Shelf sponge reefs: A seafloor Jurassic Park
If you were able to dive in a submersible on the continental shelf in the ocean that used to cover much of southern Europe in water depths between 50 and 150m you would see reefs that looked a lot like the sponge reefs we see today on the BC shelf. The seabed in these areas would have been carbonate (limestone) or marl - that is a limestone with some mud added. The time of greatest distribution of the glass sponge reefs was the late Jurassic - about 145 million years ago. The ocean covering this part of the world was known as the Tethys and in the Jurassic the continents looked quite different. Abundant organisms from that period were many species of Ammonites, which are often found in the fossilized sponge reef rock formations, and marine reptiles such as pleisiosaurs that also swam in the late Jurassic seas.
Paleogeology
| Era | Age | Organisms |
|---|---|---|
| Late Devonian | 355-330 Ma | First representatives of the subclass Hexactinosa (group of sponges which include the BC reef builders appear) |
| Mid Triassic | 235-220 Ma | First hexactinellid sponge reefs in Poland, decimetre scale in size |
| Late Triassic | 220-206 Ma | First larger siliceous sponge reefs in China, up to 10m in size, very similar to those in Jurassic |
| Early Jurassic | 206-185 Ma | Sponge reefs found on southern margin of Tethys Sea |
| Mid Jurassic | 185-165 Ma | Bioherms more widespread (India, Iran, Spain) |
| Late Jurassic | 165-144 Ma | Sponge reef facies distribution facies culminated on northern shelf of Tethys Sea and adjacent North Atlantic basins forming discontinuous deep water reef belt over 7000km long; found in Romania, Poland, Germany, Switzerland, France, Spain, Portugal, Caucasian Mountains, off Newfoundland, Oklahoma, etc. Largest bioconstruction ever built on earth! |
| Cretaceous | 144-65 Ma | restricted to Central and Western Europe; secondary to rudist and coral reefs |
| Post-Cretaceous | 65Ma- | Distribution of siliceous sponge reefs declines |
| Today | Thought to have completely disappeared until 1987-88 when towed high-resolution sub-bottom geophysical profiling and seafloor sampling led to their discovery off the West coast of British Columbia, Canada; base of oldest sponge reef ~9000 years bp |
