[Aztlan] Age of Yukatan beach ridges

[Bruce Rogers]

Listeros,

In response to questions about the age of some of 
the sedimentary deposits/rocks bordering the 
Yukatán Peninsula, I offer these dates taken from 
the 1982 New Orleans Geological Society Guidebook 
No.  10 ["No. 10"] and other sources:

"Sheraton Hotel, Cancun; Stop 5-4. Outcrop of 
Holocene dunerock under reconstructed Mayan ruin. 
"The limestone is less than 3,000 years old. 
Some of the strata exposed at the base of the 
Holocene eolian [dune] ridge on the northern part 
of the island [Cancun] contain lenticular layers 
of rounded pebbles and cobbles of corals, 
Strombus gigas [Queen conch, part of a family of 
shells around for about 144 million years since 
the Cretaceous Period {Age of dinosaurs}], and 
the pelecypods [clams] mixed with angular blocks 
of oolitic limestone [looks like white-colored 
BB's-made up of pearl-like accretionary grains 
indicative of beach environments] (intraclasts). 
A Strombus from one of these layers has a 
radiocarbon age of 2730 + 75 years before 
present." [No. 10]


"Intersection of Hwy 180 (Merida highway) and Hwy 
307 (Carillo Puerto Highway), Cancun.
"STOP 2-1.  Park car near powerline post no. 
1818.  Quarry to east exposes part of the belt of 
Upper Pleistocene calcarenite [calcareous sand 
dunes/incipient sandstone] that stretches 150 km 
along the Caribbean coast of the Yucatan 
Peninsula. These highly porous limestones were 
deposited on a progradtional strand plain (= 
beach] that developed during the Sangamon 
sea-level high 120,000-125,000 years ago. ..." 
[No. 10]


Now submerged by sea level are several terraces 
that were former beaches.  Among them is a fairly 
wide spread terrace at about 96 feet below sea 
level that is assumed to be the 8,000 year old 
terrace.  Deeper is a series of terraces between 
168 and 210 feet below sea level that are assumed 
to be the 11,000 year old beach level.  For 
further details, see: Curray, J.R., 1960, 
Sediments and history of the Holocene 
transgression, northwest Gulf of Mexico, in 
Recent sediments, northwest Gulf of Mexico, 
1951-58: Tulsa, OK, Am. Assoc. Petroleum 
Geologists, 394 p.

Even deeper along the margin of Yukatán Peninsula 
and around the Gulfo de Mexico are more submerged 
terraces.  Between -300 and -450 feet are a 
series of flats dated to between 18,000 and 
14,000 years old, thus they span the time segment 
between the youngest Pleistocene (locally called 
the "Tazewll subStage of the Wisconsin 
glaciation" in the US Midwest and Great Lakes 
area) and older Holocene epochs (rocks/sediments 
less than 10,000 years old).  For further 
elucidation, see:  Fairwell, R.W., 1961, Eustatic 
changes in sea level, in Physics and chemistry of 
the Earth: New York, v.4.


Thus we have essentially two major sets of beach 
ridges/sand dunes/ stuff: those belonging to the 
later part of the Ice Ages clustering around 
120,000 years old and the much younger ones 
clustering between 8,000 and 3,000 years old. 
And if one were to brave snorkeling among the 
Great White sharks, one would see more submerged 
dunes and deposits clustering abound . . . now! 

Two excellent references to the general geology of the Yukatán Peninsula:
Weidie, A. E. 1985, Geology of the Yucatan 
platform, in Geology and hydrogeology of the 
Yucatan and Quaternary geology of northeastern 
Yucatan Peninsula, Ward, W. C., A. E. Weidie, and 
W. Back, eds: New Orleans, New Orleans Geological 
Society, p. 1-19.

Ward, W. C., 1985, Quaternary geology of 
northeastern Yukatán Peninsula, in: Geology and 
Hydrogeology of Northeastern Yukatán and 
Quaternary Geology of Northeastern Yucatan, W. 
C. Ward, A. E. Weidie, W. Back, eds: New Orleans, 
New Orleans, New Orleans Geological Society (153 
p), p. 23-95.

Cheers,
  Bruce Rogers, earth scientist on a good, dune-filled day