F dolomite replacing limestone micrite of the
Galena Group (locality 45). Note the preservation of a
crystal face junctin where two large rhombs (center)
join. Plane polarized light, scale bar a 0.1 mm.
Figure 33. Idiotopic-P dolomite (dark) replacing calcite micrite
(light) of the Galena Group (same sample as in Fig. 32).
Note the groove in the large rhomb (upper center) created
by the crystal face junction of an adjoining dolomite
crystal which was removed during fracturing of the
sample. SEM backscattered electron image, scale bar
= 0.1 mm.
65
Figure 34 . Idiotopic-S Galena dolomite from locality 20. Note the
straight intergrain boundaries and preserved crystal face
junctions. Partly crossed polars, scale bar = 0.05 mm.
F igure 35 . Idiotopic-S Galena dolomite from locality 20. Note the
rhombic shape of many of the grains and relatively
straight intergrain boundaries. This sample is from a
weathered exposure and shows signs of dissolution. SEM,
scale bar = 0.05 mm. Figure 35 67 porous varieties, intercrystalline pore space is often filled by brown to black opaque material which coats the surface of the crystals. This intergranular material is probably mostly amorphous iron .hydroxide with some organic material and iron sulfides judging from its colors in reflected light. Sphalerite was observed as in intergranular filing in a few samples. A curious aspect of some of the coarse grained porous idiotopic-E and C dolomite is that it tends to have undulose extinction, and close examination sometimes reveals slightly curved crystal faces (fig. 36). Possibly this represents a transiton between idiotopic dolomite and xenotopic-C or saddle dolomite. Undulose idiotopic dolomite is found in association with xenotopic-A dolomite.
SEM shows the basic rhombic shape of the crystals making up the idiotopic dolomites and the relatively straight compromise boundaries that form between crystals (figs. 33 and 35). The surfaces of these grains appear corroded and somewhat pitted (fig. 35). This is probably not due to crystal growth but rather due to outcrop weathering. Mast of the Galena outcrops were moderately to strongly weathered making "fresh" samples difficult to obtain.
Because of the abundant intergranular opaque material and widespread patchy distribution of xenotopic dolomite; well developed uniform idiotopic-S dolomites are rare in the Galena Group. At locality 20 near Mine Point, Wisconsin the cleanest and most uniform fine grained idiotopic dolomites were found; but even these were associated with some xenotopic-C dolomite cement filling fossil molds. Xenotopic-A dolomites are observed in the Galena Formation varying in size from scattered patches of one to several centimeters Figure 36. Idiotopic-E Galena dolomite from locality 28. Some of
the crystals have curved edges. Note the clear-rim,
cloudy-center zoning and abundant inter-crystalline
opaque material. Plane polarized light, scale bar = 0.1
mm. 69 across in size mixed with idiotopic dolomite, to whole portions of outcrops composed entirely of xenotopic texture. No outcrops were sampled that are entirely xenotopic dolomite; however, at localities
13, 15, 19, 29 and 31 it is the dominant textural type (Appendix 3).
At locality 45 just south of Guttenburg, Iowa, patches of xenotopic-A dolomite and some xenotopic-P dolomite replace a biomicrite (figs. 37 and 38).
In thin section, the xenotopic-A Galena dolomites are characterized by curved, irregular intergrain boundaries, and the grains usually have undulose extinction under crossed polars (fig.
39). The xenotopic-A dolomite grains range in size from 0.05 to 0.4 mm, mostly about 0.2mm. Barely, xenotopic anhedra had clear-rim, cloudy center zoning; however, this kind of zoning was most often found in the coarse grained idiotopic dolomites. Both idiotopic and xenotopic Galena dolomites had the same cathodoluminescent properties. Cathodoluminescent zoning was not observed in the Galena d o lo m ites.
At most locations xenotopic textures are associated with some stylolites. Vuggy pore space in xenotopic dolomites are usually lined with ldiotopic-C dolomites. These grains have rhombic terminations where adjacent to pore spaces or matrix material but irregular boundaries with neighboring dolomite grains, just as described in the recrystallized portions of the Hueco dolomite.
SEM examination of xenotopic-A dolomtes from the Galena Group show the very rough surface texture of the grains, irregular intergrain boundaries and few rhombic shaped grains (figs. 38 and
40). 70
F igure 37 . Xenotopic-A Galena dolomite from locality 45 which
replaces a biomicrite (not shown). The few rhomb shaped
grains may be from a precursor idiotopic-D dolomite (as
in Fig. 32 which was collected nearby). Partially
crossed polars, scale bar = 0.1 mm.
Figure 38 . Xenotopic-A Galena dolomite (dark) replacing calcite
(light) from locality 45 (same sample as in Fig. 37).
Note the irregular intergrain boundaries as comprared to
idiotopic dolomite collected a few ft. away (Fig. 33).
SEM backscattered electron image, scale bar = 10 pm. F igure 37
Figure 38 72
F igure 39 . Xenotopic-A Galena dolomite from locality 19. Partially
crossed polars, scale bar =0.5 mm.
Figure 40 . Xenotopic-A Galena dolomite from locality 19 (same sample
as in Fig. 39). Note the irregular intergrain boundaries
and "Swiss cheese" structure. SEM, scale bar = 0.1 mm.
74
Good examples of xenotoplc dolomite texture were found throughout the Galena Group in southwestern Wisconsin. No aerial or stratigraphic patterns of this distribution were established. Such patterns, however, may become apparent with more detailed study.
Petrography, SEM and Distribution of the Trenton Dolomites
Generalized petrographic descriptions of the Trenton Formation cores used in this study are found in Appendix 4. Dolomites of the
Trenton Formation of the Michigan Basin replaced biomicrites very similar to the limestones of the Galena Group. Fossil content of the
Trenton limestones consists primarily of brachiopod and echinoid fragments and occasional bryozoan, gastropod and trilobite fragm ents.
Fine grained idiotopic-S dolomites (0.01 to 0.1 mm) are found in the "cap" of the Trenton and in the regional Trenton dolomite in the western part of the basin (fig. 4). This dolomite preferentially replaced micrite (fig. 41). SEM shows the rhombic shape of the crystals and their relatively smooth intergrain boundaries (fig. 42).
Idiotopic-P (floating rhombs in micrite) can be found throughout the limestone facies of the Trenton Formation.
Xenotopic-A dolomite is the principal type of texture associated with the fracture related dolomites (fig. 43) and is also found in the "cap”, and the western regional dolomites. Stylolitization sometimes gives fine grained idiotopic dolomite of the "cap" and regionally dolomitized areas a "crushed" or "ground-up" appearance.
In these stylolitized areas the fossils were replaced by coarse, undulose xenotopic-A dolomite. This undulose xenotoplc dolomite also 75
Figure 41. Idiotopic-S dolomite replacing micrite in the "cap" of
the Trenton Formation (Br core). The fossil in the upper
right is unreplaced calcite. The dolomite contains
abundant intercrystalline opaque material. Plane
polarized light, scale bar - 0 .1 mm.
Figure 42. Idiotopic-S dolomite from the "cap" of the Trenton
Formation (same sample as in Fig. 41). Arrows show
preserved crystal face junctions. SEM, scale bar = 0.05
mm.
77
Figure 43. Xenotopic-A fracture related dolomite from the Trenton
Formation (L12 core). Partially crossed polars, scale
bar = 0.3 mm. 78
replaced Che remaining micriCe which had not previously been replaced
by the fine grain idiotoplc dolomite. The fabric of the precursor
biomicrlte is, therefore, preserved in these rocks; the fossils
replaced by coarse grained xenotopic-A dolomite and the micrite
primarily by fine grained idiotopic-S dolomite (fig. 44). The fossil
replacing xenotopic-A dolomite grains range in size from 0.1 to 0.5 mm and have undulose extinction. SGM examination of these and other xenotopic dolomites in the Trenton Formation show their irregular intergrain boundaries (fig. 45).
Xenotopic-A dolomite that destructively replaced biomicritic limestones of the Trenton Formation is associated with fracture systems such as the northeast-southwest trending Albion Scipio and
Northville fields, and with fracturing in regional dolomites in the western part of the Michigan Basin (fig. 43). In thin section the anhedra range in size from 0 .1 to 0 .8 mm; most of the grains being near 0.5 mm and having undulose extinction. Cathodoluminescent zoning was observed in some of the fracture related xenotopic-A dolomites but not in the texturally similar fossil replacing d o lo m ites.
There is no observed textural or grain size variation in the fracture-related xenotopic-A dolomites that would suggest a difference in nucleation or growth between the grains replacing fossils and those replacing micrite. In some samples ghosts of former fossil fragments can be seen "floating1 in the dolomite without affecting grain size or morphology. In these cases the dolomite replaced the precursor biomicrlte with a uniform xenotopic-A texture, completely obliterating the original limestone fabric. 79
Figure 44. Dolomitized biomicrlte from the "cap" of the Trenton
Formation (El core). The fossil (center) is replaced by
xenotopic-A dolomite and the micrite mostly by
idiotopic-S dolomite, preserving the original fabric of
the biomicrlte. Partially crossed polars, scale bar **
0 .5 mm.
Figure 45. Xenotopic-A dolomite replacing a fossil in the "cap" of
the Trenton Formation (same sample as in Fig. 44).
Arrows show g ra in b o u ndaries. SEM, s c a le bar = 10 pm. 80
Figure 45 81
Several salient characteristics are common to all of the coarse,
destructive xenotoplc-A dolomites found in the Trenton: 1) They may
replace the limestone over a vertical distance of several hundred
feet, as observed from comparing petrographic descriptions with core
and gamma ray-neutron logs from the Northville field. However, there
seems to be very little lateral continuity to these dolomites as an
o il well as near as 0.25 miles away may not encounter the same
dolomitized section. [Compare, for example, the Gl, E2 and Br cores
in Appendix 4 and see Shaw (1974, fig. 10)]. 2) They are often
associated with fracture and vuggy porosity lined with xenotopic-C
(saddle) dolomite cement (fig. 46) and often filled by anhydrite. 3)
In five of the cores where these dolomites were encountered (L12, Hi,
W2, El and Brl, see fig. 4 and Appendix 4) they were associated with
notations of oil or gas shows in the core descriptions. Fracture
related dolomite was also observed in the HS well in Sanilac Co.,
Michigan for which no descriptions or completion reports were
a v a ila b le , so i t i s not known i f th i s dolom ite contained
hydrocarbons.
Coarse grained idiotopic-E and S dolomites (0.1 to 0.5 mm) are
often associated with the destructive xenotopic-A dolomites
associated with the fracturing. These dolomites have undulose
extinction and sometimes slightly curved crystal faces. They are
petrographically analogous to the similar dolomite textures found in
the Galena group (fig. 36). Also, like their counterparts in the
Galena Group, pore space is often filled by opaque material and
individual crystals are often coated by the opaques.
Floating dolomite anhedra or xenotopic-P dolomite were observed
i Figure 46. Saddle shaped xenotopic-C dolomite growing into fracture
porosity of the Trenton Formation (El core). Partially
crossed polars, scale bar *» 0.5 mm. 83 in the limestone facies and in areas of partial dolomitization of the
Trenton Formation. The xenotopic-P dolomites range in size from 0.05 to 0.5 mmm and have undulose extinction as do the xenotopic-A dolomites. Xenotopic-P dolomite was often observed partially replacing fossils and micrite in areas that are also partially replaced by the fine grained idiotopic dolomite (fig. 47). In thin section the only way xenotopic-P dolomite can be easily distinguished from calcite pseudospar, which also replaces fossils and micrite in the Trenton, is with a stain.
In most of the cores investigated, the xenotopic-P dolomites was found near the fracture related dolomites (fig. 48). However, in the JF1 core in southwestern Shiawassee Co., xenotopic-P and fine grained idiotopic-S and P dolomite were all that were observed.
Possibly fracture related dolomites are nearby but were not encountered by this core due to their limited lateral extent.
Summary of the Occurrence of Xenotopic Dolomite
Undulose Xenotopic-A dolomite was synthesized, at 250°C and
300°C, from aragonite shell fragments. Xenotopic-C (saddle) dolomite cements were also synthesized in the same experiments. Synthesis of dolomite from HMC and recrystallization of nonstoichiometrlc dolomite produced dolomite crystals with poorly developed faces.
Xenotopic-A dolomite was observed in the Hueco Formation at
Marble Canyon, Texas as the result of progressive recrystallization of an idiotopic dolomite near an igneous intrusion. This recrystallization occurred at temperatures > 250°C. 84
Figure 47. Anhedra of xenotopic-C dolomite (light) partially
replacing a calcite brachiopod fragment (stained dark) in
the Trenton Formation "cap" (Br core). Note the fine
grained idiotopic dolomite replacing micrite on the lower
left. Plane polarized light, scale bar = 0.5 mm.
Figure 48. Xenotopic-C dolomite (light) replacing limestone micrite
(stained dark) along a fracture in the Trenton Formation
(L12 core). Partially crossed polars, scale bar = 0.1
mm. 85
I F igure 47
F igure 48 86
In the Galena Group of Wisconsin and Iowa, xenotopic-A dolomites were scattered as patches from several centimeters to several meters
in size mixed with idiotopic dolomite. At one outcrop xenotopic-A and C dolomite was observed replacing biomicrite. The xenotopic-A dolomites in the Galena Group are sometimes associated with saddle shaped xenotopic-C cements.
In the Trenton Formation of the Michigan Basin xenotopic-A dolomite replaces fossils surrounded by stylolitized fine grain
idiotopic dolomite. This relationship is observed most often in the
"cap" and the regionally dolomitized western portion of the Trenton
Formation. Destructive "whole rock" replacement of the Trenton by xenotopic-A dolomite is associated with fracturing in the Trenton
Formation. These dolomites are found with xenotopic-C (saddle) cements, anhydrite and petroleum. Floating anhedra of xenotopic-C dolomite are found replacing fossils and micrite in limestones near the fracture related dolomites.
Mb xenotopic dolomite was found in the Pliocene dolomites from the Netherlands Antilles or in Pleistocene supratidal dolomites from
Andros Island.
Results of Stable Isotope Analyses
Stable oxygen and carbon isotope analyses were made on 11 selected samples of dolomite from the Hueco Limestone. Table 6 shows their 8^0 and 8^C values relative to the PDB scale. The samples were selected at intervals from 30 ft. from the intrusion to
1200 ft. from the intrusion. Table 6 also shows the maximum temperature which these samples are estimated to have been subjected 87
Table 6 Stable Isotope Values for Hueco Dolomites
Approximate ^g 13 Sample Distance from Maximum Metamorphic 5 0 6 r u o rUiJ Intrusion Temperature per mil. per mil.
MH52 1200 f t . * 100°C +0.13 +4.95
MH18 820 f t . * 100°C +2.15 -1 .2 4
MH20 720 f t . * 100 °C -0.13 +5.22
MH24 660 f t . -v 100°C -0 .8 1 +4.89
MH15 490 ft. 150°C - 0 .6 6 +5.29
MH27 480 f t . 150°C +0.33 +4.67
M1LL1 400 f t . 240°C +0.13 +3.99
-0 .2 8 +4.59
MH 9 340 f t . 325°C -1 .2 9 +4.84
MH 6 250 f t . 475°C -0 .8 2 +4.35
MH41 140 f t . 640°C -2 .5 8 +4.35
MH 2 30 f t . 825°C -0 .1 8 +1.33 88 approx. T^C distance 8 6 0 in feet
7 2 0
5 7 0 200
4 3 0
2 8 0 4 0 0
1 3 0
6 0 0
8 0 0
1000
1200
2 -1 0 1 2 5,80 per mil. (PDBI
1 8 Figure 49. 6 0 values for Hueco dolomite plotted against
temperature and distance from the intrusion. 18 The two S 0 values plotted with open circles 13 had anomalous S C values and were not used to calculate r. 89 to during metamorphism. The temperature estimates are based on the mineral assemblage data of Bridge (1966).
Figure 49 plots the values obtined for the Hueco dolomites relative to distance from the intrusion and estimated maximum temperature. The lowest fi^O value (-2.58 per m il.) was obtained for MH41 which was collected 140 ft. from the intrusion
(650°C). This sample was recrystallized into a xenotopic-A dolomite.
The fil80 values increase with increasing distance from the intrusion to about 400 ft. away. At this point and at distances further away most of the values are clustered around a mean of - 0 .12 per mil. The distance of 400 ft. from the intrusion coincides with the furthest distance at which recrystallization of dolomite is petrographically observable in thin section.
All but two of the samples had 6 ^ 0 values clustered around a mean value of +4.72 per mil. with a high of +5.29 and low of +3.99 per mil. Recrystallization had no effect on stable carbon isotope v a lu e s .
Anomalous 6^0 and S^C values were obtained for tm of the samples that were analysed, MH2 and MH18 (table 4 and fig. 54).
MH2 was collected 30 ft. from the intrusion and its values probably reflect the late redolomitization of periclase marble by magmatic water as reported by Bridge (1966). The second anomalous value was for MH18 which was collected from the alteration zone of a hydrothermal vein. These two samples were subject to a more complex alteration history than the other 9 samples which were subjected only to metamorphic recrystallization.
Stable isotope values obtained for 21 dolomite samples and 1 90
Table 7. Stable isotope values for the Galena Group. The samples are grouped into 11 pairs containing one xenotopic and one idiotopic dolomite collected in close proximity to one another. Samples marked with an asterisk (*) are listed in more than one pair. The single limestone micrite sample is paired with a dolomite collected form the same locality.
Lo cation-Sample T exture 8180 813C
PDB PDB 20-1 Idiotopic-S -3.44 +0.79 19-3 Xenotopic-A -4.37 -0 .0 8
31-8 Idiotopic-E and S -4 .8 4 - 0.0 2 31-3 Xenotopic-A -5.25 -4.07
33-3 Idiotopic-S -5 .1 8 -0 .7 5 34-2* Xenotopic-A -5.45 -5.22
34-1 Idiotopic-E (slightly undulose) -5.06 +0.33 34-2* Xenotopic-A -5.45 -5.22
45-17 Idiotopic-E and S -3.39 -0 .9 0 45-11 Xenotopic-A -5 .2 1 -0 .1 8
14-1 Idiotopic-S -3.31 - 0.2 0 14-4 Xenotopic-A -3.63 -0.09
15-1 Idiotopic-E and S (zoned) -2 .9 0 -0 .2 8 15-2 Xenotopic-A (unzoned) -6.28 -0 .0 9 30-1 Idiotopic-S -5 .5 2 +0.31 3 0 -la Xenotopic-A -5.63 -0.04 uD 30-3 Idiotopic-E undulose w/some curved xls.-5.90 +0.16 30-3a* Xenotopic-A -5.31 -0.34
30-3° Idiotopic-S undulose -5 .5 4 +0 .0 1 30-3 * Xenotopic-A -5.31 -0.34 c
28-3 Idiotopic-E undulose w/some curved x ls .-6 .3 2 -0.37 28-5 Xenotopic-B -3 .1 0 -0 .5 6 50-1 Idiotopic-P -4 .3 8 +0.31 50-1* Limestone m icrite -5 .6 8 -0.49 91 limestone sample from the Galena Group are listed in table 6. The dolomite samples are grouped into 11 pairs of idiotopic dolomite matched with xenotoplc dolomite. The two samples in each pair were collected in close proximity to one another. In all but two cases, pairs were collected from the same outcrop and, in two cases, from th e same hand sample.
Specimens were carefully examined in thin section and only regions of textural homogeneity were Isolated for isotope analysis.
The 8^®0 and S^C values reported in table 7 are relative to the PDB scale.
In 8 of the pairs 8*®0 values for the xenotopic-A dolomite were lower by an average of 0.95 per mil. than the matched idiotopic-B dolomite. In two of the pairs, an idiotopic-E dolomite had a lower
1*0 value than the xenotopic dolomite. In both of these cases the lower values were for porous idiotopic-A dolomites that have undulose extinction and some curved crystal faces (fig. 36). One idiotopic-S dolomite was found with a 8 ^ 0 value lower than its paired xenotopic-A dolomite.
Stable carbon isotope values ranged between 8^C of -5.22 per mil. and +0.79 per mil. Nineteen of the values were between
-0.90 and +0.79 per mil. with a mean of -0.14 per mil. Two of the values were anomalously low (table 7). No relationships were observed between dolomite texture and stable carbon isotope values.
Stable carbon and oxygen isotope analyses were not obtained for dolomites of the Trenton Formation as part of this study. However, oxygen isotope values for the Trenton dolomite were obtained by
Taylor (1982 and personal communication, 1982) and w ill be discussed below. 92
X-ray Diffraction of Hueco, Galena and Trenton Dolomites
Selected samples of idiotopic and xenotopic dolomite from the
Hueco, Galena and Trenton dolomites were analysed with x-ray diffraction. The purpose was to see if a difference in ordering or stoichiometry existed between xenotopic and idiotopic dolomites. All of the samples analysed from the Hueco and Galena dolomites proved to be stoichiometric, ordered dolomite regardless of texture.
Fine grained idiotopic-S dolomite from the "cap" of the Trenton formation was calculated to be enriched with calcium (54% CaCC^).
In addition, the (006) and (110) reflections were broad and diffuse and the (015) reflection nearly nonexistent, indicating poor ordering. By contrast, fracture related xenotopic-A dolomite samples collected from within 5 ft (in one case from the same hand sample) were stoichiometric and had well developed ordering reflections. The poor ordering and nonstoichiometry of the idiotopic dolomite in the
"cap" of the Trenton is probably due to its high iron content relative to the xenotopic fracture related dolomite. The distribution and origin of iron in the Trenton dolomites is thoroughly discussed by Taylor (1982).
DISCUSSION
Application of Crystal Growth Theory to Carbonate Texture
A remarkable textural sim ilarity exists between xenotopic-A dolomites from the Trenton Formation, the Galena Group, the Hueco dolomitic marbles and the synthetic dolomites produced in the laboratory by replacement and recrystallization (see, for example, figs. 11, 28, 39 and 43). These dolomites are all characterized by 93
anhedral crystals with rough, curved or otherwise irregular
intergrain boundaries and undulose extinction under crossed polars.
The xenotopic-A dolomites have a strikingly different texture from
Cenozoic dolomites known to have formed at near surface temperature
( f i g s . 8, 20 and 22) and from idiotopic dolomites found in ancient
rocks (see for example figs. 24, 34 and 41).
It is hypothesized that the xenotopic texture observed in the
Trenton and Galena dolomites formed above the critical roughening
temperature (CRT) in accordance with Jackson's (1958a and 1958b)
crystal growth model. Synthetic dolomites and dolomitic marbles were
also produced by crystal growth above CRT, thereby causing
development of a texture similar to the sedimentary rocks, even
though they were exposed to higher temperatures than normal for
diagenetic conditions. The observation of anhedral grain mosaics such as observed in xenotopic-A dolomite is consistent with the idea
that these crystals formed by non-faceted growth above the CRT. If well developed crystal faces do not form during growth (see for
instance figs. 47 and 48) then idiotopic texture as defined here is impossible.
Empirical estimates of the CRT for crystal faces can be made by studying crystal morphologies produced at different temperatures in the laboratory (Jackson and Miller, 1977). In the case of dolomite this is difficult because of kinetic problems associated with synthesis of dolomite below 100°C (Gaines, 1980). Therefore, comparisons must be made between natural dolomite produced near the surface and synthetic and natural dolomite produced above 100°C.
Because temperatures a few inches below the surface in Persian Gulf 94
sabkhas can reach as high as 50°C and dolomite crystals produced
there are euhedral (Illing at al., 1965 and McKenzie, 1980, figs.
3-7) this is taken as a tentative lower temperature lim it. An upper
lim it of 100°C is suggested because this is about the average
temperature of the lead-zinc sulfide emplacement in the Galena Group
(Bailey and Cameron, 1951) and is near temperature estimates for
fracture related dolomites in the Trenton Formation (Shaw, 1975;
Taylor, 1982). Undoubtedly a large region exists where the CRT for
dolomite may reside (between 50° and 100°C).
Change in the CRT may be made by changing the enthalpy factor
(L) in equation 2. In dolomite, this is possible by substitution of
cations, such as calcium or iron for magnesium. Such a substitution may result in a mineral with a lower latent heat of dissolution than
stoichiometric dolomite, increasing the roughening factor and, thereby, decreasing the CRT. Some ferroan dolomites and calcium rich dolomites (protodolomites) may have a lower CRT than stoichiometric dolomite. Xenotopic ankerite found in concretions from the Antrim
Shale (Devonian), Michigan are possibly of low temperature (<50°C) origin (Hathon, 1979).
Dolomite crystals growing into pore space are often observed in the Trenton and Galena dolomites to have crystal faces even when they are closely associated with xenotopic dolomite and appear to have formed at the same time (fig. 36). Development of crystal faces adjacent to pore space was also observed in some of the synthetic hydrothermal dolomites (fig. 21). Lewis (1975) noted that, experimentally, crystals grown from solution above CRT often have well developed crystal faces. He concluded that "... in solution 95 growth of perfect crystal planes form because of some additional
Impediment to their growth, related to desolvation or to the adsorption of solvant" (Lewis, 1975, p. 33). This is not inconsistent with the model if one considers that in neutral to alkaline water the dolomite surface takes on a negative charge forming a diffuse double layer between the surface and bulk solution
(Parks, 1970, Stumm and Morgan, 1970, Predali and Cases, 1973). The cations, Mg^+ and Ca^+, are electrically attracted to and concentrated near the surface. In order to attach itself to the growing crystal the COj^- anion must loose its hydrating water molecules, move from the bulk solution through the diffuse double layer and overcome the repelling force of the negatively charged surface. The availability of the carbonate ion at the crystal surface is therefore rate determining when the dolomite grows into a bulk solution such as an open pore space. This may provide the impediment to growth, allowing crystal faces to develop, as suggested by Lewis (1975).
In the surface dominated thin film of solution present between crystals during a replacement or recrystallizatin reaction, double layers of the dissolving and growing grains overlap. The concentration in the thin film is controlled by the rate of its removal from th e d is s o lv in g phase. Movement o f th e m a jo rity o f the
anion from the dissolving to the growing grain is rapid and occurs without having to move it (CC^-) from the bulk solution through a diffuse double layer. In the calcite to dolomite replacement reaction Mg 2+ must diffuse through the thin film environment to the site of attachment on the growing dolomite 96
crystal. This diffusion is rapid relative to COj-^ (Pingitore,
1982) and does not impede dolomite growth. In the dolomite to dolomite recrystallizatln reaction all of the ions are provided by the dissolving phase and a bulk solution need not exist.
An alternate explanation for the difference between crystal growth by thin film solutions and bulk solutions involves the lower dielectric constant of water in the surface environment (e=78.5 in bulk solution, e =6 in surface environment). The lower dielectric constant in surface dominated environments results in lower solubilities (Murray and Dillard, 1979). In other words,AGr and
AHr are lower for the dissolution of dolomite in a surface dominated thin film than dissolution in a bulk solution. The roughness parameter (a) would be correspondingly lower (see eq. 2) .
Therefore, CRT in a thin film may be significantly lower than in a bulk solution allowing nonfaceted crystal growth at lower temperature.
Many of the undulose idiotopic dolomites in the Trenton
Formation and Galena Group, that are associated with xenotopic dolomite and are presumed to be of high temperature origin, are associated with opaque material such as hydrocarbons. Ths opaque material is very often observed coating the crystals and filling the boundary layers between grains (see, for example, fig. 36). The association of opaques with these idiotopic dolomites suggests that the presence of this material might also act as an inhibitor, retarding crystal growth, thus allowing crystal faces to develop.
One aspect of carbonate textures that is explained by this model is the observed difference between limestone and idiotopic dolomite. 97
The former is usually composed of anhedral crystals whereas the latter of euhedral rhombic crystals. The anhedral mosaics of calcite microspar and neospar produced by recrystallizaton or polymorphic inversion may be formed near 25°C (Bathurst, 1976). These limestone textures are similar to xenotopic-A dolomite and, therefore, are presumed to be produced above the CRT of calcite. Laboratory growth of calcite from aqueous solutions near 25°C sometimes produce dendritic crystals (Kahle, written and personal communication, 1981).
Dendritic crystal growth occurs above CRT (Jackson, 1958a) and, therefore, a CRT for calcite must exist below 25°C. At near surface temperatures, therefore, calcite produces anhedral mosaics characteristics of neomorphic spars while dolomite, growing below CRT at 25°C, produces euhedra with straight compromise boundaries. When 4 the temperature is elevated above the CRT for dolomite, xenotopic texture similar to calcite neospar can develop.
The synthesis of xenotopic-C (saddle) dolomite at 250°C (fig.
14) was an unexpected bonus of the experimental studies. Spry (1969) suggested that the curved faces of saddle dolomie crystals are caused by the development of low angle facets or "vincinal faces." These faces can develop because of similar bonding and surface energies to the (1011) faces. Reeder and Barber (1982) attributed distortion in saddle shaped dolomite crystals to subparallel stacking faults normal to the rhombic faces. Radke and Mathis (1980) suggested that the following conditions might be favorable to saddle dolomite growth: 1) temperature above 60°C, 2) presence of iron, 3) sulfide-reduction conditions and 4) pyroelectric effects during fluctuating temperature conditions. High temperature was the only one of these conditions 98 that existed during hydrothermal synthesis of saddle dolomite. It can be presumed, therefore, that temperature is probably important to the development of saddle crystals. It is not clear from this study what the exact role of temperature is, or if and how crystal surface morphology is important to the development of curved crystal faces and the growth of saddle-shaped crystals. The same kind of lattice distortion that produces saddle dolomite may also be responsible for the undulose extinction observed in synthetic and many natural xenotopic-A dolomites.
Xenotopic-C dolomite cements produced at 300°C, with development of (4041) and (0001) crystal forms superimposed on the (1011) rhombic crystal form can be explained by crystal growth theory. At 25°C the
( 1011) faces are undergoing "smooth" growth, other theoretically possible faces are "rough” and quickly grow themselves out of existence (see Brice, 1973, fig. 3.3). Once the temperature is raised above CRT for the (1011) faces, growth inhibition effects as discussed above begin to favor the development of the (4041), (0001) and (1011) forms. Above 250°C these faces begin to develop.
The above explanation does not preclude the development of the
(0001) and (4041) forms at lower temperature. Naiman, et al. (in press) describe polyhedral dolomite crystals with development of the
(0001), (4041) and other forms, from the Permian of West Texas.
These crystals appear to have formed between 25° and 45°C and are associated with halite. An extensive search of the literature by R.
L. Folk (personal communication, 1982) indicates that perhaps all naturally occurring polyhedral dolomite crystals are associated with evaporites. There seems to be little in common between the 99
conditions described by Naiman, et al. and those of the hydrothermal
syntheses in this study except for high chloride concentrations.
However, other evaporite (high chloride) related dolomites do not
develop the (0001) or (4041) forms (see for example fig. 20) and the
"polyhedral" forms were synthesized only at 300°C not at 250°C
indicating a relationship to temperature. Therefore the mechanism discussed above may be sufficient to explain the hydrothermal
synthesis of the unusual crystal forms. At lower temperatures some chemical species present in certain evaporitic solutions may act to inhibit growth in the (0001), (4041) and other crystal planes allowing these faces to form. This occurs in other crystals, for instance, halite will develop octahedral faces when Li+ is added in small quantities to a growing solution (Brice, 1973).
Neomorphism of Dolomite
Theoretically, a chemical drive for neomorphism in carbonates can be provided by either: 1) a phase change, i.e ., inversion from an unstable phase such as aragonite to a stable phase such as calcite or nonstoichiometric to stoichiometric dolomite, 2) low ering su rfa c e energy, whereby a coarser grain size is energetically more favorable than a fine grain size, and 3) strain recrystallization (Folk, 1965 and Bathurst, 1976). In the sedimentary environment these reactions involve dissolution and reprecipitation in the presence of water
(Folk, 1965). In the case of the metamorphism of the Hueco dolomite water was the pore filling fluid during recrystallization (H.
Jackson, 1973a and 1973b).
The results of the hydrothermal bomb experiments demonstrated 100
that surface energy alone is not enough to recrystallize dolomite at
the temperatures and the reaction times allowed. Fine grained (less
than ljim) stoichiometric* ordered dolomite (fig. 18) failed to show
any sign of increased grain size even after long reaction times at
300°C. This difficulty in recrystallization also proved to be true
for fine grain calcites (fig. 19). Chai (1974) recrystallized
calcite in the presence of water at 500°C and 2 kilobars of pressure.
Much longer times available in nature may allow recrystallization to occur at lower temperatures. Ordered, stoichiometric dolomite at
Marble Canyon underwent recrystallization at temperatures in the
range of those used in hydrothermal bombs for this study.
The experimental recrystallization of supratidal dolomite and dolomitized coralline algae from Aruba, N.A. (figs. 21 and 23) was most likely driven by the nonstoichiometry of the dolomites
(Mg++/Ca++ + Mg'1-1" =0.4 and 0.46 respectively) and high surface energy caused by their small grain size. Coarser grained
(20 #tm) nonstoichiometric dolomite failed to recrystallize under similar conditions, possibly due to the stabilizing effect of its
lower surface energy.
The stoichiometry of ancient dolomites was studied by Lumsden and
Chlmalusky (1980). They found the following two relationships: 1) stoichiometric dolomite tended to have a larger grain size than nonstoichiometric (calcium enriched) dolomite; and 2) stoichiometric dolomite was more often associated with evaporitic depositional environments than nonstoichiometric dolomite. The dolomites associated with modern evaporite settings tend to be very fine grained (<5 fim) and calcium enriched (Deffeyes et a l., 1965; Tiling 101
et al., 1965; Shinn et a l., 1965; McKenzie, 1981). Neomorphism may
explain this apparent discrepancy between recent and ancient
supratidal dolomites. The nonstoichiometry and high surface energy
(due to small grain size) of supratidal dolomites should make them
particularly susceptible to neomorphic recrytallization. Coarser
grained dolomite formed in other diagenetic settings may be more resistent to neomorphism. Therefore, ancient supratidal dolomites
are those that have most likely undergone neomorphism. At the same time they attain stoichiometry. Recrystallized dolomites rould also be more likely to have larger grain sizes, on the average, than unrecrystallized dolomites. Xenotopic dolomite texture should only result from neomorphism if it occurred at elevated temperatures.
Recrystallization of nonstoichiometric to stoichiometric dolomite may occur near the surface at near 25°C producing an idiotopic texture in accordance with crystal growth theory.
As it has been shown that high temperature replacement of limestone by dolomite and high temperature recrystallization of dolomite both can produce the same texture, a problem exists in differentiating textures of neomorphic origin from those of replacement origin. A comparison of dolomitic marble of replacement origin (figs. 30 and 31) to dolomitic marble of recrystallization origin (figs. 28 and 29) illustrates this point. Theoretically, no difference in crystal morphology is to be expected if the same growth mechanisms produce both dolomites.
What follow s i s an attem pt to show how xenotopic dolom ite produced by neomorphism may be distinguished from that produced by replacement in t«o ancient dolomites. Unfortunately most of the 102
evidence used Is equivocal and, therefore, conclusions drawn are open
to question.
The Origin of Xenotopic Dolomite: Galena Group
Regional dolomitizatlon of the Galena Group probably occurred near the surface during early diagenesis (Badiozamani, 1972 and
Delgado, written communication, 1980). tost of the xenotopic
dolomites in the Galena Group are interpreted as probably resulting from neomorphism of these early dolomites at elevated temperatures, probably during the epigenetic event associated with sulfide ore genesis in this area. This interpretion is based on the results of stable isotope studies of the Hueco and Galena dolomites as well as other petrographic evidence.
Stable Isotope Studies:
Recrystallization over a temperature gradient of about 400°C at
Marble Canyon produced a 8^®0 depletion of 2.46 per mil. (Table
6). Assuming an ocean water compostion in an open system, homogeneous pore fluid content, and using Matthews and Katz dolomite-water fractionation equation, a depletion of about 20 per mil. vrould be expected for a 400°C temperature gradient (Matthews and
Katz, 1977).
At Marble Canyon about 100 pore volumes of water are estimated to have moved through the country rock during metamorphism (H. Jackson,
1973a). Therefore, the system that existed during the Marble Canyon metamorphlc event was rock probably dominated and only partially open with respect to fluid. The restricted amount of *®0 isotope in 103 the system limited the extent to which the recrystallizing dolomite could fractionate. Recrystallization in a closed system with a small volume of water will therefore result in a small regardless of temperature (Spooner et a l., 1977).
Generally small differences in 5*®0 were observed between idiotopic and xenotopic Galena dolomite samples (averaging less than
1 per m il.) with the xenotopic dolomites having more negative values
(Table 6). This is consistent with a neomorphism (recrystallization) origin of the xenotopic dolomites at elevated temperature if a rock dominated system, as at Marble Canyon, is presumed. However, dolomite has been observed to replace limestone, even at high temperature, retaining the oxygen isotope signature of the precursor limestone (Epstein et a l., 1964; Friedman and Hall, 1963).
Therefore, although a neomorphic origin of some of the Galena dolomite is consistent with the oxygen isotope data, a replacement origin also fit these data. Undulose idiotopic-E dolomites (fig. 36) had some of the lowest fil®0 values (Table 7). These are in areas of high porosity and are interpreted as having equilibrated in a system more open with respect to the hot water. Sverjensky (1981) showed theoretically that 0*® depletion of carbonates near ore bodies in the Upper Mississippi Valley zinc-lead district could best be explained by increased porosity near the ore bodies rather than higher temperatures.
Other Evidence for Neomorphism of Galena Dolomites:
Much of the xenotopic-A dolomite in the Galena Group is patchy in distribution mixed with idiotopic dolomite textures. Some of 104
these idiotopic dolomites may represent the precursor dolomite,
particularly the ones that are non-undulose, finer grained than
nearby xenotopic dolomite and/or zoned (compare, for instance,
localities 19 and 20 in Appendix 3). The patchy distribution of the
xenotopic-A dolomites would be expected in a partial
recrystallization of an idiotopic dolomite. This is one of the
petrographic criteria discussed by Bathurst (1976) as evidence for
neomorphism in limestones.
Neomorphism would be expected to destroy zoning in the dolomite
being recrystallized. Xenotopic dolomites in the Galena Group are
rarely zoned and when they are, do not have the well developed zoning of the idiotopic dolomites. Possible subtle changes in chemical conditions during recrystallization can result in a zoned xenotopic dolomite (Fairchild, 1980); however, a recrystallization would normally be expected to have a homogenizing effect in a chemically closed system.
If a replacement rather than neomorphic origin for xenotopic dolomite in the Galena Group is postulated, one is faced with the problem of movement of large amounts of Mg^+. it is possible to mobilize Mg^+ Qn a local scale by pressure solution of nearby dolomite. This may have happened on a local scale in the Galena
Group; however, there is no evidence of pressure solution on the scale necessary to produce the volume of xenotopic dolomite found in the Galena Group.
Mg2+ possibly was introduced during the period of hydrothermal ore genesis. The presence of dolomite cements associated with ores suggests that the ore fluids were capable of 105
producing dolomite (Bailey and Cameron, 1951; Heyl et a l., 1959; and
Hall and Friedman, 1969). However, the volume of xenotopic dolomite
(nearly 1/4 of the Galena Group if sampling for this study is
representative) indicates that this mechanism would be insufficient
to transport enough Mg^+ for widespread epigenetic
dolomitization. Therefore, the best explanation for most of the
widespread xenotopic texture in the Galena Group is that is resulted
from the neomorphism of a pre-existing dolomite.
' The Origin of Xenotopic Dolomite: Trenton Formation
Mbst of the xenotopic dolomite in the Trenton Formation of the
Michigan Basin probably formed during the migration, along fractures
of warm brines associated with one or several epigenetic events
(Shaw, 1975; Taylor, 1982). Xenotopic-A dolomite associated with
fracturing in the Trenton Formation is interpreted as probably being
of high temperature replacement origin. This interpretation is
supported by th e fo llo w in g :
1) The bodies of xenotopic dolomite often are observed to
laterally and vertically grade into biomicritic limestones (Shaw,
1975, fig. 10) that are partially replaced by xenotopic-P dolomite
(figs. 47 and 48). This indicates a direct replacement of biomicrite
by the xenotopic dolomite rather than a recrystallization by a
precursor dolomite. No evidence of such a precursor is observed.
2) The fracture systems in the Michigan Basin offer conduits
for introduction of Mg^+ rich, warm solutions from deeper in the
basin as suggested by Shaw (1975). The limited areal extent of these
dolomites diminishes the problem of moving enough Mg 2+ to 106
accomplish the dolomltization. Fluid inclusion studies by Shaw
(1975) on pore filling dolomites of the Albion Scipio field indicated
a temperature in excess of 80°C during the precipitation of these
d o lo m ites.
Taylor (1982) reported 81 80 values averaging -8.9 per mil.
(PDB) for bulk samples of fracture related Trenton dolomite from the
Northville, Albion-Scipio, Reading and Pentwater oil fields. This average was based on 51 samples ranging from = -6.9 per mil.
to 8180 = -11.4 per mil. Four values for xenotopic-C (saddle) dolomite cements ranged between 8^80 = -8.8 and -11.2 per mil
(PDB) and averaging -9.8 per mil. Temperature estimates were based on these dolomite cements because it is more likely that they equilibrated in a system open with respect to water than the bulk
samples. The temperature estimates range between 50°C, assuming
Ordovician sea water of 8*80 = -5 per m il., to >80°C, assuming oil field brine of fil80 = -1.4 per mil. (Taylor, 1982).
Possibly some neomorphism of an earlier dolomite occurred near the fractures during the epigenetic activity. This most likely would have occurred in areas of regional dolomltization in the western part of the Michigan Basin (fig. 4). No evidence for this was found, possibly because of a lack of control in this area.
Fossils in biomicrites which are replaced by xenotopic-A dolomite and are surrounded by idiotopic dolomite (fig. 44) are also interpreted as replacement in origin. This relationship is found both in the "cap" and regional dolomites of the Trenton. The dolomltization of these biomicrites possibly proceeded as follows: 1)
The m icritic portion is replaced by fine grained idiotopic dolomite 107
(fig. 41), 2) fossils and unreplaced micrite are replaced by
xenotopic dolomite (fig. 47) presumably at higher temperature,
finally resulting in 3) a dolomite in which the original biomicrite
fabric is preserved (fig. 44).
Xenotopic-A dolomite replacing fossils in two samples from the
Trenton "cap" in the Northville oil field (fig. 4) were found to have
6 ^ values 1 per mil. lower than that of the surrounding
idiotopic-S dolomite replacing micrite (Taylor, personal
communication, 1982). A similar relationship was noted above in the
Galena dolomites. A closed system can be postulated during the
dolomltization of the fossils if a local source of Mg^+ existed.
Stylolitization of the fine grain idiotopic dolomite surrounding the
dolomitized fossils suggests that the Mg^+ necessary for the dolomltization of the fossils may, in some cases, have been derived
locally by pressure solution.
Significance and Further Study
This study attempts to demonstrate that observation that xenotopic dolomite may be an important indicator of epigenetic dolomltization of limestone or neomorphism of a pre-existing dolomite. The temperatures postulated for the origin of xenotopic dolomite (>50°C) coincide with those necessary for petroleum maturation and associated with Mississippi Valley Type sulfide mineralization. Therefore, identification of xenotopic texture may be of value as an exploration tool when more precise paleotemperature indicators are unavailable.
The low porosity of the xenotopic dolomites examined in this study 108 suggest that recrystallization of dolomite may have a detrimental effect on reservoir quality.
Further lines of research that may be of interest are: 1)
Investigation of sabkha dolomites forming near 50°C to see if they could be characterized as xenotopic. The temperatures here may approach or even be above CRT. The unequivocal discovery of low temperature (25°C) stoichiometric xenotopic-A dolomite vrould put the high temperature hypothesis in jeopardy. Ideally, the place to look for such dolomite is in unaltered post-Cretaceous rocks that have not been buried deeply.
2) Rapid crystal growth may favor the development of inclusions as compared to slow growth (Spry, 1969). It seems reasonable, therefore, that xenotopic dolomites should have more inclusions than idiotopic dolomites if they indeed grew faster. Although the xenotopic dolomites in this study had numerous inclusions so apparently did many of the idiotopic dolomites. An attempt should be made to quantify the relative number of inclusions in xenotopic and idiotopic dolomites.
3) The dolomite classification system developed for this study proved quite useful in categorizing dolomite textures in the rocks studied. It remains, however, for this classification system to be applied to a broad range of dolomites from a number of stratigraphic and geographic locations before it can be universally applied.
CONCLUSIONS
Xenotopic dolomite texture is defined as a mosaic of dolomite anhedra with curved, interlocking, irregular intergrain boundaries.
Xenotopic dolomite usually has undulose extinction and is similar in 109 appearance to neomorphic textures observed in some limestones.
Xenotopic dolomite textures were observed in the Galena Group
(Ordovician), Wisconsin, the Trenton Formation (Ordovician), Michigan dolomite marble and high temperature synthetic dolomite. Xenotopic dolomite texture was not observed in any of the Cenozoic dolomites
investigated in this study.
Xenotopic dolomite textures contrast with idiotopic textures that are common in Cenozoic as well as ancient dolomites. Idiotopic dolomite texture is characterized by mostly euhedral to subhedral grains and smooth, straight intergrain compromise boundaries.
A statistical mechanical model for crystal growth proposed by
Jackson (1958a and 1958b) predicts that at low temperatures crystals grow by the addition of atoms layer by layer on a crystal face. This kind of growth requires surface nucleation sites, such as dislocations and results in smooth crystal faces and euhedral grain mosaics. Above a "critical roughening temperature" surface nucleation does not require dislocations and atoms are added randomly to the crystal face as a rough surface is energeticaly more favorable than a smooth surface.
This results in non-faceted crystals and can produce an interlocking mosaic of anhedra such as is observed in xenotopic dolomite.
It is hypothesized that a "critical roughening temperature" exists for dolomite between 50°C and 100°C. Xenotopic dolomite textures are produced by the replacement of limestone by dolomite and/or neomorphic recrystallization of dolomite at elevated temperature after burial. Idiotopic dolomites are produced below the
"critical roughening temperature" by near surface dolomltization processes. Calcite, in contrast to dolomite, has a "critical 110
roughening temperature" below 25°C, thus anhedral grain textures,
characteristic of neomorphic limestone, can be produced during early
diagenesis at low temperature.
Synthetic xenotopic dolomite was produced in laboratory
hydrothermal bombs by dolomltization of aragonite and calcite
skeletal fragments and by recrystallizatlon of nonstoichlometric
Cenozoic dolomites. These syntheses were performed at temperatures of 250° and 300°C.
Xenotopic dolomite resulted from the metamorphic recrystallization of idiotopic dolomites in the Hueco Limestone
(Permian), Culbertson Co., Texas, near the Marble Canyon intrusion.
The recrystallization occurred at temperatures ranging from 250° to
600°C. Late hydrothermal dolomltization of periclase-calcite marble adjacent to the intrusion also had xenotopic texture.
Xenotopic dolomite in the Galena Group was produced by neomorphism of a pre-existing idiotopic dolomite. This recrystallization probably occurred during the emplacement of the
Upper Mississippi Valley lead-zinc sulfides at temperatures of between 50°C and 120°C (Bailey and Cameron, 1951). Stable oxygen isotope data for the Galena dolomite are consistent with the recrystallization model.
Xenotopic dolomite in the Trenton Formation was produced by replacement of limestone at elevated temperature; Most of this probably occurred during migration Mg2+ rich brines along fracture systems. Minor replacement of limestone possibly occurred during pressure solution of nearby idiotopic dolomites, which provided a source of Mg^+. Appendix 1
Designs for Hydrothermal Bombs Used in the Experiments 3 copper gasket
Stainless Steel Hydrothermal Bomb 6.4CC Capacity SCALE IN INCHES
Stainless steel hydrothermal bomb (design by Morey, 1953) with an 18.5 ml capacity. A seal is effected by a copper washer and the screw cap is made of molybdnum steel. Appendix 2
Experimental Results 113
1) High Magnesium Calcite to Dolomite
Unaltered Sample: Gonlolithon sp. skeletal material, 25 mole percent
MgC03
Fluid Content: 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca = .25
Reaction Time: 98 hrs.
Reaction Temperature: 300°C
Product: Mineralogy: 100% well ordered dolomite, 49 mole percent
MgC03
Texture: Replacement by ovad shaped crystals with in
distinct faces (.3 to .5 /im). Algal concepticles
sometimes filled with dolomite cement; basal
terminated steep rhombic form (4041) and (0001)
superimposed onto normal rhombic form. 114
2) High Magnesium Calcite to Dolomite
Unaltered Sample: Goniolithon sp. skeletal material, 25 mole percent
MgC03
Fluid Content: 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca * .25
Reaction Time: 839 hrs.
Reaction Temperature: 300°C
Product: Mineralogy: 100% well ordered dolomite, 50 mole percent
MgC03
Texture: Same as in experiment 1 115
3) High Magnesium Calclte to Dolomite
Unaltered Sample: Gonlollthon sp. skeletal material, 25 mole percent
MgC03
Fluid Content; 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca = .25
Reaction Time: 120 hrs.
Reaction Temperature: 250°C
Product: Mineralogy: 100% well ordered dolomite, 51 mole percent
MgC03
Texture: Same as in experiment 1 116
4) High Magnesium Calcite to Dolomite
Unaltered Sample; Echinoid skeletal material, 13 mole percent
MgC03
Fluid Content; 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca = ,239
R eaction Time; 168 h rs .
Reaction Temperature; 300°C
Product; Mineralogy: 100% well ordered dolomite, 50 mole percent
MgC03
Texture: Pseudomorphic replacement of HMC including
preservation of C-axis orientation. Individual
dolomite crystals too small to resolve with SEM. 117
5) High Magnesium Calcite to Low Magnesium Calcite
Unaltered Sample: Goniolithon sp. skeletal material, 25 mile percent
MgC03
Fluid Content: 1 Molar CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca = .04
Reaction Time: 335 hrs.
Reaction Temperature: 300°C
Product: Mineralogy: LMC, 4 mole percent MgC03
Texture: Replacement texture similar to experiment 1 (.5
to 1 Jim). Algal concepticals filled with hexa
gonal scalenohedral calcite cement. 118
6) High Magnesium Calcite to Low Magnesium Calcite
Unaltered Sample: Goniolithon sp. skeletal material, 25 mile percent
MgCC>3
Fluid Content: 1 Molar CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca ■ .05
Reaction Time: 1516 hrs.
Reaction Temperature: 300°C
Product: Mineralogy: LMC, 4 mole percent MgCOg
Texture: Same as in experiment 5 7) Aragonite to Dolomite
Unaltered Samples Strombus sp. shell fragments, aragonite
Fluid Content: 1 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca = .15
Reaction Time: 136 hrs.
Reaction Temperature: 300°C
Product: Mineralogy: LMC with trace of dolomite.
Texture: Not examined with SEM 120
8) Aragonite to Dolomite
Unaltered Sample: Strombus sp. shell fragments, aragonite
Fluid Content: 1 Malar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca ■ .157
R eaction Time: 23 h r s .
Reaction Temperature: 300°C
Product: Mineralogy: LMC with trace of dolomite.
Texture: Not examined with SGM 121
9) Aragonite to Dolomite
Unaltered Sample: Strombus sp. shell fragments, aragonite
Fluid Content: 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Hg/Mg+Ca » .30
Reaction Time: 239 hrs.
Reaction Temperature: 250°C
Product: Mineralogy: 43% well ordered dolomite (49 mole percent
MgC 03) 57% calcite (2 mole percent MgCOj)
Texture: Interlocking mosaic of dolomite anhedra, charac
teristic of xenotopic texture, in a reaction rim,
around a core of inverted calcite also with an
interlocking anhedral texture, characteristic of
neomorphic spar. In thin section with a
polarizing microscope the aragonite is replaced
by xenotopic dolomite with strongly undulose
extinction surrounding a core of neomorphic
calcite spar with undulose extinction and
fascicular texture. 10) Aragonite to Dolomite
Unaltered Sample; Area ponderosa shell fragments, aragonite
Fluid Content; 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca ■ .298
Reaction Time: 334 hrs.
Reaction Temperature: 250°C
Product: Mineralogy: 73% well ordered dolomite (50 mole percent
MgCO-j) 27% c a lc ite (4 mole percen t MgCOg)
Texture: The same as in experiment 9. Some of the smaller
shell fragments appeared to have been entirely
replaced by dolomite with no calcite present.
M icroprobe*: C a lc ite , CaO = 99.21% MgO = .47% FeO =* .32%**
Dolomite, CaO = 48.55% MgO = 51.17% 8 FeO =
.28%
* Microprobe results are the average values for at least four peaks taken from at least tw locations on each sample.
** The ARL microprobe used consistently reads a few tenths of a percent high in iron, therefore the actual iron content is probably less than one tenth of a percent. 123
11) Aragonite to Dolomite
Unaltered Sample: Area ponderosa shell fragments, aragonite
Fluid Content: 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca “ .298
Reaction Time: 23 hrs.
Reaction Temperature: 250°C
Product: Mineralogy: 44% dolomite with rather diffuse ordering
peaks (44 mole percent MgCOj)
56% calcite (5 mole percent MgCOj)
Texture: The same as in experiment 9. Very well developed
saddle dolomite crystals observed in some of the
pore space. Thin section examination of the
products of experiments 10 and 11 indicate a
slightly larger grain size with longer cooking
times under the same condition.
Microprobe: Calcite CaO = 98.21% MgO - 1.53% FeO = .26% %
Dolomite CaO = 54.64% MgO = 45.05% FeO = .31 I
124
12) Aragonite to Dolomite
Unaltered Sample; Area ponderosa shell fragments, aragonite
Fluid Content: 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca = .23
Reaction Time: 979 hrs.
Reaction Temperature; 300°C
Product: Mineralogy: 44% Well ordered dolomite (50 mole percent
MgC03)
56% calcite (4 mole percent MgC 03 )
Texture: The same as in experiment 10. 125
13) Aragonite to Dolomite
Unaltered Sample: Area ponderosa shell fragments, aragonite
Fluid Content: 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca ■ .24
Reaction Time: 24 hrs.
Reaction Temperature: 300°C
Product: Mineralogy: 43% ordered dolomite (49 mole percent
MgC03)
57% calcite (3 mole percent MgC03)
T exture: The same as in experim ent 10. 126
14) Aragonite to Calcite
Unaltered Sample; Strombus sp. shell fragments, aragonite
Fluid Content; 1 Molar CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca - 0
Reaction Time: 120 hrs.
Reaction Temperature: 300°C
Product: Mineralogy: 100% LMC
Texture: Interlocking mosaic of calcite anhedra apparently
maintaining the original c-axis orientation of
the replaced aragonite. Similar to calcite
produced in experiment 9. 127
15) Aragonite to Calcite
Unaltered Samplei Area ponderosa shell fragments, aragonite
Fluid Content: 1 Molar CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca ■ 0
R eaction Time: 167 h r s .
Reaction Temperature: 250°C
Product: Mineralogy: 100% LMC
Texture: Interlocking mosaic of calcite anhedra apparently
not maintaining the original c-axis orientation
of the replaced aragonite. 128
16) Dolomite Recrystallization
Unaltered Sample; Supratidal sediment from Andros Island, Bahamas
containing aragonite and protodolomite; poorly
ordered with (015) reflection absent, 41% MgC03
grain size <3 m
Fluid Content; 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca = .246
R eaction Time; 311 h r s .
Reaction Temperature; 300°C
Product: Mineralogy: 100% well ordered dolomite, 50 mole percent
MgC03
Texture: A porous aggregate of basal terminated steep
rhomb crystals [(4041) and (0001) forms] less
than 10 jjm. Where crystals interfere with one
another, irregular intergrain boundaries form
characteristic of xenotopic dolomite texture. 129
17) Dolomite Recrystallization
Unaltered Sample: Supratidal sediment from Andros Island, Bahamas.
Same sample as in experiment 16.
Fluid Content: 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca =* .25
Reaction Time: 120 hrs.
Reaction Temperature: 300°C
Product: Mineralogy: 100% well ordered dolomite, 50 mole percent
MgC03.
Texture: The same as in experiment 16. 130
18) Dolomite Recrystallization
Unaltered Sample: Dolomitized coralline algae from Bonaire, N.A.
Mg++/ Ca++ = .46
grain size <3 m
Fluid Content: 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca = .25
Reaction Time: 2184 hrs.
Reaction Temperature: 300°C
Product: Mineralogy: 100% ordered dolomite, 50 mole percent
MgC03
Texture: Recrystallized with definite evidence of grain
growth. Intergrain boundaries are curved.
Development of (4041) and (0001) crystals forms
(basal terminations and steep rhombic form).
Crystals mostly between 2 m and 5 m.
Note: Bomb failure caused termination of the experiment and
only a trace of fluid was recovered. 131
19) Dolomite Recrystallization
Unaltered Sample: Dolomitized coralline algae from Bonaire, N.A.
Mg/Ca+Mg = .46
grain size <3 m
Fluid Content; 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca = .25
R eaction Time: 120 h r s .
Reaction Temperature: 300°C
Product: Mineralogy: 100% ordered dolomite, x-ray indicates 2
phases of dolomite present; 50 mole percent
MgC03 and less than 48 mole percent MgC 03«
Texture: Recrystallization as in experiment 18. 132
20) Dolomite Recrystallization Attempt
Unaltered Sample: Dolomitized mud from Bonaire, N.A.
Mg++/Mg'H‘ + Ca++ = .46
grain size 20 m
Fluid Content: 2 Molar Mg/CaCl 2 s o lu tio n
Ratio of Total Cations (sample and solution): Mg/Mg+Ca ■ .25
Reaction lime: 720 hrs.
Reaction Temperature: 300°C
Product: Mineralogy: Unchanged
Texture: Unchanged Appendix 3
Petrographic Descriptions for the Galena Group 133
The following are general petrographic descriptions of the
Galena Group sections which were sampled. The descriptions are based on thin section descriptions from the 131 samples which were collected during September, 1980. The 16 outcrops which were visited are among those described by David Delgado for his Ph.D. dissertation at the University of Wisconsin. The numbering system used here for the localities (outcrops) is that of David Delgado.
L o c a lity 13
SW, NW, 10, IN, 4E, Lafayette Co., Wis.
Road c u t, n o rth sid e o f highway N.
4 samples collected.
Nonporous dolomites are almost entirely xenotopic-A with undulose extinction (0.05 to 0.3 mm) and unzoned. Idiotopic-E dolomite is zoned 75% of the pore space is filled by opaque non-carbonate matrix.
Contains abundant rounded quartz (feldspar?) sand grains.
Locality 14
NE, NE, 9, IN, 4E, Lafayette Co., Wis.
Road c u t, both sid e s o f highway 11
6 samples collected
Nonporous xenotopic-A dolomite about 0.3 mm in patches of 1 to 2 cm. surrounded by idiotopic-E and S dolomite, zoned with cloudy center-clear rim. A few stylolites are present in idiotopic dolomite. Abundant rounded quartz (feldspar) sand and some chalcedony. 134
Locality 15
Center line ME, ME, 24, IN, IE, Lafayette, Co., Wis.
Roadcut, both sides of road, County highway 1.
3 samples collected.
Xenotopic-A dolomite predominates in all 3 samples (0.1-0.4 mm), in a few samples it appears to the zoned, but for the most part it is very dirty and unzoned. Patches of Idiotopic-S dolomite 0.05-0.3 mm with good zoning and a few stylolites. Abundant rounded quartz
(feldspar?) sand.
Locality 16
NW, NE, 14, IN, IE, Lafayette Co., Wis.
Road c u t, n o rth sid e o f highway 11.
9 sam ples c o lle c te d .
Xenotopic-A dolomite (0.1-0.8 mm) in scattered patches and near stylolites 1 to 2 cm across in idiotopic-E and S dolomite (0.1 to 0.5 mm), idiotopic-C dolomite surrounding vuggy porosity (fossil molds).
A few horizontal stylolites are present.
Locality 19
Nl/2, SW, 16, 4N, 2E, Iowa Co., Wis.
Road c u t, both s id e s o f U.S. 151.
10 samples collected.
Dense nonporous dirty xenotopic-A dolomite (0.1-0.5 mm) predominates with some xenotopic-C (saddle)cements (up to 1 m) in pore space.
Zoned idiotopic-E and S dolomite (0.05-0.3 mm) some with undulose extinction in porous areas. Horizontal stylolites are common in 135 xenotopic dolomites, opaque material commonly coats idiotoplc dolomite crystals. Vuggy porosity often filled by opaque material and/or chalcedony. Opaque to partly translucent mineral filing some pore space is apparently sphalerite.
L o c a lity 20
NE30, 6N, 2E, Iowa C o., H is.
Road cut, both sides of U.S. 151.
11 samples collected.
Lower part of section predominantly fine grained (about 0.1 mm) idiotopic-S dolomite with small amounts of undulose xenotopic-C dolomite cement filing fossil molds. In upper part of the section dirty coarser grain idiotopic-S dolomite predominates (0.05 to 0.2 mm) with patches and horizontal bands of xenotopic-C dolomite. Some idiotopic-C dolomite as (late?) fracture filling. Vuggy and fracture porosity filled by opaque material, chert and/or calcite. Rounded quartz sand common in upper part of the section.
Locality 23
NE30, 6N, 2E, Iowa C o., H is.
Road cut, south side of U.S. 18 and quarry 0.5 mi. north of highway.
9 samples collected.
Idiotopic-E and S dolomite predominate (about .2 mm) with patches
(0.1 mm - 1 cm) of xenotopic-A dolomite (0.1 to 0.4 mm).
Intergranular porosity in idiotopic dolomite filled with brown opaque material. Fossil mold porosity common and a few stylolites p re s e n t. 136
Locality 28
Wl/2, SW, NW, 33, 7N, 3E, Iowa Co., Wis.
Road c u t, both sid e s o f Highway 23.
5 sam ples.
Horizontal bands and patches of xenotopic-A dolomite (0.1 to 0.5 mmm)
with layers of idiotopic-S dolomite (about 0.1 mm) along stylolites
and coarser undulose idiotopic-E and S dolomite (some with slightly
curved crystal edges) (0.1 to 0.5 mm) which predominates. One sample
consisted entirely of coarse, undulose xenotopic-A (0.3 to 0.6 mm).
Vuggy porosity common, vuggy and intergranular space often filled by
reddish brown opaque material often coating dolomite euhedra.
L o c a lity 29
Center N line, 33, 7N, 3E Iowa Co., Wic.
Road cut, both sides of Highway 23
4 samples collected
Xenotopic-A dolomite predominates (0.1 to 0.4 mm) sometimes interlayered with idiotopic-S dolomite (0.05 to 0.2 mm) some of which are zoned. Stylolites are present throughout section; vuggy porosity is common, often filled with opaque material. Bounded quartz
(feldspar?) sand throughout.
L o c a lity 30
NW, NE, 28, 7N, 3E, Iowa Co., Wis.
Boad c u t, both sid e s o f highway 23
9 samples collected
Mostly idiotopic-E and S dolomite (0.05 to 0.6 mm) with patches of I
137
xenotopic-A dolomite throughout (0.1 to 0.4 mm), especially in areas
which are predominantly finer grain idiotopic-S dolomite. Vuggy
porosity is common as are opaques, rounded sand grains, and fossils
preserved by chert. A few stylolites are present.
Locality 31
NE10, 6N, SE Iowa C o., Wis.
Quarry on north side of highway 151.
8 sam ples c o lle c te d .
Predominantely xenotopic-A dolomite (0.05 - 3 mm) with some vuggy
porosity filled by idiotopic-C dolomite (0.1 to 0.5 mm) and black
(organic?) opaque material. A few stylolites are present as well as
vertical fractures filled by black opaques.
Locality 33
SE, 1, and NW, NE, 12, 2N 2W, Grant Co., Wis.
Koad cut, north side of U.S. 151.
13 sam ples c o lle c te d .
Mostly idiotopic-E and S dolomite (0.1 to 0.6 mm) some zoned, and vug
filling idiotopic-C dolomite some of which is undulose and associated
with vug filling black opaque material. A few patches (1-3 cm) of
undulose xenotopic-A dolomite were observed (grains about 0.3 mm and
unzoned) surrounded by idiotopic dolomite. Stylolites appear to be
common and vuggy porosity is often filled by calcite cement. Bounded
sand grains are common throughout the section. 138
Locality 34
Sl/2, NW and NE, NE, SW, 7, 2N, 2W, Grant Co., Wis.
4 road cuts on east side and 1 road cut on west side U.S. 61.
5 samples collected from 3rd roadcut on east (while facing south).
Mastly idiotopic-S dolomite (0.05-0.4 mm) with patchy xenotopic-A dolomite (0.1 to 0.5 mm) as at locality 33. Vuggy porosity, stylolites, opaques and sand as at locality 33.
Locality 37
NW, SW, 31, 5N, 3W, Grant C o., Wis.
Road cut on both sides of County Highway V.
8 sam ples c o lle c te d .
Limestone biomicrites and biosparites with idiotopic-D and patches of idiotopic-E and S dolomite replacing micrite. Stylolites common with some asociated neospars replacing micrite.
Locality 45
El/2, SW, 28, 92N, 2W, Clayton Co., Iowa.
Road cut, west side of U.S 52 along Mississippi River.
23 samples collected.
Biomicrite interbedded with and partially replaced by idiotopic-E, S and P dolomite (0.05-0.2 mm). A few patches (1-3 cm) of xenotopic-A dolomite (undulose and up to 0.5 mm) replace both biomicrite and idiotopic-S dolomite. Scattered anhedra of xenotopic-P dolomite are also present in some limestones. Stylolites are common, especially in area s th a t are d o lo m itized , dolom ite rhombs are commonly associated with stylolites in limestones. 139
Locality 50
NE, NE, 26 and SE23, 98N, 8W, Winneshiek Co., Iowa.
Goad cuts, both sides of highway 9
4 sam ples c o lle c te d .
Biomicrite and biosparite with scattered rhombs of idiotopic-P dolomite (0.01 to 0.1 mm). In one specimen a warm trail was replaced by idiotopic-S dolomite. Sounded to subangular quartz (feldspar?) sand common throughout section. APPENDIX 4
Petrographic Descriptions for the Trenton Formation 140
The following are general petrographic descriptions of the
Trenton Formation cores studied. The general descriptions are based on 130 individual thin sectin descriptions and, where noted, an oil
company core descriptions and drilling logs. The numbering system used for the Trenton cores is simply an abbreviation of the well name and number. Depth is given in feet below kelly bushing. In most of the subsurface records (logs, core or sample descriptions) of the
Michigan Basin the Trenton Formation is not differentiated from the underlying Black River Formation, which is lithologically sim ilar.
In the following descriptions, therefore, both units are referred to as "Trenton Formation.”
1. HS (Hewett-Shadd No. 1) Perm it Number: 30974 Location: SW, Se, 20, 12N, 15# Sanilac Co., Michigan Field: none, Dry hole E lev atio n : Top of Trenton Formation: 6,370 ft. Number of thin sections: 11, no core description available
Depth in Feet Lithology 6392-6412 Sparse biomicrite with a few stylolites and idiotopic-P dolomite and a few scattered patches of xenotopic-P dolomite.
6412-6452 Biomicrite partially replaced by xenotopic-P dolomite, xenotopic-C dolomite cements filling vugs and fractures, and coarse undulose idiotopic-S dolomite.
6452-6572 Biomicrite with scattered patches of fine grain idiotopic-S and idiotopic-P dolomite.
6512-6562 Biomicrite with scattered fine grain idiotopic-P dolomite euhedra and scattered xenotopic-P dolomite euhedra some apparently related to fractures filled with xenotopic-C dolomite cem ent.
2. E—1 (Evans PRoducts Co. No. 1) Perm it Number: 26075 Location: SE, NE, 25, 15, 8E. Wayne Co., Michigan. 141 Field: Northville Elevation: 718 feet above sea level (K.B.) Top of Trenton Formation: 3651 ft. Number o f th in s e c tio n s : 31
Depth in Feet Lithology 3651-3656 Fine grain idiotopic-S dolomite (<0.1 mm), highly stylolitized, replacing micrite. Coarser grained (0.1-0.5 mm) xenotopic-A dolomite replacing fossils and patches of micrite. Gore description reported bleeding oil from vertical fractures and horizontal p a rtin g s .
3656-3658 Destructive replacement by coarse (0.1 to 1 xenotopic-A dolomite (obliterating biomicrite fabric). Core description reported bleeding oil as above.
3659-3779 Biomicrite with scattered euhedra of fine grain idiotopic-P dolomite (<0.1 mm) replacing micrite scattered patches of xenotopic-A and P dolomite replacing micrite and fossils. Some stylolites.
3779-3825 Destructive replacement by coarse xenotopic-A dolomite (0.2 to 1mm) and coarse undulose idiotopic-E and S dolomite associated with opaque material. Finer grain dolomite near sylolites. Fractures filled with xenotopic-B (saddle) dolomite cements, anhydrite and opaque material. Gore description reported gas shows.
3825-4028 Core description reports medium grain dolomite, with sucrosic, vuggy and fracture porosity. Fractures are often lined with coarse, white dolomite cements. The dolomites are interbedded with fossiliferous „ , limestones. This section has numerous shale. partings. Anhydrite, pyrite and oil shows are reported associated with the dolomites.
4028 Base of core
4150 T.D.
3. E-2 (Evans Products Co. No. 2) Perm it Number: 26100 Location: Nl/2, NE, 25, IS, 8E Wayne Co., Michigan Field: Northville Elevation: 772 feet above sea level (K.B.) Top of Trenton Formation: 3678 ft. Number o f th in s e c tio n s : 10
Depth in feet Lithology 3678-3681 Fine grain idiotopic-S dolomite (<0.1 mm) with stylolites replacing micrite. Coarse xenotopic-A 142
dolomite (0.2 to 0.6 mm) replacing fossils and about a small amount of the m icrite, preserving biomicrite fabric. Some of the biomicrite is destructively replaced (obliterating biomicrite fabric) by coarse xenotopic-A and undulose ldiotopic-G and S dolomite (up to 1 mm) with intercrystalline opaque material. Vuggy and v e r ti c a l fra c tu re p o ro s ity are f i l l e d by xenotopic-C (saddle) dolomite cement and anhydrite.
3681-3693 Mostly biomicrite with some stylolites partly replaced by fine grain idiotopic-P dolomite (0.05 to 0.2 mm) and scattered patches of xenotopic-A and P dolomite (0.1 to 0.3 mm).
3693-4152 Gore description reports only limestone, no samples tak en .
4152-4285 Core description reports "medium crystalline” dolomite interbedded with limestone. Oil and gas shows in the dolomite.
4285 T. D.
4. BR (Burroughs Corporation No. 1) Perm it Number: 25808 Location: Nl/2, NW, NW, 25, IS, 8E Field: Northville Elevation: 725 feet above sea level (K.B.) Top of Trenton Formation: 3651 ft. Number o f th in s e c tio n s : 9
Depth in feet Lithology 3651-3654 Fine grain idiotopic-S dolomite (<0.1 mm) with ...... stylolites replacing micrite. Fossils partially to wholely replaced by xenotopic-A and P dolomite (0.1 to 0.5 mm). Some destructive replacement (obliterating biomicrite fabric) by coarse grain xenotopic-A and undulose idiotopic-E and S dolom ite.
3654-3660 Biomicrite with fine grain idiotopic-S dolomite (as above) partially replacing micrite and scattered patches of xenotopic-P dolomite replacing micrite and fossils.
3660-4022 Core description reports interbedded finely crystalline, fossiliferous limestones interbedded with porous petroliferous dolomites (vuggy, fracture and intercrystalline porosity).
4022 Base of Trenton core.
4264 T. D. 143
5. W2 (Robert and Dorothy Whitaker No. 2) Permit Number: 28407 Location: SW, NW, NE, 29, 75, 4W Field: Reading Elevation: 1061 feet above sea level (K.B.) Top of Trenton Formation: 3043 ft. Number o f th in s e c tio n s : 9
Depth in feet Lithology 3043-3063 Gore d e s c rip tio n re p o rts medium c r y s ta llin e , slightly fossiliferous dolomite with a few shale partings and trace of pyrite.
3063-3068 Biomicrite limestone with fine grain idiotopic-P dolomite (<0.1 mm) partially replacing micrite and patches of coarser xenotopic-P dolomite (0.1-0.5 mm) replacing both fossils and micrite. Xenotopic dolomite appears more abundant near stylolites in the limestone.
3068-3092 Coarse grain slightly undulose idiotopic-E and S dolom ite (up to 0.8 mm), many c ry s ta ls a re m icely zoned, black and brown opaque material filling intercrystalline space, and coarse grain xenotopic-A dolomite (up to 0.8 mm). Stylolites are common and dolomite near them tends to be finer grain, rhombic and associated with more opaque material than elsewhere. Gore description reports oil shows in this section.
3092-3300 Core description reports fine to coarse crystalline dolomites with pinpoint, inter-crystalline, vuggy and fracture porosity, fractures filled by coarse dolomite cements. Dolomites interbedded with dense, fine grain fossiliferous limestone.
3308 T. D.
6. JF (Jelinek-Ferris Unit No. 1) Perm it Number: 27907 L ocation: SE, SE, NW, 5, 5N, 2E Shiawassee C o., Michigan Field: None, on Northville-Howell-Fowlervllle trend, Dry Hole Elevation: 843 feet above sea level (K.B.) Top of Trenton Formation: 6101 ft. Number of th in s e c tio n s : 19
Depth in feet Lithology 6101-6164 Biomicrite limestone with micrite partially replaced by fine grain idiotopic-P dolomite and patches of idiotopic-E and S dolomite (0.02-0.1 mm). Fossils and micrite are partially replaced by patches and individual anhedra of xenotopic-P dolomite (0.05 - 0.6 mm). Stylolites are common in the biomicrites, idiotopic-C dolomites are most common in and adjacent to the stylolites. 144
6164-6190 No core description available.
6190 Base of Trenton core.
7056 T. D.
7. HI (Elmer M. Haab Unit NO. 1) Perm it Number: 19231 Location: SW, SE, NW, 8, 3S, 4E Washtenaw Co., Michigan Field: Freedom Field, Dry Hole Elevation: 963 feet above sea level (K.B.) Top of Trenton Formation: 3760 ft. Number o f th in s e c tio n s : 9
Depth in feet Lithology 3760-3771 Coarse grain xenotopic-A dolomite (0.2-0.7 mm) containing ghosts of echinoid fossils. Finer grain idiotopic-E and S dolomite (<0.1 mm), with abundant intercrystalline opaque material, near stylolites. Core description reports fracture filled with coarse white dolomite cement and anhydrite.
3771-3832 Echinoidal biomicrite with fine grain idiotopic-D dolomite replacing micrite, especially near stylolites. Coarse grain xenotopic-P dolomite (up to 0.5 mm) replacing micrite and fossils. Some samples are up to 50% replaced by dolomite. Fracture porosity is lined by xenotopic-B (saddle) dolomite (up to 0.5 mm) and filled with anhydrite. Gore description reports interbedded dolomite and fossiliferous limestone and shows of oil.
3832-4365 Gore description reports fossiliferous limestone with one thin bed of dolomite (l17" thick) at 3930. The limestone contains shale partings chert nodules and o il shows throughout.
4365-4408 Core description reports fine to coarse crystalline dolomite with vertical fracturing (sometimes filled with gypsum), vuggy, pinpoint and intercrystalline porosity, and o il shows throughout.
4408 Base of Trenton core.
4691 T. D.
8. L12 (Jack Lauber No. 12) Permit number: 17549 L ocation: Center SE, SE, 6, 16N, 17W Oceana C o., Michigan Field: Pentwater Field, deep test, dry hole Elevation: 684 feet above sea level (K.B.) Top of Trenton Formation: 4947 ft. Number o f th in s e c tio n s : 32 145
Depth in feet L ithology 4947-4996 Fine grained idiotopic-S dolomite (<0.1 mm), with numerous stylolites giving it a "ground-up" appearance, replaing micrite. Coarse grained xenotopic-A dolomite (.2-.4 mm) replacing fossil fragments. Some areas of destructive replacement (obliterating biomicrite fabric) by coarse grained xenotopic-A (up to 0.5 mm) dolomite. Black opaque material was observed in some samples surrounding coarse (up to 0.5 mm) undulose idiotopic-E dolomite. Saddle shaped xenotopic-C dolomite cement oserved as vug fillings.
4996-5078 No core taken, drilling log records dense fossiliferous dolomites, dolomitic limestones, traces of gypsum, and o il shows.
5078-5105 Dolomite as in 4947-4996 section including coarse, undulose idiotopic-A dolomite (up to 1 mm) coated with black opaque material. A section of biomicrite partially replaced by xenotopic-P dolomite was observed at 5103'. The core description notes bleeding oil and gas throughout this section of co re.
5106-5109 Biomicrite partially replaced by xenotopic-P dolomite (up to 0.3 mm) and idiotopic-P dolomite (<0.1 mm).
5109-5152 No core taken, drilling log records dense, shaley lim estones and medium c r y s ta llin e d o lo m ite, some fossiliferous.
5152-5202 Mostly coarse grained xenotopic-A dolomite (up to 1 mm) and idiotopic-E and S dolomite (up to 1 mm, usually coated by opaque material) destructively replacing biomicrites. A few areas of fine grain idiotopic-S dolomite replacing micrite and coarser xenotopic-A replacing fossils were observed, particularly in the interval of 5190-5201 associated with unreplaced biomicrite. The core description notes oil shows throughout this section.
5202-5243 Gore description records dense, shaley fine crystalling dolomite.
5243 Base of Trenton core.
5382 T.D. BIBLIOGRAPHY 146
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