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By Dr. Charles Shaw, Geologist and former Director of Centro Ecológico Akumal
As the last ice age took hold of the Earth 100,000 years ago, the level of the sea fell 600 feet, in lock step with the permanent accumulation of snow and ice on the northern continents. Powerful ice-age winds blew the newly exposed sand on the continental shelf of the Yucatan Peninsula into a series of large sand dunes spread along the peninsula’s eastern edge. The two highest of these dunes today form the island of Isla Mujeres and the site of the ancient Maya city of Tulum.
Figure 1
The map in Figure 1 shows the distribution of dunes built during the last ice age when sea level was 600 feet lower than today. Arrows show the wind direction indicated by the shape and structure of the dunes.
Figure 2
Figure 2 is based on data from the Vostok Ice Core taken in Antarctica by a joint French-Russian team and can be used to see the timing of the two most recent glacial periods and the warm periods between. The warm period can be seen as peaks on the blue temperature curve at 140,000 years and 10,000 years. Between these times the temperature was low and carbon dioxide and methane in the atmosphere also were low compared to the warm periods.
Transgressive and Regressive Dunes
Dunes form whenever the continental shelf is exposed to the atmosphere. Winds blow the loose sand into dunes both as the sea falls, called a marine regression, and when the sea rises, it is called, you got it, a marine transgression. It follows that we should be able to divide the dunes into two groups, transgressive dunes and regressive dunes. The question is how to tell the difference.
Figure 3 shows a transgressive dune on Isla Mujeres. The cross-bedded structure is clearly shown and is common to both kinds of dunes. To see the cross-beds compare the rather thick, flat layer near the center of the photo with layers immediately below and above. The beds slope to the right, are thin and of a lighter color than the thick, flat bed. Below the thick bed, trace the thin layers to the left. Observe that they are cut off one by one by the overlying flat layer. Note the layers below the feet of the person. These too can be followed to the left where they converge toward the flat layer. Similar sloping beds can be seen near the green bush at the left side of the picture. These beds slope in the direction of the wind and are called foreset beds. The slope of the foreset beds indicate the direction of the wind, which here was toward the right of the picture.
However, there is nothing in the picture to indicate if the dune formed during the last sea level rise 140,000 years ago (transgressive), or during the sea level fall at the beginning of the last glaciation starting around 110,000 years ago (regressive).
Figure 4 provides a clue. It is a dune where the crossbeds have been disturbed by roots. The vertical, or near vertical, tube-like structures beneath the hard capping rock are sand casts of roots from vegetation that grew on the dune more than 125,000 years ago. Dr. John Mylroi, of Mississippi State University, who took this photograph, calls these “veggiemorphs”. This is a nice inclusive term that covers all the fossil roots and we don’t have to spend time trying to figure out a palm root from a shrub. The interpretation stays the same in any case and is this: The dune is regressive because if the sea was rising it would overtop the dune or erode it and the plants would have no time to take root and grow to the large extent shown in the photograph.
Figure 5 shows a feature of transgressive dunes. The picture shows a cave on the seaward side of a dune at Copal Resort on the coast south of the Tulum Ruins. This cave, partly filled with modern beach sand, is one of a string of several caves along the dune length. All are close to the same altitude and presumably were formed at the same time.
The string of “flank margin caves” is interpreted as evidence of a transgressive dune. The argument is this: the caves are dissolved out of the dune rock, the dune, obviously, must have already been present when the sea level rose and dissolution took place. Therefore, the dunes must have formed earlier during the marine transgression. This is consistent with the absence of well developed veggiemorphs.
Dune at the Tulum Ruins
Figure 6 shows a portion of the sea cliff below the Maya ruins at Tulum. If you look carefully you will see cross beds in the cliff. The set of cross beds at the right side of the photograph is particularly clear, as are layers that slope toward the left at the top of the cliff.
Well developed veggiemorphs can be seen nearby, though not in this picture. These features are clear indications that the upper part of the Tulum dune is regressive, having been built as the sea dropped 125,000 years ago. It follows that the dune was exposed to the atmosphere for the 100,000 years of the last glacial epoch and that post-glacial sea level rise has only recently (the last 3000 to 5000 years) reached the foot of the dune.
The two nearly horizontal layers with a reddish cast seen in the lower half of the cliff are interpreted as terra rosa soil horizons developed during two earlier periods of dune building These would have to be associated with the previous two glacial epochs at 140,000 years and over 230,000 years, respectively. That these soils had a long time to form and have not been eroded away, indicate two earlier periods of dune formation during the regressive part of two earlier sea level cycles (because the soils are preserved).
We might ask why regressive dunes were built repeatedly at this site, while transgressive dunes were preserved just a short distance south along the coast at Copal Resort and elsewhere. I have no answer, so will fall back on the scientist’s favorite response: This is a question for further investigation.
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