The Relation of Dolomite Associated with 

Faults to the Stratigraphy and Structure of 

Central Kentucky

G ;S O L d G I G A L SURVEY t R O P B S S I O fi A L PAPER 'lit 1 * A

Prepared in cooperation with the 

Kentucky Geological Survey

The Relation of Dolomite Associated with Faults

to the Stratigraphy and Structure of Central Kentucky

By DOUGLAS F. B. BLACK, WILLIAM C. MAcQUOWN, JR., and RONALD J. DEHAAS 

CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1151-A

Prepared in cooperation with the 

Kentucky Geological Survey

A discussion of field and laboratory studies 

of newly discovered fault-associated 

dolomite bodies found as a result 

detailed geologic mapping

UNITED STATES GOVERNMENT PRINTING OFFICE, W ASH I N GT O N : 1981
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Doyle G. Frederick, Acting Director

Library of Congress Cataloging in Publication Data

Black, Douglas F B

The relation of dolomite associated with faults to the stratigraphy and structure of central Kentucky.

(Contributions to the geology of Kentucky) (Geological Survey professional paper ; 1151-A) Bibliography: p.

Supt. of Docs. no. : I 19.16:1151-A

1. Dolomites Kentucky. 2. Faults (Geology) Kentucky. 3. Geology Kentucky. I. MacQuown, William Charles, Jr.

1915- joint author. II. -DeHaas, Ronald J., joint author. III. Kentucky. University. Geological Survey.

IV. Title. V. Series. VI. Series:

United States. Geological Survey. Professional paper ;

Library of Congress Cataloging in Publication Data

1151-A.

QE471.15.D6B55 552'.5 79-607049

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402
CONTENTS

	Page

Abstract _____________________________ Al

Introduction ____________________________ 1

Dolomite classification and features used in field identification 2

Regional stratigraphy ______________________ 2

Lithologic description of dolomite types _____________ 3

Regional geologic structure and relation to dolomite _____ 3

Fault-related dolomite occurrences _______________ A9

Representative examples of fault-related dolomite _______ 9

Laboratory investigations ____________________ 16

Summary and conclusions ____________________ 17

References cited _________________________ 18

ILLUSTRATIONS

Page

FIGURE 1. Index map showing regional structural features in relation to study area ____________________________ A2

2. Schematic cross section, south-central to northern Kentucky showing stratigraphic relations and nomenclature of

	near-surface formations and schematic plot of fault-associated dolomite occurrences ___________________ 4

3. Map showing fault-associated dolomite localities in relation to geologic structural fabric in the study area ___________ 6

4. Total-intensity aeromagnetic map of the study area shown in relation to dolomite occurrences and surface faults _______ 8

5. Geologic map of part of the Winchester quadrangle that includes dolomite localities 8 and 11 _________________ 12

6. View west at sample array BW-1 to BW-9 ____________________________________________ 13

7. T-dolomite body at locality 8b ______________________________________________________ 14

TABLES

TABLE 1. Structurally related dolomite occurrences 

2. Chemical analyses of samples __________

Page

A10 

17

in

CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY

THE RELATION OF DOLOMITE ASSOCIATED WITH FAULTS 

TO THE STRATIGRAPHY AND STRUCTURE OF CENTRAL KENTUCKY

By DOUGLAS F. B. BLACK, WILLIAM C. MACQUOWN, Jr. 1 , and RONALD J. DEHAAS2

ABSTRACT

Two modes of origin have been determined for fault-related dolomite 

in limestone formations of central Kentucky. Dolomite bodies have 

been emplaced as grabens downfaulted from regionally dolomitized 

formations much higher in the stratigraphic section. Offset on these 

faults is as much as 800 feet, much greater than displacements of other 

faults in the area. Other dolomite bodies associated with faults resulted 

from inplace alteration of limestone country rock by solutions conducted along the faults. These bodies are elongate in plan view and are 

adjacent to the conduit faults, commonly on the downthrown sides.

The fault-related dolomite bodies are in the central Kentucky 

mineral district located on the highly faulted central part of the' 

Jessamine Dome of the Cincinnati Arch where barite has been mined 

in the past from veins that also contain minor amounts of fluorspar, 

lead, and zinc. The veins occur in limestone and have a northerly 

strike. The dolomite is found chiefly along faults that strike northwest 

and contains only trace amounts of vug-fill barite locally.

Some of the dolomite lies along the major fault systems of the area 

that are known to displace Proterozoic basement rocks and to have 

been active intermittently during their history. The remaining 

dolomite bodies, including those found at each of two cryptoexplosive 

structures in the study area, lie along lineaments defined by geologically mapped structural features, such as swarms of faults, grabens, small 

steep-walled structural basins, and linear folds. We suggest that these 

structurally expressed lineaments, though defined by relatively minor 

disruption of the surface rocks, also reflect basement faults along 

which displacement has taken place periodically.

Circular cryptoexplosive structures, such as the Jeptha Knob and 

Versailles structures in this area, have been attributed to meteorite 

impacts by some geologists and to subterranean explosions by others 

who call the resulting structures cryptovolcanic. At the center of the 

Jeptha Knob disturbance, a horst was discovered that consists of 

dolomitized limestone, uplifted about 450 feet from its normal 

stratigraphic position. The horst is in direct fault contact with undolomitized carbonate rock. The dolomitic rock is derived from a unit 

that is nowhere else known to be dolomitized. In the absence of contradicting evidence, this fact, coupled with the apparent coincidence of 

both the Jeptha Knob and Versailles features and structural 

lineaments presumed to reflect deep-seated faults, suggests that the 

dolomitization is related to the structure, that it took place at depth

1 University of Kentucky, Lexington, Ky. 40506

2 University of Michigan, Ann Arbor, Mich. 48109

before uplift of the horst block, and that these structures are thereiore 

more likely to be of cryptovolcanic rather than of meteorite-impact 

origin.

We further suggest that the structural lineaments defined by detailed geologic mapping may be loci for mineral deposits and (or) 

petroleum entrapments at depth, especially in areas where thick impervious shale formations cover the brittle solution-susceptible carbonate rock formations that are exposed and locally dolomitized in the 

central part of the Jessamine Dome. Minor amounts of oil were found 

in two zones in a shallow drill core and as vug fillings seen in the 

dolomite outcrop at the Stoner Branch locality.

INTRODUCTION

Anomalous local bodies of dolomite were found 

associated with many faults in the predominantly 

limestone terrane of central Kentucky (fig. 1) during the 

detailed geological mapping program conducted 

cooperatively by the Kentucky Geological Survey and 

the U.S. Geological Survey (USGS). Fossils found in 

some of these dolomite bodies during the early work led 

the geologists to think that all these bodies were 

downfaulted blocks derived from overlying Upper Ordovician and younger dolomite formations. Black (1974) 

determined that in some places the dolomitization was 

due to inplace alteration of limestone by solutions moving along faults and fractures. This interpretation was 

made first at the Stoner Branch locality in the Winchester Quadrangle where a body of dolomite is bounded 

on only one side by a fault. The dolomite is underlain by 

a quartz siltstone, the Garrard Formation, which, 

though its interstitial calcite cement has been locally 

dolomitized, was traceable along four stream valleys 

beneath the dolomitized limestone of the overlying formation. Thus, the possibility that the dolomite body had 

been downfaulted from higher dolomitic formations was 

ruled out. Syndepositional dolomitization was also considered, but this hypothesis was discarded later when an 

excavation revealed the nature of lateral gradation from 

dolomite to limestone.

Al
A2 CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY

EXPLANATION 

Cryptoexplosive structure 

Fault

Ordovician and Sflurian 

outcrop area

100 MILES

FIGURE 1. Index map showing regional structural features in relation to study area. Central Kentucky faults sketched from recent geologic 

maps; others from Jillson (1929); Grenville Front in Ohio and southern Kentucky from Bayley and Muehlberger (1968). Map also shows 

location of dolomite localiaty T24 .

Two means of emplacement are now recognized for 

the bodies of dolomite associated with faults in the 

region: grabens faulted down from younger dolomite 

formations and inplace dolomitization of limestone. We 

must distinguish between the two origins of dolomite to 

understand the geology of the area. This study deals 

chiefly with field relations and observations pertaining 

to the dolomites. It also briefly summarizes laboratory 

investigations being conducted by W. C. MacQuown, 

Jr., and R. J. DeHaas, and thermal maturity studies by 

A. E. Harris (USGS) and S. R. Jacobson (Chevron, U. S. 

A., Inc.). Results of chemical analysis are also 

presented.

DOLOMITE CLASSIFICATION AND FEATURES 

USED IN FIELD IDENTIFICATION

Dolomite in the study area has been divided into three 

types which are based in part on the classification 

system of Dunbar and Rodgers (1957). Stratiform 

regionally dolomitic formations and the dolomite blocks 

downfaulted from them are called S-dolomite. Tectonically related dolomitized rock is designated 

T-dolomite. The term "U-dolomite" is used for faultrelated dolomite of uncertain origin.

We now believe that, during the early part of the mapping program, certain dolomite bodies, such as those 

shown in the Coletown (Black, 1967) and Nicholasville 

(MacQuown, 1968) Quadrangles, were mapped incorrectly. Fault-associated bodies enclosed by Middle Ordovician formations exposed in the central Bluegrass

area of the Jessamine Dome are very similar to the Upper Ordovician to Middle Devonian S-dolomites in outlying areas. This, coupled with a Silurian- or Devonian-age 

determination based on fossils at one of the Nicholasville 

localities, caused us to map all these bodies as grabens 

derived from the younger rocks. Subsequently, other 

bodies were found, and they, too, were all mapped as 

grabens until the discovery of T-dolomite was made at 

Stoner Branch in the Winchester Quadrangle. The 

earlier discovered bodies have since been reevaluated, 

and all subsequent discoveries evaluated by means of 

certain characteristics on which their present classification is based. These include (1) identifiable key fossils in 

the dolomite, (2) lithologies and relict features not completely destroyed by dolomitization, (3) position in 

recognizable stratigraphic sequences, (4) proximity to 

and obvious association with other dolomite bodies of 

known origin, and (5) observed gradation at boundaries 

between dolomite and limestone. Less positive classifications, which are queried in this report, are based on 

poorly preserved fossils and on the similarity of relict 

textures and sedimentary structures to like features in 

positively identified stratigraphic units.

REGIONAL STRATIGRAPHY

Rocks of Middle Ordovician age, the oldest exposed in 

Kentucky, crop out in the south-central part of the 

Jessamine Dome of the Cincinnati Arch (fig. 1). Successively younger strata, extending to rocks of Pennsylvanian age, crop out in an irregularly concentric pat-
STRATIGRAPHY AND STRUCTURE OF CENTRAL KENTUCKY A3

tern around the dome or extend southward and northward along the flanks of the arch. Figure 2 shows the 

lithologies and stratigraphic relationships of Ordovicianto Devonian-age formations that include the S-dolomite 

units discussed herein. The section, not drawn to scale, 

is a schematic representation of the surface and nearsurface stratigraphy between the south-central part of 

the dome northeastward to the Ohio River. Detailed 

descriptions of rocks along the belt of outcrop are given 

in the USGS geologic quadrangle reports.

The stratigraphic section shown in figure 2 is divided 

into three sequences on the basis of S-dolomite content. 

The lower sequence is made up partly of bedded 

dolomite and dolomitic limestone in that part of the 

High Bridge Group exposed in Kentucky and extends 

down to include thick subsurface dolomitic formations in 

the lower Paleozoic (not shown). These subsurface rocks 

are not germane to the subject of this paper except as a 

possible source of magnesium for tectonically controlled 

hydrothermal alteration of limestone to dolomite and 

are not discussed further. The outcropping part of the 

lower dolomitic sequence contains thick-bedded 

stratiform dolomite, chiefly in the Oregon Formation. It 

also includes micritic limestone that contains dolomite 

pods and stringers occurring as fillings of burrows and 

tracks in the original sediments or as thin laminae in 

parts of the Tyrone and Camp Nelson Limestones. The 

dolomite of the lower sequence probably formed during 

early diagenesis in a tidal-flat, supratidal, and highintertidal environment (Cressman and Noger, 1976).

The upper sequence includes S-dolomite of Late Ordovician, Silurian, and Devonian age. To the south, it 

consists of dolomite, dolomitic mudstone, and dolomitic 

shale. Northward, the sequence thins by intertonguing 

of the lower part with limestone units of the middle sequence as shown in figure 2. This sequence has been 

eroded in the central area of the Jessamine Dome but is 

exposed around the flanks. In these rocks, traces of 

fossils and relict bedding suggest that much of the 

dolomite was originally limestone deposited in a shallowwater subtidal environment very similar to that of the 

limestone and shale sequence below. Mud cracks in some 

beds indicate, however, that some of the sediment may 

have been dessicated by subaerial exposure at times 

and, thus, would have been partly intertidal or 

supratidal.

The middle sequence is virtually devoid of S-dolomite. 

It is made up of bioclastic limestone and shale. This sequence contains the isolated fault-related bodies of 

T-dolomite that are the principal subject of this report.

LITHOLOGIC DESCRIPTION OF DOLOMITE

TYPES

S-dolomite constitutes a suite of several lithologic 

varieties. Some stratigraphic units are composed chiefly

of indurated, very fine to fine sand-sized crystalline 

dolomite that crops out as minor ledges. Fresh rock is 

gray but has a slight greenish cast and locally is flecked 

with patches of bright green. It is generally weathered 

yellow, orange, and brownish red. In unweathered rock 

where the green color is indistinct, the dolomite is 

difficult to distinguish from limestone, and identification 

requires chemical staining or acid etching. The presence 

of interstitial calcite in some of the dolomitic rocks 

causes them to effervesce in HCI even more vigorously 

than fine-grained limestone varieties. Etched dolomite 

when treated with a solution of dilute HCI and alizarin 

red-S stain shows a rhombic-crystal mosaic pattern in 

which unstained rhombs are outlined by carmine-red 

staining of the interstitial calcite.

Other S-dolomite units are argillaceous carbonate 

rocks and can be divided into nonfissile dolomitic 

mudstone and dolomitic shale. These argillaceous rocks 

commonly weather to unctuous clay of various shades of 

greenish gray or locally reddish gray. The dolomitic 

mudstone contains very fine to fine rhombic dolomite 

crystals scattered through a matrix of aphanitic 

argillaceous material and silt-sized dolomite and sparse 

quartz particles. The apparent fissility of the dolomitic 

shale varies with the degree of weathering. In addition 

to clay minerals, the shale also contains scattered 

dolomite crystals and some quartz silt.

S-dolomite of the upper sequence is readily 

distinguishable by its coarser crystal size and darker color from the very finely crystalline to microcrystalline 

yellow dolomite of the lower sequence. Within the upper 

sequence, however, lithologic similarities make it 

difficult to differentiate the various formations and 

members except where they contain definitive fossils or 

locally definitive chert or where there is sufficient outcrop to establish a recognizable stratigraphic sequence.

T-dolomite is remarkably similar to the S-dolomite of 

the upper sequence but is unlike the limestone from 

which it is derived. The descriptions given for the evenbedded resistant dolomite, dolomitic mudstone, and 

dolomitic shale of the upper S-dolomite sequence apply 

equally well to rocks found in the T-dolomite bodies. A 

minor exception is found in some of the T-dolomite 

where local concentrations of pink to white coarse 

saddle-shaped crystals line cavities in finely crystalline 

rhombic dolomite and in partly dolomitized limestone.

REGIONAL GEOLOGIC STRUCTURE AND 

RELATION TO DOLOMITE

The location of the study area with respect to the major structural features of Kentucky is shown in figure 1. 

The area encompassses the confluence of several major 

fault systems, the crest of the Jessamine Dome of the 

Cincinnati Arch, part of the east-northeast-trending 

38th Parallel Lineament, the northwestern flank of the
A4 CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY

FIGURE 2. Stratigraphic relations and nomenclature of near-surface formations and schematic plot of fault-associated dolomite by genetic type 

tions of dolomite bodies with respect to adjacent rocks are shown by the letter symbols, queried where appropriate and keyed to locality numbers 

graben-emplaced S-dolomite and horst-emplaced T-dolomite; dots show original stratigraphic position of these bodies. Arbitrary lateral contacts

Rome Trough, the Jeptha Knob and Versailles Cryptoexplosive Structures and, in the basement, the northnortheast-trending Grenville Front. Regional dip off the 

arch is gentle, as much as 30 feet per mile toward the 

Appalachian Basin to the southeast and about 15 feet 

per mile northwesterly toward the Illinois Basin.

The shaded-relief image of geologic structure shown 

in figure 3 was constructed by means of modified dataprocessing techniques (Chavez and others, 1979)

developed for the U.S. space program by the imageprocessing facility of the U.S. Geological Survey at 

Flagstaff, Ariz. Structure contour maps drawn on 

various stratigraphic horizons at a contour interval of 20 

feet were traced on a vectoral digitizer, raster-scan con-' 

verted to a pixel array wherein vertical values were 

assigned varying shades of gray, areally analyzed for 

steepness and direction of slope, and processed to portray the chosen direction and angle of illumination. Most
STRATIGRAPHY AND STRUCTURE OF CENTRAL KENTUCKY A5

(S, stratigraphic; T, tectonic; U, unknown), south-central and northern Kentucky. Shaded areas show regionally dolomitized units. Present posigiven in figures 1 and 3 and table 1. Dashed lines show range of T-dolomite occurrences. Wavy dashed lines show range of displacement of 

between units whose lithologies intergrade are shown by vertical dotted lines.

of the image expresses structure contours drawn on the 

top of the Tyrone Limestone of Middle Ordovician age. 

The various structure contour horizons shown in the 

southern part were extrapolated approximately to this 

horizon.

The major fault systems and linear folds that crosscut 

the study area are shown in figure 3. Some of these are 

known from drill-hole and geophysical data to extend into the Proterozoic basement. The fault systems divide

the area into major blocks that in turn are subdivided into minor blocks by structural lineaments. Some 

lineaments are also discernible in figure 4.

A variety of structural features are responsible for the 

lineaments. They include rectilinear to somewhat 

sinuous normal faults, swarms of en echelon normal and 

strike-slip faults, linear folds expressed by alined bends 

in the structure contours, and alinements of grabens 

and small closed structural depressions. The structural
A6 CONTRIBUTIONS TO THE GEOLOGY OP^ KENTUCKY
85°00'

38°00't»-'

'23X

37°45'

JEPTHA KNOB

CBYPTOEXPLOStVE

STRUCTURE

37°30'

VERSAILLES 

YPTOEXPlOSiVE 

STRUCTURE

15 KILOMETERS

FKJURE 3.—Fault-associated dolomite localities shown in relation to geologic structure of central Kentucky. Traces of mapped faults, showing sense of displacement by bar and ball symbols on downthrown side, shown above. Dolomite localities are shown by triangles below and are keyed to table 1. The surface 

generated as a modified computer-derived shaded-relief image is not topographic. It represents folding and faulting of a structurally contoured datum 

horizon, the top of the Tyrone Limestone of Ordovician age, extrapolated from 20-foot-interval structure contour maps drawn on various stratigraphic horizons. Symbols showing horizons contoured: tOt—Tyrone Limestone; tDna, bDna—top or base New Albany Shale; bMn—base Newman Limestone. Angle of illumination is 30° from the northwest.
A8 CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY

37°22'3G"

FIGURE 4.—Total-intensity aeromagnetic map of the study area shown in relation to dolomite occurrences and surface faults. Contour interval 

100 gammas. Hachures indicate closed areas of lower magnetic intensity. Crosses show corners of 7.5-minute quadrangles. Labels designate 

sample localities of table 2. Triangles show dolomite localities.

lineaments locally are also expressed topographically by 

alined patterns of sink holes (not shown) in areas 

underlain by limestone. Some of these coincide with 

mapped faults, and others probably formed along joint 

concentrations.

The orthogonal patterns of the lineaments, together 

with structure contour patterns in areas between them, 

suggest that the surface-rock deformation has been controlled by differential movement along faults that, in 

some places, are not exposed. From drilling, we know
STRATIGRAPHY AND STRUCTURE OF CENTRAL KENTUCKY A9

that some of these surface faults coincide with subsurface faults of much greater displacement than that 

shown in surface rocks. Thus, these lineaments are 

believed to define deep-seated blocks that apparently 

have shifted, both laterally and vertically with respect to 

one another, at various times during the complex tectonic history of the region.

Basement faults coincident with surface faults are 

also expressed on aeromagnetic and Bouguer gravity 

maps. Figure 4 is an aeromagnetic map of the study area 

by R. W. Johnson, Jr. (in Black and others 1976; 1977a, 

b, c, d). The pronounced change in magnetic fabric 

across the north-northeast-trending Lexington Fault 

System shows a major discontinuity in the basement 

rock that projects toward, and is probably continuous 

with, the Grenville Front in southern Ohio, discussed by 

Bass (1960) and shown by Bayley and Muehlberger 

(1968) and Lidiak and Zeitz (1976). The changes in contour spacing show a marked decrease in magnetic intensity from north to south across a band that closely corresponds to the Kentucky River Fault System. This 

masking of basement-rock-induced magnetism is caused 

by increased thickness of the lower Paleozoic section on 

the downthrown side of that fault system, known from 

drill holes that extend to basement. The Kentucky River 

Fault System forms the northern limit of the Rome 

Trough. The Irvine-Paint Creek Fault System to the 

south is also known to be a growth fault, one in which 

concurrent displacement and sedimentation has occurred, and has an even greater increase in thickness in 

the lower Paleozoic section on its downthrown side. 

Limited aeromagnetic coverage prevents clear definition of this feature, however. As shown in figure 4, many 

of the dolomite bodies east of the Grenville Front appear 

to be clustered about several of the positive magnetic 

anomalies. Some dolomite bodies have no obvious 

associated magnetic anomalies, however, and any possible genetic relationship between the dolomite and rocks 

responsible for these anomalies could be determined only by deep drilling.

The dolomite bodies of the area are clearly related to 

faults of the major fault systems and structural 

lineaments. The T-dolomitization almost certainly has 

been effected by solutions conducted along faults and 

fractures and further extended along bedding planes. 

The source of the solutions is not known. They might 

have percolated downward from the same sources that 

caused the S-dolomitization of the Upper Ordovician to 

Middle Devonian carbonate rocks (fig. 2) or, just as 

reasonably, risen from subsurface sources along faults 

that we suggest have been propagated upward from 

basement-block margins. As noted earlier, thick sequences of dolomite that could be potential sources of 

magnesium are present in the subsurface. Evidence for

and against each of the above hypotheses will be considered.

In either hypothesis, the association between dolomite 

and deep-seated faults indicated by the structural 

lineaments suggests that:

1. During the complex tectonic history of the region, 

periodic displacements along linear zones of 

weakness have created open spaces in relatively 

brittle carbonate rocks, some of which have been 

selectively dolomitized along the lineaments.

2. These same linear zones might be suitable for the 

emplacement of ore minerals, or entrapment of 

petroleum, or both, especially where thick, relatively plastic, and probably impervious shale overlies 

the solution-susceptible carbonate rocks. 

The area of dolomite occurrence lies within the Central Kentucky Mineral District (Jolly and Heyl, 1964; 

Fohs, 1913; Robinson, 1931). An example of oil and gas 

accumulation along an elongate fault-related body of 

locally dolomitized and petroliferous rock in otherwise 

barren limestone is the Albion-Scipio field in Michigan 

(Levorsen, 1967).

FAULT-RELATED DOLOMITE OCCURRENCES

Thirty-three dolomite occurences related to faults are 

known in central Kentucky (table 1). All but one, 

reported by Jillson (1945) and shown as locality T24 in 

figure 1, were discovered during the cooperative mapping program and are shown in figure 3. The mode of 

emplacement for each was determined by the field 

geologist who mapped the body.

REPRESENTATIVE EXAMPLES OF 

FAULT-RELATED DOLOMITE

Six of the 24 dolomite localities shown in table 1 have 

been selected as representative of dolomite occurrences 

in central Kentucky. Five of these are found in the area 

of the Winchester (Black, 1974) and Hedges (Black, 

1975) Geologic Quadrangles. In addition, the dolomite at 

the center of the Jeptha Knob Cryptoexplosive Structure in the Waddy and Shelbyville Quadrangles 

(Cressman, 1975a, b) is discussed.

T-DOLOMITE: STONER BRANCH LOCALITY 11

At this locality, an elongate body of dolomite lies along 

the Boonesborough Fault of the Kentucky River Fault 

System where it is crossed by Stoner Branch (fig. 5). The 

dolomitized equivalent of the Galloway Creek Limestone 

and Garrard Siltstone below are downthrown about 300 

feet against the middle and upper parts of the Lexington 

Limestone and the overlying lower part of the Clays 

Ferry Formation. Steeply dipping drag-folded beds of
A10 CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY

TABLK I.—Structurally related dolomite occurrences

(Locality numbers anil symbols depicting dolomite type art' keyed to tigs. 1-3)

Locality No., dolomite 

type geologic quadrangle, and reference

Structural occurrence Stratigraphic relations

1, T, Cardwell 

(Peterson, 1977).

2. U, Danville 

(Cressman, 

1972a).

3. S, Bryantsville 

(Wolcott and 

Cressman, 1971) 

and Stanford 

(Shawe and 

Wigley, 1974).

4. T, Nicholasville 

(MacQuown, 

1968).

5, S. Nicholasville 

(MacQuown, 

1968).

6, T?, Coletown 

(Black, 1967).

7, S, Ford 

(Black, 1968).

8, T, Winchester 

(Black, 1974).

9. U, Winchester 

(Black, 1974).

10. T?, Winchester 

(Black, 1974),

11, T, Winchester 

(Black, 1974).

Narrow steep-walled 

structural basin, 

trend WNW

Graben, trend E.

Graben, trend NW.

Fault blocks of Lexington Fault 

System, trend N.

Graben, trend N.

Two connecting 

grabens, trend 

WNW and NW.

Breccia-filled graben, 

trend NW.

Graben (8a) and 

nearby zone of 

minor faults and 

fractures (8b), trend 

NW.

Graben, trend NW.

_do_

Boonesborough fault, 

trend ENE.

Galloway Creek Limestone, fossiliferous, 

dolomitized in part.

Dolomite and dolomite mudstone of unknown 

stratigraphy downthrown against upper part of

Lexington Limestone and Clays Ferry Formation.

Preachersville and Rowland Members of Drakes 

Formation downthrown against middle and 

upper part of Lexington Limestone, 

throw 650 ft maximum.

Mostly dolomitized Clays Ferry Formation and 

Lexington Limestone displaced max. 150 ft 

against middle part of Lexington Limestone, 

Clays Ferry Formation, and Garrard Siltstone.

Dolomite of Silurian or Devonian age downthrown against middle part of Lexington 

Limestone and Clays Ferry Formation, throw 

between 700 and 800 ft.

Dolomite, probably altered Clays Ferry 

Formation and upper part of Lexington 

Limestone displaced no more than a few tens 

of feet against middle to upper parts of 

Lexington Limestone.

Dolomite and limestone breccia; fragments 

range from upper part of Drakes Formation 

to Ashlock Formation. Greatest displacement 

slightly more than 800 ft against lowermost 

part of Lexington Limestone and Tyrone 

Limestone.

Dolomitized middle and upper parts of Lexington Limestone downthrown against middle 

part of Lexington; graben at 8a displaced 

several tens of feet, negligible displacement 

of spheroidal body at 8b (figs. 3, 5, and 7).

Dolomite of unknown stratigraphy downthrown 

against upper part of Lexington Limestone, 

displacement not known.

Undolomitized Clays Ferry Formation and 

dolomitized upper part of Lexington 

Limestone(?) downfaulted 100 ft(?) against 

Lexington Limestone.

Extensive elongate body that includes dolomitized Galloway Creek Limestone, Garrard 

Siltstone, and Clays Ferry Formation 

downthrown 300 ft against undolomitized 

Lexington Limestone. Dolomite grades 

laterally to undolomitized equivalent lithologies at varying distances from fault (figs. 3 

and 5).

On structural lineament that includes 

Bardstown monocline. Locality 

includes three separate dolomite 

bodies.

Originally mapped as Drakes Formation requiring throw of about 

600 ft.

At intercept of NW-trending lineament 

and Brumfield fault.

Grabens within Lexington fault system originally mapped as Drakes 

Formation requiring displacement of 

400-600 ft. Small displacement now 

assumed. Five separate dolomite 

bodies.

Lithology resembles that of Brassfield Dolomite or dolomitic Bisher 

Limestone but could be Boyle 

Dolomite. Included fossils tentatively 

identified as Silurian or Devonian.

Relict bedding character and possible 

local intergradation with limestone 

suggest T-dolomite. Originally 

mapped as Drakes Formation 

requiring displacement of about 

600 ft. Small displacement now 

assumed. Two separate but adjoining 

dolomite bodies.

Part contains identifiable fossils, part 

unidentifiable dolomite. Two separate 

dolomite bodies.

Ascending solutions dolomitized 

spheroidal body in fault zone. Graben 

originally mapped as U-dolomite. See 

text discussion. Two separate 

dolomite bodies.

Lies along connecting fault that also 

adjoins locality T10(?). See text 

discussion.

Fault relations are obscure. Lexington designation based on lithology 

and brachiopod indentification. 

Bryozoan identification suggests 

possible displacement from 

Calloway Creek Limestone. See 

text discussion. Two separate 

dolomite bodies.

Local petroleum hydrocarbons in 

vugs. Section dolomitized to total 

depth penetrated by diamond-drill 

core hole. See text discussion.
STRATIGRAPHY AND STRUCTURE OF CENTRAL KENTUCKY 

TABLE I.—Structurally related dolomite occurrences—Continued

All

Locality No., dolomite 

type geologic quadrangle, and reference

Structural occurrence Stratigraphic relations Comments

12, T, Winchester 

(Black, 1974) 

and Hedges 

(Black, 1975).

13. T, Hedges 

(Black, 1975).

14. S, Hedges 

(Black, 1975).

15, U, Hedges 

(Black, 1975).

16. T, Austerlitz 

(Outerbridge, 

1975).

17. T?, Salvisa 

(Cressman, 

1968).

18, T?, Salvisa 

(Cressman, 

1968).

19. S? Salvisa 

(Cressman, 

1968).

20. T?, Versailles 

(Black, 1964).

21, U, Versailles 

(Black, 1964).

22. T?, Lawrenceberg (Cressman, 

1972b).

23. T, Waddy 

(Cressman, 

1975a).

24, T, Monterey 

(Moore, 1977).

Eagle Nest fault, 

trend NE.

Ruckerville fault,

trend NE. 

Megabreccia graben,

trend NW.

Graben, trend NW.

Austerlitz Fault Zone, 

curvilinear, trend 

NW to N. to NE.

Graben, trend NW.

Graben, trend NW.

__do_____ 

__do________

Versailles Cryptoexplosive Structure.

Graben, trend NW.

Jeptha Knob cryptoexplosive structure.

Normal fault, trend

NW.

Multiple outcrops of Reba Member of Ashlock 

Formation, selectivity dolomitized as much 

as 4,000 ft from fault, grades laterally to 

limestone, underlying units not dolomitic, 

overlying units S-dolomite. Downfaulted 200 ft 

against undolomotized Galloway Creek 

Limestone.

Reba Member of Ashlock selectivity dolomitized 

as above (locality T12).

Several large blocks of S-dolomite and limestone 

derived from recognized formations from 

Brassfield Dolomite to Grant Lake Members of Ashlock Formation downthrown max. 

of 500 ft against lower part of Galloway Creek 

Limestone and Garrard Siltstone.

Dolomitic shale, dolomitic mudstone, and dolomite of unknown stratigraphy displaced 

against upper part of Lexington Limestone.

Dolomite in elongate body mapped at high angle 

to fault, replaces Clays Ferry Formation and 

upper part of Lexington Limestone.

Undolomitized Clays Ferry Formation.

Dolomitized middle part of Lexington Limestone(?) downthrown 10-20 ft against 

undolomitized middle part of Lexington 

Limestone.

Drakes Formation. Downthrown max. 550 ft 

against Clays Ferry Formation and upper part 

of Lexington Limestone.

Partly dolomitized(?) Clays Ferry Formation and 

dolomitized upper part of Lexington Limestone downthrown as much as 100 ft against 

middle part of Lexington Limestone.

Dolomite as large float block of unknown 

stratigraphy overlying cryptoexplosive breccia 

in ring graben that displaces upper part of 

Lexington Limestone max. of 80-100 ft 

against middle part of Lexington Limestone 

peripheral to the structure.

Dolomitized Clays Ferry Formation(?) downthrown 50-100 ft? against Clays Ferry Formation and upper part of Lexington 

Limestone.

Dolomitized lower part of Lexington Limestone 

upfaulted 300-450 ft against Clays Ferry 

Formation. Uncomformably overlain by 

Brassfield Dolomite that is not affected by 

structure. Brassfield normally more than 800 ft 

higher in section

Dolomitized Kope Formation and Clays Ferry 

Formation downthrown against Kope and 

Clays Ferry, minor throw.

Faulting and dolomitization pattern 

similar to, and en echelon with, 

locality T13 . See text discussion.

See above, locality T12 . See text

discussion. 

See text discussion.

Dolomitic shale dominant, lithologically 

similar to Drakes Formation (S-dolomite?) or could be dolomitized Clays 

Ferry Formation (T-dolomite).

Relict bedding and poorly preserved 

fossils in dolomite resemble those 

found in adjacent Lexington Limestone on upthrown sides and also in 

graben.

Originally mapped as Drakes Formamation. Present interpretation based 

on similarity of relict bedding and 

fossils to adjacent middle part of 

Lexington Limestone.

Interbedded dolomitic mudstone and 

dolomite similar to that of Drakes 

Formation.

T-dolomite interpretation based on 

relict bedding and character of 

poorly preserved fossils.

Breccia not dolomitized where 

sampled (BV-1, table 2).

Interbedded dolomite and dolomitic 

mudstone. Originally mapped as 

Drakes Formation S-dolomite.

Identification of lower part of Lexington (Logana Member) based on 

fossils and lithology. Logana not 

dolomitized elsewhere. Dolomitizaation probably structurally related. 

Rocks surrounding horst are not 

dolomitized. See text discussion.

T-dolomite classification based on 

USGS fossil identifications. Body 

originally described by Jillson (1945) 

as Silurian dolomitic formations. 

Cited fossil identifications (Berry and 

Boucot, 1970, p. 240) also gave 

Silurian age. Their fossil collections 

not located, however.
A12 CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY

CONTOUR INTERVAL 10 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

EXPLANATION

Ashlock Formation 

Oagl. Grant Lake Member 

Oat, Tate Member

| Occj|

*Calloway Creek Limestone

*Garrard Siltstone

Olm'

Olb|

Olgc'

*Clays Ferry Formation and

*Lexington Limestone 

^ C Ocf, C/ays Ferry Formation 

o Olm. Millersburg Member 

1jj < Olt Tanglewood Limestone Member 

g Olb. Brannon Member 

-i I Olgc, Grier and Curdsuille Limestone 

Members

* Patterned where dolomitized

Tyrone Limestone

Contact

Fault 

Bar and ball on downthrown side

Structure contours

Drawn on base of Brannon Member of Lexington Limestone 

north of Boonesborough fault; on top of Garrard Siltstone 

to south. Arrows indicate direction of dip. Contour 

interval 10 feet (3.28 meters). Only index contours 

shown locally.

QDDH 

Diamond-drill hole U.S.G.S Alien No. 1

FIGURE 5.—Part of the Wihchester Quadrangle that includes dolomite localities 8a, 8b,and 11.
STRATIGRAPHY AND STRUCTURE OF CENTRAL KENTUCKY A13

FIGURE 6.—View west at sample array BW-1 to BW-9. Samples to left 

are essentially unaltered Galloway Creek Limestone; middle 

samples are partly dolomitized; samples to right are completely 

dolomitized. Throughgoing shale interbeds were dug out to 

establish exact correlation of sampled limestone and dolomite 

beds. Stadia rod is 13 feet long.

micrograined limestone immediately adjacent to the 

fault may belong to the Clays Ferry Formation.

The limestone on the upthrown side contains widely 

disseminated small pods of dolomite, but in quantities 

not unusual regionally. On the downthrown side, however, the Galloway Creek Limestone is almost completely dolomitized throughout an area, about 2 miles long 

and as much as 0.5 mile wide, that parallels and abuts 

the fault. A tongue of dolomite locally extends as much 

as 4,000 feet from the fault into the enclosing undolomitized limestone. The quartz silt of the underlying 

Garrard Siltstone is unaffected by the dolomitization, 

but its normally calcitic cement is locally dolomitized. 

About 40 feet of the underlying Clays Ferry Formation 

also is dolomitized to total depth of penetration in the 

USGS Alien No. 1 diamond-drill core (fig. 5). Thus 

dolomitization may extend to much greater depth.

Field relations indicated that the dolomite body was 

not a graben. The Garrard was traced in four drainages 

beneath the almost completely dolomitized Galloway 

Creek Limestone. The continuity of the Garrard showed 

that the dolomite could not have been downfaulted from 

S-dolomite formations of the upper dolomite sequence.

The possibilty that these field relations could be explained as a structurally controlled depositional 

dolomitic facies of the Galloway Creek was also examined. Tongues of limestone, bounded by dolomite above 

and below, extend as much as 1,500 feet into the 

dolomitic zone from the unaltered part of Galloway 

Creek. These limestone tongues, also found in the drill 

core, are shown on the map and cross section (fig. 5). 

However, close inspection of outcrops close to the area

of total dolomitization showed partial dolomitization of 

the limestone and an abrupt lateral gradation between 

the two rock types. This was best shown in a stock-pond 

excavation shown in figure 6. Here, limestone in a given 

bed grades laterally to dolomite within a few yards. 

Three separate resistant beds and intervening shale 

beds were traced across the transition zone between 

essentially unaltered limestone to the south (left set of 

three samples in sample array of fig. 6) and the completely dolomitized, stratigraphically equivalent beds 

about 40 feet to the north. Chemical analyses of the 

resistant beds, samples BW-1 through BW-9, are 

shown in table 2.

The Galloway Creek Limestone where not dolomitized 

includes a suite of lithologic types, generally some shade 

of gray and coarsely to finely bioclastic, that contains 

common to abundant whole fossils and large fossil 

fragments in most beds and partings and very thin interbeds of gray calcareous shale. Much of the rock is irregularly bedded to nodular, but some zones consist of 

even-bedded limestone and very thin shale interbeds. At 

Stoner Branch, the coarser limestone is altered to a 

medium- to coarse-crystalline mosaic of rhombic 

dolomite cemented by calcite; locally, coarse sparry 

calcite fillings and replacements postdate the dolomite. 

Most dolomite in surface exposures is weathered orange 

or yellow, at places flecked with red and green patches. 

Where fresh, the dolomite is gray and very similar in color to unaltered limestone. Megafossils are commonly 

obliterated in the dolomite or are seen as sparse ghosts 

outlined by opaque mineral concentrations, commonly 

pyrite. Crinoid columnals and plates and rare colonial 

corals, though dolomitized, are locally preserved as are 

phosphatic fillings of small gastropods and bryozoan 

zooecia and locally abundant phosphatic conodonts. The 

finer grained limestone is altered to finely crystalline 

dolomite or, where argillaceous, to dolomitic mudstone 

or locally very fissile dolomitic shale containing fine 

rhombic crystals disseminated throughout. Mudstone 

and shale are medium to dark gray and weather 

greenish gray or locally reddish gray. Original bedding 

in the limestone is irregular, hackly, or nodular, in part 

varying with size and type of enclosed megafossils and 

quantity of mud support. It is largely destroyed in the 

dolomitized equivalent, which is mostly thin and even 

bedded. Relict bedding is seen locally where ferruginous 

partings are preserved.

Cavity fillings of coarsely crystalline minerals that include barite, dolomite, and calcite have been found in 

outcrop at a few localities in the dolomitized rock. Barite 

veins were found north of the Stoner Branch area (fig. 5) 

but are regional in occurrence and not demonstrably 

related to the dolomitization. One mineralized cavity 

containing barite was found at a depth of 171 feet in the
A14 CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY

'KK 7.-T-dolomite body at locality 8b. Minor fault displaces the Millersburg (Olm) and Tanglewood Limestone (Olt) Members of the Lexmgton Limestone, downthrown side to the left. View northwest, Kentucky Highway 627, Winchester Quadrangle, Ky.

Alien No. 1 drill core. Two oil-bearing zones were 

penetrated by the drill, and tarry accumulations were 

found in cavities in outcropping dolomite and partly 

dolomitized limestone.

T-DOLOMITE: LOCALITY 8

The dolomite body at locality 8a (fig. 5) was originally 

mapped as a graben containing U-dolomite. Subsequent 

to the publication of the geological quadrangle (Black, 

1974), a new roadcut in the Millerburg and Tanglewood 

Limestone Members of the Lexington Limestone exposed the dolomite body at locality 8b (fig. 7). As at 

Stoner Branch, field relations here show that this body, 

too, is T-dolomite and that dolomitizing solutions have 

been conducted by faults, but, in addition, there is 

strong indication that the solutions responsible for 

dolomitization have risen from below.

The dolomitized body at 8b appears to be roughly 

hemispherical or lensoid around a fault of small displacement that is restricted to the lower part of the 

roadcut. In contrast to the general observation that the 

downthrown sides of faults are preferentially dolomitized in most Kentucky occurrences, very little displacement is indicated here, and both sides are altered at approximately equal distances from the fault. We have no 

explanation for the apparent preference. The fact that 

the conduit fault and fracture system here does not offset the overlying surface rocks suggests that the 

faulting and associated dolomitization died out at this 

point. It is also possible that such a small displacement 

may have resulted from collapse caused by shrinkage 

brought about by dolomitization.

Features indicating shrinkage include curved fractures peripheral to the body and calcite-filled gash fractures at a slightly greater distance from it; increased 

porosity near its center where open, as well as partly 

filled, fractures and vugs are present; and synclinal sag 

of throughgoing beds above the body.

Minerals lining cavities in this body include dolomite,
STRATIGRAPHY AND STRUCTURE OF CENTRAL KENTUCKY A15

mostly as pink to white large saddle-shaped crystals, 

some sparry calcite, barite, pyrite, and limonite 

pseudomorphous after pyrite. Most of the dolomite is of 

the rhombic-mosaic variety. Near the center of the body, 

some of the rhombic dolomite shows draped bedding, interpreted as eroded dolomite crystals that make up a 

sediment that has been redeposited above a constriction 

in a solution-enlarged cavity. The bedding in the sediment is locally drag folded, presumably as a result of further settling. The roadcut face opposite the illustrated 

exposure contains a narrow zone of dolomitization along 

a joint.

Though the dolomitizing solutions could have descended elsewhere and moved laterally along the fault 

and associated fractures, no evidence was found along 

strike that suggested this. A more reasonable assumption is that the fault conduits feeding both the bodies at 

localities 8a and 8b are interconnected at depth and that 

the dolomitizing solutions, probably hydrothermal, rose 

from below. For this reason, after the discovery at 

locality 8b, the body at 8a was reclassified as T-dolomite. 

The reclassification was supported by the presence of 

chert float of the Brannon Member, a unit in the middle 

of the Lexington Limestone, now thought to be nearly in 

place and preserved as residuum. The Brannon occurs 

close by on the upthrown side of one of the bounding 

faults and is also preserved in the downthrown dolomite 

and dolomitic mudstone at locality 8a. Maximum 

displacement of no more than a few tens of feet is, 

therefore, indicated for this graben, and major displacement from S-dolomite formations above is ruled out.

T-DOLOMITE: LOCALITIES 12 AND 13

At these localities, the Reba Member of the Ashlock 

Formation has been selectively dolomitized in two areas 

adjacent to the Eagle Nest and Ruckerville Faults, en 

echelon faults of the Kentucky River Fault System (Winchester and Hedges Quadrangles; Black, 1974, 1975). 

Complete dolomitiz;ation extends laterally to a maximum of 4,000 feet from the faults and partial 

dolomitization of the member extends more than 1,000 

feet farther. The overlying rocks are all regional 

S-dolomites extending as high as a dolomitic member 

(the "Duffin" or "Ravenna" of earlier workers) in the 

basal part of the Devonian New Albany Shale (fig. 2). 

The Terrill Member of the Ashlock immediately underlying the Reba is also an S-dolomite consisting of dolomitic 

mudstone and shale. The Grant Lake Member of the 

Ashlock that directly underlies the Terrill is nowhere 

dolomitized, not even immediately adjacent to the 

faults, though its lithology would appear to be as susceptible to dolomitization as the Reba.

The Reba Member in this area consists of a basal 

micrograined limestone (calcisiltite to micrite) commonly containing ostracodes and some cephalopods. The 

micrograined limestone is overlain by a zone of nodular 

brachiopodal and bryozoan limestone and some shale 

partings. This zone is locally overlain by a few feet of 

fossil-fragmental calcar'enite. The argillaceous nodular 

limestone is dolomitized at greatest distance from the 

fault, the upper calcarenite at intermediate distance, 

and the lower micrograined limestone at least distance. 

The mapped extent of dolomitization represents only the 

area in which the rock is completely dolomitized (Black, 

1975).

The suite of dolomite types found in the dolomitized 

Reba resembles types found in the upper S-dolomite sequence just above. However, the dolomitization in the 

Reba is classified as tectonic because of its obvious affinity to the two faults. Undolomitized Reba crops out in the 

graben at locality 14 to the north.

We do not know the time or means of emplacement of 

dolomite in the formations of the upper S-dolomite sequence. Because of the widespread distribution of 

dolomite, the possibility that it could be the product of 

fault-related hydrothermal solutions seems unlikely. 

Early diagenetic or syndepositional regional dolomitization would seem more probable. Yet, selective 

dolomitization of the Reba Member, occurring as it does 

sandwiched between the dolomitic Terrill mudstone and 

the remainder of the overlying upper S-dolomite sequence, suggests that the same dolomitizing solutions 

responsible for alteration of the upper S-dolomite sequence might have percolated downward along the 

Eagle Nest and Ruckerville Faults and been responsible 

for the local dolomitization. The downward migration of 

the solution may have been restricted by an impervious 

seal afforded by the shaly Terrill Member. This contrasts with the upward migration indicated at locality 8.

FAULT-RELATED S-DOLOMITE: LOCALITY 14

The west-northwest-trending graben at this locality 

contains a megabreccia consisting of huge blocks, some 

several hundred feet across, downfaulted against the 

lower part of the Galloway Creek Limestone and upper 

part of the Garrard Siltstone. The blocks are partly 

S-dolomite and dolomitic shale and partly limestone. 

They are tilted and separated by crossfaults within the 

graben. At places, the stratigraphic sequence within a 

single block can be determined. Dolomitic formations 

identified include from oldest to youngest: the Terrill 

Member of the Ashlock Formation, the Rowland 

Member and part of the Preachersville Member of the 

Drakes Formation, and the Brassfield Dolomite (fig. 2). 

Limestone units are the Reba and Grant Lake Members 

of the Ashlock Formation. Identifications are based on 

included fossils, lithologic characteristics, and 

stratigraphic sequence. Maximum displacement is prob-
A16 CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY

ably between 450 and 500 feet and is based on 

stratigraphic intervals measured about 3 miles to the 

south. Though this is not the greatest stratigraphic 

throw recorded in this area, it is exemplary of this type 

of structure and is very large considering the small areal 

extent of the graben.

U-DOLOMITE: LOCALITY 9

In this graben, neither field relationship nor fossil 

evidence was sufficient to determine if the dolomite body 

was altered country rock or S-dolomite strata emplaced 

by faulting. The graben is bounded by the upper part of 

the Lexington Limestone, dominantly nodular limestone 

and shale of the Millersburg Member. The body lies 

along a northwest-trending normal fault of small 

displacement, downthrown on the northeast. This fault 

also bounds another dolomite body (locality 10) about 1.5 

miles southeast of locality 9. That occurrence has been 

'tentatively classified as T-dolomite (table 1), but the 

distance between the two occurrences is judged to be too 

great for any assumed genetic relationship. The bounding fault on the northeast of the graben at locality 9 was 

inferred only on the basis of dolomite in outcrop and as 

float southwest of the fault versus limestone in nearest 

outcrops to the northeast. A fault would be present if 

the dolomite is derived from S-dolomite formations of 

the upper sequence; however, if the body is T-dolomite, 

no fault at this location would be requisite. No definitive 

fossils were seen at this locality, but phosphatic insoluble residues might yield conodonts that could roughly 

establish stratigraphic affinities. This has not been attempted for this or for the other U-dolomite bodies listed 

herein.

T-DOLOMITE: LOCALITY 23

One additional T-dolomite locality, that at the center 

of the Jeptha Knob Cryptoexplosive Structure, is 

worthy of special mention because of its potential 

significance to the understanding of circular structural 

features variously referred to as Cryptoexplosive, cryptovolcanic, or impact structures. The term for the Kentucky structures chosen here, "Cryptoexplosive," suggests only that the structures were formed by an explosive event of unknown cause. The term "cryptovolcanic" implies origin by explosion emanating from 

within the Earth but of unknown cause. The term "impact structure" indicates origin by impact from an extraterrestrial body, a meteorite, or a comet.

Considerable controversy exists concerning the origin 

of such circular features, and many detailed investigations have been made elsewhere in attempts to understand their genesis. Advocates of both the endogenetic 

and exogenetic schools lay claim to the Kentucky structures, though tentatively of course, because there is not 

yet sufficient evidence to prove their origin.

Three circular structures have been found in northcentral Kentucky. They are the Jeptha Knob and Versailles features in the study area and the Muldraugh 

Dome, about 40 miles to the west of the study area. 

Another such feature, the Middlesboro structure, is in 

southeastern Kentucky. All are circular in plan view, are 

highly brecciated, and are not mineralized at the surface; at one, the Middlesboro structure, shatter cones 

have been found. No pronounced gravity or magnetic 

anomalies are associated with either the Jeptha Knob or 

the Versailles features. All four appear to be closely 

associated with faults or with structural lineaments that 

may indicate the presence of deep-seated faults beneath 

: the structures. The Muldraugh Dome is underlain by an 

upthrust plug of Knox Dolomite (Cambrian and Ordovician) that pierces later Ordovician strata and is overlain 

by Silurian and Devonian dolomite and by domed strata 

of Devonian and Mississippian age (A. L. Bryant, 

Louisville Gas and Electric Co., Louisville, Ky., written 

commun., 1978); thus, origin by impact is unlikely.

The mode of occurence of the dolomite at locality 23 at 

the center of the Jeptha Knob structure suggests, but 

does not prove, that the explosions that caused the 

structure originated at depth. The dolomite is shown on 

the geologic map (Cressman, 1975a) as a fault-encircled 

horst, though the exposures are insufficient to determine the detailed relations to the surrounding rock. The 

uplifted block was identified by Cressman (1975a, b) on a 

lithologic basis as the Logana Member of the Lexington 

Limestone. This was subsequently confirmed by paleontologic identifications. This rock regionally contains a 

zone of silicified brachiopods that helped in identification 

of the unit at Jeptha Knob where the silicified 

brachiopods are enclosed by dolomite. Dolomite has not 

been found in this unit elsewhere throughout the region, 

either in outcrop or in drill cores. The Logana block is 

surrounded by limestone of the Clays Ferry Formation 

that locally has been deformed but not dolomitized. The 

Logana block has been uplifted about 450 feet from its 

normal stratigraphic position on the basis of 

stratigraphic intervals measured at considerable 

distance from the structure. The dolomite in this horst is 

an exception to the observation that Kentucky 

T-dolomites occur mostly on the downthrown sides of

faults.

Regional S-dolomite formations cap the structure but 

are not deformed and thus postdate the disturbance. 

Two generations of dolomitization are, therefore, indicated, one that dolomitized the Logana block either 

during or before deformation and another that involved 

only the undeformed S-dolomite above.

The Logana dolomitization apparently is restricted to, 

and therefore related to, the structure. We cannot show 

incontrovertibly that the dolomitizing solutions did not 

come from above and selectively dolomitize only the
STRATIGRAPHY AND STRUCTURE OF CENTRAL KENTUCKY A17

Logana Member, but, inasmuch as the adjacent rocks of 

the Clays Ferry Formation are not dolomitized, a more 

reasonable assumption is that the Logana dolomitization 

occurred at depth before the fault emplacement of the 

Logana block. Therefore, more than one deformation 

event would be indicated, and intervening time must be 

allowed to accomplish the dolomitization. These reasons 

indicate that this structure probably resulted from 

several endogenetic events occurring between Late Ordovician and Middle Silurian time, the age of the oldest 

structurally deformed rocks and of the youngest 

undeformed rocks, respectively.

LABORATORY INVESTIGATIONS

The field work on fault-related dolomites has been supplemented by several laboratory studies. These include 

petrographic and cathodoluminescense studies, X-ray 

and chemical analyses, and thermal maturity determinations. Most of the laboratory work has been concentrated at the Stoner Branch locality because of clear-cut 

field relations and good exposures there. The studies are 

briefly summarized here from the work of W. C. MacQuown, Jr., and R. J. DeHaas.

Petrographic and cathodoluminescence studies of the 

dolomite at Stoner Branch were made by MacQuown; 

X-ray analyses and field and microscopy studies were 

made by DeHaas. Their combined work shows that both 

sodic and potassic feldspars occur as silt-sized to finesand-sized crystals at the centers of many dolomite 

rhombs. This was first determined from X-ray studies 

and the intracrystalline position established by 

microscopic and .cathodoluminescence examination. The 

feldspar grains do not appear to be abraded and are 

believed to be authigenic crystals that formed during an

early stage in the dolomitization. Separations of 

authigenic feldspars preparatory to attempts at Rb-Sr 

and K-Ar age-dating of the dolomite were made by 

DeHaas. This dolomite-enclosed feldspar may prove to 

be unique to T-dolomite. Extensive petrographic study 

of regional S-dolomite occurrences in Kentucky by 

Wayne A. Pryor (oral commun., 1979) of the University 

of Cincinnati found no nucleic feldspar in the dolomite 

samples collected.

The dolomite at Stoner Branch is ferroan, and crystals 

show successive growth stages identified by variations 

in iron content, which is discerned as overgrowth bands 

of different colors under cathodoluminescence. Latestage intercrystalline filling and partial replacement of 

dolomite by calcite was determined from paragenetic 

relations seen during the microscopic study.

Semiquantitative spectrographic and colorimetric 

analyses that include 11 samples from the Stoner 

Branch locality are shown in table 2. Samples BW-1 

through BW-9 are from the collection array illustrated 

in figure 6. Three resistant beds were sampled across 

the zone of transition from essentially unaltered 

limestone (BW-1, -2, -3), through partially dolomitic 

limestone (BW-4, -5, -6), to completely dolomitized 

material (BW-7, -8, -9). Ca:Mg (not given) decrease as 

would be expected, and the concentrations of the tested 

elements fall within expectable limits for normal marine 

carbonate rocks (Donald A. Brobst, 1978, oral 

commun.). The remainder of the analyses are for 

dolomite samples (localities shown in fig. 4) from the 

Winchester (BW), Lawrenceburg (BL), Versailles (BV), 

and Salvisa (BS) Quadrangles, except that sample BV-1 

is from a limestone breccia from the Versailles Cryptoexplosive Structure. A possible explanation for the

TABLE 2.—Chemical analyses of samples

[Semiquantitative spectrographic analysis by E. L. Mosier; colorimetrif analysis hy M. S. Erickson. N, not detected at limit of detection or at value shown; L, detected, but lie-low limit of determination or below value shown]

Field No.

BW-1 __

BW-2 __

BW-3 __

BW-4 __

BW-5 __

BW-6 __

BW-7 __

BW-8 __

BW-9 __

BW-10 _

BW-11 _

BL-1 _

BV-1 __

BV-2 ___

BV-3 _

BS-1 _

BS-2 _

BS-3 _

Semiquantitative spectrographic analysis 6-step D.C. arc method

Fe

0.5

.7

.5

.3

.7

1.5

2

1.5

3

3

2

2

1

1

2

1.5

3

2

MR

(percent)

0.7

2

1

7

2

5

7

5

10

7

5

7

.5

3

5

3

5

5

Ti

0.05

.1

.02

.1

.03

.1

.07

.03

.2

.1

.07

.07

.05

.005

.1

.05

.07

.07

Mn

1,000

1,000

700

1,000

1,000

700

1,000

700

1,000

1,000

1,000

1,000

700

500

700

700

1,000

1,000

B

5

10

L(5)

30

5

20

20

15

70

50

20

20

7

N(5)

20

10

20

15

Ba Be

50 N(l)

70 N(l)

20 N(l)

150 1

50 N(l)

150 L(l)

100 L(l)

50 N(l)

1000 1

100 1

150 L(l)

100 L(l)

50 N(l)

N(20) N(l)

100 L(l)

50 N(l)

100 L(l)

70 L(l)

Bi Cd

N(l) 2

N(l) 10

N(l) N(2)

N(l) 3

N(l) 3

N(l) 3

N(l) 5

N(l) N(2)

N(l) 2

N(l) 5

N(l) 2

N(l) N(2)

N(l) N(2)

N(l) N(2)

N(l) N(2)

N(l) N(2)

N(l) N(2)

N(l) N(2)

Co

N(5)

N(5)

N(5)

15

N(5)

5

7

5

5

10

20

10

N(5)

N(5)

10

N(5)

30

15

Cr

5

7

5

10

5

7

7

7

20

20

20

10

7

5

10

7

7

5

Cu

(ppm)

7

5

5

15

5

10

10

10

10

15

15

10

5

7

20

10

20

10

Ni

7

5

N(5)

20

7

10

10

7

10

15

15

15

7

10

15

10

20

10

Pb

7

10

7

20

15

20

20

10

20

10

10

5

20

5

10

5

10

5

Sr

500

700

300

100

200

200

N(100)

N(100)

100

N(100)

N(100)

N(100)

500

N(100)

N(100)

N(100)

N(100)

N(100)

V

10

20

N(5)

50

10

50

20

10

70

50

20

20

15

10

20

15

20

20

Y

N(10)

10

N(10)

20

L(10)

15

15

N(10)

20

20

10

15

50

U10)

20

10

30

30

Zr

50

70

50

150

20

100

50

20

100

50

30

20

50

N(20)

100

20

30

20

Ga

1

1.5

1

5

1

5

3

2

7

5

5

3

2

1

5

2

5

3

Colorimetric

analysis

F

(ppm)

400

300

200

350

350

600

400

300

1,000

650

900

300

1,700

800

800

700

600

600
A18 CONTRIBUTIONS TO THE GEOLOGY OF KENTUCKY

high fluorine content of that sample might be that an unnoticed vug containing fluorspar was sampled. No indications of intensive mineralization in outcrop at that 

locality were found, though barite float occurs at a locality about 300 yards north of the structure.

Conodonts from three surface and two core-hole 

samples from the dolomite at Stoner Branch were 

analyzed colorimetrically by Anita Epstein Harris 

(USGS) to establish the thermal history of the rock. The 

method, described by Epstein and others (1977), involves the comparison of the color of conodonts from the 

sample with those heated to form a set of controlled 

standards. On exposure to increasing temperatures, the 

color of the organic material in the conodonts irreversibly changes; this change indicates the maximum 

temperature to which the rock has been subjected. The 

results show that the samples at Stoner Branch never 

exceeded a temperature of 50°C. Acritarchs from the 

same samples analyzed by Stephen Jacobson of 

Chevron, U.S.A., Inc., indicated that the temperature 

never exceeded 65°C. These analyses suggest that the 

fault-conducted dolomitizing solutions were either cool 

downward percolating waters or moderately hot hydrothermal solutions. Therefore, the direction of fluid 

migration was not determined. A sample of T-dolomite 

collected at the Jeptha Knob Cryptoexplosive Structure 

(locality 23) yielded the same results.

SUMMARY AND CONCLUSIONS

Detailed mapping in central Kentucky has revealed 

numerous occurrences of fault-related dolomite. Field 

evidence supported by some laboratory analyses indicates two modes of emplacement for these dolomite 

bodies. Some are small grabens displaced from regionally dolomitic (S-dolomite) formations much higher in the 

stratigraphic section. A second mode consists of inplace 

dolomitization of limestone country rock (T-dolomite) 

adjacent to faults that acted as conduits for the 

dolomitizing solutions. The mode of emplacement of a 

few bodies in the study area (U-dolomite) could not be 

definitely classified.

Some general field observations pertaining to 

T-dolomite occurrences are as follows:

1. Dolomite is commonly restricted to the downthrown 

sides of faults.

2. Rocks on the upthrown side show only trace amounts 

of disseminated dolomite similar to amounts found 

in unaltered limestone of the region.

3. Dolomitization appears to affect fine-grained argillaceous limestone at a greater distance from the 

source faults than it does the coarser grained 

limestone.

4. Dolomitization can obliterate all traces of most fossils 

even in rocks wholly composed of fossils. Crinoids 

and corals locally retain their original form even 

though dolomitized. Phosphatic fossils such as conodonts and phosphatic fillings of bryozoan zooecia 

and small gastropods appear commonly to withstand dolomitization as do ferruginous coatings of 

fossils and bedding features.

5. Dolomitization modifies original bedding to such a 

degree that very irregularly bedded fossilfragmental limestone may be converted to evenbedded dolomite.

6. Porosity and permeability are increased, especially 

where the limestone is only partly dolomitized.

7. Mineral- and petroleum-filled vugs appear to be 

commonly associated with this mode of dolomite 

emplacement.

Laboratory studies on some of the T-dolomite bodies 

revealed the common occurrence of feldspathic nuclei in 

banded ferroan dolomite crystals, indicating that 

T-dolomitization of the original limestone occurred as a 

series of crystal growth stages. Lack of rounding of 

many of the feldspar crystals suggests that they may be 

authigenic, and their nucleic position suggests that they 

may have formed very early in the dolomitization process. Though additional study is needed, early indications suggest that the nucleic feldspar found in 

T-dolomite does not occur in S-dolomite-bearing formations in Kentucky, and, thus, its presence may indicate 

fault-associated replacement origin. Thermal maturity 

studies show that the dolomitizing solutions, if 

hydrothermal, were only moderately hot. All samples 

tested had never been subject to temperatures higher 

than 50°C.

Evidence for the origin of the dolomitizing solutions 

appears to be conflicting. At some localities, upward 

migration of the solutions is suggested along faults, 

some of which are known to extend to basement and to 

have been periodically active. At other localities, 

however, downward migration would seem more likely. 

Further comparative studies between regional 

S-dolomite formations and the T-dolomite bodies might 

find some means by which they could be distinguished 

from one another, or, if they are truly indistinguishable, 

their presumed common heritage might be revealed. 

Conodont studies of the remaining U-dolomite bodies 

might identify these as T- or S-dolomites.

The recognized association between local dolomites 

and both Mississippi Valley-type mineral deposits and 

petroleum accumulations elsewhere suggest that drilling along structural lineaments where these dolomite 

bodies are found might be fruitful, other conditions being favorable. 

If the dolomitization of the horst at the center of the
STRATIGRAPHY AND STRUCTURE OF CENTRAL KENTUCKY A19

Jeptha Knob Cryptoexplosive Structure could be shown 

conclusively to be related to both the circular feature 

and to the deep-seated fault believed to underlie it, 

meteorite-impact origin could be ruled out, and, because 

this feature can be accurately dated as latest Ordovician, another age determination would be added to our 

knowledge of the regional tectonic history.

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*U.S. GOVERNMENT PRINTING OFFICE: 1981 — 314-614///0