The Chazy Reef Formation is a mid- Ordovician limestone deposit in northeastern

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

. It consists of some of the oldest reef systems built by a community of organisms rather than the deposit of a limited range of similar organisms, such as Stromatolite mounds deposited by ancient cyanobacteria. The reef structure was formed largely by cryptostome and trepostome bryozoa, some of the oldest known bryozoans, but corals made an early appearance, and

stromatoporoid Stromatoporoidea is an extinct clade of sea sponges common in the fossil record from the Ordovician through the Devonian. They were especially abundant and important reef-formers in the Silurian and most of the Devonian.Stock, C.W. 2001, Stro ...

s. The formation is named for the small town of Chazy, New York, where the reef was noted by James Hall in ''Palaeontology of New York'' (vol. I, 1847) and the fossils first studied by the Canadian paleontologist Elkanah Billings (1858, 1859). The reef extends from Tennessee to Quebec and

Newfoundland Newfoundland and Labrador (; french: Terre-Neuve-et-Labrador; frequently abbreviated as NL) is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region ...

, but its most easily studied outcropping is at Goodsell Ridge, Isle La Motte, the northernmost island in Lake Champlain; there, gentle uplift has tilted the sediments: the

bedding plane In geology, a bed is a layer of sediment, sedimentary rock, or pyroclastic material "bounded above and below by more or less well-defined bedding surfaces".Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' ...

s now dip slightly to the north, revealing sequences of horizons in exposed rock. The black limestone of Isle La Motte takes a polish, revealing the white "bird's-eye" markings of embedded fossil shells, notably the spirals formed by sliced

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

shells. Rock of the Chazy Formation was quarried from the nineteenth century at the Fisk Quarry, Isle La Motte, the oldest quarry in Vermont.The blackest stone may be seen in the polished "marble" revetments—actually unmetamorphosed limestone— inside

Radio City Music Hall Radio City Music Hall is an entertainment venue and Theater (structure), theater at 1260 Sixth Avenue (Manhattan), Avenue of the Americas, within Rockefeller Center, in the Midtown Manhattan neighborhood of New York City. Nicknamed "The Showplac ...

, New York, and in floors in the Vermont State House. Portions of the exposed reef on several islands in Lake Champlain were dedicated as the Chazy Fossil Reef National Natural Landmark in 2009. The system formed in warm tropical waters of the

Iapetus Ocean The Iapetus Ocean (; ) was an ocean that existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale (between 600 and 400 million years ago). The Iapetus Ocean was situated in the southern hemisphere, between the paleoco ...

, in low latitudes of the Southern Hemisphere. The site was in shallow waters of a

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...

: the continental craton of Laurentia lay to the west, beyond the lagoon system that formed behind the protective reef. The time was some 480-450 Ma (million years ago), at a period when global paleoclimate was so warm that the planet was all but ice-free. Atmospheric carbon dioxide was fourteen to sixteen times more plentiful than it is today. The Chazy Reef Formation, which built up vertically from a muddy base catching fine dark silt as it grew, began as mounds deposited by bryozoans that stabilized a muddy bottom, then built up into the water column to such an extent that the connected mounds modified their surrounding environment (Mehrtens). As the reef aged, it began to offer an increasing variety of ecological niches, which fostered the first rich local biodiversity that has characterized all reef systems ever since. One dominant reef-building organism took the place of another, in a slowly evolving

faunal succession The principle of faunal succession, also known as the law of faunal succession, is based on the observation that sedimentary rock strata contain fossilized flora and fauna, and that these fossils succeed each other vertically in a specific, r ...

. The Chazy Reef Formation has given its name to the contemporaneous mid-Ordovician faunal stage, the Chazyan.

Notes

External links

(Lamotte Preservation Trust) The Chazy Reef on Isle La Motte
* ttp://www.lcbp.org/wayside/PDFS/Chazy_Reef/Chazy_Reef.pdf Chazy Reef Formation(pdf file)

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Further reading