The fossil
forests of Joggins, Nova Scotia, have baffled and amazed scientists since geologist
Sir Charles Lyell visited in 1842. Lyell called the cliffs where the
giant tides of the Bay of Fundy are continually revealing the Carboniferous
world of 300 million years ago the most wonderful phenomenon perhaps
that I have ever seen.
The giant Joggins cliffs in Nova Scotia,
which are world famous for their expansive buried fossilized forests, dwarf
people walking along its beaches. Some geologists now think they know how the
trees became preserved in the first place, some 300 million years ago. Courtesy
of Michael Rygel.
The up-to-6-meter-tall (19 feet) fossil trees that were buried and preserved
intact, as well as 76 coal seams, lie in the Joggins Formation, part of the
Joggins Section, one of the thickest Carboniferous-aged sections in the world.
Oddly, the rapidly deposited section 4.2 kilometers of sediment piled
up in just 4 million years also changed substantially in thickness, an
indication of subsidence.
The dramatic thinning, combined with sediment deposition rates rarely seen on
land, left geologists puzzled about what drove the basin to sink steadily and
repeatedly, burying 63 layers of fossilized trees in the process. Now researchers
say that they have found the answers to some of the mysteries of the formation
in a simple substance: salt.
The fact that there are standing trees is not terribly unique; whats
unique is that it happened more than 60 times, says sedimentologist Michael
Rygel of the University of Nebraska in Lincoln. We needed a way to explain
how the conditions stayed just right for millions of years.
Geologists first suggested the involvement of the underlying Mississippian-aged
salt decades ago, but the high-quality seismic data needed to investigate it
only recently became available. Rygel and John Waldron, a structural geologist
at the University of Alberta in Edmonton, turned to seismic lines recorded in
2002 by Devon Canada, an oil company prospecting in the Cumberland Basin.
We can show, from the seismic profiles, that those areas were actually
subsiding because salt was flowing out from underneath them, says Waldron,
who reported the results with Rygel in the May Geology.
The seismic profiles show a salt weld beneath Joggins,
he says. When sediments are deposited on top of salt, the added weight squeezes
the salt out, Waldron says, and the newly deposited layers then weld to older
sediments that had underlain the salt.
Additionally, the new study shows how salt, usually associated with the trapping
of oil deposits after they have formed, could be related to the formation of
coal. A basin needs to be sinking rapidly enough to allow plant layers to accumulate,
but not so rapidly that the layers drown in deep water conditions that
would be produced by the removal of salt layers, the researchers say. To
the best of our knowledge, this is the first time that salt withdrawal and coal
formation have been linked, Rygel says.
Until now, it was a bit of a mystery how such a thick and complete sedimentary
succession accumulated in a relatively short interval of time, says Howard
Falcon-Lang, a paleobotanist at the University of Bristol, United Kingdom, who
has studied the fossil trees of Joggins. What the Waldron and Rygel paper
does is provide a mechanism for creating that huge amount of accommodation space
needed for all those sediments to accumulate.
The Joggins site, described in Lyells 1872 Principles of Geology
(a book that helped found the field of modern geology) and in Darwins
1859 On the Origin of Species, is particularly famous for fossilized
hollow trunks, inside which were found the oldest known fossils of true reptiles,
the first four-legged creatures to lay eggs on dry land.
Because of this historic and geologic significance, Canada is asking the United
Nations to declare the Joggins section as the UNESCO World Heritage Site to
represent the Carboniferous, which Rygel says the new research supports. This
will add to a growing body of literature that proves that this is an important
place.
Falcon-Lang agrees, saying that the new study explains how such a complete
and important succession could build up. So its quite fundamental,
he says, from the point of view of the preservation and formation of this
very unusual window on the Carboniferous.
Sara Pratt
Geotimes contributing writer
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