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For example, chalk is made up partly of the microscopic calcium carbonate skeletons of marine plankton, the deposition of which induced chemical processes to deposit further calcium carbonate. Similarly, the formation of coal begins with the deposition of organic material, mainly from plants, in anaerobic conditions.
Chalk is a soft, white, porous ... or aragonite that rapidly converted to the more stable low-magnesium calcite after deposition, ... are a notable example of ancient ...
The Chalk Group is now divided into a White Chalk Subgroup and a Grey Chalk Subgroup, both of which are further subdivided into formations.These modern divisions replace numerous earlier divisions, references to which occur widely on geological maps and in other geological literature.
The area consists of a layer of fine sand approximately 82 ft (25 m) deep resting on a chalk platform belonging to the same geological feature that incorporates the White Cliffs of Dover. More than 2,000 ships are believed to have been wrecked on the Goodwin Sands because they lie close to the major shipping lanes through the Straits of Dover .
The putative galloanseran bird Austinornis lentus has been found in the Austin Chalk. [2] [3] The general absence of dinosaurs is a reflection of the Austin limestone being marine in origin, primarily composed of microscopic shell fragments from floating sea organisms known as "coccolithophores" (the same organisms that contributed to the White Cliffs of Dover, on the south coast of England). [4]
Chalk – Soft carbonate rock; Chert – Hard, fine-grained sedimentary rock composed of cryptocrystalline silica; Claystone – Clastic sedimentary rock composed primarily of clay-sized particles; Coal – Combustible sedimentary rock composed primarily of carbon; Conglomerate – Sedimentary rock composed of smaller rock fragments
Pachyrhizodus caninus skeletons (casts) Pseudoperna congesta fossil oysters encrusting a large Platyceramus platinus bivalve shell, Smoky Hill Chalk member of Niobrara Formation. Photo in place at Castle Rock chalk badlands, Kansas. During the time of the deposition of the Niobrara Chalk, much life inhabited the seas of the Western Interior Seaway.
Niobrara Chalk was weathered and opalized in the Valentine phase of the Ogallala Formation. The Niobrara Formation / ˌ n aɪ. ə ˈ b r ær ə /, also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late ...