The higher terrace – home to the Hato Caves

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On entering the Hato Caves, you also visit the ‘insides’ of a fossil coral reef. This we already mentioned before. But there is much more to the story. Let’s investigate these fossilized remains a little more in detail.

The modern reef system

Apart from looking at fossils of reef organisms, it is always a good thing to take a look at the modern day coral reefs around the Island as well, as a reference. Today the Island is basically surrounded by what is known as a fringing reef, a narrow zone of coral growth along the submarine slopes around the Island. Reef building corals occur up to water depths of 60 meters or so, because the coral polyps live in close cooperation with algae and these need sunlight to survive. The reefs start in shallow water, but are most lush along what is known as the drop-off zone, the steep slope that at 80 meters depth flattens out a little. In fact, you could visualize Curacao as the top of a mountain rising up from the depths with fairly steep slopes. And only the upper 60 meters before reaching the surface are home to the coral reefs we admire so much.

Massive fossil Elkhorn coral fragments

The comparison

Over time a fringing reef only produces coral deposits of a few tens of meters wide, at most. This is totally distinct from the lower terrace along the north coast, which consists of fossilized reef remains of about 50.000-25.000 years old. This terrace is quite wide, even up to almost 600 meters in places. One thing is clear, we can not visualize the way this terrace has been shaped by comparing it to the modern fringing reefs. Fossils to the rescue!
The lower terrace is relatively young, so most information is still in place. The older the rocks are, the more they have been altered by processes of deformation, weathering and erosion. The lower terrace is therefore the perfect candidate for a paleontological reconstruction. This has been done by several scientists who worked on these deposits, and the first thing they noticed were the fossils of massive Elkhorn Corals (Acropora palmata) within the rocks closest to the sea. Further inland they found fossils of corals like the Staghorn Coral (Acropora cervicornis). Why is this significant?

Fragile Staghorn coral remains

The reconstruction

Elkhorn corals like to live in a high energy environment, with heavy wave action. Staghorn corals, on the other hand, would be beaten to pieces in such an environment, so this species requires a low energy environment. This fact, together with additional significant information like the shape of the former coastline, which is preserved as a deep undercut ledge along the foot of the middle terrace, being totally different from the current water edge, led the researchers to a surprising conclusion: during the period the lower terrace was deposited the coastline looked like a barrier reef with a relatively wide, but shallow lagoon behind it. The barrier reef, which was constructed by the massive Elkhorn corals, protected the lagoon against the fury of the sea, and that’s why the Staghorn corals could survive (together with lots of other marine species, like the Queen Conch – Strombus gigas).

Generalized cross-section of the Pleistocene limestone terraces of Curacao

Further analysis of the older terraces on the Island showed, that this was also the case during the times that those terraces were deposited, but at different levels, of course. Only the middle terrace II is different, because this one was formed during an intermediate phase of rapid sea level fluctuations which were too swift (geologically speaking) to allow a barrier reef to form. The fact that nowadays we don’t have a barrier reef as well is probably due to a way too rapid sea level rise after the last glacial period. A pity, really, because those lagoons must have been beautiful!

Areal picture of the lower (left), middle and higher terrace. The oval structure on top of the middle terrace is a fossilized dune system which was active during the deposition of the lower terrace.

The higher terrace

The higher terrace, home of the Hato Caves, has an age of at least 900.000-1.2 million years. Geologically speaking this is not that old. But limestone weathers and erodes pretty easily and fast, so a significant part of the originally wide terrace has been removed over the years. The entire barrier reef zone is gone. That means that a visit to the Hato Caves will lead you through the former lagoon deposits. Enjoy, and don’t forget to ask the guide to show you the fossil of the marine gastropod (snail), one of the species that thrived in the ancient shallow lagoon!

Leon Pors

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