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The Volcanogenic Mount Rogers 

Formation and the Overlying 

Glaciogenic Konnarock 

Formation-Two Late Proterozoic 

Units in Southwestern Virginia 

By DOUGLAS W. RANKIN 

The Mount Rogers Formation, largely continental rift-facies 

volcanic rocks, is redefined, and the name Konnarock 

Formation is introduced for the overlying glaciogenic deposits 

of diamictite, arkose, and laminite. Four rhyolite members of 

the Mount Rogers are named and described 

U.S. GEOLOGICAL SURVEY BULLETIN 2029 

/ 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1993 

For sale by 

U.S. Geological Survey, Map Distribution 

Box 25286, Bldg. 810, Federal Center 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Rankin, Douglas W. 

The volcanogenic Mount Rogers Formation and the overlying glaciogenic 

Konnarock Formation-Two Late Proterozoic units in southwestern 

Virginia I by Douglas W. Rankin. 

p. em. - (U.S. Geological Survey bulletin ; 2029) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:2029 

1. Geology, Stratigraphic-Proterozoic. 2. Volcanic ash, tuff, etc.-

Virginia. 3. Rocks, Sedimentary-Virginia. 4. Mount Rogers Formation. 

5. Konnarock Formation. I. Title. II. Series. 

QE75.B9 no. 2029 

[QE653.5] 

557.3 s-dc20 

[551.7'15'09755] 92-11110 

CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 3 

Previous Work 3 

Regional and Structural Setting 6 

Mount Rogers Formation (Revised Name) 7 

Volcanic Centers 10 

Fees Rhyolite Member (New Name) 10 

Mt. Rogers Volcanic Center 10 

Buzzard Rock Member (New Name) 12 

Whitetop Rhyolite Member (New Name) 12 

Wilburn Rhyolite Member (New Name) 14 

Lower Contact 16 

Upper Contact 17 

Konnarock Formation (New Name) 19 

Name, Type Area, and Thickness 19 

Description 19 

Upper Contact and Age 22 

Discussion and Summary 24 

References Cited 25 

FIGURES 

1-3. Geologic maps of: 

1. Parts of the central and southern Appalachian orogen 2 

2. The Blue Ridge tectonic province in southwestern Virginia and adjacent 

North Carolina and Tennessee showing thrust faults, Late Proterozoic volcanic centers, and location of the Mt. Rogers area 4 

3. The Mt. Rogers area 8 

4-13. Photographs of: 

4. Greenstone in the lower part of the Mount Rogers Formation 12 

5. Flow-layered lava, flow breccia, and lithophysae in lava of the Whitetop 

Rhyolite Member of the Mount Rogers Formation 13 

6. Welded tuff in the Wilburn Rhyolite Member of the Mount Rogers 

Formation 15 

7. Xenoliths in basal greenstone of the Mount Rogers Formation 17 

8. Massive diamictite in the Konnarock Formation 19 

9. Rhythmite in the Konnarock Formation 20 

10. Rhythmite and interlayered arkose bed in the Konnarock Formation 21 

11. Rhythmite containing dropstones in the Konnarock Formation 21 

12. Bedded diamictite in the Konnarock Formation 22 

13. Paraconformable contact between the Konnarock and Unicoi 

Formations 23 

Contents Ill 
CONVERSION FACTORS 

Measurements in this report are given in metric units except that elevations are given in feet for ease in 

referring to U.S. Geological Survey topographic maps of the 7.5-minute quadrangles. Conversion factors are 

provided below. 

Multiply By To obtain 

foot (ft) 0.3048 meter (m) 

millimeter (mm) .03937 inch (in.) 

centimeter (em) .3937 inch (in.) 

meter (m) 3.281 foot (ft) 

kilometer (km) .6214 mile (mi) 

cubic kilometer (km3

) .2399 cubic mile (mi3

) 

IV Conversion Factors 
The Volcanogenic Mount Rogers Formation and the 

Overlying Glaciogenic Konnarock Formation -Two 

Late Proterozoic Units in Southwestern Virginia 

By Douglas W. Rankin 

Abstract 

In the Mt. Rogers area of southwestern Virginia and 

adjacent North Carolina and Tennessee, the billion-yearold Grenvillian basement of the Blue Ridge anticlinorium 

is nonconformably overlain on the northwest limb by a 

stratified cover sequence that is disconformably overlain 

by the Lower Cambrian Unicoi Formation. This cover 

sequence is here divided into two formations. The older, 

dominantly volcanic unit is called the Mount Rogers 

Formation (revised name). The younger, which overlies 

the Mount Rogers, is the glaciogenic Konnarock Formation (new name). 

The volcanic rocks of the Mount Rogers Formation 

are a bimodal suite of basalt and rhyolite in which the 

younger rhyolites have peralkaline affinity. The rhyolites 

are concentrated in thick masses thought to have been 

volcanic centers. The erosion of the volcanic edifices of 

older centers, which may or may not be partially preserved in the area, produced extensive volcanic conglomerates that make up much of the lower part of the Mount 

Rogers Formation in the type area. The Fees Rhyolite 

Member (new name), near the base of the Mount Rogers 

Formation, was probably erupted from one of these earlier centers, and clasts of the Fees are recognizable in the 

conglomerates. Rhyolites of the Mt. Rogers volcanic center constitute the upper part of the Mount Rogers Formation in the type area. These include the Buzzard Rock 

Member (new name) which is lava, the Whitetop Rhyolite 

Member (new name), also largely lava, and the climactic, 

zoned, welded ash-flow sheet of the Wilburn Rhyolite 

Member (new name). The rhyolites of the Mt. Rogers 

volcanic center erupted about 760 Ma and are thought to 

be related to an early aborted episode of lapetan continental rifting. 

The Konnarock Formation consists of maroon massive diamictite (tillite), bedded diamictite, varvelike rhythmite that contains dropstones in places, and pink arkose. 

The Konnarock is mostly structurally isolated from the 

Mount Rogers Formation but in three areas is seen to be 

stratigraphically above it. In one of these areas, the 

Manuscript approved for publication April 3, 1992. 

contact appears to be unconformable; in the second, 

concordant; and, in the third, the nature of the contact is 

indeterminate but is probably concordant. The deformation that caused the unconformity could be synvolcanic or 

postvolcanic block faulting, which is common in volcanic 

terranes. 

INTRODUCTION 

The rocks described in this report are exposed in the 

Mt. Rogers area in southwestern Virginia and adjacent 

North Carolina and Tennessee (fig. 1). The Mount Rogers 

Formation, as used by Rankin (1970) for rocks in this area, 

consists of interlayered and interfingering bimodal volcanic 

and clastic sedimentary rocks, all metamorphosed under 

low to moderate greenschist-facies conditions. The prefix 

"meta" is omitted in the following discussion but should be 

assumed for these rocks. The strata nonconformably overlie 

Middle Proterozoic Laurentian basement, are overlain with 

apparent conformity by the basal Cambrian clastic sequence 

of the Chilhowee Group, and are interpreted to be of Late 

Proterozoic age. The strata crop out in three major thrust 

sheets and two or more areally restricted slices. Rankin 

(1967, 1970) noted that these strata could be roughly 

divided into three parts: the lower part is interbedded clastic 

sedimentary rock, greenstone, and rhyolite; the middle is 

dominantly rhyolite; and the upper part is sedimentary and 

includes extensive glaciogenic deposits. The lower two 

parts form continuous sequences in more than one thrust 

sheet but are generally separated by thrust faults from the 

upper part, which is stratigraphically beneath the Chilhowee 

Group. Those relations were not understood in 1970. 

The purpose of this paper is to redefine the Mount 

Rogers Formation to exclude the upper sedimentary part, to 

introduce the name Konnarock Formation for that upper 

sedimentary part, to briefly delineate the stratigraphic and 

structural setting of the Konnarock and redefined Mount 

Rogers Formations, and to introduce several names for 

volcanic members of the Mount Rogers. 

Introduction 
82° 

PENNSYLVANIA 

---,----... --::.:,,~: 

I '"---' ~.., I ,,f t, 

'/ !"-- \," I ', 

;" 

;I 

/'J 

I 

f'" I I 

( \ I 

/ ~ 

/ 

I 

Edge of ___.. 

KENTUCKY Coastal Plain 

TENNESSEE 

.Knoxville 

VIRGINIA 

NORTH CAROLINA 

,.....,.,-------------- 100 

SOUTH CAROLINA 

EXPLANATION 

CJ Rift-facies stratified rocks (Lower Jurassic and Upper Triassic) 

D Stratified rocks of the Jefferson accreted terrane (Lower Ordovician and Cambrian) 

CJ Rift-facies stratified rocks of Laurentia (lowermost Cambrian and Late Proterozoic) 

CJ Grenvillian crystalline basement (Middle Proterozoic) 

- --- Contact 

----Fault 

"----'--• Normal fault-Hachures on downthrown side 

.~ Thrust fault-Sawteeth on upper plate. Half arrows show direction of movement of -...::-- strike-slip component 

GM Grandfather Mountain window 

MC Mountain City window 

200 KILOMETERS 

Figure 1. Generalized geology of parts of the central and southern Appalachian orogen. Box outlines Mt. Rogers area 

shown in figure 2. Significant geographic features are labeled. 

2 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
Acknowledgments 

Over a period of many years, I have discussed the 

geology of the Mt. Rogers area with ma~y colleagues, m~st 

of whom have visited the field area With me. Those with 

whom discussions have been particularly helpful include 

R.L. Smith, J.M.G. Miller, J.C. Crowell, and the late 

D.W. Elliott, D.R. Wones, and M.D. Crittenden, Jr. For 

most field seasons in the Mt. Rogers area, I was assisted on 

an almost daily basis by my wife Mary Louise. Her help is 

gratefully acknowledged. The manuscript was improved by 

reviews by W.C. Burton and P.T. Lyttle. 

PREVIOUS WORK 

Interbedded volcanic and sedimentary rocks in southwestern Virginia were named the Mount Rogers series by 

Stose and Stose (1944) after exposures on Mt. Rogers, the 

highest mountain in Virginia. 1 The name was modified to 

Mount Rogers Volcanic Group by Rodgers (1953), and that 

usage was followed by King and Ferguson (1960). Because 

the unit is relatively restricted geographically to parts of ten 

7 .5-minute quadrangles (see fig. 2 for locations of the seven 

most important quadrangles) and because rock types intertongue locally and are repeated throughout the unit, Rankin 

( 1970) reduced the name to formation status as the Mount 

Rogers Formation. Although volcanogenic deposits constitute more than 50 percent of the unit, the modifier "volcanic" was dropped because of a large component of nonvolcanic strata in the unit. 

Most early reports on the geology of the Mt. Rogers 

area make no mention of volcanic rocks, probably because 

ancient volcanic rocks were generally not recognized by 

early American geologists (see Williams, 1894). As 

recently as 1916, the geologic map of Virginia included 

rocks of the Mount Rogers Formation in "crystalline schists 

and gneisses, chiefly micaceous, intruded with granite and 

basic igneous materials" (Watson, 1916). As early as 1869, 

however, Safford recognized a sequence of rocks in northeastern Tennessee (now including the Mount Rogers) that 

were intermediate in stratigraphic position and degree of 

metamorphism between the crystalline basement and the 

Chilhowee Group. In fact, Safford (1869) even suggested 

"referring them to" (including these rocks in) his Ocoee 

Group. The observation of intermediate degree of metamorphism between the Chilhowee and basement is generally correct and is because the Chilhowee Group generally 

1

Mt. Rogers, elevation 5,729 ft, formerly called Balsam Mountain, 

was named for William Barton Rogers, the first State Geologist of Virginia 

( 1835), tater founder and first president of the Massachusetts Institute of 

Technology ( 1862) and third president of the National A~ade~y of 

Sciences ( 1879) (Roberts, 1936). Elevations in this report are g1ven m feet 

for ease in referring to U.S. Geological Survey topographic maps of the 

7 .5-minute quadrangles. 

crops out northwest of the Mount Rogers and the grade of 

Paleozoic metamorphism increases to the southeast. 

About 60 km south of Mr. Rogers, Keith (1903) 

mapped metamorphosed basalt and rhyolite in what is now 

called the Grandfather Mountain Formation (Bryant and 

Reed, 1970). He also delineated a belt of "metarhyolite" at 

the northern border of the Cranberry 30-minute quadrangle 

about 4 km east of Payne Gap (fig. 2). He interpreted this 

body of metarhyolite to be thin sheets and dikes intrusive 

into his Cranberry Granite (later modified as Cranberry 

Gneiss). That body of metarhyolite is here interpreted to be 

part of the stratified sequence of the Mount Rogers Fo~ation above a thrust fault. Hence, Keith should be credited 

with the first recognition of a volcanic or hypabyssal 

component of the Mount Rogers. 

The first major study of the Mount Rogers Formation 

was by A.J. and G.W. Stose for the Virginia Geological 

Survey (Jonas and Stose, 1939). Their most extensive 

discussion of the unit is contained in the report on the 

Gossan Lead district and includes a detailed map of the 

eastern part of the Mount Rogers outcrop area (Stose and 

Stose, 1957). Stose and Stose (1957) subdivided their 

Mount Rogers Volcanic Group into a lower Flat Ridge 

Formation (with its Cinnamon Ridge Member and Cornett 

Basalt Member) and an upper unnamed rhyolite unit. These 

units were poorly defined and not useful as map units for the 

present study. The Flat Ridge and its members were 

abandoned by Rankin ( 1970). 

In their comprehensive report on the geology of 

northeasternmost Tennessee, King and Ferguson ( 1960) 

included several important observations concerning the 

Mount Rogers Formation, but their study of the formation 

was largely incidental to mapping the northeast end of 

the Mountain City window. Carrington ( 1961) reported 

rhythmically layered sedimentary rocks (herein called the 

Konnarock Formation but at that time included within the 

upper part of the Mount Rogers Volcanic Group) t~at 

contain large, scattered, well-rounded to subrounded gramtoid clasts as much as 1.25 m across. He concluded that the 

rhythmites were deposited in quiet water and reflected 

volcanic cycles, not glaciation. Rankin (1967, 1969) suggested a glacial origin for the laminated pebbly mudstones 

and associated muddy-matrix conglomerates (diamictites), 

a conclusion supported by Blondeau and Lowe (1972), 

Schwab (1976), and Miller (1986, 1989a). 

The present study of the Mt. Rogers area began in 

1962 with geologic mapping at a scale of 1:24,000. The 

project followed logically from the work of King and 

Ferguson (1960) in northeastemmost Tennessee and overlapped in time the end of the study of the Grandfather 

Mountain area, North Carolina, by Bryant and Reed (1970). 

The detailed Mt. Rogers study was suspended for several 

years in 1965 in favor of reconnaissance mapping of the 

Winston-Salem 1 °X2° quadrangle, which contains the Mt. 

Rogers area. The regional study provided a broader perPrevious Work 3 
Jefferson • 

£c 10 KILOMETERS 

Figure 2. Geology of the Blue Ridge tectonic province in southwestern Virginia and adjacent North Carolina 

area mapped in figure 3. 

4 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
Yc 

VJRGINIA 

NORTH -CAROLiNA 

o£a 

/ / 

/ 

' 

B 

Undivided 

Contact 

BLUE 

Northwest Limb 

[B} Chilhowee Group 

Disconformity 

0 

Konnarock Formation 

lzmml 

Mt. Rogers 

volcanic 

center 

~ ~till Lower Pond 

stratified Mountain 

rocks volcanic 

center 

EXPLANATION 

RIDGE 

Lower 

Cambrian 

I -z,.Qt.l 

Razor Ridge 

volcanic 

center 

Mount Rogers Formation 

Nonconformity 

Fault 

~-4. 

Axis of syncline 

Thrust fault-Dashed 

where uncertain 

81 °45' 

36° 

45 

~{-

«:-0 ~'?' 

0~ ~ 

<§ <t- ~A...~ ~~ 

'v<v Q'?' 

:0" 

"~0 

o+ <( 

~v\<v+- .t<vvq:. 

81 °15' 

36° 

45' 

rrerm-~ ----- --- r--'0 __ ( 0~ c;. / -l...Cj 

~'Y" «:-+- q_"t "'?"<?~+- 0~v~«; 

36° 

30 ' 

81 °45' INDEX SHOWING LOCATIONS OF 7.5-MINUTE QUADRANGLES 

DISCUSSED IN TEXT 

ANTICLINORIUM 

Core Southeast Limb 

~ 

Thrust 

~F~n l 

£Zw 

Wills Ridge 

Formation 

Cranberry Gneiss 

~-~ Probable thrust fault-Boundary 

between ultramafic-bearing strata 

(Ashe Formation) of the Jefferson terrane and nonultramafic-bearing strata (Wills Ridge Formation) of 

Laurentia 

NC 

36° 

30' 81 °15' 

Lower 

Ordovician 

Cambrian 

and Tennessee. Map shows thrust faults, Late Proterozoic volcanic centers, and location of the Mt. Rogers 

Previous Work 5 
spective in which to complete the field work in the Mt. 

Rogers area in 1973 and 1974 (Rankin, 1970, 1975, 1976). 

Detailed maps of the Mt. Rogers area have not yet 

been published, but the distribution of rock types and the 

regional setting are shown on the west half of the Winston 

Salem 1°X2° quadrangle at a scale of 1:250,000 (Rankin 

and others, 1972). The impetus for this paper is the recent 

documentation of two episodes of continental rifting near 

the end of the Proterozoic separated by as much as 200 m. y. 

(Aleinikoff and others, 1991). The older of the two 

episodes is represented by rhyolites of the Mount Rogers 

Formation, which are dated at about 760 Ma. Thus, roughly 

200 m.y. elapsed between eruption of the Mount Rogers 

rhyolites and the deposition of the siliciclastic rocks of the 

basal Cambrian Chilhowee Group. A reevaluation of the 

field data indicates that the glaciogenic deposits stratigraphically beneath the Chilhowee can be interpreted to be 

separated from the rhyolites by thrust faults or, in a few 

places, to be stratigraphically above them. Because of the 

long time interval between the rhyolites and the Chilhowee, 

the permissiveness of the field relations, and the current 

interest in Late Proterozoic glaciation, it seems wise to 

define a new stratigraphic unit for the glaciogenic deposits 

rather than to continue to include them as part of the Mount 

Rogers Formation. In order to demonstrate the need for a 

new formation as well as to provide stratigraphic names for 

map units within the redefined Mount Rogers Formation, 

the stratigraphy of the Mount Rogers will be discussed first 

after a review of the regional and structural setting of the 

Mt. Rogers area. 

REGIONAL AND STRUCTURAL SETTING 

The thrust sheets or slices that carry the Mount 

Rogers and Konnarock Formations are along the leading or 

northwestern edge of the composite Blue Ridge crystalline 

thrust sheet. The composite sheet experienced its last major 

movement (Alleghanian) in the late Paleozoic. The sole 

thrust, which has different local names along strike, carries 

Middle Proterozoic crystalline basement and its Late 

Proterozoic stratigraphic cover over little-metamorphosed 

to nonmetamorphosed Paleozoic strata as young as Mississippian of the Valley and Ridge province. In the Mt. Rogers 

area, the sole thrust is traditionally thought to be the 

Holston Mountain-Iron Mountain fault, which is the sole 

thrust of the Shady Valley thrust sheet and at its leading 

edge carries the Lower Cambrian Chilhowee Group over 

rocks as young as the Middle Ordovician clastic sequence 

(see Rankin, Drake, and Ratcliffe, 1989). In the interpretation of Rankin and others ( 1991), however, the fault that 

carries rocks exposed in the Mountain City window (probably the Pulaski fault) also cuts into the basement (fig. 1). 

The crystalline basement is mostly gneissic granite 

and augen gneiss included within the Cranberry Gneiss 

(roughly 1.2 to 1.0 Ga). These rocks are interpreted to be 

part of the Grenville province of Laurentia exposed in an 

outlier within the Appalachian orogen. The outlier, called 

the French Broad massif, is one of several external massifs 

of Grenvillian rocks exposed along the western margin of 

the Appalachian internides between Georgia and Newfoundland. The Mount Rogers and Konnarock Formations 

are the basal part of the cover sequence on the northwest 

flank of the French Broad massif, and the Wills Ridge 

Formation is interpreted to be the basal part of the cover 

sequence locally exposed on the southeast flank of the 

massif (Rankin and others, in press) (fig. 2). Hence, 

although the basement and cover are part of an imbricate 

thrust stack, the term "Blue Ridge anticlinorium" is still 

applicable. 

The Wills Ridge Formation was recently defined by 

Rankin and others (in press) as a new formation free of 

ultramafic rocks. The rocks of the Wills Ridge were 

formerly included in the Ashe Formation at its structural 

base. The juxtaposition of the Wills Ridge and Ashe 

Formations is thought to be along a thrust fault. The age of 

the Wills Ridge Formation is poorly constrained. It is 

bracketed between the Middle Proterozoic formation of 

Grenvillian basement and the Ordovician Taconian closing 

of the Iapetus Ocean. Whisonant and Tso (1992) concluded 

from a field and petrographic study of the Wills Ridge 

Formation (their "lower Ashe Formation") that it was 

deposited in subaqueous parts of fan or braid deltas that 

prograded into relatively deep water on Laurentian crust. 

Whether the absence of ultramafic bodies in the Wills Ridge 

is because the depositional site was on the Laurentian 

margin or because the age of the deposits is predrift is not 

known. The Wills Ridge Formation, therefore, is here 

assigned a Late Proterozoic to Cambrian age. Note that on 

figure 2, the Ashe Formation is indicated as being Lower 

Ordovician and Cambrian in agreement with the interpretation of Rankin and others (in press) that the Ashe formed as 

an accretionary wedge during subduction of Iapetan oceanic 

crust. Hence the age of the Ashe Formation is bracketed 

between the opening of Iapetus, roughly at the beginning of 

the Cambrian, and the Taconian closing of Iapetus. 

Figures 2 and 3 are generalized tectonic and geologic 

maps of the Mt. Rogers and surrounding areas that show the 

present interpretation of the configuration of thrust faults 

and their relation to the stratigraphic units under discussion. 

The interpretation differs in important ways from that 

portrayed in Rankin ( 1967, 1970) and Rankin and others 

(1972). Details of those differences will have to await 

publication of the detailed maps, but differences important 

for this report are noted briefly here. 

1. The east-northeast-trending thrust on the south side of 

Whitetop Laurel Creek separates glaciogenic deposits on 

the north in the Shady Valley thrust sheet from rocks of 

the Chilhowee Group on Chestnut and Lost Mountain to 

the south and dips steeply north where exposed by 

6 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock FormatKm 

u 
digging along Virginia Highway 859 (lat 36°38.85'N., 

long 81°40.05'W.) (fig. 3). It is reinterpreted to be the 

northeastern continuation of the Iron Mountain fault 

rather than the Catface fault of earlier interpretations. 

The reinterpretation for this particular segment of the 

fault is the same as that shown by King and Ferguson 

(1960). The reinterpretation puts this southern block of 

Chilhowee and the stratigraphically underlying glaciogenic deposits along Green Cove Creek (see Rankin, 

1967) within the Mountain City window. 

2. The Catface fault is now interpreted to go through 

McQueen Gap and separate the glaciogenic. deposits of 

the Konnarock Formation to the north along Green Cove 

Creek from the sheared felsites and sedimentary rocks of 

the Mount Rogers Formation to the south. The sharp 

transition from little-sheared glaciogenic deposits to 

strongly sheared volcanogenic rocks in outcrops along 

the Norfolk and Western (abandoned) railroad cuts was 

noted by Rankin (1967). To the east in the Park 

quadrangle, the Catface fault swings south and is overridden by the Stone Mountain fault, which carries the 

main body of the French Broad massif as well as the best 

studied and most complete section of the Mount Rogers 

Formation as here redefined. The trace of the Stone 

Mountain fault to the south to meet the well-defined 

position of the Stone Mountain fault southwest of U.S. 

Highway 421 (23 km south of the Virginia State line) 

has been determined by a combination of field mapping 

and the trace of a conspicuous aeromagnetic gradient 

(the high magnetic values are in the Stone Mountain 

thrust sheet) (U.S. Geological Survey, unpub. data). 

3. The previously unnamed thrust along the lower north 

slopes of Mt. Rogers that carries rhyolite of the Mount 

Rogers Formation over glaciogenic deposits of the 

Konnarock-Trout Dale valley continues eastward into 

the Middle Fox Creek quadrangle and carries clastic and 

volcanic sedimentary rocks of the Mount Rogers Formation over the eastward continuation of the glacio genic 

deposits. This fault is here named the Trout Dale fault. 

To the west, the Trout Dale fault overrides the Iron 

Mountain fault east of Konnarock and is in tum overridden on the north slope of Whitetop Mountain by the 

Stone Mountain fault. Thus, it forms the northeast 

termination of the Mountain City window (here an 

eyelid window) and is an out-of-sequence thrust. The 

Stone Mountain fault must also be an out-of-sequence 

thrust because of relations described in point 2 as well as 

here. 

MOUNT ROGERS FORMATION (REVISED 

NAME) 

The most studied and best understood area of the 

redefined Mount Rogers Formation is in the Stone Mountain thrust sheet. In that sheet, the lower two parts of the 

tripartite division of the formation as formerly used are 

exposed in continuous section (this does not imply continuous outcrop!) from the nonconformity above the Middle 

Proterozoic (Grenvillian) Cranberry Gneiss up to the 

Wilburn Rhyolite Member (named below), the youngest 

known rhyolite. The triangular area in the Stone Mountain 

thrust sheet from Whitetop Mountain to Pine Mountain in 

Virginia south to Nella, N.C., is designated as the type area 

of the Mount Rogers Formation (fig. 3). This type area 

includes the summit of Mt. Rogers. No stratigraphic unit 

younger than the Wilburn Rhyolite Member is preserved in 

the type area, and thus the top of the Mount Rogers is not 

preserved in the type area or, in fact, in the Stone Mountain 

thrust sheet. The glaciogenic deposits underlying the Unicoi 

Formation (the lowest of the three formations constituting 

the Chilhowee Group in the Mt. Rogers area) are nowhere 

preserved in the Stone Mountain thrust sheet and are 

herewith excluded from the Mount Rogers Formation. The 

Mount Rogers in the type area is on the order of 3,000 m 

thick, although structural complexities in the lower part of 

the formation make this estimate a crude one. 

The lower part of the Mount Rogers Formation 

consists of complexly folded graywacke, phyllite, conglomerate, arkose, greenstone, and rhyolite; only a major rhyolite has been broken out here. The clastic sedimentary rocks 

are about twice as abundant as the volcanic rocks. Most 

conglomerates are clast-supported volcanic conglomerates 

in which most of the clasts are porphyritic felsite. These are 

probably derived from the erosion of a volcanic edifice or 

edifices that predate the Mt. Rogers volcanic center (see 

below). Locally, basement clasts predominate. Where the 

conglomerates are deformed, the felsite clasts tend to be 

more flattened and elongated than the granitic basement 

clasts, suggesting that the felsites may have been pumiceous. The greenstone is in part nonporphyritic and in part 

plagioclase-phyric. The latter commonly has a crudely 

aligned to turkey-track (fig. 4) arrangement of the plagioclase. Some greenstones are amygdaloidal. In the type area, 

the major felsite in the lower part of the Mount Rogers is at 

or near the base of the formation; it is described in the next 

section. 

Metamorphic minerals in the matrix of the graywacke 

include biotite and stilpnomelane. In some of the arkose, 

detrital plagioclase has been replaced by epidote and quartz, 

and the basement-derived microcline is reddish orange. 

The resulting rock looks much like unakite, locally developed by similar metamorphic reactions in the underlying 

Cranberry Gneiss. The greenstone has typical greenschistfacies assemblages (albite-epidote-actinolite-chlorite, mostly 

with sphene and with or without quartz). 

Deformation is locally intense. Folding is polyphase, 

and clasts in the conglomerates, even up to boulder size, are 

flattened and elongated. Where shearing is intense, it is 

commonly difficult to distinguish between arkose and 

Mount Rogers Formation (Revised Name) 7 
8 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
) Zmw 

25.-1/141'" 7 '- 'Olin.;:--· ---... ----!J_,t a tlf ,_ Zmwp \ <" ) 

148'o.; vf'CUIJ(I 

.,Peal( 

36° 

37' 

30" 

Figure 3A. Geology of the Mt. Rogers area. 

Geology mapped in 1962-74. Base modified 

from U.S. Geological Survey 1 :100,000-scale 

map of the Wytheville 30' X60' quadrangle, 

Virginia, North Carolina, and Tennessee, 1982. 

See figure 38 for explanation and index of 

7.5-minute quadrangles. 

Mount Rogers Formation (Revised Name) 9 
graywacke of the Mount Rogers and granitic gneiss of the 

Cranberry. A microscopic aid in identifying the cover rocks 

and thus confirming the location of a field contact is the 

presence in the cover rocks of clasts of highly perthitic 

alkali feldspar derived from phenocrysts in the rhyolites. 

Enough of this volcanic perthite texture survives the shearing to be distinguishable from the basement microcline 

perthite. 

The Mount Rogers Formation is intruded by diabase 

(greenstone) dikes, rhyolite dikes, and a body of granophyre. The diabase and rhyolite dikes are not shown in 

figure 3. The diabase dikes intrude rocks as young as the 

Wilburn Rhyolite Member. Several dikes crop out on 

Wilburn Ridge, are roughly vertical, trend northeast, and 

are as much as 10 m thick. They could be feeder dikes for 

the metabasalts in the Unicoi Formation but have not been 

observed to intrude either the Konnarock or Unicoi Formation. The granophyre is poorly exposed and deeply weathered near the western border of the Trout Dale quadrangle 

about 1 km south of Fairwood (fig. 3). The mapped area, 

about 0.4 X0.8 km, is determined largely from float. 

Phenocrysts of perthite as large as 13 mm, quartz as large as 

4 mm, and equant opaque minerals as large as 1 mm are set 

in a reddish aphanitic groundmass that in places is micrographic. Because the granophyre appears to intrude the 

Whitetop Rhyolite Member (named below) (fig. 3) and 

because it has a phenocryst assemblage similar to that in the 

Wilburn Rhyolite Member, it is interpreted to be the 

intrusive equivalent of the Wilburn. 

Volcanic Centers 

Rhyolite makes up 50 to 60 percent of the Mount 

Rogers Formation. Where analyzed, it is mostly high-silica 

rhyolite, but some low-silica rhyolite is present. The rhyolites are metaluminous and have high concentrations of Nb, 

Y, and rare earth elements and low contents of Ba and Sr. 

Some rhyolite is thought to have been peralkaline at the 

time of eruption. Mafic phenocrysts are sparse, and fluorite 

is a common accessory mineral. Most of the rhyolite is 

concentrated in three thick masses interpreted to be the sites 

of volcanic centers that erupted 500 to 1 ,000 km3 of 

rhyolite. From east to west, these centers are here named 

Razor Ridge, Mt. Rogers, and Pond Mountain volcanic 

centers (fig. 2). 

Rhyolites of the Mt. Rogers volcanic center constitute 

the upper part of the redefined Mount Rogers Formation in 

the Stone Mountain thrust sheet and will be discussed more 

fully below. Rhyolites of the Mt. Rogers center also crop 

out in at least two {)ther and probably four other thrust 

sheets or slices. In the current interpretation, the Razor 

Ridge volcanic center is east of the Mt. Rogers center in the 

Stone Mountain thrust sheet and may crop out only in that 

thrust sheet. Because the rhyolite of that center is petrographically similar to the Fees Rhyolite Member, named 

below, in the lower part of the Mount Rogers Formation in 

the type area, the Razor Ridge center is probably older than 

the Mt. Rogers center. The Pond Mountain center is in the 

Catface thrust sheet southwest of and structurally below the 

Stone Mountain thrust sheet. Rhyolite of the Pond Mountain center has the same phenocryst assemblage as the Fees 

Rhyolite Member, but the phenocrysts constitute as much as 

40 percent of the rock, and feldspar phenocrysts are 

commonly a centimeter and may be as much as 3 em across. 

These coarsely porphyritic rhyolites could be hypabyssal 

intrusive rocks. The Razor Ridge and Pond Mountain 

volcanic centers will not be discussed further here. 

Fees Rhyolite Member (New Name) 

A prominent porphyritic rhyolite near the base of the 

Mount Rogers Formation in the type area is here named the 

Fees Rhyolite Member for exposures on the southern 

extension of Fees Ridge in the northeastern part of the Park 

quadrangle (fig. 3). The Fees is characterized by a phenocryst assemblage of quartz, perthite, and plagioclase. 

Plagioclase constitutes a variable, commonly small percentage of the phenocryst assemblage, but its presence with 

quartz phenocrysts distinguishes the Fees from the rhyolites 

of the overlying Mt. Rogers volcanic center. The groundmass is aphanitic and typically has a grayish-purple or 

maroon tinge as do most rhyolites of the Mount Rogers 

Formation. The rock is highly sheared. Phenocrysts typically make up 35 percent of the rock, and the feldspars are 

about 5 mm across. Mapping of float is interpreted to 

indicate that locally the Fees is in contact with the Cranberry Gneiss. Elsewhere the Fees is within the lower part of 

the Mount Rogers. These relations suggest that the basement topography was uneven. The Fees Rhyolite Member is 

on the order of 100 m thick. 

Mt. Rogers Volcanic Center 

The upper part of the Mount Rogers Formation in the 

type area in the Stone Mountain thrust sheet consists of a 

thick mass of extrusive rhyolites interpreted to be the site of 

a volcanic center called the Mt. Rogers volcanic center. The 

rhyolites have been mapped as three units, which are here 

named as members of the Mount Rogers Formation. From 

oldest to youngest, these are the Buzzard Rock Member 

(formerly unit C of Rankin, 1967, and Rankin and others, 

1974), Whitetop Rhyolite Member (formerly unit B), and 

Wilburn Rhyolite Member (formerly unit A). The Buzzard 

Rock and Whitetop consist mostly of lava flows, and the 

upper or Wilburn Rhyolite Member is a welded ash-flow 

sheet. 

The relatively simple structure of the rhyolites of 

this center contrasts with the polyphase tight folding 

observed toward the base of the formation. No major 

structural discordance was observed between the rhyolites 

10 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
c 

0 

:0 

Ill 

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0 

u... 

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OJ 

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2 

c 

(l) 

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Undivided u 

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EXPLANATION 

Stratified Rocks 

I £e ... l 

Erwin Formation 

Hampton ~ Formation 

Unicoi ~ Formation 

Disconformity 

rn 

Undivided Massive diamictite 

Konnarock Formation 

lzmwl lzmwbl 

Welded tuff Breccia Phenocryst-poor 

welded 

-

tuff 

Wilburn Rhyolite Member 

Lava Lapilli tuff 

Whitetop Rhyolite Member 

lzmbl 

Buzzard Rock Member 

Lower 

~ part of 

formation , undivided 

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Fees Rhyolite 

Member 

Intrusive Rocks 

Granophyre 

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Contact- Dashed 

where gradational 

Fault 

Thrust fault 

0 

-+- ___JE ...cryo 

Vertical Inclined Overturned 

Strike and dip of bedBall indicates top of 

bed known from 

sedimentary features 

KONNAROCK WHITETOP TROUT 

MTN DALE 

0 36 

37 

30 "------ TENN / GRASSY PARK 

( f--- CREEK 

-~A 

1 GRAYSON l Nc--

1'' ----

81 °30 ' 

INDEX SHOWING LOCATIONS 

OF 7.5-MINUTE QUADRANGLES 

DISCUSSED IN TEXT 

36° 

37' 

30" 

Cranberry ~ Gneiss 

0 

Compaction foliation 

BR 

Buzzard Rock 

c 

Ill 

·s:: 

.D 

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Figure 38. Explanation for figure 3A and index of 7.5-minute quadrangles discussed 

in text. 

Mount Rogers Formation (Revised Name) 11 
Figure 4. Greenstone with turkey-track arrangement of 

plagioclase phenocrysts from lower part of the Mount 

Rogers Formation. Float block located along Appalachian 

Trail on northwest side of Elk Garden Ridge about 1 km 

northeast of the height of land on Virginia Highway 600, 

Whitetop Mountain quadrangle (fig. 3). U.S. penny for 

scale. 

and the underlying greenstone and sedimentary package. 

The Buzzard Rock Member is discontinuous beneath the 

Whitetop Rhyolite Member, but at present there is no 

evidence that this discontinuity is structural. Indeed the 

Buzzard Rock Member interfingers with both the greenstone and sedimentary and tuffaceous rocks along the 

southern slopes of Mt. Rogers and Haw Orchard Mountain. 

The contrast in structural style is probably the result of the 

massiveness of the rhyolite pile relative to the inhomogeneous nature of the lower part of the Mount Rogers 

Formation. The massive rhyolite, however, is penetratively 

deformed as evidenced by foliation of variably developed 

intensity, deformed phenocrysts and spherulites, and compaction foliation (eutaxitic texture) in welded tuffs systematically oblique to the axes of columnar joints. 

Buzzard Rock Member (New Name) 

The Buzzard Rock Member is named for exposures 

on Buzzard Rock, a shoulder of Whitetop Mountain, in the 

southwest corner of the Whitetop Mountain quadrangle (fig. 

3). It is the lowest and thinnest of the three rhyolite units of 

the Mount Rogers Formation in the Mt. Rogers volcanic 

center. The rock is characterized by an aphanitic, commonly grayish-purple or maroon groundmass containing 5 

to 20 percent phenocrysts of perthitic alkali feldspar and 

plagioclase in subequal amounts. The phenocrysts are 

typically 2 to 4 mrn across. Growth aggregates of the two 

feldspar species are common; monomineralic aggregates of 

plagioclase are particularly common. The reddish perthite 

and greenish-white plagioclase phenocrysts give the rock a 

distinctive appearance easily recognized in mapping outcrop or float. 

Scattered observations of flow layering indicate that 

the unit consists of lava flows. Flow layering is not obvious 

at the type locality on Buzzard Rock but is conspicuous in 

roadcuts between the elevations of 4,760 and 4,880 ft on the 

Grayson Highlands State Park Road on the west side of 

Haw Orchard Mountain (fig. 3). In one large float block 

along this stretch of road, the flow layering and folding are 

brought out by layered concentrations of purple-fluoritefilled amygdules a few millimeters long. The amygdules, 

which also contain quartz, albite, epidote, and sphene, 

make up about 25 percent of the rock, which may be the 

scoriaceous margin of a flow. The weathered surface of the 

rhyolite is commonly mottled with whitish spots; in some 

thin sections it can be seen that the spots are poorly 

preserved spherulites. The alkali feldspar phenocrysts are 

highly perthitic mesoperthite in which the two phases tend 

to be in coarse patches, a feature shared with the alkali 

feldspar phenocrysts in the two overlying rhyolite members. 

The Buzzard Rock Member is a low-silica rhyolite. The 

normalized volatile-free silica content of four samples 

ranges from 68.9 to 70.5 percent, which is distinctly lower 

than that of the two overlying high-silica rhyolites (Rankin, 

unpub. data, 1975). 

On the southwest side of Haw Orchard Mountain, the 

Buzzard Rock Member is about 300m thick and consists of 

at least two flows separated by maroon arkose. In the thrust 

slice above the Trout Dale fault, the Buzzard Rock Member 

is about 350 m thick on the east side of Elk Ridge. 

Whitetop Rhyolite Member (New Name) 

Phenocryst-poor rhyolite that crops out on the summit 

of Whitetop Mountain (about 5,530 ft and the second 

highest mountain in Virginia) in the southwestern corner of 

the Whitetop Mountain quadrangle is here named the 

Whitetop Rhyolite Member of the Mount Rogers Formation 

(fig. 3). The rhyolite is characterized by an aphanitic 

groundmass, commonly grayish purple or maroon, containing 0 to 10 percent phenocrysts of quartz and perthite in 

varying proportions. Commonly quartz is more abundant 

than perthite. The phenocrysts are typically small (less than 

2 mm across) and inconspicuous. Some samples are aphyric 

on the scale of a thin section. Some of the quartz phenocrysts are embayed, and the perthite is a mesoperthite in 

which the two phases tend to be in patches. Locally, 

in the upper drainage basin of Wilson Creek, lava of 

the Whitetop contains microphenocrysts of arfvedsonite. 

These arfvedsonite-bearing lavas are directly overlain by 

arfvedsonite-bearing, phenocryst-poor welded tuff at the 

base of the Wilburn Rhyolite Member. This stratigraphy 

suggests that, toward the end of the Whitetop lava cycle, a 

period of relative quiescence ensued long enough for some 

upper parts of the magma chamber to evolve peralkaline 

12 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
liquids and to build up enough volatile components to 

produce the climactic ash-flow eruption of the Wilburn 

Rhyolite Member. The Whitetop is a high-silica rhyolite. 

Six samples of lava have silica contents ranging from 76.72 

to 78.47 percent, normalized volatile free. 

Many outcrops or parts of outcrops are massive and 

provide no diagnostic textures indicative of the eruptive 

mechanism. Furthermore, in numerous outcrops or parts of 

outcrops, the rhyolite is bleached white (silicified), and in 

some areas the rhyolite contains significant pyrite. Textures 

are best seen on weathered outcrop surfaces and weathered 

joint surfaces. Flow layering, commonly chaotic, is 

observed in many outcrops (fig. SA) and indicates that most 

of the unit consists of lava flows. Flow breccias are also 

observed (fig. SB). Flow layering is commonly emphasized 

by the concentration in layers of relict spherulites. Relict, 

thin-walled, multishelled lithophysae have also been 

observed at several localities. Two of the best of these are 

( 1) at an elevation of about 5,220 ft on the road up 

Whitetop Mountain and (2) at an elevation of about 4,800 

Figure 5. Whitetop Rhyolite Member of the Mount Rogers 

Formation. A, Flow-layered lava. Outcrop, north peak 

Haw Orchard Mountain, Whitetop Mountain quadrangle 

(fig. 3). Pencil for scale. B, Flow breccia. Outcrop in 

pasture in 5,170-ft saddle between Brier Ridge and Mt. 

Rogers, Whitetop Mountain quadrangle (fig. 3). Pencil for 

scale. C, Multishelled lithophysae in lava. Outcrop at 

elevation of about 4,800 ft on ridge extending south from 

Wilburn Ridge toward Haw Orchard Mountain (lat 

36°37.25'N., long 81°30.90'W.), Whitetop Mountain quadrangle (fig. 3). Scale shows centimeters. 

ft on the ridge extending south from Wilburn Ridge (fig. 

5C). At the Whitetop Mountain locality, the lithophysae are 

as much as 5 em across and contain up to 40 shells. Both 

spherulites and lithophysae may be deformed so that the 

long axes of the ellipsoids plunge southeast at moderate 

angles. 

The Whitetop Rhyolite Member also includes a variety of pyroclastic rocks. Typically these occur as isolated 

outcrops and are difficult to relate to a regional pattern with 

two exceptions. A tuff breccia (not shown on fig. 3) at the 

base of the Whitetop on the northwest side of the saddle 

between Buzzard Rock and Whitetop Mountain contains 

unsorted clasts as much as 30 em across in an aphanitic 

phenocryst-poor matrix. The clasts include pieces of the 

Buzzard Rock Member, pieces of the Whitetop Rhyolite 

Member, and sparse clasts of basement gneissic granite. In 

thin section, scattered fragments of quartz, perthite, microcline, and plagioclase are xenocrysts in the matrix. A strong 

foliation has destroyed any primary structure in the matrix. 

The tuff breccia may be the initial air-fall deposit of the 

Mount Rogers Formation (Revised Name) 13 
Whitetop Rhyolite Member deposited on the underlying 

Buzzard Rock Member. A pyroclastic unit of phenocrystpoor rhyolite several meters thick has been mapped much of 

the way, but not all of the way, around the upper part of 

Whitetop Mountain (fig. 3). Some of this material is 

composed of lithic lapilli tuff breccia in which the fragments are sorted by size; some is probably welded tuff, and 

some was suggested by R.L. Smith (U.S. Geological 

Survey, oral commun., 1973) to be near-vent agglutinate. 

The last crops out in a series of ledges at an elevation of 

about 5,000 ft on the east shoulder of Whitetop Mountain 

(lat 36°38.50'N., long 81°35.45'W.) and is characterized 

by flattened lapilli for which the compaction foliation is 

wavy, not planar. Tuff breccia in the lower part of this unit 

on the north side of the saddle through which the road to the 

summit of Whitetop Mountain passes, at an elevation of 

more than 4,800 ft, contains clasts of lava from the 

Whitetop Rhyolite Member and clasts of the Buzzard Rock 

Member, porphyritic rhyolite resembling the Fees Rhyolite 

Member, greenstone, and gneissic granite. Most of the 

clasts are pebble to granule size, but pieces of the Whitetop 

are as much as 30 em across. 

As determined from the map pattern and attitudes of 

flow layering and compaction foliation, the Whitetop Rhyolite Member on Whitetop Mountain has an estimated 

minimum thickness of about 500 m (fig. 3). The top of the 

Whitetop is not preserved in this section. The Whitetop 

Rhyolite Member between the Buzzard Rock Member on 

Haw Orchard Mountain and the Wilburn Rhyolite Member 

is estimated to be about 750 m thick. 

Wilburn Rhyolite Member (New Name) 

Porphyritic rhyolite on Wilburn Ridge2 in the southeastern part of the Whitetop Mountain quadrangle is here 

named the Wilburn Rhyolite Member of the Mount Rogers 

Formation (fig. 3). The rock is characterized by an 

aphanitic groundmass, commonly grayish purple or 

maroon, containing about 30 percent quartz and perthite 

phenocrysts in the main body of the unit. The phenocrysts 

are mostly 2 to 3 mm across; some perthite phenocrysts are 

as large as 6 mm. Quartz phenocrysts are commonly 

embayed, and the perthite, as in the Buzzard Rock Member, 

is a mesoperthite in which the two phases are present as 

patches within the phenocryst outline. Typically, perthite is 

more abundant than quartz in a ratio of about 3:2, but in 

2

Named for Wilburn Waters, a prominent 19th century local figure 

who is buried on a hill overlooking Big Horse Creek in nearby North 

Carolina (Park quadrangle). A marker along the stream pointing to his 

grave reads: 

WILBURN WATERS 

1812-1878 

PREACHER 

BEAR HUNTING PIONEER 

KILLED 120 BEARS 

many rocks they are present in subequal amounts. The 

lower few tens of meters of the Wilburn Rhyolite Member 

contain fewer and smaller phenocrysts, typically about 10 

percent quartz and perthite in subequal amounts. 

The Wilburn is a welded ash-flow sheet interpreted to 

have been emplaced by the culminating eruptions from the 

Mt. Rogers volcanic center, which probably led to a caldera 

collapse. It is thought to have been erupte9 and emplaced 

subaerially. Diagnostic features of welded ash-flow tuffs 

such as columnar joints and fiamme (flattened pumice 

lumps) (fig. 6A) are present throughout the member. The 

columnar joints have been modified by Paleozoic tectonism 

but are still convincing (fig. 6B). Devitrified and flattened 

shards are visible in numerous thin sections (fig. 6C). 

Irregularly shaped, calcite-filled vugs arranged in crudely 

columnar zones perpendicular to the compaction foliation 

may be fossil fumaroles (R.L. Smith, oral commun., 1973). 

The main body of the ash-flow sheet is highly compacted. 

Fiamme range in length from 1 or 2 em to a maximum of 

about 20 em and have a flattening ratio of about 10:1. No 

variation in flattening ratio through the main body of the 

sheet was observed. In many thin sections, the spherulitic 

centers and axiolitic borders of the fiamme are preserved. In 

most thin sections, however, the fiamme are simply coarser 

grained mosaics of quartz and feldspar than the rest of the 

groundmass. In numerous thin sections as well as numerous 

outcrops, no fiamme are visible, presumably because the 

rocks are recrystallized. 

In 1973 a suite of samples was collected from two 

sections through the Wilburn Rhyolite Member on Wilburn 

Ridge. Both of these sections started at the basal contact 

with the Whitetop Rhyolite Member and extended stratigraphically upward through the Wilburn as high as unequivocal compaction foliation indicated a consistent direction of 

dip. The stratigraphic thickness of the thicker section as 

thus determined is 760 m. Topographic profiles of each 

section combined with the attitude of compaction foliation 

near each sample site determined the stratigraphic position 

of each sample in the ash-flow sheet. Petrography and 

chemistry of these samples indicate that the Wilburn Rhyolite Member is a zoned ash-flow sheet. Preliminary results 

of this study were presented by Novak and Rankin (1980). 

The basal 30m of the Wilburn, in addition to having 

only about 10 percent quartz and perthite phenocrysts, is 

characterized by microphenocrysts of zincian arfvedsonite 

and sparse acmite. Weathered outcrops in this zone show a 

mottling of small (millimeter-size) black spots or lenses that 

are aggregates of microphenocrystic arfvedsonite. The 

acmite is restricted to the basal zone, but the arfvedsonite 

persists upward in decreasing amounts. In the top third of 

the studied sections, minor biotite is the sole Fe-Mg 

groundmass mineral. In the basal zone, the fiamme are 

somewhat smaller and more compacted. In fact, some of the 

eutaxitic texture approaches flow layering, suggesting that 

parts of the alkalic lower part of the Wilburn flowed during 

14 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
Figure 6. Welded tuff, Wilburn Rhyolite Member of the 

Mount Rogers Formation. A, Fiamme (collapsed pumice 

lumps) in welded tuff. Float boulder, at elevation of about 

3,480 ft about 0.6 km south of Fairwood, Trout Dale 

quadrangle (fig. 3). Dime for scale. B, Columnar jointing in 

outcrop at elevation of about 5,000 ft on ridge extending 

south from Wilburn Ridge, Whitetop Mountain quadrangle (fig. 3). Backpack is about 0.5 m tall. C, Photomicrograph showing phenocrysts of perthite (P) and quartz (Q) 

in welded tuff. Devitrified and flattened shards are visible 

in the groundmass. Plane-polarized light. Thin section is 

of sample R-TC2-14, one of two samples of rhyolite from 

the Mount Rogers Formation dated by T.W. Stern (Rankin 

or after compaction. Lithic inclusions are common in the 

basal zone and are found scattered well up into the main 

body of the Wilburn. In places, lithic fragments in the basal 

10 m or so of the Wilburn constitute almost 50 percent of 

the rock in which defonned fiamme may be seen between 

the lithic fragments. Excellent exposures of the fragmentrich, basal ash-flow tuff are at an elevation of about 4,900 

and others, 1969). Sample is from elevation at about 3,620 

ft on ridge northwest of Solomon Branch on the north 

slope of Stone Mountain (Pine Mountain on fig. 3), Trout 

Dale quadrangle (lat 36°41.46'N., long 81°27.75'W.). Horizontal field of view about 3 mm. D, Fiamme deformed by 

steeply dipping regional cleavage. Cleavage surfaces are 

poorly defined here, but they regionally strike northeast 

and dip moderately southeast. Outcrop is basal, 

phenocryst-poor, arfvedsonite-bearing welded tuff at elevation of about 4,960 ft on ridge extending south from 

Wilburn Ridge, Whitetop Mountain quadrangle (fig. 3). 

Dime for scale. 

ft on the southern extension of Wilburn Ridge in the 

Whitetop Mountain quadrangle (lat 36°38.19'N., long 

8l 030.94'W.) and are at about 4,500 ft on the south side 

of Wilburn Ridge in the Trout Dale quadrangle a short 

distance above the Grayson Highlands State Park Road 

(lat 36°38.27'N., long 8l 0

29.80'W.) (fig. 3). Lithic 

clasts include Whitetop Rhyolite Member, Buzzard Rock 

Mount Rogers Formation (Revised Name) 15 
Member, arfvedsonite-bearing Wilburn Rhyolite Member, 

coarsely porphyritic rhyolite, granite, and gneissic granite. 

At the exposure in the Trout Dale quadrangle, nearly 75 

percent of the fragments are of the Whitetop Rhyolite 

Member (as large as 35 X 50 em), and most of the rest are 

granite. Clearly the basal Wilburn ash flows overran a 

rubbly surface of Whitetop Rhyolite Member but carried 

with them fragments of older units including basement, 

probably derived from the vent walls. Zones rich in lithic 

fragments higher up in the Wilburn probably mark distinct 

flow units. These zones were not traced in mapping. 

Analyses of 18 samples from the two sections show 

that the Wilburn Rhyolite Member is a high-silica rhyolite 

having a mean Si02 content of 76.0± 1. 7 weight percent 

dry (Novak and Rankin, 1980). The mineralogic and 

chemical zonation through the ash-flow sheet reflects a 

magma chamber in which Ti02 decreased upward, whereas 

F, the differentiation index, the molecular (Na+ K)/A1 

ratio, and the Fe/(Fe+ Mg) ratio increased upward. The 

basal zone representing the upper part of the magma 

chamber, although containing peralkaline minerals (acmite 

and arfvedsonite), is not now peralkaline (molecular 

(N a+ K)/ A 1 > 1) but probably was prior to regional metamorphism. 

Breccia cropping out over areas as large as 2X0.8 km 

has been mapped at the base of the Wilburn Rhyolite 

Member on the north slopes of Mt. Rogers and Pine 

Mountain (Whitetop Mountain and Trout Dale quadrangles, 

respectively, fig. 3). The matrix of the breccia is quartzand perthite-phyric tuff of the Wilburn. Devitrified shards 

can be seen in some thin sections. No sodic amphibole or 

acmite microphenocrysts have been observed. The breccia 

fragments include lava and flow breccia of the Whitetop 

Rhyolite Member in fragments up to several meters across, 

welded tuff of the Wilburn, and rare granite. In most 

outcrops, the Whitetop blocks are the dominant clasts and 

may constitute 75 to 80 percent of the rock. At first 

appearance, these outcrops look like flow breccia of the 

Whitetop Rhyolite Member unless the matrix, of which 

there may not be much, is examined carefully. Perhaps 

these matrix-poor breccias of the Wilburn Rhyolite Member 

are parts of the collapsed caldera wall infiltrated with ash of 

the Wilburn. The breccias crop out at or near the leading 

edge of the Stone Mountain thrust sheet and only in that 

thrust sheet. Hence, other direct evidence for the caldera is 

not preserved. 

Much of the Wilburn has been penetratively 

deformed. Phenocrysts, particularly the quartz, have been 

sheared. The sheared quartz typically has "ribbon" extinction. Regional foliation has produced a crinkling of fiamme 

in places (fig. 6D) and an alignment of microlites (opaque 

minerals or arfvedsonite commonly) in the groundmass. In 

most outcrops, the compaction foliation is at a high angle, 

but not at right angles to the axes of columnar joints. 

Lower Contact 

The Mount Rogers Formation nonconformably overlies the Middle Proterozoic (Grenvillian) Cranberry Gneiss. 

The actual contact is rarely exposed. Recognizable clasts up 

to boulder size of granite and gneissic granite of the 

Cranberry Gneiss are scattered throughout the Mount Rogers Formation in both sedimentary and volcanic rocks. 

Their presence indicates that the Grenvillian basement 

granite had been uplifted and was exposed to erosion at the 

time of deposition of the Mount Rogers. Four localities 

illustrating features observed at or near the contact are 

described below. At the first locality, the contact is a fault; 

at the other localities, the contact is stratigraphic. 

First area providing data on lower contact. -

A saprolite roadcut at an elevation of about 3,200 ft a little 

more than 1 km north of Flat Ridge in the northwestern part 

of the Middle Fox Creek quadrangle (fig. 2, lat 

36°42.53'N., long 81 °20.95'W.) exposes a fault contact 

between the Mount Rogers Formation to the north and the 

Cranberry Gneiss to the south. The fault strikes N. 70° W. 

and is vertical. A zone of blastomylonite about a meter wide 

separates light-greenish-gray, quartz-pebble-bearing conglomerate saprolite from orange saprolite of gneissic granite. This fault is interpreted to be the Stone Mountain fault, 

but it is also possible that the structure here is more 

complicated than deduced from reconnaissance mapping in 

1965. Early movement on this fault could be related to 

Iapetan continental rifting. 

Second area providing data on lower contact. -In 

the northeastern part of the Middle Fox Creek quadrangle 

(fig. 2, lat 36°44.55'N., long 81°16.35'W.; elevation about 

3,100 ft), a residual regolith separates the Cranberry Gneiss 

(here a little-deformed granite) from the Mount Rogers. 

This locality was noted by Stose and Stose ( 1957) and 

shown on their plate 60. They considered the regolith to be 

a conglomerate, or perhaps there is a conglomerate along 

strike that was not visited by me. At the above locality, the 

regolith is overlain by a few meters of medium- to coarsegrained graywacke with a maroon matrix, then by laminated 

maroon argillite, which grades through purple tuffaceous 

rocks into greenstone. About 0. 7 km to the southeast, on 

Cinnamon Ridge (fig. 2), greenstone forms the basal Mount 

Rogers Formation. The basal part of the greenstone is 

oxidized (maroon color) and contains abundant littledeformed xenoliths of subangular to subrounded granite 

(Cranberry Gneiss) as large as 0.7 m (fig. 7). Superficially 

this rock resembles diamictite of the Konnarock Formation 

described below, but the hand specimens have noticeably 

high specific gravity (heft), and the maroon "matrix" 

contains abundant hematite and abundant, randomly oriented, millimeter-size, white-weathering plagioclase laths. 

This area has been visited in reconnaissance only, but the 

oxidized basal zone appears to grade up into green 

amygdaloidal greenstone without granite clasts. 

16 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
Figure 7. Basal oxidized zone of maroon greenstone at 

base of the Mount Rogers Formation. Xenoliths of billionyear-old Cranberry Gneiss probably picked up from 

underlying regolith. Note that xenoliths are littledeformed granite. They were incorporated into the basalt 

prior to Paleozoic deformation that profoundly affected 

the main body of the Cranberry Gneiss farther southeast. 

Float block on Cinnamon Ridge in northeast corner of 

Middle Fox Creek quadrangle (fig. 2). Scale shows centimeters. 

Third area providing data on lower contact. -A 

large poison-ivy-covered greenstone outcrop near the eastern border of the Park quadrangle about 260 m north of the 

North Carolina State line is interpreted to be the basal 

outcrop of the Mount Rogers Formation. The outcrop, 

about lO m high, is above the road on the east side of a 

stream (fig. 3, lat 36°34.93'N., long 8l 0

30.30'W.). In the 

upper part of the outcrop, the greenstone is free of basement 

clasts, but these become increasingly abundant in the 

greenstone toward the bottom of the outcrop, where they 

constitute about 50 percent of the rock. The basement clasts 

are gneissic granite, granite, and quartz in rounded and 

stretched pebbles to boulders. The largest clast observed is 

35 em across perpendicular to the stretching. Basement is 

present in float across the stream and in nearby exposures of 

this structural1y complex area. The actual contact was not 

observed. 

Fourth area providing data on lower contact.-

Finally, basement-cover relations may be studied near 

Buzzard Rock, southwestern Whitetop Mountain quadrangle (fig. 3). This area was described by Rankin (1967). 

Buzzard Rock is about 5,120 ft in elevation. At an elevation 

of 4,980 ft on the south side of Buzzard Rock, ledges of 

Cranberry Gneiss are as close as 3 m horizontally and less 

than 1 m vertically from ledges of the Buzzard Rock 

Member of the Mount Rogers Formation. The Cranberry 

here is a medium-grained mylonitic gneiss with a strong 

subhorizontal foliation. In addition to sheared and fragmented quartz, microcline, and plagioclase , the rock contains chlorite, epidote, abundant sericite, and minor 

stilpnomelane. Although the foliation in the Cranberry 

Gneiss is pronounced, lineation is not obvious. The Cranberry is overlain by about 20 m of strongly sheared 

tuffaceous material. Although the shearing partially 

obscures the lithologies, the tuff is a mixture of basement 

granitic rocks and disaggregated granite (quartz and orange 

basement microcline fragments are easily visible), and 

Buzzard Rock Member rhyolite in an aphanitic felsic 

matrix. The tuff may be water-reworked air-fall material of 

the Buzzard Rock and basement debris. The percentage of 

recognizable basement material in the tuff decreases 

upward, and, at an elevation of about 5,040 ft, outcrops are 

of sheared rhyolite of the Buzzard Rock Member. The 

strong deformation of this rock is probably because the 

nonconformity here is only about 90 m above the Stone 

Mountain fault (fig. 3). 

About 300m around to the northwest, on the Buzzard 

Rock side of the saddle between Buzzard Rock and Beech 

Mountain, the highest Cranberry Gneiss outcrops contain 

pieces of the Buzzard Rock Member. These outcrops are 

interpreted to be an admixture of regolith of the Cranberry 

Gneiss and pieces of rhyolite of the Buzzard Rock Member. 

Apparently shearing has obscured the fragmental nature of 

the Cranberry. No tuffaceous interval is present here 

between the Cranberry and homogeneous Buzzard Rock. 

In summary, for the three localities where the contact 

is stratigraphic, the Mount Rogers Formation is nonconformably above the Cranberry Gneiss. Mount Rogers units in 

contact with the Cranberry include a variety of sedimentary 

rocks, greenstone, and the Fees Rhyolite Member of the 

lower part of the formation and the Buzzard Rock Member 

of the Mt. Rogers volcanic center. My earlier statements 

that at one place or another nearly every unit from the 

bottom to the top of the formation is in contact with the 

Cranberry (Rankin, 1967; Rankin, Drake, and others, 1989) 

need to be revised in accordance with removing the glaciogenic deposits from the Mount Rogers Formation. However, the stratigraphic range of Mount Rogers units that are 

in contact with the Cranberry Gneiss, as well as the 

coarseness of many conglomeratic units, does indicate that 

the Mount Rogers was deposited on a basement topography 

of considerable relief such as one might find in continental 

rifts. 

Upper Contact 

As noted above, the upper contact of the Mount 

Rogers Formation is not preserved in the Stone Mountain 

thrust sheet. The upper contact is interpreted to be preserved 

in three areas in underlying thrust sheets or slices. 

Mount Rogers Formation (Revised Name) 17 
First area providing data on upper contact. -Near 

the community of Konnarock astride the border of the 

Konnarock and Whitetop Mountain quadrangles, a much 

thinned and geographically restricted section of the Mount 

Rogers is preserved above an inlier of Cranberry Gneiss 

(fig. 3). These rocks are interpreted to be in the Shady 

Valley thrust sheet immediately above the Iron Mountain 

fault (fig. 2) and stratigraphically below maroon clastic 

sedimentary rocks here included in the Konnarock Formation. In this area, the Mount Rogers includes a very thin 

section of greenstone and sedimentary rocks of the lower 

part of the formation as well as all rhyolite members of the 

Mt. Rogers volcanic center in their proper stratigraphic 

order (that is, Buzzard Rock, Whitetop, and Wilburn). 

Mapping of float suggests that both greenstone and lower 

sedimentary rocks are in stratigraphic contact with the 

Cranberry Gneiss basement. 

The contact between the Wilburn Rhyolite Member 

and maroon sedimentary rocks of the Konnarock Formation 

is exposed in a roadcut along Virginia Highway 603 just 

west of the highway bridge over Big Laurel Creek and about 

400 m east of the Konnarock-Whitetop Mountain quadrangle boundary (fig. 3). On the west side of this outcrop, 

arkose overlies the Wilburn and is separated from it by 2 to 

4 em of sheared rock. Above the shear zone, the arkose 

appears undeformed and is overlain by maroon rhythmites 

(graded sets of siltstone or sandstone and mudstone). The 

contact strikes N. 40° E. and dips 45° N. On the east side of 

the outcrop is a volcanic conglomerate in steep contact with 

the Wilburn and beneath the projection of the bedding in the 

now-eroded arkose. The conglomerate is composed dominantly of clasts of the Wilburn, but it also contains clasts of 

the Whitetop Rhyolite Member and rhythmite. A thin 

section of the matrix of this rock contains basement microcline. The Wilburn Rhyolite Member in the outcrop is much 

fractured, and many surfaces are slickensided. The contact 

has been exposed by digging in a road bank on its strike 

projection to the southwest across Big Laurel Creek. There 

the contact is sharp and vertical and strikes N. 75° E. The 

basal arkose of the Konnarock is again overlain by maroon 

rhythmite. 

On the basis of evidence from both sides of Big 

Laurel Creek, the contact between the Wilburn Rhyolite 

Member and the Konnarock Formation is interpreted to be 

a stratigraphic one in which the Konnarock covered an 

irregular surface marked by rubble-filled pockets. Contacts 

between the Konnarock and other units of the Mount Rogers 

are not exposed. Outcrops are generally poor, and much of 

the mapping is based upon float. The resulting map pattern, 

however, is interpreted to delineate an unconformity 

beneath the Konnarock Formation along which maroon 

rhythmite or arkose of the Konnarock is in contact with and 

stratigraphically above all units from the Cranberry Gneiss 

to the Wilburn Rhyolite Member. If the above interpretation 

is correct, at least some deformation, even i~ it was simply 

,• 

warping or faulting related to the volcanism, must have 

. occurred between the Mount Rogers eruptions and deposition of the Konnarock. In addition, the Mount Rogers in the 

inlier near Konnarock must have been distal to the main 

body of the formation. 

Second area providing data on upper contact. -

The upper contact of the Mount Rogers may be preserved 

on the north slope of Whitetop Mountain in rocks that are 

interpreted to be in the Mountain City window (fig. 3). 

Field relations are equivocal, but bedding in maroon arkose 

and rhythmite immediately north of an outcrop of the 

Wilburn Rhyolite Member dips northwest away from the 

rhyolite. The sedimentary rocks are not unusually 

deformed, although the rhythmites do have a cleavage. In 

roadcuts near an elevation of 3,700 ft along Virginia 

Highway 600 to Elk Garden, arkose, grit, and mudstone are 

interlayered in the Wilburn. Channeling of grit in the 

mudstone indicates that tops are to the northwest and that 

the beds are overturned (bedding dips 30° to the southeast). 

An alternate interpretation, not favored, is that this thin 

section of the Wilburn including the interlayered sedimentary rocks is at the top of the more or less complete section 

of the Mount Rogers Formation that appears to be overturned and stratigraphically attached to the Cranberry 

Gneiss southeast of Elk Garden. That interpretation would 

require the fault framing the Mountain City window to be 

either between the Wilburn Rhyolite Member and rocks to 

the northwest here included in the Konnarock Formation or 

farther northwest within the Konnarock. The last possibility 

would, of course, still make the Wilburn-Konnarock contact here a stratigraphic one. 

Third area providing data on upper contact.- Conglomerate, grit, and siltstone closely resembling those in the 

lower part of the Mount Rogers Formation crop out structurally beneath diamictite, arkose, and rhythmite here 

included in the Konnarock Formation along and south of 

Fox Creek between Fairwood and Trout Dale (fig. 3), in the 

west-central part of the Trout Dale quadrangle. Bedding in 

both suites dips moderately north, and graded bedding in 

the laminites close to the conglomerate, grit, and siltstone 

indicates that beds young to the north. The conglomerate, 

grit, and siltstone are thought to be in stratigraphic contact 

with and beneath the diamictite, arkose, and rhythmite. The 

conglomerate contains far less argillaceous matrix than the 

diamictite, and clasts in the conglomerate are more rounded 

and dominantly rhyolite as opposed to granite. The conglomerate, grit, and siltstone are here considered to be part 

of the Mount Rogers Formation. J.M.G. Miller (Vanderbilt 

University, written commun., 1989) reported a concordant 

contact, which I have since visited with her, between 

bedded diamictite (Konnarock Formation) and conglomerate (Mount Rogers Formation) on the west side of Virginia 

Highway 739 along Solomon Branch south of the junction 

with Virginia Highway 603 (lat 36°41.85'N., long 

81°27. 55'W.). The suites of rocks described above are in 

18 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
the Shady Valley thrust sheet. Field relations here are again 

equivocal, but the map pattern suggests that this relatively 

thin section of the Mount Rogers Formation is overridden 

from the south by phenocryst-poor rhyolite lava resembling 

the Whitetop Rhyolite Member, grit, siltstone, and phyllite, 

all of the Mount Rogers Formation. 

A second exposure in this general area (lat 

36°4l.8l'N., long 8l 028'W.) is in a brush-covered roadcut 

on the south side of Virginia Highway 603 about 0.8 km 

west of the intersection with Virginia Highway 739. This 

exposure was also shown to me by J.M.G. Miller. Diamictite of the Konnarock Formation overlies volcanic conglomerate of the Mount Rogers; the contact is sharp and 

dips steeply to the north. Although this outcrop is near the 

concordant contact just described on Virginia Highway 739, 

bedding is not visible in this outcrop. Excellent exposures 

of the volcanic conglomerate may be seen in the adjacent 

roadcut to the west and in outcrops along Fox Creek below 

that roadcut. 

KONNAROCK FORMATION (NEW NAME) 

Name, Type Area, and Thickness 

Maroon diamictite, rhythmite, and pink arkose in the 

vicinity of Konnarock, Va., in the Konnarock quadrangle, 

are here called the Konnarock Formation (fig. 3). These 

rocks were formerly considered to be the upper part of the 

Mount Rogers Formation. They are here defined as a new 

formation because, as discussed in the previous sections, 

they could be significantly younger than the Mount Rogers 

Formation and are locally unconformably above the Mount 

Rogers; the rocks are largely glaciogenic and nonvolcanic, 

whereas the Mount Rogers, as redefined, is largely volcanic; the unit is stratigraphically overlain by the Lower 

Cambrian Unicoi Formation; and, except locally, it is 

structurally isolated from the Mount Rogers Formation. 

Typical exposures are along Whitetop Laurel Creek and 

adjacent roadcuts and railroad (now abandoned) cuts from 

northwest of the end of Virginia Highway 728 at Creek 

Junction (lat 36°39.08'N., long 81°40.81'_W.) to Konnarock (fig. 3). The Konnarock Formation is on the order of 

a kilometer thick. 

Description 

Lithologies in the Konnarock include massive and 

bedded diamictite, rhythmite, rhythmite containing dropstones, massive argillite, arkose, and minor conglomerate. 

Aspects of these rocks have been described in publications 

by Carrington (1961), Rankin (1967), Blondeau and Lowe 

(1972), Schwab (1976), and Miller (1989a,b) and in three 

Master's theses by Blondeau (1975), Rexroad (1978), and 

Whithington (1986). Most rocks are maroon with the 

Figure 8. Massive diamictite (tillite) in Konnarock Formation. Outcrop along Whitetop Laurel Creek about 250 m 

northwest of end of Virginia Highway 728 at Creek Junction at lat 36°39.08'N., long 81°40.81'W. (fig. 3, note that 

Creek Junction is not shown). 

exception of gray or greenish sandy or silty bases to the 

graded sets in the rhythmites and the pink or greenish-gray 

arkose. 

The massive diamictite (fig. 8) is a matrix-supported 

conglomerate or conglomeratic mudstone in the terminology of Blondeau and Lowe (1972). Grain size ranges 

continuously from clay to boulders more than a meter 

across, although, as noted by Schwab (1976), clay and silt 

account for up to 80 percent of the rock. From 70 to 83 

percent of the clasts are basement rocks, mostly granite and 

porphyritic granite. Very few clasts are recognizable metamorphic rocks (gneiss, schist, amphibolite). The balance of 

the clasts are from the Mount Rogers Formation and include 

rhyolite, basalt, arkose, and argillite (the last two types 

could also be material from the Konnarock Formation). The 

matrix of the diamictite is typically reduced (greenish) 

adjacent to basalt clasts. Rhyolite, including both porphyritic and phenocryst-poor types, generally does not constitute more than 10 percent of the clasts. The clasts were 

incorporated in the diamictite as unweathered rock. The 

massive diamictite is nonbedded and occurs in layers 3 to 30 

m thick (Schwab, 1976). Massive diamictite is particularly 

common toward the top of the formation, and a unit that is 

dominantly massive diamictite has been mapped as a more 

or less continuous unit, as thick as 400 m (Miller, 1989a), 

for a strike distance of nearly 40 km in the Shady Valley 

thrust sheet (figs. 2 and 3). The massive diamictite also 

forms a thick unit here interpreted to be in the Mountain 

City window on the slopes of Beech Mountain. 

Rankin (1967) suggested that the massive diamictite 

is a tillite and later (Rankin, 1969) suggested that the tillite 

Konnarock Formation (New Name) 19 
might have been the result of alpine glaciation related to 

volcanic edifices. As discussed below, that latter idea no 

longer seems reasonable. Miller (1986, 1 989a,b) proposed 

that the diamictite is a lodgment tillite that was deposited 

from ice that advanced across a glacial lake. Schwab (1976) 

reported very rare striated clasts in the diamictite; I 

observed none. Both Rankin (1969) and Schwab (1976) 

noted the absence of a striated pavement, but pointed out 

that the diamictite everywhere rests on coeval subaqueous 

sediments (also implied in the lodgment till interpretation). 

Rhythmite, rhythmite containing dropstones (laminated pebbly mudstone), and bedded diamictite are variants 

of units deposited by turbidity currents and grade into one 

another. Rhythmites are repetitions of graded sets or couplets of gray-green siltstone or fine-grained sandstone bases 

and maroon mudstone tops. Typically the siltstone part of 

the couplet is thinner than the mudstone. In many outcrops, 

the cleavage is refracted across the repeated graded beds so 

that the cleavage surface has the superficial appearance of 

ripple marks. Typically the couplets range in thickness from 

a few millimeters to a few centimeters (fig. 9). 

The couplets may be simple, consisting of a single 

graded set, or complex, where a generally fining upward 

cycle is composed of a series of thinner couplets. Carrington (1961) described and illustrated sets of microrhythmites (as thin as 0.1 mm) within rhythmites. He also 

described thicker cycles that he called megarhythmites that 

had periodicities of 0.5 to 3.5 m. 

Even relatively thin couplets are remarkably continuous laterally. In an old, small quarry on the north side of 

Virginia Highway 603 about 3.8 km east of Virginia 

Highway 600 (lat 36°41.81'N., long 81°34.15'W.), couplets as thin as 1 em are continuous for 6 m and show no 

variation in thickness. Sandy bottoms may show load casts 

in the underlying mudstone (fig. 10). Mudstone rip-ups are 

common, as are slump structures and penecontemporaneous 

small-scale faulting. Locally the mudstone is greenish along 

bedding planes or in irregular patches as a result of 

reduction. The rhythmites were probably deposited by 

glaciolacustrine turbidity currents and may in fact be varves 

(Rankin, 1967; Blondeau and Lowe, 1972; Schwab, 1976; 

Miller, 1986, 1989a). 

In several extensive exposures (fig. 3), the rhythmites 

contain outsized angular to subangular clasts interpreted to 

be ice-rafted dropstones. Places where these may be seen 

include the railroad cut on the north side of the trestle over 

Whitetop Laurel Creek at the junction with Green Cove 

Creek (fig. 11A); a small quarry on the northeast side of the 

intersec6on of Virginia Highways 600 and 603, Whitetop 

Mountain quadrangle; pavement exposures in Big Laurel 

Creek about 1.2 km east of the junction of Virginia 

Highways 600 and 603, Whitetop Mountain quadrangle 

(Rankin, 1975, plate 1B; the caption to that plate erroneously gives the locality west of the road junction); pavement 

outcrops in Big Laurel Creek about 3 km east of the 

Figure 9. Rhythmite in Konnarock Formation. Bedding 

strikes N. 2So E. and dips sse N. Beds are right side up and 

are younger toward the northwest (toward the hammer 

head). Weathered outcrop in road gutter at elevation of 

about 3,180 ft on east side of Virginia Highway 600 about 

0.7 km southeast of junction with Virginia Highway 603, 

Whitetop Mountain quadrangle (fig. 3). Photograph taken 

in 1965. Gutter is now lined with concrete; no outcrop is 

visible. 

intersection of Virginia Highways 600 and 603, Whitetop 

Mountain quadrangle (fig. 11B); and artificial pavement 

exposures east of the junction of Virginia Highways 603 

and 16, Trout Dale, Va. No statistical study has been made 

of the dropstones, but as with the clasts in the massive 

diamictite, most of them are basement granitoids. Most 

dropstones range in size from granules to cobbles. One 

boulder 1 m across was observed in the rhythmite at an 

elevation of about 3, 760 ft on the west slope of Beech 

Mountain, Konnarock quadrangle (lat 36°38.65'N., long 

81 °38.17'W.). The boulder is exposed on the wall of an 

open pit in saprolite of the rhythmite (a manganese prospect 

pit?). 

20 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
Figure 10. Rhythmite and interlayered arkose bed, which is about 20 em thick, in Konnarock 

Formation. Sandy bottoms of arkose bed and successive couplets show load casts accentuated by regional cleavage (which strikes N. 60° W. and dips 25° S.). Cleavage is refracted 

through couplets. Beds are younger to the right. Photograph taken looking north along steep 

east limb of asymmetrical east-verging anticline to which cleavage is axial planar. Fold axis 

trends N. 85° E. and plunges 15° W. Roadcut on north side of Virginia Highway 603 about 0.5 

km south of intersection with Virginia Highway 16, Trout Dale quadrangle (fig. 3). 

Figure 11. Rhythmite containing dropstones (laminated 

pebbly mudstone) in Konnarock Formation. A, Subangular 

granitoid dropstone in rhythmite. Right (south) side of 

outcrop is bedded diamictite. Bedding strikes N. 65° E. and 

dips 60° N. Photograph taken looking east; beds are 

younger to left. Railroad cut on north side of trestle over 

Whitetop Laurel Creek at junction with Green Cove 

Creek, Konnarock quadrangle (fig. 3). Quarter for scale. B, 

Several granitoid dropstones in rhythmite. Bedding strikes 

N. 55° E. and dips 35° N. Beds are younger in direction of 

pencil eraser. Weathered pavement outcrop along Big 

Laurel Creek about 3 km east of junction of Virginia 

Highways 600 and 603 (180 m west of Bethel Church), 

Whitetop Mountain quadrangle (fig. 3). 

Konnarock Formation (New Name) 21 
Figure 12. Bedded diamictite in Konnarock Formation. 

Float block along Big Laurel Creek 1.2 km east of junction 

of Virginia Highways 600 and 603, Whitetop Mountain 

quadrangle (fig. 3). 

In places, thin beds of grit alternate with mudstone or 

sandy mudstone. Grading may or may not be obvious. 

Outsized clasts are scattered throughout, but some may be 

associated with specific grit beds. Mudstone rip-ups and 

slump structures are common. These are the bedded diamictites of Miller (1989a) (fig. 12). 

Bedded pink or greenish-gray arkose is scattered 

throughout the Konnarock Forn1ation. The beds may be 

several centimeters thick, interlayered repeatedly in the 

rhythmites and somehow part of the periodicity, or in more 

massive beds as thick as 6 m that are probably part of the 

fluvial lithofacies of Schwab (1976). 

Blondeau and Lowe (1972) and Mi11er (1989a) presented generalized stratigraphic columns for the rocks now 

called the Konnarock Formation. The section of Blondeau 

and Lowe ( 1972) includes quartz-pebble conglomerate 

within the Konnarock. Blondeau (1975, p. 14) placed the 

top of the upper division of the Mount Rogers Formation 

(here the Konnarock Formation) at the base of a "regionally 

extensive basalt unit" above the quartz-pebble conglomerates because of the absence of orthoquartzite (characteristic 

of parts of the Chilhowee Group) beneath the basalt and the 

presence of maroon mudstone interlayered with both the 

quartz-pebble conglomerate and the diamictite. 

King and Ferguson (1960) did include quartz-pebble 

conglomerate exposed on Big Hill (about 1.5 km due south 

of Konnarock) along U.S. Highway 58 and along Green 

Cove Creek in the Mount Rogers Formation. These areas 

were at the very northeastern limit of their very large map 

area, however, and were peripheral to their main study. 

King and Ferguson (1960, p. 31) noted the similarity of 

those rocks to the lower division (usage of King and 

Ferguson, 1960) of the Unicoi Formation. The stratigraphy 

established by King and Ferguson (1960) for their large map 

area includes most of the basalt in the lower division of the 

Unicoi. Typically the uppermost basalt is at the top of the 

lower division of the Unicoi. 

In my mapping, I extended this stratigraphy into the 

Mt. Rogers area. I used the uppermost basalt to define the 

top of the lower division of the Unicoi and mapped the 

quartz-pebble conglomerate as the lower division of the 

Unicoi (Rankin, 1967). My mapping demonstrated that the 

quartz-pebble conglomerates on Big Hill and along Green 

Cove Creek are stratigraphically beneath basalt (Rankin, 

1967), a relation not discovered by King and Ferguson 

( 1960). Because of the continuity of the stratigraphy of the 

Mt. Rogers area with that in northeastemmost Tennessee 

and because of striking contrast between the quartz-pebble 

conglomerate and the diamictite (see below), the placement 

of the lower contact of the Unicoi Formation above the 

quartz-pebble conglomerate as suggested by Blondeau 

(1975) and Blondeau and Lowe (1972) should not be 

adopted. Furthermore, the maroon mudstone interlayered 

with quartz-pebble conglomerate is different from that in 

the Konnarock Formation. The maroon shales in the Unicoi 

Formation have detrital muscovite on the bedding surfaces. 

Detrital muscovite is not visible on the bedding surface of 

pelitic rocks in the Konnarock Formation (Rankin, 1967). 

This distinction was found to be a useful mapping criterion 

even in mudstone chips in the soil. 

Upper Contact and Age 

The contact as here defined between the Konnarock 

Formation and lower division of the Unicoi Formation in 

weathered outcrop is exposed on U.S. Highway 58 on Big 

Hill about 1.5 km due south of Konnarock (figs. 3, 13) and 

was described by Rankin ( 1967). At the time of my 

mapping, this was a fresh roadcut. The lower 15 m of the 

Unicoi consists mostly of feldspathic grit, but includes a 

bed 0.5 m thick of quartz-pebble conglomerate at its base. 

This basal bed of the Unicoi is atypical in that it contains 

numerous small granite pebbles (most clasts in the conglomerates of the Unicoi lower division are monomineralic: 

either quartz or microcline). The basal conglomerate bed is 

in knife-sharp contact with maroon laminated mudstone and 

arkose of the Konnarock Formation. Beds on both sides of 

the contact strike N. 80° E. and dip 60° N. The contact in 

this outcrop appears to be conformable but is probably 

paraconfom1able. The laminated mudstone and arkose lack 

obvious graded bedding and are hence not the typical 

rhythmite. One graded bed was observed several centimeters below the contact, and its presence confirms that beds 

are younger toward the north toward the Unicoi. Massive 

diamictite crops out about 100 m to the south. 

22 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
Figure 13. Paraconformable contact between laminated mudstone and arkose of the Konnarock Formation on the left and the lower division of the Unicoi Formation; view looking west. 

Shovel of entrenching tool rests on contact, which strikes N. 80° E. and dips 60° N. Weathered 

roadcut at elevation of about 3,280 ft on U.S. Highway 58 on Big Hill about 1.5 km south of 

Konnarock (fig. 3, lat 36°39.58'N., long 81 °38.31'W.). Photograph taken in 1969. Roadcut is 

now vegetated, but contact may be exposed by vigorous digging. 

In railroad cuts along Green Cove Creek, 3.5 km 

southwest of the outcrop described above, the massive 

diamictite is directly overlain by the Unicoi Formation, 

lower division (Rankin, 1967) (fig. 3). The actual contact is 

covered in a steep gully a few meters wide, but bedding in 

the Unicoi (strike N. 45° E.; dip 70° S.) appears to be 

parallel to the contact. Crossbedding in the quartz-pebble 

conglomerate of the Unicoi just north of the contact 

confirms that, although the beds dip steeply to the southeast, they are younger to the northwest. The diamictite is 

not bedded. The Green Cove Creek locality is important 

because, upsection along the creek, Simpson and Sundberg 

(1987) reported the Early Cambrian (Tommotian?) marine 

trace fossil Rusophycus in the upper division of the Unicoi. 

The fossil locality is about 400 m stratigraphically above the 

Konnarock. Rankin (1967) noted the sharp lithologic contrast between the diamictite, in which roughly 75 percent of 

the c1asts are rock fragments, and the quartz-pebble conglomerate, in which roughly 90 percent of the clasts are 

monomineralic (quartz, microcline, or black chert). This 

contrast implies a major shift in provenance between the 

times of deposition of the two units and probably a 

significant time gap. The Konnarock Formation is, therefore, thought to be of Late Proterozoic age. The Konnarock 

is stratigraphically above the Mount Rogers Formation, 

which is dated at roughly 760 Ma, and beneath the Lower 

Cambrian Unicoi Formation; thus, the Konnarock is in the 

proper time interval to be part of the global episode of 

glaciation that, according to Miller (1989b), happened 

between 700 and 650 Ma. 

Plumb and James ( 1986) noted that there were several 

episodes of widespread Late Proterozoic continental glaciation in all tectonic regimes and on all continents. They 

stated that this glaciation reached its widest extent during 

the interval 800 to 700 Ma and that, in many areas, 

glaciation continued into the latest Proterozoic. Subsequently, in recognition of the episodes of cold climate, the 

Subcommission on Precambrian Stratigraphy recommended 

and the International Union of Geological Sciences adopted 

the name Cryogenian for a formal period (850 to 650 Ma) 

within the Neoproterozoic Era (roughly equivalent to the 

Late Proterozoic but beginning 1000 Ma rather than 900 

Ma) (Plumb, 1991). The closest reasonably well dated Late 

Proterozoic glaciogenic sedimentary rocks to the Mt. Rogers area are the Squantum Member of the Roxbury Conglomerate of the Boston Bay Group in Massachusetts. The 

Squantum is stratigraphically above the basal Mattapan 

Volcanic Complex, also of the Boston Bay Group, and is 

Konnarock Formation (New Name) 23 
conformably below the fossiliferous Lower Cambrian Weymouth Formation (Socci and Smith, 1990). The Mattapan is 

dated at 602±3 Ma (Kaye and Zartman, 1980). The 

Squantum is in the Avalon terrane of proto-Gondwanaland 

and in Late Proterozoic time was probably far removed 

from the site of the Konnarock glaciation in Laurentia. It is, 

thus, not possible at present to better constrain the age of the 

Konnaro~k Formation by correlation with other Late 

Proterozoic, glacio genic deposits. ',, 

DISCUSSION AND SUMMARY 

In southwestern Virginia and adjacent North Carolina 

and Tennessee, the billion-year-old Grenvillian basement is 

overlain by a stratigraphic cover sequence older than the 

Lower Cambrian Unicoi Form-ation. This cover sequence is 

here divided into two formations: an older, dominantly 

volcanic unit called the Mount Rogers Formation (revised 

name) is overlain by the glacio genic Konnarock Formation 

(new name). The volcanic rocks of the Mount Rogers 

Formation are a bimodal suite of basalt and rhyolite in 

which the younger rhyolites have peralkaline affinities. The 

volcanic rocks and consanguineous plutonic rocks that 

intrude the Grenvillian basement to the south are part of the 

Crossnore Complex and are thought to be related to an early 

aborted phase of Iapetan continental rifting. 

The rhyolites are concentrated in thick masses 

thought to have been volcanic centers. The erosion of the 

volcanic edifices of older centers, which may or may not be 

partially preserved, produced extensive volcanic conglomerates that make up much of the lower part of the Mount 

Rogers Formation in the type area. The Fees Rhyolite 

Member (new name) near the base of the Mount Rogers 

Formation was probably erupted from one of these earlier 

centers, and clasts of the Fees are recognizable in the 

conglomerates. 

Rhyolites of the Mt. Rogers volcanic center constitute 

the upper part of the Mount Rogers Formation in the type 

area. These include the basal Buzzard Rock Member (new 

name), which is lava, the Whitetop Rhyolite Member (new 

name), also largely lava, and the climactic, zoned, welded 

ash-flow sheet of the Wilburn Rhyolite Member (new 

name). The rhyolites of the Mt. Rogers volcanic center 

are dated at about 760 Ma (Aleinikoff and others, 1991; 

R.E. Zartman, U.S. Geological Survey, written commun., 

1992). 

The thin basalts in the Unicoi Formation may correlate with the volcanic rocks, mostly metabasalt, of the 

Catoctin Formation, which overlies Grenvillian rocks of the 

Blue Ridge anticlinorium from central Virginia to South 

Mountain, Pennsylvania (rift-facies stratified rocks of Laurentia on fig. 1). Rhyolite of the Catoctin Formation has 

recently been dated at about 570 Ma (J.N. Aleinikoff, U.S. 

Geological Survey, written commun., 1992). The basalts in 

the Unicoi Formation are about 145 m stratigraphically 

below the Early Cambrian (Tommotian?) marine trace fossil 

along Green Cove Creek. Hence the correlation of the 

basalts in the Unicoi with the Catoctin Formation is at least 

permissible. The Catoctin volcanism is thought to mark the 

second and successful episode of continental rifting that led 

to the opening of the Iapetus Ocean. 

A couple of other points should be made in conclusion. Because the Konnarock Formation generally crops out 

northwest of the Mount Rogers Formation, it is less 

intensely deformed than much of the Mount Rogers. Granitoid basement clasts in the Konnarock are typically without 

foliation or have only a crude foliation subparallel to the 

regional cleavage in the Konnarock. There are no clasts of 

mylonitic gneiss or augen gneiss such as those that constitute the Cranberry Gneiss beneath the more highly sheared 

basal units of the Mount Rogers Formation further southeast. The intense deformation of the Grenvillian basement 

in the Blue Ridge is, therefore, a Phanerozoic event. Many 

of the granitic clasts in the Konnarock are porphyritic and 

contain large phenocrysts of microcline. This texture supports the idea that many of the augen gneisses in the 

Cranberry were derived by shearing and recrystallizing 

porphyritic granite. 

The clast makeup of the Konnarock diamictite is 

markedly different from that of conglomerates of the Mount 

Rogers Formation, even where bedded diamictite is in 

stratigraphic contact with the conglomerate of the Mount 

Rogers west of Trout Dale. Clasts in the diamictite are 

dominantly basement granitoids; clasts in conglomerate of 

the Mount Rogers are dominantly volcanic rocks, mostly 

rhyolites from the Mount Rogers. This clast distribution 

argues against the Konnarock glaciation being alpine on 

volcanic highlands. The glaciers were clearly moving over 

exposed basement rocks, but basement rocks in the outcrop 

area of the Konnarock Formation were mostly covered by 

the Mount Rogers Formation. 

The contact between the Konnarock and Unicoi 

Formations on Big Hill is concordant. The Mount Rogers 

and Konnarock Formations thin to featheredges both to the 

northeast and southwest beneath the Unicoi Formation. For 

reasons not clear, both the Mount Rogers and Konnarock 

feather out at about the same places. East of the disappearance of the two formations, where the Unicoi Formation is 

nonconformably above Grenvillian basement, greenstone 

dikes cutting the basement rocks terminate at the nonconformity, indicating an erosional interval between the time of 

intrusion of the dikes and the deposition of the Unicoi 

(Rankin, 1970). Regionally, therefore, the Unicoi Formation is disconformably above the Konnarock Formation. It 

must be emphasized, however, that no metamorphic episode or folding event is represented by that disconformity. 

Rankin (1976) speculated that the rhyolite of the 

Mount Rogers Formation was generated at a triple junction 

of three intersecting continental rifts. Apparently this epi24 The Volcanogenic Mount Rogers Formation and the Overlying Glaciogenic Konnarock Formation 
sode of rifting did not proceed to continental separation and 

the generation of oceanic crust (Aleinikoff and others, 

1991). One of these rifts was hypothesized to have extended 

roughly northwest from the Mt. Rogers area toward the 

craton. If this rift or failed arm did exist, most of the 

evidence for it was eroded along with the thrust sheets that 

project into the air northwest of Mt. Rogers, or the evidence 

may be buried deeply far to the southeast in autochthonous 

Laurentian crust. Support for the former existence of this 

failed arm or aulacogen comes from the great thickness of 

the Chilhowee Group in the Mt. Rogers area relative to the 

thickness of that group northeast and southwest of Mt. 

Rogers (Rankin, 1976). Perhaps the simultaneous thinning 

and disappearance of both the Mount Rogers and Konnarock Formations to the northeast and southwest can also be 

attributed to the presence of this northwest-trending trough 

at the time of their deposition. 

Of the three areas in which the Konnarock Formation 

is interpreted to be in stratigraphic contact with the Mount 

Rogers Formation, in one, the contact is an unconformity; 

in another, the contact is concordant; and in the third, the 

nature of the contact is indeterminate. In the area south of 

Konnarock where the map pattern suggests an unconformity, block faulting and rotation of blocks, such as one 

finds near volcanic centers, could produce the map pattern. 

No orogenic episode is required. In other areas within the 

Mt. Rogers volcanic center in the Stone Mountain thrust 

sheet, combinations of normal faulting and volcanic topography could produce the outcrop areas of older volcanic 

rocks and volcanic conglomerates that show through the 

veneer of the Wilburn Rhyolite Member. Two such areas 

are in the Brier Ridge-Mt. Rogers saddle and in the upper 

basin of Wilson Creek (fig. 3). 

Two of the areas of stratigraphic contact between the 

Konnarock and Mount Rogers Formations are in the Shady 

Valley thrust sheet (fig. 2). Rocks of the Shady Valley 

thrust sheet are thought to have always been palinspastically 

northwe~t (using modem directions) of those in the Stone 

Mountain thrust sheet (Rankin, 1970). The thin Mount 

Rogers section in the inlier south of Konnarock and the 

presence of volcanic conglomerate at the top of the Mount 

Rogers west of Trout Dale indicate that the palinspastic 

position of those outcrops of the Mount Rogers Formation 

was distal to and north of the Mt. Rogers volcanic center. 

REFERENCES CITED 

Aleinikoff, J.N., Zartman, R.E., Rankin, D.W., Lyttle, P.T., 

Burton, W.C., and McDowell, R.C., 1991, New U-Pb 

zircon ages for rhyolite of the Catoctin and Mount Rogers 

Formations-More evidence for two pulses of Iapetan rifting 

in the central and southern Appalachians [abs.]: Geological 

Society of America Abstracts with Programs, v. 23, no. 1, 

p. 2. 

Blondeau, K.M., 1975, Sedimentation and stratigraphy of the 

Mount Rogers Formation, Virginia: Baton Rouge, Louisiana 

State University, M.S. thesis, 117 p. 

Blondeau, K.M., and Lowe, D.R., 1972, Upper Precambrian 

glacial deposits of the Mount Rogers Formation, central 

Appalachians, U.S.A.: International Geological Congress, 

24th, Canada, 1972, Proceedings, Section 1, p. 325-332. 

Bryant, Bruce, and Reed, J.C., Jr., 1970, Geology of the 

Grandfather Mountain window and vicinity, North Carolina 

and Tennessee: U.S. Geological Survey Professional Paper 

615, 190 p. 

Carrington, T.J., 1961, Preliminary study of rhythmically layered, 

tuffaceous sediments near Konnarock, southwestern Virginia: 

Virginia Polytechnic Institute Mineral Industries Journal, v. 

8, no. 2, p. 1-6. 

Jonas, A.l., and Stose, G.W., 1939, Age relation of the preCambrian rocks in the Catoctin Mountain-Blue Ridge and 

Mount Rogers anticlinoria in Virginia: American Journal of 

Science, v. 237, p. 575-593. 

Kaye, C.A., and Zartman, R.E., 1980, A late Proterozoic Z to 

Cambrian age for the stratified rocks of the Boston Basin, 

Massachusetts, U.S.A., in Wones, D.R., ed., Proceedings of 

the Caledonides in the USA, International Geological Correlation Program, Project 27, Caledonide orogen: Virginia 

Polytechnic Institute and State University, Department of 

Geological Sciences, Memoir 2, p. 257-262. 

Keith, Arthur, 1903, Description of the Cranberry quadrangle 

[North Carolina-Tennessee]: U.S. Geological Survey Geologic Atlas, Folio 90, 9 p., 4 map sheets, scale 1:125,000. 

King, P.B., and Ferguson, H.W., 1960, Geology of northeasternmost Tennessee: U.S. Geological Survey Professional Paper 

311, 136p. 

Miller, J.M.G., 1986, Upper Proterozoic glaciogenic rift-valley 

sedimentation; upper Mount Rogers Formation, southwestern 

Virginia [abs.]: American Association of Petroleum Geologists Bulletin, v. 70, p. 621. 

---1989a, Glacial and glaciolacustrine sedimentation in a rift 

setting: Upper Proterozoic Mount Rogers Formation, S. W. 

Virginia: U.S.A. [abs.]: 28th International Geological Congress, Abstracts, v. 2, p. 2-436 to 2-437. 

--1989b, Stop 4.4-A: Kannarock [sic] West and Stop 4.4-B: 

Kannarock [sic] East, in Dennison, J.M., compiler, Paleozoic sea-level changes in the Appalachian basin-Field Trip 

Guidebook T354 for the 28th International Geological Congress: Washington, D.C., American Geophysical Union, p. 

37-38. 

Novak, S.W., and Rankin, D.W., 1980, Mineralogy and geochemistry of an ash-flow tuff of peralkaline affinity from the 

Mt. Rogers Formation, Grayson Co., VA [abs.]: Geological 

Society of America Abstracts with Programs, v. 12, no. 4, p. 

203-204. 

Plumb, K.A., 1991, New Precambrian time scale: Episodes, v. 

14, p. 139-140. 

Plumb, K.A., and James, H.L., 1986, Subdivision of Precambrian time; Recommendations and suggestions by the 

Subcommission on Precambrian Stratigraphy: Precambrian 

Research, v. 32, p. 65-92. 

Rankin, D.W., 1967, Guide to the geology of the Mt. Rogers 

area, Virginia, North Carolina, and Tennessee-Field trip 

References Cited 25 
guidebook, 1967: [Durham, N.C.] Carolina Geological Society, 48 p. 

---1969, Late Precambrian glaciation in the Blue Ridge 

province of the southern Appalachian Mountains [abs.]: 

Geological Society of America Special Paper 121, p. 246. 

---1970, Stratigraphy and structure of Precambrian rocks in 

northwestern North Carolina, in Fisher, G. W., Pettijohn, 

F.J., Reed, J.C., Jr., and Weaver, K.N., eds., Studies of 

Appalachian geology-Central and southern: New York, 

Interscience Publishers, p. 227-245. 

---1975, The continental margin of eastern North America in 

the southern Appalachians: the opening and closing of the 

proto-Atlantic Ocean: American Journal of Science, v. 

275-A, p. 298-336. 

---1976, Appalachian salients and recesses: Late Precambrian 

continental breakup and the opening of the Iapetus Ocean: 

Journal of Geophysical Research, v. 81, p. 5605-5619. 

Rankin, D.W., Dillon, W.P., Black, D.F.B, Boyer, S.E., 

Daniels, D.L., Goldsmith, Richard, Grow, J .A., Horton, 

J.W., Jr., Hutchinson, D.R., Klitgord, K.D., McDowell, 

R.C., Milton, D.J., Owens, J.P., and Phillips, J.D., 1991, 

E-4, Central Kentucky to Carolina trough: Boulder, 

Colorado, Geological Society of America Centennial Continent/Ocean Transect # 16, 2 sheets with text, scale 

1:5,000,000. 

Rankin, D.W., Drake, A.A., Jr., Glover, Lynn, III, Goldsmith, 

Richard, Hall, L.M., Murray, D.P., Ratcliffe, N.M., Read, 

J.F., Secor, D.T., Jr., and Stanley, R.S., 1989, Pre-orogenic 

terranes, in Hatcher, R.D., Jr., Thomas, W.A., and Viele, 

G.W., eds., The Appalachian-Ouachita orogen in the United 

States, v. F-2 of The geology of North America: Boulder, 

Colo., Geological Society of America, p. 7-100. 

Rankin, D.W., Drake, A.A., Jr., and Ratcliffe, N.M., 1989, 

Geologic map of the U.S. Appalachians showing the Laurentian margin and the Taconic orogen, in Hatcher, R.D., Jr., 

Thomas, W.A., and Viele, G.W., eds., The AppalachianOuachita orogen in the United States, v. F-2 ofThe geology 

of North America: Boulder, Colo., Geological Society of 

America, pl. 2, scale 1:1,538,000. 

---in press, Proterozoic North American (Laurentian) rocks of 

the Appalachian orogen, in Reed, J .C., Jr., Bickford, M.E., 

Jr., Houston, R.S., Link, P.K., Rankin, D.W., Sims, P.K., 

and Van Schmus, W.R., eds., Precambrian: Conterminous 

United States, v. C-2 of The geology of North America: 

Boulder, Colo., Geological Society of America. 

Rankin, D.W., Espenshade, G.H., and Neuman, R.B., 1972, 

Geologic map of the west half of the Winston-Salem quadrangle, North Carolina, Virginia, and Tennessee: U.S. Geological Survey Miscellaneous Geologic Investigations Map 

I-709-A, scale 1:250,000. 

Rankin, D.W., Lopez-Escobar, Leopold, and Frey, F.A., 1974, 

Rhyolites of the upper Precambrian Mount Rogers (Virginia) 

volcanic center: Geochemistry and petrogenesis [abs.]: 

American Geophysical Union Transactions, v. 55, no. 4, p. 

475. 

Rankin, D.W., Stem, T.W., Reed, J.C., Jr., and Newell, M.F., 

1969, Zircon ages of felsic volcanic rocks in the upper 

Precambrian of the Blue Ridge, central and southern Appalachian Mountains: Science, v. 166, no. 3906, p. 741-744. 

Rexroad, R.L., 1978, Stratigraphy, sedimentary petrology, and 

depositional environments of tillite in the upper Precambrian 

Mount Rogers Formation, Virginia: Baton Rouge, Louisiana 

State University, M.S. thesis, 164 p. 

Roberts, J.K., 1936, William Barton Rogers and his contribution 

to the geology of Virginia: Virginia Geological Survey 

Bulletin 46-C, p. 25-28. 

Rodgers, John, 1953, Geologic map of east Tennessee with 

explanatory text: Tennessee Division of Geology Bulletin 58, 

pt. 2, 168 p. 

Safford, J.M., 1869, Geology of Tennessee: Nashville, State of 

Tennessee, 550 p. 

Schwab, F.L., 1976, Depositional environments, provenance, and 

tectonic setting of the Late Precambrian Mount Rogers 

Formation, southwestern Virginia: Journal of Sedimentary 

Petrology, v. 46, p. 3-13. 

Simpson, E.L., and Sundberg, F.A., 1987, Early Cambrian age 

for synrift deposits of the Chilhowee Group of southwestern 

Virginia: Geology, v. 15, p. 123-126. 

Socci, A.D., and Smith, G.W., 1990, Stratigraphic implications 

of facies within the Boston Basin, in Socci, A.D., Skehan, 

J.W., and Smith, G.W., eds., Geology of the composite 

A val on terrane of southern New England: Geological Society 

of America Special Paper 245, p. 55-74. 

Stose, A.J., and Stose, G.W., 1957, Geology and mineral 

resources of the Gossan Lead district and adjacent areas in 

Virginia: Virginia Division of Mineral Resources Bulletin 72, 

233 p. 

Stose, G.W., and Stose, A.J., 1944, The Chilhowee Group and 

Ocoee Series of the southern Appalachians: American Journal 

of Science, v. 242, p. 401-416. 

Watson, T.L., 1916, A geological map of Virginia: Charlottesville, Virginia Geological Survey, scale 1:500,000. 

Whisonant, R.C., and Tso, J.L., 1992, Field trip number 2, 

Stratigraphy and stru~ture of the lower Ashe Formation 

(upper Precambrian) along the Fries fault in southwestern 

Virginia, in Geologic field guides to North Carolina and 

vicinity: University of North Carolina at Chapel Hill, Department of Geology, Geologic Guidebook 1, p. 15-28. 

Whithington, D.B., 1986, Petrography and provenance of sedimentary rocks from the upper Mount Rogers Formation, 

upper Proterozoic, southwestern Virginia: Nashville, Tenn., 

Vanderbilt University, M.S. thesis, 102 p. 

Williams, G.H., 1894, The distribution of ancient volcanic rocks 

along the eastern border of North America: Journal of 

Geology, v. 2, p. 1-31. 

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