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The Cerrejón Formation is a geologic formation in Colombia dating back to the Middle-Late Paleocene. It is found in the El Cerrejón sub-basin of the
Cesar-Ranchería Basin The Cesar-Ranchería Basin ( es, Cuenca Cesar-Ranchería) is a sedimentary basin in northeastern Colombia. It is located in the southern part of the department of La Guajira and northeastern portion of Cesar. The basin is bound by the Oca Faul ...
of
La Guajira La Guajira () is a department of Colombia. It occupies most of the Guajira Peninsula in the northeast region of the country, on the Caribbean Sea and bordering Venezuela, at the northernmost tip of South America. The capital city of the departm ...
and Cesar. The formation consists of
bituminous coal Bituminous coal, or black coal, is a type of coal containing a tar-like substance called bitumen or asphalt. Its coloration can be black or sometimes dark brown; often there are well-defined bands of bright and dull material within the seams. It ...
fields that are an important economic resource. Coal from the Cerrejón Formation is mined extensively from the
Cerrejón Cerrejón is a large open-pit coal mine in Northern Colombia owned by Glencore. At Cerrejón, low-ash, low-sulphur bituminous coal from the Cerrejón Formation is excavated. At over the mine is one of the largest of its type, the largest in Lati ...
open-pit Open-pit mining, also known as open-cast or open-cut mining and in larger contexts mega-mining, is a surface mining technique of extracting rock or minerals from the earth from an open-air pit, sometimes known as a borrow. This form of mining ...
coal mine, one of the largest in the world. The formation also bears fossils that are the earliest record of Neotropical rainforests.Wing et al., 2009


Definition

The formation was first named Septarias Formation and in 1958 renamed to Cerrejón Formation by Thomas van der Hammen, probably based on an earlier report by Notestein.Rodríguez & Londoño, 2002, p.163


Geology

The Cerrejón Formation, with an assigned total thickness of , is subdivided into lower, middle, and upper groups based on the thickness and distribution of coal beds. On average the coal beds are thick, and range from to in thickness. The thickest beds are in the upper part of the formation. The Cerrejón Formation is laterally equivalent with the Los Cuervos and
Bogotá Formation The Bogotá Formation ( es, Formación Bogotá, E1-2b, Tpb, Pgb) is a geological formation of the Eastern Hills and Bogotá savanna on the Altiplano Cundiboyacense, Eastern Ranges of the Colombian Andes. The predominantly shale and siltstone form ...
s to the south; Llanos Orientales and
Altiplano Cundiboyacense The Altiplano Cundiboyacense () is a high plateau located in the Eastern Cordillera of the Colombian Andes covering parts of the departments of Cundinamarca and Boyacá. The altiplano corresponds to the ancient territory of the Muisca. The Al ...
respectively. The formation is also time-equivalent with the Marcelina Formation of the Venezuelan Serranía del Perijá and the Catatumbo Formation of the southwestern Colombian part of the
Maracaibo Basin The Maracaibo Basin, also known as Lake Maracaibo natural region, Lake Maracaibo depression or Lake Maracaibo Lowlands, is a foreland basin and one of the eight natural regions of Venezuela, found in the northwestern corner of Venezuela in South ...
, the Catatumbo Basin.Ojeda & Sánchez, 2013, p.69 The formation has also been described as laterally equivalent to the Mostrencos and
Santa Cruz Formation The Santa Cruz Formation is a geological formation in the Magallanes/Austral Basin in southern Patagonia in Argentina and in adjacent areas of Chile. It dates to the late Early Miocene epoch, and is contemporaneous with eponymous Santacrucian ...
s of Venezuela.Rodríguez & Londoño, 2002, p.165 Based on lithofacies associations and paleofloral composition, the
depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...
fluctuated from an estuarine-influenced
coastal plain A coastal plain is flat, low-lying land adjacent to a sea coast. A fall line commonly marks the border between a coastal plain and a piedmont area. Some of the largest coastal plains are in Alaska and the southeastern United States. The Gulf Co ...
at the base of the formation to a fluvial-influenced coastal plain at the top. In the geologically recent past, some coal in the formation has spontaneously and naturally combusted to form clinker, red and brick-looking burnt coal. These rocks outcrop irregularly and are up to thick. Clinker is found near deformed zones such as faults or tight folds, and is older than the deformities themselves. They are thought to have combusted after the development of the Cerrejón
thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...
and
alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...
.


Paleoenvironment

Fossils found from the Cerrejón Formation are the earliest record of Neotropical rainforests, with an abundance of plant macrofossils and
palynomorph Palynology is the "study of dust" (from grc-gre, παλύνω, palynō, "strew, sprinkle" and ''-logy'') or of "particles that are strewn". A classic palynologist analyses particulate samples collected from the air, from water, or from deposits ...
s. The Cerrejón Formation also records a riverine vertebrate fauna that includes lungfish, turtles, snakes, and
crocodyliform Crocodyliformes is a clade of crurotarsan archosaurs, the group often traditionally referred to as "crocodilians". They are the first members of Crocodylomorpha to possess many of the features that define later relatives. They are the only pseu ...
s. Based on these fossils and the stratigraphy of the formation, the Cerrejón Formation was likely formed on a coastal plain, covered in a wet tropical rainforest and incised by a large river system.Head et al., 2009 The rainforest is estimated to have been around 5°N paleolatitude. During the Paleocene, equatorial temperatures were much higher than they are today. Based on the size of the giant boid ''
Titanoboa ''Titanoboa'' (; ) is an extinct genus of very large snakes that lived in what is now La Guajira in northeastern Colombia. They could grow up to , perhaps even long and reach a body mass of . This snake lived during the Middle to Late Paleo ...
'', specimens of which have been found in the Cerrejón Formation, the mean annual temperature of Paleocene equatorial South America was between and . This is the minimum annual temperature range that a
poikilotherm A poikilotherm () is an animal whose internal temperature varies considerably. Poikilotherms have to survive and adapt to environmental stress. One of the most important stressors is temperature change, which can lead to alterations in membrane ...
as large as ''Titanoboa'' could live. It is consistent with Paleocene climate models which predict greenhouse temperatures and an atmospheric pCO2 concentration of around 2,000 parts per million. Paleotemperature estimates based on fossil leaf assemblages from the Cerrejón Formation predict the mean annual temperature to be lower than other estimates. However, such temperature estimates based on riparian and wetland rainforest paleoflora have been considered underestimates. Mean annual temperatures of are considered to be too high for modern tropical forests, but the Cerrejón rainforest could have been maintained by increased atmospheric pCO2 levels and the high regional rainfall, which is estimated to have been around per year.


Flora

The floral record of the Cerrejón Formation is well known, with many identifiable and well preserved plant microfossils having been found from the Cerrejón mine. The fossils are well preserved, and in some cases their cell structure is intact. In comparison to modern Neotropical rainforests, the diversity of plants is quite low. This may be an indication of the early stage of Neotropical diversification, or a delayed recovery period following the
Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With th ...
. Many plants from the Cerrejón Formation belong to families that are still common today in modern Neotropical rainforests. There is a diverse variety of palms and legumes in the formation. In addition to palms and legumes, much of the biomass of the Paleocene forest consisted of
laurales The Laurales are an order of flowering plants. They are magnoliids, related to the Magnoliales. The order includes about 2500-2800 species from 85-90 genera, which comprise seven families of trees and shrubs. Most of the species are tropical ...
, malvales, menisperms,
aroids The Araceae are a family of monocotyledonous flowering plants in which flowers are borne on a type of inflorescence called a spadix. The spadix is usually accompanied by, and sometimes partially enclosed in, a spathe (or leaf-like bract). Also ...
, and zingiberaleans. Studies of fossil plants from Cretaceous-age sites indicate that the floral composition below the
Cretaceous–Paleogene boundary The Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K–T) boundary, is a geological signature, usually a thin band of rock containing much more iridium than other bands. The K–Pg boundary marks the end of ...
(K–T boundary) was very different from that of the Paleocene. Legumes are absent from Cretaceous strata, and likely appeared or diversified during the Paleocene. The presence of these types of flora in Paleocene strata shows that plants characteristic of modern Neotropical rainforests have existed for geologically long periods of time, being able to withstand climatic and geographic changes in South America. It has been suggested that today's Neotropical rainforests are the result of environmental changes brought about by Quaternary glacial cycles (i.e. the recent ice age). These cycles would have caused fluctuation in the diversity and extent of rainforests. If this was the case, the current diversity of the
Amazon rainforest The Amazon rainforest, Amazon jungle or ; es, Selva amazónica, , or usually ; french: Forêt amazonienne; nl, Amazoneregenwoud. In English, the names are sometimes capitalized further, as Amazon Rainforest, Amazon Forest, or Amazon Jungle. ...
would be a recent speciation in a changing environment. However, the floral record from the Cerrejón Formation shows that the current diversity of the Amazon Rainforest can be traced back into the early Cenozoic.


Fauna

Feeding damage from insects is evident on some of the plant macrofossils from the Cerrejón Formation. One survey of plant macrofossils showed that around half of the studied specimens had been attacked by herbivorous insects. The insects that damaged the leaves were predominantly generalist feeders, unlike modern Neotropical insects that are mainly specialist herbivores. There is no evidence of the elevated insect-feeding diversity or host-specialized feeding associations that are seen in later Neotropical forests. The insect diversity in the Cerrejón Formation is low in comparison to the diversity of insects in Neotropical rainforests today, and it is likely that leaf damage was made by relatively few species. Remains of the giant boid '' Titanoboa cerrejonensis'' have been found from a gray
claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too sma ...
layer underlying Coal Seam 90 in the Cerrejón mine. ''Titanoboa'' is the largest known snake to have ever existed, reaching an estimated length of . ''Eunectes'', the
anaconda Anacondas or water boas are a group of large snakes of the genus '' Eunectes''. They are found in tropical South America. Four species are currently recognized. Description Although the name applies to a group of snakes, it is often used t ...
, is likely to be a close living analogue of ''Titanoboa''. A
dyrosaurid Dyrosauridae is a family of extinct neosuchian crocodyliforms that lived from the Late Cretaceous (Maastrichtian) to the Eocene. Dyrosaurid fossils are globally distributed, having been found in Africa, Asia, Europe, North America and South Ame ...
crocodylomorph Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cro ...
called '' Cerrejonisuchus improcerus'' was described in 2010 from the Cerrejón Formation in the same layer as ''Titanoboa''. It was a small dyrosaurid, and had the shortest snout length relative to its skull length of any dyrosaurid. Most dyrosaurids were marine, with long snouts adapted for catching fish. The short snout of ''Cerrejonisuchus'' is likely to have been an adaptation for a more generalized diet in a transitional aquatic environment. It is possible that ''Cerrejonisuchus'' was a food source for ''Titanoboa'', as the two inhabited the same riverine environment. The anaconda has been documented consuming
caiman A caiman (also cayman as a variant spelling) is an alligatorid belonging to the subfamily Caimaninae, one of two primary lineages within the Alligatoridae family, the other being alligators. Caimans inhabit Mexico, Central and South America f ...
s, a feeding habit that is similar to the inferred habit of ''Titanoboa''. A second dyrosaurid, ''
Acherontisuchus ''Acherontisuchus'' is an extinct genus of dyrosaurid neosuchian from Middle to Late Paleocene deposits of Colombia.
'', was named in 2011 from the formation. With a large body and long snout, it resembles most other dyrosaurids.Hastings et al., 2011 A third dyrosaurid, ''
Anthracosuchus ''Anthracosuchus'' (meaning "coal crocodile" in Greek) is an extinct genus of dyrosaurid crocodyliform from the Paleocene of Colombia. Remains of ''Anthracosuchus balrogus'', the only known species, come from the Cerrejón Formation in the Cerr ...
'', was named in 2014. Unlike other dyrosaurids, it had blunt teeth and a short skull. ''Anthracosuchus'' likely had massive jaw muscles allowing it to feed on large turtles, a behavior which is corroborated by predation marks found at the site.


Coal resources

The Cerrejón Formation contains extensive coal seams that are mined primarily at the Cerrejón mine. The coal is desirable for its low
ash Ash or ashes are the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non-gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash ...
and sulfur content and for its resistance to caking. Cerrejón is Colombia's largest coal producing mine, with most of the production exported to Europe. It is the largest coal mining operation in Latin America, with an estimated 28.4 million tons mined in 2006.


Itaboraian correlations


References


Bibliography

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Further reading

* * * * * {{DEFAULTSORT:Cerrejon Formation Geologic formations of Colombia Formations Paleogene Colombia Paleocene Series of South America Itaboraian Peligran Thanetian Stage Coal in Colombia Coal formations Mudstone formations Sandstone formations Siltstone formations Fluvial deposits Reservoir rock formations Source rock formations Formations Formations