|Photo by http://www.flickr.com/people/fanny/ used under Creative Commons license.|
The White Cliffs of Dover, in southeastern England, are certainly one of the most famous exposures of Cretaceous rocks in the world. Close to vertical in many places, and more than 100 meters or 350 feet high, the cliffs are chalk, the sedimentary product of accumulations of microscopic coccoliths, the calcium carbonate plates of coccolithophores, floating algae that must have overwhelmed the seas of Europe at times during the Cretaceous.
The chalk extends in the subsurface and near surface across much of southeastern Britain, and equivalents are found on the continent too. Most of it is of relatively late Cretaceous age, but I wanted to mention it early in the month because it is an iconic expression of chalk and known around the world. High-standing sea levels made much of Europe a shallow shelf where the algae apparently thrived. The land areas around the shallow sea were low and small, so there was not much input of clastics – sand, silt, and mud – to the depositional basin. That’s one reason the chalk is so pure.
There are black streaks in the White Cliffs of Dover, formed by strings of flint nodules. Flint is the same as chert, cryptocrystalline quartz. The source of the silica in flint and chert is debatable and may be from multiple origins. It might be that within the calcareous ooze on the sea floor, made up of all those coccoliths, there were zones where siliceous ooze was produced by, for example, accumulations of sponge spicules or perhaps from algae or other microorganisms that secreted siliceous shells as the coccolithophores secreted calcite plates. An alternative is that the silica came in later, during the lithification process that turned the sediment into rock, or even later, after the rock had solidified. Either way, the black flint nodules scattered through the white chalk make for a distinctive rock.
Besides the microscopic algae that make up most of the rock, other fossils are fairly common in the chalk of southeastern Britain, including especially echinoids or sea urchins. Shark’s teeth, brachiopods, bivalves, and crinoid stems are also present.
In the North Sea, the chalk is an important oil and gas reservoir in places, but it also served as a tight seal over older reservoirs, keeping the hydrocarbons in place. As friable as the chalk is, unless it is fractured or has undergone dissolution, the tiny coccoliths that make it up can be very tightly interlocking, reducing the permeability greatly. Permeability is the interconnectedness of the porosity or open space in a rock.
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I missed Alfred Wegener’s birthday the other day. He was born November 1, 1880, in Berlin. He worked as a meteorologist and polar scientist, but he is most remembered as the formulator of the first real theory of continental drift, in 1912. His ideas were at least tacitly accepted in Europe and South Africa by the 1930s, but rejected and ridiculed in the United States until the 1960s, when evidence from fields as diverse as paleontology and paleomagnetism began to convince scientists of the validity of the concept.
—Richard I. Gibson
Photo by http://www.flickr.com/people/fanny/ used under Creative Commons license.