[Aztlan] Correction: Mayapan clay (and plate tectonics of the Yukatan and Florida Platforms and Campeche Bank)

[Bruce Rogers]

Listeros,

In response to a note from Jack Sulak, I am posting this 
geographically corrected note to my post about clays in the Yukatan.

Yes, the State of Campeche is located along the western side of the 
Yukatan Peninsula.  Yukatan State occupies the central and northern 
area while Quintana roo State covers the eastern part.  These, 
however, are political states and have only a cursory relation to the 
bedrock geology of the peninsula.  The Campeche Bank, a huge 
submerged bank, fits against the Yukatan Peninsula like an 
northwest-tilted beret.  So, yes, while typing that post, it was late 
at night and I typed in "eastern" instead of "western."  You caught 
me!  :-)

The tectonic layout of the Caribbean is rather complicated.  About 
220 million years ago North America started drifting away from South 
America and Africa as the super continent Pangea broke up.  The 
resulting shallow basin was the beginnings of what was to become the 
Gulf of Mexico.  After the formation of salt deposits, as result of 
shallow sea covering the region periodically drying up,  the Yukatan 
block moved off south from North America as the entire Gulf of Mexico 
coast of Mexico separated from the Florida area of the US.  True 
ocean crust then formed in the basin area starting about 165 million 
years ago.

The Gulf as we know it today then formed as a gradually deepening 
basin in response to crustal cooling and shrinking plus floundering 
under the weight of sediments being dumped into the basin (the 
Sigsbee Deep east of the Olmec heartland is a little less than 4400 m 
deep).  Among the relics left from the parting of the ways, so to 
speak, is a north-trending fault that is now buried under the Cozumel 
Channel, which is at the eastern margin of the Yukatan Peninsula. 
Both the Florida and Yukatan Platforms were above seal level until 
about 145 million years ago, then both areas were submerged and thick 
deposits of limey sediment were laid down.

See the website below for a good rendition of the Bank Bay and other 
features in the Gulfo de Mexico.

<http://www.gulfbase.org/facts.php>

As you can see from the illustration on the website, the Campeche 
Shelf makes up about a third of the shallow (<20 m depth) waters of 
the Gulf of Mexico.  It does extend along the western and northern 
edges of the Yukatan Peninsula.  The eastern margin of Yukatan is 
bounded by the Cozumel Channel, a 50 km long,  18 km wide, and 400 m 
deep trough filled with fast currents.

Cheers,
Bruce Rogers, earth scientist on a good day, but (continental) 
marginal late at night.