The Mawson Formation is a

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

, dating to roughly between 182 and 177

million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...

and covering the

Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toarcian ...

stages Stage or stages may refer to: Acting * Stage (theatre), a space for the performance of theatrical productions * Theatre, a branch of the performing arts, often referred to as "the stage" * ''The Stage'', a weekly British theatre newspaper * S ...

of 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 Mya. The J ...

Period in the

Mesozoic Era The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising ...

. Vertebrate remains are known from the formation. The ''Mawson Formation'' is the South Victoria Land equivalent of the Karoo Large Igneous Province in

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...

(including the upper

Clarens Formation The Clarens Formation is a geological formation found in several localities in Lesotho and in the Free State, KwaZulu-Natal, and Eastern Cape provinces in South Africa. It is the uppermost of the three formations found in the Stormberg Group of ...

desertic interbeds), as well the Lonco Trapial Formation and the

Cañadón Asfalto Formation The Cañadón Asfalto Formation is a Lower Jurassic to Late Jurassic geologic formation, from the Jurassic period of the Mesozoic Era. Its age is controversial, uranium-lead dating of the volcanic tuff beds having given various different ages. A ...

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

Geology

The thin lacustrine interbeds of the Mawson Formation have received several names in literature, being known as either Carapace Sandstone or Carapace Formation, being a series of Freshwater environments developed during times when the Kirkpatrick Basalt stopped invading the zone. The lava flow deposits of the Kirkpatrick Basalt belong to the Ferrar Large Igneous Province, developed in a linear belt along the

Transantarctic Mountains The Transantarctic Mountains (abbreviated TAM) comprise a mountain range of uplifted (primarily sedimentary rock, sedimentary) rock in Antarctica which extend, with some interruptions, across the continent from Cape Adare in northern Victoria La ...

, from the

Weddell Sea The Weddell Sea is part of the Southern Ocean and contains the Weddell Gyre. Its land boundaries are defined by the bay formed from the coasts of Coats Land and the Antarctic Peninsula. The easternmost point is Cape Norvegia at Princess Martha ...

region to

North Victoria Land North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north'' is ...

, covering approx. 3,500 km in length. This event was linked with the initial stages of the breakup of the

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...

n part of

Pangea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...

, concretely with the rifting of East Antarctica and

Southern Africa Southern Africa is the southernmost subregion of the African continent, south of the Congo and Tanzania. The physical location is the large part of Africa to the south of the extensive Congo River basin. Southern Africa is home to a number of ...

, developing a magmatic flow controlled by an Early Jurassic zone of extension related to a triple junction in the proto-

region at approximately 55°S. This eruptions phase includes the

Dufek Intrusion The Dufek Intrusion is a mafic layered intrusion that was emplaced into present-day Antarctica approximately 183 million years ago. It comprises two outcropping sections called the Dufek Massif and the Forrestal Range that are thought to be connecte ...

, the

Ferrar Dolerite sills Ferrar is a surname. Notable people with the surname include: * Ada Ferrar (1867–1951), British actress * Beatrice Ferrar (1876–1958), British actress * Bill Ferrar (1893–1990), English mathematician * Catherine Ferrar (born 1940), American ...

and dikes, extrusive rocks consisting of pyroclastic strata, and the Kirkpatrick Basalt lava flows, with a total thickness variable, but exceeding 2 km in some places. This Volcanism is not limited to the Antarctica, as it was recorded also in

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

and

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

, suggesting that these area where connected back then. __TOC__

Paleoenvironment

The Mawson Formation was described originally subdivided in two sections, that where identified as separate units. This, is due to a clear differentiation of two kinds of deposits: the so-called "Mawson Tuffs", representing lithified pyroclastic material and the "Carapace sandstones", alluvial/lacustrine, both deposited in a setting defined by Ballance and Watters (1971) as composed by “shallow, northeast flowing, ephemeral streams on a subsiding alluvial plain”. Beyond alluvial settings, ancient lakes, with

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

influence, where developed thanks to the relationships with the overliying Kirckpatrick Basalt. This deposits mark the know locally as "Mawson Time", a section of the sedimentological evolution of the Ferrar Range, where volcanic material deposited in

Allan Hills The Allan Hills are a group of hills at the end of the Transantarctic Mountains System, located in Oates Land and Victoria Land regions of Antarctica. They are mainly ice free and about long, lying just north-west of the Coombs Hills near th ...

and Coombs Hills, while the Carapace Sandstones hosted an alluvial plain that recovered all the volcanic detritus, being latter flooded and developing a lacustrine ecosystem. The Formation includes two main locations: Carapace Nunatak in South Victoria Land, representing a deposit of interbeds dominated by sandstones of fluvial to lacustrine origin. The main outcrop of this location is notorious for the presence of a 37 m Hialoclastite, volcanic material accumulated, likely on a local lake of the same depth. This lake layers, called "Lake Carapace", host the only relatively complete fish remains recovered in the whole formation, and was likely feed by seasonal streams that brought the volcanic materials from sources located far away of the alluvial setting. The "Lake Carapace" also shows temporal exposed paleosoils, with and without roots, as well with muds cracks, indicating seasonal drougths. This lacustrine-type deposit is also found on the second main fossiliferous outcrops of the formation, being in the

Queen Alexandra Range The Queen Alexandra Range is a major mountain range of the Transantarctic Mountains System, located in the Ross Dependency region of Antarctica. It is about long, bordering the entire western side of Beardmore Glacier from the Polar Plateau to ...

in the Central Transantarctic Mountains. Sedimentary interbeds deposited over lava flows of the Kirkpatrick Basalt during the Early Jurassic splitting of Gondwana represent unusual freshwater paleoenvironments, with hotter conditions that allow to the diversification of the microbes (

Archea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeb ...

). According to Barrett, "...the basalt-dominated Mawson

Formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondar ...

and

tholeiitic The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma ...

flows (Kirkpatrick Basalt)...are included in the Ferrar Group." The Mawson Formation consists of diamictites, explosion breccias, and lahar flows, evidence of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

entering water-saturated

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

s. The Kirkpatrick Basalts (180 Ma) have interbedded lake sediments with plant and fish fossils.

Fossil content

There abundant Fossils of microorganisms, as members of the group

and other who take advantage of the hydrothermal activity The Acuatic fauna, dominated by invertebrates, includes a diversity of species complete enough to establish Trophic chains: there are traces of feeding, including a coprolite of uncertain affinity with a fish scale, conchostracan valves with traces of possible biotic borings and palynological residues linked with Ostracodan valves.

Demospongiae

Crustacea

Insects

Fossil insect wings not described to the genus level are known from the formation. The overall record of local insects include up to 50 specimens all recovered in lacustrine deposits.

Fish

Fungi

Palynology

Mostly of the samples recovered at Carapace Nunantak are characterised by dominance of the Cheirolepidaceous '' Classopollis'' and '' Corollina''. Two taxa, the Araucariaceous '' Callialasporites dampieri'' and the

Pteridaceae Pteridaceae is a family of ferns in the order Polypodiales, including some 1150 known species in ca 45 genera (depending on taxonomic opinions), divided over five subfamilies. The family includes four groups of genera that are sometimes recogni ...

'' Contignisporites cooksoni'' are also common palynological residues in local samples.

Megaflora

One of the best preserved fossil flora of the Antarctic. Nearly all the floral remains where recovered from Siliclastic interbeds, being mostly of them Silidified.

References

{{reflist, 30em Geologic formations of Antarctica Jurassic System of Antarctica Pliensbachian Stage Toarcian Stage Mudstone formations Tuff formations Lacustrine deposits Paleontology in Antarctica