Western Michigan University ScholarWorks at WMU
Master's Theses Graduate College
8-1981
Dolomitization of the Brassfield ormationF (Lower Silurian) in Adams County, Ohio
Lisa L. Varga
Follow this and additional works at: https://scholarworks.wmich.edu/masters_theses
Part of the Geology Commons
Recommended Citation Varga, Lisa L., "Dolomitization of the Brassfield ormationF (Lower Silurian) in Adams County, Ohio" (1981). Master's Theses. 1843. https://scholarworks.wmich.edu/masters_theses/1843
This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. DOLOMITIZATION OF THE BRASSFIELD FORMATION (LOWER SILURIAN) IN ADAMS COUNTY, OHIO
by
Lisa L. Varga
A Thesis Submitted to the Faculty of the Graduate College in partial fulfillment of the requirements for the Degree of Master of Science Department of Geology
Western Michigan University Kalamazoo, Michigan August 1981
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. WUU&yJuSUB!
DOLOMITIZATION OF THE BRASSFIELD FORMATION (LOWER SILURIAN) IN ADAMS COUNTY, OHIO
Lisa L. Varga, M.S.
Western Michigan University, 1981
The Lower S ilurian Brassfield Formation which outcrops in the
T ri-sta te area o f Ohio, Indiana and Kentucky is a transgressive sequence consisting of a series of interbedded shales, limestones
and dolostones. Evidence from depositional environments, petro
graphy and spatial relationships of dolomitized and undolomitized
rock suggests dolom itization in southwestern Ohio was a two-stage
process. Initial dolomitization was restricted to the basal Bel
fa s t Member and probably occurred penecontemporaneously on small
supratidal islands in a manner analagous to that which occurs in the modern sabkha environment of the Persian Gulf. Regional dolo
mitization was a later diagenetic event related to the formation of a fresh-seawater mixing zone beneath a landmass created by up-
warping of the Cincinnati Arch at the close of the Silurian. In
tensity of dolomitization in the outcrop belt is controlled by the
proximity of the original carbonate to the source o f dolomitizing
fluids in the mixing zone.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. >•» —— «»«. -Tvte* iw n i
ACKNOWLEDGEMENTS
The w rite r would very much lik e to thank Dr. W illiam B. Harri
son III for his advice and assistance on this project. Dr. Harri
son was most generous with his time and his u n fa ilin g good humor
was greatly appreciated. Acknowledgement is made to Dr. Thomas
Straw and Dr. John Grace who kindly read an early d ra ft and made
helpful suggestions.
F in a lly, I am grateful fo r the encouragement and moral supp
ort provided by my family and friends. I thank you a ll.
The research for this study was partially funded by a grant
from the Graduate College o f Western Michigan U niversity.
Lisa L. Varga
11
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INFORMATION TO USERS
This was produced from a copy of a document sent to us for microfilming. While the most advanced technological means to photograph and reproduce this document have been used, the quality is heavily dependent upon the quality of the material submitted.
The following explanation of techniques is provided to help you understand markings or notations which may appear on this reproduction.
1. The sign or “target” for pages apparently lacking from the document photographed is “Missing Page(s)”. If it was possible to obtain the missing page(s) or section, they are spliced into the film along with adjacent pages. This may have necessitated cutting through an image and duplicating adjacent pages to assure you of complete continuity.
2. When an image on the film is obliterated with a round black mark it is an indication that the film inspector noticed either blurred copy because of movement during exposure, or duplicate copy. Unless we meant to delete copyrighted materials that should not have been filmed, you will find a good image of the page in the adjacent frame. If copyrighted materials were deleted you will find a target note listing the pages in the adjacent frame.
3. When a map, drawing or chart, etc., is part of the material being photo graphed the photographer has followed a definite method in “sectioning” the material. It is customary to begin filming at the upper left hand corner of a large sheet and to continue from left to right in equal sections with small overlaps. If necessary, sectioning is continued again—beginning below the first row and continuing on until complete.
4. For any illustrations that cannot be reproduced satisfactorily by xerography, photographic prints can be purchased at additional cost and tipped into your xerographic copy. Requests can be made to our Dissertations Customer Services Department.
5. Some pages in any document may have indistinct print. In all cases we have filmed the best available copy.
Uni I International 300 N. ZEEB RD.. ANN ARBOR. Ml 48106
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I 1317257 ! j VARGA, LISA LOUISE DQLQMITIZATION OF THE BRASSFIELD FORMATION I (LOWER SILURIAN) IN ADAMS CDUNTY, OHIO•
WESTERN MICHIGAN UNIVERSITY, M .S., 1981
U n i v e r s e Microfilms International 300 N. ZEEB RD., ANN ARBOR, Ml 48106
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLEASE NOTE:
In all cases this material has been filmed in the best possible way from the available copy. Problems encountered with this document have been identified here with a check mark V
1. Glossy photographs or pages.
2. Colored illustrations, paper or print
3. Photographs with dark background ^
4. Illustrations are poor copy.
5. Pages with black marks, not original copy.
6. Print shows through as there is text on both sides of page.
7. Indistinct, broken or small print on several pages______
8. Print exceeds margin requirements_____
9. Tightly bound copy with print lost in spine______
10. Computer printout pages with indistinct print.
11. Page(s)______lacking when material received, and not available from school or author.
12. Page(s)______seem to be missing in numbering only as text follows.
13. Two pages numbered______. Text follows.
14. Curling and wrinkled pages _
15. Other ______
University Microfilms International
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS
ACKNOWLEDGEMENTS ...... 1i
LIST OF FIGURES ...... v
INTRODUCTION ...... 1 Geologic Setting ...... 1
Stratigraphic Nomenclature ...... 3
METHODOLOGY ...... ! ...... 7
Field and Laboratory Methods ...... 7 Terminology...... 7
STRATIGRAPHY ...... 14
General Statement ...... 14
Description of Stratigraphic Units 17
Belfast Member ...... 17
LowerMassively.Bedded B ra s s fie ld ...... 20
Thin Bedded Unit ...... 24 Upper Massive Unit ...... 24
Upper Thinly Bedded Brassfield ...... 24 DEPOSITIONAL ENVIRONMENTS ...... 30
Introduction ...... 3 0
Tectonic Setting and Regional Paleogeography ...... 3 0
Stratigraphic Units and Facies Interpretations ...... 32
Supratidal to In te rtid a l and Lagoonal Facies . 32 Subtidal Shoal Facies ...... 37
Subtidal Open Marine Facies ...... 38
i i i
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Summary of Depositional Environments ...... 39
DOLOMITIZATION • • ...... • • • • . . . . * • • 41 Introduction ...... 41 Hypersaline Models of Dolom itization...... 42
Nonhypersaline Models of Dolomitization ...... 4 5
Brassfield Dolomite ...... 48
Distribution ...... 48
Petrography ...... 54 Suggested Origin o f Dolomite ...... 56 Timing o f Dolomitization ...... 65
SUMMARY AND CONCLUSIONS ...... 68
BIBLIOGRAPHY ...... 69
' ' 1v ' 1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGUBES
Figure Page
1. Brassfield outcrop in the Cincinnati Arch area . . . . . 2
2. Regional d is trib u tio n of Lower S iluria n rocks in the Cincinnati Arch area ...... 4
3. Stratigraphic nomenclature fo r the Brassfield Formation in the Ohio Valley area ...... 6
4. Study area and location o f sample sections ...... 8
5. Textural classification of carbonate rocks ...... 10
6. Photomicrograph showing p a rtia l dolom itization ..... 11
7. Photomicrograph showing strong d o lo m itiz a tio n . . . . . 11
8. Photomicrograph showing strongly dolomitized rock with only relict original texture preserved ...... 12
9.. Generalized stratigraphic column 15
10. Belfast Member exposed at the West Union section.... 18
11. Paraconformable contact between the Drakes Formation and the Brassfield at the West Union section ...... 18
12. Lower massively bedded Brassfield a t the Manchester section ...... 19
13. Bioturbation of lower massively bedded Brassfield . . . 19
14. Thin bedded u n it exposed at the West Union section . . .2 1
15. Close-Up o f figure 14 ...... 22 16. Chert replacing a burrow in the th in bedded u n it .... 23
17. Upper massive u n it exposed a t the West Union section ...... 25
18. Upper th in ly bedded Brassfield exposed at the West Union section ...... 26
19. Megaripple in the upper thinly bedded Brassfield . . . . 27
v
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I '
J
Figure Pa9e 20. Surface o f the bead bed ...... 29
21. Lateral distribution of Brassfield depositional environments ...... 33
22. Gypsum casts in the Belfast Member . . . . . « . . . . . 34
23. Laminae and b ird 's eye structures in the Belfast Member . 34
24. C lassification o f dolomite types, environments and mechanisms o f formation ...... 43 25. Distribution of Brassfield dolomite in the Cincinnati Arch area ...... 49
26. Photomicrograph showing the dolom itization o f fin e grained matrix 51
27. Dolomitization o f burrow fillin g s and geopetal mud . . . 51
28. Photomicrograph showing the dolom itization o f the Belfast Member ...... 53
29. Photomicrograph showing preferential dolom itization along s ty o lite s ...... 53
30. Photomicrograph showing fine-grained anhedral dolomite . 55
31. Photomicrograph showing s o lita ry rhombs of euhedral dolomite ...... 55
32. Photomicrograph showing loosely intergrown euhedral dolomite ...... 57
33. Sabkha model fo r dolom itization ...... 59
34. Mixing zone model for dolomitization ...... 62
vi
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION
The principal aim of this study was to investigate the occur-
ance, and propose a process fo r the genesis o f dolomite in the Brassfield Formation (Lower S iluria n) o f Adams County, Ohio. The
Brassfield rocks are a series of interbedded shales, limestones and
dolostones which outcrop in the Tri-S tate area of Ohio, Indiana
and Kentucky (Fig. 1). They are unique in the study area because
the rocks are not uniformly dolomitized. This irregularity in the
dolomite distribution is a major concern.
The enigmatic nature of ancient dolomite formation has been
considerably discussed within the geologic community in recent
years, and several models concerning the environments o f dolom iti
zation have been advanced; fo r instance, seepage refluxion (Adams
and Rhodes, 1960), evaporative pumping (Hsu and Siegenthaler,
1969) and mixed water (Land and Epstein, 1970). The dolom itizing
mechanism o f the Brassfield Formation w ill be considered in view
o f the existing models.
Geologic Setting
The shales, limestones and dolostones of the Brassfield For
mation form an arcuate outcrop belt on the flanks of the Cin cinnati Arch and overlie Upper Ordovician rocks in a regional dis-
conformity. The Brassfield is thinner and less shaley on the
western side of the arch than on the eastern side, and the section
. 1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2
WAYNE- MONTGOMERY V . ’-PREB^E'-.
GREENE./-.I*.
FRANKLIN
CINCINNATI • w • • HIGHLAN %
ADAMS/.; JENNINGS; SWIT ZERLAND OHIO
KY ^ e iv tu c^
ESSF » S BRASSFIELD aflem in g OUTCROP
sv MONTGOMERY
Vv-ESTILL
BOYLE ---MARIO 0 5 10 20 30 40
Fig. 1- B Lssfield outcrop in the C incinnati Arch area. M,*-ES
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3
thins southwestward from the study area to a pinchout near Stan
ford, Kentucky.
The Brassfield rocks were deposited in an epeiric sea during
the Early to Middle Llandoverian (Berry and Boucot, 1970), (Fig.
2). Carbonate deposition at th is time was widespread and rela
tively continuous although local evidence for short periods of
subareal exposure is present on the eastern side o f the arch.
According to recent reconstructions of paleogeographic positions,
the study area was situated between 10-20° south latitude (Dott
and Batten, 1976; Irving, 1979), within a climate and setting
which exists in many modern carbonate producing areas, such as
the Gulf o f Mexico and the Carribean. Islands in the Appalachian
belt known as'Taconian land shed sand and fine mud that was carried
westward influencing Brassfield sedimentation.in Ohio and Kentucky.
Local tectonism in the vicinity of the Cincinnati Arch was subtle
although gentle uplifts may have influenced Brassfield deposition
and diagenesis (McDowell and Peterson, 1980).
Stratigraphic Nomenclature
August Foerste (1885, 1896, 1904) was the pioneer worker
who form ally named the Brassfield Limestone fo r rocks exposed at
the type section in Madison County, Kentucky. Previously these
rocks had been miscorrelated with the Clinton group (Middle S il
urian) o f New York by Orton (1890). Foerste recognized that
although the Ohio "Clinton" represented similar facies, it was not
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. So. Ohio Eastern Southern W estern system series Adams, Co. Kentucky Indiana Tennessee
PEEBLES FM. ci .5 LAUREL LAUREL IS LS. LS. o o LILLEY "E FM. » -r i - w 0)i % BISHER FM. BISHER FM. OSGOOD OSGOOD CRAB CRAB FM. FM. o ORCHARD ORCHARD Z Q. ; < a FM. FM. * • ^ WiM cc D .ItlStll _l BRASSFIELD FM. W c CO BRASSFIELD BRASSFIELD * •aw FM. FM. o BRASSFIELD > Middle FM. o r '■o ■i * ; e □ Ik BELFAST BELFAST k MEM> MEM. M Lower ^Spf
c■ RICHMOND RICHMOND RICHMOND RICHMOND ■aai * > GROUP GROUP GROUP GROUP a>• c■ ■ac C)• ■aE C.> ORDOVICIAN
Fig. 2- Generalized regional distribution of Lower Silurian rocks in the Cincinnati Arch area (modified from Berry and Boucot, 1970).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5
time equivalent with the New York Clinton. Above the Brassfield
Limestone, Foerste combined two formations, the Indian Fields and
Alger Formations, to form the Crab Orchard Group.
The term Brassfield Limestone was used as o rig in a lly proposed
u n til Rexroad, et a i- (1965) made several changes in the c la s s ifi
cation (Fig. 3). Additionally O'Donnell (1967) further amended the
Brassfield Formation to include four general lithologic units in
cluding the Noland Member. This study recognizes three of O'Donnell's proposed units;
1. Belfast Member, 2. lower massively bedded Brassfield, 3.
upper thinly bedded Brassfield. In Adams County additional thin
bedded and massively bedded units can be found between O'Donnell's
"lower massively bedded Brassfield" and "upper thinly bedded
B rassfield". Since these units may not be widely persistant or
recognizable, they w ill be treated informally within the scope
of this study (Fig. 3).
The Noland Member, designated as a formation by Rexroad, et
a l. (1965), is d ifficu lt to trace or correlate north of Bath County,
Kentucky and is not recognized in Ohio (Harrison, personal comm.).
Furthermore, the jo in t U.S. Geological Survey and Kentucky Geolog
ica l Survey did not include the Noland in th e ir mapping program
(Peck, 1967). Therefore, the Noland is disregarded by this study
in favor o f the Crab Orchard Formation fo r rocks which overlie
the Brassfield Formation.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a* u n it u n it Lower Lower Upper Upper Lower Lower Upper Upper Belfast Plum Plum Creek M br./unit Brassfield Brassfield th in ly bedded th in ly bedded massively bedded massively bedded THIS THIS STUDY ■n m r~ CO t—1 00 ■5. CRAB Fm. ORCHARD Lower Upper Upper Noland Belfast massive Brassfield Brassfield M br./unit th in ly bedded WT o'l T " T l 2 C/) CO t—4 m r- 00 DAYTON • Mbr. Fm. Belfast Plum Creek Plum ~n CO CO »—i m r- REXRQjygjil. . CO Fm. NOLAND Mbr. • Plum Plum Creek m ■n 30 CO r~ CO ►—i m 03 • Fm. INDIAN FIELDS Fig. 3- Stratigraphic nomenclature for the Brassfield Fm. in the Ohio valley area.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. METHODOLOGY
Field and Laboratory Methods
Three outcrop sections o f the Brassfield Formation were
measured and described in detail from Adams County, Ohio on the
eastern flank o f the Cincinnati Arch (Fig. 4). Several additional
outcrops were examined and these observations were used to supple
ment data from the three major sections. Sections were measured
with a jacob's staff and a Stratex surveying micro-altimeter was
used to crosscheck elevations. Samples were collected from every
major carbonate bed, while shales were described in situ. One
hundred polished slabs were made and representative thin sections
from each measured section were examined. Both slabs and thin
sections were etched and stained with Alizarin Red S (Dickson,
1965) to more easily distinguish calcite from dolomite. Selected
slabs were stained with a solution of potassium ferricyanide (Dick
son, 1965) so the type and amount of ferroan dolomite could be
assessed. A modified Wentworth grain-size scale (Folk, 1962) was
used fo r a ll grain measurements.
Terminology
Dolomitic rocks of the Brassfield Formation retain much of
their original depositional texture despite diagenesis. This is
particularly true in Ohio where the rocks are not as strongly al
tered as they are in Kentucky. Textural properties and composit
ional data are prerequisites for interpretation of depositional
7
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8
ADAMS COUNTY OHIO
Belfast section
Ohio
PEEBLES
Ohio 247
** West Union section
WEST UNION Ohio 125 Ohio
C* Manchester section Ohio
flhio jy
______. — measured-sampled section
mi^es ° * observed section Fig. 4- Study area and location of sample sections.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. environments,- therefore, the terminology o f Dunham (1962) is used
in this study (Fig. 5). Following the suggestion of Gordon (1980),
Dunham's terms w ill be followed by a Roman numeral which indicates
the corresponding "Energy Index" as proposed by Plumley e t a l.
(1962) and modified by Catalov (1972), (Fig. 5). The energy index
provides a genetic classification based on the energy that existed
in the depositional environment. Textural data of important con
sideration are size, sorting, roundness of the allochems, and the
degree of winnowing. The four textural levels of the energy in
dex are related to the degree of water agitation.
The combination of Dunham's textural classification and the
energy index concept provide a more detailed description o f the
physical environment o f deposition. For example, packstone ( I I I )
indicates the original sediment was grain-supported, and deposited
in moderately agitated water.
Where do!omitic rocks are discussed Dunham's terminology
(along with the corresponding energy index) w ill be prefixed by
terms that indicate the degree of dolomitic alteration. Powers (1962) offered the terms, "partially dolomitized" for rocks which
display 10-75% discrete dolomite rhombs and "strongly dolomitized"
where 25-75% of the rock is interlocking dolomite (Figs. 6 and 7).
Dolomite with "re lic o riginal texture" is more than 75% in te rlo ck
ing dolomite where original texture is s till recognizable (Fig.
8). Usually only ghosts o f original texture remain. "C rystalline
dolomite" has no recognizable original texture and consequently
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CRYSTALLINE DOLOMITE
o ri gi nal texture o b ii terated
DOLOMITE WITH RELIC ORIGINAL TEXTURE
more than 75% interlocking dolomite
STRONGLY DOLOMITIZED
25-75% interlocking dolomite
PARTIALLY DOLOMITIZED
10-75% discrete dolomite rhombs
LjJ CC • a UJ UJ 3 z CO UJ CO X ■"—» z - a UJ E o c z c U l CM o 3 z H- *1 11 o • r — I— CO 1— E o CO t o (O 1— (O t o 1—’ c UJ S- t o S_ — 1 «—I • z CO t o • r — X c n a O l eC «. »—1 JC X rO o = 3 a o +-» «=c OS n>E § ro C o o *—i JZ •COCL o r-H H 1— C a A V to»-* o 3 O '— a . grain-supported mud-supported
■ cn ■ ( 9 --- e c
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 6- Photomicrograph o f sample WU- 24 showing p a rtia l dolo- . mitization of matrix (x 11.2).
Fig. 7- Photomicrograph o f sample WU- 12 displaying strong dolomitic alteration (x 11.2).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 8- Photomicrograph o f sample WU- 5 showing strong dolomiti- zation. Note the relict original texture preserved By the dark laminae (x 11.2).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 13
cannot be cla s sifie d on the basis o f depositional fa b ric although
some generalizations can usually be made based on crystal size or
relict structures.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STRATIGRAPHY
General Statement
The Brassfield Formation as defined by th is study, in Adams
County, Ohio consists of fiv e lith o lo g ic un its, only one o f which
is formally recognized as a member (Figure 3). The Brassfield
Formation at the West Union location is the thickest exposure of
Brassfield strata (16 meters) in the entire outcrop belt. South-
westward along strike, the rocks thin to a point near Stanford,
Kentucky where the formation is truncated by pre-Devonian erosion.
To the north near Dayton, Ohio and on the western side of the
Cincinnati Arch in Indiana, the formation thins to average thick
nesses of 10 meters and 3 meters, respectively. Consequently,
in most sections outside Adams County some of the component stratigraphic units recognized in this study are missing or un
recognizable. For example, the Belfast Member attains its maximum
thickness at the West Union section and pinches out near the Ohio
River at the Manchester section. At the Manchester section the
lower massive Brassfield directly overlies Ordovician rocks. There
fore, the composite stratigraphic section (Fig. 9) illu s tra tin g
Brassfield stratigraphy in the study area cannot be s tr ic tly
applied to other Brassfield outcrops outside southwestern Ohio.
In general, the Brassfield is disconformable with the under
lying Cincinnatian Formations of the Richmond Group in the Ohio
valley region. Contacts are sharp and well-defined. However, in
14
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. /
15
i
AGE ONLAP-OFFLAP TEXTURE FM. METERS
packstone —■
— ID m
— 5
MckMtone SSJS
■udstone L- 0
Fig. 9- Generalized stratigraphic column of the Brassfield Formation in southwestern Ohio.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16
Adams County, notably at the West Union and Belfast sections of
this study, the contact hetween the Ordovician Preachersville
Member of the Drakes Formation and the Belfast Member o f the Brass
fie ld Formation is more subtle. The shales and siltstones o f the
two formations were deposited in sim ilar environments without any
pronounced physical evidence fo r a break in sedimentation. This
apparent paraconformity and the evidence about water depth i t
provides has been discussed by Gray and Boucot (1972). Data based
on acritarchs and spore tetrads indicates water depths no deeper
than in te rtid a l, or possibly even nonmarine environments at the
top of the Ordovician and in the lower portion of the Belfast.
Water depth increases higher into the Belfast. Depth zonation
for higher Brassfield beds which contain megafossils can be a-
ssigned by c rite ria proposed by Ziegler (1965) and Ziegler and
Boucot (1970). In the study area the top of the Brassfield is defined by a
prominant, biostratigraphic marker bed, in the upper shales of
the upper th in ly bedded Brassfield. The "bead bed", as i t is
commonly known (Rexroad et al_., 1965), is a distinct megarippled
bed containing large crinoid columnals. Geographically the beds are
found on the east side o f the Cincinnati Arch from Dayton, Ohio to
Birmingham, Alabama (O'Donnell, 1967). The contact w ith the Brass
field and the overlying limestones and shales of the Crab Orchard
Formation is gradational.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17 Description of Stratigraphic Units
Belfast Member
The Belfast Member consists of a series of thinly-bedded
dolomitic, light gray siltstones and shales (Fig. 10). It is 1-2
meters th ick and rests paraconformably on Ordovician shales (Gray
and Boucot, 1972). There is a slight lithologic and color change
marking the contact (Fig. 11). Si Itstone beds are fla t or slightly
undulate and are usually finely (less than 1cm) laminated. The
laminations have a thin (organic?) film separating each pair.
Quartz s ilt is common in some beds. A distinctive 4-8 cm thick
siltstone has flute or load casts on its base which trend to the
northwest. Since th is bed was only observed at two nearby locat
ions, it must have been a very localized pulse of energy. Fossils
other than very sparse vertical burrows are rare. Petrographically
the dolomitized Belfast Member displays only re lic t original texture
but represents former argillaceous mudstones (I).
Lower Massively Bedded Brassfield
The lower massive Brassfield is a widely persistant, blocky,
re la tiv e ly homogeneous, ledge-former above the Belfast Member
(Fig. 12). I t sometimes appears as a single massive bed 1.5-2
meters th ic k , although near the top o f the u n it very thin shale partings separate irregular bedding surfaces. The unit is ex
tremely bioturbated (Fig. 13). Megafossils, including
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18
Fig. 10- Interbedded shales and siltstones o f the upper Belfast Member exposed at the West Union section. Note the gently undulating bedding.
Fig. 11- Paraconformable contact between the Ordovician Drakes Fm. and the Belfast Member of the Brassfield Fm. Hammer pick rests on the contact.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19
Fig. 12- Lower massively bedded Brassfield at the Manchester section. Hammer rests on sandy lens.
Fig. 13- Horizontal branching burrows characteristic of the lower massively bedded Brassfield.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20
disarticulated crinoid columns, and solitary rugose coral, occur
w ithin the shale partings. The contact between the lower massive
Brassfield and the thin bedded unit is gradational, but a zone
of glauconite and manganese oxide (black staining) is character istic of the uppermost lower massive Brassfield. Petrographically,
the strongly dolomitized lower massive Brassfield is also a form
er mudstone ( I) .
Thin Bedded Unit
The thin bedded unit ,is a sequence of alternating gray,
splintery shales and partially dolomitized limestone beds (2-10
cm th ic k ), which is approximately 1.5 meters in to ta l thickness
(Fig. 14). Many limestone beds appear lumpy or nodular due to
bioturbation by large organisms (Fig. 15). Other limestones are
finely laminated (less than 1 cm) and appear to be deposited by
interruptive bursts of higher energy. Fossils other than traces,
become more common and assemblages, more diverse upward. Crin-
oids, s o lita ry rugose cora l, brachiopods and bryozoa are p le n tifu l
in certain layers. Higher in the section discontinuous chert
stringers within the nodular beds may be related to burrowing
(Fig. 16). Petrographically these rocks are mainly wackestones
( I I ) and packstones ( I I I ) and are among the least dolomitized
rocks in the study sections.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 14- Thin bedded unit exposed at the West Union section.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 15- Close-up of a portion of the thin bedded unit at the West Union section showing lumpy or nodular bedding.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 16- Large burrow in the thin bedded unit pre fe re n tia lly replaced by chert.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24
Upper Massive Unit
The upper massive u n it is a d is tin c t chert-bearing u n it which
is 1.2-1.6 meters th ick in the study area (Fig. 17). Although
the unit appears massively-bedded, thin shale partings occur at
30 to 40 cm intervals. Bioturbation is very localized. Surfaces
o f fa lle n slabs appear nobby and mounds o f diverse fauna represent
isolated well-developed conmunities. Occasionally these mounds
are wholly or partially replaced by chert.
Generally, the white chert is globose, associated with glau
conite, and often richly fossiliferous. Some nodules contain
pyrite . O'Donnell (1967) found th a t, going from north to south,
the chert is found progressively lower in the section. The unit
is strongly dolomitized and probably represents former wackestones
( I I ) , packstones ( I I I ) and grainstones (IV ).
Upper Thinly Bedded Brassfield
The interbedded shales and dolomitic limestones above the.
upper massive u n it are known as the upper th in ly bedded Brass
fie ld (Fig. 18). Limestones rarely exceed thicknesses o f 30 cm.
Some display high-energy features including large-scale ripples
(wave-lengths reaching 100 cm) (Fig. 19), cross-bedding and scour
and f ill structures. Shale interbeds are much thicker (up to
100 cm) than shales in lower Brassfield units. Bioturbation is
restricted to a few muddy horizons. This unit contains the most
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 17- Upper massive unit exposed at the West Union section. Note the large globose masses o f white chert.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 18- Upper th in ly bedded Brassfield exposed at the West Union section. Note the thick shale beds and the megarippled horizon at the meter s tic k .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27
Fig. 19- Megarippled bed characteristic o f the upper upper th in ly bedded Brassfield. Wave-length is approximate ly 100 cm.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 28
fossiliferous strata, including the "bead bed", the conventional
marker fo r the top o f the Brassfield, which is a megarippled
horizon composed of a variety of quite large (0.5-2.5 cm in dia
meter) crinoid columnals (Fig. 20). Beds of hematite coated bio clasts and intraclasts occur near the top of the section and were
once mined locally as an iron ore. The limestones are partially
to strongly dolomitized. They represent wackestones ( I I ) through
grainstones (IV ), w ith packstones ( I I I ) being most predominant.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29
Fig. 20- Surface o f the megarippled "bead^bed" exposed at the West Union section. Note the variety of crinoid columnals.
*
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. )
DEPOSITIONAL ENVIRONMENTS
Introduction
Different lithostratigraphic units often represent different
environments o f deposition or facies. Each may undergo d iffe re n t
diagenetic histories producing different textures and mineralogies.
Original sedimentary texture is indicative o f the energy which ex
isted in the depositional environment while diagenetically altered
textures reflect post-depositional environments. Altered textures
of the Brassfield units show a relationship between original
sedimentary texture and the type and extent of dolomitization.
Therefore study of the Brassfield depositional environments pro
vides clues which help to clarify many of the complexities of
diagenetic alteration.
Tectonic Setting and Regional Paleoqeoqraphv
The Brassfield rocks were deposited in a shallow, northwest
wardly transgressing sea across a gently sloping, slightly irregular shelf. A world-wide Llandoverian (Early Silurian) transgression
due to melting of Late Ordovician-Early Silurian glaciers in the
southern hemisphere has been documented by Berry and Boucot (1973)
and McKerrow (1979). In addition, minor eustatic fluctuations in
sea level continued throughout the Silurian resulting in several
short pulses o f sea level change (Sheehan, 1975).
The platform on which the Brassfield was deposited in the
30
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ohio valley region was generally of low relief although locally
several high areas may have existed and influenced sedimentation.
Foerste (1904) and Rexroad (1967) discussed the Ripley Island
or Ripley positive area in southwestern Indiana where Brassfield
strata are missing apparently due to nondeposition. Gordon (1980)
also postulated a high area in east-central Kentucky where the
absence o f the B elfast Member may be evidence o f subareal exposure
during in itia l Brassfield deposition. The Cincinnati Arch must
have affected sedimentation although scattered outliers across the
crest suggest inundation o f th is area during at least part o f
Brassfield deposition (Foerste, 1906). Thinning of section west-
wardly across the arch and the absence of elastics on the western
flank suggest the arch must have acted as some sort of barrier to
sedimentation. In southwestern Ohio this influence is negligible,
but.in Kentucky, flexure of the insipiant arch had a pronounced
e ffe ct on Brassfield sedimentation (Gordon, 1980). Basal con
glomerates, strom atolites, mudcracks and the dominance o f high-
energy very near-shore environments suggest synsedimentary u p lift
south of Bath County, Kentucky which did not exist to the north
along depositional strike. This implies a shallow-water to deep-
water facies change, south to north, controlled by flexure of the
insipiant arch which may have sim ilarly affected Upper Ordovician
formations in the area (Weir & Peck, 1968). The facies change
along strike northeastward into Ohio. Individual units become
thicker, more shaley and less dominated by high-energy regimes.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32 Overall Brassfield sedimentation in the study area was continuous
in a steadily transgressing sea that was only slightly affected
by the gentle upwarping to the southwest.
S tratigraphic Units and Facies Interpretations
The vertical sequence of facies in the Brassfield represent
transgressing environments which migrated la te ra lly over each
other through time. In ascending order the B elfast Member, lower
massively bedded B rassfield, thin bedded u n it, upper massive unit
and upper thinly bedded Brassfield display depositional textures,
structures and fo s s ils that suggest these units were deposited
as three major facies representing deepening marine environments.
These facies are 1.) supratidal to intertidal and lagoonal, 2.)
subtidal shoal and, 3.) subtidal open marine (Fig. 21).
Supratidal to In te rtid a l and Lagoonal Facies
The laminated, non-fossiliferous mudstones (I) of the Bel
fast Member are indicative of deposition in a very quiet low-en
ergy environment (Fig. 21). In fa ct the presence of gypsum casts
(Fig. 22) in the basal Belfast combined with evidence of land-
derived spore tetrads (Gray and Boucot, 1972) suggests an in itia l
phase of supratidal conditions. The supratidal environment was
however, short-lived as the bulk of the Belfast rocks represent in
tertidal to very shallow subtidal environments. Do!omitic mudstones
(I) with brown algal (?) films between thin (0.2 cm) laminae and
bird's eye structures are interpreted to record middle to upper
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. >IE LAGOONAL Brassfield bedded unit unit bedded Brassfield ' , Belfast massive Lower Lower th in ly Upper massively Upper th in ly bedded MUDSTONE MUDSTONE MUDSTONE PACKSTONE WACKESTONE LAMINATED BURROWED WACKESTONE GRAINSTONE STONE PACK SUPRATIDAL SUPRATIDAL INTERTIDAL SHOAL SUBTIDAL Fig. 21- Lateral distribution of Brassfield facies equivalents.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34
Fig. 22- Gypsum casts preserved in the lower Belfast Member
Fig. 23- Laminae (A) and bird 's eye structures (B) in the lower Belfast Member.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 in te rtid a l environments where algal mats developed on carbonate
muds (Fig. 23). Although no desiccation cracks were observed in
the fie ld some irre g u la rly shaped b ird 's eye structures, obvious
in polished slabs, may be internal or "healed mudcracks". These
are believed to originate from internal shrinkage of the muds
resulting from their dehydration during subareal exposure (Laporte,
1967). Often fine-grained layers in the lower Brassfield mudstones
are selectively dolomitized. This preferential dolomitization
of fine-grained laminae may be the result of evaporation at the
algal mat surface bringing magnesium-rich sea water upward through
the sediment by ca p illa ry action. This process suggested by Shinn
et al_. (1965), results in dolomite being precipitated at the mat
surface as a fine-grained layer. This in itia l stage of dolomiti
zation produced d is tin c t dolomitic laminae in the algal sediments
of the lower Brassfield and supports the shallow intertidal in
terpretation. Oscillatory ripple marks and increased infaunal activity
characterize the lower intertidal zone and marks the gradational
contact between the Belfast Member and lower massively bedded
Brassfield. The lower massively bedded Brassfield represents
lower intertidal to shallow subtidal facies (Fig. 21). It is
intensely burrowed and bioturbated (Fig. 13). Murray (1975) in
his study of trace fossils in the Brassfield attributed the pro
lific browsing and burrowing activity in this unit to deposit
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36 feeding worms: These organisms e xp lo it n u trie n t-ric h layers.
Their horizontal, branching burrows form tie rs in the sediment
and consequently erase any additional intertidal imprint.
Occasionally the mottled mudstones (I) of the lower massively
bedded Brassfield are interrupted by liminated, grainy, lensoid
beds (not more than 25 cm th ic k ). These beds may represent in te r
tid a l channels which form wedges o f sandy sediment brought onto
the in te rtid a l zone by storms rather than tides (James, 1979).
The re su lt is a layer o f coarser, sandy debris le f t behind as a lag
deposit. Small (1-2 cm) s o lita ry rugose coral, scattered crinoid col-
umnals and higher-energy rock textures suggest subtidal environ
ments in the upper lower massively bedded Brassfield. The unit
grades into wackestone ( I I ) and packstones ( I I I ) o f the thin
bedded un it.
Since the intertidal rocks of the lower Brassfield exemplify
a muddy low-energy system, i t follows that deposition must have
occurred in a protected location; that is protected from waves and
swells which would produce a high-energy beach zone. A protective
ba rrie r such as a shoal (Fig. 21) dissipates wave energy and in
the lee of this barrier, provides a protected environment, a lagoon.
The Belfast and lower massively bedded Brassfield were deposited in
a muddy intertidal to logoonal environment protected from wave
processes by a seaward shoal. The lim ited areal extent o f the units
in southwestern Ohio and northern east-central Kentucky also suggests
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 37
a restricted depositional environment. Supratidal flats were
not extensive and were only developed lo c a lly in southwestern Ohio.
Subtidal Shoal Facies
The depositional textures and sedimentary structures of the
upper massive unit are suggestive of a high-energy or shoaling
environment above wave base. The packstones ( I I I ) and grain-
stones (IV) of this unit are highly fossiliferous. In fact, mounds
of diverse invertebrate fauna often in life position which are dom
inated by crinoids, s o lita ry rugose coral, and associated bryozoan
assemblages may represent bioherms which could flo u ris h in the
agitated environment. Wilson (1975) discussed the baffling action
produced by thickets of crinoids. The organic activity usually
begins below wave base. Binding, trapping and encrusting o f
organic debris and sediment by these organisms eventually builds
mounds which are colonized by successive generations of crinoids
and bryozoans. Continued b a fflin g and building results in a mound
which intersects wave base and dissipates open ocean energy. The
large masses of chert which characterize this unit seem to be a
replacement type preferentially altering all or portions of these
fossiliferous buildups. Sheltered areas between the mounds, and
sediment deposited on the lee side of the shoal (represented by
rocks o f the th in bedded u n it) are muddier wackestones ( I I ) . In-
faunal activity was minimal. A few large burrows, probably pro
duced by trilobites or other arthropods (Murray, 1975), penetrated
the muddier sediments o f the back shoal.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38 These rocks are interrupted by several stratified grainstone
(IV) beds (10 cm th ic k ). The beds are often graded and contain
well-rounded quartz s ilt. The sedimentologic character of these
rocks suggests rapid deposition, probably during storms.
The th in bedded u n it and upper massive u n it represent a shoal
ing bank which was dissected with channels o f wel1-winnowed
sketeletal sands deposited during storms. The fossil communities
which thrived on these banks were necessarily capable of withstand
ing high-energy conditions.
Subtidal Open Marine Facies
A change in regime is indicated where rocks o f the upper massive
unit grade in to the upper th in ly bedded Brassfield. Interbedded
dolomitic limestone beds are separated by th ick (up to 100 cm)
fis s ile shale sequences. The shales indicate a return to quieter
energy conditions. Original wackestone ( I I ) and packstone ( I I I )
textures suggest that this environment was only occasionally agi
tated. Sedimentary structures such as crossbedding, graded bedding
and scours are best developed in the lower portion o f the u n it. Thicknesses o f shale interbeds and the abundance o f burrowers
greatly increase toward the top. This suggests a gradual deepening
of the Brassfield subtidal environment resulting in a transition
from shallow to deeper open marine facies (Fig. 21). Fossil
d ive rs ity is moderate to high and again, crinoids dominate. Sev
eral megarippled horizons contain fossil allochems which are well-
rounded and abraded. These beds may represent transported and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39 mixed assemblages which were deposited as major storms lowered
wave base and disrupted the quieter energy conditions which pre
vailed on the shelf. The fossiliferous "bead bed" is a megarippled
packstone ( I I I ) formed during such a high-energy event. The "bead
bed" interrupts a thick shale sequence in the uppermost Brassfield
that was deposited as the Brassfield sea reached its maximum level
of inundation.
The depositional environment represented by the upper thinly
bedded Brassfield was a steadily deepening, open marine, shallow
shelf environment where quiet energy conditions were predominant
as water depth increased.
Summary o f Depositional Environments
The Brassfield Formation represents a transgressive sequence
of carbonate rocks. Facies existed contemporaneously and migrated
northwestward across the Ohio valley region (for a detailed des
c rip tio n of depositional environments in east-central Kentucky,
see Gordon, 1980). In southwestern Ohio small islands o f low
r e lie f, lying ju s t above normal high tid e level were covered by
algal mats. Prim itive vascular plants may have colonized these
supratidal fla ts as algal growth stablized the sediment. The
supratidal zone sloped gently, eventually becoming alternately
inundated and drained by tides. Browsing and burrowing organisms
exploited the rich organic layers of the intertidal flats. Tidal
waters flooding these fla ts cut channels or creeks which during
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40 storms would deposit coarser-grained sediment, as a lag deposit
on the fine-grained muds. The adjacent subtidal (lagoonal) areas
were sites of slow, low-energy deposition suggesting the existance
of a barrier shoal. The lagoon which existed in the lee o f th is
barrier was protected from the fu ll force of ocean waves and
swells. The shoal was formed by a belt of crinoid gardens, sup
porting a diverse fauna which could thrive in the high-energy en
vironment. The crinoid bars were not continuous, but were breached
by channels tha t may have migrated la te ra lly through time. The
shelf on which these bars existed gradually deepened seaward, and
quiet, low-energy conditions were established below normal wave-
base. Deposition of thick shale sequences was periodically in
terrupted by storms which lowered wave base and deposited mega
rippled bodies o f abraded skeleltal sands. When the storms passed
quiet, deeper water sedimentation resumed.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DOLOMITIZATION
Introduction
Dolomite is a complex carbonate mineral which seems to form
under a variety of natural conditions, where the dolomitizing
solution is oversaturated with respect to dolomite. Many chemical environments should be conducive fo r dolomite formation. However,
no one has yet synthesized dolomite under temperature and pressure
conditions typical of modern sedimentary environments. Synthesis
of dolomite at elevated temperatures (greater than 100°C) has been
accomplished and is re la tiv e ly uncomplicated (Gaines, 1980). The
most s ig n ific a n t advances made on the problem of dolomite genesis
have come from the studies of natural modern dolomites and their
ancient analogs.
It is generally agreed that most dolomite is a replacement
product. This "secondary" dolomite is formed during early or late
stages o f diagenesis by replacement of some calcium carbonate
sediment precusor. The small amount o f dolomite considered "p ri
mary" is formed by direct precipitation usually in hypersaline
lakes. Knowledge of the depositional setting is useful in dis -
tinguishing primary and secondary dolomites as well as the timing
of the secondary event. Evidence fo r the m ultiple origins of large bodies o f dolo-
mitized rock is extensive and several models have been proposed to
account for the variety of geologic settings and formative processes
41
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42 which y ie ld dolomite (.Fig. 24). Discovery o f "Recent" dolomite
in the Persian Gulf ■ tillin g 'e t aL., 1965) and on Andros Island
(Shinn et al_., 1965) helped reconcile the discrepancy between the
large amounts of Proterozoic and lower Paleozoic dolomite and the
lack of any significant amounts of late Cenozoic dolomite. In
addition, the Holocene discoveries helped fuel the popular concept
of the necessity of hypersaline solutions for dolomitizing processes.
Hypersaline Models of Dolomitization
Dolomite formed by brines with a high Mg/Ca ratio are associated
with evaporite deposits. Original evaporite minerals may be pre
served, but often these minerals are removed or obscured by later
dissolution or diagenetic change. However, evidence of vanished
evaporitic minerals has been found and has been discussed in
d e tail by Friedman (1980).
Two methods for the formation of dolomite by hypersaline brines
have been favored in the literature. Both involve the transpor
tation of brines through carbonate sediment. Adams and Rhodes
(1960) proposed the seepage refluxion model in which reflu x of brines through the sediment by gravitational forces caused dolom itization
along porous zones where the brines seeped through the underlying
sediment. Development of a restricted lagoon or tidal fla t where
evaporation concentrated the brine and caused evaporites to form
is a prerequisite fo r the model. Sears and Lucia (1980) have also
suggested a refluxing mechanism fo r one phase o f dolom itization
in the pinnacle reefs of Northern Michigan. The seepage reflux
model has also been applied by other authors.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. metamorphlc Late (teleogenetlc) r i 1 hydrothermal b u ria l SECONDARY Intermediate (nesogenetlc) re flu x seepage E a rly organic dolom ite (eogenetlc) zone m ixing QNITIZATION j DOL (hu m id). coroonicoroong PENECONTEHPORANEOUS (a rid ) sabkha lakes PRIMARY hypersaline Fig. 24- Classification of dolomite types, environments of and mechanisms formation.
with permission of the copyright owner. Further reproduction prohibited without permission.
o CD Q.
- 5 O Q. C CD ■ o 44
Evaporation at the sediment-air interface in arid supratidal
zones concentrates pore fluids into hypersaline brines. These
fluids subsequently rise from the shallow water table by capillary
forces causing the precipitation of gypsum (and other evaporites)
and the replacement o f aragonitic sediment by dolomite. The
"sabkha" or evaporative pumping model has been suggested fo r the
penecontemporaneous, modern dolomites o f the Persian Gulf region
and has been investigated by many workers ( Illin g e t a]_., 1965;
Kinsman, 1969; McKenzie, et al_. 1980). A sabkha is a s a lt f la t ,
and the broad supratidal flats of the Persian Gulf are areas of
arid, evaporitic conditions where periodic marine flooding augments
the water supply lost to evaporation. In contrast, the Coorong
district of southern Australia is a humid region where evaporitic
dolomite is formed in ephemeral, alkaline lakes situated behind
a modern coastal dune barrier or on the inner portion of a broad
coastal plain (von der Borch, 1965; Muir et al_., 1980).
The Coorong model o f dolom itization is an example o f penecon
temporaneous evaporitic dolomitization in a humid climate rather
than an arid sabkha environment and expands the premise preferred
by Friedman (1980) tha t most dolomites form under conditions of
hypersalinity. Although it is clear that many dolomite formations
in the recent and ancient geologic record owe their origin to re
action of calcium carbonate sediment with hypersaline brines, this
model needs to be reconciled with widespread dolomitic facies
which represent shallow marine, subtidal environments and are in
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45 no way associated with evaporitic minerals or their traces. Al
though subtidal deposition does not necessarily preclude later
dolom itization by hypersaline waters, where there is no evidence
of later hypersaline conditions in the rock record other dolo
m itizing mechanisms must be considered.
Nonh.ypersaline Models o f Dolomitization
Studies by RunnelIs (1969), Land and Epstein (1970), Badio-
zamani (1973) and Folk and Land (1975) have shown that dolomite
can form in fresh or brackish waters with Mg/Ca ratios as low as
1:1 provided a slow rate of crystallization. This "dorag" or mixed
water model fo r dolomitization has been applied by Hanshaw et al_.,
(1971) fo r Florida and Yucatan, Badiozamani (1973) fo r the Ordo
vician of Wisconsin, Land (1973a) for the Pleistocene of Jamaica,
and others for nonhypersaline dolomites associated with shallow
epicontinental shelves or structural highs. The mechanism involves
the creation of a zone of brackish water saturated with dolomite
(undersaturated with calcite) due to mixing of fresh meteoric and
sea waters. Folk and Land (1975) suggest tha t dolomite can form
readily under these conditions, but as salinity and crystallization
rates increase the Mg/Ca ra tio at which dolomite can f ir s t form
rises u n til values o f 5:1 or 10:1 are reached in hypersaline
environments. Therefore, the lower the salinity, the easier dolo
mite forms, since the concentration o f competing ions is reduced.
Work on Andros Island by Gebelein et al., (1980) has verified
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46 the existance of disordered "protodolmite" in peritidal sediments
below palm hammocks that are underlain by fresh water lenses. The
dolom itic sediment does not s tr ic tly coincide with the fresh or
mixed water zones but may be related to burrows, desiccation cracks or organic material that allow the migration of dolomitizing
waters. Other work by Gebelein and Hoffman (1973) indicates that
dolomitic laminae in ancient carbonates may also be related to an
organic origin. Algal mats may act as a source of magnesium for
penecontemporaneous or secondary dolom itization along alg a l-rich
layers. This mechanism has been suggested fo r recent dolomitic
laminae in the Bahamas (Shinn et a l., 1965) and for ancient rocks
by Gebelein and Hoffman (1973).
Other aspects of dolomitization include cannibalization of
pre-existing Mg-rich sediment. This process produces small amounts
o f dolomite and has been reported in the Bahamas by Goodell and
Garman (1969).
Deep burial, late diagenetic dolomitization can occur on a
local scale as long as there is a source of magnesium-bearing
flu id s such as a shale basin (Mattes and Mountjoy, 1980). This
latter type of dolomite grades into hydrothermal, essentially meta-
morphic dolomite, often associated w ith base metal ore deposits
(Engle e ta ^ ., 1958).
It is clear from the preceeding discussion that dolomite is
known to form in a variety of environments and at varying stages
in the history of a sediment body. I t seems no one model or so
lu tio n can be applied universally to the many types and occurances
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of dolomite. Even w ithin the various models, sediment or fabric
selectivity can affect the degree of dolomitization.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Brassfield Dolomite
The carbonate rocks o f the Brassfield Formation are not uni
formly dolomitized. Dolomitization ranges from 100% near the southern-most exposure of the outcrop b e lt to less than 5% in
some beds of the northern portion (Fig. 25). Dolomitization is
most pronounced in east-central Kentucky. Rocks in the study area
are partially dolomitized and seem to represent a transition be
tween a dolomite suite to the south and a limestone suite to the
north. It is likely that the rocks in southwestern Ohio lie on
some sort o f boundary between where dolomitizing flu id s had a pro
found effect in Kentucky and where rocks were minimally or unaffect
ed in Ohio and Indiana.
Distribution
The distribution of dolomite in the Brassfield Formation may
be controlled by the physical chemical nature of the original
texture of the individual rock units or by the availability of dolomitizing solutions. In as much as the Brassfield rocks studied
in Adams Co., Ohio occur along the boundary between dolomitized and
undo!omitized rocks i t could be assumed that the sediment had lim ited
access to the dolomitizing waters. In addition preferential dolo
mitization of the Brassfield carbonates was apparently a fabric
selective process.
The most striking correlation between dolomitization and de-
positional fabric is the strong preference for dolomite to replace
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49
• >5
22
> 9 5 > 95
100 88
A.Shub Fig. 25- Median percentage o f dolomite (carbonate fra ctio n ) inMedian the Brassfield Fm. (modified from O'Donnell, 1967).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission rocks which were originally deposited as lime muds. Murray and
Lucia (1967) noted the su sce p tib ility o f fine-grained sediments
to preferential dolomitization in Mississippian rocks in Alberta,
Canada. They suggested a number o f in te ra ctive physical factors
to explain this selectivity. Such factors as permeability, and
particle size, reflective of the surface area available to react
with fluids, are important aspects. Since coarser-grained units
of the Brassfield must have had greater in titia l permeabilities
than the muddy sediments, early calcitic cementation or U n ific a
tion of the coarser-textured rocks must have occurred prior to, or
in greater amounts than, cementation o f the mudstones. Another
approach would be to consider the combined effects o f a larger
p a rtic le surface area and the presence o f a more soluble carbonate
in the muds. Recent carbonate muds are aragonitic which tends to
be more soluble than ca lcite . Murray and Lucia (1967) suggest
this solubility difference may be enough to cause dolomite select
iv ity . In addition to the strong dolomitization of mudstone beds,
fine-grained matrix in wackestones and packstones is also dolo
mitized, usually when the allochemical grains are not (Fig. 26).
Burrow-fi 11ings and geopetal muds are dolomitized as well (Fig.
27). Bioturbation may have aided dolomitization in two ways.
Infaunal organisms tend to increase the porosity o f the sediment
by th e ir a c tiv itie s and, the organic matter le f t behind is rich
in magnesium which could react with dolom itizing pore waters dur
ing diagenesis, much like the dolomitization of algal mats
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51
Fig. 26- Photomicrograph o f sample WU- 12 showing the preferential dolomitization of fine-grained matrix surrounding the mollusk fragment (x 11.2).
•*»*.?■*•.V
Fig. 27- Etched and stained slab (sample M-l) showing the preferen tia l dolom itization o f geopetal mud w ithin the rugose coral (A) and fine-grained burrow muds (B).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. described by Gebelein and Hoffman (1973). Dolomite which pre ferentially replaces organic-rich algal laminae in the intertidal facies of the Belfast Member has been previously mentioned.
Facies of the Brassfield which were deposited in low-energy, quiet regimes where abundant fine-grained sediments were deposited, are the most intensely dolomitized. This corresponds with the
Belfast Member, the lower massively bedded Brassfield and quiet water facies of the upper thinly bedded Brassfield (Fig. 28). In terparticle muds and burrow-fillings were preferentially dolomitiz ed in the coarser-textured, higher-energy environments of the thin bedded u n it, upper massive u n it and the upper th in ly bedded Brass fie ld . Organic a c tiv ity by infaunal organisms and blue-green algae aided this process by acting as additional sources of magnesium.
Another fabric selective aspect of Brassfield dolomitization involves the preferential replacement of fo s s il allochems that have high-magnesium hard parts. Dolomitization o f crinoid debris and mollusks is actually rather minor, but is worthwhile to note.
This type of dolomitization is most common in the higher-energy textures o f the wackestone ( I I ) , packstone ( I I I ) , and grainstone
(IV) facies where fo s s il grains are most abundant and dolomite is scarse. An association exists between dolomite and styolites and fractures in the Brassfield. Typically styolites are dolomitized
(Fig. 29). Alteration concentrated around these seams in relatively undolomitized rocks indicates that compaction and s ty o litiz a tio n occurred prior to dolomitization. The styolites acted as avenues
of the copyright owner. Further reproduction prohibited without permission. 53
Fig. 28- Photomicrograph of sample WU- 1 showing the fine-grained dolomite typical o f the Belfast Member,deposited in a low- energy environment. Note the dark algal laminae (x 3.2).
Fig. 29- Photomicrograph o f sample WU- 31 showing the preferential dolomitization of styolites (x 11.2).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54 for dolomitizing fluids and consequently selective alteration
occurred along these accesses.
To summarize, the most volum etrically important amounts of
dolomite in the Brassfield Formation are associated with carbonate
muds. Whether the fine-grained sediment comprises an entire u n it
or facies, or on a petrographic scale is only a bit grain sheltered
sediment, preferential dolomitization is favored. Biological
activity, usually related to the muddy facies, enhances the pro
cess.. Selective alteration of high-magnesium fossil grains and
dolomitization along styolites occurs to a lesser extent.
Petrography
Dolomite in the Brassfield Formation occurs as two dis
tin ct crystal sizes ranging from 0.01-0.04mm and 0.06-0.4mm in
long diagonal size approximating the intermediate axis.
The finer-grained dolomite is restricted to the siltstones * of the Belfast Member of the basal Brassfield (Fig. 30). This
group o f crystals has an average diameter between 0.01mm and 0.04mm.
The crystals are anhedral, irregularly-meshed and usually only
relict original textures are discernable. The centers of the crys
tals are cloudy, probably due to abundant inclusions.
The second group o f crystals is found throughout the Brass
field section. These crystals are relatively large, 0.06-0.4mm,
well-ordered, euhedral rhombs (Fig. 31). The centers appear clear
and limpid and a few crystals seem to be zoned. These coarser
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 30- Photomicrograph o f sample WU- 5 showing the fine-grained anhedral dolomite characteristic of the basal Brass.field produced in hypersaline brines (x 32).
Fig. 31- Photomicrograph o f sample WU- 41 showing s o lita ry rhombs of euhedral dolomite replacing allochems surrounded by blocky calcite cement (x 32).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. crystals partially to strongly replace the original carbonate sed
iment with original textures well-preserved. The crystals appear
as solitary rhombs (Fig. 31) or as loosely intergrown mosiacs
(Fig. 32) replacing calcitic cements, carbonate mud matrix or
fossils, sometimes selectively. For instance, grain she!ted muds
are sometimes preferentially replaced. ~
Staining with potassium ferricyanide reveals the abundance
of ferroan dolomite. The presence of ferroan dolomite is note
worthy, because as Choquette (1971) suggests, it is characteristic
of la te r epigenetic dolom itization. The source o f iron fo r Brass
field ferroan dolomites may be beds of iron-coated bioclasts with
in the Brassfield and the overlying Crab Orchard Formation, or
iron-rich clays.
Cementation o f the original Brassfield sediments occurred
prior to dolomitization. Equant calcite is the most common in
itia l pore fillin g cement and is indicative of freshwater phreatic diagenesis (Longman, 1980). Cement is d istrib u te d evenly around
the grains. Sometimes the grains are not in contact, suggesting
the former presence o f now recrystallized mud. The secondary
dolomite crystals partially or wholly replace portions of this
cement (Fig. 31).
Suggested Origin o f Dolomite
Evidence from depositional environments, petrography and
regional relationships of dolomitized rock in the Brassfield
suggests m ulti-stage dolom itization in southwestern Ohio.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 32- Photomicrograph o f sample WU- 27 showing loosely in te r grown mosiac euhedral dolomite (x 11.2).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58
The dolomite limited to basal Brassfield (Belfast Member) is
the typically fine-grained, dirty dolomite produced by hypersaline
brines. The crystal form is anhedral, irregularly-meshed, and
m icritic. Dolomite replaces 100% of the original carbonate sediment.
Analyses o f depositional environments suggests an in it ia l
phase of supratidal conditions at the onset of Brassfield deposit
ion. Small islands, probably lying just above normal high tide
level, existed on a very localized basis. Evidence of these supra
tidal flats is not known to exist outside the study area. The pres
ence o f gypsum casts suggests th is environment was suitable fo r the
formation of evaporites and was therefore hypersaline. Since the
gypsum casts are preserved, the environment must have also been
subject to protracted periods of drying. Dissolution of the gypsum
would lik e ly have occurred in a more humid setting.
The Brassfield supratidal environment must have been similar
to the classic sabhka o f the Persian Gulf and i t can be assumed
that dolom itization was a penecontemporaneous process not unlike
the modern sabkha (Fig. 33). Flood waters from storm tides in
filtrated the fla ts, as evaporation commenced the precipitation of
gypsum and anhydrite depleted the ground-water brine of Ca re
sulting in an enrichment of Mg. The Ca/Mg ratio of the residual
brine would be >7 allowing the formation of dolomite (McKenzie,
et al_, 1980). As the hydrologic cycle continued, dolomitization
could proceed as "evaporative pumping" of the ground-water artesians
brought fresh solutions upward into the evaporitic zone. This
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10 CJl Mg.> Mg.> 7 ). 1980 evaporation zone zone of dolomite formation water circulation patternwater table SABKHA lagoon storm recharge dolomitization. Diagram Illustrates the circulation pattern during times of evaporation and dolomite formation (modified from etMcKenzie al_., Fig. 33- Diagramatic representation of the sabkha or "evaporative pumping" model for
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60
process was enhanced by blue-green mat-forming algae that pro
vided an additional source of magnesium.
As transgression deepened the Brassfield seas the supratidal
environment required for sabkha-style dolomitization was replaced
by intertidal and fin a lly open marine subtidal environments.
Evaporitive dolomitization was halted as the bulk of the Brass
fie ld rocks were deposited. No other hypersaline environments ex
isted during deposition o f the Brassfield Formation or the over-
lying Crab Orchard Formation which could provide brines fo r fu r
ther dolomitic alteration. Additionally, the most strongly dolo
mitized portion of the Brassfield outcrop belt in east-central
Kentucky (Fig. 25), exhibits none of the restricted hypersaline
characteristics o f an evaporitic environment (Gordon, 1980). The
Brassfield paleoenvironments in.Kentucky were exclusively shallow
marine (Gordon, 1980). The intense dolom itization of these rocks
was due to their proximity to a structural high which was subareally
exposed following Silurian deposition.
Several authors have suggested that a major regression at the
onset o f the Devonian restricted marine deposition to a few basins
and along marginal mobile belts. Much of the continental interior
was exposed as a lowland for several m illion years (Dott and Batten,
1971). Evidence of this major Devonian unconformity exists in east-
central Kentucky where Silurian strata are truncated by Devonian
erosion. McDowell and Peterson (1980) suggest upwarping o f the
Cincinnati Arch, was initiated during the Early Silurian and be
came fu lly developed before Middle Devonian time.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 61
The exposure of a relatively high area along the axis of the
Cincinnati Arch would have caught meteoric water and provided a
site for the formation of a paleoaquifer in the Brassfield lime
stones. Underlying this fresh water lens, mixing with magnesium-
bearing marine pore water could produce a chemical environment
conducive for the formation of regionally extensive dolomite by
fresh-seawater mixing. Consequently, regional dolomitization of
the Brassfield rocks was probably a eogenetic secondary process
resulting from the admixture of marine interstitial waters and
meteoric water that occurred in a brackish mixing zone below an
exposed landmass (Fig. 34).
Dolomitization by fresh-seawater mixing has been suggested by
several authors. Badiozamani (1973) proposed the mixing zone model
for the dolomitization of Ordovician limestones in Wisconsin. Bad-
iozamani suggested tha t u p lif t o f the Wisconsin Arch provided a
structurally high, subareally exposed element that absorbed meteoric
water establishing a fresh water phreatic lens in a manner seemingly
analagous to the dolomitization of the Brassfield. Dolomitization
occurred where this fresh water lens intersected marine-saturated
carbonates (Fig. 34). The dolostone-limestone boundary followed
the lower margin o f the ground-wa te r lens. Where dolomi tic rocks
are present, mixing occurred, whereas limestones suggest mixing did
not occur. Any shifts in this boundary are due to sea-level fluct
uations caused by transgressions and regressions, or tectonic flex
ures. Dunham and Olson (1978, 1980) in th e ir studies of the mixed
water dolom itization o f the Hanson Creek Formation in Nevada point
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CTl rvj OCEAN I! n • • watir n I! a H v p• r * * p• r v H (modified et from Hanshaw al.., 1971). show generalizedshow flow paths. Dolomitization occurs in the brackish mixing zone Fig. 34- Diagramatic representation of the mixing zone process for dolomitization. Arrows
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63 out the importance of paleogeographic control in the formation of
dolomite. Like the Brassfield, the Hanson Creek Formation includes
dolomitized and undolomitized rocks. Portions of the formation
which were dolomitized had access to meteoric-derived groundwater
with in te rstitia l sea water acting as the magnesium source. The
sections that remained limestone were d ista l to the area of fresh
water recharge. The degree o f dolom itization o f the Brassfield Formation
reflects the proximity of the original carbonate sediment to a
zone o f fresh-seawater mixing. I t has been mentioned that the
most strongly dolomitized rocks occur in east-central Kentucky,
coincident with the axis o f the Cincinnati Arch and the area of
fresh water recharge. The Kentucky do!ostones grade into limestones
in the northern portion of the outcrop belt (Fig. 25). This trend
reflects the position of the paleoaquifer and the zone of mixing.
Apparently the mixing zone did not extend far into Ohio or Indiana
since these Brassfield rocks remain basically unaltered. The
partial dolomitization of the rocks in the study area may indicate
the lower margin of the ground-water lens where effective dolomiti
zation was limited.
Rock selectivity may have had a greater bearing on the extent
o f dolom itization in southwestern Ohio, where the dolomitizing
fluids were contaminated with excessive marine waters. Periodic
minor fluctuations in sea level may have aided the dolomitization
process along th is interface by lowering or extending the in flu
ence of the ground-water lens.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64 Dolomite crystals o f the Brassfield Formation not associated '
with hypersaline brines, are euhedral, loosely intergrown rhombs
usually larger than 0.1mm. The dolomite is typically limpid and
is sometimes zoned. The significance o f lim pid dolomite produced
at slow crystallization rates in dilute solutions has been dis
cussed by Folk and Land (1975). These well-ordered crystals are
the signatures of mixing zone dolomitization.
Trace elements such as strontium and sodium and stable isotope
composition also provide clues to the nature of dolomitizing waters.
Attention has been focused on the elements Sr and Na because Sr
is an important precusor of the original carbonate sediment and
Na, being the most abundant cation in marine water, should roughly
indicate the s a lin ity of the marine-derived solutions (Land, 1980).
Although laboratory analysis of the trace element and isotopic
geochemistry of the Brassfield dolomite was not possible in this
study, some general comments can be made. Sr and Na
substitute for Ca in carbonate minerals more readily in hypersaline
solutions or by rapid c ry s ta lliz a tio n . Consequently evaporitic
sabkha dolomite is enriched in Sr and Na relative to dolomite
formed from dilute fluids. However, this simple relationship is
complicated by several factors, including nonhomogeneous crysta lli
zation rates, later neomorphism of the in itia l dolomite, and con
tamination by new flu id s injected during burial (Land, 1980).
Oxygen isotope composition shows a sim ilar relationship to
that of Sr. £^80 values are higher in hypersaline dolomites be
cause evaporation preferentially depletes theCaCOg sediment of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the lighter isotopes. Although dolomite formed in hypersaline
18 0 18 conditions is enriched in 0 the § 0 of dolomite depends jn 0 18 part on the (y 0 of the water, the temperature at which dolomite a 18 formed, and Q 0 o f the precusor sediment (Land, 1980). Interpretation of absolute values fo r trace elements or iso
topes is d ifficu lt because of the variables that affect natural
systems. However prudent application o f the re la tive values of
these factors can be useful.in the systematic study of dolomite.
Timing of Dolomitization
Dolomitization o f the Brassfield Formation in southwestern
Ohio was a two-stage process. In itia l dolomitization of the basal
Brassfield supratidal islands was a penecontemporaneous event.
As the supratidal environment was replaced by shallow open marine
conditions as the Brassfield seas transgressed however, this phase
of dolomitization was halted. Large-scale dolomitization of the
formation took place sometime later, and hypersaline brine was
likely not the agent of dolomitization. Although evidence for the timing of major dolomitizing events
is often equivocal (Choquette and Steinen, 1980), it is possible
to bracket the timing of the event by the relationship of dolomite
to other diagenetic features, and in the case of the Brassfield,
to tectonic activity.
Equant calcite is the most common non-dolomitic Brassfield
cement. Large blocky crystals w ill f i l l the pore spaces between
grains. However, where dolomite is present, calcitic cement is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 66 replaced by single euhedral, or .mosaics of dolomite crystals.
Dolomite formation is cle a rly secondary to c a lc ite cementation
(Fig. 31).
Apparently the Brassfield sediments underwent physical com
paction which produced styolites before dolomitization, as seen by
the preferential dolomitization along styolites and fractures (Fig.
29). The styolites acted as conduits for dolomitizing fluids
which formed crystals and replaced the original carbonate.
S ilicification of the original carbonate sediment also occurred
prior to dolomitization. Preservation of undolomitized high mag
nesium b io cla stic debris in chert nodules precludes dolom itization
of the original sediment before silicification since altered bio
clasts occur in the surrounding dolomitized sediment. The s ilic i-
fie d sediments preserve the original textures sometimes obscurred
by later dolomitization.
It appears that regional dolomitization of the Brassfield
Formation occurred post-depositionally a fte r c a lc ite cementation,
compaction and s ilic ific a tio n of the carbonate sediment. The pro
posed mixing zone model for Brassfield dolomitization is based on
the existance of a subareally exposed landmass that contained
fresh water in a paleoaquifer available to react with marine pore
waters. It has been suggested that this landmass existed along the
axis of the Cincinnati Arch in east-central Kentucky, and that sub
areal exposure, resulting in a major unconformity, was developed
before Middle Devonian time (McDowell and Peterson, 1980). Sedi
mentation resumed by the Upper Devonian resulting in the deposition
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of the Ohio Shale. Therefore, dolomitization of the Brassfield
which was controlled by the position of the paleoaquifer in the
landmass along the Cincinnati Arch must have occurred sometime
within the Middle Silurian and Upper Devonian hiatus represented
by the regional unconformity. Dolomitization by the mixing zone
process can only take place at depths shallow enough to be influenc
ed by a fresh groundwater lens. This constraint coupled with the
spatial trend in dolomitic alteration related the proximity of
Brassfield sediments to the paleoaquifer suggests dolomitization
was associated with flexure of the Cincinnati arch probably at the
onset o f the Devonian.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SUMMARY AND CONCLUSIONS
Evidence from depositional environments, petrography and
regional relationships of doloraitized and undolomitized rock in
the Brassfield Formation suggests two stages dolom itization in
Adams County, Ohio. In itia l dolom itization o f the basal Brass fie ld took place penecontemporaneously on supratidal islands that
existed in what is now southwestern Ohio. Dolomitization probably
occurred in a manner analagous to tha t which occurs in the modern
sabkha environment of the Persian Gulf. Dolomite crystals in the
basal Brassfield are small, 0.01-0.04mm, anhedral, dirty rhombs
typical of hypersaline environments.
Since the Brassfield rocks are representative of an Early
Silurian transgression the supratidal environment was replaced by
shallow open marine conditions. Regional dolom itization of the
Brassfield Formation can best be explained as a la te r epigenetic
event that occurred in a brackish mixing zone of fresh and sea
waters associated with the creation of a landmass due to upwarping
of the Cincinnati Arch at the close of the Silurian. The dolomite
crystals formed by this process are large (up to 0.4mm), euhedral,
limpid rhombs characteristic of dilute dolomitizing solutions.
Intensity of dolomitization is directly related to the proximity of
the original carbonate to the source of dolomitizing fluids in the
mixing zone.
68
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BIBLIOGRAPHY
Adams, J.E ., & Rhodes, M.L. Dolomitization by Seepage Refluxion. American Association o f Petroleum Geologists B u lle tin , 1960, 45, 1712-1920. Badiozamani, K. The Dorag dolom itization model: application to the Middle Ordovician of Wisconsin. Journal o f Sedimentary Petrology, 1973, 43, 965-984.
Berry, W.B.,& Boucot, A.J. Correlation of the North American Silurian rocks (Special Paper No. 102). New York: Geological Society of America, 1970.
Berry, W.B., & Boucot, A.J. Glacio-eustatic control of Late Ordovician-Early Silurian platform sedimentation and fauna! changes. Geological Society o f America B u lle tin , 1973, 84, 275-284.
Catalov, G.A. An attempt at energy index analysis of the Upper • Anisian, Ladinian and Carnian carbonate rocks in the Tetevan . anticlinorium . Sedimentary Geology, 1972, JJ, 159-175.
Choquette, P.W. Late ferroan dolomite cement, Mississippian carbonates, Illin o is Basin, U.S.A. In O.P. Bricker (Ed.), Carbonate Cements. Baltimore: John Hopkins Press, 1971.
Choquette, P.W., & Steinen, R.P. Mississippian non-supratidal dolomite, Ste. Genevieve Limestone, Illin o is Basin: evidence for mixed-water dolomitization. In D.H. Zenger, J.B. Dunham, and R.L. Ethinqton (Eds.), Concepts and models o f dolom iti zation (Special Publication No. 28). Tulsa, Oklahoma: Society of Economic Paleontologists and Mineralogists, 1980.
Dickson, U.A.D. A modified staining technique fo r carbonates in th in section. Nature,, 1965, 205, 587.
Dott, R.H. Jr., & Batten, R.L. Evolution of the earth. New York: McGraw-Hill, 1976.
Dunham, R.J. Classification of carbonate rocks according to depositional texture. Journal o f Sedimentary Petrology, 1962, 32, 108-121.
Dunham, J.B.,& Olson, E.R. Diagenetic dolomite formation related to Paleozoic paleogeography of the Cordilleran miogeocline in Nevada. Geology, 1978, 556-559.
69
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70 Dunham, J.B., & Olson, E.R. Shallow subsurface dolomitization of sub tida lly deposited carbonate sediments in the Hanson Creek Formation (Ordovician-Silurian) of central Nevada. In D.H. Zenger, J.B. Dunham and R.L. Ethington ( Eds. ) , Concepts and models of dolomitization (Special Publication No. 28). Tulsa, Oklahoma: Society o f Economic Paleontologists and Mineralo g is ts , 1980.
Engle, A.E., Clayton, R.N., & Epstein, S. Variations in isotopic composition of oxygen and carbon in Leadvilie limestones (Miss issippian, Colorado) and in its hydrothermal and metamorphic phases. Journal of Geology, 1958, 66_, 364-393.
Foerste, A.F. The Clinton Group of Ohio. Dennison University Science Laboratory B u lle tin , 1885, 1_, 63-120.
Foerste, A.F. Middle Silurian of Ohio and Indiana. Cincinnati Society o f Natural H istory, 1896, T_8, 161-200.
Foerste, A.F. The Ordovician-Silurian contact in the Ripley Island area of southern Indiana, with notes on the age of the Cincinnati geanticline. American Journal o f Science Series 4, 1904, 18, 321-342.
Foerste, A.F. The S ilu ria n , Devonian and Irvine Formations in east-central Kentucky. Kentucky Geological Survey B u lle tin , 1906, 7, 1-369.
Folk, R.L. Spectral subdivision of limestone types. In W.E. Ham (Ed.), Classification of carbonate rocks - a symposium (Memoir No. 1). Tulsa, Oklahoma: American Association of Petroleum Geologists, 1962.
Folk, R.L., & Land, L.S. Mg/Ca ratio and salinity: two controls over the crystallization of dolomite. American Association of Petroleum Geologists Bulletin, 1975, 59, 60-68.
Friedman, G.M. Dolomite is an evaporite mineral: evidence from the rock record and from sea-marginal ponds of the Red Sea. In D.H. Zenger, J.B. Dunham and R.L. Ethington ( Eds. V. Con cepts and models of dolomitization (Special Publication No. 28). Tulsa, Oklahoma: Society of Economic Paleontologists and Mineralogists, 1980.
Gaines, A.M. Dolomitization kinetics: recent experimental studies. In D.H. Zenger, J.B. Dunham and R.L. Ethington (Eds.), Concepts and models of dolomitization (Special Publication. No. 28). Tulsa, Oklahoma! Society of Economic Paleontologists and Mineralogists, 1980.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Gebelein, C. & Hoffman, P. Algal origin of dolomite laminations in stro m a to litic limestone. Journal of Sedimentary Petroloqy, 1973, 43, 603-613.
Geblein, C., Steinen, R.P., Garrett, P., Hoffman, E.J., Queen, J .'M ., & Plummer, L.N. Subsurface dolom itization beneath the tidal flats of central west Andros Island, Bahamas. In D.H. Zenger, J.B. Dunham and R.L. Ethington (Eds.), Concepts and models of dolomitization (Special Publication No. 28). Tulsa, Oklahoma: Society o f Economic Paleontologists and Mineralo g is ts , 1980.
Goodel!, H.G., & Garman, R.K. Carbonate geochemistry of superior deep te st w e ll, Andros Island, Bahamas. American Association o f Petroleum Geologists B u lle tin , 1969, 53, 513-536.
Gordon, L.A. Stratigraphy and Paleoecology of the Brassfield Formation (Llandoverian) in east-central Kentucky. Unpublished Master's thesis, University of Kentucky, 1980.
Gray, J ., & Boucot, A.M. Palynological evidence bearing on the Ordovician-Si1urian paraconformity in Ohio. Geological Society o f America B u lle tin , 1972, 83, 1299-1314.
Hanshaw, B.D., Back, W., & Deike, R.G. A geochemical hypothesis for dolomitization by groundwater. Economic Geologist, 1971, 66, 710-724.
Harrison, W.B. Personal communication, June 12, 1979.
Hsii, K .J., & Siegenthaler, C. Preliminary experiments and hydro- dynamic movement induced by evaporation and th e ir bearing on the dolomite problem. Sedimentology, 1969, 1J2, 11-25.
Illin g , L.V., Wells, A.J., & Taylor, J.C. Penecontemporaneous dolomite in the Persian Gulf. In L.C. Pray and R.C. Murray (Eds.), Polomitization and limestone diagenesis - a. symposium (Special Publication No. 12). Tulsa, Oklahoma: Society of Economic Paleontologists and Mineralogists, 1965. Irvin g , E. Paleopoles and paleolatitudes o f North America and speculations about displaced terrains. Canadian Journal of Earth Science, 1979, 26, 669-694.
James, N.P. Facies models 10., shallowing upward sequences in carbonates. In R.G. Walker (Ed.), Facies models (Reprint Series 1). Toronto: Geological Association of Canada, 1979.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72 Kinsman, D.J. Modes of formations, sedimentary associations, and diagnostic features of shallow-water and supratidal evaporites. American Association o f Petroleum Geologists B u lle tin , 1969, 53, 830-840.
Land, L.S. Contemporaneous dolomitization of Middle Pleistocene reefs by meteoric water, North Jamaica. B u lle tin of Marine Science, 1973a, 23, 64-92.
Land, L.S. The isotopic and trace element geochemistry o f dolo mite: the state of the art. In D.H. Zenger, J.B. Dunham and R.L. Ethington (Eds.), Concepts and models of dolomitization (Special Publication No. 28). Tulsa, Oklahoma: Society of Economic Paleontologists and Mineralogists, 1980.
Land, L.S,, & Epstein, S. Late Pleistocene diagenesis and dolo m itiza tion , North Jamaica. Sedimentology, 1970, 14, 187-200.
Laporte, L.F. Carbonate deposition near mean sea level and re sultant facies mosiac: Manlius Formation (Lower Devonian) o f New York state. American Association o f Petroleum Geologists B u lle tin , 1967, 51_, 73-101.
Longman, M.W. Carbonate diagenetic textures from nearsurface diagenetic environments. American Association of Petroleum Geologists B u lle tin , 1980, 64, 461-487.
Mattes, B.W., & Mountjoy, E.W. Burial dolomitization of the Upper Devonian Miette Buildup, Jasper National Park, Alberta. In D.H. Zenger, J.B. Dunham and R.L. Ethington (Eds.), Concepts and models of dolomitization (Special Publication No. 28). Tulsa, Oklahoma: Society o f Economic Paleontologists and Mineralogists, 1980.
McDowell, R.C., & Peterson, W.L. Stratigraphy of the Silurian outcrop belts in Kentucky: Evidence of gentle tectonism. Geological Society o f America Abstracts with Programs South eastern Section (Annual Meeting) , Birmingham, Alabama, 1980, 184. (Abstract) McKenzie, J .A ., Hsu, K .J., & Schneider, J.F. Movement o f sub surface waters under the Sabkha, Abu Dhabi, UAE, and its re la t ion to evaporite dolomite genesis. In D.H. Zenger, J.B. Dunham and R.L. Ethington (Eds.), Concepts and models o f dolom itization (Special Publication No. 28). Tulsa, Oklahoma: Society of Economic Paleontologists and Mineralogists, 1980.
McKerrow, W.S. Ordovician and S ilurian changes in sea le ve l. Journal o f the Geological Society, 1979, 136. 137-143.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Muir, M., Lock, D., & von der Borch, C. The Coorong model fo r penecontemporaneous dolomite formation in the Middle Proterozoic McArthur Group, Northern T erritory, A ustralia. In D.H. Zenger, J.B. Dunham and R.L. Ethington (Eds.), Concepts and models of dolomitization (Special Publication No. 28). Tulsa, Oklahoma: Society of Economic Paleontologists and Mineralogists, 1980.
Murray, E.J. Trace fossils of the Brassfield Formation, Lower Silurian, in south-central Ohio and north-central Kentucky. UnpublishedTMaster's thesis, Western Michigan University, 1975.
Murray, R.C., & Lucia, F.J. Cause and control of dolomite dis tribution by rock selectivity. Geological Society of America B u lle tin , 1967, 78, 21-36. O'Donnell, E. The lith ostratigraphy o f the Brassfield Formation ( Lower S iluria n) in the Cincinnati Arch area. Unpublished doctoral dissertation, University of Cincinnati, 1967.
Orton, E. F irs t annual report o f the geological Survey of Ohio. 1890. Peck, J.H. Geologic map of the Tollesboro Quadrangle, Lewis and Fleming Counties, Kentucky. Washington, D.C.: U.S. Geological Survey, 1967.
Plumley, W.J., Risley, G.A., Graves, R.W., & Kaley, M.E. Energy Index for limestone interpretation and classification. In W.E. Ham (Ed.), Classification of carbonate rocks - a symposium (Memoir No. 1). Tulsa, Oklahoma: American Association of Petroleum Geologists, 1962.
Powers, R.W. Arabian Upper Jurassic carbonate reservoir rocks. In W.E. Ham (Ed.), Classification of carbonate rocks - a. symposium (Memoir No. l ) . Tulsa, Oklahoma: American Assoc iation of Petroleum Geologists, 1962. Rexroad, C.B Stratigraphy and conodont paleontology of the Brass field (Silurian) of the Cincinnati Arch area. Indiana Geolog ica l Survey B u lle tin , 1967, 36, 1-64. Rexroad, C.B., Branson, E.R., Smith, M.D., Summerson, D., & Boucot, A.J. The S iluria n Formations of east-central Kentucky and adjacent Ohio. Geological Survey of Kentucky Series X, 1965, 2, 1-34. Runnells, D.D. Diagenesis, chemical sediments, and the mixing of natural waters. Journal of Sedimentary Petrology, 1969, 39, 1188-1201.
permission of the copyright owner. Further reproduction prohibited without permission 74 Sears, S.O., & Lucia, F.J. Dolomitization o f Northern Michigan reefs by brine refluxion and freshwater/seawater mixing. In D.H. Zenger, J.B. Dunham and R.L. Ethington (Eds.), Concepts and models of dolomitization (Special Publication No. 28). Tulsa, Oklahoma: Society of Economic Paleontologists and Min eralogists, 1980. Sheehan, P.M. Brachiopod synecology in a time o f c ris is . Paleo biology, 1975, T_* 205* Shinn, E.A., Ginsberg, R.N., & Lloyd, R.M. Recent supratidal dolomite from Andros Island, Bahamas. In L.C. Pray and R.C. Murray (Eds.), Dolomitization and limestone diagenesis-a symposium (Special Publication No. 12). Tulsa, Oklahoma: Society of Economic Paleontologists and Mineralogists, 1965.
von der Borch, C.C. The distribution and preliminary geochemistry o f modern carbonate sediments of the Coorong area, South Aus tr a lia . Geochimica et Cosmochimica Acta, 1965, 29, 781-799.
Weir, G.W., & Peck, J.H. Lithofacies of Upper Ordovician rocks exposed between Maysvilie and Stanford, Kentucky, 162-168. Geological Survey research 1968 (U.S. Geological Survey Pro fessional Paper 60(M)n New York: U.S. Geological Survey, 1968. Wilson, J.L. Carbonate facies in geologic history. New York: Springer Verlag, 1975. Ziegler, A.M. S iluria n marine communities and th e ir environ mental significance. Nature, 1965, 207, 270-272.
Z iegler, A.M., & Boucot, A.J. North American S ilurian animal communities. In W.B. Berry & A.J. Boucot (Eds.), Correlation of North American Silurian rocks (Special Paper No. 102). New York: Geological Society of America, 1970.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.