Chapter II: Part 2: Physiographic Provinces 11

Introduction 11
Nomenclature and classification of deep-sea relief 11
Units of depth and slope 12
Continent and ocean 16
Continental margin 17
Definition and general categories 17
General categories 17
Category I provinces 17
Category II provinces 18
Category III provinces 19
Regional description of continental margin 21
Benches and terraces of the continental margin 41
Geology and geophysics of continental-margin physiographic
provinces 51
Seismicity of the continental-margin provinces 51
Magnetic anomalies and continental-margin physiographic
provinces 51
Crustal structures and continental-margin provinces 52
Sediment distribution and physiographic provinces of the
continental margin 53
Past, present, and future of the continental-margin
provinces 53
Ocean-basin floor 55
General description 55
Abyssal plains 55
General discussion 55
Regional description 57
Abyssal hills 61
Definition and distribution 61
Regional description 63
Origin of abyssal-hills topography 65
Other major features of the abyssal floor 66
Abyssal gaps 66
Mid-ocean canyons 66
Regional description 66
Origin of abyssal-floor topography 74
Oceanic rises 74
Definition and distribution 74
Regional description 74
Seamounts of the ocean-basin floor 78
Seismicity of the ocean-basin floor 80
Ocean-basin floor provinces and crustal structure 80
Mid-oceanic ridges 83
Definition 83
Mid-Atlantic Ridge 83
Provinces of the Mid-Atlantic Ridge 84
General statement 84
Crest provinces 84
Flank provinces 95
Azores Plateau 97
Azores-Gibraltar Ridge 98
Atlantis-Plato-Cruiser-Great Meteor Seamount Chain 98
Geology and geophysics of Mid-Atlantic Ridge physiographic
provinces 98
Seismicity of the Mid-Atlantic Ridge 98
Sediments and physiographic provinces of the
Mid-Atlantic Ridge 99
Rocks of the Mid-Atlantic Ridge 99
Crustal structure of the Mid-Atlantic Ridge provinces 100
Origin of the Mid-Atlantic Ridge 103
Sub-bottom reflections recorded on PDR records and
physiographic provinces 05
Summary of province characteristics 107

BIBLIOGRAPHY 109

INDEX 115

ILLUSTRATIONS

PLATES

FOLLOWING
PLATE PAGE

1. Physiographic diagram: Atlantic Ocean, Sheet 1 (In separate tube)

2. Preliminary chart of Hudson Submarine Canyon 20

3. Representative PDR records from continental rise of
northeastern United States 32

4. PDR records from continental slope and Blake Plateau 32

5. PDR records from outer ridge east of the Blake Plateau and
from the Blake-Bahama Abyssal Plain 32

6. PDR record of outer ridge showing sub-bottom horizon 32

7. Tracings of PDR records across continental slope and part
of the Blake Plateau 36

8. PDR records European continental rise and Biscay Abyssal
Plain 38

9. Representative PDR records of continental margin and
abyssal floor southwest of Canary Islands 38

10. PDR record of abyssal hills, southeast of Bermuda Rise 38

11. Ocean bottom photographs of the continental margin and
ocean basin floor 38

12. PDR record of Biscay Abyssal Plain 52

13. PDR records of Madeira Abyssal Plain and the Madeira Rise 52

14. PDR records of Mid-Ocean Canyon No. 2 and canyons at Vema
Gap 58

15. Ocean-bottom photographs of seamounts 58

16. Representative PDR records from Bermuda Rise 78

17. PDR record, Bermuda Scarp Zone 78

18. PDR record, Upper Step Mid-Atlantic Ridge 78

19. Ocean-bottom photographs; Mid-Atlantic Ridge 78

20. Physiographic provinces, Atlantic Ocean 122

21. Control chart 122

22. Six trans-Atlantic topographic profiles 122

23. Index chart showing locations of profiles of Plates 24,
25, 26 and Figure 45 122

24. Thirty-four profiles of the continental margin: western
North Atlantic 122

25. Twenty-three profiles of the continental margin of Europe
and Africa 122

26. Crustal structure and continental-margin provinces 122

27. Five topographic profiles, western North Atlantic 122

28. Distribution of deep-sea sands in relation to
physiographic provinces 122

29. Earthquake epicenters, North Atlantic 122

30. Location of PDR records and bottom photographs reproduced
as illustrations. 122

FIGURES

PAGE

FIGURE

1. Method of preparation of physiographic diagram 4

2. Precision depth recorder (PDR) sounding lines obtained by
research vessels 6

3. Good, but nonprecision sounding lines obtained by research
vessels 7

4. Major basins of the North Atlantic, after Wüst (1940) 12

5. Sound-velocity corrections for echo soundings 13

6. Conversion diagram for degrees, per cent grade, feet per
statute mile, and gradient 14

7. Gradients from 1:5 to 1:8000 shown at 40:1 vertical
exaggeration 15

8. Outline of submarine topography 15

9. Major morphologic divisions: North Atlantic Ocean 16

10. Three categories of continental-margin provinces 17

11. Two east-west profiles of Southeast Newfoundland Ridge 22

12. Laurentian Channel 23

13. Eastern Channel, Gulf of Maine 23

14. Submarine canyons off the Scotian Shelf 24

15. Continental-margin provinces: Type profile off northeastern
United States 26

16. Tracings of PDR records of continental and insular slopes 28

17. Cross sections of Hudson Submarine Canyon 32

18. Tracing of PDR record of Blake Escarpment 34

19. Continental-margin provinces: Type profiles off northwest
Africa 40

20. Georges Bank canyons 44

21. Two projected profiles of Georges Bank canyons 45

22. Geologic section at Cape Hatteras, Virginia 46

23. Correlation of structural benches off northeast United States 48

24. Geologic section: Western Europe based on refraction
measurements 49

25. Index chart, location of abyssal hill profiles illustrated
in Figures 26 and 27 62

26. Eleven profiles, Abyssal Hills Province, western Atlantic 62

27. Eight profiles, Abyssal Hills Province, eastern Atlantic 63

28. Tracing of PDR record, abyssal hills, southeast of
Bermuda Rise 64

29. Northwest Atlantic Mid-Ocean Canyon 67

30. Eight east-west profiles showing Northwest Atlantic
Mid-Ocean Canyon 68

31. Forty-eight cross sections of Northwest Atlantic
Mid-Ocean Canyon 69

32. Long profile of Northwest Atlantic Mid-Ocean Canyon 70

33. Tracing of PDR record across Theta Gap 73

34. Natural scale profile, Kelvin Seamounts 77

35. Crustal sections in various physiographic provinces,
determined by seismic-refraction measurements 81

36. Index to natural-scale Mid-Atlantic Ridge profiles
reproduced in Figures 37-41 84

37. Natural-scale Mid-Atlantic Ridge profile 1A 85

38. Natural-scale Mid-Atlantic Ridge profile 1B 86

39. Natural-scale Mid-Atlantic Ridge profile 1C 87

40. Natural-scale Mid-Atlantic Ridge profile 2A 88

41. Natural-scale Mid-Atlantic Ridge profile 2B 89

42. Type profile, provinces of the Mid-Atlantic Ridge 90

43. Tracing of PDR record, Rift Valley, Rift Mountains, High
Fractured Plateau, and Upper Step 91

44. Tracing of PDR record, Western Rift Mountains 92

45. Twenty-six rift valley profiles, Mid-Atlantic Ridge 93

46. Five representative profiles, crest and western flank of
Mid-Atlantic Ridge 94

47. Axial profile of the Mid-Atlantic Ridge 96

48. Profile of total magnetic intensity and topography,
Mid-Atlantic Ridge 101

49. Physiographic provinces and trans-Atlantic structure 102

TABLES

TABLE PAGE

1. Characteristics of continental rise, northeastern United States 27

2. Characteristics of continental rise, northwest Africa 41

3. Depths of prominent continental-shelf terraces 42

ABSTRACT

The Physiographic Diagram: Atlantic Ocean, Sheet 1, which portrays the North Atlantic between 17° and 50° North Latitude, is the first of a projected series of diagrams. The diagram is based on continuous echo-sounding traverses made by research vessels. The relief shown on the profiles was sketched in perspective using the technique introduced by Lobeck. Between sounding profiles the relief is speculative, based on extrapolation of trends noted in the profiles.

The area of the diagram is divided into three major physiographic regions which are in turn subdivided into the following categories of provinces.

CONTINENTAL MARGIN

_Category I_
Continental Shelf
Epicontinental Seas
Marginal Plateaus

_Category II_
Continental Slope
Marginal Escarpments
Landward Slopes of Trenches

_Category III_
Continental Rise
Marginal Trench-Outer Ridge Complex
Marginal Basin-Outer Ridge Complex

OCEAN BASIN FLOOR

_Abyssal Floor_
Abyssal Plains
Abyssal Hills
Abyssal Gaps and Mid-Ocean Canyons

_Oceanic Rises_

_Seamount Groups_

MID-OCEANIC RIDGE

_Crest Provinces_
Rift Valley
Rift Mountains
High Fractured Plateau

_Flank Provinces_
Upper Step
Middle Step
Lower Step

Each province is defined, briefly described, and illustrated with profiles and photographs of echo-sounding records.

The boundaries of the physiographic provinces, defined solely by bottom topography, show good correlation with variations in crustal structure as determined by seismic-refraction measurements and with anomalies of the gravity and magnetic fields. In addition, the province boundaries correlate well with distribution patterns of bottom sediments. The physiographic provinces are thus really morpho-tectonic provinces. The precise correlation of topographic provinces and structure observed in specific sections can thus be extrapolated along province boundaries to deduce the geology in large areas where no geophysical work has been done. The tectonic map of the Atlantic prepared in this manner will be presented in a subsequent publication.