TisILeclerc wrote:Some people think that 'chalk is weird' so perhaps others are out there wondering about it. Perhaps it's like the fish in water who doesn't know what water is all about?
Agreed, chalk is weird for a rock, but the flint in the chalk is even weirder. No-one seems to know where flint really came from, just that "it's there", with lots of assumptions that it was formed by some kind of marine lifeform, but there's bugger-all evidence for a what that lifeform might have been.
Flint is chert that occurs in chalk, that is to say cryptocrystalline silica, black, shiny and with conchoidal fractures. It is extremely common, but no-one really knows how it forms. The silica doesn’t come from sand like in normal sediments, oh no, it is thought to come from sponge spicules, diatoms and other biological sources. It is formed somehow during diagenesis. Often it infills burrows or surrounds fossils, suggesting a role for micro-environments with unusual (weird?) chemistry that allow the silica to precipitate out as a gel. Sometimes soft-sediment deformation is seen to deform flints, so they are soft during early stages of diagenesis. Also flint sometimes infills early faults/fractures to form sheet flints. It can also directly replace chalk, rather than filling cavities. As a resistant erosional product, flint is ubiquitous in Southern England (it forms the gravel drive of my house) yet nobody really knows how it forms.