1 LECTURE 2 WHAT IS A CONTINENTAL MARGIN? CEE 598, GEOL 593 TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS.

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Presentation transcript:

1 LECTURE 2 WHAT IS A CONTINENTAL MARGIN? CEE 598, GEOL 593 TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS

2 EVERY CONTINENT IS SURROUNDED BY A MARGIN The margin consists of a shelf (out to ~ 100 m), slope, and rise (which tapers off into deep water). shelf slope rise continental plain (subaerial) shoreline

3 THE POSITION OF THE SHORELINE VARIES WITH SEA LEVEL Glacial low stand Interglacial high stand

4 DURING THE LAST GLACIAL MAXIMUM, THE SHORELINE MOVED OUTWARD TOWARD THE SHELF-SLOPE BREAK NASA World Wind Shoreline at low stand (lowest sea level elevation during glaciation)

5 shelf slope rise continental plain (subaerial) SO SEA LEVEL FLUCTUATIONS STRONGLY AFFECT CONTINENTAL MARGINS

6 CONTINENTAL MARGINS COME IN TWO BASIC FLAVORS DEPENDING ON THE TECTONIC SETTING: ACTIVE AND PASSIVE MARGINS Passive margins tend to be broad, with extensive shelves. Active margins tend to be narrow, with constricted shelves.

7 CONTINENTAL PLATE TECTONICS

8 MANTLE CIRCULATION DRIVES PLATE TECTONICS

9 online.com/Geoware CentralNAtlanticMap. jpg SPREADING MARGIN: ATLANTIC OCEAN

10 PERU-CHILE MARGIN

trench.gif PERU-CHILE MARGIN

12 Pratson and Haxby, 1997 CONTINENTAL MARGINS AROUND THE UNITED STATES

13 CONTINENTAL SLOPE OFF NEW JERSEY Dissected passive siliciclastic margin.

14 CONTINENTAL SLOPE OFF GULF COAST FLORIDA Carbonate margin: life is the sediment factory.

15 CONTINENTAL SLOPE, NORTHERN GULF OF MEXICO Here the continental slope is governed by salt tectonics.

16 CONTINENTAL SLOPE OFF NORTHERN CALIFORNIA This is a characteristic active margin.

17 CONTINENTAL SLOPE OFF OREGON Active margin with complicated tectonics.

18 EACH MARGIN HAS ITS OWN SPECIFIC HISTORY AND ORIGIN EXAMPLE: SALT TECTONICS!

19 THE NORTHERN CONTINENTAL SLOPE OF THE GULF OF MEXICO IS RIDDLED WITH DIAPIRIC MINIBASINS

20 HOW WAS THIS BASIN-AND-RIDGE TOPOGRAPHY CREATED?

21 UNDERNEATH A LAYER OF SEDIMENT ~ 10 KM THICK IS A LAYER OF SALT ~ 1 KM THICK Salt tectonism: Salt is a Newtonian fluid with a very high viscosity! Overlying sediment (mud) ~ a Bingham fluid! Salt is lighter than mud! Salt flows up, sediment flows down. Result: minibasins on surface. Minibasins

22 TURBIDITY CURRENTS HELP TO BOTH BUILD AND DISSECT CONTINENTAL MARGINS

23 REFERENCE Pratson, L.F. & Haxby, W.F. ( 1997) Panoramas of the seafloor. Scientific American, 276(6),