Many have enjoyed reading Ric Hajovsky’s website and his books, with the latest being The TRUE History of Cozumel. Recently a National Geographic documentary on Chicxulub peaked interest on how and when the island of Cozumel was formed. The best lead for information came from an article Ric had written about Geology, Pedology, and Hydrology of Cozumel. After meeting with Ric, he was happy to share his updated article on Cozumel formation on our Cozumel My Cozumel website!
Geology, Pedology, and Hydrology of Cozumel Copyright 2019, Ric Hajovsky
The Yucatan peninsula is a karstic terrain (meaning partially-dissolved carbonate bedrock) made up of marine calcareous sediments (limestone) lain down during the Jurassic period between 200 and 145 million years ago. This layer of limestone was later overlain with a second 1,000-meter-thick layer of calcareous marine deposits to the seabed during the Cretaceous. Still later, another 15 meters was deposited on top of that during the Miocene and Pliocene.
The limestone of Cozumel Island’s surface was laid down during two different periods of submergence during the Pleistocene. Two older Pleistocene, ten late Pleistocene and one Holocene shallow-water depositional layers are present. The two older Pleistocene layers are made up of the remnants of patch reefs and sandy banks. The younger late Pleistocene layers are remnants of reefs, reef passes, and a seagrass-stabilized backreef, oolite shoal, storm channel, beach and dune environment. Beachrock deposited during the Holocene is also found on the island.
Around 66 million years ago, an asteroid estimated to be around 6 miles in diameter struck the ocean surface just offshore from the present-day town of Chicxulub, Yucatan, with the force of 96 teratons of TNT (4×1023Joules), creating a crater in the seabed 122 miles in diameter. The ejecta from this crater are currently theorized to be the reason for the abrupt climate change that killed off the dinosaurs at the end of the CretaceousPeriod. The impact also shook the seabed (an area that would later become the Yucatan peninsula when the sea levels fell millions of years later) so hard that two other things happened: One was that the roofs of many nearby undersea caverns collapsed, creating a ring of blue-holes (cenotes) around the perimeter of the crater, and; The block of land that was later to become the island of Cozumel was squeezed up from between two fault lines while the surrounding sea bed subsided, resulting in a horst-block formation, or a table of uplifted seabed.
the upper surface of this block was still below sea level at the time it was
extruded, later sea level fluctuations exposed the top surface of the block and
it became what we now call Cozumel Island.
the Pleistocene, the sea level rose and fell twice, depending on the amount of
the Earth’s water that was sequestered in the glaciers that covered most of
Northern America, Europe, Asia and the southern parts of South America and
Africa at the time. Twice the island was
covered by water (the last time was 125,000 years ago) and twice during this
period, the sea level dropped to around 130 meters (422 feet) lower than it is
today. That put the surface of Cozumel
and the adjacent Quintana Roo coast around 130 meters above sea-level during
those ice ages and that meant the island was surrounded by sheer cliffs on all
sides. During that time, the rainwater
that fell on the Yucatan peninsula and Cozumel percolated through the exposed, porous
limestone, dissolving cracks and fissures that enlarged over millennia into the
karst topography of deep caves, caverns and sinkholes.
The peninsula today is a tectonically stable
platform. No movement of the
island relative to the mainland has occurred in the past 125,000 years.
The thin covering of soil on the island is mainly rendzina, or humus rich soil made up of weathered limestone. The name originates from a Polish colloquial word “rzędzić,” which means “to chat:” the soil of this type contains a significant amount of gravel and stones, which, during plowing, produce various sound effects (clicking, screeching, etc.) so that they seem to “talk” to the ploughman. Areas near the beaches on Cozumel are mostly regolsol, or a weakly developed mineral soil made up of unconsolidated material, in this case eroded calcareous sandstone. Layers of sascap, (unconsolidated rock, or aragonite that never crystalized into calcite) can be found interspersed between layers of bedrock and are often exposed in open quarries.
Because of the many
pores, cracks, faults, cavities, and caverns in the limestone bedrock and the high
porosity of the surface of Cozumel, rain water quickly percolates down to
underground aquifers before having a chance to form surface streams, rivers or
lakes. This lower-density freshwater
rides on top of the denser saline groundwater, forming a freshwater lens, or a
karstic aquifer. This freshwater lens is
deeper in the center of the island and shallower towards the edges of the
island. Most of Cozumel’s water comes
from water wells in the center of the island that tap into this freshwater
lens. The freshwater lens shrinks and
expands in relationship with the rainy season and dry season.
When the percolation of water down through the bedrock erodes the surface enough to cause it to collapse into the underlying karstic caverns, it forms a sinkhole, or cenote, a word from Yucatec Mayadzonot or ts’onot, meaning “well.” Many of the cenotes in Cozumel are connected together via an underground anchialine cave system (a system that connects the freshwater terrestrial cave system with a saltwater marine cave system) that has been mapped sufficiently to show that it is the fifth largest such system in the world.