Limestone

Limestone

Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate)

Limestone often contains variable amounts of silica in the form of chert or flint, as well as varying amounts of clay, silt and sand as disseminations, nodules, or layers within the rock. The primary source of the calcite in limestone is most commonly marine organisms. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as pelagic ooze or alternatively is conglomerated in a coral reef (see lysocline for information on calcite dissolution). Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves). This produces speleothems such as stalagmites and stalactites. Another form taken by calcite is that of oolites (oolitic limestone) which can be recognized by its granular appearance. Limestone makes up about 10% of the total volume of all sedimentary rocks[citation needed].
Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases.
When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures.
Karst topography and caves develop in carbonate rocks due to their solubility in dilute acidic groundwater. Cooling groundwater or mixing of different groundwaters will also create conditions suitable for cave formation.
Coastal limestones are often eroded by organisms which bore into the rock by various means. This process is known as bioerosion. It is most common in the tropics, and it is known throughout the fossil record (see Taylor and Wilson, 2003).
Pure limestone is almost white. Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestones exhibit different colors, especially on weathered surfaces. Limestone may be crystalline, clastic, granular, or massive, depending on the method of formation. Crystals of calcite, quartz, dolomite or barite may line small cavities in the rock. Folk and Dunham classifications are used to describe limestones more precisely.
Travertine is a banded, compact variety of limestone formed along streams, particularly where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution that is supersaturated with chemical constituents of calcite. Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells.
During regional metamorphism that occurs during the mountain building process (orogeny) limestone recrystallizes into marble.
Limestone is a parent material of Mollisol soil group.

Blastoids

Blastoids

Blastoids (class Blastoidea) are an extinct type of stemmed echinoderm. Often called sea buds, blastoid fossils look like small hickory nuts. They originated, along with many other echinoderm classes, in the Ordovician period and reached their greatest diversity in the Mississippian epoch of the Carboniferous period. Blastoids persisted until their extinction at the end of Permian, about 250 million years ago. Although never as diverse as their contemporary relatives, the crinoids, blastoids are quite common fossils, especially in many Mississippian-age rocks. They can be found in Illinois, Kentucky, and Indiana.
Like most echinoderms, blastoids were protected by a set of interlocking plates of calcium carbonate, which formed the main body, or theca. In life, the theca of a typical blastoid was attached to a stalk or column made up of stacked disc-shaped plates. The other end of the column was attached to the ocean floor, very much like stalked crinoids. The mouth was located at the summit of the theca. Radiating like flower petals from the center were five food grooves, or ambulacra. Each ambulacrum had many long, thin, fine structures called brachioles, which were used to trap food particles and bring them to the mouth. Brachioles were delicate structures, and in fossils are not usually preserved in place. A series of five spiracle plates surrounded the star-shaped mouth, which included the anus, mouth and entrances to a set of five complex, folded respiratory organs known as hydrospires. These spiracles prevented mixing of the various fluids. Waste elimination was through the anispiracle, an opening formed by the fusing of anus and adjacent spiracles.
Like crinoids, blastoids were high-level, stalked suspension feeders (feeding mainly on planktonic organisms) that inhabited clear-to-silty, moderately-agitated ocean waters from shelf to basin. The food gathering system of blastoids consisted of several types of ambulacra. Food entered the brachiolar ambulacra, was transferred to the side ambulacra through the brachiolar pit, then transferred to the main (median) ambulacra, and finally entered the mouth. Each of these ambulacra were roofed by cover plates. The cover plates of the brachiolar groove were movable and could open, allowing food to enter, or close as needed. Other cover plates may also have been movable.
Blastoids are subdivided into two subclasses: Fissiculata, which are characterized by direct entrance to the individual hydrospires by way of slits; and Spiraculata, which are characterized by indirect entrance to the hydrospires through canals by way of pores. The earliest blastoid yet found, Macurdablastus from the Middle Ordovician of Tennessee, cannot be classified as either subclass.

The Solnhofen limestone

The Solnhofen limestone

The Solnhofen limestone is a Jurassic Konservat-Lagerstätt that preserves a rare assemblage of fossilized organisms, some of which, such as sea jellies, don't ordinarily fossilize at all. Others, like the early bird Archaeopteryx are preserved in such detail that they are among the most famous and most beautiful fossils in the world.
The Solnhofen beds lie in the German state of Bavaria, halfway between Nuremberg and Munich. During late Jurassic times, this area was an archipelago at the edge of the Tethys Sea. This included placid lagoons that had limited access to the open sea and where salinity rose high enough that the resulting brine could not support life. Since the lowest water was devoid of oxygen, many ordinary scavengers were absent. Any organism that fell, drifted, or was washed into the lagoons from the ocean or the land became buried in soft carbonate mud. Thus, many delicate creatures avoided consumption by scavengers or being torn apart by currents. The wings of dragonflies, the imprints of stray feathers, and terrestrial plants that washed into the lagoons were all preserved. The fossils are not numerous, but some of them are spectacular, and their range gives a comprehensive picture of a local Jurassic ecosystem.
At times, the lagoons almost dried out, exposing sticky carbonate muds that trapped insects and even a few small dinosaurs. Over 600 species have been identified, including twenty-nine kinds of pterosaur ranging from the size of a sparrow to 1.2 m (4 ft) in length have been found.
The fine-grained texture of the mud silt forming the limestone from the Solnhofen area (which is composed mainly of the towns of Solnhofen and Eichstätt) is ideal for making lithographic plates, and extensive quarrying in the 19th century revealed many fossil finds, as commemorated in the name Archaeopteryx lithographica, all the specimens of which come from these deposits.

The Triassic

The Triassic

The Triassic was generally dry, a trend that began in the late Carboniferous, and highly seasonal, especially in the interior of Pangaea. Low sea levels may have also exacerbated temperature extremes. With its high specific heat capacity, water acts as a temperature-stabilizing heat, and land areas near large bodies of water—especially the oceans—experience less variation in temperature. Because much of the land that constituted Pangaea was distant from the oceans, temperatures fluctuated greatly, and the interior of Pangaea probably included expansive areas of desert. Abundant evidence of red beds and evaporites such as salt support these conclusions.
Sea levels began to rise during the Jurassic, which was probably caused by an increase in seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 200 m more than today, which flooded coastal areas. Furthermore, Pangaea began to rift into smaller divisions, bringing more land area in contact with the ocean by forming the Tethys Sea. Temperatures continued to increase and began to stabilize. Humidity also increased with the proximity of water, and deserts retreated.
The climate of the Cretaceous is less certain and more widely disputed. Higher levels of carbon dioxide in the atmosphere caused the world temperature gradient from north to south to become almost flat: temperatures were about the same across the planet. Average temperatures were also higher than today by about 10°C. In fact, by the middle Cretaceous, equatorial ocean waters (perhaps as warm as 20 °C in the deep ocean) may have been too warm for sea life, and land areas near the equator may have been deserts despite their proximity to water. The circulation of oxygen to the deep ocean may also have been disrupted. For this reason, large volumes of organic matter accumulated because they were unable to decompose and were eventually deposited as "black shale".
Not all of the data support these hypotheses, however. Even with the overall warmth, temperature fluctuations should have been sufficient for the presence of polar ice caps and glaciers, but there is no evidence of either. Quantitative models have also been unable to recreate the flatness of the Cretaceous temperature gradient.[citation needed]

The Mesozoic Era

The Mesozoic Era

The Mesozoic Era is one of three geologic eras of the Phanerozoic eon. The division of time into eras dates back to Giovanni Arduino, in the 18th century, although his original name for the era now called the 'Mesozoic' was 'Secondary' (making the modern era the 'Tertiary'). Lying between the Paleozoic and the Cenozoic, Mesozoic means 'middle animals', derived from Greek prefix meso-/μεσο- for 'between' and zoon/ζωον meaning animal or 'living being'. It is often called the 'Age of the Dinosaurs', after the dominant fauna of the era.
The Mesozoic was a time of tectonic, climatic and evolutionary activity. The continents gradually shifted from a state of connectedness into their present configuration; the drifting provided for speciation and other important evolutionary developments. The climate was exceptionally warm throughout the period, also playing an important role in the evolution and diversification of new animal species. By the end of the era, the basis of modern life was in place.

Cambrian fauna

Cambrian fauna

Of those modern animal phyla that fossilize easily, all save the bryozoans appear to have representatives in the Cambrian, and of these most (except the considerably older sponges) seem to have originated near the start of the period. Many extinct phyla and odd animals that have unclear relationships to other animals also appear in the Cambrian. The apparent "sudden" appearance of very diverse faunas over a period of no more than a few tens of millions of years is referred to as the "Cambrian Explosion". Also, the first possible tracks on land, such as Protichnites and Climactichnites, dating to about 530 mya and found in Ontario, Canada, and northern United States, appeared at this time. The conodonts, small predatory primitive chordates known from their fossilised teeth, also appeared during the Furongian epoch of the Cambrian period. The conodonts thrived throughout the Paleozoic and the early Mesozoic until they completely disappeared during the Late Triassic period when the first mammals were evolving.
The best studied sites where the soft parts of organisms have fossilized are in the Burgess shale of British Columbia. They represent strata from the Middle Cambrian and provide us with a wealth of information on early animal diversity. Similar faunas have subsequently been found in a number of other places — most importantly in very early Cambrian shales in the People's Republic of China's Yunnan Province (see Maotianshan shales). Fairly extensive Precambrian Ediacaran faunas have been identified in the past 50 years, but their relationships to Cambrian forms are quite obscure.

Cambrian

Cambrian

Cambrian continents are thought to have resulted from the breakup of a Neoproterozoic supercontinent called Pannotia. The waters of the Cambrian period appear to have been widespread and shallow. Gondwana remained the largest supercontinent after the breakup of Pannotia. It is thought that Cambrian climates were significantly warmer than those of preceding times that experienced extensive ice ages discussed as the Varanger glaciation. Also there was no glaciation at the poles. Continental drift rates in the Cambrian may have been anomalously high. Laurentia, Baltica and Siberia remained independent continents since the break-up of the supercontinent of Pannotia. Gondwana started to drift towards the South Pole. Panthalassa covered most of the southern hemisphere, and minor oceans included the Proto-Tethys Ocean, Iapetus Ocean, and Khanty Ocean, all of which expanded by this time.