Encrinite

Encrinite within the Carmel Formation (Middle Jurassic) exposed at Mt. Carmel Junction, Utah. This is one of the youngest encrinites.

Encrinite (crinoidal limestone) from the Mississippian Ft. Payne Formation of southern Kentucky

Example encrinite from the Silurian Brassfield Formation of Ohio. Note different sizes of ossicles present.

In its modern usage, which was established by Bissell and Chilingar, ^[1] an encrinite is a crinoidal limestone containing crinoid ossicles in excess of 50 percent of the bulk of the rock. In terms of Dunham's classification ^[2] of carbonate sedimentary rocks, it could be either a crinoidal packstone or crinoidal grainstone. In terms of Folk's classification ^[3] of carbonate sedimentary rocks, an encrinite would be either a crinoidal biomicrite, crinoidal biosparite, crinoidal biomicrudite, or crinoidal biosparudite. In older literature, the word is sometimes used to refer to a crinoid, especially a fossil crinoid belonging to or like one belonging to the genus Encrinus . However, this usage is obsolete. ^[4]

Ausich ^[4] regards the term encrinite as being of very limited usefulness in modern sedimentology. Although it certainly describes a specific type of sedimentary rock, it ignores specific characteristics of crinoid-rich limestones, e.g. whether it has a mud matrix or spar cement, that are essential to recognizing their facies and origins. As a result, the term encrinite lacks usefulness in the current classification and nomenclature of sedimentary rocks and is, at best, a relict of the early nomenclatural history of sedimentology. ^[4]

Types of encrinites

Depending on their areal extent and thickness, the accumulation of encrinites can be subdivided into either "regional encrinites" ^[5] or "local encrinites". ^[6] A regional encrinite is an extensive accumulation of limestone that is at least 50% pelmatozoan ossicles at least 5 to 10 m (16 to 33 ft) thick and has an areal extent of at least 500 km² (190 sq mi). By definition, crinoid ossicles are the predominate component of encrinites. However, the ossicles of other pelmatozoans, e.g., rhombiferans or diploporans during the Ordovician and Silurian or [[[blastoid]]s during the Lower Mississippian, can locally be the predominate bioclast. The classic example of an encrinite is the Burlington and Keokuk limestones of Iowa, Missouri, and Illinois. Additional examples of geologic formations that contain regional encrinites are the Lower Muschelkalk, Switzerland (Triassic); Redwall Limestone, Arizona (Mississippian); Coeymans, Keyser, and New Creek limestones, Pennsylvania (Devonian); Edgecliff Limestone, New York (Devonian); Brassfield Formation, Ohio (Silurian); and Chicotte Formation, Anticosti Island, Quebec (Silurian). Regional encrinites are only found in the stratigraphic record from the Ordovician through the Jurassic. They are most abundant in Lower Mississippian strata. ^{[4] [5] [7]} Encrinites which do not meet the criteria of regional encrinites are termed local encrinites. They typically occur as individual lenses or beds 2 to 5 m (6.6 to 16.4 ft) thick of very limited lateral extent. ^[6]

Regional encrinites accumulated where the offshore continental shelf was nearly completely dominated by crinoids (and in some cases other stalked echinoderms) for extensive periods of time. At the time of the accumulation of encrinites, the seafloor of the shallow continental sea consisted entirely of a coarse, poorly sorted pelmatozoans bioclasts lying within the storm-wave base. Because of the low-density and coarse nature of these bioclasts, the sea floor was episodically mobile during periodic storms. This made the sea floor an unfavorable habitat for most non-pelmatozoan benthic organisms. However, this type of seafloor was it was well suited for pelmatozoans to anchor their rhizoid holdfasts. This created a positive taphonomic feedback in which the disarticulation of pelmatozoans provided abundant bioclastic debris that created a seafloor that encouraged the establishment and growth of pelmatozoans, but discouraged the establishment and growth of other benthic organisms. ^{[4] [5] [7]}

Distribution

Encrinites form in areas where disaggregated crinoid skeletal debris becomes concentrated, typically shallow warm euhaline seas. They are therefore very common in the Paleozoic rock record of North America and Eurasia, particularly during the Silurian through Early Devonian and the Early Mississippian age, when high sea levels created widespread epeiric seas. Some encrinites are also known from the Jurassic of North America and Western Europe. ^[5]

See also

Fossil hash

References

1. Bissell, H.J., and Chilingar, G.V., 1967. Classification of sedimentary carbonate rocks. In Chilingar, G.V., Bissell, H.J., and Fairbridge, R.W., eds. pp. 87–168. Carbonate Rocks Origin, Occurrence and Classification. Developments in Sedimentology, vol. 9. part A. Amsterdam, Elsevier, 471 pp. ISBN   978-0-444-53315-9
2. Dunham, R.J., 1962. Dunham classification of carbonate rocks according to depositional texture. In Ham, W.E., ed., pp. 108–121. Classification of Carbonate Rocks--A Symposium, American Association of Petroleum Geologists Memoir, vol. 1. Tulsa, Oklahoma, American Association of Petroleum Geologists. 279 pp. ISBN   978-1-629-81236-6
3. Folk, R.L., 1959. Practical petrographic classification of limestones. American Association of Petroleum Geologists Bulletin, 43(1), pp.1-38.
4. Ausich, W.I., 2003. Encrinites. In Middleton, G.V., Church, M.J., Coniglio, M., Hardie, L.A., Longstaffe, F.J. eds., Encyclopedia of Sediments and Sedimentary Rocks. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. ISBN   978-1-402-00872-6
5. Ausich, W.I., 1997. Regional encrinites: a vanished lithofacies. In Brett, C.E., and Baird, G.C., eds., pp. 509–519. Paleontological Events Stratigraphic, Ecological, and Evolutionary Implications. New York, New York, Columbia University Press, 604 pp. ISBN   978-0-231-08250-1
6. Hunter, A.W. and Clark, N.D., 2009. The palaeoecology of two Scottish encrinites: Jurassic crinoid assemblages from the Trotternish Peninsula, Isle of Skye, Scotland. Scottish Journal of Geology , 45(2), pp.169-176.
7. Ausich, W.I., 2021. Disarticulation and preservation of fossil echinoderm: recognition of ecological-time information in the echinoderm fossil record. Elements of Paleontology Series. Cambridge, United Kingdom, Cambridge University Press, 50 pp. ISBN   978-1-108-89337-4