The Bonneterre Formation is an

Upper Cambrian The Furongian is the fourth and final epoch and series of the Cambrian. It lasted from to million years ago. It succeeds the Miaolingian series of the Cambrian and precedes the Lower Ordovician Tremadocian Stage. It is subdivided into three sta ...

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

which outcrops in the

St. Francois Mountains The St. Francois Mountains in southeast Missouri are a mountain range of Precambrian igneous rock, igneous mountains rising over the The Ozarks, Ozark Plateau. This range is one of the oldest exposures of igneous rock, igneous rock (geology), rock ...

of the

Missouri Missouri (''see #Etymology and pronunciation, pronunciation'') is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it border ...

Ozarks The Ozarks, also known as the Ozark Mountains, Ozark Highlands or Ozark Plateau, is a physiographic region in the U.S. states of Missouri, Arkansas, and Oklahoma, as well as a small area in the southeastern corner of Kansas. The Ozarks cover ...

. The Bonneterre is a major host rock for the

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

ore Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...

s of the Missouri Lead Belt.

Description

The formation is dominantly dolomite with areas or layers of pure

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

. A

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

y or

glauconitic Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color which is very friable and has very low weathering resistance. It crystallizes with a monoclinic geometry. Its name is derived from the Greek () m ...

zone occurs in the lower portion and the base contains sand and conglomerate or breccia where the formation overlaps the Lamotte and lies directly on the

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

of the mountain core.

Stratigraphy

Early geologists offered a variety of names for what is now known as the Bonneterre Formation. In 1894, Missouri state geologist Arthur Winslow proposed St. Francois limestone as a name for thick limestone beds, including everything between what are now known as the Lamotte Sandstone and the St. Peter Sandstone. He described the lower part of that formation (now comprising the Bonneterre and the Elvins Group) separately as the St. Joseph limestone. Charles Rollin Keyes's Fredericktown limestone included everything between the Lamotte and the

Potosi Dolomite The Potosi Formation is a geologic formation in Missouri, Illinois and Indiana. It preserves fossils dating back to the Cambrian period. Paleofauna Monoplacophora * '' Gayneoconus'' : ''G. echolsi'' See also * List of fossiliferous stratigraph ...

when he first described it in 1896, but his later uses of the name were in a more restricted sense equivalent to the modern Bonneterre. In 1901, Frank Lewis Nason was the first to apply the name Bonneterre (originally spelled Bonne Terre) to these rocks, identifying a

type section In geology, a stratotype or type section is the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. If the stratigraphic unit is layered, it is called a stratotype, whereas the ...

near the city of

Bonne Terre, Missouri Bonne Terre ( ) is a city in St. Francois County, Missouri, St. Francois County, Missouri, United States with a population of 6,864 at the 2010 United States Census, 2010 census. Situated in the Southeast Missouri Lead District, lead mining shape ...

Contacts

The Bonneterre is conformably overlain by the Davis Formation. The Bonneterre Formation lies conformably on the Lamotte Sandstone and in places lies directly on the

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

core of the mountains.

Thickness

In the outcrop area the Bonneterre has an average thickness of 375 to 400 feet. It is present in the subsurface throughout Missouri and has a maximum recorded thickness of 1580 feet under Pemiscot County in the

Missouri Bootheel The Missouri Bootheel is a Salient (geography), salient (protrusion) located in the southeasternmost part of the U.S. state of Missouri, extending south of 36°30′ north latitude, so called because its shape in relation to the rest of the sta ...

Fossils

The dolomites and limestones of the Bonneterre Formation contain fossils of late Cambrian invertebrates. Algal

stromatolites Stromatolites ( ) or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria) ...

echinoderms An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larv ...

, and microfossils such as ''

Girvanella ''Girvanella'' is a fossil thought to represent the calcified sheath of a filamentous cyanobacterium known from the Burgess Shale and other Cambrian fossil deposits. Specimens are also known from the Early Ordovician San Juan Formation, Argentin ...

'' are associated with fringing reef paleoecosystems surrounding the Cambrian islands that are now the

. A variety of

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

fossils have been reported from the Bonneterre, including '' Coosella'', '' Holcacephalus'', '' Meteoraspis'', '' Tricrepicephalus'', and '' Welleraspis''. Several

monoplacophora Shell of Monoplacophora Monoplacophora , meaning "bearing one plate", is a polyphyletic class of molluscs with a cap-like shell, inhabiting deep sea environments. Extant representatives were not recognized as such until 1952; previously they wer ...

ns have also been described from the Bonneterre; one, '' Hypseloconus bonneterrense'' is named for the formation.

References

Bibliography

* * * * * * * {{Chronostratigraphy of Missouri Geologic formations of Illinois Geologic formations of Minnesota Geologic formations of Missouri Cambrian System of North America Cambrian Missouri Cambrian Minnesota Cambrian Illinois Guzhangian Dolomite formations of the United States Reef deposits Cambrian northern paleotemperate deposits Paleontology in Missouri Landforms of the Ozarks St. Francois Mountains Shale formations of the United States Limestone formations of the United States Sandstone formations of the United States