File:Chalk (Niobrara Formation, Upper Cretaceous; Great Plains, USA) 1.jpg

English: Chalk from the Cretaceous of the American Great Plains. (outdoor public display, Ashfall Fossil Beds State Historical Park, Nebraska, USA)

Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by lithification (also known as diagenesis), which involves deposition, burial, compaction, dewatering, and cementation. Most sedimentary rocks have a clastic texture, but some are crystalline.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions.

Limestone is a common biogenic sedimentary rock composed of the mineral calcite (CaCO3), which bubbles in acid. Many geologically young limestones are composed of aragonite (also CaCO3). Numerous varieties of limestone exist - e.g., fine-grained limestone/micritic limestone/lime mudstone, coquina, chalk, wackestone, packstone, grainstone, rudstone, rubblestone, coralstone, calcarenite, calcisiltite, calcilutite, calcirudite, floatstone, boundstone, framestone, oolitic limestone, oncolitic limestone, etc. Most limestones represent deposition in ancient warm, shallow ocean environments.

Chalk is distinctive variety of limestone that is soft, whitish, and powdery. The most spectacular chalk locality on Earth is the White Cliffs of Dover (farm1.static.flickr.com/119/290719612_5a27cbaf61.jpg), along the southern shores of Britain. The rocks there are Cretaceous in age (“creta” means “chalk”).

Chalk is a biogenic sedimentary rock, but it is not obvious how this white powdery material represents the remains of once-living organisms. When examined under a scanning electron microscope, chalk powder is seen to be composed of exceedingly small microfossils, principally coccoliths (www.soes.soton.ac.uk/staff/tt/eh/pics/lith2.gif). Coccoliths are calcitic plates that once covered a living cell (upload.wikimedia.org/wikipedia/commons/d/d9/Emiliania_hux...). The cell was an organism called a coccolithophorid (Kingdom Protista, Phylum Chrysophyta, Class Coccolithophorida). Coccolithophorids are unicellular, photosynthetic organisms. They are often called “algae”, but they’re better called photosynthetic protists. When they die, the cell degrades and the hard calcitic plates covering the cell fall to the ocean bottom.

Chalk forms in moderately deep marine environments, but it cannot form in the deepest ocean depths. In chalk-forming facies, abundant coccolith plates can accumulate on seafloors as sediments - "calcareous ooze". Such facies require no calcite dissolution and no significant dilution by muddy or sandy sediments washed in from the continents.

Stratigraphy: Niobrara Formation, Upper Cretaceous

Locality: undisclosed, but apparently derived from the American Great Plains, possibly Nebraska