Prolific life in ancient seas

2008-06-27 00:00

Coral fronds wave gently in the ocean currents, occasionally more agitated by the passing of a larger swell nearing the shoreline. Brightly coloured fish, swimming in vast schools, dart among the coral heads, while their larger counterparts swim sedately along the drop-offs, silhouetted against the deep blue of the ocean depths. Eels lurk in the caves, making occasional forays out into clear water on the never-ending quest for prey. The reefs are encrusted with star fish and sea urchins, and the long tentacles of crayfish protrude from beneath the rocky overhangs. In the upper sunlit zones, ammonites jet their way through the water, their large, squid-like eyes taking in the details of their watery realm, their tentacles trailing behind them as they propel themselves through the warm, blue seas. Sharks proliferate, circling in schools near the surface or hunting in solitary stealth in the deep blue vastness of the oceans. A plesiosaur swims by, momentarily blocking out the darting rays of the sun as it surfaces and surveys the airy world above the waves. On the sand banks between the reefs, marine snails or gastropods and bivalves make their living on the sandy substrate.

These are Cretaceous seas — the time period stretching from 142 to 65 million years ago. It is more famously known for its dinosaurs — Tyrannosaurus Rex, Triceratops and Brachiosaurus to name just three — but life here in the oceans was no less prolific, with coral reefs covering vast areas of shallow oceans. Global sea levels were elevated during this period thanks to high global temperatures and much of the low-lying regions of the continents were covered by ocean. The term Cretaceous, which has been assigned to this geological period, derives from the Greek word for chalk. Coral reefs are made up predominantly of calcium carbonate and over time they may become preserved in the geological record as limestones and chalk beds, perhaps the most famous of which are the White Cliffs of Dover.

In South Africa, a narrow seaway formed along the eastern margins of the continent due to the ongoing movement of the tectonic plates. Tropical seas encroached upon the land from just south of Durban to north of the Mozambique border, and regions of the eastern and southern Cape coasts. Boreholes drilled into the Cretaceous sediments in Durban harbour and Richards Bay reveal thick accumulations of shells and other marine organisms. One doesn’t need to drill boreholes at St Lucia, as the Cretaceous is exposed along the lake shores and at Hell’s Gate. Fossil ammonites occur in abundance here and a fine example is to be found outside the entrance to the offices at Charters Creek. Ammonites ranged in size from a few centimetres across to massive specimens up to one metre in diameter, but, to our eternal loss, the ammonites went extinct, along with the dinosaurs, at the end of the Cretaceous and the only surviving member of the species is the nautilus – that beautifully spiralled shell which graces the shelves of tourist shops.

Cretaceous rocks form reservoirs for the entrapment of oil and much exploration has taken place along the South African coastlines since 1965. Although much of the drilling has been unfruitful in terms of oil, the borehole data has been highly instrumental in understanding Cretaceous geology. Gas reserves were found off Mossel Bay and these are currently being exploited, while ongoing exploration for oil continues on the continental shelf.

The idea that the dinosaurs were wiped out by a massive meteor impact has become part of popular culture. It is one of those cataclysmic, apocalyptic endings which we as a species seem particularly fond of, which is perhaps the reason for its mass appeal. Geological theory is based on the theory of Uniformitarianism: the processes which make our world take place over millions of years and the processes in operation today are similar to those of the past. In the early days of the science, there was a protracted battle between the Uniformitarians and the Catastrophists, who ascribed to the theory that the Earth was created by a series of catastrophic events. The Uniformitarians won in the end, but the notion of a catastrophic impact leading to the demise of the dinosaurs and marine life is part of the Catastrophist creed, and it is irony indeed that, 150 years later, geologists had to accept the role of catastrophes in the geological evolution of the Earth.

The supposed site of the impact which ended the Cretaceous Period is the Chicxulub Crater in the Yucatan Peninsula of Mexico. The jury is still out to some extent whether this is the smoking gun of mass extinction, but it is accepted that some catastrophic event — or a series of them — led to the extinctions.

• Allan Davie runs weekend geological workshops covering the fundamentals of geology and which include visits to relevant sites. He can be contacted via e-mail at geologist@netactive.co.za

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