Times of the Islands Summer 2016

green pages newsletter of the department of environment & maritime affairs
the calcium carbonate is released, much of it as sandsized
particles that eventually wash landward to become
beaches. Once on the beach, the wind can sweep up these
sands to make dunes, called eolian calcarenites, which
means literally “windblown calcium carbonate sand.”
All land in the TCI above about 6 m (20 ft) is eolian
calcarenite. In the lagoon, sands are deposited along with
the coral reefs. When sea level is higher than today, as
it was 120,000 years ago during the previous interglacial,
these lagoon deposits were placed where we can see
them now, as what was under shallow water then is above
sea level today. Fossil reefs and related lagoon features
are found all over the TCI a few meters above modern sea
level. These all formed 120,000 years ago, a time called
the last interglacial.
On the exposed land existing today, and across the
entire exposed banks when sea level is low during a glaciation,
soils form. The TCI is a 100% calcium carbonate
environment (NaCl, halite, and CaSO 4 •2H 2 O, gypsum,
called evaporate minerals, are sometimes found). The
soils are red, brown or tan because they contain iron
oxides, plus other oxides such as Al 2 O 3 , that arrive on
the bank as wind-blown dust all the way from the Sahara
Desert. When these soils become fossilized, they turn
into a very hard and dense crust called a terra rossa
paleosol. These paleosols cover all the rocks that formed
on interglacials older than today. The young eolian calcarenites
that have formed in the past few thousand years
of the current interglacial are too young to have collected
significant Saharan dust, and so lack a thick red soil. The
coastal rocks of the cays from Little Water Cay north to
Parrot Cay are all eolian calcarenites and related rocks
less than 5,000 years old.
The geology of the TCI is simple. When sea level is
high and the bank tops are flooded, a variety of limestones
are made. When sea level is low, for a duration ten
times longer than for sea level highstands, the main geological
events are erosion and soil formation. There have
been ten or more cycles of glaciation and interglacials
during the last two million years, and the rocks produced
by the last three or four cycles are preserved as dry land
in the TCI today. The only fossil reefs and related lagoon
deposits found today formed 120,000 years ago. Such
deposits from earlier high sea levels have not been found.
Either the sea level wasn’t above modern, the platform has
subsided slightly, or those deposits have eroded away.
Top: Eolianite can be seen in this road cut near Lorimers, Middle
Caicos. Bottom: This close-up shows eolianite in a road cut near Long
Bay, Providenciales.
Top: This fossil coral was found on the Crossing Place Trail in Middle
Caicos. Bottom: This close-up shows fossil conch shells.
22 www.timespub.tc