Printed on recycled paper
Cover. View of Trunk Bay on the northern coast of St. John, U.S. Virgin Islands. Trunk Cay is located just to

the left of the shoreline. Along the skyline, from left to right, are Jost Van Dyke and Great Thatch Islands of

the British Virgin Islands, and Mary Point, which is to the northeast on St. John.
U.S. Department of the Interior

U.S. Geological Survey

Geology of St. John, U.S. Virgin Islands

By Douglas W. Rankin

Stratigraphy, structure, and geologic history of St. John as

related to the evolution of the northeastern corner of the

Caribbean plate

Prepared in cooperation with the

National Park Service

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1631
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

CHARLES G. GROAT, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 2002

Published in the Eastern Region, Reston, Va.

Manuscript approved for release March 26, 1998.

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.

Copies of this report are for sale by:

U.S. Geological Survey

Information Services

Box 25286, Federal Center

Denver, CO 80225

or call the toll-free telephone number

1–888–ASK–USGS

Library of Congress Cataloging in Publication Data

Rankin, Douglas W., 

Geology of St. John, U.S. Virgin Islands / by Dougls W. Rankin.

p. cm. — (U.S. Geological Survey professional paper ; 1631)

Includes bibliographical references.

1. Geology—Virgin Islands—Saint John. I. Title. II. Geological Survey professional paper ; 1631.

QE75 .P9 vol. 1631

[QE226.V]

557.3 s—dc21

[557.297’22]

00-052820
III

CONTENTS

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Geologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Previous Work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Stratigraphy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Lameshur Volcanic-Intrusive Complex (Here Named) . . . . . . . . . . . . . . . . . . . . . 5

Water Island Formation (Revised) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Keratophyre. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Basalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Volcanic Wacke and Bedded Tuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Radiolarian Chert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Careen Hill Intrusive Suite (Here Named). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Intrusive Keratophyre. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Trondhjemite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Gabbro. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Sheeted Dikes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Virgin Island Group (Not Used) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Louisenhoj Formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Outer Brass Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Tutu Formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Picara Member (Here Named) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Mandal Member (Here Named) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Congo Cay Limestone Lens (Revised). . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Diabase, Gabbro, and Diorite Dikes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Biotite-Hornblende Tonalite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Hornblende Lamprophyre. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Explosion Breccia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Red Hook Tonalite Porphyry (Here Named) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Folds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Geologic History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Appendix A.—Description of Map Units. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

PLATE

[Plate is in pocket]

1. Geologic map of St. John, U.S. Virgin Islands

FIGURES

1.–23. Photographs of—

1. Water Island Formation, northwest side of Ram Head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

2. Nodular phenocryst-poor keratophyre of the Water Island Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

3. Flow breccia of plagioclase-phyric keratophyre of the Water Island Formation . . . . . . . . . . . . . . . . . . . . . . . . .7

4. Pillow basalt of the Water Island Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

5. Pillow fragment in basaltic pillow breccia of the Water Island Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

6. Basaltic hyaloclastite of the Water Island Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
IV CONTENTS

7. Amygdaloidal basalt of the Water Island Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

8. Bedded volcanic wacke, tuff breccia, and tuff of the Water Island Formation . . . . . . . . . . . . . . . . . . . . . . . . . . .9

9. Columnar joints in trondhjemite of the Careen Hill Intrusive Suite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

10. Mafic sheeted dikes of the Careen Hill Intrusive Suite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

11. Bedded, boulder volcanic conglomerate of the Louisenhoj Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

12. Graded bed showing transition from volcanic conglomerate to wacke of the Louisenhoj Formation . . . . . . . . .14

13. Boulder bed in the Louisenhoj Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

14. Foliated andesitic tuff breccia of the Louisenhoj Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

15. Conformable contact between the Water Island and Louisenhoj Formations . . . . . . . . . . . . . . . . . . . . . . . . . . .15

16. Conformable contact between the Water Island and Louisenhoj Formations . . . . . . . . . . . . . . . . . . . . . . . . . . .15

17. Foliated and contact-metamorphosed volcanic conglomerate of the Louisenhoj Formation . . . . . . . . . . . . . . .16

18. Fluted outcrop of marble of the Outer Brass Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

19. Thin-bedded turbidite of the Mandal Member of the Tutu Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

20. Wave-cut notch in the Congo Cay Limestone Lens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

21. Gently dipping tonalite dike intruding vertical beds of the Mandal Member . . . . . . . . . . . . . . . . . . . . . . . . . . .20

22. Vertical dike of explosion breccia intruding hyaloclastite of the Water Island Formation . . . . . . . . . . . . . . . . .22

23. Exposed surface of the Guinea Gut fault in the Louisenhoj Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

TABLES

1. Stratigraphic nomenclature used previously and in this report for St. Thomas, St. John,

and the British Virgin Islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

2. Conventional K-Ar ages and three 40Ar/39Ar ages from the Virgin Gorda batholith and The Narrows pluton . . . . . . .21

METRIC CONVERSION FACTORS

Multiply By To Obtain

millimeter (mm) 0.03937 inch (in)

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

square kilometer (km2

) 0.3861 square mile (mi2)
1

ABSTRACT

The rocks of St. John, which is located near the eastern

end of the Greater Antilles and near the northeastern corner

of the Caribbean plate, consist of Cretaceous basalt,

andesite, keratophyre, their volcaniclastic and hypabyssal

intrusive equivalents, and minor calcareous rocks and chert.

These rocks were intruded by Tertiary mafic dikes and

tonalitic plutons. The oldest rocks formed in an extensional

oceanic environment characterized by abundant keratophyre

and sheeted dikes. Subduction-related volcanism of the

east-west-trending marine Greater Antilles volcanic arc,

began on St. John near the transition between the Early and

Late Cretaceous. South-directed compression, probably

caused by the initial collision between the Greater Antilles

arc of the Caribbean plate and the Bahama platform of the

North American plate, deformed the Cretaceous strata into

east-west-trending folds with axial-plane cleavage. Late

Eocene tonalitic intrusions, part of the Greater Antilles arc

magmatism, produced a contact aureole that is as much as

two kilometers wide and that partly annealed the axial-plane

cleavage. East-west compression, possibly related to the relative eastward transport of the Caribbean plate in response

to the beginning of spreading at the Cayman Trough, produced long-wavelength, low-amplitude folds whose axes

plunge gently north and warp the earlier folds. A broad

north-plunging syncline-anticline pair occupies most of St.

John. The last tectonic event affecting St. John is recorded

by a series of post-late Eocene sinistral strike-slip faults

related to the early stages of spreading at the Cayman

Trough spreading center and sinistral strike-slip accommodation near the northern border of the Caribbean plate.

Central St. John is occupied by a rhomb horst bounded by

two of these sinistral faults. Unlike other parts of the Greater

Antilles, evidence for recent tectonic movement has not

been observed on St. John.

INTRODUCTION

St. John is one of three major islands of the U.S. Virgin

Islands. St. Thomas lies about 4 km to the west of St. John

and St. Croix about 60 km to the south. The British Virgin

Islands lie north and east of St. John across the Sir Francis

Drake Channel, which is as narrow as 1 km immediately

north of St. John where the channel is called The Narrows.

The British Virgin Islands, St. John, St. Thomas, and Puerto

Rico are subaerial topographic highs on the Puerto Rico

Bank, a submerged platform generally defined by the 183-

m-depth contour (Rogers and Teytaud, 1988). During the

Pleistocene low stand of sea level, a continuous subaerial

landmass extended eastward from Puerto Rico across the

British Virgin Islands. Today, a drop in sea level of about 30

m would reestablish a land bridge between St. Thomas and

Puerto Rico, about 60 km to the west (National Oceanic and

Atmospheric Administration (NOAA) Chart 25650, 1983).

Deep water and steep slopes surround the Puerto Rico Bank.

The Puerto Rico Trench, with water depths as great as 9,166

m, is to the north; the Virgin Islands Basin, with water

depths commonly in excess of 2,500 m (NOAA Chart

25641, 1983), separates St. John from St. Croix to the south.

St. John is an irregularly shaped island covering about

48 km2 that is characterized by steep slopes and relief of as

much as 390 m. Slopes in excess of 30 percent cover more

than 80 percent of the island (CH2M Hill, 1979). Bedrock

is well exposed in outcrops and cliffs along the irregular

shoreline, in some of the water courses (locally called guts),

along some ridge crests, and in roadcuts. Cliffs are commonly interrupted by deep cuts caused by the erosion of

nearly vertical dikes. In places, cliffs drop into deep water

and the cliff-dry forest interface above is typically exceedingly thorny. Elsewhere, exposures are limited and, in

numerous places, contacts are mapped based on float.

Vegetation in the dry forest that covers much of the island is

thorny and where thorny vines are abundant, the dry forest

is essentially impenetrable. The guts, however, are largely

vegetation free because of flash floods (Rankin, 1984, fig.

4B). Much of the island was cleared for sugar plantations in

GEOLOGY OF ST. JOHN, U.S. VIRGIN ISLANDS

By Douglas W. Rankin1

1Scientist Emeritus, U.S. Geological Survey, Reston, VA, 20192.
2 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

colonial times. Narrow, rock-walled terraces are still present

on some of the steep, forest-covered slopes. The Virgin

Islands National Park covers much of the island and

includes substantial offshore area as well.

The islands of St. Thomas and St. John and nearby cays

(islands) were mapped at a scale of 1:24,000 between 1983

and 1988 with brief visits in 1990 and 1995. Most of the

time spent in the field, starting in 1985, was spent on St.

John. The mapping began as part of a U.S. Geological

Survey (USGS) program to assist the government of the

U.S. Virgin Islands (USVI) by providing it with a modern

geological data base. Initial funding was provided by the

Office of the Assistant Secretary for Territorial and

International Affairs of the Department of the Interior.

Major- and trace-element analyses (including rare-earth element, or REE, analyses) of about 100 samples from St.

Thomas and St. John were made in the USGS laboratories

as part of this geologic mapping project. An extended discussion of the chemistry of these rocks is not in order here,

but reference will be made to the data set as appropriate.

Although the base-map elevations are in feet, all other scientific measurements are in metric units (see conversion

table in the Contents section).

ACKNOWLEDGMENTS

I would like to thank Thomas W. Donnelly (State

University of New York at Binghamton) for numerous discussions that helped to introduce me to the geology of St.

Thomas and St. John and for encouragement to pursue the

study. I also acknowledge helpful discussions with Henry V.

Alminas, who shared with me the geochemical data generated as part of the mineral-resource assessment; with

Richard D. Krushensky on the geology of Puerto Rico; with

E-an Zen on prehnite-pumpellyite identification; and with

Thomas R. Armstrong on numerous aspects of the geology

and mineralogy. Michael J. Kunk made 40Ar/39Ar determinations for four samples. Charles D. Blome studied

Radiolaria from two samples, both from St. Thomas. An

early version of the geologic map and explanation was

reviewed by William C. Burton and Avery A. Drake, Jr. The

revised geologic map and accompanying text were reviewed

by William C. Burton and Arthur P. Schultz. Personnel of

the Virgin Islands National Park were helpful in many ways

during the fieldwork. I particularly thank Caroline Rogers,

now with the U.S. Geological Survey, and Ranger Bernard

Stoffel. Some assistance was provided in the later part of the

fieldwork by the Island Resources Foundation. The friendship of Angela and Michael Ebner made life on St. John

even more enjoyable. Finally, I thank my wife, Mary

Louise, for her help as a volunteer field assistant under conditions that were not always pleasant.

GEOLOGIC SETTING

The Virgin Islands constitute the eastern extremity of the

Greater Antilles. The Puerto Rico Bank is the geomorphic

expression of the Puerto Rico-Virgin Islands microplate

located within the northern boundary zone between the

Caribbean and North American plates. The Caribbean plate

is bounded on the east and west by subduction zones, and on

the north and south by strike-slip faults. Current thinking is

that the Caribbean plate began as an oceanic plateau of overthickened oceanic crust of Jurassic and Cretaceous age in

the Pacific Ocean and moved into the gap between North

and South America formed by the opening, about 170 Ma,

of the North Atlantic Ocean (Burke and others, 1978;

Pindell, 1990; Pindell and Barrett, 1990; Montgomery and

others, 1994; Draper and Pindell, 1995). According to Hall

(1995), the Caribbean plate entered the Atlantic about 75 Ma

and since then has been moving roughly eastward with

respect to both North and South America. In response to the

relative eastward movement of the Caribbean plate, westdipping subduction of the North American plate has produced magmatism preserved as a continuous record from the

Late Cretaceous to Paleogene in the submarine Aves Ridge

volcanic-arc complex and from the Eocene to Holocene in

the Lesser Antilles arc (Pindell, 1990). The active Lesser

Antilles arc terminates at the island of Saba, about 160 km

southeast of St. John. Calc-alkaline basalt dated at 4 Ma has

been dredged from the Noroit Seamount about 110 km north

of Saba at the Anegada Passage, which is the eastern boundary of the Puerto Rico-Virgin Island microplate northeast of

the Virgin Island Basin (Bouysse and others, 1985). The

seamount is interpreted to be the extinct north end of the

Lesser Antilles arc.

The following overview is taken largely from Pindell

and Barrett (1990). The Greater Antilles, including the

Virgin Islands, are interpreted to be part of the same magmatic arc, which is probably built on oceanic crust.

Subduction-related magmatism began in the Early

Cretaceous and continued until the Eocene. The early polarity of this arc is not known. In places, Jurassic to Early

Cretaceous oceanic crust, much of it thought to be samples

of early oceanic crust of the Caribbean plate, has been tectonically emplaced in the Greater Antilles (and in northern

South America and La Desirade, a small island east of

Guadeloupe). The Bermeja Complex in southwestern

Puerto Rico is one such body interpreted to be serpentinized

upper mantle rocks and metamorphosed Caribbean oceanic

crust (Mattson, 1973; Schellekens and others, 1990; Evans

and others, 1997) tectonically interleaved with radiolarian

chert ranging in age from Early Jurassic (Pliensbachian) to

Late Jurassic (Tithonian) (Montgomery and others, 1994).

Mattson (1973) and Mattson and Pessagno (1979) interpreted the complex to have been obducted above a south-dipping subduction zone prior to the Campanian. After the
STRATIGRAPHY 3

Santonian or Campanian, however, the Greater Antilles arc

was definitely north-facing in Cuba and Hispaniola (Pindell

and Barrett, 1990).

Donnelly and Rogers (1980) recognized two major

magma series in the northeastern Caribbean island arcs and

elsewhere in the greater Caribbean area. They called the

older one (of which the Water Island Formation of the

northern Virgin Islands was the initial example) the “primitive island arc (PIA) series;” it is characterized by low thorium (Th) and uranium (U) contents and low Th/U ratios.

The PIA also included the Louisenhoj Formation of the

northern Virgin Islands and rocks older than the Robles

Formation of Puerto Rico. Many of the PIA series rocks

have been called spilites and keratophyres. The younger

series was called the “calc-alkaline (CA) series” which, in

places, included high-potassium (K) units near the base. In

Puerto Rico, the CA series began with rocks of the Robles

Formation and Rio Orocovis Group (Albian, possibly

extending upward to the Turonian according to Donnelly

and others, 1990) and extended upward to the youngest

(Eocene) volcanic rocks in Puerto Rico. The dominant rock

type is andesite. The relationship of the current work to

these observations will be discussed later.

Aspects of plate motion thought to be important for this

study are summarized from Pindell and others (1988). From

the Late Triassic(?) to the Late Cretaceous, South America

moved relatively southeastward away from North America,

opening a gap into which the Caribbean plate moved progressively. Since the Campanian, no significant deformations of the Caribbean plate can be attributed to relative

motions of North and South America. Although the northern boundary of the Caribbean plate with the North

American plate is complex and has probably shifted in location through time, it is essentially a sinistral strike-slip fault

for much of its length. The Puerto Rico-Virgin Islands

microplate, at the northeast corner of the Caribbean plate, is

in the transition region between west-dipping subduction of

the North American plate beneath the Lesser Antilles arc

and strike-slip motion along the northern plate boundary.

Speed and Larue (1991) argue that motion along the

Caribbean-North American plate boundary zone here has

been left transtensional for the past 15 to 20 m.y., resulting

in a boundary-normal component of extension that is the

principal cause of the Puerto Rico Trench. Their model uses

a vector of motion across the boundary of N. 82°-83° W.,

1.1 to 1.5 cm/yr. Additional consequences of this vector are

a counterclockwise rotation of the Puerto Rico-Virgin

Islands microplate, south-directed thrusting in the Muertos

Trough (south of Puerto Rico), and the opening of the

Virgin Island Basin by extension. The fault pattern in southwestern Puerto Rico supports the model of counterclockwise rotation of the microplate (Meltzer and others, 1995).

However, Deng and Sykes (1995) argue strongly that the

azimuth of plate motion along most of the northern boundary of the Caribbean plate is S. 70°±4° W. (95 percent confidence) and therefore do not accept an extensional component of motion in the Puerto Rico Trench; they suggest that

Puerto Rico and the northern Virgin Islands are moving with

the main Caribbean plate rather than as a separate

microplate.

PREVIOUS WORK

The only previous detailed geologic mapping of St.

John was by Donnelly (1966) as part of a Ph.D. dissertation

at Princeton University under the guidance of Harry H.

Hess, one of the pioneers of modern Caribbean geological

research. Donnelly’s 1:62,500-scale map covers both St.

Thomas and St. John and is based on field work done mostly in 1956 and 1957. Fragmentary earlier work is well summarized by Donnelly (1966). The British Virgin Islands

were mapped by Helsley (1960, 1971), another Hess student. Longshore (1965) studied the chemical and mineralogical variations of the Virgin Islands batholith and wall

rocks. Hekinian (1971) did a petrologic and geochemical

study of spilites of the Water Island Formation from a core

hole drilled near Ram Head, St. John. The petrology and

geochemistry of Caribbean volcanic rocks, including rocks

of the northern Virgin Islands, are described by Donnelly

(1963, 1972), Donnelly and Rogers (1978, 1980), and

Donnelly and others (1971). Griscom and Geddes (1966)

published a 1:2,500,000-scale aeromagnetic map of Puerto

Rico, the Virgin Islands, and a large offshore area. The

results of a mineral-resource assessment of the Virgin

Islands were published by Alminas and others (1994) prior

to the completion of the new geologic maps. As part of the

mineral-resource assessment, Tucker (1987) made a geochemical study of St. John.

STRATIGRAPHY

Basalt, andesite, keratophyre, and their volcaniclastic

equivalents make up most of St. John as well as St. Thomas

and the offshore cays. Calcareous rocks and cherts are

minor but important components. Stratified rocks, except

the surficial deposits, are of latest Aptian or Albian to late

Turonian or late Santonian (late Early Cretaceous to Late

Cretaceous) age; at least most of the section is marine. The

stratified rocks are several kilometers thick. The oldest volcanic rocks (Water Island Formation) are of oceanic affinity

emplaced in an extensional environment. Volcanic rocks

higher in the section (Louisenhoj Formation and higher), are

increasingly of island-arc affinity. Hypabyssal intrusive

rocks related to the Water Island Formation are common.

All pre-surficial stratified rocks are intruded by dikes and

small plutons ranging in composition from gabbro to gran-
4 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

ite. Some of the plutons are as young as late Eocene and

have contact metamorphic aureoles. In northern and eastern

St. John, the contact effects extend out a few kilometers

from the small exposed plutons, suggesting that larger plutons, including The Narrows pluton and the Virgin Gorda

batholith, underlie The Narrows and the Sir Francis Drake

Channel. Surficial deposits include beach deposits,

beachrock, bay-mouth bars, playa deposits, alluvium, boulder fields, and artificial fill. Only the alluvium is likely to

contain material as old as Pleistocene.

For the most part, the stratigraphic units established by

Donnelly (1966) were found to be valid in the course of the

new geologic mapping. Table 1 compares the formal stratigraphic units of Helsley (1971) and Donnelly (1966) with

revisions developed during the current work. As will be

discussed later, I do not use the terms Virgin Island Group

or Hans Lollik Formation. The latter was not mapped on

St. John by Donnelly (1966), but my reassignment of the

rocks at the type locality on Hans Lollik Island to the

Louisenhoj Formation has bearing on the regional structural interpretation.

Donnelly (1966) defined the Water Island Formation,

the oldest unit recognized in the northern Virgin Islands, as

a heterogeneous unit of keratophyre (80 percent), spilite,

and radiolarian chert along with hypabyssal intrusive rocks.

He established the age as Albian or older, but probably late

Early Cretaceous, and selected Water Island in the harbor of

Charlotte Amalie, St. Thomas as the type locality. More

recently Emile A. Pessagno, Jr. (University of Texas at

Dallas) dated the top of the Water Island (just north of

Haypiece Hill, Central St. Thomas quadrangle) at probably

late Aptian or earliest Albian based on Radiolaria (in

Donnelly and others, 1990; oral commun., March 1997).

Several conventional K-Ar ages for both intrusive and extrusive keratophyre, mapped by Donnelly as the Water Island

Formation, were published by Alminas and others (1994).

The ages range from 65.8±2.9 Ma to 30.9±0.3 Ma. Some of

the rocks probably are devitrified. These whole-rock ages

are not considered to be reliable and cannot be reconciled

with the geologic history presented here. Detailed field work

has shown that intrusive rocks interpreted to be comagmatic

with the keratophyres and spilites are more varied and abundant than recognized by Donnelly. The assemblage is here

grouped into three stratigraphic units: the Lameshur

Volcanic-Intrusive Complex consisting of the Water Island

Formation and Careen Hill Intrusive Suite.

Table 1. Stratigraphic nomenclature used previously and in this report for St. Thomas, St. John, and the British Virgin Islands.

Donnelly (1966) Helsley (1971) This report

(St. Thomas and St. John) (British Virgin Islands) (St. John)

Virgin Island Group Virgin Island Group

Necker Formation

Tortola Formation

Shark Bay Member

Sage Mountain Member

Hans Lollik Formation Hans Lollik Member

possible unconformity

Tutu Formation Tutu Formation Tutu Formation

Mandal Member

Congo Cay Limestone Member Congo Cay Limestone Member Congo Cay Limestone Lens

Picara Member

conformity unconformity conformity

Outer Brass Limestone Outer Brass Limestone Outer Brass Limestone

Louisenhoj Formation Louisenhoj Formation Louisenhoj Formation

unconformity unconformity conformity

Lameshur Volcanic-Intrusive Complex

Water Island Formation Water Island Formation Water Island Formation and Careen Hill 

Intrusive Suite
STRATIGRAPHY 5

LAMESHUR VOLCANIC-INTRUSIVE COMPLEX

(HERE NAMED)

The Lameshur Volcanic-Intrusive Complex (Klm) consists dominantly of keratophyre and volcaniclastic rocks

derived from them, but also includes basalt, basaltic

andesite, an unknown volume of trondhjemite (as defined by

Barker, 1979) and gabbro, and minor chert. The igneous

rocks are distinctly bimodal in silica content. The terms

“keratophyre” and “spilite” are ingrained in the literature of

the Water Island Formation of Donnelly (1966) and, as

described, are part of the primitive island arc (PIA) series of

the Caribbean (Lidiak, 1965; Donnelly, 1966; Donnelly and

others, 1971, 1990; Hekinian, 1971; Donnelly and Rogers,

1980). Keratophyre is a name applied to igneous rocks,

either intrusive or extrusive, that have an aphanitic or finegrained groundmass, are silicic (commonly >70 percent

SiO2),2 and are sodic rather than potassic. Donnelly (1966)

used the term “spilite” for the mafic rocks of the Water

Island because he observed that the rocks contained albitic

plagioclase and other secondary minerals. The term “spilite”

also has been used for mafic rocks that are sodic rather than

calcic. The initial magmatic chemistry of the mafic rocks is

difficult to assess because of the highly amygdaloidal nature

of most rocks and the presence of pervasive secondary minerals. Analyses of the least amygdaloidal rocks indicate that

they are basalts and basaltic andesites (see also Donnelly

and others, 1971; and Hekinian, 1971). Analyses made as

part of this study show that in bulk chemistry they are not

particularly sodic as opposed to calcic relative to basalts and

andesites in general. They do not plot in the field of spilites

on a plot of Na2O against CaO (weight percent) as shown by

Mullen (1983). These rocks are called basalts in this report

for ease of expression, although (a) they do include some

basaltic andesites; (b) in many, the primary igneous mineralogy has been altered; and (c) in the northern part of St.

John, the rocks are metamorphosed to grades in the amphibolite facies.

Keratophyre occurs in several varieties that are distinguished by the size and mineralogy of phenocrysts and by

the mode of emplacement. Keratophyre occurs as lava

flows, breccia, layered tuff, dikes, and small hypabyssal

intrusive bodies. In all varieties of keratophyre, quartz phenocrysts (if present) are embayed, and phenocrysts of plagioclase (albite as determined by Donnelly, 1963, 1966) and

quartz (if present) form growth aggregates either singly or

together. Plagioclase phenocrysts are either not zoned or

weakly zoned. The groundmass of the freshest keratophyre

is medium grayish blue (5PB 4/2) to medium dark gray

(N4).3 The rocks become progressively lighter in color as

the content of whitish-weathering albite phenocrysts and

(or) the degree of weathering increases. The more mafic

rocks of the complex occur as pillow lavas, pillow breccias,

dikes, and rare small plutons. In the extrusive rocks, phenocrysts are generally inconspicuous. All rocks may exhibit local intense deuteric hydrothermal alteration, which may

include silicification, oxidization, and sulfidization. The

development of boxwork textures and relict gossan on

Bordeaux Mountain are weathering products that formed

relatively recently from previously altered rocks.

The name Lameshur Volcanic-Intrusive Complex is

used in reference to the assembly of comagmatic igneous

rocks and the volcaniclastic rocks derived from them. The

unit is mapped undivided where the size and mineralogy of

phenocrysts in the keratophyre varies over short distances

and the distinction between extrusive and intrusive rocks is

not readily apparent. The name is taken from the locality of

Lameshur in southern St. John. Along the coast of Great and

Little Lameshur Bays, a variety of keratophyres (extrusive

and intrusive) and basalt are exposed. The complex crops

out only on St. Thomas, St. John, offshore cays, and the

British Virgin Islands. The complex is thought to have been

generated in an oceanic extensional environment. The stratified rocks are interpreted to be submarine deposits

(Donnelly, 1966, and this report).

WATER ISLAND FORMATION (REVISED)

The name Water Island Formation is retained for stratified rocks, but is revised from the usage of Donnelly (1966)

to exclude intrusive rocks. The unit consists dominantly of

extrusive keratophyre and related volcaniclastic rocks (80

Figure 1. Water Island Formation, northwest side of Ram Head.

Porphyritic keratophyre (Kwp) overlying hyaloclastite (Kws).

Person is standing on the contact that strikes 20° and dips 40° SE.

2Unless otherwise stated, major oxide values are weight-percent normalized volatile free. In general, the rock classification of Le Maitre

(1989) is used.

3Throughout the text, alphanumeric symbols following color names

are taken from the rock-color chart of Goddard and others (1970).
6 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

percent), basaltic lava and breccia (about 20 percent), and

minor chert (fig. 1). No internal stratigraphic order has been

recognized. In places, mixed blobby masses of keratophyre

and basalt indicate that the two lava types were extruded

simultaneously. An example may be found in shoreline outcrops at the northeast corner of Limestone Bay, Water

Island, Central St. Thomas quadrangle. In other places,

clasts of basalt are present in keratophyre, and clasts of keratophyre in basalt.

The various units of the Water Island Formation are

interlayered and any lithology may be in contact with any

other. The attitude of these contacts, along with bedding in

the volcanic wacke, flow layering in keratophyre (used cautiously), and “bedding” in pillowed basalt aid in defining the

map pattern and overall structure. Donnelly (1966) apparently mismapped numerous younger diabase dikes as extrusive basalt and plotted the trends of these dikes as layering

in the Water Island Formation. As a result, our structural

interpretations of layering (bedding) in eastern St. John are

nearly perpendicular to each other. The base of the Water

Island is not exposed, although it is presumably underlain

by oceanic crust (Donnelly, 1989, p. 305). A 715-m-deep

corehole on Ram Head, thought at the time to have reached

lowest stratigraphic level of the formation, bottomed in

basalt and keratophyre of the Water Island (Hekinian,

1971). The present mapping shows that Ram Head is structurally isolated and probably not near the base of the formation. On St. John, the Water Island Formation is at least

2 km thick and could be more than twice that.

KERATOPHYRE

The groundmass of the keratophyre is locally

spherulitic. Much of the keratophyre on eastern St. John

weathers so that ovoid bodies (fig. 2A), some as large as 4

cm, stand out on the weathered surface. In thin section,

these ovoids are commonly rimmed with limonite-filled

fractures. The larger ovoids contain phenocrysts of the same

species and in about the same abundance as the rest of the

rock. The extremely fine grained quartzofeldspathic material within the ovoids lacks any radial structure and looks

identical to the material in the matrix around them. If anything, the ovoids are more homogeneous than the surrounding matrix. The ovoids are interpreted to be spherulites

recrystallized by the metamorphism that affected northern

and eastern St. John. Typically, these nodular keratophyres

of eastern St. John contain fine-grained metamorphic chlorite, muscovite, and brown biotite. Phenocryst-poor keratophyre, without obvious nodules, along the crest of the 320-

ft ridge northwest of Haulover contains metamorphic biotite

and sillimanite. In places, the ovoids are roughly spherical,

but more commonly they are oblate spheroids and the rock

has a foliation seen in thin section by the alignment of phyllosilicates. Weathered surfaces of foliated albite-phyric keratophyre (Kwf) between elevations of 100 and 200 ft on the

slope north of the East Base horizontal control marker in the

town of Coral Bay show that oblate spheroids a few centimeters across make up at least 30 percent of the rock (fig.

2B); on fresh surfaces of this rock, the spheroids are nearly

invisible.

A

B

Figure 2. Nodular phenocryst-poor keratophyre (Kwa) of the

Water Island Formation. The nodules are interpreted to be metamorphosed spherulites. A, West side of Turner Point north of cobble beach. Kwa is not mapped separately here, but is included in

undivided rocks of the Lameshur Volcanic-Intrusive Complex

(Klm). Dime for scale. B, View looking east at weathered, nearvertical surface perpendicular to foliation. Nodules are oblate

spheroids flattened in plane of foliation (85°, dipping 70° N.).

Located at elevation of about 150 ft, north of East Base, Coral Bay.

Dime for scale.
STRATIGRAPHY 7

Several varieties of keratophyre are distinguished by the

size and mineralogy of phenocrysts; all have hypabyssal

equivalents in the Careen Hill Intrusive Suite. Ash-flow

tuffs are notably sparse. Phenocryst-poor keratophyre (Kwa)

is characterized by up to 10 percent small phenocrysts (generally less than 2 mm across) of quartz and plagioclase in an

aphanitic groundmass. Many outcrops lack visible primary

structural features, whereas others display flow layering

(Rankin, 1999, fig. 4), flow folds, or brecciation. Lava, flow

breccia, some bedded tuff (Rankin, 1999, fig. 5), and volcaniclastic rock, in which the clasts are phenocryst-poor

keratophyre, have been recognized. Unit Kwa is the dominant unit of the Water Island Formation on St. John and is

well exposed between an elevation of 200 and 350 ft in Fish

Bay Gut. Porphyritic keratophyre (Kwp) is characterized by

obvious quartz and plagioclase phenocrysts, typically less

than 5 mm across, in an aphanitic groundmass. Phenocrysts

constitute 10 to 25 percent of the rock. Flow layering was

observed in a few outcrops and even fewer outcrops are

breccias. Unit Kwp is superbly exposed in a series of cascades (typically dry) between an elevation of 100 and 400 ft

in Battery Gut. The mapped distribution is approximate, but

the unit appears to constitute less than 10 percent of the

Water Island Formation on St. John. Plagioclase-phyric keratophyre (Kwf) contains 5 to 10 percent small (1- to 3-mmlong) plagioclase phenocrysts. The groundmass is commonly a felted mass of flow-oriented aphanitic feldspar laths and

interstitial quartz. Flow layering is visible in enough outcrops to conclude that this unit consists mostly of lava flows

(fig. 3). Unit Kwf is a minor part of the Water Island

Formation on St. John; however, because the phenocrysts

are small and sparse, some areas mapped as Kwa could be

Kwf. On St. John, Kwf is largely restricted to the area of

Great and Little Lameshur Bays.

BASALT

The unit mapped as basalt (Kws) consists of amygdaloidal pillow basalt, pillow breccia, and hyaloclastite;

some rocks probably are andesites. Although Donnelly

(1966) reported that pillows were uncommon, textural evidence for pillows, pillow breccia, or hyaloclastite is present

in most outcrops. Pillows typically are about 1 m across.

Figure 3. Flow breccia of plagioclase-phyric keratophyre (Kwf) of

the Water Island Formation, head of peninsula (locally known as

Yawzi Point) between Little and Great Lameshur Bays. Folding of

flow layering is visible in one fragment. Knife is 8.3 cm long.

A

B

Figure 4. Pillow basalt (Kws) of the Water Island Formation,

Harbor Point in Coral Bay. Layering strikes 310° and dips 35° NE.

A, View looking east. Pillows are as large as a meter across.

Pillow asymmetry indicates that layers are right side up. B, View

looking north shows that pillows in A are cross sections of tubes

elongated downdip. Hammer is 28.5 cm long.
8 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

Amygdules are commonly in concentric shells within the

pillows. Interstices between the pillows are, in places, filled

with calcite or epidosite. In good exposures, bedding and

topping direction can be determined from the pillow asymmetry. Topping directions have been determined from pillowed basalt at Harbor Point (fig. 4A), the point between

Elk Bay and Haulover (Rankin, 1999, fig. 6), and eastern

Red Point, all in eastern St. John. Other excellent exposures

of bedding in pillowed basalt are on the southwest side of

Dittlif Point on the south coast of St. John, on the point

between Saltpond Bay and Ram Head, and on Turner Point

in eastern St. John. In places, such as at Harbor Point, some

pillows are actually tubes as long as 3 m (fig. 4B).

In many outcrops, the basalt is fragmented (Rankin,

1999, fig. 7). The fragments are irregularly shaped and have

either angular or rounded edges. Many are molded against

one another and some are folded; these fragments were

clearly soft when incorporated into the deposit. The fragments range in size from vanishingly small to many centimeters across. These “lumpy greenstones” (a field term)

are interpreted to be pillow breccias grading into hyaloclastites. On the south-southeast-trending coast between

Saltpond Bay and Ram Head, a bent, tubular mass, of which

an area 20 by 60 cm is exposed, has a rounded termination

and is enclosed within a large expanse of pillow breccia and

hyaloclastite. Several shells of small amygdules parallel the

borders of the tube, follow the borders around the bend, and

then around the rounded termination. This body is the termination of a small lava tube, now probably isolated as a

clast in the breccia. In another part of this outcrop, breccia

fragments contain layers of amygdules that are parallel to

one margin of the clast but terminate at other margins of the

clast (fig. 5). This is further evidence for brecciated pillows.

Other good exposures of pillow breccia and hyaloclastite

are found on Dittlif Point (fig. 6A) and Contant Point. In

loose blocks at Contant Point, deformed breccia fragments

are in a limestone matrix (fig. 6B). A rudist or solitary coral

fossil is present in the limestone matrix of one of the

blocks. In the freshest exposures, breccia fragments are

commonly reddish brown and the matrix is greenish, yet the

color difference, even in the same outcrop, is not restricted

to fragmental borders and resulted from rock alteration.

Donnelly (1966) and Hekinian (1971) noted the color difference, called the rocks “patchy spilite,” and discussed the

alteration. In some altered rocks, the clasts have been

replaced by epidosite.

Figure 5. Pillow fragment in basaltic pillow breccia (Kws) of the

Water Island Formation on headland between Salt Pond and

Booby Island. The fragment contains truncated concentric layers

of amygdules. Penny for scale at bottom edge of pillow fragment.

A

B

Figure 6. Basaltic hyaloclastite (Kws) of the Water Island

Formation. A, East side, base of peninsula of Dittlif Point. Knife

is 8.3 cm long. B, Fragments, deformed while soft, in a carbonate

matrix. Dime for scale. Float block on west side of Contant Point.
STRATIGRAPHY 9

In central and southern St. John, the groundmass of the

least altered basalt consists of typically randomly oriented

plagioclase microlites in a dark mesostasis that, except for

opaque dust, is microscopically irresolvable. Phenocrysts

are sparse, small (about 1 mm), and consist of plagioclase

(typically in growth aggregates) and clinopyroxene.

Epidosite segregations are common; in places, pillows have

epidosite cores. Secondary minerals in the southern St. John

area include prehnite and pumpellyite (both identified optically only), chlorite, calcite, and leucoxene. To the north

and east, with increasing intensity of metamorphism, pale

amphibole, and then, finally, pleochroic amphibole appear

in the groundmass. Clinopyroxene is replaced by epidote

and pale amphibole and farther north by pleochroic amphibole. Plagioclase phenocrysts include myrmekitic intergrowths with quartz. Basalt pillow breccia adjacent to the

tonalite body at the Please horizontal control marker in the

Eastern St. John quadrangle is strongly foliated, the breccia

fragments are flattened and elongated downdip, and the

metamorphic assemblage includes garnet and diopside in

addition to quartz, plagioclase, hornblende, and calcite.

Most of the basalt, including the breccia fragments, is

amygdaloidal. On Kiddel Point, flattened amygdules are as

long as 10 cm (fig. 7). Some basalt is scoriaceous with small

(a few millimeters) amygdules making up 30 percent or

more of the rock. Amygdules are filled with various combinations of quartz, calcite, chlorite, epidote, and plagioclase,

as well as local prehnite and pumpellyite. Toward the north

and east, pale amphibole and clinozoisite in radiating fibers

appear in the amygdules.

VOLCANIC WACKE AND BEDDED TUFF

Volcanic wacke and bedded tuff are minor components

of the Water Island Formation and constitute unit Kww (fig.

8). Volcanic wacke consists of subrounded to angular clasts

of keratophyre of all varieties, basalt, and granitoid in a

matrix of comminuted keratophyre that includes xenocrystic

quartz and plagioclase. The constituents are poorly sorted,

up to more than a meter across, and bedding is commonly

poorly defined. Graded beds are rare but present in some of

the finer grained rocks between Turner Bay and Contant

Point. Generally, the keratophyre clasts have sharp rather

than ragged edges, are not pumiceous, and were probably

derived from lava. Keratophyre clasts in bedded rocks along

the shore between Grootpan and Kiddel Bays on the south

coast have wispy ends and are probably pumice; these rocks

may be submarine pumice flows. Trondhjemite clasts as

large as boulders have been found in two areas on the East

End: (1) in the cove along Long Bay about 250 m northeast

of Long Point and (2) the easternmost part of Privateer

Point. The trondhjemite clasts have granophyric texture and

are discussed in the section on the Careen Hill Intrusive

Suite. The bedded tuff in unit Kww contains finer grained,

better bedded, and better sorted beds of sand- to silt-size

material. It is difficult to determine whether the finest material is ash or chert. No shards have been seen in the few thin

sections available. Some of the fine-grained beds have

slump folds. Unit Kww crops out on the East End from

Europa Bay to Saltpond Bay, and forms the top of the Water

Island Formation in southwestern St. John.

Figure 7. Amygdaloidal basalt (Kws) of the Water Island

Formation, Kiddel Point, south shore, St. John. Knife in center of

photograph is 8.3 cm long.

Figure 8. Bedded volcanic wacke, tuff breccia, and tuff (Kww) of

the Water Island Formation, Red Point, East End. Clasts include

both porphyritic and phenocryst-poor keratophyre. Bedding

strikes 330° and dips 60° NE.
10 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

RADIOLARIAN CHERT

Radiolarian chert (Kwr) is mapped where fossil

Radiolaria have been observed. Beds of light-blue-gray to

dark-gray chert crop out near Contant Point in unit Kww (but

are too thin to show on the map) and on Kiddel Point. On

Kiddel Point, where it crops out on the shore of Saltpond Bay,

the unit contains well-preserved Radiolaria. They were only

observed in thin section after the completion of field work.

CAREEN HILL INTRUSIVE SUITE (HERE NAMED)

The Careen Hill Intrusive Suite consists of hypabyssal

trondhjemite, intrusive coarsely porphyritic keratophyre,

intrusive equivalents of all of the keratophyre units of the

Water Island Formation, and fine-grained gabbro thought to

be the intrusive equivalent of basalt of the Water Island.

These intrusive rocks are found isolated on islands off St.

John and as dikes and small plutons intruding each other and

the Water Island Formation. No order of intrusion has been

recognized and the intrusions are found throughout the stratigraphic thickness of the Water Island. Coarsely porphyritic

keratophyre intrudes the Water Island within about 35 stratigraphic meters of the overlying Louisenhoj Formation northwest of Boatman Point on the south coast. Columnar jointing is typical of the small plutons and dikes and is one of the

criteria used to identify a rock body as intrusive. Rocks of the

Careen Hill Intrusive Suite have been subjected to the same

local hydrothermal alteration as those of the Water Island.

The larger keratophyre intrusive bodies form sharp hills

along the south coast of St. John and around the harbor of

Charlotte Amalie, St. Thomas. The name is taken from

Careen Hill, which is held up by intrusive coarsely porphyritic keratophyre (Kcp), in Charlotte Amalie.

INTRUSIVE KERATOPHYRE

Several varieties of intrusive keratophyre have been

mapped based on the size and mineralogy of the phenocrysts, as was true for the Water Island Formation.

Phenocryst-poor keratophyre (Kca) contains up to about 10

percent small (generally less than 2 mm across) phenocrysts

of quartz and plagioclase in an aphanitic groundmass. This

unit forms a small pluton that holds up the sharp 272-ft-high

hill of Bovocoap Point and forms dikes such as the one at

the end of Dittlif Point that is 20 m thick. Porphyritic keratophyre (Kcp) is characterized by about 20 percent conspicuous phenocrysts of quartz and white-weathering plagioclase up to 3 mm across in a medium-dark-bluish-gray

(5B 4/1), aphanitic groundmass. Unit Kcp forms a small pluton about 140 m across on the 160-ft-high hill at the head of

Europa Bay and forms dikes elsewhere. In thin section, the

Europa Bay body looks remarkably unaltered except for

some secondary sericite and chlorite; yet, the presence of

unzoned albite and the alkali ratio suggest metasomatic

alteration. Many phenocrysts are in growth aggregates and

some quartz has embayments and inclusions of devitrified

glass. Plagioclase is twinned, nonzoned, clear albite. Some

plagioclase aggregates are surrounded by granophyric intergrowths of quartz and plagioclase. The groundmass consists

of randomly oriented plagioclase laths about 0.1 mm long,

interstitial quartz, and magnetite dust. Apatite and allanite

are sparse accessories. In major element chemistry, the rock

consists of 77.9 percent SiO2, 6.54 percent Na2O, and 0.15

percent K2O. Yet, the presence of unzoned albite and the

alkali ratio suggest metasomatic alteration. Plagioclasephyric keratophyre (Kcf) consists of 5 to 15 percent plagioclase phenocrysts (1 to 3 mm long) in a groundmass of typically felted plagioclase microlites and interstitial quartz.

This unit forms small plutons on Ram Hill and on the peninsula leading to Dittlif Point, and occurs as dikes. The Ram

Hill pluton is a laccolith dipping moderately to the east and

is cut off by a fault on the south. Relatively small columnar

joints, 10 to 20 cm across and plunging 40° WNW. may be

seen just north of the saddle between Ram Hill and Ram

Head.

Coarsely porphyritic keratophyre (Kcc) is characterized

by prominent quartz and plagioclase phenocrysts, typically 5

to 10 mm across, that constitute 20 to 30 percent of the rock.

In some rocks, the groundmass is exceedingly fine grained

and similar to that of the extrusive keratophyres; in others,

plagioclase laths are up to 0.3 mm long. Toward the north and

east where contact metamorphism has affected the rocks,

coarse epidote appears in the plagioclase phenocrysts and

epidote, pale amphibole, and shreddy olive biotite appear in

the groundmass. At an elevation of 320 ft along Center Line

Road northwest of Coral Bay, blue-green amphibole is present in the groundmass. Small plutons of Kcc hold up the sharp

hills north of Cocoloba Cay, near Hart Bay, and Maria Bluff.

Dikes of Kcc are widespread in the Water Island Formation

and cut the trondhjemite (Kct) of Flanagan Island, where the

groundmass of Kcc consists of granophyric spherulites. The

body of Kcc on the east slope of Bordeaux Mountain is interpreted to be a sill or laccolith based on the southwest-plunging columnar joints, and on the map pattern of Kcc, which

shows that it extends northeastward downslope, as shown in

guts. The northeast dip of the body is consistent with the attitude of the Water Island Formation in eastern St. John. Unit

Kcc has no extrusive equivalent.

Unit Kcc is distinguished from the Tertiary Red Hook

Tonalite Porphyry (Tr) by the generally smaller size of the

quartz phenocrysts, the absence of barrel-shaped biotite

phenocrysts, the absence of strong zoning in plagioclase

phenocrysts, the absence of a yellowish cast to the rock

color, the presence of some secondary epidote, the presence

locally of intense hydrothermal alteration, and the trace-element chemistry. The coarsely porphyritic keratophyre, like
STRATIGRAPHY 11

most of the keratophyres of the Lameshur, has an essentially flat rare-earth element (REE) chondrite-normalized pattern with a small negative europium (Eu) anomaly. In unit

Kcc, REE’s are enriched 10 to 15 times those in chondrites.

Chondrite-normalized light REE’s in the Red Hook (only

two samples) decrease from 20 to 30 times that of chondrites for lanthanum (La) to 3 to 4 times that of chondrites

for terbium (Tb). Heavy REE’s are 3 to 4 times that of chondrites; there is no Eu anomaly. The coarsely porphyritic keratophyre that crops out in roadcuts on the steep slope leading down to the northeast side of Hart Bay is unusually

fresh. Plagioclase phenocrysts are albite, not zoned, have

sharp euhedral borders, and minor sericitic alteration. The

groundmass is homogeneous and ultrafine grained (generally less than 0.05 mm), but does contain some secondary calcite, chlorite, and sericite. The major oxides include 76.2

percent SiO2, 6.25 percent Na2O, and 0.10 percent K2O.

Volatile components reported in the analysis include 0.81

percent H2O+, 0.12 percent H2O-

, and 0.38 percent CO2.

Again, the unzoned albite and the alkali ratio suggest metasomatic alteration.

TRONDHJEMITE

Trondhjemite (Kct) is fine to medium grained and commonly porphyritic with phenocrysts of quartz and plagioclase. In the freshest rocks, the plagioclase is albite, nonzoned, and twinned, but with some sericitic alteration. The

groundmass is typically a granophyric intergrowth between

quartz and albite. Commonly granophyric intergrowths

radiate from albite phenocrysts. Chlorite forms 5 to 7 percent of the rocks; in one rock, chlorite is clearly replacing

biotite. Trondhjemite forms Leduck Island, where columnar

jointing is prominent (fig. 9), and Flanagan Island, where it

is not. Trondhjemite also forms a 4-m-thick dike on the

small peninsula leading to Blinders Rocks, southwest of

Haulover in the Eastern St. John quadrangle. On Leduck

Island, the trondhjemite is light bluish gray (5B 7/1) and

coarsely porphyritic; the groundmass has a less well developed granophyric texture. Albite tends to be subhedral and

the quartz is equant; the grain size is about 0.1 mm. The

trondhjemite on Flanagan Island is medium bluish white

(5B 9/1) and contains a few percent clinozoisite in addition

to chlorite. The stratigraphic position of the trondhjemite on

these two islands cannot be determined directly because of

the absence of stratified rocks. Donnelly (1966) and Helsley

(1960, 1971) included the trondhjemite on Flanagan Island

in the younger suite of plutonic rocks that includes the

Virgin Gorda batholith. The trondhjemites are here included

in the Careen Hill because of the mineralogy, texture, and

chemistry. The chondrite-normalized REE patterns for the

trondhjemites are essentially the same as that of the keratophyres in the rest of the Lameshur Volcanic-Intrusive

Complex. The consistent, steep, west-southwest plunge of

the columnar joints on Leduck Island suggests a tabular

body that dips gently east-northeast and is roughly conformable with the bedding in the Water Island Formation

around Privateer Bay to the northeast.

Two zones of trondhjemite clasts up to boulder size in

volcanic wacke of the Water Island Formation were

observed on the East End. One is on the shore of Long Bay,

about 250 m northeast of Long Point, and the other is on the

easternmost part of Privateer Point. At the latter locality, one

boulder clast is more than 1 m across. As mapped, the two

zones of clasts are at different stratigraphic levels.

Trondhjemites in these clasts are typically porphyritic with

phenocrysts of quartz and nonzoned plagioclase in a granophyric crystalline groundmass that contains several percent chlorite, probably after biotite. They are interpreted to

be clasts of trondhjemite of the Careen Hill Intrusive Suite.

These observations, as well as the outcrops of trondhjemite,

indicate that the Lameshur Volcanic-Intrusive Complex

includes heretofore unrecognized silicic plutons, and that

some of these were either exposed to erosion at the time of

volcanism related to the Water Island Formation, or were

solidified enough to have survived transport upward through

a volcanic vent and subsequent incorporation into a volcanic

wacke.

GABBRO

Dark-greenish-gray, fine-grained gabbro (Kcm) forms a

small intrusive body on Bovocoap Point in southwestern St.

John. The gabbro is porphyritic with small (2 mm or less),

subhedral phenocrysts of clinopyroxene and plagioclase in a

Figure 9. Columnar joints in trondhjemite (Kct) of the Careen

Hill Intrusive Suite on southwestern shore of Leduck Island.

Columns plunge 75° with a bearing of 255°. Dark band to right

of person is a northwest-trending diabase dike (Td) dipping 75°

to the southwest.
12 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

groundmass of randomly oriented plagioclase laths about

0.5 mm long, clinopyroxene, chlorite, leucoxene, and

opaque minerals. The gabbro is cut by small plagioclasepyribole pegmatites. The gabbro has the same REE pattern

as the basalts of the Water Island Formation (light REE’s

enriched 3 to 5 times that of chondrites; heavy REE’s

enriched 5 to 8 times that of chondrites). The gabbro physically occurs within the area of the Lameshur VolcanicIntrusive Complex, and is here included within the Careen

Hill Intrusive Suite. Some of the mafic dikes associated with

keratophyre dike swarms, particularly those that show

hydrothermal alteration, are also probably of the Careen

Hill Intrusive Suite, but are not mapped separately.

Trondhjemite on the southwest coast of Flanagan Island

forms a network of veins in a large area of diorite or gabbro.

These dark rocks may be mafic rocks of the Careen Hill

Intrusive Suite.

SHEETED DIKES

Sheeted dikes were observed at a number of localities

on St. John and on St. Thomas (where they are better developed). The sheeted-dike complexes on St. Thomas are

unusual in that they consist dominantly of keratophyre but

also include diabase. The dikes trend northeast, dip moderately to steeply northwest and include both diabase with

columnar jointing and keratophyre. Helsley (1960, 1971)

mapped a series of abundant north-northwest-trending porphyry dikes cutting the Water Island Formation on the

Figure 10. Mafic sheeted dikes of the Careen

Hill Intrusive Suite seen in bluff at southwest

corner of Saltpond Bay. The dikes display

columnar jointing, strike northeast, and dip

northwest. The dikes are intruded by a sill of

coarsely porphyritic keratophyre also of the

Careen Hill Intrusive Suite (lower right) that

is 9 m thick at right edge of photograph, and

by a younger diabase dike (unit Td) in left

center. The diabase dike trends 40° and dips

70° NW.

Figure 11. Bedded, boulder volcanic conglomerate of the Louisenhoj Formation, south

end of Trunk Cay. Beds strike 80° and dip 65°

N. Hammer resting on largest boulder is 28.5

cm long.
STRATIGRAPHY 13

southern tier of the British Virgin Islands between St. John

and Virgin Gorda. The porphyry dikes are not present on the

northern tier of the British Virgin Islands where younger

strata (Tutu Formation and younger, as mapped by Helsley,

1960, 1971) are present. The dikes are highly altered porphyritic “microgranites” with granophyric texture and are

almost completely lacking in mafic minerals (Helsley, 1960;

Longshore, 1965). Helsley (1971) interpreted the dikes to be

of post-Eocene age and related to the opening of the

Anegada Trough. Granite, also mapped on the southern tier

of the British Virgin Islands, possibly might be trondhjemite

of the Careen Hill Intrusive Suite such as that on Flanagan

Island, and the porphyry dikes might be part of the sheeteddike complex. On St. John, sheeted dikes were observed on

the buttress forming the south side of Saltpond Bay (fig. 10)

and at Harbor Point.

Keratophyres are not normally a major component of

sheeted-dike complexes. An exception is the Canyon

Mountain Complex of Oregon, where albite granite and keratophyre dikes locally are dominant in the sheeted-dike unit

(Thayer and others, 1981). Avé Lallemant (1976) suggested

that, because of the abundance of silicic rocks in the Canyon

Mountain Complex, the ophiolite might have formed either

near a volcanic island arc or in a spreading marginal basin.

The Lameshur Volcanic-Intrusive Complex certainly

formed in a marine extensional environment. The REE pattern of the mafic rocks is most similar to that of normal (Ntype) mid-ocean ridge basalt (MORB) (Sun and others,

1979; Sun and McDonough, 1989).

VIRGIN ISLAND GROUP (NOT USED)

Donnelly (1966) included the stratified rocks of St.

Thomas and St. John that overlie the Water Island

Formation in his Virgin Island Group (table 1). He considered the lowest unit of the group, the Louisenhoj Formation,

to lie unconformably above the Water Island and to be part

of the primitive island arc (PIA) series. The dominant volcanic components of the Louisenhoj and Tutu Formations,

however, are distinctly of island arc character, and, except

for the lowest basalt flows of the Louisenhoj, contrast with

those of the more oceanic Water Island Formation. Because

of the different chemistry and interpreted different tectonic

settings, I would not include the Water Island and

Louisenhoj in the same “series.” The stratigraphic contact

between the Water Island and Louisenhoj is here interpreted to be conformable. The youngest unit of the Virgin Island

Group in the northern U.S. Virgin Islands was named the

Hans Lollik Formation by Donnelly (1966). In the northern

U.S. Virgin Islands, it crops out only on Hans Lollik Island,

Little Hans Lollik Island, and Pelican Cay, all north of St.

Thomas. The rocks on these islands are here interpreted to

be the Louisenhoj Formation (Rankin, 1997), brought up

north of the Leeward Passage on the north limb of a major

syncline, as first postulated by Cleve (1882). As stated earlier, the name Virgin Island Group is not used in this report.

LOUISENHOJ FORMATION

Strongly cemented volcanic conglomerate, breccia, volcanic wacke, shale, chert, andesite, basalt, tuff, and rare

limestone of the Louisenhoj Formation (Kl) overlie the

Water Island with apparent conformity. The rock is known

locally as the “blue beach” (Cleve, 1882), or “blue bitch”

(Donnelly, 1966). The formation was named by Donnelly

(1966) for excellent exposures in roadcuts in the vicinity of

Louisenhoj north of Charlotte Amalie, St. Thomas.

Volcaniclastic rocks predominate. Primary volcanic rocks,

such as lavas and flow breccias, are present but sparse. In

weathered exposures or exposures covered with vegetation,

distinguishing between epiclastic breccia and aa lava is difficult (for example, in the Camelberg Peak area). Other

units of massive conglomerate and breccia consisting of

large boulders and slabs of basalt or andesite, such as those

at America Point, appear to have been deposited near the

source rocks. Conglomerate, breccia, and coarse volcanic

wacke are typically pale green, greenish gray, or grayish

blue green. The formation is at least 1.5 km thick on the

west coast of St. John, but thins to the east. At Leinster Bay,

it may be as thin as 0.5 km.

The age of the Louisenhoj cannot be constrained more

closely than Albian to Santonian. The latter is the younger

age limit of the overlying Outer Brass Limestone (Ko).

Donnelly (1966, 1989) considered the Louisenhoj to be of

Albian age because of his interpreted Albian age of the Tutu

Formation. Helsley (1960, 1971) assigned a Cenomanian

age to the Louisenhoj, but did not present any evidence.

Bedding characteristics of the Louisenhoj are heterogeneous. The unit is dominated by thick beds of conglomerate (fig. 11), but these are typically interlayered with

sequences a few to several meters thick of silt- to sand-size

material and even some laminated radiolarian chert at

Dorothea Point and Kastel Point in the Central St. Thomas

quadrangle. Graded beds are common (Rankin, 1999, fig.

9), some as thick as 4 to 6 m (fig. 12). Crossbedding is

present but less common. Soft sediment deformation and

broken-up, finer grained beds were observed locally.

Massive beds of unsorted sand- (or smaller) to boulder-size

material are tens of meters thick at the point forming the

northeastern end of Hawksnest Bay. The larger clasts in the

conglomerates tend to be more rounded than the smaller

ones. Clasts larger than 0.5 m across are abundant throughout the formation (figs. 11 and 13). The largest subrounded

clast observed on St. John is about 3 m across. Breccia

fragments may be larger, but these appear to be closely
14 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

related to lava flows. The conglomerates may be either

clast supported or matrix supported.

Clasts of clinopyroxene andesite and basalt constitute

the bulk of the Louisenhoj Formation. Some of these rocks

have overgrowths of hornblende on the pyroxene phenocrysts, and a few also have primary hornblende phenocrysts. Small amygdules are pervasive in some clasts. The

andesite and basalt clasts in the Louisenhoj are characterized by stubby, zoned plagioclase and clinopyroxene phenocrysts that occur together or singly. In most rocks, the

phenocrysts are obvious, constitute about 20 percent of the

rock, and commonly attain diameters of 5 to 10 mm. These

field characteristics differ markedly from those of the

basalts of the Water Island Formation. Large xenocrysts of

plagioclase and pyroxene are notable constituents of some

of the clastic rocks. Other rocks found as clasts include

devitrified andesitic glass (particularly in western St.

Thomas), pumice (not common), limestone, keratophyre,

phenocryst-poor basalt similar to that of the Water Island

Formation. Differences in clast populations suggest local

sources. Keratophyre and phenocryst-poor basalt clasts

locally are abundant near the basal contact with the underlying Water Island Formation. Limestone clasts occur on

Rata and Ramgoat Cays; some of these clasts are fossiliferous with individual identifiable forms as well as fossil hash.

Small collections have been made from these limestone

boulders. On the west side of Rata Cay, the limestone clasts

are in two layers separated along strike by about 0.7 m of

conglomerate; they probably represent dismembered beds

of limestone. Elsewhere, calcite cement is common in the

conglomerate of the Louisenhoj.

In some places, breccia fragments (fig. 14) coalesce into

massive andesite (field term, only); in other places, only

massive andesite appears to be present. These areas are

mapped as porphyritic andesite lava (Kla). The rocks may be

vesicular and are characterized by stubby plagioclase and

pyroxene phenocrysts. Unit Kla at the north end of Ramgoat

Cay is pillowed. Highly brecciated Kla on Camelberg Peak

could be aa lava. Chemically, Kla includes basalts as well as

andesites (SiO2 content of six samples ranges from 49.7 to

59.0 percent). The REE contents range from 10 to 30 times

that of chondrites for lanthanum (La) to 5 to 15 times that of

chondrites for lutetium (Lu). Porphyritic, locally amygdaloidal basalt (Klb) forms the basal unit of the Louisenhoj

in the vicinity of Rendezvous Bay and near Ajax Peak. The

basalt is included in the Louisenhoj because of the presence

of prominent phenocrysts of stubby plagioclase and (or)

pyroxene. The basalt is chemically transitional between

basalt of the Water Island Formation and andesite of the

Louisenhoj. The chondrite-normalized REE pattern is

essentially flat and is about 10 times that of chondrites. No

evidence has been found to support the idea of Donnelly

(1966) that a paleovolcano in Pillsbury Sound between St.

Thomas and St. John was the source of the volcanic material in the Louisenhoj Formation.

The lower contact of the Louisenhoj with the Water

Island is well exposed in a few places on St. John and can

be traced around Contant Point. On the nose of the point, the

contact is gradational over about 15 cm from thinly bedded

volcanic wacke and chert or ash of the Water Island up into

massive conglomerate of the Louisenhoj. The same contact

Figure 13. Boulder bed in the Louisenhoj Formation, looking

northwest, north shore of St. John on point between Denis and

Jumbie Bays. Clasts are andesite. Beds strike 85° and dip 60° N.

Hammer is 28.5 cm long.

Figure 12. Graded bed 4 m thick showing transition from volcanic

conglomerate to wacke of the Louisenhoj Formation, west side,

Jumbie Bay. Pyroxene andesite clasts at base are commonly 10 cm

and as much as 15 cm across. Matrix contains abundant obvious

detrital pyroxene. Beds strike 285° and dip 45° N.; foliation (not

obvious in photograph) trends 285° and dips 80° N.
STRATIGRAPHY 15

is sharp at the base of a ledge on the northwest shore of

Great Cruz Bay. Graded beds in the Water Island volcanic

wacke dip northeast about 30° and are upright. The contact

is somewhat irregular and an outlying patch of Louisenhoj

conglomerate (Kl) is preserved on a flat exposure of volcanic

wacke (Kww) a few meters shoreward of the ledge. At the

time of field work in 1986, construction for a new resort

hotel at the head of Great Cruz Bay exposed an excellent

section across the conformable contact (290°, 50° N.) (fig.

15). The section extends up from amygdaloidal basalt

through bedded ash or chert and lithic tuff of the Water

Island to conglomerate of the Louisenhoj. A 10-m-thick sill

of hornblende lamprophyre intrudes the volcanic wacke

(Kww) about 6 m below the contact; a 20-m-thick slightly

discordant sill (80°, 70° N.) of fine-grained gabbro intrudes

the Louisenhoj above the contact. The contact is now covered by outbuildings and landscaping of the resort. The 200-

m sinistral offset of this northeast-dipping contact at sea

level, determined by projecting the exposed contact along

strike into Great Cruz Bay, was the first evidence developed

for the Guinea Gut fault. The contact is again exposed on

the peninsula between Monte and Klein Bays where amygdaloidal basalt containing stubby phenocrysts of plagioclase

and pyroxene (Klb) overlies volcanic wacke (Kww) containing blobby masses of flow-layered, phenocryst-poor keratophyre. Protrusions of basalt cut down into the volcanic

wacke. Finally, a conformable contact between underlying

flow-layered, brecciated, phenocryst-poor keratophyre

(Kwa) and bedded conglomerate (Kl) is exposed on the north

end of Cocoloba Cay (fig. 16). Cobbles of keratophyre are

abundant in the lower 3 m of the conglomerate.

As the degree of metamorphism increases to the northeast, biotite, pale amphibole, and epidote appear in the

andesite, and epidote, hornblende, and lesser amounts of

biotite appear in the basalt. The calcite cement in the conglomerate from Cinnamon Bay eastward also contains epidote, brown anisotropic garnet, and, farthest east, diopside.

From central St. John to the north and east, a foliation is

present in most rocks, particularly in the finer grained sedimentary rocks and in the matrix of the finer grained conglomerates. Northeast of Cinnamon Bay and particularly

Figure 14. Foliated andesitic tuff breccia of the Louisenhoj

Formation, buttress between Jumbie and Trunk Bays. Dark spots

are relict clinopyroxene phenocrysts. Dime for scale.

Figure 15. Conformable contact between bedded ash or chert and

lithic tuff (Kww) of the Water Island Formation below and volcanic sandstone and conglomerate of the Louisenhoj Formation

above. The head of the three-pound hammer rests on the contact,

which strikes 290° and dips 50° N. Photograph taken in 1986 at a

construction site low on the west side of Roman Hill at the head of

Great Cruz Bay. The site is now covered by landscaping of a resort

hotel.

Figure 16. Conformable contact between phenocryst-poor keratophyre tuff (Kwa) of the Water Island Formation below and volcanic conglomerate of the Louisenhoj Formation above, north end

Cocoloba Cay. Contact dips 30° NE. Hammer is 28.5 cm long.
16 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

east of Annaberg Point, the clasts in the conglomerates are

distinctly flattened and locally elongated (fig. 17). Flattened

boulders can be seen in float blocks of conglomerate of the

Louisenhoj along the shore of Leinster Bay at Annaberg

Point. Deformation is thought to precede the thermal peak

of metamorphism.

OUTER BRASS LIMESTONE

The Outer Brass Limestone (Ko) (fig. 18) is a relatively

minor, thin unit on St. John stratigraphically separating the

conglomeratic Louisenhoj Formation below from the less

conglomeratic Tutu Formation (above). The formation was

named by Donnelly (1966) for exposures on the western

seacliff side of Outer Brass Island north of St. Thomas. The

thin outcrop belt extends from Maho Bay to Brown Bay on

St. John. The belt is discontinuous because of numerous

faults and it is cut by a Tertiary pluton (Tt) east of Leinster

Hill. The map pattern portrayed on Leinster Hill is without

field control due to impenetrable vegetation, chiefly a

thorny vine known locally as come-along. Neither the base

nor the top of the formation is exposed on St. John. The

Outer Brass is interpreted to be part of a conformable

sequence from the Louisenhoj Formation through the Tutu.

The conformable upper contact of the Outer Brass with the

Picara Member of the Tutu Formation is exposed on Outer

Brass Island. The only exposure of the Outer Brass’s lower

contact on St. Thomas is a mylonite zone. The Outer Brass

is on the order of 100 m thick. Pessagno (1976; oral commun., March 1997) identified planktonic Foraminifera of

late Turonian to late Santonian age in thin sections of the

Outer Brass from St. Thomas.

Outcrops of the formation on St. John are within the

metamorphic aureole of the Tertiary plutons. The Outer

Brass Limestone consists of thin-bedded calc-silicate beds

of white to blue-gray calcite marble (H2S odor when broken), and matrix-supported cobble conglomerate consisting

of andesite clasts in a carbonate matrix. A unit of clean calcite marble at least 3 m thick and with a fluted weathering

surface is exposed on the western nose of the small ridge at

the Annaberg Sugar Mill Ruins (fig. 18, this report; fig. 16

of Rogers and Teytaud, 1988, mislabeled as bedded siltstone of the Tutu Formation). Clean calcite marble is also

found on the low hill east of Maho Bay and along the trail

from Waterlemon Bay to Brown Bay near the 240-ft ridge

crest. Heterogeneous calc-silicate rock, exposed in the

bluff on Annaberg Point, includes calcite, plagioclase, epidote, brown isotropic garnet, diopside, vesuvianite, and

wollastonite in various assemblages. A carbonate matrix

conglomerate containing flattened clasts of andesite crops

out on the low hill east of Maho Bay and at the Annaberg

ruins. Coastal exposures east of Leinster Hill are of tonalite

containing abundant calc-silicate xenoliths, which are

inferred to be more or less in-place remnants of the Outer

Brass Limestone.

TUTU FORMATION

The Tutu Formation (Kt) consists of volcanic wacke,

shale, conglomerate, calcareous siltstone, sparse limestone,

and rare basalt and andesite or their metamorphosed equivaFigure 17. Foliated and contact-metamorphosed volcanic conglomerate of the Louisenhoj Formation, looking east on America

Point. Meta-andesite clasts are flattened in a plane striking 285°

and dipping 90° and elongated downdip. Knife in center of photograph is 8.3 cm long.

Figure 18. Fluted outcrop of bluish-gray, clean, calcite marble of

the Outer Brass Limestone, looking east on nose of ridge at

Annaberg ruins. Beds strike 85° and are vertical. Knife is 8.3 cm

long.
STRATIGRAPHY 17

lents. Many rocks are calcareous. The formation was named

by Donnelly (1966) for exposures around Tutu Bay on the

north shore of St. Thomas, Eastern St. Thomas quadrangle.

Donnelly also defined the Congo Cay Limestone Member

for exposures on Congo Cay at the west border of the

Western St. John quadrangle. In this report, the Tutu is divided into a basal Picara Member and an overlying Mandal

Member on the basis of field work on St. Thomas. The

Congo Cay has been reduced to a lens within the Mandal

Member. Donnelly (1966) concluded that the Tutu was of

Albian age on the basis of the faunal assemblage in limestone clasts in a megabreccia exposed at Coki Point, Eastern

St. Thomas quadrangle. Young (1972) identified ammonites

of early late Albian age in the Tutu of St. Thomas. If the

Turonian to Santonian age for the Outer Brass Limestone is

correct, the ammonites in the Tutu are probably reworked

from an older unit as noted by Pessagno (1976).

The Tutu Formation is more completely exposed on St.

Thomas and on the chain of islands that forms the northern

border of Pillsbury Sound. The Tutu underlies northernmost

St. John and the chain of islands north of Pillsbury Sound

that extends into the Western St. John quadrangle. The basal

contact of the Tutu with the Outer Brass Limestone is not

exposed on St. John, but bedding in adjacent outcrops is

essentially parallel. The contact is interpreted to be conformable. The Congo Cay Limestone Lens of the Mandal

Member is the youngest stratified bedrock unit in the map

area, but stratigraphic contacts are under water. Helsley

(1960) portrays the Congo Cay to be within the Tutu on

Great Thatch Island and Tortola of the British Virgin Islands.

In the map area of this report, the Tutu Formation is within

the contact aureole of the Tertiary plutons. Metamorphic

recrystallization is most noticeable in the calcareous rocks.

Former limestones are now marbles, and calcareous siltstones and sandstones contain obvious garnet, diopside, and,

in places, wollastonite.

Graded beds and slump folds are common in the Tutu, as

are disrupted slabs of metasandstone and metasiltstone in

metaconglomerate (Rankin, 1999, fig. 10). A spectacular

example of slump folds in the Mandal Member is exposed in

a steeply north dipping bed of interlayered metasiltstone and

metasandstone 3 m thick on Thatch Cay, just east of Mother

East Point, Eastern St. Thomas quadrangle. Most of the

material is thought to have been deposited by turbidity currents. The extent of soft sediment deformation combined

with the evidence for turbidity current deposition indicates

that the Tutu was deposited on an unstable slope such as a

trench wall leading to an accretionary wedge. Tectonism has

produced boudins of metasiltstone in marble and of

metasandstone in metasiltstone and marble. Tight to isoclinal

tectonic folds with axial-plane cleavage also are common.

Marble is typically deformed in a plastic manner.

Distinguishing between soft sediment and tectonic folds is

difficult in places because the two appear to grade into one

another.

PICARA MEMBER (HERE NAMED)

The Picara Member (Ktp) on St. John consists of metamorphosed thick- to thin-bedded volcanic wacke, conglomerate, siltstone, limestone, and rare andesite or basalt. Many

rocks have a calcareous matrix. The member, minus limestone and basalt, is well exposed on the long peninsula on the

north side of St. Thomas that ends in Picara Point, from

which it takes its name. The Picara Member on St. John is

well exposed in shoreline outcrops around Maho, Leinster,

and Threadneedle Points. Clasts in the conglomerate are

metamorphosed andesite, basalt, and limestone. In the limited exposures on St. John, pebble and cobble conglomerates

are more common than boulder conglomerates.

Metasiltstone and fine-grained metawacke are commonly

weathered out and produce ribby outcrops. In places, the carbonate content is concentrated in nodules. Marble conglomerate and calc-silicate beds north of Maho Point are shown

on the geologic map as Ktpl. Calc-silicate assemblages

include plagioclase-biotite-hornblende and plagioclasehornblende-diopside-calcite. Porphyritic meta-andesite

(Ktpv) crops out on the south side of Maho Point and on the

hill to the east. Fresh rock is foliated, medium dark gray (N

4) and contains relict phenocrysts of glomerophyric plagioclase and prominent clinopyroxene as large as 3 by 6 mm.

Metamorphic amphibole replaces the clinopyroxene phenocrysts. Fibrous amphibole and chlorite are abundant in the

groundmass and impart a foliation to the rock.

Two calc-silicate layers, 5 and 10 m thick separated by

about 4 m of siliceous hornfels, crop out on the west end of

Waterlemon Cay. The calc-silicate assemblage includes

brown garnet, some of which is anisotropic, diopside, wollastonite, sphene, calcite, monazite, and sulfide. In places,

the beds are layered with each layer dominated by one mineral. In other places, the monomineralic layers form concentric zones of lenses of calcareous minerals. A typical

sequence within the lens consists of calcite cores surrounded by a layer of garnet, surrounded by a layer of diopside.

The garnets may be euhedral and as large as 2 cm; wollastonite blades in sheaves are as long as 2 cm. Some euhedral

garnets are multiply zoned and in thin section are seen to

consist of alternating shells of garnet and diopside. These

observations suggest open-space crystallization; the rocks

are probably skarns.

MANDAL MEMBER (HERE NAMED)

The Mandal Member (Ktm) is characterized by finer

grained, thinner bedded material than the Picara (fig. 19).

The unit is named for rocks on the east side of Mandal

Point, Eastern St. Thomas quadrangle, that overlie those of

the Picara Member with apparent conformity. The Mandal
18 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

Member makes up most of Whistling, Lovango, and Congo

Cays, Carval Rock, and Mary Point, St. John. The Mandal

protoliths were noticeably calcareous in the Western St.

John quadrangle. The siltstones were typically calcareous

and the sandstones commonly had a calcareous matrix as

shown by the present calc-silicate mineralogy.

Conglomerates are sparse, but can be as much as 3 m thick;

most are composed of limestone (marble) clasts. Areas of

limestone conglomerate, abundant calc-silicate beds, and

marble are mapped as Ktml on both sides of Fungi Passage.

Marble clasts in the conglomerate are composed of calcite,

plagioclase, tremolite, brown garnet, epidote, and opaque

minerals. Metamorphosed calcareous siltstone and sandstone contain brownish garnet (locally as large as 3.5 cm),

diopside, tremolite, plagioclase, calcite, opaque minerals,

and minor quartz. Eight meters below a 6-m-thick tonalite

dike at the northeast corner of Whistling Cay, garnet-rich,

bedded calc-silicate rock contains isotropic reddish garnet,

bright-green diopside, plagioclase, wollastonite, phlogopite,

sphene, pleochroic amphibole, and quartz. Dark-grayish-red

(5R 3/2), very fine grained metasandstone contains abundant reddish-brown biotite and cordierite clouded with tiny

inclusions. Some metasandstones contain tiny muscovite

crystals as well as biotite and cordierite.

CONGO CAY LIMESTONE LENS (REVISED)

The Congo Cay Limestone Lens (Ktmc) consists of vertical beds of calcite marble interlayered with some reddishbrown, siliceous hornfels and calc-silicate rock on Congo

Cay (fig. 20) and Carval Rock. The marble is white to medium light bluish gray (5B 6/1) and consists of 98 percent

calcite plus accessory plagioclase, white mica, and opaque

minerals; in other rocks, the accessories are plagioclase,

tremolite, wollastonite, sphene, and opaque minerals. The

calcite grain size is typically 1 to 3 mm, but the grain size in

some marbles is as large as 1 cm. The stratigraphic position

of the Congo Cay within the Tutu is not known because the

rocks are isolated by water. The Congo Cay is interpreted to

be continuous underwater with the calcareous rocks of the

Mandal Member (Ktml) on Whistling Cay. Helsley (1960)

and Donnelly (1966) interpreted the lens to be continuous

underwater with carbonate beds on Great Thatch Island of

the British Virgin Islands. The beds on Great Thatch Island

could well be the Congo Cay repeated on the north limb of

the syncline whose axis passes between Picara Point and

Hans Lollik Island, Central St. Thomas quadrangle.

DIABASE, GABBRO, AND DIORITE DIKES

Dikes of diabase, gabbro, and diorite (Td) are abundant.

They range in thickness from a few centimeters to several

tens of meters. Chilled margins are typical. Some form

complex bodies of multiple dikes; some have internal layering parallel to the walls. Most dikes do not display columnar jointing. Color index, grain size, and texture vary; most

are diabase or, if thick enough, gabbro with diabasic margins. Some dikes are amygdaloidal. Primary minerals in the

dikes are plagioclase, clinopyroxene, commonly hornblende, magnetite, and (or) pyrite, and in some, quartz. The

dikes are commonly porphyritic with phenocrysts of plagioclase, clinopyroxene, and (or) hornblende. Thicker bodies

commonly contain epidote-quartz (epidosite) nodules and

considerable disseminated epidote. Sampled dikes contain

Figure 19. Thin-bedded turbidite of the Mandal Member of the

Tutu Formation in the zone of contact metamorphism on the north

shore of Francis Bay. Bedding strikes 90° and dips 70° N.

Figure 20. Wave-cut notch in the Congo Cay Limestone Lens of

the Mandal Member of the Tutu Formation, looking west along

southern shore of Congo Cay. Bedding strikes 80° and dips 80° N.

Isoclinal folds are visible in beds in foreground.
STRATIGRAPHY 19

secondary pale, commonly fibrous amphibole, chlorite, and

epidote. To the north and east, most dikes are progressively

metamorphosed to greenschist facies; farther north they are

at amphibolite facies.

The dikes intrude all stratified units, are intruded by

tonalite (Tt), and pieces of the dikes are xenoliths in the

tonalite. Dikes cut clasts in conglomerates of the Louisenhoj

and Tutu Formations, which indicates that the dike intrusion

occurred after the conglomerates were cemented. In the

northern part of the island, many dikes exhibit the same

penetrative cleavage as the stratified rocks; some dikes are

broken into segments aligned parallel to the cleavage. At

Mandal Point on St. Thomas and on the south shore of

Thatch Cay, west of Stony Bay in the Eastern St. Thomas

quadrangle, segments of mafic dikes as thick as 15 m in the

Mandal Member occur as boudins. Many of the northern

dikes were affected by the metamorphic aureole of the

tonalites. Some dikes appear to have intruded after cleavage

formed. Abundant fresh, noncleaved diabase and gabbro

dikes cut and are chilled against noncleaved tonalite of

unknown age on the Capella Islands, about 3 km south of

eastern St. Thomas. Most diabase and gabbro dikes that

intrude the Lameshur Volcanic-Intrusive Complex are

mapped as unit Td. Those mafic dikes that show the same

hydrothermal alteration as the adjacent keratophyre are

included in the Careen Hill Intrusive Suite. Some of those

Careen Hill mafic dikes have columnar joints.

The dikes mapped as Td are almost certainly of more

than one age. No systematic study of the dikes was made

and no distinctive field characteristics were noted that could

document an age assignment. Some dikes, particularly those

that are cleaved, may be intrusive equivalents of the enclosing or overlying stratified units. A few cut hornblende lamprophyre (Tl) and are interpreted to be among the youngest

rocks in the map area. Donnelly and others (1971) and

Donnelly and Rogers (1980) noted that the diabase dikes on

St. John are part of a dike swarm extending from San Juan,

Puerto Rico, to St. John. In Puerto Rico, that dike swarm

cuts Paleocene strata (Pease, 1968) and is overlain by

Oligocene coastal plain strata (Donnelly and Rogers, 1980).

Donnelly and others (1971) suggested that the dike swarm

was related to the Tertiary plutons (here represented by the

tonalite unit, Tl). However, Donnelly and others (1990) later

concluded that the dike swarm was an independent magmatic event largely because of its wide distribution and lack

of localization around the plutons. A large mass of mafic

and intermediate dikes on St. John that (1) generally lack

columnar joints and hydrothermal alteration, (2) have

chilled margins, (3) are near vertical, and (4) cut folded

rocks as young as the Tutu Formation are interpreted to represent an independent magmatic episode older than the

tonalite. The tonalite, however, locally grades into gabbro

and some dikes may be early phases of the tonalitic magmatism. The dikes, therefore, are grouped into a single map

unit and assigned a Tertiary age.

Gabbroic dikes (Td) 3 m or more in thickness are shown

on the geologic map where space permits. They are nearly

vertical and trend northeast; a subset of gabbroic dikes, also

nearly vertical, strikes northwest. Thinner dikes of unit Td

are prominent in most coastal exposures; in some exposures, dike complexes occupy as much as 20 percent of the

outcrop. The inset map of dike distribution and attitude on

plate 1 shows a more complete data set of all dike attitudes,

including those assigned to the Careen Hill Intrusive Suite

and younger dikes; the dikes are separated into those more

than 3 m thick and those less than 3 m thick. The inset map

also indicates that, when all dikes are included, attitudes are

more varied. Presumably the real density of dikes in the

interior is the same as it is on the coast.

BIOTITE-HORNBLENDE TONALITE

Biotite-hornblende tonalite (Tt) is very light gray to

medium light gray, medium grained, and nonfoliated. The

unit also includes minor gabbro, diorite, granite, and pegmatite. The tonalite (58.8 to 60 percent SiO2) is composed

of interstitial quartz (10 to 15 percent); subhedral strongly

and complexly zoned plagioclase; biotite, partly altered to

chlorite; hornblende; and magnetite. The gabbro is composed of plagioclase, hornblende, clinopyroxene, apatite,

and pyrite and (or) magnetite. The granite (not sampled for

chemistry) is very light gray, fine to medium grained, and

contains quartz; orthoclase (about 35 percent); strongly

zoned plagioclase; biotite, partly altered to chlorite; zircon;

and magnetite. The pegmatite is white with a locally greenish copper stain and contains quartz, slightly perthitic orthoclase (poikilitic crystals up to 1.5 cm across), reddish

isotropic garnet, and small amounts of diopside, sphene,

allanite, and calcite.

Unit Tt crops out for about 2 km along the north shore

of Mary Point where it intrudes the Mandal Member of the

Tutu. Between Threadneedle Point and Brown Bay, unit Tt

is interpreted to intrude the sequence from the Picara

Member of the Tutu to the Louisenhoj Formation. The unit

also occurs in a smaller area, not shown by Donnelly

(1966), at the Please horizontal control marker in the

Eastern St. John quadrangle, where it intrudes the Water

Island Formation. Granite is found within unit Tt in a small

area on the north shore, north of the eastern high point (520

ft) of the ridge on Mary Point. Small pegmatites cut calc-silicate beds of the Mandal Member about 350 m south of the

tonalite contact at the west end of Mary Point. The contact

of the body on Mary Point with the tightly folded Mandal

Member is exposed both at Fungi Passage and at Anna

Point. At both localities, the contact is sharp and roughly

parallel to the nearly vertical bedding in the Mandal; locally, the contact crosscuts bedding. At both localities, the

tonalite is choked with xenoliths, indicating some stoping
during intrusion. At Anna Point, most xenoliths are micarich contact rocks of the Tutu; the xenoliths are roughly

aligned 290° and dip about 90° as much as 200 m from the

contact. Adjacent to Fungi Passage, xenoliths of gray, finegrained granofels, gabbro, calc-silicate rock, and siliceous

hornfels constitute 5 to 10 percent of the tonalite as far as

200 m from the contact. A reaction zone about 5 cm thick

separates the tonalite body from bedded calc-silicate rocks

of the Tutu Formation. Near the contact, the tonalite is cut

by small leucocratic pegmatites and granite veins and the

tonalite contains segregations a few centimeters across of

large hornblende crystals. Some blocks of bedded calc-silicate rocks in the tonalite are internally tightly folded.

Outcrops and float boulders along the western half of the

north shore of Mary Point contain numerous masses of

pyrite up to 30 cm across, calc-silicate xenoliths, and pockets of limonite-stained skarn. The masses of skarn commonly are zoned and may contain open space. In some, an

outer rim of coarse-grained epidote crystals surrounds intergrowths of epidote, brown garnet, and calcite. Toward the

center of the skarn bodies are masses of sulfide (mostly

pyrite) and free-growing quartz crystals. Some rocks have a

green copper stain.

Exposures on the eastern end of Whistling Cay just west

of Fungi Passage are useful in working out age relationships. At the northeast corner of the cay, a gently dipping 6-

m-thick tonalite dike cuts nearly vertical calc-silicate beds

of the Mandal Member (fig. 21). The beds are tightly folded and have a cleavage that is axial planar to those folds;

fold axes are subhorizontal. Thermal metamorphism has

partly annealed the cleavage. The dike has sharp contacts, is

not internally deformed, and contains about 5 percent xenoliths of the country rock. Nearby diabase dikes have been

deformed into boudins whose axes are subhorizontal.

Locally, thin diabase dikes are offset by cleavage. The

sequence of events here is (1) deposition of the Mandal

Member of the Tutu Formation, (2) tight folding with the

development of axial-plane cleavage, (3) intrusion of nearly

vertical diabase dikes in the later stages of this deformation

and the development of boudins, and (4) intrusion of

tonalite accompanied by thermal metamorphism.

Tonalite and related rocks (Tt) of St. John are part of The

Narrows pluton that also crops out on the north side of The

Narrows from Great Thatch Island to western Tortola and is

hypothesized to underlie much of The Narrows (Helsley,

1960; Donnelly, 1966). The Narrows pluton may or may not

be connected to the Virgin Gorda batholith, a composite

batholith that underlies eastern Tortola, Virgin Gorda, and

adjacent islands. Helsley (1960) states that the batholith has

a total land area of only about 34 km2, but that the total area

including the subaqueous parts is on the order of 260 km2. A

major reason for hypothesizing the large size relative to the

area of the exposed pluton is the extent of the contact aureole. Certainly this seems valid for The Narrows pluton.

The Narrows pluton intrudes the Mandal Member of the

Tutu (Turonian or younger) and is post-kinematic relative to

cleavage and tight folding of the Tutu. The Virgin Gorda

batholith (sensu stricto) in eastern Tortola intrudes the

Tortola Formation of middle Eocene age (Helsley, 1971).

Published K-Ar ages for rocks of the Virgin Gorda batholith

(sensu lato) are listed in table 2. As part of this study, several samples of rocks from St. John, including three samples

of tonalite and gabbro from The Narrows pluton, were analyzed by the 40Ar/39Ar age-spectrum dating technique by

Michael J. Kunk (U.S. Geological Survey, written commun., March 1997 and August 1998). Sample VI-85-193 is

a medium-grained quartz-plagioclase-hornblende-biotitemagnetite tonalite (59.8 percent SiO2) collected near the

Please horizontal control marker in the Eastern St. John

quadrangle. Sample VI-85-218 is a medium-grained quartzplagioclase-hornblende-biotite-magnetite-pyrite gabbro

(53.8 percent SiO2) collected from the northwestern shore

of Mary Point in the Western St. John quadrangle. Sample

VI-85-222 is a medium-grained quartz-plagioclase-hornblende-biotite-magnetite tonalite with less than 1 percent

potassium feldspar (60.0 percent SiO2) from the northeastern corner of Mary Point. Plateau ages from hornblende

separates from these samples, all close to 39 Ma, also are

shown in table 2. These new data, together with most of the

other ages in table 2 (except VI-7 and VIJ-451), suggest a

late Eocene age for the Virgin Gorda batholith (sensu lato)

according to the time scale of Harland and others (1989),

and is consistent with stratigraphic and structural control of

the age of this batholith. The young hornblende age (24.3

Ma) for The Narrows pluton reported by Kesler and Sutter

(1979) is the most puzzling anomaly. Their sample came

from the north shore of Mary Point about midway between

Fungi Passage and Anna Point (Stephen E. Kesler, written

20 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

Figure 21. Gently dipping (25°, dipping 20° W.) tonalite dike (Tt),

6 m thick, intruding vertical beds (280°, dipping 85° N.) of the

Mandal Member of the Tutu Formation at the northeastern corner

of Whistling Cay. View looking south from a boat.
STRATIGRAPHY 21

commun., April 1997). That location is certainly within the

body that I map as The Narrows pluton. Pindell and Barrett

(1990), presumably referring to this 24.3 Ma age, suggested

that Oligocene magmatism in the Virgin Islands might be

related to subduction beneath the Lesser Antilles arc rather

than beneath the Greater Antilles arc. The problem of the

24.3 Ma age is unresolved.

HORNBLENDE LAMPROPHYRE

Dark, aphanitic dikes characterized by abundant large

hornblende phenocrysts (up to 3 by 1 cm) are scattered

across both St. Thomas and St. John. They typically contain

smaller plagioclase phenocrysts and disseminated magnetite; some contain centimeter-sized clinopyroxene phenocrysts. Some of these dikes have columnar joints. They

intrude all stratified units, but have not been observed to cut

tonalite (Tt) on St. John. Many cut diabase and gabbro dikes

(Td) but a few of the coarsely hornblende-phyric dikes are

cut by Td dikes. It is not clear that the coarsely hornblendephyric dikes represent an independent intrusive event. In

places, coarsely hornblende-phyric dikes grade texturally

into diabase. Donnelly (1966) recognized the hornblendephyric dikes and called them lamprophyres, but did not map

them except at Haypiece Hill, St. Thomas. These distinctive

rocks are here mapped separately as hornblende lamprophyres (Tl). One such dike thick enough to show on the map

cuts gabbro of the Careen Hill Intrusive Suite south of

Devers Bay. This dike has chilled margins, contains

clinopyroxene phenocrysts as well as hornblende up to 3 by

1.2 cm, plagioclase up to 3 mm long, and zones of epidote

alteration. Magnetite constitutes about 15 percent of the

SAMPLE ROCK AND UNIT LOCATION MINERAL AGE (Ma) REFERENCE

NUMBER

V-G-1 Granodiorite, Virgin Gorda batholith Southern Virgin Gorda Hornblende 34.2±1.6 Cox and others (1977)

V-G-1 Granodiorite, Virgin Gorda batholith Southern Virgin Gorda Biotite 36.4±0.9 Cox and others (1977)

VI-8 Granodiorite, Copper Mine Point pluton, Southern Virgin Gorda Biotite 32.7±0.4 Kesler and Sutter (1979)

Virgin Gorda batholith

VI-7 Quartz diorite, The Narrows pluton Mary Point, St. John Hornblende 24.3±0.3 Kesler and Sutter (1979)

VI 29 Diorite, The Narrows pluton Fort Recovery, Tortola Whole rock 42.80±2.14 Vila and others (1986)

TTL 459 Virgin Gorda batholith Beef Island Whole rock 38.85±1.94 Vila and others (1986)

VIJ 453 Granite, Virgin Gorda batholith The Baths, Virgin Gorda Whole rock 35.37±1.77 Vila and others (1986)

VIJ 451 Gabbro, Virgin Gorda batholith Near Virgin Peak, Whole rock 60.36±3.02 Vila and others (1986)

Virgin Gorda

VI-85-193 Tonalite (Tt), The Narrows pluton Please horizontal control Hornblende, 38.81±0.14 This report1

marker, Eastern St. John plateau age

quadrangle

VI-85-218 Quartz gabbro (Tt), The Narrows pluton Northwestern shore, Mary Hornblende, 39.29±0.22 This report2

Point, Western St. John plateau age

quadrangle

VI-85-222 Tonalite (Tt), The Narrows pluton Northeast corner, Mary Hornblende, 38.67±0.22 This report2

Point, Western St. John plateau age

quadrangle

1Michael J. Kunk, U.S. Geological Survey, written commun., March 1997.

2Michael J. Kunk, U.S. Geological Survey, written commun., August 1998.

Table 2. Conventional K-Ar ages and three 40Ar/39Ar ages from the Virgin Gorda batholith and The Narrows pluton.
groundmass. The hornblende lamprophyre of Devers Bay is

cut by a diabase dike of unknown age, but indistinguishable

from those of unit Td.

EXPLOSION BRECCIA

A mafic dike (Te), 15 to 20 cm thick, that contains 60

percent or more xenoliths of keratophyre, greenstone, gabbro, and tonalite intrudes hyaloclastite of the Water Island

Formation on the east side of the middle of the peninsula

of Dittlif Point (fig. 22). No other occurrence of this unit

is known and it is shown only on the inset map of dike distribution and attitude on plate 1. The dike can be traced for

about 20 m across the outcrop and cuts a diabase dike that

is interpreted to be unit Td, but could be gabbro (Kcm) of

the Careen Hill Intrusive Suite.

RED HOOK TONALITE PORPHYRY

(HERE NAMED)

The Red Hook Tonalite Porphyry (Tr) consists of dikes

and small hypabyssal intrusions of yellowish-gray (5Y 7/2),

pinkish-gray (5YR 8/1), and grayish-yellow-green (5GY

6/2) porphyritic tonalite characterized by large, bipyramidal

quartz and barrel-shaped biotite phenocrysts. The groundmass is aphanitic to fine grained and locally spherulitic.

Apatite crystals typically are prominent and may be as large

as 1 by 0.3 mm. Phenocrysts, in places as abundant as 45

percent, consist of quartz, plagioclase, biotite, and less commonly hornblende. Quartz and plagioclase are commonly in

growth aggregates, either singly or together. Plagioclase

phenocrysts are complexly zoned with many reversals.

Bipyramidal and embayed quartz phenocrysts, commonly 1

cm to as much as 3 cm across, and euhedral, barrel-shaped

biotite phenocrysts are distinguishing features of these

rocks. Vermiculite is typically a pseudomorph of the biotite.

Some coarsely porphyritic keratophyre (Kcc) of the Careen

Hill Intrusive Suite resembles the Red Hook (see previous

discussion about unit Kcc). The unit takes its name from Red

Hook, Eastern St. Thomas quadrangle, where several

tonalite porphyry dikes intrude hydrothermally altered keratophyres of the Water Island Formation. At Red Hook, the

tonalite porphyry dikes are not affected by the alteration.

The Red Hook Tonalite Porphyry intrudes stratified rocks as

young as the Mandal Member of the Tutu (on Grass Cay,

Eastern St. Thomas quadrangle). On the Capella Islands,

Eastern St. Thomas quadrangle, the Red Hook intrudes

tonalite that may or may not be biotite-hornblende tonalite

(Tt). On St. John, three small bodies of the Red Hook are

interpreted to be intruded along the Great St. James fault. A

small body of Red Hook crops out just west of the catchment basin above Caneel Bay Plantation, but the large dike

of quartz andesite porphyry (Red Hook of this report)

mapped by Donnelly (1966) at Caneel Bay could not be

located. The Red Hook is assigned a Tertiary age because it

intrudes the Late Cretaceous Tutu Formation and may be the

youngest bedrock unit on St. John.

STRUCTURE

The rock units on St. John and adjacent cays form a

north- or northeast-dipping homoclinal sequence plicated

by folds of various degrees of intensity whose axes generally trend roughly parallel to the strike of local segments of

the homocline. The structure is complicated by younger

long-wavelength, low-amplitude cross folds and faults. The

assembly of the structural elements of St. John into a coherent package, indeed even identifying what those elements

are, draws heavily on evidence developed on St. Thomas

and, to a certain extent, on the British Virgin Islands

(Helsley, 1960, 1971). Some of the features mentioned in

this discussion are shown on Donnelly’s (1966) geologic

map or on the generalized geologic maps of St. Thomas

(Rankin, 1984) and the northern U.S. Virgin Islands

(Rankin, 1999).

FOLDS

Numerous observations of topping directions confirm

that the units young to the north or northeast. Deformation

increases in intensity northward across St. John. Moderate

dips are typical in the southern part of the island; steep dips

are common in the northern part. A few outcrop- or mapscale open folds with generally east-west axes, mostly in the

22 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

Figure 22. Vertical dike 15 to 20 cm thick trending 275° of explosion breccia (Te) intruding hyaloclastite of the Water Island

Formation, east side of the middle of the peninsula of Dittlif Point.

Dime for scale.
Louisenhoj Formation, were observed or deduced from

reversals of bedding attitudes. Most fold axes plunge gently

east or west; a few steep plunges were observed. No consistent direction of tectonic transport was determined. The

folds are cut by an axial-plane cleavage that typically dips

more steeply (most commonly to the north) than the bedding and increases in intensity to the north. Horizontal beds

cut by vertical cleavage crop out in the western part of the

head of Hawksnest Bay. Graded beds and channels indicate

that the beds are upright. Bedding attitudes nearby indicate

that the bedding-cleavage intersection in the beachside outcrop is parallel to the axial trace of a syncline that strikes

280°. At places along the north shore from Caneel Bay to

Maho Bay, and as far south as just northwest of elevation

marked 662 ft on Center Line Road south of Susannaberg,

clasts (pumice lumps at the Susannaberg locality) in the

Louisenhoj conglomerates are flattened in the plane of

cleavage. In a few of those outcrops, the clasts are elongated downdip. In a seacliff outcrop on the east side of

Hawksnest Point, boulder conglomerate of the Louisenhoj

dips north 60° and is cut by vertical cleavage. Boulders, several as large as 1.5 m across, are aligned in the cleavage.

Penetrative cleavage passes through boulders as large as 2 m

in the Louisenhoj on the west side of Cinnamon Bay.

Boulders flattened in and elongated down the dip of the vertical cleavage may also be seen in steeply north dipping

conglomerate beds in coastal outcrops on America Point.

Pillows and pillow fragments are flattened and elongated

downdip in coplanar, steeply southwest dipping cleavage

and layering in metabasalt of the Water Island Formation

near the Please horizontal control marker.

Mesoscopic folds, interpreted to be of the same generation as the open folds described above, become tighter to

the north. Minor folds in the Outer Brass Limestone and

Tutu Formation are tight to isoclinal with axial-plane cleavage that generally dips steeply north and is either parallel to

or dips more steeply than bedding, except in the noses of

folds. The few observations made suggest a shear sense in

which the north side moved up. The geometry of boudins of

sandy beds within limy beds of the Mandal Member on the

south side of Whistling Cay and of the Outer Brass

Limestone on Outer Brass Island also indicate a shear sense

of north side up.

The same progressive tightening of folds and the development of axial-plane cleavage is observed northeastward

across St. Thomas from Charlotte Amalie to Mandal and

Coki Points. The intensity of deformation on St. Thomas

increases markedly near the base of the Outer Brass

Limestone. At Mandal on Route 42 at the western edge of

the Eastern St. Thomas quadrangle, a mylonite forms the

base of the Outer Brass. The more intense deformation may

extend half a kilometer or so southwest into the Louisenhoj

Formation, but outcrops are sparse.

The rocks on St. John and adjacent cays are interpreted

to be on the south limb of an east-west syncline as first proposed by Cleve (1882). The axis of the syncline passes

between St. Thomas and Hans Lollik Island, probably

through The Narrows, and may continue through the Sir

Francis Drake Channel. I map the rocks (mostly volcaniclastic conglomerate) on Hans Lollik Island, Little Hans

Lollik Island, and Pelican Cay (Central St. Thomas quadrangle) as the Louisenhoj Formation. Donnelly (1966) interpreted these strata to be a continuation of the north-dipping

homoclinal sequence and defined them as his youngest

stratigraphic unit, the Hans Lollik Formation (table 1).

Helsley (1960) included the Hans Lollik as a member in the

Tortola Formation and reported middle Eocene fossils in it

at an unspecified locality on Jost Van Dyke Island, some 7

km north of Mary Point. The Hans Lollik Formation is not

used here as a stratigraphic unit. The conglomerates on

Hans Lollik Island are broken up by the intrusion of abundant diabase dikes, and, in most places, bedding is obscure.

At the southern end of Hans Lollik Island, however, beds

dip 55° to 75° S. and in one outcrop, graded and channeled

beds top to the south. Elsewhere, the north limb of the syncline is in the British Virgin Islands. Helsley (1960, 1971)

showed that bedding in the southern part of Tortola generally dips steeply to moderately (as gently as 40°) south. He

interpreted the beds to be overturned but gave no evidence

for this. The only mention of penetrative cleavage in the

British Virgin Islands by Helsley (1960) is a statement that

schistosity is well developed in metamorphic rocks and is

always parallel to bedding. No fold axes are shown on his

map. Bedding on Jost Van Dyke Island and in northern

Tortola generally dips steeply to moderately north. The

internal stratigraphy of the Tortola Formation is poorly constrained because of the lack of throughgoing marker beds or

reported topping direction indicators. The bedding attitudes

on Tortola indicate a major east-west-trending anticline.

The major syncline south of Hans Lollik Island and the

major anticline through Tortola are here interpreted to be of

the same generation as the minor folds with generally eastwest axes on St. John.

The intensity of deformation decreases north of St.

Thomas. Conglomerates interpreted to be in the Louisenhoj

on Hans Lollik and Little Hans Lollik Islands are deformed

in open folds with roughly east-west-trending axes. Dips on

Guana Island north of Tortola are mostly moderate to the

north, also suggesting a decrease in the intensity of deformation with either time or place.

The east-west fold axes and associated axial-plane

cleavage may represent the initiation of the collision

between the Caribbean plate and the North American plate

in the time interval between the end of the deposition of the

Mandal Member (Santonian, about 83 Ma) and the intrusion

of The Narrows pluton (about 39 Ma). Further studies of the

mafic dikes in the northern U.S. Virgin Islands as well as

structures in the British Virgin Islands may help to constrain

the time of the initial collision. At least some of the dikes

that intrude the Mandal Member were intruded before

STRUCTURE 23
north-south compression. Other dikes, thought to be related

to these, cut clasts in conglomerates of the Louisenhoj and

Tutu Formations; those dikes did not intrude unconsolidated sediments. Finally, in Puerto Rico, where penetrative

cleavage has not been reported, similar dikes cut Paleocene

strata and are overlain by nonconformably Oligocene

coastal plain strata. The Puerto Rico Trench is the present

boundary between the Caribbean and North American

plates in the vicinity of the Virgin Islands. Rocks of the

North American plate on the north wall of the Puerto Rico

Trench are basalts and serpentinite geochemically similar to

oceanic tholeiites (MORB) and ultramafic rocks (Perfit and

others, 1980). In the interval between 84 and 39 Ma, however, the Caribbean plate was relatively farther west, and

rocks of the Virgin Islands may have been in collision with

the crust of the Bahama platform. The available structural

information indicates a south vergence; most tight folds and

the asymmetry of boudins indicate a north-side-up shear

sense and most cleavage dips north. These observations are

most compatible with a south-facing arc, following the definition of Pindell and Barrett (1990), whereby a south-facing arc overlies a north-dipping subduction zone. On the

other hand, by the Late Cretaceous, the Greater Antilles arc

in Hispaniola was north facing (Lewis and others, 1990;

Pindell and Barrett, 1990). There, Late Cretaceous to

Eocene subduction complexes were thrust over the shelf

carbonates of the Bahama bank in the early Paleogene.

Given the regional setting, the best guess as to the time of

development of penetrative cleavage in the northern Virgin

Islands, also interpreted to be the time of collision with the

North American plate, is Eocene and prior to intrusion of

The Narrows pluton at 39 Ma.

The structures related to the north-south compression

possibly are of more than one generation. Although most

cleavage on St. John dips north or is vertical, about 25 percent of the recorded cleavages dip to the south. Float blocks

of marble and calc-silicate rocks along the shore of Leinster

Bay, from below the Annaberg ruins to Annaberg Point, display tight to isoclinal folds with a poorly developed axialplane cleavage. In some float blocks, the axial surfaces of the

isoclines are folded. No folded axial surfaces have been

observed in outcrop. Finally, on the east side of Mandal

Point, Eastern St. Thomas quadrangle, graded bedding and

refracted cleavage show that beds of the Mandal Member of

the Tutu Formation that dip 65° S. are overturned (Rankin,

1984, fig. 6). Cleavage in the slaty rocks dips 80° S. Either

these outcrops are on limbs of downward-facing folds or the

overturning of the beds is not related to the cleavage. The

overturning may be related to large-scale slumping, but no

other evidence for slumping on that scale was found; the next

outcrops to the northeast across strike are several kilometers

away across open water and in the British Virgin Islands.

Structures resulting from the north-south compression

are warped by long-wavelength, low-amplitude folds whose

axes plunge gently north. These folds are most easily seen

by the attitudes of bedding in central and western St.

Thomas as shown by Donnelly (1966) and Rankin (1984)

and by the broad arc of the Tutu Formation from Picara

Point on St. Thomas to Mary Point on St. John. The wavelength of these folds ranges from a few to many kilometers.

Bedding attitudes in the Water Island and Louisenhoj

Formations show that a broad, north-plunging syncline-anticline pair disrupted by later faulting occupies most of St.

John. That conclusion reflects my observations that stratigraphic layering (bedding, pillow layering, and flow layering) from Great and Little Lameshur Bays across Bordeaux

Mountain to the East End dips northeastward, roughly at

right angles to the layering shown by Donnelly (1966) (see

discussion of the Water Island Formation in this report). The

curvature, concave to the north, of the Sir Francis Drake

Passage northeast of St. John may reflect a broad, northplunging syncline east of St. John. Cleavage related to the

north-south compression also is warped across the northsouth axes, although in southern St. John, where cleavage is

marginally discernible, relations are poorly understood. The

age of the north-south folding relative to the age of The

Narrows pluton is not known. No north-plunging fold axes

have been observed in outcrop nor is there a north-southstriking cleavage. There are, however, numerous north- to

north-northeast-striking, nearly vertical, brittle faults of

unknown but probably minor displacement. Numerous

anastomosing limonite-calcite veins are commonly associated with and subparallel to the faults. In places, northtrending limonite zones are present where there is no apparent fault. Most of these minor brittle faults were not recorded in the field, but may be readily observed on the chain of

islands north of Pillsbury Sound.

FAULTS

The two generations of folds are cut by northeast-striking, nearly vertical, brittle faults. Some, such as the ones

shown in the southeastern part of St. John, are defined by the

distribution of rock units and the fault pattern established farther west. The most convincing evidence for the northeaststriking fault set comes from the offset of the Water IslandLouisenhoj stratigraphic contact as mapped along the highly

irregular southwestern coast of St. John. One of the fault

zones is exposed in a long, quarry-like exposure of weathered Louisenhoj Formation on the northwest side of Guinea

Gut, from which the fault takes its name, about 200 m

upstream from Highway 104 (fig. 23). The exposure is parallel to the fault zone, which is at least 2 m thick. The fault

zone consists of punky-weathering limonite-stained breccia

containing open cavities parallel to the fault zone that are

partly filled with calcite. Northeast-trending limonite zones

up to 10 cm thick in the country rock parallel the fault zone

on the southeast side of the gut. The sense of movement

24 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS
could not be determined. As noted earlier, however, the

Water Island-Louisenhoj contact has 200 m of sinistral offset across Great Cruz Bay. Southeast of Great Cruz Bay, the

Water Island-Louisenhoj contact is progressively offset to

the northeast another 1.5 km, and a series of faults is shown

on the geologic map to account for this.

Geologic mapping of central St. John for this report produced a significantly different distribution of units from that

shown by Donnelly (1966). The Water Island-Louisenhoj

stratigraphic contact reappears south of L’Esperance where

it dips northeast parallel to the dip of the contact along the

southwestern coast, and it reappears again near Ajax Peak

where it dips northwest. Between the southwestern coast

and Ajax Peak, three additional contacts between the Water

Island and Louisenhoj are interpreted to be high-angle faults

with very little deflection of strike across steep topography.

The faults southwest of Battery Gut and northeast of

Camelberg Peak are upthrown on the northeast. The third

fault, southwest of Ajax Peak, is upthrown on the southwest.

An attempt to integrate these elements must also accommodate observations from southeastern St. Thomas and adjacent cays where the Water Island-Louisenhoj stratigraphic

contact also is offset by a series of northeast-striking, nearly vertical faults. These faults may be dip-slip faults creating a series of graben and horsts as portrayed by Donnelly

(1966), or most of them could be strike-slip faults extending

the pattern deduced for southwestern St. John. In the

Eastern St. Thomas quadrangle, the southeasternmost exposure of the stratigraphic contact is on Great St. James Island

where it dips northeast and is cut off on the southeast by a

northeast-trending fault. My interpretation is that, between

Great St. James Island and Great Cruz Bay, the Water

Island-Louisenhoj contact is offset 4 km along a sinistral

fault, here called the Great St. James fault after Great St.

James Cut (the open water between Great and Little St.

James Islands) on the southeast side of Great St. James

Island. The Great St. James fault projects into Turner Bay

and Enighed on St. John. No fault has been recognized

there, yet the Great St. James fault must be a major fault.

Farther northeast, this fault projection merges with the fault

that I had earlier mapped as a continuation of the Guinea

Gut fault. The axis of an open S-style fold that plunges 35°

NE in volcanic wacke of the Water Island Formation south

of Turner Bay suggests that there was also a dip-slip component to the movement along the Great St. James fault.

Movement along the Great St. James fault appears to

decrease toward the northeast. The Outer Brass Limestone

is offset by only about 1 km, and Helsley (1960, 1971)

shows no fault on Tortola along the projection of the Great

St. James. This complex region of faulting in central St.

John is essentially bounded on the southeast by the Brown

Bay fault, which is named for Brown Bay on the north coast

of St. John. The Brown Bay fault is interpreted to be a significant sinistral strike-slip fault that terminates the outcrop

belt of the Louisenhoj on St. John. The Brown Bay fault

projects northeastward to a northeast-trending stretch of the

coast of Tortola that marks the eastern end of the outcrop

belt of the Tutu Formation as mapped by Helsley (1960).

Movement along the Brown Bay fault appears to decrease

southwestward to less than 100 m at Fish Bay on the south

coast of St. John. I suggest that this zone, from Great St.

James Island through central St. John, represents regional

sinistral movement across a block that is bounded by rightstepping, en-echelon, sinistral strike-slip faults, as illustrated by Aydin and Nur (1982, fig. 1) and Ramsay (1987, fig.

23.39). The sinistral movement along each bounding fault

decreases along the zone of overlap to no movement beyond

the zone of overlap. Rocks within the zone of overlap are

uplifted under compression. The northwest-trending faults

near Battery Gut and Camelberg Peak, within the zone of

overlap, are interpreted to be high angle reverse faults. As

shown on cross section A-A ́, the Water Island-Louisenhoj

contact is brought up on the order of 2,700 m along the fault

near Battery Gut. A brittle fault within the Water Island

Formation is exposed along Center Line Road southwest of

Ajax Peak. The fault dips 80° NE., and kinematic indicators

suggest downward movement of its northeast side. This

fault is interpreted to be the northeastern bounding fault of

the zone of overlap between the Great St. James and Brown

Bay faults. This fault is shown on the geologic map and on

cross section A-A ́ as a high-angle normal fault. An alternative interpretation is that the overall dip of the fault is

steeply to the southwest and that it moves the block of overSTRUCTURE 25

Figure 23. Exposed surface of the northeast-striking, near-vertical, left-lateral, strike-slip Guinea Gut fault in the Louisenhoj

Formation, looking southwest on northwest side of lower end of

Guinea Gut, where dip symbol is plotted on map (plate 1).
lap up and to the northeast. The change in attitude of the

Water Island-Louisenhoj contact between L’Esperance and

Ajax Peak is attributed to a broad, north-plunging syncline

that formed during the episode of mild east-west compression (discussed earlier) that occurred prior to the strike-slip

faulting.

The sinistral strike-slip faulting is the youngest major

structural episode recognized on St. John. Two minor northwest-trending faults near Ajax Peak offset the Water IslandLouisenhoj stratigraphic contact. The westernmost fault

probably also offsets the Great St. James fault and appears

to offset the intrusive contact of The Narrows pluton on

Mary Point. These youngest northwest-trending faults are

interpreted to be dextral strike-slip faults and probably represent some minor adjustment within the Puerto RicoVirgin Islands microplate to stress at the northeastern corner

of the Caribbean plate.

The age of the late brittle faulting, including the major

sinistral strike-slip faulting, is interpreted to be younger

than the intrusion of The Narrows pluton. The Red Hook

Tonalite Porphyry appears to be intruded along the strikeslip faults. If these interpretations are correct, then the sinistral faulting is younger than The Narrows pluton and older

than the Red Hook.

Sinistral strike-slip faulting is widely documented in

Puerto Rico, particularly along the North and South fault

zones that have been interpreted to divide the island into

three structural blocks (Glover, 1971; Mattson and others,

1990). Faults in both zones cut Eocene strata; it is not

known how much earlier than Eocene the sinistral faulting

may have begun. Both fault zones are covered unconformably by middle Oligocene and younger beds at their

northern extremities, but near its southern margin, the South

fault zone displaces Oligocene and Miocene rocks (Mattson

and others, 1990). Thus, a major episode of sinistral strikeslip faulting took place in Puerto Rico in late Eocene to

early Oligocene time, which is compatible with a post-39

Ma (age of The Narrows pluton) timing for the sinistral

faulting in St. John. Sinistral strike-slip motion of the northern border of the Caribbean plate relative to the North

American plate probably began with the initiation of the

Cayman Trough spreading center. Pindell and Barrett

(1990) conclude that there has been 1,050 to 1,100 km of

offset along the Cayman Trough since the Eocene. Other

estimates for the time of initiation of the spreading center

and the amount of offset include 1,400 km since 38 Ma

(Sykes and others, 1982), 1,100 km since 30±5 Ma (Wadge

and Burke, 1983), and a time of initiation of 45 to 50 Ma

(Rosencrantz and others, 1988). I suggest that the sinistral

strike-slip faulting on Puerto Rico, St. Thomas, and St. John

represents strike-slip accommodation near the northern border of the Caribbean plate in the early stages of the Cayman

Trough spreading and the transition from an east-westtrending arc to an east-facing arc. Perhaps the gentle folding

on north-south axes on St. Thomas and St. John was caused

by compression at the earliest stage of Cayman Trough

spreading prior to the strike-slip faulting.

Finally, the strike-slip faults in St. John strike northeast

to east-northeast, whereas those in Puerto Rico strike eastwest to west-northwest. The difference between the average

orientation of these Tertiary sinistral faults is about 45°. If

the two sets of faults represent conjugate shears, a possible

cause of the difference in orientation of these two shear sets

is the difference in location of the two islands relative to the

curvature of the arc and the vector of plate motion at the

time of shearing.

GEOLOGIC HISTORY

The geologic history of St. John began with the eruption

of submarine keratophyres and amygdaloidal pillow basalts

of the Water Island Formation (at least 2 km thick) in the late

Early Cretaceous, probably onto oceanic plateau crust. At

that time, the Caribbean plate was in the western Pacific. The

submarine volcanism was accompanied by the deposition of

radiolarian cherts and by the intrusion of small hypabyssal

plutons and abundant dikes of the Careen Hill Intrusive

Suite, again mostly keratophyres but including mafic rocks.

The presence of sheeted dikes indicates that the igneous

activity was in an extensional environment. Presumably, the

high Na2O/K2O ratio of the high-silica rocks (keratophyres)

is secondary; the abundance of the keratophyres in an oceanic environment is problematic. Perhaps the rocks represent a

back-arc basin environment, but the high silica content is

still a problem. Local deuteric hydrothermal alteration is

probably responsible for the metal anomalies reported by

Alminas and others (1994) in rocks and outcrop areas of the

Lameshur Volcanic-Intrusive Complex.

The igneous compositions changed through a relatively

brief interval to more island arc-like clinopyroxene andesite

and basalt that are preserved mostly as clasts in the volcaniclastic Louisenhoj Formation (as thick as 1.5 km) which

conformably overlies the Water Island. Primary volcanic

units are sparingly present and on Ramgoat Cay are pillowed. The variation of clast lithology with location suggests that there were local sources, such as lavas from different vents, for the clasts. No specific sources for the clasts

have been identified, but the sources cannot be far away

because of the prevalence of boulder-size clasts throughout

the formation. The coarser units probably originated as

debris flows on relatively steep slopes. Final emplacement

was in a submarine environment as indicated by the com26 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS
mon grading of even the coarsest material.4 Radiolarian

chert is present, but rare, on St. Thomas. Local submarine

pyroclastic flows are indicated by the local abundance of

pumiceous clasts. Calcite cement becomes increasingly

common in the upper part of the formation and rare fossiliferous limestone beds are present.

A brief interval of volcanic quiescence followed in the

late Turonian to late Santonian with the deposition of the

Outer Brass Limestone, which includes clean calcite limestone (now marble), calc-silicate rocks, and conglomerate

with calcareous cement and clasts of marble and (or)

andesite. No structural discordance is seen either at the base

or top of the Outer Brass.

Volcanism then resumed as indicated by the deposition

of volcaniclastic turbidites and rare pyroxene basalt to

andesite flows in the overlying Tutu Formation in Late

Cretaceous time. The volcanic source for the Tutu was farther away than for the Louisenhoj because boulder conglomerates in the Tutu are sparse and silt- to sand-size material dominates. Sediments become finer upward from the

basal Picara Member to the overlying Mandal Member, suggesting increasingly distal sources. Carbonate cement is

typical and limestone beds (now marble and calc-silicate

rock) and conglomerate are prominent. Soft sediment deformation throughout the Tutu, including some striking slump

folds in the Mandal Member, suggests deposition on an

unstable slope, perhaps a trench wall leading to an accretionary wedge.

Volcanism that occurred during the deposition of the

Louisenhoj and Tutu is interpreted to be island-arc volcanism that was part of the Greater Antilles arc volcanism. No

direct evidence for the polarity of this arc is known on St.

John. The Tutu Formation is the youngest stratified unit on

St. John. Volcanism continued in the British Virgin Islands

into the middle Eocene. Volcanism on St. John was followed by a variety of intrusive activities that began with the

intrusion of abundant diabase dikes. The dikes probably are

part of the same suite that continues east to Puerto Rico

where they are dated stratigraphically as post-Paleocene

and pre-Oligocene. The age of the dike swarm on St. John

(nearly vertical dikes that trend mostly northeast and northwest) relative to the middle Eocene volcanism on Tortola is

not known.

North-south compression followed the major pulse of

dike intrusion and produced broad folds that originally had

east-west axes. The folds are interpreted to be related to the

initiation of the collision between the Greater Antilles arc of

the Caribbean plate and the Bahama platform of the North

American plate. A synclinal axis north of St. Thomas and

St. John accounts for the roughly north-dipping homoclinal

sequence from the Water Island to Tutu Formations on those

islands. The intensity of deformation related to this northsouth compression increases northward across St. John.

Tight folds have a penetrative axial-plane cleavage that

mostly dips steeply north or is vertical. The asymmetry of

both the tight folds and boudins mostly indicates a northside-up sense of shear.

Intrusion of the dominantly tonalitic The Narrows pluton and Virgin Gorda batholith followed the episode of

north-south compression, produced a contact metamorphic

aureole a couple of kilometers wide, partly annealed the

axial-plane cleavage within the aureole, and probably produced the metal anomalies in the Louisenhoj and younger

formations reported by Alminas and others (1994). The

Narrows pluton and the Virgin Gorda batholith are part of

the Greater Antilles arc magmatism. The age of intrusion of

The Narrows plutons is about 39 Ma (late Eocene) based on

an 40Ar/39Ar plateau age on hornblende.

East-west compression produced long, broad, open

folds of bedding and the east-west fold axes on a new set of

axes that plunge gently north. Widely spaced, nearly vertical, north- to northeast-striking, brittle fractures associated

with anastomosing calcite-limonite veins may be related to

these late folds. The age relationship of these late folds to

the tonalitic plutons is not known. The broad north-plunging

late folds may be related to the relative eastward transport of

the Caribbean plate in response to the beginning of spreading at the Cayman Trough. If so, the north-south folds

should be younger than The Narrows pluton.

The last major tectonic event recorded on St. John is a

series of post-39 Ma sinistral strike-slip faults interpreted

to be related to the initiation of spreading at the Cayman

Trough center and the sinistral strike-slip accommodation

near the northern border of the Caribbean plate. A structural block that extends from Great St. James Island, Eastern

St. Thomas quadrangle, northeastward across central St.

John is bounded by northeast-trending, right-stepping, en

GEOLOGIC HISTORY 27

4On a field visit in March 2001, Richard S. Fiske (Smithsonian

Institution) noted the similarity of the Louisenhoj Formation graded

sequences to those of the Pliocene Shirahama Group of the Izu Peninsula,

Japan (Fiske, 1986). There, larger lithic clasts in proximal deposits, like

those in the Louisenhoj, are better rounded than associated finer ones and

finer ash matrix is sparse. Fiske suggested that major parts of both units

might have been generated by shallow submarine explosive eruptions such

as were observed during the 1952–1953 eruption of Myojinsho, 420 km

south of Tokyo, Japan. Myojinsho exploded 1000 times during that eruption (Fiske and others, 1998). The products of those explosions are below

present-day sea level and have never been observed, but are likely to consist of thick sequences of graded beds on the volcano’s submarine flanks.

The many breccias and conglomerates of the Louisenhoj Formation are

interpreted to have been produced by shallow submarine explosive eruptions from numerous centers. The volcanic clasts in the Louisenhoj therefore are interpreted to be primary (or only slightly) reworked volcanic

products.
echelon, sinistral strike-slip faults. The zone of overlap

between the bounding faults is uplifted on at least two

northwest-trending, high-angle reverse faults that dip

northeast. The structure is thus the inverse of a pull-apart

basin (that is, a rhomb horst, a term introduced by Aydin

and Nur, 1982). The stratigraphic contact between the

Water Island and Louisenhoj Formations is sinistrally offset 5.7 km across this block. The Water Island-Louisenhoj

contact is brought up about 2.7 km on one of the high-angle

reverse faults within the block in the zone of overlap. The

amount of uplift on the second high-angle fault cannot be

determined.

Unlike other parts of the Greater Antilles, evidence for

recent tectonic movement has not been observed. I have

seen no uplifted beaches, no uplifted wave-cut platforms,

and no fault scarps. Hypocenters of shallow earthquakes

(depth <50 km) are mostly associated with the Puerto Rico

Trench and the north wall of the Virgin Islands Basin

(Frankel and others, 1980). Sea level appears to have been

roughly at the same level since the Pleistocene. Most bays

are blocked by bay-mouth bars consisting of modern reef

debris that entraps a salt pond or playa landward. Much of

the cliffy shoreline of Congo Cay is undercut by a wave-cut

notch at sea level (fig. 20).

Finally, two geomorphic features are worthy of mention. First, on the north side of Congo Cay toward its eastern end, a relict sinkhole is exposed at the top of a sea cliff.

The sinkhole formed after the beds on Congo Cay had been

deformed into their nearly vertical orientation and erosion

had reduced the region to roughly the present topography.

The lower part of the sinkhole is filled with disoriented

blocks of marble and hornfels in reddish-orange caliche

showing horizontal stratification. The rest of the country

rock (marble) in which the sinkhole formed has been eroded away. Second, isolated pillars and blocks of gossan are

present on the wooded summit of Bordeaux Mountain. The

gossan presumably formed from weathering of hydrothermally altered Water Island Formation more recently than the

time of the deuteric alteration.

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30 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS
APPENDIX A
DESCRIPTION OF MAP UNITS

[Dikes 3 m or more in width are shown where space permits.

Alphanumeric symbols following color names are taken

from the rock color chart of Goddard and others (1970).

Minerals are not necessarily listed in order of abundance.]

Surficial deposits (Quaternary)—Alluvium,

swamp deposits, beach deposits, beachrock (sedimentary rock formed in tidal zone), and artificial fill. Only alluvium is likely to contain material as old as Pleistocene

Red Hook Tonalite Porphyry (new name)

(Tertiary)—Dikes and small hypabyssal intrusions of yellowish-gray (5Y 7/2), pinkish-gray

(5YR 8/1), and grayish-yellow-green (5GY 6/2)

porphyritic tonalite characterized by large

bipyramidal quartz and barrel-shaped biotite phenocrysts. Groundmass aphanitic to fine grained

and locally spherulitic. Phenocrysts, as abundant

as 45 percent, of quartz, plagioclase, biotite, less

commonly hornblende, and prominent apatite.

Quartz and plagioclase commonly in growth

aggregates, either singly or together. Bipyramidal

and embayed quartz phenocrysts commonly 1 cm

to as much as 3 cm across. Plagioclase phenocrysts complexly zoned with many reversals.

Biotite typically completely replaced by vermiculite. Named for prominent dikes at Red Hook,

Eastern St. Thomas quadrangle, where dikes are

intruded along late brittle faults

Explosion breccia (Tertiary?)—Mafic dike 15 to

20 cm thick contains 60 percent or more xenoliths of keratophyre, greenstone, gabbro, and

tonalite. Observed only on east side of Dittlif

Point where it can be traced for about 20 m

across outcrop of Water Island Formation. Cuts

diabase dike that is interpreted to be unit Td but

could be unit Kcm. Shown only on inset map

showing dike distribution and attitude on plate 1

Hornblende lamprophyre (Tertiary)—Dark,

magnetite-bearing, aphanitic dikes with prominent hornblende phenocrysts as large as 3 cm by

1 cm and, in places, clinopyroxene phenocrysts

as large as 1 cm. Typically smaller plagioclase

phenocrysts are present. Most Tl dikes are

younger than dikes of unit Td, but some Td mafic

dikes cut Tl

Biotite-hornblende tonalite (Tertiary, late

Eocene)—Very light gray to medium-light-gray,

medium-grained, nonfoliated biotite-hornblende

tonalite and minor gabbro, diorite, granite, and

pegmatite. Tonalite composed of interstitial

quartz (10 to 15 percent); subhedral, strongly

and complexly zoned plagioclase; biotite partly

altered to chlorite; hornblende; and magnetite.

Gabbro composed of plagioclase, hornblende,

clinopyroxene, apatite, and pyrite and (or) magnetite. Granite is very light gray, fine to medium

grained and contains quartz, orthoclase (about

35 percent), strongly zoned plagioclase, biotite

altering to chlorite, zircon, and magnetite. White

pegmatite contains quartz, slightly perthitic

orthoclase (poikilitic crystals up to 1.5 cm

across), reddish isotropic garnet, and minor

diopside, sphene, allanite, and calcite. Shows

copper stain. Contains many xenoliths of stratified country rock (gray, fine-grained granofels,

siliceous hornfels, and calc-silicate) and gabbro

(unit Td?); also contains pockets of skarn with

massive sulfide and copper stain. Sharp contacts;

post-kinematic to tight folding and cleavage in

country rock. Produces contact-metamorphic

aureole as wide as 2 km

Diabase, gabbro, and diorite (Tertiary)—Nearly

vertical dikes ranging in thickness from a few

cm to several tens of meters and complex bodies

of multiple dikes, some with internal layering

parallel to dike walls. Chilled margins typical;

columnar jointing not typical. Color index, grain

size, and texture vary; mostly diabase or gabbro

with diabase margins. Amygdules present locally. Primary minerals include plagioclase,

clinopyroxene, commonly hornblende, magnetite, and (or) pyrite, and, in some rocks,

quartz. May be porphyritic with phenocrysts of

plagioclase, clinopyroxene, and (or) hornblende.

Thicker bodies commonly contain epidosite

nodules and disseminated epidote. Secondary,

pale, commonly fibrous amphibole, chlorite, and

epidote are common. In northern half of St.

John, progressively metamorphosed to amphibolite-facies assemblages. Also toward the north,

many dikes carry the same penetrative cleavage

as the enclosing stratified rocks; some broken

into segments aligned parallel to cleavage.

Other dikes are post-cleavage. Dikes intrude all

stratified units except surficial deposits and are

intruded by tonalite (Tt). Probably includes

dikes of more than one age. Some may be intrusive equivalents of extrusive rocks in enclosing

or overlying stratified units. Others may be part

of tonalite (Tt) intrusive cycle, and a very few cut

hornblende lamprophyre (Tl). Most interpreted

to be part of early Tertiary pre-tonalite (Tt) magmatic cycle, as suggested by Donnelly and others (1990)

Tutu Formation (Upper Cretaceous, upper

Turonian, upper Santonian, or younger)—

Volcanic wacke, shale, conglomerate, calcareous

siltstone, sparse limestone, and rare basalt and

andesite or their metamorphosed equivalents.

Many rocks are calcareous. All rocks are within

32 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

Qs

Tr

Te

Tl

Tt

Td
contact aureole of tonalite (Tt); hornfels, calc-silicate rock, and marble common. Clasts in conglomerate dominantly basalt and andesite; limestone clasts locally abundant; clasts of plutonic

rocks rare, but present. Graded beds, slump

folds, and disrupted slabs of metasandstone and

metasiltstone in metaconglomerate indicate deposition by turbidity currents on an unstable

slope, perhaps a trench wall. Tight tectonic folds

with axial-plane cleavage also present, as well as

boudins of competent thermally metamorphosed

sandstone and silt beds in marble.

Distinguishing between soft-sediment and tectonic folds difficult in places where cleavage is

not apparent. Basal contact not exposed; in

apparent conformity with underlying Outer

Brass Limestone (Ko)

Mandal Member (new name)—Medium- to thinbedded, metamorphosed sandstone, siltstone,

shale, and limestone. Siltstone typically calcareous and sandstone commonly with a calcareous

matrix. Sparse conglomerate, mostly limestone

conglomerate in beds as thick as 3 m. Marble

contains calcite, plagioclase, tremolite, brown

garnet, epidote, and opaque minerals.

Metamorphosed calcareous siltstone and sandstone contain brownish garnet (locally as large

as 3.5 cm), diopside, tremolite, plagioclase, calcite, opaque minerals, and minor quartz. On

Whistling Cay, bedded, garnet-rich calc-silicate

rock contains isotropic reddish garnet, brightgreen diopside, plagioclase, wollastonite, phlogopite, sphene, pleochroic amphibole, and

quartz. Dark-grayish-red (5R 3/2), very fine

grained metasandstone contains abundant reddish-brown biotite and cordierite clouded with

tiny inclusions. Named for exposures on the east

side of Mandal Point, Eastern St. Thomas quadrangle. About 750 m exposed on St. John

Congo Cay Limestone Lens—Tightly

folded, vertical beds of marble, calcsilicate rock, and siliceous hornfels

exposed on Congo Cay and Carval

Rock. Dominantly white to mediumlight-bluish-gray (5B 6/1) marble

that is 98 percent calcite and either

accessory plagioclase, white mica,

and opaque minerals, or accessories

plagioclase, tremolite, wollastonite,

sphene, and opaque minerals. Calcite

grain size typically 1 to 3 mm, but

may be as large as 1 cm.

Stratigraphic position of rocks on

Congo Cay within the Tutu

Formation (Ktm) not known. Here

correlated with similar rocks on

Whistling Cay and Mary Point that

are within the Mandal Member

Marble, calc-silicate rock, and marble

conglomerate

Picara Member (new name)—Thick- to mediumbedded, metamorphosed volcanic wacke, conglomerate, siltstone, limestone, and rare andesite

or basalt. Locally calcareous. Clasts in conglomerate of metamorphosed andesite, basalt, and

limestone. Pebble and cobble conglomerate

more common than boulder conglomerate.

Ribby outcrops of metasiltstone and metawacke

common; calcareous nodules locally present.

Named for exposures on the peninsula of Picara

Point, Central St. Thomas quadrangle. About

600 m thick on St. John

Marble conglomerate and calc-silicaterich beds north of Maho Point—

Calc-silicate assemblages include

plagioclase-biotite-hornblende and

plagioclase-hornblende-diopside-calcite. Two layers of marble and calcsilicate rock on Waterlemon Cay not

mapped separately. Assemblage

there includes brown garnet (some

anisotropic and as large as 2 cm),

diopside, wollastonite (blades as long

as 2 cm), sphene, calcite, monazite,

and sulfide

Medium-dark-gray (N4), porphyritic

meta-andesite south of Maho Point—

Contains relict phenocrysts of glomerophyric plagioclase and prominent

clinopyroxene as large as 3 by 6 mm.

Metamorphic amphibole replaces

clinopyroxene; fibrous amphibole

and chlorite abundant in groundmass

and impart foliation

Outer Brass Limestone (Lower Cretaceous,

upper Turonian to upper Santonian)—Thinbedded calc-silicate rock, beds of white to bluegray calcite marble (H2S odor when broken) as

thick as 3 m, and metamorphosed, matrix-supported cobble conglomerate with andesite clasts

in carbonate matrix. Heterogeneous calc-silicate

at Annaberg Point includes, in various assemblages, calcite, plagioclase, epidote, brown

isotropic garnet, diopside, vesuvianite, and wollastonite. About 100 m thick

Louisenhoj Formation (Cretaceous, Albian to

Turonian or Santonian)—Strongly cemented

volcanic conglomerate, breccia, volcanic wacke,

shale, chert, andesite, basalt, tuff, and rare limestone. Overlies Water Island Formation with

apparent conformity. Detrital units dominantly

volcaniclastic. Lavas and flow breccias present

but sparse. Conglomerate, breccia, and coarse

sandstone typically pale green, greenish gray, or

grayish blue green. Matrix may be calcareous.

APPENDIX A 33

Ktm

Ktmc

Ktml

Ktp

Ktpl

Ktpv

Ko

Kl
Xenocrysts of plagioclase and pyroxene are

notable constituents of some clastic rocks.

Dominated by thick, clast- or matrix-supported

conglomerate beds typically interlayered with

sequences a few to several meters thick of siltstone and sandstone. Graded beds as thick as 6

m; some crossbedding. Soft sediment deformation and rafts of broken, finer grained beds common. Clasts larger than 0.5 m abundant throughout the formation; largest subrounded clast 3 m

across. Breccia blocks may be as large as tens of

meters. Clasts of porphyritic clinopyroxene

andesite and basalt constitute bulk of formation.

Typically contain about 20 percent prominent

phenocrysts, commonly 5 to 10 mm across, of

clinopyroxene and stubby, zoned plagioclase,

either singly or together. In some rocks, pyroxene

phenocrysts overgrown by hornblende; a few

rocks have hornblende phenocrysts as well.

Small amygdules pervasive in some clasts. Other

clast lithologies include pumice (not common),

limestone, keratophyre, and phenocryst-poor

basalt like that in Water Island Formation.

Differences in clast populations suggest local

sources. Keratophyre and phenocryst-poor basalt

are locally abundant near basal contact.

Limestone clasts, some fossiliferous, on Rata and

Ramgoat Cays. Metamorphism increases to the

northeast. Biotite, pale amphibole, and epidote

appear in andesite; epidote, hornblende, and lesser amounts of biotite appear in basalt. From central St. John to the north and east, most rocks are

foliated. At least 1.5 km thick on west coast of St.

John; may be as thin as 0.5 km at Leinster Bay

Porphyritic, locally vesicular andesite

lava—Phenocrysts of stubby plagioclase and pyroxene. Pillowed on

Ramgoat Cay. Highly brecciated on

Camelberg Peak and could be aa

lava. May be more abundant than

shown on map because of difficulty

in the field distinguishing between

conglomerate/breccia and homogeneous lava

Porphyritic, locally amygdaloidal basalt

and pillow basalt—Abundant (as

much as 40 percent) stubby plagioclase phenocrysts, typically 3 to 4

mm but as much as 15 mm long; and

less abundant clinopyroxene. Near

Ajax Peak, recrystallized to greenschist-facies assemblages. Appears

to be restricted to base of formation 

Lameshur Volcanic-Intrusive Complex (new

name) (Lower Cretaceous, Aptian to lowermost Albian)—Includes Water Island

Formation and Careen Hill Intrusive Suite

Undivided—Dominantly keratophyre and volcaniclastic rock derived from it, but includes

basalt and basaltic andesite, an unknown volume

of trondhjemite and gabbro, and minor chert.

Several varieties of keratophyre distinguished by

size and mineralogy of phenocrysts and mode of

emplacement. Keratophyre occurs as lava flows,

breccia, layered tuff, dikes, and small

hypabyssal intrusive rocks. In all varieties,

quartz phenocrysts embayed, if present, and

phenocrysts of plagioclase (albite as determined

by Donnelly, 1963, 1966) and quartz form

growth aggregates, either singly or together.

Plagioclase phenocrysts not zoned or weakly

zoned. Groundmass medium grayish blue (5PB

4/2) to medium dark gray (N4). Keratophyres

become progressively lighter colored as content

of albite phenocrysts and (or) weathering

increases. Mafic rocks occur as pillow lava, pillow breccia, dikes, and rare small plutons. All

rocks may exhibit local intense, deuteric,

hydrothermal alteration, which may include silicification, oxidation, sulfidization, and development of boxwork textures and, locally, gossan.

Named for locality of Lameshur on the southern

coast of St. John. Mapped where size and mineralogy of phenocrysts vary over short distances

and distinction between extrusive and intrusive

rocks is not readily apparent

Careen Hill Intrusive Suite (new name) (Early

Cretaceous)—Hypabyssal trondhjemite (Kct),

coarsely porphyritic keratophyre (Kcc), intrusive

equivalents of all keratophyre units of Water

Island Formation, gabbro and sheeted dikes

intrusive into the Water Island Formation or isolated on islands. No sequence of intrusion recognized. Except for some bodies of Kct and Kcc,

columnar jointing characterizes keratophyre and

is used as one criteria for mapping these bodies

as intrusive rocks. Larger keratophyre intrusive

bodies form sharp, isolated hills along south

coast of St. John and around harbor of Charlotte

Amalie, St. Thomas. Name is taken from Careen

Hill in Charlotte Amalie

Trondhjemite—Fine- to medium-grained, commonly porphyritic with quartz and twinned, nonzoned albite phenocrysts. Groundmass typically

granophyric intergrowth between quartz and

albite. Common granophyric intergrowths radiate from albite phenocrysts. Forms Leduck

Island (where columnar jointing is prominent)

and Flanagan Island. Also occurs as clasts in tuff

breccia of Water Island Formation in eastern St.

John. On Leduck Island, is light bluish gray (5B

7/1) and coarsely porphyritic with less welldeveloped granophyric texture in groundmass

that contains subhedral albite and equant quartz.

Grain size is about 0.1 mm. On Flanagan Island,

is medium-bluish-white (5B 9/1); contains a few

percent clinozoisite and chlorite

34 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

Kla

Klm

Kct

Klb
Coarsely porphyritic keratophyre—Consists of 20

to 30 percent prominent quartz and plagioclase

phenocrysts, typically 5 to 10 mm across.

Groundmass varies from exceedingly fine

grained, similar to extrusive keratophyre, to

coarser grained with plagioclase laths up to 0.3

mm long. At higher metamorphic grade, coarse

epidote appears in plagioclase phenocyrsts and

epidote, pale amphibole, and shreddy biotite

appear in groundmass. Blue-green amphibole is

in groundmass along Center Line Road at elevation of 320 ft. Widespread as dikes in Water

Island Formation; cuts tonalite (Kct) of Flanagan

Island. Plugs form sharp hills of Maria Bluff,

northeast of Chocolate Hole and north of

Cocoloba Cay

Porphyritic keratophyre—About 20 percent conspicuous quartz and albite phenocrysts up to 3

mm across in a medium-dark-bluish-gray (5B

4/1), aphanitic groundmass of randomly oriented

plagioclase laths 0.1 mm long, interstitial quartz,

and magnetite dust. Accessory apatite and allanite. Glomerophyric plagioclase surrounded by

granophyric intergrowths. Forms small unaltered

plug west of Lameshur and sparse dikes

Phenocryst-poor keratophyre—May contain up to

10 percent small (generally less than 2 mm

across) phenocrysts of quartz and plagioclase.

Forms small pluton north of Bovocoap Point and

sparse dikes

Plagioclase-phyric keratophyre—Consists of 5 to

15 percent plagioclase phenocrysts, 1 to 3 mm

long, in groundmass of typically felted plagioclase microlites and interstitial quartz. Forms

small plutons on Ram Hill and on peninsula of

Dittlif Point, and sparse dikes

Gabbro—Dark-greenish-gray, fine-grained, porphyritic, with small (2 mm or less) subhedral

phenocrysts of clinopyroxene and plagioclase in

groundmass of randomly oriented plagioclase

laths about 0.5 mm long, clinopyroxene, chlorite,

leucoxene, and opaque minerals. Forms intrusive

body on Bovocoap Point; some mafic dikes in

Water Island Formation are probably unit Kcm.

May be more abundant than shown on map

Sheeted dike complex—Dominantly keratophyre,

but includes diabase as well. Dikes trend northeast (as does the diagrammatic overprint pattern)

and dip moderately to steeply northwest.

Mapped southwest of Salt Pond and at Harbor

Point. Better developed on St. Thomas than on

St. John. (Shown in magenta on plate1)

Water Island Formation (Lower Cretaceous)—

Dominantly (80 percent) extrusive keratophyre

and volcaniclastic rock derived from it; about 20

percent basaltic lava, breccia, and hyaloclastite;

minor radiolarian chert. No internal stratigraphic order recognized. Groundmass of keratophyre

commonly spherulitic. With increasing metamorphic grade to the north and east, spherulites

are recrystallized and stand out as ovoids, some

as large as 4 cm, on weathered surface. Lower

contact of formation not exposed. Interpreted to

have been deposited on oceanic plateau crust.

Upper contact conformable and is exposed at

several localities in southwestern St. John and

nearly exposed south of Ajax Peak. Thickness at

least 2 km. Several varieties of keratophyre distinguished by size and mineralogy of phenocrysts; all have intrusive equivalents in Careen

Hill Intrusive Suite. Ash-flow tuffs notably

absent

Porphyritic keratophyre—Consists of 10 to 25 percent conspicuous quartz and plagioclase phenocrysts, typically less than 5 mm across, in

aphanitic groundmass. Flow layering present

locally. Mapped distribution is approximate;

appears to constitute less than 10 percent of formation on St. John

Phenocryst-poor keratophyre—May contain up to

10 percent small (generally less than 2 mm

across) phenocrysts of quartz and plagioclase.

Some rocks are aphyric on the scale of thin section. Many outcrops untextured but may display

flow layering and flow folds. Includes lava, flow

breccia, some bedded tuff, and some volcaniclastic rock. Dominant unit of Water Island

Formation on St. John

Plagioclase-phyric keratophyre—Consists of 5 to

10 percent plagioclase phenocrysts 1 to 3 mm

long in groundmass that is commonly felted

mass of oriented aphanitic feldspar laths. Flow

layering visible in some outcrops. Exposed in

vicinity of Great and Little Lameshur Bays.

Because phenocrysts are small and sparse, some

areas mapped as phenocryst-poor keratophyre

(Kwa) could be Kwf

Basalt and basaltic andesite—Amygdaloidal pillow lava, pillow breccia, and hyaloclastite.

Called spilite by Donnelly (1966). Na2O and

CaO contents, however, are in the range of

basalt. Pillows typically about 1 m across.

Tubular bodies as long as 3 m have been

observed. Amygdules commonly in concentric

shells within pillows. Interstices between pillows filled with calcite or epidosite. Bedding and

topping direction determined from pillow asymmetry at Harbor Point, peninsula between Elk

Bay and Haul Over, and eastern Red Point, all in

eastern St. John. Bedding, but no topping direction, observed at several other localities.

Excellent exposures of pillow breccia and hyaloclastite found in coastal outcrops between

Saltpond Bay and Ram Head and on Dittlif and

Contant Points. Fragments of breccia are comAPPENDIX A 35

Kcc

Kcp

Kca

Kcf

Kcm

Kwp

Kwa

Kws

Kwf

See Plate

1 for pattern
monly deformed against one another.

Groundmass of least altered basalt consists of

plagioclase microlites (typically randomly oriented) and opaque dust in a cryptocrystalline

mesostasis. Small (millimeter range), sparse

phenocrysts of glomerophyric plagioclase and

clinopyroxene. Epidosite segregations common

and, in places, pillows have epidosite cores.

Secondary minerals in southern St. John include

prehnite and pumpellyite (identified optically

only), chlorite, calcite, and leucoxene. Zeolites

probably the result of burial metamorphism. To

the north and east toward Tertiary plutons, pale

amphibole and finally pleochroic amphibole

appear in groundmass. Clinopyroxene replaced

by epidote and pale amphibole; farther north by

pleochroic amphibole. Metamorphic assemblage

adjacent to tonalite at Please horizontal control

marker, north of More Hill in eastern St. John,

includes quartz, plagioclase, hornblende, diopside, garnet, and calcite. Most basalt, including

breccia fragments, is amygdaloidal; some is scoriaceous (amygdules a few millimeters across

make up 30 or more percent of rock). Flattened

amygdules on Kiddel Point are as long as 10 cm.

Amygdules filled with combinations of quartz,

calcite, chlorite, epidote, and plagioclase, as well

as local prehnite and pumpellyite; at higher

grade, filled with pale amphibole and clinozoisite in radiating sheaves

Volcanic wacke and bedded tuff—Volcanic wacke

consists of subrounded to angular clasts of keratophyre of all varieties, basalt, and trondhjemite

in a matrix that is probably comminuted keratophyre but includes xenocrysts of quartz and plagioclase. Constituents poorly sorted and up to

more than 1 m across; bedding may be poorly

defined. Graded bedding rare but present.

Pumiceous keratophyre clasts rare except

between Grootpan and Kiddel Bays where submarine pumice flows may be present.

Trondhjemite clasts up to boulder size, thought

to be from Kct, found in cove 250 m northeast of

Long Point and easternmost Privateer Point.

Bedded tuff locally; no shards seen in the few

thin sections available. Difficult to distinguish

between tuff and chert in the field

Radiolarian chert—Light-blue-gray to dark-gray,

bedded chert containing Radiolaria. Mapped

where Radiolaria have been observed, such as at

Kiddel Point on the shore of Saltpond Bay

36 GEOLOGY OF ST JOHN, U.S. VIRGIN ISLANDS

Kww

Kwr