Gulf of Mexico Geology

Just thinking how much general knowledge most of us have of this most important geographic location.

  

Diapir
http://en.wikipedia.org/wiki/Diapir
A diapir (pronounced /ˈdaɪ.əpɪər/ [1]) (French, from Greek diapeirein, to pierce through) is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh-Taylor instability-type structures in regions with low tectonic stress such as in the Gulf of Mexico to narrow dikes of material that move along tectonically-induced fractures in surrounding rock. The term was introduced by the Romanian geologist Ludovic Mrazek, who was the first to understand the principle of salt intrusion and plasticity. The term “diapir” may be applied to igneous structures, but it is more commonly applied to non-igneous, relatively cold materials, such as salt domes and mud diapirs……………………………………………….Diapirs commonly intrude vertically upward along fractures or zones of structural weakness through denser overlying rocks because of density contrast between a less dense, lower rock mass and overlying denser rocks.[citation needed] The density contrast manifests as a force of buoyancy. The process is known as diapirism. The resulting structures are also referred to as piercement structures.[citation needed]
In the process, segments of the existing strata can be disconnected and pushed upwards. While moving higher, they retain much of their original properties such as pressure, which can be significantly different from that of the shallower strata they get pushed into. Such overpressured Floaters pose a significant risk when trying to drill through them. There is an analogy to a Galilean thermometer.[3]
Rock types such as evaporitic salt deposits, and gas charged muds are potential sources of diapirs. Diapirs also form in the earth’s mantle when a sufficient mass of hot, less dense magma assembles. Diapirism in the mantle is thought to be associated with the development of large igneous provinces and some mantle plumes.
Explosive, hot volatile rich magma or volcanic eruptions are referred to generally as diatremes. Diatremes are not usually associated with diapirs, as they are small-volume magmas which ascend by volatile plumes, not by density contrast with the surrounding mantle.

File:1990s Mathmos Astro.jpg

An original Mathmos Astro lava lamp. A lava lamp illustrates Rayleigh–Taylor instability-type diapirism in which the tectonic stresses are low.
Diapirs or piercement structures are structures resulting from the penetration of overlaying material. By pushing upward and piercing overlying rock layers, diapirs can form anticlines, salt domes and other structures capable of trapping petroleum and natural gas. Igneous intrusions themselves are typically too hot to allow the preservation of preexisting hydrocarbons.[Schlumberger Oilfield Glossary]

Geological Setting

Bob Carney  Louisiana State University

This expedition’s primary focus will be to explore and better understand bacteria and animal communities found where sulfide and/or methane are seeping from the deep-sea floor. Neither of these energy-rich compounds normally is abundant in the deep ocean. So their presence requires special geological processes. Therefore, we have a special interest in the geology of the Gulf of Mexico. The slope of the Gulf displays three general types of bottom. Very tall, steep cliffs called escarpments are on the east and south sides. We know that chemosynthetic communities exist at the base of the west Florida escarpment, and we suspect that they may be found off Yucatan. Across the northern Gulf and in the southwest corner, thick sediments are being sculpted by deep salt layers. We know that such areas have chemosynthetic communities. The rest of the Gulf slope on the west and a small bit in the northeast corner display more normal types of seafloor. The geology of the area suggests that no chemosynthetic communities will be found there, but we are not certain of this.

Image courtesy of Gulf of Mexico 2002, NOAA/OER.
The slopes of the Gulf of Mexico are unlike those found in the adjacent Atlantic. In two areas (type 1) there are steep carbonate escarpments. On the north and a bit on the south salt movement greatly influences the chemistry and geology (type 2). Only in the west and a small area of the northeast are “normal” deep habitats found (type 3).

 Five structures give an indication of geological history and active processes. Geologists suspect that the Gulf and Caribbean complex were created when continental fragments (Structure 1) collected between the Pacific and Atlantic. Later, similar fragments (Structure 2) sealed the region off from the Pacific. The volcanic arch (Structure 3) on the east and the deep trench in the middle (structure 4) are both indicators of continued seafloor activity. Most fascinating is a very small seafloor spreading center (Structure 5) at the bottom of the trench. Image courtesy of Gulf of Mexico 2002, NOAA/OER.

While the Gulf is geologically inactive,

   Asphalt flows from deep-      sea volcanoes

      New kind of  volcano discovered
           in the Gulf of Mexico

Underwater volcanoes that spew asphalt instead of lava: they were discovered in the Gulf of Mexico during an expedition of the research vessel SONNE, led by Prof. Gerhard Bohrmann of the DFG Research Center Ocean Margins. On these volcanoes the multinational team of scientists encountered a previously unknown highly diverse ecosystem at a water depth of 3,000 meters. The prominent scientific journal Science reports the spectacular discovery in its issue of 14 May 2004.]

the modern Caribbean and adjacent regions show strong evidence of continued plate tectonics. Above water, one can see the volcanic islands of the Aves arch separating the Caribbean from the Atlantic. On the northern end of this arch, the Puerto Rican trench lies on the Atlantic side. At the southern end lies the Barbados accretionary prism. An unusual feature of the Caribbean is the 6000-m deep Cayman Trough. The floor of this deep trough is quite complex. Although it is quite small, there is a spreading region at the bottom of the trough, and its ecology remains unexplored. “

http://oceanexplorer.noaa.gov/explorations/02mexico/background/currents/currents.html

Currents in a Cul-de-Sac 

Bob Carney  Louisiana State University  We are all familiar with cul-de-sacs— neighborhoods where you have to turn you car around to get out. The circulation of the Gulf of Mexico and Caribbean is similar to a cul-de-sac. Because ocean life depends on currents to transport larvae, it is possible that the Gulf’s fauna may be controlled to some extent by these unusual current patterns.

Gulf Surface Currents

Surface currents are ocean currents in which the moving water lies between the surface and a maximum depth of about 500m. Currents that are no deeper than 200m are usually caused by the wind pushing on the water. Currents as deep as 500m usually are caused by forces associated with the rotating Earth and are called geostrophic (Earth-turned) currents. In our exploration of the Gulf of Mexico we are concentrating our research on the ecology below 500m and are very interested in the Gulf Loop, an example of geostrophic flow that strongly influences our exploration area. The Gulf Loop flows in through the straits of Yucatan and exits through the straits of Florida. Sometimes it is confined to the coast of Cuba. At other times, it flows along a long loop to the North before turning south and eventually exiting through the straits of Florida. This elongated loop is unstable and pinches off large eddies that spin clockwise as they drift westward. The eddies eventually spin down in the western Gulf. They sweep over the bottom and may have a great influence on the ecology.

The general current flow of the Gulf of Mexico and Caribbean is greatly influenced by the narrow gaps in the eastern wall and by the depth of inter-basin sills. This can be seen by examining bathymetry of two transects. The wall transect runs along the islands of the eastern wall, and the floor transect runs through the center of the basins. Bathymetry data were obtained from the NOAA National Geophysical Data Center ETOPO2 Global Elevation Data CDROM. Image courtesy of Gulf of Mexico 2002, NOAA/OER.
Gulf Deep Currents  We know little about circulation below 500m, but we are studying it extensively at this time. We do know, however, where the deep water enters the Gulf. This information leads us to suspect that animals in the Gulf may be somewhat different than those in the adjacent Atlantic.
“The Gulf is rather isolated, and we know that it is 3600m deep. The Yucatan Strait is about 2000m deep, but the Florida Strait is only about 800m deep. This means that the deep water in the Gulf flows in from the Caribbean, not directly from the Atlantic. In effect, the islands of the eastern Caribbean form a very leaky wall with many shallow gaps, but only a few deep gaps. Just as this wall limits deep water flow, it might partially isolate animal populations in the deep Gulf from the populations in the larger deep Atlantic.”
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