I. Importance

A. Transition Between Continent and Ocean

B. Major Depocenters - Thick Sediments (10-15 km)

II. Classification of Margins

A. Types

1. Atlantic = divergent = passive = aseismic

2. Pacific = convergent = active = seismic

3. Transform = translation

B. Atlantic

1. NOT a plate boundary

2. Typical of margins in the Atlantic and Indian Oceans

C. Pacific

1. Plate boundaries

2. Typical of margins in the Pacific Ocean

D. Transform

1. May or may not be a plate boundary

2. Occur in all oceans

a. Atlantic - south of Grand Banks, north of Falkland Plateau, equatorial Africa, southern South Africa

b. Indian - south of Iran, west coast of India, east coast of Madagascar

c. Pacific - southern California

I. Introduction

A. Most conspicuous feature is thick sediments

1. average: 7-8 km, as thick as 15 km (maybe 22 km)

B. Result from rifting processes

C. Possess interesting geophysical anomalies

II. Thick Sediments

A. All shallow-marine (<200 m) or landward facies

B. How?

1. 200 m deep shelf can only accumulate 500 m of sediment through loading

a. a 3000 m deep basin can accumulate 7500 m of sediment through loading, but only the uppermost sediments will be shallow-marine or landward - most of the section will be deep-marine

2. So need active subsidence

3. But also need to thin the crust somehow

a. without thinning, subsidence will only bring then top of the crust back to sealevel

4. Ways to thin the crust

a. subaerial erosion

b. metamorphism of lower crust

c. subcrustal flow

d. necking or stretching

C. Subsidence

1. Results from thermal cooling of thinned crust

a. exponential like oceanic lithosphere

2. Early work

a. McKenzie

b. Watts & Ryan

i. south coast of France

ii. east coast of U. S.

3. Later work

a. Watts & Steckler

i. BACKSTRIPPING method

Y = S* x [rm-rs/rm-rw] + Wd - DSL x [rm/rm-rw]

ii. accounts for changes in original sediment thickness due to compaction, original water depth and changes in sealevel

iii. east coast of North America

iv. south coast of France

v. small to moderate effect if flexural effects considered

b. Theoretical stratigraphies of Atlantic type passive margins

i. see below for comparison with modern margins off east coast

III. Rifting Processes

A. Data Base

1. East African Rift, other rift valleys aborted on land

2. Young continental margins - Red Sea, Gulf of Aden, Mediterranean

3. Old continental margins - Bay of Biscay, North America

4. Ancient continental margins in mountain belts

B. Normal faulting

1. Listric in nature (like in MOR rift valley)

2. Affects only upper brittle part of crust (upper 10 km)

a. lower part of crust deforms plastically

C. Rift valley fill

1. Sediments - generally non-marine

a. Redbeds

i. conglomerate - alluvial fans

ii. sandstone - braided streams

iii. shale - lacustrine

b. Evaporites (non-marine to shoreline) common (orogenic rain shadow)

2. Basalt

a. volcanic flows

b. intrusive sills (& dikes)

3. Generally inclined

D. Onset of Spreading

1. After the crust has been thinned enough, it breaks & seafloor spreading commences

a. Marked by POST-RIFT UNCONFORMITY

i. uplift from thermal pulse & subsequent erosion

ii. often associated with final burst of basaltic igneous activity

b. Post-rift unconformity overlain by marine sediments deposited by transgression as post-rift subsidence commences

2. Propagating rifts

a. Spreading does not initiate at the same time everywhere along a margin

b. Parts of the margin may still be rifting even though other parts may be adjacent to spreading centers

E. Bay of Biscay margins illustrate this well (sediment starved margin)

IV. Geophysical Anomalies Associated with Passive Continental Margins

A. Result from Contrasts Between Continental & Oceanic Crust

B. Magnetic Anomalies

1. Often a positive magnetic anomaly on the oceanic side

a. Basalt is more magnetic than continental crust

C. Gravity Anomalies

1. Free-air gravity anomalies

a. positive on continent side, negative on ocean side

b. "edge" effect

2. Isostatic gravity anomalies

a. generally positive on ocean side

b. oldest ocean crust is slightly thicker than younger crust

c. oceanic crust is slightly denser than continental crust

V. Evolution of U. S. East & Gulf Coast Continental Margin

A. East Coast

1. Northern

a. Marginal carbonate reef during Late Jurassic & Early Cretaceous

i. middle shelf mixed carbonate & terrigenous

ii. inner shelf is terrigenous

b. Erosion of slope & retreat of shelf edge during the Oligocene

i. continental rise develops in Miocene

2. Southern

a. Carbonate platform forms outer & middle shelf from Late Jurassic to Late Cretaceous

i. inner shelf is terrigenous

b. Platform drowns & steps back in Latest Cretaceous time

i. slope is terrigenous, but Blake Plateau is carbonate

B. Gulf Coast

1. Significant progradation, particularly in the Tertiary

2. Lots of evaporites & salt

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