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CEE 598, GEOL 593
TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS
LECTURE 2
WHAT IS A CONTINENTAL MARGIN?
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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).
continental plain
shoreline
(subaerial)
shelf
slope
rise
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THE POSITION OF THE SHORELINE VARIES WITH SEA
LEVEL
Interglacial high stand
Glacial low stand
http://www.teachingboxes.org/seaLevel/lessons/lesson4_SeaLevelCurveGraph.htm
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DURING THE LAST GLACIAL
MAXIMUM, THE SHORELINE
MOVED OUTWARD TOWARD THE
SHELF-SLOPE BREAK
NASA World Wind
Shoreline at low stand (lowest sea
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level elevation during glaciation)
SO SEA LEVEL FLUCTUATIONS STRONGLY AFFECT
CONTINENTAL MARGINS
continental plain
(subaerial)
shelf
slope
rise
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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.
http://sio.ucsd.edu/png/science/
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CONTINENTAL PLATE TECTONICS
http://www.geoware-online.com/GeowareMercatorMap.jpg
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MANTLE CIRCULATION DRIVES PLATE TECTONICS
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http://www.eoearth.org/media/draft/b/b5/Cross_section_of_the_earth,_tectonic_plates.gif
SPREADING
MARGIN: ATLANTIC
OCEAN
http://www.geowareonline.com/Geoware
CentralNAtlanticMap.
jpg
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PERU-CHILE
MARGIN
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http://web1.hrhost.net/~huttonco/Other/Latinia/SAm_P&A750.jpg
PERU-CHILE MARGIN
http://astro.wsu.edu/worthey/earth/html/im-geology/peru-chiletrench.gif
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http://www.clas.ufl.edu/users/jmeert/sam.jpg
CONTINENTAL MARGINS AROUND THE UNITED STATES
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Pratson and Haxby, 1997
CONTINENTAL SLOPE OFF NEW JERSEY
Dissected passive siliciclastic margin.
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CONTINENTAL SLOPE OFF GULF COAST FLORIDA
Carbonate margin: life is the sediment factory.
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CONTINENTAL SLOPE, NORTHERN GULF OF MEXICO
Here the continental slope is governed by salt tectonics.
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CONTINENTAL SLOPE OFF NORTHERN CALIFORNIA
This is a characteristic active margin.
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CONTINENTAL SLOPE OFF OREGON
Active margin with complicated tectonics.
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EACH MARGIN HAS ITS OWN SPECIFIC HISTORY AND
ORIGIN
EXAMPLE: SALT TECTONICS!
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THE NORTHERN CONTINENTAL SLOPE OF THE GULF OF
MEXICO IS RIDDLED WITH DIAPIRIC MINIBASINS
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HOW WAS THIS BASIN-AND-RIDGE TOPOGRAPHY
CREATED?
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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
SEAFLOOR
SEDIMENT LAYER
SALT LAYER
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TURBIDITY CURRENTS HELP TO BOTH BUILD AND
DISSECT CONTINENTAL MARGINS
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REFERENCE
Pratson, L.F. & Haxby, W.F. ( 1997) Panoramas of the seafloor. Scientific American,
276(6), 82-87.
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