Carbonate hardgrounds

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 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, oysters, barnacles, cornulitids, hederelloids, microconchids and crinoids), borings of organisms produced through bioerosion, 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 techniques like side-scan radar.

Cretaceous hardground
Cretaceous hardground from Texas with encrusting oysters and Gastrochaenolites borings. The scale bar is 1.0 cm.

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 Period to today (Taylor and Wilson, 2003).

Middle Jurassic hardground (Carmel Formation) with encrusting oysters and borings.
Scientific papers on hardgrounds by period. Serves as a proxy for hardground abundance over time. Aragonite and calcite sea intervals are plotted on the time axis.

Carbonate hardgrounds were most commonly formed during calcite sea intervals in Earth history, which were times of rapid precipitation of low-magnesium calcite and the dissolution of skeletal aragonite (Palmer and Wilson, 2004). The Ordovician-Silurian and the Jurassic-Cretaceous Systems have the most hardgrounds (sometimes hundreds in a single section) and the Permian-Triassic 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 and echinoderms, the latter by oysters and deep bivalve (Gastrochaenolites) 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)

Liberty Hardground 090114

Hardground in the Liberty Formation (Upper Ordovician) of southern Ohio.


Cross-section of an Upper Ordovician hardground from Kentucky. The light-colored vertical elements are borings (Trypanites) filled with dolomite. The scale bar is 1.0 cm.

Ordovician hardground Utah

A Middle Ordovician hardground from the Kanosh Formation of Utah with echinoderm holdfasts cemented to its upper surface. The scale bar is 1.0 cm.

Petroxestes borings Ordovician

Petroxestes borings in an Upper Ordovician hardground, southern Ohio.


Trypanites borings in an Upper Ordovician hardground, Indiana.


Carbonate hardground with an encrusting bryozoan; Kanosh Formation (Middle Ordovician) of Utah; scale in mm.


Cross-section of a carbonate hardground encrusted by oysters and bored by bivalves (Gastrochaenolites); Carmel Formation (Middle Jurassic) of southern Utah.


Trypanites borings in an Upper Ordovician hardground from northern Kentucky.

Ora hardground MR

Carbonate hardground; Ora Formation, Upper Cretaceous (Turonian), southern Israel.

Hardground oblique Ordovician 071514c
A carbonate hardground surface with a bryozoan and vertical borings (Trypanites) from the Upper Ordovician of Kentucky.


  • Bodenbender, B.E.; Wilson, M.A.; Palmer, T.J. (1989). "Paleoecology of Sphenothallus on an Upper Ordovician hardground". Lethaia. 22 (2): 217–225. doi:10.1111/j.1502-3931.1989.tb01685.x.
  • Brett, C.E.; Liddell, W.D. (1981). "Preservation and paleoecology of a Middle Ordovician hardground community". Paleobiology. 4: 329–348.
  • Fürsich F.T., Kennedy, W.J., Palmer, T.J. (1981). "Trace fossils at a regional discontinuity surface: the Austin/Taylor (Upper Cretaceous) contact in central Texas". Journal of Paleontology. 55: 537–551.CS1 maint: multiple names: authors list (link)
  • Fürsich, F.T.; Oschmann, W.; Singh, B.; Jaitly, A.K. (1992). "Hardgrounds, reworked concretion levels and condensed horizons in the Jurassic of western India: their significance for basin analysis". Journal of the Geological Society of London. 149 (3): 313–331. doi:10.1144/gsjgs.149.3.0313.
  • Malpas, J.A.; Gawthorpe, R. L.; Pollard, J.E.; Sharp, I.R. (2004). "Ichnofabric analysis of the shallow marine Nukhul Formation (Miocene), Suez Rift, Egypt: implications for depositional processes and sequence stratigraphic evolution". Palaeogeography, Palaeoclimatology, Palaeoecology. 215 (3–4): 239–264. doi:10.1016/j.palaeo.2004.09.007.
  • Palmer, T.J. Fürsich, F.T. (1974). "The ecology of a Middle Jurassic hardground and crevice fauna". Palaeontology. 17: 507–524.CS1 maint: multiple names: authors list (link)
  • Palmer, T.J.; Palmer, C.D. (1977). "Faunal distribution and colonization strategy in a Middle Ordovician hardground community". Lethaia. 10 (3): 179–199. doi:10.1111/j.1502-3931.1977.tb00608.x.
  • Palmer, T.J.; Wilson, M.A. (2004). "Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas". Lethaia. 37 (4): 417–427 [1]. doi:10.1080/00241160410002135.
  • Palmer, T.J. (1978). "Burrows at certain omission surfaces in the Middle Ordovician of the Upper Mississippi Valley". Journal of Paleontology. 52: 109–117.
  • Palmer, T.J. (1982). "Cambrian to Cretaceous changes in hardground communities". Lethaia. 15 (4): 309–323. doi:10.1111/j.1502-3931.1982.tb01696.x.
  • Pope, M.C.; Read, J.F. (1997). "High-resolution surface and subsurface sequence stratigraphy of the Middle to Late Ordovician (late Mohawkian-Cincinnatian) foreland basin rocks, Kentucky and Virginia". AAPG Bulletin. 81: 1866–1893. doi:10.1306/3b05c654-172a-11d7-8645000102c1865d.
  • Taylor, P.D.; Wilson, M.A. (2003). "Palaeoecology and evolution of marine hard substrate communities". Earth-Science Reviews. 62: 1–103 [2]. Bibcode:2003ESRv...62....1T. doi:10.1016/S0012-8252(02)00131-9.
  • Vinn, O.; Wilson, M.A. (2010). "Microconchid-dominated hardground association from the late Pridoli (Silurian) of Saaremaa, Estonia". Palaeontologia Electronica. 2010 (2): 13.2.9A. Retrieved 2012-09-16.
  • Vinn, O.; Toom, U. (2015). "Some encrusted hardgrounds from the Ordovician of Estonia (Baltica)". Carnets de Géologie. 15 (7): 63–70. doi:10.4267/2042/56744. Retrieved 2015-06-18.
  • Wilson, M.A.; Palmer, T.J. (1992). "Hardgrounds and hardground faunas". University of Wales, Aberystwyth, Institute of Earth Studies Publications. 9: 1–131.
  • Wilson, M.A.; Wolfe, K.R.; Avni, Y. (2005). "Development of a Jurassic rocky shore complex (Zohar Formation, Makhtesh Qatan, southern Israel)". Israel Journal of Earth Sciences. 54 (3): 171–178 [3]. doi:10.1560/71EQ-CNDF-K3MQ-XYTA.

Further reading


Aulopora is an extinct genus of tabulate coral characterized by a bifurcated budding pattern and conical corallites. Colonies commonly encrust hard substrates such as rocks, shells and carbonate hardgrounds.

Calcite sea

A calcite sea is one in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic carbonate precipitates. The Early Paleozoic and the Middle to Late Mesozoic oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and the Cenozoic (including today) are characterized by aragonite seas ).

The most significant geological and biological effects of calcite sea conditions include rapid and widespread formation of carbonate hardgrounds , calcitic ooids , calcite cements, and the contemporaneous dissolution of aragonite shells in shallow warm seas. Hardgrounds were very common, for example, in the calcite seas of the Ordovician and Jurassic, but virtually absent from the aragonite seas of the Permian.Fossils of invertebrate organisms found in calcite sea deposits are usually dominated by either thick calcite shells and skeletons, were infaunal and/or had thick periostraca, or had an inner shell of aragonite and an outer shell of calcite. This was apparently because aragonite dissolved quickly on the seafloor and had to be either avoided or protected as a biomineral.Calcite seas were coincident with times of rapid seafloor spreading and global greenhouse climate conditions. Seafloor spreading centers cycle seawater through hydrothermal vents, reducing the ratio of magnesium to calcium in the seawater through metamorphism of calcium-rich minerals in basalt to magnesium-rich clays. This reduction in the Mg/Ca ratio favors the precipitation of calcite over aragonite. Increased seafloor spreading also means increased volcanism and elevated levels of carbon dioxide in the atmosphere and oceans. This may also have an effect on which polymorph of calcium carbonate is precipitated. Further, high calcium concentrations of seawater favor the burial of CaCO3, thereby removing alkalinity from the ocean, lowering seawater pH and reducing its acid/base buffering.

Flood geology

Flood geology (also creation geology or diluvial geology) is the attempt to interpret and reconcile geological features of the Earth in accordance with a literal belief in the global flood described in Genesis 6–8. In the early 19th century, diluvial geologists hypothesized that specific surface features were evidence of a worldwide flood which had followed earlier geological eras; after further investigation they agreed that these features resulted from local floods or glaciers. In the 20th century, young Earth creationists revived flood geology as an overarching concept in their opposition to evolution, assuming a recent six-day Creation and cataclysmic geological changes during the Biblical Deluge, and incorporating creationist explanations of the sequence of rock strata.

In the early stages of development of the science of geology, fossils were interpreted as evidence of past flooding. The "theories of the Earth" of the 17th century proposed mechanisms based on natural laws, within a timescale set by the biblical chronology. As modern geology developed, geologists found evidence of an ancient Earth, and evidence inconsistent with the notion that the Earth had developed in a series of cataclysms, like the Genesis flood. In early 19th-century Britain, "Diluvialism" attributed landforms and surface features such as beds of gravel and erratic boulders to the destructive effects of this supposed global Deluge, but by 1830 geologists increasingly found that the evidence only showed relatively local floods. Attempts were made by so-called scriptural geologists to give primacy to literal Biblical explanations, but they lacked background in geology and were marginalised by the scientific community, as well as having little influence on the church.

Flood geology was revived as a field of study within creation science, which is a part of young Earth creationism.

Proponents hold to a literal reading of Genesis 6–9 and view its passages to be historically accurate, using the Bible's internal chronology to place the Flood and the story of Noah's Ark within the last five thousand years.The key tenets of flood geology are refuted by scientific analysis. Flood geology contradicts the scientific consensus in geology, stratigraphy, geophysics, physics, paleontology, biology, anthropology, and archeology. Modern geology, its sub-disciplines and other scientific disciplines utilize the scientific method. In contrast, flood geology does not adhere to the scientific method, making it a pseudoscience.

Glossary of geology

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.


The Jurassic Period ( juu-RASS-ik; from the Jura Mountains) is a geologic period and system that spanned 56 million years from the end of the Triassic Period 201.3 million years ago (Mya) to the beginning of the Cretaceous Period 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles. The start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, and the Tithonian event at the end; neither event ranks among the "Big Five" mass extinctions, however.

The Jurassic period is divided into three epochs: Early, Middle, and Late. Similarly, in stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, and Upper Jurassic series of rock formations.

The Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period were first identified.

By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses: Laurasia to the north, and Gondwana to the south. This created more coastlines and shifted the continental climate from dry to humid, and many of the arid deserts of the Triassic were replaced by lush rainforests.

On land, the fauna transitioned from the Triassic fauna, dominated by both dinosauromorph and crocodylomorph archosaurs, to one dominated by dinosaurs alone. The first birds also appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, and the evolution of therian mammals, including primitive placentals. Crocodilians made the transition from a terrestrial to an aquatic mode of life. The oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates.


Limalok (formerly known as Harrie or Harriet) is a Cretaceous-Paleocene guyot/tablemount in the southeastern Marshall Islands, one of a number of seamounts (a type of underwater volcanic mountain) in the Pacific Ocean. It was probably formed by a volcanic hotspot in present-day French Polynesia. Limalok lies southeast of Mili Atoll and Knox Atoll, which rise above sea level, and is joined to each of them through a volcanic ridge. It is located at a depth of 1,255 metres (4,117 ft) and has a summit platform with an area of 636 square kilometres (246 sq mi).

Limalok is formed by basaltic rocks and was probably a shield volcano at first; the Macdonald, Rarotonga, Rurutu and Society hotspots may have been involved in its formation. After volcanic activity ceased, the volcano was eroded and thereby flattened, and a carbonate platform formed on it during the Paleocene and Eocene. These carbonates were chiefly produced by red algae, forming an atoll or atoll-like structure with reefs.

The platform sank below sea level 48 ± 2 million years ago during the Eocene, perhaps because it moved through the equatorial area, which was too hot or nutrient-rich to support the growth of a coral reef. Thermal subsidence lowered the drowned seamount to its present depth. After a hiatus lasting into the Miocene, sedimentation commenced on the seamount leading to the deposition of manganese crusts and pelagic sediments; phosphate accumulated in some sediments over time.


The Ordovician ( or-də-VISH-ee-ən, -⁠doh-, -⁠VISH-ən) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, and placed them in a system of their own. The Ordovician received international approval in 1960 (forty years after Lapworth's death), when it was adopted as an official period of the Paleozoic Era by the International Geological Congress.

Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events. Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event considerably increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, and those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today.

Planetary surface

A planetary surface is where the solid (or liquid) material of the outer crust on certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets (including Earth), dwarf planets, natural satellites, planetesimals and many other small Solar System bodies (SSSBs). The study of planetary surfaces is a field of planetary geology known as surface geology, but also a focus of a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences, and astronomy. Land (or ground) is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.

In differentiated bodies, the surface is where the crust meets the planetary boundary layer. Anything below this is regarded as being sub-surface or sub-marine. Most bodies more massive than super-Earths, including stars and gas giants, as well as smaller gas dwarfs, transition contiguously between phases, including gas, liquid, and solid. As such, they are generally regarded as lacking surfaces.

Planetary surfaces and surface life are of particular interest to humans as it the primary habitat of the species, which has evolved to move over land and breathe air. Human space exploration and space colonization therefore focuses heavily on them. Humans have only directly explored the surface of Earth and the Moon. The vast distances and complexities of space makes direct exploration of even near-Earth objects dangerous and expensive. As such, all other exploration has been indirect via space probes.

Indirect observations by flyby or orbit currently provide insufficient information to confirm the composition and properties of planetary surfaces. Much of what is known is from the use of techniques such as astronomical spectroscopy and sample return. Lander spacecraft have explored the surfaces of planets Mars and Venus. Mars is the only other planet to have had its surface explored by a mobile surface probe (rover). Titan is the only non-planetary object of planetary mass to have been explored by lander. Landers have explored several smaller bodies including 433 Eros (2001), 25143 Itokawa (2005), Tempel 1 (2005), and 67P/Churyumov–Gerasimenko(2014).


Pleurodictyum is an extinct genus of tabulate corals, characterized by polygonal corallites. Colonies commonly encrust hard substrates such as rocks, shells and carbonate hardgrounds.

Resolution Guyot

Resolution Guyot (formerly known as Huevo) is a guyot (tablemount) in the underwater Mid-Pacific Mountains in the Pacific Ocean. It is a circular flat mountain, rising 500 metres (1,600 ft) above the seafloor to a depth of about 1,320 metres (4,330 ft), with a 35 kilometres (22 mi) wide summit platform. The Mid-Pacific Mountains lie west of Hawaii and northeast of the Marshall Islands, but at the time of its formation the guyot was located in the Southern Hemisphere.

The guyot was probably formed by a hotspot in today's French Polynesia before plate tectonics shifted it to its present-day location. The Easter, Marquesas, Pitcairn and Society hotspots, among others, may have been involved in the formation of Resolution Guyot. Volcanic activity has been dated to have occurred 107–129 million years ago and formed a volcanic island that was subsequently flattened by erosion. Carbonate deposition commenced, forming an atoll-like structure and a carbonate platform.

The platform emerged above sea level at some time between the Albian and Turonian ages before eventually drowning for reasons unknown between the Albian and the Maastrichtian. Thermal subsidence lowered the drowned seamount to its present depth. After a hiatus, sedimentation commenced on the seamount and led to the deposition of manganese crusts and pelagic sediments, some of which were later modified by phosphate.


Thecideida is an order of cryptic articulate brachiopods characterized by their small size and habit of cementing their ventral valves to hard substrates such as shells, rocks and carbonate hardgrounds. Thecideides first appear in the Triassic (Jaecks and Carlson, 2001) and are common today (Lüter, 2005; Lüter et al., 2007).

Trace fossil

A trace fossil, also ichnofossil ( ; from Greek: ἴχνος ikhnos "trace, track"), is a geological record of biological activity. Ichnology is the study of such traces, and is the work of ichnologists. Trace fossils may consist of impressions made on or in the substrate by an organism: for example, burrows, borings (bioerosion), urolites (erosion caused by evacuation of liquid wastes), footprints and feeding marks, and root cavities. The term in its broadest sense also includes the remains of other organic material produced by an organism—for example coprolites (fossilized droppings) or chemical markers—or sedimentological structures produced by biological means—for example, stromatolites. Trace fossils contrast with body fossils, which are the fossilized remains of parts of organisms' bodies, usually altered by later chemical activity or mineralization.

Sedimentary structures, for example those produced by empty shells rolling along the sea floor, are not produced through the behaviour of an organism and not considered trace fossils.

The study of traces - ichnology - divides into paleoichnology, or the study of trace fossils, and neoichnology, the study of modern traces. Ichnological science offers many challenges, as most traces reflect the behaviour—not the biological affinity—of their makers. Accordingly, researchers classify trace fossils into form genera, based on their appearance and on the implied behaviour, or ethology, of their makers.


Trypanites is a narrow, cylindrical, unbranched boring which is one of the most common trace fossils in hard substrates such as rocks, carbonate hardgrounds and shells (Bromley, 1972). It appears first in the Lower Cambrian (James et al., 1977), was very prominent in the Ordovician Bioerosion Revolution (Wilson and Palmer, 2006), and is still commonly formed today. Trypanites is almost always found in calcareous substrates, most likely because the excavating organism used an acid or other chemical agent to dissolve the calcium carbonate (Taylor and Wilson, 2003). Trypanites is common in the Ordovician and Silurian hardgrounds of Baltica (Vinn et al. 2015).


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