HOME

TheInfoList



OR:

Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor (Wilson and Palmer, 1992). A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in
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) ...
sequences and distinguished from later-lithified sediments by evidence of exposure to normal marine waters. This evidence can consist of encrusting marine organisms (especially
bryozoans Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...
,
oysters Oyster is the common name for a number of different families of Seawater, salt-water bivalve molluscs that live in Marine (ocean), marine or Brackish water, brackish habitats. In some species, the valves are highly Calcification, calcified, a ...
,
barnacles Barnacles are arthropods of the subclass Cirripedia in the subphylum Crustacea. They are related to crabs and lobsters, with similar nauplius larvae. Barnacles are exclusively marine invertebrates; many species live in shallow and tidal water ...
, cornulitids, hederelloids, microconchids and crinoids), borings of organisms produced through
bioerosion Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, ...
, early marine calcite cements, or extensive surfaces mineralized by iron oxides or calcium phosphates (Palmer, 1982; Bodenbender et al., 1989; Vinn and Wilson, 2010; Vinn and Toom, 2015). Modern hardgrounds are usually detected by sounding in shallow water or through
remote sensing Remote sensing is the acquisition of information about an physical object, object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring inform ...
techniques like side-scan sonar. Carbonate hardgrounds often host a unique fauna and flora adapted to the hard surface. Organisms usually cement themselves to the substrate and live as sessile filter-feeders (Brett and Liddell, 1982). Some bore into the cemented carbonate to make protective domiciles (borings) for filter-feeding. Sometimes hardgrounds are undermined by currents which remove the soft sediment below them, producing shallow cavities and caves which host a cryptic fauna (Palmer and Fürsich, 1974). The evolution of hardground faunas can be traced through the Phanerozoic, from the
Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
Period to today (Taylor and Wilson, 2003). Carbonate hardgrounds were most commonly formed during calcite sea intervals in Earth history, which were times of rapid precipitation of low-magnesium
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
and the dissolution of skeletal
aragonite Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...
(Palmer and Wilson, 2004). The
Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
-
Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...
and the
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
-
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
Systems have the most hardgrounds (sometimes hundreds in a single section) and the
Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
-
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
Systems have the least (usually none). This cyclicity in hardground formation is reflected in the evolution of hardground-dwelling communities. There are distinct differences between the Paleozoic and Mesozoic hardground communities: the former are dominated by thick calcitic
bryozoans Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...
and
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 ...
, the latter by
oysters Oyster is the common name for a number of different families of Seawater, salt-water bivalve molluscs that live in Marine (ocean), marine or Brackish water, brackish habitats. In some species, the valves are highly Calcification, calcified, a ...
, deep
bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
(''
Gastrochaenolites ''Gastrochaenolites'' is a trace fossil formed as a clavate (club-shaped) boring in a hard substrate such as a shell, rock or carbonate hardground. The aperture of the boring is narrower than the main chamber and may be circular, oval, or dumb-be ...
'') and sponge ('' Entobia'') borings (Taylor and Wilson, 2003). Stratigraphers and sedimentologists often use hardgrounds as marker horizons and as indicators of sedimentary hiatuses and flooding events (Fürsich et al., 1981, 1992; Pope and Read, 1997). Hardgrounds and their faunas can also represent very specific depositional environments such as tidal channels (Wilson et al., 2005) and shallow marine carbonate ramps (Palmer and Palmer, 1977; Malpas et al., 2004)


Gallery

Image:Liberty Hardground 090114.jpg, Hardground in the Liberty Formation (Upper Ordovician) of southern Ohio. Image:Trypanites01.jpg, Cross-section of an Upper
Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
hardground from
Kentucky Kentucky (, ), officially the Commonwealth of Kentucky, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Illinois, Indiana, and Ohio to the north, West Virginia to the ...
. The light-colored vertical elements are borings ('' Trypanites'') filled with dolomite. The scale bar is 1.0 cm. Image:Ordovician_hardground_Utah.jpg, A Middle
Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
hardground from the Kanosh Formation of
Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...
with
echinoderm 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 ...
holdfasts cemented to its upper surface. The scale bar is 1.0 cm. Image:Petroxestes_borings_Ordovician.jpg, '' Petroxestes'' borings in an Upper
Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
hardground, southern Ohio. Image:LibertyBorings.jpg, ''Trypanites'' borings in an Upper
Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
hardground,
Indiana Indiana ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Michigan to the northwest, Michigan to the north and northeast, Ohio to the east, the Ohio River and Kentucky to the s ...
. Image:KanoshHardground.jpg, Carbonate hardground with an encrusting bryozoan; Kanosh Formation ( Middle Ordovician) of Utah; scale in mm. Image:CarmelHardgroundSection.jpg, Cross-section of a carbonate hardground encrusted by oysters and bored by bivalves (''Gastrochaenolites''); Carmel Formation (Middle
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
) of southern Utah. Image:Trypanites02.jpg, ''Trypanites'' borings in an Upper Ordovician hardground from northern Kentucky. Image:Ora hardground MR.jpg, Carbonate hardground; Ora Formation, Upper Cretaceous (Turonian), southern Israel.


References

* * * * * * * * * * * * * * * *


Further reading

* * * *{{cite journal , doi = 10.3176/earth.2016.01 , title = A sparsely encrusted hardground with abundant ''Trypanites'' borings from the Llandovery of the Velise River, western Estonia (Baltica) , year = 2016 , author = Vinn, O. , author2 = Toom, U. , journal = Estonian Journal of Earth Sciences , volume = 65 , pages = 19–26 , url = http://www.kirj.ee/public/Estonian_Journal_of_Earth_Sciences/2016/issue_1/earth-2016-1-19-26.pdf , accessdate = 2016-03-05, doi-access = free Limestone Sedimentology Historical geology