[Aztlan] Plaster and the general geology of the Yukatan Peninsula

[Bruce Rogers]

Listeros,

The term "plaster" can mean any building substance that is applied in 
a plastic state and can made from either gypsum or limestone.  In the 
Mayab adjoining the Olmec heartland, there is neither, so mud or 
adobe was used as mortar and plaster (and sun-dried building blocks). 
However, most commonly the term "plaster" refers to Plaster of Paris 
made from gypsum, but I believe what we are talking about here is 
mortar and/or "plaster" made of burnt limestone, lime sand, and 
possibly other materials.

(Side bar: Larger unit of geologic time: Eras; medium-sized, Periods; 
smaller yet, Epochs.)

The general geology of the Yukatan Peninsula is a basement of mostly 
metamorphosed Paleozoic Era sedimentary rocks (ca. 540 - 245 million 
years old), with an unknown thickness (but probably several thousand 
meters thick). At the end of the Paleozoic, wide spread mountain 
building and then erosion planed off the landscape.

Atop these older rocks is a series of early Mesozoic Era (ca. 245-144 
million years old - Age of Reptiles) "redbeds"  of evaporite rock; 
i.e. rocks made of arid land sediments such as dune sandstone, 
gypsum, salt, iron oxide-rich slit, etc. that is about 100 meters 
thick.

On top of all this is another Mesozoic-aged thick blanket of 
limestone dating to the Cretaceous Period (136 to 65 million years 
old -  Age of Dinosaurs) with a thickness of about 1300 meters.

A further covering of very pure limestone was deposited in the 
Tertiary Period (65 million to 10,000 years old) that totals about 
1000 meters in thickness.

The uppermost units of these rocks were deposited during the 
Oligocene to Pliocene Epochs (ca. 38 to 5 million years ago) and 
includes the Carrillo Puerto Formation that is about 15 m thick.

As a frosting on the cake, the Quaternary Epoch (the 2 million to 
10,000 years old Ice Ages) limestone is about a few score meters 
thick with a finishing cover of Holocene Epoch (less than 10,000 
years old) limestone less than 10 meters thick.

Thus the most common building material in the Mayab is limestone. 
There is some gypsum in part of the Cretaceous limestone as well as a 
few very thin lenses of gypsum in the Carrillo Puerto Formation, but 
this is extremely small in volume compared to the limestone.  Gypsum 
is very water soluble, thus the tropical rainfall quickly washes any 
surface exposures of gypsum away.

The Mayab limestone is burned to make quick lime that is, in turn, 
used as a cement or plaster to hold building blocks together and to 
cover them, respectively.


For more technical reading, I suggest the following references:

Back, W., B. B. Henshaw, J. S. Herman, and J. N. Van Driel. 1986, 
Differential dissolution of a Pleistocene reef in the ground-water 
mixing zone of coastal Yucatan, Mexico: Geology, v. 14, p. 137-140.

Coke, J. G. IV, E.C. Perry, and A. Long. 1991, Sea level curve: 
Nature, v. 353, p. 25.

Holthuis, L. B. 1973, Caridean shrimps found in land-locked saltwater 
pools at four Indo-West Pacific localities (Sinai Peninsula, Funafuti 
Atoll, Maui and Hawaii Islands), with the description of one new 
genus and four new species: Zoologische Verhandelingen, v 128,  p. 
1-48.

Ward, W. C., and A. E. Weidie, eds. 1978, Geology and hydrogeology of 
northeastern Yucatan: New Orleans, LA New Orleans Geological Society.

Ward, W. C. 1985, Quaternary geology of northeastern Yucatan 
Peninsula, Part 2, p. 23-53 in Ward, W. C., A. E. Weidie, and W. 
Back, eds., Geology and hydrogeology of the Yucatan and Quaternary 
Geology of northeastern Yucatan Peninsula: New Orleans, LA, New 
Orleans Geological Society.

Weidie, A. E. 1985. Geology of the Yucatan Platform, Part 1, p. 1-19 
in Ward, W. C., A.E. Weidie, and W. Back, eds., Geology and 
hydrogeology of the Yucatan and Quaternary  Geology of northeastern 
Yucatan Peninsula: New Orleans, LA, New Orleans Geological Society.

Cheers,
Bruce Rogers, earth scientist on a good day