A Comparison of Two Atlantic-type 

Continental Margins

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1167
A Comparison of Two Atlantic-type 

Continental Margins

By JOHN S. SCHLEE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1167

Stratigraphy and structure of the continental margin of eastern 

North America and West Africa their history as restricted 

rift basins and later as broad troughs

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Schlee, John Stevens, 1928-

A comparison of two Atlantic-type continental margins.

(Geological Survey professional paper ; 1167)

"Stratigraphy and structure of the continental margins of eastern North America and West Africa-their history as restricted

rift basins and later as broad troughs." 

Bibliography: p.

Supt. of Docs. no. : I 19.16:1167 

1. Continental margins-Atlantic coast (North America) 2. Continental margins-Atlantic coast (Africa) 3. Geology-Atlantic

coast (North America) 4. Geology-Atlantic coast (Africa) I. Title. II. Series: United States. Geological Survey.

Professional paper ; 1167. 

GC84.2.N65S34 551.46'08'093 80-607047

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

Washington, D.C. 20402
CONTENTS

	Page

Abstract ______________________________ 1

Introduction ____________________________ 1

Acknowledgments _______________________ 3

West Africa ____________________________ 4

Structural elements of the region ______________ 4

Division of the continental margin _____________ 5

Transform margin ____________________ 5

Divergent margin _____________________ 7

North America ___________________________ 19

Structural elements of the region ______________ JQ

Division of the continental margin ______________ U

Page

Margin south of Long Island ______________ 12

Margin between Long Island and Newfoundland ___ 14

Margin north of the Newfoundland lineament ____ 15

Discussion _____________———————————————— j5

Similarities and dissimilarities in the structure of

matching margins _________———————————— 15

Stratigraphic sequences ____________—————— jg

Oceanic-margin ties ______________________ •, „

Divergent-transform margins ________________ -„

References cited __________———————————————— - o

ILLUSTRATIONS

Page

FIGURE 1. Index maps showing geography and continental margin bathymetry of eastern North America and West Africa _______ 2

2. Generalized geologic map of West Africa north of Liberia showing major structural framework _________________ 3

3. Map of the coatinental margin off Liberia and parts of Sierra Leone and Ivory Coast showing major oceanic fracture zones, inferred and mapped mafic intrusive rocks, coastal and shelf sedimentary basins, and the modifications of 

trend in bathymetry caused by the intersections of the fracture zones with the margin ____________________ 4

4. Cross sections and location map of a shelf basin off Liberia _____________________________________ 6

5. Cross section across the continental margin from West Sahara northwest to the eastern Canary Islands ____________ 8

6. Schematic cross sections through the West African and eastern North American margins at the locations indicated in figures

2 and 8 _________________________________________________________________ 9

7. Map showing structural and stratigraphic elements of the eastern North American continental margin and the adjacent

western Atlantic basin^Cape Hatteras to Newfoundland _____________________________________ 10

8. Isopach map of Jurassic(?) and younger sediments on the Atlantic continental margin, Chesapeake Bay to Newfoundland __ 12

III

A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS

By JOHN S. SCHLEE

ABSTRACT

The tectonic framework and deposits of the northwest 

African and eastern North American continental margins 

are similar; they reflect the complex interplay of differential 

subsidence, buildup of carbonate deposits, deposition of clastic 

sediments, and local tectonism. Early history was marked by 

block faulting or downwarping to form a series of basins 

at the margin edge in which continental clastic sediments 

were deposited. Where climatic and oceanographic conditions 

permitted, these beds were covered by evaporite deposits and 

shallow-water limestone. Off Liberia, volcanic rocks and 

clastic sediments dominate, whereas further northwest 

(Guinea-Bissau, Morocco), evaporite deposits and limestones 

are also in the section; a similar sequence of evaporite deposits and red beds of Late Triassic to Middle Jurassic age 

characterized the eastern Canadian margin. Salt forms 

diapirs, mainly off Canada and West Africa. The evaporite 

deposits are interbedded with or are overlain by limestone 

and dolomite that formed platforms and reefs adjacent to 

the newly formed Atlantic Ocean. The reef complexes are 

discontinuous features that appear to have acted as partial 

sediment dams during the Jurassic and Early Cretaceous 

as the North Atlantic Ocean widened. For some margins, 

these platforms and reefs have provided the foundation to 

which later physiographic features of the shelf and slope 

broadly conform. Fine-grained clastic sediments from ancient deltas overwhelmed the carbonate deposits during the 

Cretaceous to prograde the Atlantic margins seaward and 

build a gentle slope to the deep ocean. This slope was cut 

and steepened during the Tertiary and Quaternary.

The intersection of oceanic fracture zones with the continent 

can have different structural formats, depending on whether 

the margin has undergone divergent or transform movement. 

In areas where transform motion is dominant, fracture zones 

intersect areas marked by faulting, intrusion, and shallowly 

buried Precambrian rocks; narrow fault basins can be tucked 

in the shelf-slope areas between the intersection of these 

zones with the continent. On divergent margins, areas of 

present margin offset seem to be related to ancient continental zones of crustal offset and faulting, and the present offsets 

appear to be associated with zones of volcanism in the oceanic 

crust. These relations lend support to the idea that marked 

irregularities in the outline of the crustal break (leading to 

transform motion of crustal blocks) can lead to the formation 

of zones of weakness in the newly formed oceanic crust and 

then to the formation of seamounts and volcanic islands.

INTRODUCTION

In this paper, I examine the structure and 

stratigraphy of Atlantic-type continental margins 

(as described by Mitchell and Reading, 1969) along 

eastern North America and West Africa (fig. 1) to 

see the similarities and dissimilarities in patterns 

of continental-margin response to crustal separation. Also of interest are the possible ties between 

oceanic and continental structures and the different 

effects divergent and transform motion have had 

on the tectonic framework of the margin.

Since 1965, many offshore studies of West Africa 

and eastern North America (Beck and Lehner, 

1974; Behrendt, Schlee, and Robb, 1974; Behrendt, 

Schlee, Robb, and Silverstein, 1974; Delany, 1971; 

Emery and others, 1975; Given, 1977; Institute of 

Petroleum and Geological Society of London, 1965; 

Jansa and Wade, 1975a, b; Lehner and de Ruiter, 

1976, 1977; Mattick and others, 1974; Schlee and 

others, 1974, 1976; Sheridan, 1974a, b; Uchupi and 

others, 1976; Van der Linden and Wade, 1975) 

have revealed much about the margin structure and 

stratigraphy. Major summaries of the regional 

geology ashore (Dillon and Sougy, 1974; Fisher and 

others, 1970; Van Houten, 1977; Zen and others, 

1968) in the two areas have aided in a comparison 

of the margins. Broad geophysical surveys of the 

eastern Atlantic (Emery and others, 1975; Uchupi 

and others, 1976) have added much new data on 

oceanic crustal structure there. Stratigraphic well 

data have been compiled for the eastern Canadian 

margin (Jansa and Wade, 1975a) and have been 

correlated to reflectors on multichannel seismicreflection profiles (Given, 1977); such correlation 

is underway for the U.S. Atlantic margin (Smith 

and others, 1976; Scholle, 1977). Multichannel reflection profiles (Beck and Lehner, 1974; Lehner 

and de Ruiter, 1977; Schlee and others, 1976; Grow
A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS

^V 15°W 10°W 5°W

THE GRAND

BANKS OF

NEWFOUNDLAND

W7fggA#fA# S£A-(f<

AREA OF FIGURE 3 

POMANCHE

^^

GUINEA 

BASIN

+ 

I

35°N

30°N

25°N

20°N

15°N

10°N

5°N

0°N

A B

FIGURE 1.—Index maps showing geography and continental margin bathymetry. A, Eastern North America. B, West

Africa. Base modified from Uchupi (1971).

and others, 1979) have allowed much to be in- (Behrendt and Klitgord, 1976; Emery and others,

ferred about deep structures beneath the shelves 1970; Haworth, 1975; Klitgord and others, 1976;

off West Africa and off the Eastern United States. Taylor and others, 1968), and gravity data (Grow

In addition, refraction surveys (Grow and Sheridan, and others, 1975) have contributed to the picture

1976; Keen and others, 1975), magnetic studies of crustal-margin configuration.
INTRODUCTION

25°W 10°W 5°W

35°N

30°N

25°N

15°N

10°N

AFRICAN

20°M

FIGURE 2.—Generalized geologic map of West Africa north of Liberia showing major structural framework. Compiled from the Association of African Geological Surveys and United Nations Educational, Scientific, and 

Cultural Organization (1968), and Uchupi and others (1976, figs. 3 and 35). Locations of African sections 

(figures 5 and GB and D) are indicated here.

ACKNOWLEDGMENTS

Many of my colleagues have contributed to this 

paper through their comments and discussions 

with me: W. P. Dillon, K. D. Klitgord, J. A. Grow, 

and J. C. Behrendt of the U.S. Geological Survey; 

Elazar Uchupi and K. 0. Emery of the Woods Hole

Oceanographic Institution; and L. H. King of the 

Bedford Institute of Oceanography, Dartmouth, 

Nova Scotia. David A. Ross of the Woods Hole 

Oceanographic Institution and James E. Case and 

William P. Dillon of the U.S. Geological Survey read 

the paper and helped it to attain its final form.
A COMPARISON OP TWO ATLANTIC-TYPE CONTINENTAL MARGINS

Kevin King and Michael Roy prepared the illustrations.

WEST AFRICA 

STRUCTURAL ELEMENTS OF THE REGION

The major structural elements of West Africa 

(figs. 2 and 3) are the West African Shield, the 

Mauritanides, and the coastal basins that border 

the Mauritanides or West African Shield. Major 

structural features transverse to the margin are 

the Alpine fold belt of Morocco (fig. 2), a zone of 

intrusions associated with the Canary Islands, and 

faults through the Guinea marginal plateau (fig. 

IB). The West African Shield consists of Precambrian metamorphic rocks and granite, and it extends from the Ghana-Ivory Coast-Liberia area

north to the Anti-Atlas Mountains of Morocco (Dillon and Sougy, 1974) and westward to near the 

coast in a few places, although it is covered in some 

areas by platform deposits of Paleozoic age (fig. 2). 

The main outline of the West African Shield had 

achieved its present size and shape by 1,000 m.y. 

(million years) ago. Subsequent epeirogenic movements have warped it to create the Reguibat uplift 

of northwest Africa, and the Anti-Atlas Mountains 

of Morocco and Algeria. Adjacent to these uplifts, 

the shield is covered by varying thicknesses of marine platform deposits of late Precambrian and 

Paleozoic age in several broad basins (Dillon and 

Sougy, 1974; Association of African Geological 

Surveys and United Nations Educational, Scientific, 

and Cultural Organization, 1968).

Adjacent to the shield and its associated basins

14°W 12°W 10°W 8°W

Freetown

8°N

SIERRA 

LEONE

? ' ~

1 %:^x I I l^-~ C- f1 xI

\

\ \

• GUINEA) ' r—

SIERRA

IVORY 

COAST

6°N

4°N

.4000/7?..

EXPLANATION 

• Oceanic fracture zone

Inferred and mapped mafic

intrusive rocks 

Inferred faults; arrow shows

direction of fault-plane dip 

Coastal and shelf sedimentary

basins

Depth below mean sea level, — 

given in meters

ST. PAUL FRACTURE ZONE

100 200 

I

300 KILOMETERS 

_J

FIGURE 3.—Continental margin off Liberia and parts of Sierra Leone and Ivory Coast showing major oceanic fracture zones, inferred and mapped mafic intrusive rocks, inferred faults and the direction of fault-plane dip, coastal and shelf sedimentary basins, and the modifications of trend in bathymetry caused by the intersections of the 

fracture zones with the margin. Area of figure is shown in figure IB.
WEST AFRICA

are the Mauritanides, a belt of discontinuous folded 

mountains that crop out from Algeria in the north 

to Sierra Leone in the south. Sediments as young as 

Late Devonian in age were deformed during the 

Hercynian orogeny (Carboniferous); the presence 

of volcanic detritus and ophiolites caused Dillon 

and Sougy (1974) to speculate that the Mauritanides may have been part of a middle Paleozoic 

subduction zone, prior to the Hercynian orogeny. 

In the south, the Mauritanides split into two 

branches (Dillon and Sougy, 1974); the western 

branch goes along the southern side of the Mauritania-Senegal basin and the other goes into Sierra 

Leone.

North of the South Atlas fault (fig. 2), the Hercynian orogenic belt continues in the High Atlas 

and the Moroccan Meseta (fig. 2), where thick 

eugeosynclinal sediments were folded and thrust 

in an orogenic wave that moved east during the 

Devonian and Carboniferous. In contrast, miogeosynclinal sequences make up the Mauritanides south 

of the South Atlas fault.

West Africa and its margin are crossed by two 

major structural lineaments that divide the area 

into three subregions, both physiographically and 

technically. In the north, the South Atlas fault and 

the Alpine fold belt associated with it separate the 

fragmented crust (basins and adjoining mesetas) to 

the north from the series of seaward-opening coastal 

basins to the south (fig. 2). The same zone also 

marks a change in width of the shelf from an 

average 54 km in the south to 37 km in the north. 

South of the Mauritania-Senegal basin (fig. 2), the 

coastal area and margin are characterized by faulting, arching, and intrusion near the intersection of 

the Guinea fracture zone. A zone of mafic intrusive 

rocks (dolerites) trends northeast from the coast of 

Guinea (fig. 2).

The South Atlas fault (a right-lateral fault) is 

the latest expression of an ancient structural zone 

of weakness along which movement has taken place 

from the early Paleozoic to the present age (Rod, 

1962). It juxtaposes eugeosynclinal rocks of early 

Paleozoic age and miogeosynclinal rocks of the same 

age. Indeed, the fault area may well have been a 

shelf edge during the early to middle Paleozoic 

(Dillon and Sougy, 1974). In the Tertiary, the fault 

zone formed the southern limit of orogenic movements that created the Atlas Range, Rif Mountains, 

and Tell Range of Morocco and Algeria. Rod (1962) 

speculated that the fault zone continued offshore 

toward the Canary Islands. The transverse structural zone off Guinea shows up as (1) an offset of

the continental margin of about 230 km (Krause, 

1964) in an east-west direction, (2) a narrowing of 

the Mauritania-Senegal basin against a broad easttrending basement rise thought to underlie the 

southern edge of the Guinea marginal plateau 

(Lehner and de Ruiter, 1976) near the margin extension of the Guinea fracture zone, and (3) a 

change in the pattern of magnetic anomalies (McMaster and others, 1971) under the shelf off of 

where the northeast-trending zone of mafic intrusive rocks ashore intersects the coast. South of 

Guinea (fig. LB)> in Sierra Leone, Liberia, and the 

Ivory Coast (fig. 3), the West African Shield extends to the coast, where it is bordered by narrow 

sedimentary basins (either beneath the Continental 

Shelf or on land).

Offshore, the West African margin (fig. IB) consists of several kinds of Atlantic-type margins because the continent has been involved both in transform motion relative to South America and in crustal divergence away from North America. The 

transform motion took place along the southern part 

of West Africa (Liberia, Ivory Coast, Ghana) and 

resulted in the rifting of the West African Shield.

DIVISION OF THE CONTINENTAL MARGIN

TRANSFORM MARGIN

The Liberia-Ivory Coast part of West Africa 

(figs. 3 and 4) was the northernmost area affected 

by the separation of Africa and South America 

during the Early Cretaceous. Oceanic fracture 

zones approach the coast at low angles, subparallel 

to the trend of the southern bulge of West Africa. 

Their orientation to the coast suggests an oblique 

separation of the two crustal blocks. How has this 

oblique separation affected the shape of the coastal 

basins, and what are the ties between the equatorial 

oceanic fracture zones and coastal geology?

The northernmost of the equatorial fracture 

zones associated with the translation of Africa past 

South America is the St. Paul fracture zone, a complex of three zones (Behrendt, Schlee, and Robb, 

1974) that intersect Liberia and the Ivory Coast 

near Cape Palmas. Where they intersect the coast, 

the margin is faulted in a series of broad crustal 

blocks that are tilted seaward. The blocks trend 

east, appear to be intruded, and underlie much of 

the Continental Slope as broad ramplike steps which 

strike parallel to the fracture zone and deepen 

toward the west. The Continental Shelf adjacent to 

Cape Palmas is probably Precambrian crystalline 

rocks covered by a thin veneer of sediments
A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS

.Sea floor

LLJ

2 

P

£8UJ £ w

Is 2

3-I

VERTICAL EXAGGERATION OF 

TOPOGRAPHY ABOUT X10; 

SUBSURFACE REFLECTORS 

X5 OR LESS

20 KILOMETERS

^^--\./r>---_---

100 KILOMETERS

H

FIGURE 4.—Cross sections and location map of a shelf basin off Liberia. Note the block-faulted basement and the seaward-thickening prism of sediment. The sections are interpretations of multichannel seismic-reflection profiles. 

Modified from Schlee and others (1974).

generally less than 0.5 km thick. Onshore near Cape 

Palmas, a few mafic intrusive bodies cut isoclinally 

folded Eburnean paragneisses (-2,000 m.y. old) 

which strike east-northeast, subparallel to the trend 

of the intersecting fracture zones (Behrendt, Schlee, 

and Robb, 1974; Behrendt, Schlee, Robb, and Silverstein, 1974); in addition, coastal metamorphic rocks 

are faulted in the area where the St. Paul fracture 

zone is inferred to intersect the coast.

The other main tectonic trend in coastal Liberia 

is northwest as shown by the mafic intrusive rocks 

(fig. 3) that range in age from 176 m.y. old to 192 

m.y. old. These diabase dikes and sills were intruded 

parallel to the magnetic grain created by the Pan 

African thermotectonic event (-550 m.y. ago; Hurley and others, 1971); rocks metamorphosed in that 

event are found in a narrow belt along the coast

(White and Leo, 1969). An aeromagnetic survey of 

Liberia (Behrendt and Wotorson, 1970), revealed 

the mafic rocks to be associated with two broad 

northwest-trending belts of -50- to — 150-gamma 

linear anomalies, one in the coastal area and shelf, 

and the second associated with dikes, 90 km inland. 

Tensional forces connected with crustal rifting and 

the regional grain of Precambrian crystalline rocks 

appear to have affected the trend of mafic rocks 

intruded during the separation of crustal blocks 

(May, 1971).

Shelf basins adjacent to the intersection of the 

St. Paul fracture zone and the margin are elongate 

narrow features that open in a seaward direction 

over a block-faulted basement beneath the slope. To 

the east, the Ivory Coast basin (Spengler and Delteil, 1966; Arens and others, 1971; Lehner and
WEST AFRICA

de Ruiter, 1977) trends east parallel with the coast 

(fig. IB); the basin is bordered on the north by a 

major east-trending coastal fault (inner shelf continuation of St. Paul fracture zone) and on the south 

by the Romanche fracture zone (figs. 15 and 3). 

Beneath the Ivory Coast Shelf, Cretaceous and Tertiary sediments more than 5 km thick form a southplunging monocline that thickens under the Continental Slope and thins seaward of the slope.

Northwest of Cape Palnias, another sedimentfilled shelf basin also parallels the coast (fig. 3). 

Off central and northwest Liberia, it is built over 

an irregularly faulted basement (fig. 4) of presumed 

crystalline rocks, Paleozoic sedimentary rocks, and 

volcanic rocks (Schlee and others, 1974). The basin 

is faulted seaward, and at least as much or more 

sediment is under the rise and lower slope as is under 

the shelf. As can be seen from figure 4A, seismic 

waves passing through sediment in the coastal basin 

have two-way travel times of about 2 s; if the average velocity is assumed to be 2 km/s, this sediment is 

1-2 km thick. The shelf basin extends northwest to 

the Liberia-Sierra Leone border; off southern Sierra 

Leone, two smaller isolated basins have been outlined by McMaster and others (1975, fig. 6). Jones 

and Mgbatogu (1977) inferred that Cretaceous and 

younger sediments are 2 km thick west of Freetown 

(fig. 3).

The Sierra Leone-Guinea area bridges the change 

from transform motion in the south to divergent 

motion in the north (described in the next section 

"Divergent Margin"). Ashore, it is intruded by a 

northeast-trending belt of intrusive rocks, and offshore, it is marked by east-west offsets of the Continental Slope (see the 200- and 2000-m isobaths, 

figs. 2 and 3) where the Sierra Leone and Guinea 

fracture zones intersect the margin (McMaster and 

others, 1973, McMaster and others, 1975). McMaster 

and others (1971) postulated that at least two normal faults are transverse to the margin; one of 

these faults intersects the shelf edge near where 

Krause (1964) postulated that the Guinea fracture 

zone projects along the slope (fig. 2). Lehner and de 

Ruiter (1976, 1977) inferred that at least 6 km of 

sedimentary rocks (Paleozoic and younger) is above 

an irregular Precambrian basement (acoustic velocity of 6.1-6.5 km/s) beneath the Guinea marginal plateau; the basement rises to within 3 km 

of sea floor beneath the slope south of the plateau. 

An extensive carbonate platform covered the plateau during the Mesozoic.

To summarize, in the Liberia-Ivory Coast part of 

West Africa, the main responses to crustal separation of a shield area where Africa and South America moved away from each other at a low angle 

were: (1) the formation of narrow coastal and 

shelf sedimentary basins adjacent to the slope 

areas between fracture zones; they are built over 

block-faulted shield rocks and thicken in a seaward 

direction; (2) faulting and the intrusion of mafic 

rocks parallel to axes of the basins; and (3) the 

formation of a structurally complex ridge of tilted 

fault blocks and intrusions where extensions of the 

ocean fracture zones intersect the coast. The structural response of the Liberia-Ivory Coast area to 

rifting appears to lend support to Kinsman's 

(1975) concept of what a margin subjected to transcurrent separation should be like. The margin is 

narrow, is occupied by narrow basins, and lacks a 

thick postrifting sedimentary wedge.

DIVERGENT MARGIN

North America and West Africa, north of Sierra 

Leone, have been diverging from each other during 

the past 180 m.y. In response to the separation, 

several coastal basins (50-400 km wide) have 

formed; these basins contain Cretaceous and Cenozoic sediments deposited over a block-faulted crust 

>of Paleozoic and older rocks (Ayme, 1965; 

Spengler and others, 1966).

A map of two-way travel time of seismic waves 

to basement (fig. 2) shows that postrift sediment 

is in linear basins beneath the West African Shelf 

and Slope and that two-way travel time through this 

sediment increases seaward to more than 4 s. Data 

used to draw basement contours were compiled by 

Uchupi and others (1976) from published literature 

for land isopachs and from their marine survey for 

offshore isopachs. Land data can be converted to 

thickness in kilometers by multiplying time figures 

by 3 km/s. This computation indicates that thickening exists underneath the seaward parts of the 

basins. Sediment thickness follows, in a general 

way, basin outcrop pattern; the thickness is less 

where the shield areas approach the coast or intersect it (as off Guinea-Bissau). The trend and values 

of the contours change significantly at zones of 

crustal weakness (extension of the South Atlas 

fault) or where oceanic fracture zones approach 

the coast. The most pronounced change is in the 

vicinity of the Canary Islands, a zone of weakness 

along which Tertiary volcanic rocks were extruded 

(fig. 5). Adjacent to this zone and north of it, 

coastal basins are more restricted, the continental 

margin (particularly the shelf) is narrowed, and the 

onshore geology is characterized by block-faulted
A COMPARISON OP TWO ATLANTIC-TYPE CONTINENTAL MARGINS

NW 

KILOMETERS

g| Tertiary

200 

KILOMETERS

EXPLANATION 

Mesozoic

400

vi'riq Continental basement

E^M*

Oceanic basement basalt Tertiary volcanic rocks

FIGURE 5.—Cross section across the continental margin from West Sahara northwest to the eastern Canary Islands. 

Taken from Beck and Lehner (1974). Location of profile is shown on figure 2.

basins (of Late Triassic and Early Jurassic age) 

and basement blocks (Van Houten, 1977). Along 

the offshore extension of the South Atlas fault, intruded faulted continental basement rises in a 

manner similar to the continental basement south 

of the Guinea marginal plateau (Lehner and de 

Ruiter, 1976).

Sedimentary rocks within the coastal basins (figs. 

6B and D) are mainly continental clastic rocks of 

Mesozoic and Cenozoic age toward the east; they 

change to marine shale, limestones, and evaporites 

marked by conspicuous unconformities and diapirs 

(Dillon and Sougy, 1974; Ayme, 1965; Templeton, 

1971) in the west. The salt is inferred to be Triassic 

or Early Jurassic in age (Uchupi and others, 1976; 

Beck and Lehner, 1974), and it intrudes Cretaceous 

and younger strata in the southern part of the 

Mauritania-Senegal basin, and off Morocco and 

West Sahara.

On land, outcrops of Late Triassic age are red 

beds of conglomerate, sandstone, and mudstone associated with some salt. They are interlayered with 

basalts in the upper part and are restricted mainly 

to Morocco and Algeria (Van Houten, 1977); drilling in West Sahara has penetrated evaporite deposits of Triassic age. Dolerites intruded between 

the Carboniferous and Late Triassic are associated 

with the West African Shield (Mauritania, Mali, 

Guinea, Ivory Coast).

In the northern basins, the sedimentary section 

changes from continental red beds of Early Jurassic age to marine carbonate and evaporite deposits 

of Middle and Late Jurassic age (Dillon and Sougy, 

1974). Jurassic and younger limestones formed 

carbonate platforms off Senegal and Guinea; during 

the Jurassic, the platforms flanked evaporite basins 

(Lehner and de Ruiter, 1976), but the banks persisted into the middle Cretaceous, unlike those 

flanking the basins further north. The lower Tertiary sedimentary rocks in Senegal are thin beds 

of phosphatic glauconitic limestone, chert, and shale. 

Off part of Morocco, effective deposition ceased 

after the Eocene or Oligocene time (Summerhayes 

and others, 1971) because of upwarping which 

shifted the loci of deposition to the slope and rise.

The main response of the West African margin 

(north of Sierra Leone) to crustal divergence is a 

central zone of coastal basins built over a blockfaulted basement of deformed Paleozoic rocks; 

broad highs of Precambrian crystalline rocks separate these depocenters. The basins range widely in 

their width and appear narrowed where the West 

African Shield and Mauritanides are close to the 

coast. North of the Canary-South Atlas lineament, 

the margin is intricately faulted and deformed 

(figs. 3 and 6) in the part of Africa that has interacted with the European plate. In the south off 

Guinea, the change from a divergent margin to one 

affected by transform motion is marked by a narrowing of the basins, a lessening of subsidence, the 

absence of evaporite sequences, and a rise in the 

continental basement.
WEST AFRICA

KILOMETERS 

0-

10

^ x^ ^^^^^^

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^o/tfe/^^Bfen^_

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KILOMETERS 

0

CENTRAL SCOTIAN SHELF

Coast MOROCCO 

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MOROCCAN MARGIN

w

KILOMETERS 

0

NEW JERSEY MARGIN

Coast SENEGAL

^^•^^'Li^^r^-i''^^^

_-^-

^'Vlv--mental basement -

SOUTHERN MAURITANIA-SENEGAL BASIN

50 100 KILOMETERS

I

Sand or sandstone 

Shale

Triassic and Permian 

sedimentary rocks

VERTICAL EXAGGERATION X5

EXPLANATION 

3 Carbonate rock 

|] Shale and limestone 

3 Salt

T J Tertiary | 1 j Triassic

K | Cretaceous | L [ Lower

~J| Jurassic | u (Upper

FIGURE 6.—Schematic cross sections through the West African (B and D) and eastern North American (A and C) 

margins at the locations indicated in figures 2 and 8. A, The Scotian Shelf profile is from Bhat and others (1975). 

B, The Moroccan profile is modified from Beck and Lehner (1974). C, The New Jersey profile is from Schlee and 

others (1976). D, The Senegal profile is a composite of data from Ayme (1965) and Uchupi and others (1976). 

All sections are the same scale.
10 A COMPARISON OP TWO ATLANTIC-TYPE CONTINENTAL MARGINS

NORTH AMERICA

STRUCTURAL ELEMENTS OF THE REGION

The North American continental margin (fig. 

I A) has many of the same features just described 

as being on the West African margin—coastal 

basins, transverse structural zones, a bordering 

folded mountain belt, and a crystalline shield. An 

additional feature is a series of fault basins (Triassic to Early Jurassic in age), adjacent to much 

of the Appalachian fold belt.

The structural elements of the North American 

margin (King, 1969) are (1) the Canadian Shield 

of Precambrian crystalline rocks, (2) a platform 

cover of Paleozoic sedimentary rocks, (3) the Appalachian mountain system (extending from Alabama to Newfoundland), (4) discontinuous fault 

basins (and associated mafic volcanic rocks), and 

(5) the Atlantic Coastal Plain, the exposed part of a 

series of coastal basins and troughs filled with sediments of Mesozoic and Cenozoic age, most of which 

are under the Continental Shelf and Slope (fig. 7).

The southeastern part of the Canadian Shield is 

the Grenville Province—a sequence of marble and

quartzite associated with syenite and anorthosite 

800 m.y.-l,000 m.y. old (King, 1959). Shield rocks 

crop out along the Labrador coast, but farther 

south they are 200-600 km inland; shield rocks 

of West Africa are 50-450 km inland. Gently dipping Paleozoic sedimentary rocks, as much as several kilometers thick, overlie Precambrian crystalline rocks in the east-central United States. As in 

West Africa, these rocks have been broadly warped 

into domes, arches, and basins by epeirogenic movements which affected certain areas during the Paleozoic and later.

The Appalachian erogenic belt borders both the 

Canadian Shield and the cover of Paleozoic rocks 

of the central United States. It is exposed as a belt 

100-600 km wide (the Mauritanides are 0-150 km 

wide); in the Eastern United States, the northeasttrending belt consists of metamorphosed eugeosynclinal rocks (the Piedmont) of Paleozoic age to the 

southeast thrust northwest along with a core of 

Precambrian crystalline rocks (gneiss and granite) 

against a parallel trending trough of folded (or 

faulted) miogeosynclinal rocks also of Paleozoic 

age.

V V V V

v v v vvvvvvvvvvvvvvvvvvy

VVVVV VVVVVVVV 

V V V V V V

VVVVVVVVVVVVV LABRADOR

vvvvvvvvvvvvvvv 

vvvvvvvvvvvvvvv

VVVVVVVVVVVVV

vvvvvvvvvvv

V V VVVVV V--V-V V V V

V V V

V V V

V V V

LA HAVE 

PLATFORM

ATLANTIC 

OCEAN

300 KILOMETERS

z

EXPLANATION

ield area ''•••.. 

E22Platform deposits of Paleozoic age '••.... 

BS^Oroaenic belt (Precambrian rocks shaded) 

CUD Coastal basin

BB Jurassic and Triassic basins (onshore and nearshore bas 

E53 Mafic intrusive rocks 

CD Seamount 

Hingeline

—— Contact between geologic provinces 

Oceanic fracture zone 

Major margin fault zone 

Depth below mean sea level, given in meters

•4-J-Structural high in Atlantic Coastsl Plain 

~i COST No.B-2hole

o

So*

FIGURE 7.—Structural and stratigraphic elements of the eastern North American continental margin and the adjacent 

western Atlantic basin, Cape Hatteras to Newfoundland. A composite map from King (1969); Jansa and Wade (1975a) ; 

King (1975); Schlee and others (1976); Mattick and others (1974); Ballard and Uchupi (1975); Schultz and Grover 

(1974); Hans Schouten and K. D. Klitgord, unpub. data (1977).
NORTH AMERICA 11

To the northeast in northern New England and the 

Canadian Maritime Provinces, the general subdivisions are the same except that two additional belts 

of rocks are exposed in the southeastern part of this 

area. Bird and Dewey (1970) suggested that in 

New England and the Canadian Maritimes, island 

arcs may have been present beginning in the Early 

Ordovician, and that these arcs plus remnants of a 

proto-Atlantic oceanic crust were compressed during the Silurian and Devonian to form a belt of 

metasedimentary and metavolcanic rocks (Zone B 

of Bird and Dewey, 1970) between the Piedmont 

rocks to the northwest and a platform of Precambrian crystalline rocks, the Avalon platform, to the 

southeast (Ballard and Uchupi, 1975; Emery and 

Uchupi, 1972); fragments of the platform are exposed in southeastern Newfoundland, northern 

Nova Scotia, and southern New Brunswick, but 

much of it is covered beneath Georges Bank and 

areas of the shelf to the southwest. Rocks from this 

ancient platform exposed in Newfoundland are 

mainly Precambrian metasedimentary and volcanic 

rocks overlain by Cambrian and Ordovician rocks; 

by their faunal remains, these lower Paleozoic rocks 

show affinities with the rocks of Africa and Spain 

(Schenk, 1975), thereby suggesting that the platform may be a fragment of Europe and Africa that 

broke off and remained attached to North America 

during th£ latest opening of the Atlantic Ocean.

Stretching from Nova Scotia to the southern Appalachians are a series of elongate basins which are 

on the Acadian and Alleghenian orogenes. These 

basins contain as much as several kilometers of 

Upper Triassic to Lower Jurassic red beds (fig. 7) 

associated with basalt and diabase (Klein, 1962; 

Sanders, 1963; Faill, 1973; Ballard and Uchupi, 

1975; Van Houten, 1977). Where exposed on land, 

they form north- to northeast-trending elongate 

belts as down warps (Faill, 1973), grabens, and half 

grabens (Sanders, 1963) as much as 50 km across. 

The basin sediments were locally derived from 

adjacent highlands and were deposited as fans and 

fluvial deposits. The lowermost part of the section 

is intruded by diabase and contains basalt flows; 

the uppermost part of the section is also intruded 

by diabase.

Most recent workers think that the basins formed 

just prior to the last opening of the Atlantic, in 

response to tensional forces (May, 1971). Van 

Houten (1977, fig. 4) postulated that cratonic 

stretching in the Late Triassic may have resulted in 

faulting and continental sedimentation in basins 

along the eastern United States, Gulf of Mexico,

Bay of Fundy, and the east-trending South Atlas 

fault and Kelvin-Cornwall lineament. In the Early 

Jurassic, the rifting spread to include basin development and evaporite deposition off Nova Scotia 

and Newfoundland and Morocco, as the margin in 

these areas fragmented during the initial rifting 

of Africa and North America. Early stages of 

fault-basin formation are on the sites of earlier 

orogenies (Alleghenian and Acadian) that affected 

eastern North America. Other investigators (Faill, 

1973; Sanders, 1963; Ballard and Uchupi, 1975) 

have proposed more detailed models of crustal extension, thinning, graben formation, downwarping, 

and volcanism for certain areas of the eastern North 

American margin.

DIVISION OF THE CONTINENTAL MARGIN

Like West Africa, eastern North America has 

transverse structural lineaments (fig. 8) that subdivide the margin into three segments; one lineament is south of Long Island (Kelvin-Cornwall) 

and the other is south of the Grand Banks of Newfoundland (fig. I A). They show up by changes in 

the trend of the continental margin from northeast 

to east; the changes are 240 km southwest of 

Georges Bank and 500 km south of the Grand Banks 

of Newfoundland. The geology also shows major 

changes across both these areas (fig. 7).

Beneath the Scotian basin, a major fault zone is 

inferred to bound block-faulted basins northeast of 

Nova Scotia and possibly to continue down into the 

Bay of Fundy as the Glooscap fault zone (fig. 7). 

Southeast of this zone, Nova Scotia and its margin 

are inferred to have separated from the main part 

of North America during the Jurassic (Van Houten, 1977, fig. 4). Along the southern side of the 

Grand Banks, the Newfoundland lineament also 

originated as North America and Africa-Europe 

separated. The feature is southeast of Logans Line, 

the east-southeast-trending lineament along which 

the northern Appalachians appear to be offset 

(Drake and Woodward, 1963). The line (shown by 

dots on fig. 7) also marks a major change in the 

pattern of the magnetic and gravity anomalies beneath the Gulf of St. Lawrence (Haworth, 1975). 

Drake and Woodward (1963) speculated that the 

wrench fault (Logans Line?) that offset the Appalachians in the Gulf of St. Lawrence may tie across 

the shelf to the Newfoundland lineament. Haworth 

(1975) suggested that Logans Line may represent 

an offset in the continental edge of North America 

during the early Paleozoic.
12 A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS

\.4000mEXPLANATION

— Total sediment thickness, given in kilometers

•. Depth below mean sea level, given in meters 

Structural lineament

0 200 KILOMETERS

ATLANTIC OCEAN ^j\

FIGURE 8.—Isopach map of Jurassic (?) and younger sediments on the Atlantic continental margin, Chesapeake Bay to Newfoundland. Data sources: King (1975) and Schlee and others (1976). Locations shown for figures 6A and C.

The Kelvin-Cornwall lineament south of New 

England is thought to be a wrench fault (Drake 

and Woodward, 1963); its existence is inferred 

from bathymetric, magnetic, and seismic-refraction data offshore and from a noticeable change in 

the trend of Triassic and Jurassic basins and thickness of pre-Mississippian rocks onshore. The fault 

may tie into the New England Seamount Chain 

which trends east-southeast (fig. I A). The position 

of the lineament south of Long Island is uncertain. 

Drake and Woodward (1963) projected it obliquely 

across the shelf, and Schultz and Grover (1974) put 

it parallel to the base of the Continental Slope along 

the East Coast magnetic anomaly. Sheridan (1974a, 

b) interpreted the lineament to be a part of the edge 

of an oceanic block, defined in a seaward direction 

by oceanic fracture zones and at the margin, by the 

seaward edge of a sedimentary basin. These offshore 

studies indicate that some sort of zone of deepseated faulting probably existed along the present 

zone between the Baltimore Canyon trough and the 

Georges Bank basin (fig. 7) during the initial 

phase of crustal separation. The fault zone appears

to have formed in an area of rapid sediment thickening (fig. 8). This cross-shelf fault, like those adjacent to northwest Africa, may be "continued" 

offshore in a zone of volcanoes (seamounts) perhaps 

lined up along a zone of weakness in the oceanic 

crust (Vogt, 1973).

MARGIN SOUTH OF LONG ISLAND

Along the eastern United States, several younger 

coastal basins are built over Triassic and older 

rocks. Their emerged part is the Atlantic Coastal 

Plain (maximum width as much as 190 km in the 

study area, fig. 7) which extends southward from 

Long Island to the Gulf of Mexico. Northwest of 

Long Island, N.Y., coastal basins are all under the 

Continental Shelf. On the Atlantic Coastal Plain, 

the Mesozoic and Cenozoic sedimentary rocks attain 

a maximum thickness of 3 km near Cape Hatteras 

(fig. 1); north of there, the sedimentary rocks of 

Late Jurassic age and younger are 1-2 km thick 

and are contained in a broad series of embayments 

(Perry and others, 1975).
NORTH AMERICA 13

The sediments of the Atlantic Coastal Plain are 

broadly warped over structural highs and lows 

(embayments). Minard and others (1974) showed 

that three embayments and two structural highs are 

north of Cape Hatteras. One, the Fort Monroe high, 

is near the entrance of Chesapeake Bay, and the 

other, the South New Jersey high, is north of Delaware Bay (fig. 7). To the north, sediments of the 

Coastal Plain thin against the complexly faulted 

Long Island platform.

The Baltimore Canyon trough is 150 km wide and 

400 km long. It contains as much as 14 km of strata, 

including most of the sedimentary section under the 

Continental Shelf, Slope, and Rise (figs. 6C and 8). 

From an analysis of 3,800 km of multichannel 

seismic-reflection profiles over the Baltimore Canyon trough (Schlee, unpub. data; Grow and others, 

1979), the oldest sedimentary rocks in the trough 

are inferred to be as much as 5 km of Upper Triassfc and Lower Jurassic red beds and evaporite deposits that accumulated in a series of rifts. During 

the Jurassic, the trough expanded southward when 

as much as 3 km of nonmarine sediment (mainly 

sand) was deposited in a broad seaward-opening 

wedge. Discontinuous carbonate platforms appear 

to have bordered the trough during the Jurassic and 

Early Cretaceous (Schlee and others, 1976; Sheridan, 1976). In the northern part of the trough, the 

reef complex is a lensoid mass that has acoustic 

velocities of 3-5 km/s—similar to those velocities 

found in carbonate sequences. Well-bedded sediments grade laterally into the "reef," and the unit 

appears to be associated with a "fore reef" facies 

on the seaward side.

Diapirs are present within the trough, particularly near Cape Hatteras (Grow and Markl, 1977; 

Grow and others, 1979). Salinity gradients in some 

shallow slope holes drilled in 1967 led Manheim and 

Hall (1976) to suspect that Jurassic evaporite deposits may underlie the slope off New Jersey and 

New York.

The only deep stratigraphic information comes 

from the COST (Continental Offshore Stratigraphic 

Test) No. B-2 hole (location shown on fig. 7) 

drilled in 1975 off New Jersey (Smith and others, 

1976; Scholle, 1977). The COST No. B-2 hole bottomed in Upper Jurassic sandstone of deltaicnearshore origin at 4,772 m below the sea floor. The 

Lower Cretaceous sequence is about 1.3 km thick 

and is mainly a shelf sequence of shale and sand 

(minor carbonate rock). Upper Cretaceous rocks 

(950 m thick) are a sequence of marine shelf and

upper slope calcareous shale, sandstone, and dense 

limestone, which record at least two marine transgressions. A thin sequence (400 m) of slope-outer 

shelf limestone, claystone, and shale was deposited 

during the Eocene and Oligocene. It is overlain by 

a thick sequence (800 m) of poorly consolidated 

sand, gravel, and clay of Miocene age deposited in a 

shelf environment. Compared to the sequence of 

Coastal Plain rocks in the Island Beach, New Jersey, well, approximately 100 km away (Gill and 

others, 1963), the sequence in the COST No. B-2 

hole is thicker (the Cretaceous section is three times 

thicker, and the Tertiary is four times thicker) and 

tends to be more marine.

Gravity and magnetic studies of the deep crustal 

structure beneath the Baltimore Canyon trough 

(Grow and others, 1979; Klitgord and Behrendt, 

1979) show that the trough is probably built over 

a zone of thinned and intruded continental crust. 

Oceanic crust beneath the Continental Rise is about 

10-12 km below sea level and is overlain by 6-8 km 

of sedimentary rocks. Between the trough and the 

rise wedge is an acoustical basement ridge 25-75 

km wide that is mainly under the slope; here, 

acoustic penetration does not exceed 6 km below 

sea level. The acoustic basement ridge is marked 

by faulting and is overlain by the carbonate platform complex. In part because the ridge coincides 

with the East Coast magnetic anomaly, it is interpreted as thickened oceanic basement (layer 2) 

formed during the initial phase of sea-floor 

spreading.

In summary, the main response of the margin 

south of Long Island to separation of Africa and 

North America was subsidence, first in more restricted rift basins where probable nonmarine sediments and evaporite deposits accumulated and later 

in a broader trough as marine deposits of Late 

Jurassic and Cretaceous age were deposited. Marine strata were deposited on the seaward side of 

the trough, initially as carbonate platforms which 

formed the ancestral slope and later as fine-grained 

clastic sediments which prograded over the platform 

to create a constructional slope seaward of the 

present one (Schlee and others, 1979). An African 

counterpart would be the Mauritania-Senegal basin 

where Lehner and de Ruiter (1977, Fig. 5) indicated the presence of a carbonate shelf edge (as 

young as Cenomanian) beneath the present Continental Slope and outer shelf. There, a cluster of salt 

domes off Senegal, Gambia, and Guinea-Bissau 

indicate that a Lower Jurassic evaporite basin is 

beneath the shelf.
14 A COMPARISON OP TWO ATLANTIC-TYPE CONTINENTAL MARGINS

MARGIN BETWEEN LONG ISLAND AND 

NEWFOUNDLAND

Between the two major physiographic offsets 

(near Long Island and Newfoundland), the margin 

is characterized by several basins and platforms. 

From south to north, these are the Long Island 

platform, the Georges Bank basin, the La Have 

platform, the Scotian basin, the Avalon uplift, and 

the Flemish basin (fig. 7). Many of these features 

are further subdivided off the Canadian Maritimes 

into smaller subprovinces (ridges, subbasins, and 

banks), and the reader is referred to Jansa and 

Wade (1975a) and L. H. King (1975) for a complete description of all these features. The basins 

are open toward the sea and contain 8 to 14 km of 

Jurassic and younger sedimentary rocks. In part, 

they are built over block-faulted troughs probably 

containing Triassic or Lower Jurassic continental 

clastic sedimentary rocks (Ballard and Uchupi, 

1975; Van Houten, 1977); hence, the fragmented 

character of the continent inherited during the 

Triassic continues in later periods as exemplified 

by the configuration of younger deposits in basins 

off New England and eastern Canada (fig. 8). 

Some offshore basins are bounded on their landward 

flanks by a zone of rapid thickening—a hinge zone, 

which can also be continued as a zone of faulting 

into the overlying sediments (Jansa and Wade, 

1975a).

The platforms (Long Island, La Have) are characterized by an extensively faulted basement to 

form whole and half grabens, a thin sequence of 

postrift sediments, and a thickened wedge of sediments along their seaward flanks (Given, 1977; 

Grow and others, 1979). The grabens and half 

grabens are as much as 10-20 km wide and can 

contain several kilometers of rift sediment. The 

postrift sequence is usually less than 3-4 km over 

the platform; except for the oldest units (Upper 

Triassic (?) to Middle Jurassic), many of the same 

acoustic units appear to extend onto the platform 

though they are much thinned (Schlee, unpub. 

data). Well data (Given, 1977, fig. 6) shows that 

Lower and Middle Jurassic stratigraphic units present in the Scotian basin are missing from the La 

Have platform. The seaward edges of the platforms 

are marked by wedges of rapidly thickening postrift sediment (Jansa and Wade, 1975a; Given, 

1977; Grow and others, 1979). The seaward zone 

of the Long Island platform is presumably where 

the Kelvin-Cornwall lineament projects through the 

margin.

Georges Bank basin is likewise built across a

block-faulted basement to a thickness of more than 

10 km (Schlee, 1978). On the basis of an interpretation of 1,650 km of multichannel seismic-reflection profiles (plus correlation to Scotian Shelf and 

Massachusetts well data), the basin appears similar 

to the sequence that is found in the Baltimore 

Canyon trough. The sequence is inferred to be 

Triassic (?) and Lower Jurassic nonmarine clastic 

rocks and evaporite deposits (0-8 km thick), Middle 

and Upper Jurassic nonmarine clastic rocks and 

marine carbonate rocks (0-4 km thick), Cretaceous 

marine and nonmarine sedimentary rocks (0-2 km 

thick), and Cenozoic marine and glacial deposits 

(0.2-0.5 km thick). Georges Bank basin is bordered 

along the southern edge by a probable carbonate 

platform; though buried in most areas, the platform 

is exposed and has been sampled on the eastern end 

of Georges Bank by Ryan and others (1978). 

They collected Neocomian biohermal limestone in a 

canyon on the south side of the bank.

Under the Scotian Shelf (fig. 1), several deep 

holes revealed a dominantly carbonate-calcareous 

shale-salt sequence of Late Triassic and Jurassic 

age that changes upward to sandstone, shale, and 

mudstone of Cretaceous and Tertiary age (Williams, 

1975; Jansa and Wade, 1975a, b; Parsons, 1975; 

Given, 1977). As noted by Given (1977), the pattern of sedimentation was influenced by the configuration of the Paleozoic basement during the initial stages. As in basins already described, red 

beds, halite sequences, and dolomite (Upper Triassic and Lower Jurassic) characterized the rift 

phase of margin formation. The postrift phase of 

basin evolution (Jurassic) was characterized by 

discontinuous carbonate buildup along the seaward 

sides of the basins and by deposition of nonmarine 

clastic sediments inshore. During the Early Cretaceous, regressive deltaic wedges built over the carbonate rocks and shed detritus into the slope area. 

In the Late Cretaceous, a marine transgression 

deposited deep-water chalks and shales over the shelf 

part of the Scotian basin (Given, 1977). A regressive deltaic wedge occupied the Scotian basin during 

the Paleocene age, offlapping shales of Late Cretaceous age and contributing sediment through canyons to the Continental Rise sedimentary wedge.

The sedimentary patterns of both the Georges 

Bank basin and the Scotian basin are generally 

similar to those of the Baltimore Canyon trough. 

The Canadian rocks are documented in much better 

detail because of the many deep holes drilled on the 

Scotian Shelf and the correlation of the results to 

seismic-reflection profiles (Jansa and Wade, 1975b;
NORTH AMERICA 15

Given, 1977). As Bally (1976) and Given (1977) 

both pointed out, the patterns of subsidence and rock 

types support the Falvey (1974) model of a rift 

stage (Late Triassic and Early Jurassic) followed 

by a postrift stage (Jurassic and later) during 

which subsidence and marine transgression took 

place.

MARGIN NORTH OF THE NEWFOUNDLAND LINEAMENT

Off southern Newfoundland north of the Newfoundland lineament, the margin shows a change in 

structural style (figs. 7 and 8) in that the Scotian 

basin changes across the Avalon uplift into a series 

of narrow northeast-trending subbasins (fig. 8) that 

contain as much as 10-12 km of sediment; adjacent 

highs are covered by less than a kilometer of sediment. The basement is more fragmented beneath the 

eastern Grand Banks of Newfoundland (fig. 1A) 

and, therefore, shows a different tectonic framework 

than the one of general subsidence seen farther 

south. The Avalon uplift is a positive area that was 

active during the Early Cretaceous (Jansa and 

Wade, 1975a); it is covered by a thin sequence of 

Upper Cretaceous and Cenozoic sedimentary rocks. 

Clearly, the rifting phase (horsts and grabens) 

ceased much later in the southern Newfoundland 

area than it did to the southwest. Yet the lithological 

sequences are similar.

Beneath the Grand Banks of Newfoundland 

(Amoco Canada Petroleum Company Limited and 

Imperial Oil Limited, Offshore Exploration Staffs, 

1974), the section is Triassic nonmarine shales and 

evaporite deposits in fault-bounded troughs. They 

are unconformably overlain by Jurassic evaporite 

deposits and marine calcareous shale, limestone, and 

sandstone. The carbonate rocks (Middle and Upper 

Jurassic) formed as part of a discontinuous series 

of banks at the seaward edge of the Scotian basin, 

both under the southern Grand Banks of Newfoundland and under the Scotian Shelf. These rocks are 

overlain by sandstone, siltstone, and shale of Cretaceous age and mudstone of Tertiary age. Major 

uplift and tilting of Jurassic and older strata took 

place during the latest Jurassic and Early Cretaceous under the Grand Banks of Newfoundland.

The response of the southern Newfoundland 

margin to crustal separation has been broad subsidence along the southern edge (adjacent to the 

lineament) and restricted deep subsidence in the 

subbasins of the Grand Banks of Newfoundland. 

The pattern is similar to that in the Long Island 

Platform where northeast-trending narrow rifts extend through the Long Island Platform and sediment 

accumulation is as much as 3-4 km within the rift 

(Klitgord and Behrendt, 1979) as compared to 1-2 

km on the platform away from the rifts.

The deep crustal transition (continental to 

oceanic) beneath the Newfoundland Continental 

Slope has been inferred to be abrupt (King and 

others, 1975). It is marked by relief of as much as 

4 km in the basement—possibly caused by volcanic 

extrusion and block faulting at the time of continental separation. The abrupt transition is in contrast to 

the Scotian margin where refraction studies (King 

and others, 1975) appear to indicate a zone of crustal 

thinning that is about 60 km wide and that is mainly 

under the slope. King and others (1975) thought 

that the crustal structure of the Scotian margin was 

caused by rifting during divergence, whereas the 

structure of the margin off the southern Grand 

Banks resulted from transform faults that formed 

along old lines of weakness (Glooscap fault zone) as 

adjacent crustal fragments moved by each other 

(see Discussion section that follows).

DISCUSSION

Several features of the structural-stratigraphic 

frameworks of the North American and West African margins are similar, whereas some others appear 

uniquely related to the earlier tectonic features in 

the particular area. I wish to discuss four aspects 

of these Atlantic-type margins: (1) similarities and 

dissimilarities in patterns of continental margin 

response to crustal separation, (2) stratigraphic 

sequences formed during the crustal separation, (3) 

the relationship of oceanic and margin tectonic 

features, and (4) the effects of transform and divergent crustal motion in continental margin 

formation.

SIMILARITIES AND DISSIMILARITIES IN THE 

STRUCTURE OF MATCHING MARGINS

Parts of both margins are built over a blockfaulted continental crust marked by hinge zones, 

subparallel to the coast or subparallel to older structural lineaments inherited from ancient orogenic 

belts. The parallelism of coastal basins to ancient 

fold belts is fairly clear off the central bulge of West 

Africa and off the northeastern United States. Adjacent to New England and the Canadian Maritime 

Provinces, the pattern is complicated by the fragmented nature of the margin. The Canadian margin 

and its matching counterpart in Morocco show a
16 A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS

wider pattern of rifting and basin formation ashore 

than in some areas to the south (figs. 6A and B).

The broader pattern of basins, ridges, and platforms off eastern Canad is paralleled ashore by a 

more complex tectonic pattern in the Appalachian 

fold belt. The northern Appalachians are marked by 

Logans Line, thought by some to represent a zone of 

ancient crustal offset. They contain the Avalon platform (possibly a continental fragment of Africa at 

one time) and an additional belt of metasedimentary 

and volcanic rocks between the platform and a 

belt of eugeosynclinal rocks to the northwest. The 

complexities of offshore basin formation seem, in 

part, to mirror the onshore patterns of the tectogene. North of Newfoundland, where the margin 

trends across the northeast strike of the Appalachians, the crust has been greatly fragmented in 

formation of elongate block-faulted basins (Van der 

Linden, 1975) under the shelf and slope.

Major dissimilarities exist between the two 

margins in the magnetic anomaly patterns and in 

the distribution of postorogenic basins. Many investigators have pointed out that the magnetic quiet 

zone off Africa is narrower than the matching zone 

off North America (Pitman and Talwani, 1972; 

Vogt, 1973; Luyendyk and Bunce, 1973; Uchupi and 

others, 1976; Hayes and Rabinowitz, 1975). A 

plausible explanation for the difference is that the 

rift axis jumped eastward early in the opening of 

the Atlantic, to leave behind a defunct spreading 

center in the western Atlantic. Some investigators 

have postulated that the East Coast magnetic 

anomaly (Emery and others, 1970) may be such a 

center, though others (Uchupi and others, 1976) 

thought that the anomaly may have been caused by 

an intrusion of mafic dikes and sills prior to the 

breakup of the continents. Along the eastern United 

States is a discontinuous string of fault basins 

which apparently are not present in West Africa. 

This string of basins (of Triassic to Early Jurassic 

age) seems to suggest that eastern North America 

was Stretched more than West Africa. Though intrusive rocks are present along the coast of Africa, 

basins are not exposed except for those in Morocco 

discussed above. Some basins may underlie the 

younger coastal basins or the margins but even if 

they do, their presence there would seem to indicate 

that the present site of the West African margin 

subsidence coincided with the site of earlier rifting 

in West Africa, whereas in eastern North America, 

the rift formed earlier to the west and subsequently 

shifted to the site of the present margin in the Late 

Triassic (Van Houten, 1977).

STRATIGRAPfflC SEQUENCES

The strata deposited during rifting are similar on 

both margins and are similar to the types of deposits 

for newly rifted areas like the Red Sea. In the Red 

Sea, Miocene clastic rocks (sandstone and shale) 

are interbedded with basalt, volcanic tuff, and 

an evaporite sequence; the evaporite sequence is as 

much as 3.5 km thick and has flowed to form diapirs 

(Lowell and Genik, 1972; Ross and Schlee, 1973). 

Similar rocks have been drilled off the Canadian 

Maritime Provinces (Given, 1977), have been exposed in outcrops along the Eastern United States 

and Africa (fig. 6), or have been inferred to be offshore (Beck and Lehner, 1974; Emery and others, 

1975; Van Houten, 1977). Associated with the older 

continental clastic rocks and evaporite deposits are 

diabase and basalt, emplaced as tabular bodies subparallel to the subsiding basins offshore (Emery and 

Uchupi, 1972). Along with the subsidence and block 

faulting, these intrusions suggest possible thinning 

of continental crust as a part of the process of 

crustal breakup.

In Africa-North America, discontinuous carbonate banks and reef complexes formed during an 

early phase of continental separation-subsidence, as 

marine waters spread southward into the newly 

forming Atlantic during the Jurassic (Bhat and 

others, 1975; Jansa and Wade, 1975b; Schlee and 

others, 1976; Given, 1977). The banks acted as 

partial barriers to dispersal of sediment into the 

deep sea and as a support for the shelf edge (Lehner 

and de Ruiter, 1976). Establishment of these banks 

may relate to the block-faulted foundation inherited 

from the rifting phase. As in the Red Sea (Carella 

and Scarpa, 1963), reefs may have been established 

on the upthrown blocks and may have become quite 

large (Guilcher, 1955).

Deposition of a thick carbonate sequence appears 

to have been discontinuous. Carbonate rocks appear 

to be widespread off the Canadian Maritimes, along 

part of the Eastern United States margin, off West 

Africa (fig. 6), and in the Gulf of Mexico. Conditions did not facilitate their formation off the 

Liberia-Ivory Coast area. When the coverage of 

offshore well data becomes extensive enough in each 

area, the presence or absence of carbonate banks 

during the Late Jurassic and Early Cretaceous probably will be found to be related to the interplay of 

coastal sediment sources and bathymetry. In areas 

where the rivers delivered enough sediment to the 

shelf to construct a deltaic complex, major accumulations of lime mud would not have formed. This
DISCUSSION 17

pattern is similar to the one that Jansa and Wade 

(1975b, fig. 1) have postulated to exist off eastern 

Canada during the Late Jurassic.

Eventually the banks were overwhelmed by marine and nonmarine clastic deposits that covered 

them and prograded the shelf edge seaward. For 

both North America and Northwest Africa, the 

main phase of buildup after reef formation was 

during the Cretaceous (Jansa and Wade, 1975b; 

Beck and Lehner, 1974; Schlee and others, 1976; 

Bhat and others, 1975; Lehner and de Ruiter 1976, 

1977; Given, 1977); as much as several kilometers 

of sandstone and shale of deltaic, alluvial, and 

marine-shelf origin were deposited during the interval. During the Cenozoic, much less sediment accumulated on the shelf (generally less than 1.5 km), 

and it is composed of marine clay and sand, some of 

which is phosphatic and glauconitic.

OCEANIC-MARGIN TIES

A variety of features suggest that the initial break 

between North America and Africa was jagged and 

that irregularities in the edges of the continents 

may have influenced orientation of structures both 

in the newly formed oceanic crust and under the 

margins. Matching zones of seamounts line up with 

ancient continental lineaments—the Canary-South 

Atlas fault trend, the Guinea fracture zone-Guinea 

zone of intrusive rocks and faulting, the Newfoundland lineament-Glooscap fault zone, and the New 

England Seamount Chain—Kelvin-Cornwall lineament. Besides being marked by faulting and intrusions, these zones are marked in some places by 

offsets in the trend of the margin and by substantial 

changes of sediment thickness within the zone. The 

alinement of some of these features along a major 

change in the orientation of the margin, and along 

hypothesized zones of ancient continental offset, suggests that they are following ancient lines of weakness that crossed both continents prior to their 

separation (Wilson, 1970, fig. 7). The sliding of 

these irregular continental projections by each other 

may have been accompanied by volcanism and 

block faulting of the basement, much as King and 

others (1975) inferred for the southern edge of the 

Grand Banks of Newfoundland, and Schlee and 

others (1974) described as taking place off Liberia. 

If so, some fracture zones may be related to marginal offsets, as has been postulated by Le Pichon and 

Hayes (1971) for the South Atlantic, and by Le 

Pichon and Fox (1971) for the North Atlantic. For 

many other fracture zones, the tie to continental

features is more nebulous, particularly where the 

crustal blocks diverged and the sediment wedge over 

them is large.

DIVERGENT-TRANSFORM MARGINS

The divergent margins that have been examined 

in this paper are characterized by broadly subsided 

basins, probably built over a zone of block-faulted 

and intruded basement rocks. The critical boundary 

between continental and oceanic crust is in part 

marked by faulting, diapirism, and an extensive zone 

of Jurassic and Early Cretaceous carbonate platform deposits which are found in both Africa and 

North America. The crucial boundary may be a 

thickened zone of oceanic crust or a zone of transitional crust (marked by slivers of both types of 

crust).

The structural pattern of a divergent margin can 

change laterally from broad basins under the shelf 

(Mauritania-Senegal basin; Baltimore Canyon 

trough) to more of a fragmented margin marked by 

platforms and ridges and having basins under the 

slope rather than under the shelf (figs. 2 and 8). As 

discussed in the previous section, this change may 

reflect the irregularity of the crustal break during 

the latest stage of ocean formation. Where the break 

was subparallel to the direction of plate movement, 

transform motion would dominate and a transition 

zone of block-faulted platform would be anticipated. 

In areas like the Guinea marginal plateau (Lehner 

and de Ruiter, 1976) and the eastern Canary Islands 

(Beck and Lehner, 1974), a suggestion of a marginal 

ridge is given by the rise in the continental basement 

adjacent to the junction with oceanic crust. Along 

the southern side of the Grand Banks of Newfoundland (near the Newfoundland lineament), the abrupt 

transition from continental to oceanic crust has already been described. The clearest form of this type 

of a margin is off equatorial Africa where the sediment cover is thin and ridges and escarpments show 

through as the major tectonic features (Delteil and 

others, 1974; Emery and others, 1975; Schlee and 

others, 1974; Behrendt, Schlee, Robb, and Silverstein, 1974). The low-angle intersection of two fracture zones has resulted in two southward-facing 

escarpments separated by the elongate Ivory Coast 

basin (Emery and others, 1975; Spengler and 

Delteil, 1966; Delteil and others, 1974). The basin is 

limited laterally by these escarpments or ridges, and 

sediments in it thicken to more than 4 seconds (twoway travel time) in a seaward direction (Emery and 

others, 1975) and then thin against the southern of
18 A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS

the two escarpments; the greatest accumulation of 

sediment is adjacent to the coast, where more than 

5 km of sediment has been drilled.

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