Transcript Lecture 2
CEE 598, GEOL 593 TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS LECTURE 2 WHAT IS A CONTINENTAL MARGIN? 1 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 2 THE POSITION OF THE SHORELINE VARIES WITH SEA LEVEL Interglacial high stand Glacial low stand http://www.teachingboxes.org/seaLevel/lessons/lesson4_SeaLevelCurveGraph.htm 3 DURING THE LAST GLACIAL MAXIMUM, THE SHORELINE MOVED OUTWARD TOWARD THE SHELF-SLOPE BREAK NASA World Wind Shoreline at low stand (lowest sea 4 level elevation during glaciation) SO SEA LEVEL FLUCTUATIONS STRONGLY AFFECT CONTINENTAL MARGINS continental plain (subaerial) shelf slope rise 5 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/ 6 CONTINENTAL PLATE TECTONICS http://www.geoware-online.com/GeowareMercatorMap.jpg 7 MANTLE CIRCULATION DRIVES PLATE TECTONICS 8 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 9 PERU-CHILE MARGIN 10 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 11 http://www.clas.ufl.edu/users/jmeert/sam.jpg CONTINENTAL MARGINS AROUND THE UNITED STATES 12 Pratson and Haxby, 1997 CONTINENTAL SLOPE OFF NEW JERSEY Dissected passive siliciclastic margin. 13 CONTINENTAL SLOPE OFF GULF COAST FLORIDA Carbonate margin: life is the sediment factory. 14 CONTINENTAL SLOPE, NORTHERN GULF OF MEXICO Here the continental slope is governed by salt tectonics. 15 CONTINENTAL SLOPE OFF NORTHERN CALIFORNIA This is a characteristic active margin. 16 CONTINENTAL SLOPE OFF OREGON Active margin with complicated tectonics. 17 EACH MARGIN HAS ITS OWN SPECIFIC HISTORY AND ORIGIN EXAMPLE: SALT TECTONICS! 18 THE NORTHERN CONTINENTAL SLOPE OF THE GULF OF MEXICO IS RIDDLED WITH DIAPIRIC MINIBASINS 19 HOW WAS THIS BASIN-AND-RIDGE TOPOGRAPHY CREATED? 20 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 21 TURBIDITY CURRENTS HELP TO BOTH BUILD AND DISSECT CONTINENTAL MARGINS 22 REFERENCE Pratson, L.F. & Haxby, W.F. ( 1997) Panoramas of the seafloor. Scientific American, 276(6), 82-87. 23