Mawson Formation

The Mawson Formation is a geological formation in Antarctica, dating to roughly between 182 and 177 million years ago and covering the Toarcian stages of the Jurassic Period in the Mesozoic Era. Vertebrate remains are known from the formation. The Mawson Formation is the South Victoria Land in the Ross Dependency equivalent of the Karoo Large Igneous Province in South Africa, as well the Lonco Trapial Formation and the Cañadón Asfalto Formation of Argentina. The Volcanic material was likely sourced from the Antarctic Peninsula's Ellsworth Land Volcanic Group.

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, from the Weddell Sea region to North Victoria Land, covering approx. 3,500 km in length. This event was linked with the initial stages of the breakup of the Gondwanan part of Pangea, concretely with the rifting of East Antarctica and Southern Africa, developing a magmatic flow controlled by an Early Jurassic zone of extension related to a triple junction in the proto-Weddell Sea region at approximately 55°S. This eruptions phase includes the Dufek Intrusion, the Ferrar Dolerite sills 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 and New Zealand, suggesting that these area where connected back then. The Paleovulcanology analysis of the Mawson Formation have recovered Permian and Triassic material, which was eroded by lavas, with the presence of tachylite pyroclasts that imply rapid cooling by interaction with water.

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 as composed by "shallow, northeast flowing, ephemeral streams on a subsiding alluvial plain". The Mawson Formation was thus, heavily influenced by vulcanism, with tuff breccia deposits dropped in a <100 m paleotopography valley in Coombs Hills, probably reduced from previous erosion events, while at Allan Hills a paleovalley of up to 500 m was present. In this paleovalleys, massive production and accumulation of volcanic lahars in lowlands occur, in a similar way to more recent ones of places such as Osceola Mudflow at Mount Rainier. Over this pyroclastic sequences, lacustrine beds developed temporally. Thus, beyond alluvial settings, ancient lakes, with hydrothermal influence, where developed and latter basaltically surrounded 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 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 described lacustrine system was, like the "Chacritas Paleolake" of the sister Cañadón Asfalto Formation in Patagonia, developed following the local rift in a similar way to the modern Lake Magadi in the Kenyan Rift Valley, as proven by the discovery of Chert like the one found in this african lake, what suggest that both, Carapace and Chacritas where likely alkaline lakes that had notorious influence of hydrothermal fuids. Other more recent lacustrine/fluvial sequences have been described in new outcrops, like at Suture Bench and SW Gair Mesa, with abundant invertebrate and plant fossils.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 droughts. This lacustrine-type deposit is also found on the second main fossiliferous outcrops of the formation, being in the Queen Alexandra Range 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.
According to Barrett, "...the basalt-dominated Mawson Formation and tholeiitic flows...are included in the Ferrar Group." The Mawson Formation consists of diamictites, explosion breccias, and lahar flows, evidence of magma entering water-saturated sediments. The Kirkpatrick Basalts have interbedded lake sediments with plant and fish fossils.

Fossil content

There abundant Fossils of microorganisms, as members of the group Archea and other who take advantage of the hydrothermal activity The aquatic 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

Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Demospongiae	Indeterminate	Carapace Nunatak	All the sections	Borings in invertebrate valves	Holes of random pattern in valves. Boring Traces on local Conchostracan valves are common and suggested to resemble the boring traces of extant sponges, yet there isn't any evidence of Porifera fossils in the local beds

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.

Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Blattaria	Indeterminate	Carapace Nunatak SW Gair Mesa	Middle Section	Tegmen of a fossil cockroach Complete blattid insect	Indeterminate Blattaria Cockroaches
Caraphlebia	C. antartica	Carapace Nunatak	Middle Section	Wings	A dragonfly of the family Selenothemidae. Was found to be related with the genus Liassophlebia, but the hind wing has severalweak antenodals in addition to the two strong, primary ones.
Coleoptera	Indeterminate	Carapace Nunatak	Middle Section	Charred fragmentary beetle elytron	A Beetle with resemblance with archostematids and some adephagian beetles that have such elytra
Ephemeroptera	Indeterminate	Carapace Nunatak	Middle Section	Abdominal segments and paired cerci	Indeterminate Mayfly nymphs
Hemiptera	Indeterminate	Carapace Nunatak	Middle Section	Abdominal segments and isolated wings	Indeterminate Hemipterans

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 Contignisporites cooksoni are also common palynological residues in local samples.

Genus	Species	Location	Material	Notes	Images
Alisporites	A. similis A. spp.	Carapace Nunatak Storm Peak Coalsack Bluff	Pollen	Affinities with the families Caytoniaceae, Corystospermaceae, Peltaspermaceae, Umkomasiaceae and Voltziaceae
Aratrisporites	A. sp.	Carapace Nunatak	Spores	Affinities with Pleuromeiales. The Plueromeiales were tall lycophytes common in the Triassic. These spores probably reflect a relict genus.
Araucariacites	A. australis	Carapace Nunatak Storm Peak Coalsack Bluff	Pollen	Affinities with the family Araucariaceae in the Pinales. Conifer pollen from medium to large arboreal plants.
Baculatisporites	B. comaumensis	Carapace Nunatak	Spores	Affinities with the family Osmundaceae in the Polypodiopsida. Near fluvial current ferns, related to the modern Osmunda regalis.
Callialasporites	C. dampieri C. segmentatus C. turbatus	Carapace Nunatak Storm Peak Coalsack Bluff	Pollen	Affinities with the family Araucariaceae in the Pinales. Conifer pollen from medium to large arboreal plants.
Cibotiumspora	C. jurienensis	Carapace Nunatak	Spores	Affinities with Cyatheaceae and Dicksoniaceae inside Filicopsida.
Classopollis	C. classoides	Carapace Nunatak Storm Peak Coalsack Bluff	Pollen	Affinities with Cheirolepidiaceae inside Pinales.
Contignisporites	C. cooksoni	Carapace Nunatak Storm Peak Coalsack Bluff	Spores	Affinities with the Pteridaceae in the Polypodiopsida. Forest ferns from humid ground locations.
Corollina	C. spp.	Carapace Nunatak Storm Peak Coalsack Bluff	Pollen	Affinities with Cheirolepidiaceae inside Pinales.
Cupressacites	C. ramachandrae	Carapace Nunatak Storm Peak Coalsack Bluff	Pollen	Affinities with Cupressaceae.
Cyathidites	C. australis C. minor	Carapace Nunatak	Spores	Affinities with the family Cyatheaceae or Adiantaceae. Arboreal fern spores.
Dictyophyllitides	D. harrisi	Carapace Nunatak	Spores	Affinities with the family Schizaeaceae, Dicksoniaceae or Matoniaceae.
Exesipollenites	E. spp.	Carapace Nunatak Storm Peak Coalsack Bluff	Pollen	Affinities with Araucariaceae or Cheirolepidiaceae inside Pinales.
Ginkgocycadophytus	G. nitidus	Carapace Nunatak	Pollen	Affinities with the family Karkeniaceae and Ginkgoaceae.
Inaperturopollenites	I. limbatus	Carapace Nunatak	Pollen	Affinities with Cupressaceae.
Ischyosporites	I. crateris	Carapace Nunatak Storm Peak Coalsack Bluff	Spores	Uncertain pteridophyte affinities
Neoraistrickia	N. spp.	Carapace Nunatak	Spores	Affinities with the Selaginellaceae. Herbaceous lycophyte flora, similar to ferns, found in humid settings.
Osmundacidites	O. senectus O. wellmanii	Carapace Nunatak Storm Peak Coalsack Bluff	Spores	Affinities with the family Osmundaceae in the Polypodiopsida. Near fluvial current ferns, related to the modern Osmunda regalis.
Pinuspollenites	P. globosaccus	Carapace Nunatak Storm Peak Coalsack Bluff	Pollen	Affinities with the family Pinaceae in the Pinopsida. Conifer pollen from medium to large arboreal plants.
Podosporites	P. variabilis	Carapace Nunatak	Pollen	Affinities with the family Podocarpaceae. Occasional bryophyte and lycophyte spores are found along with consistent occurrences of Podosporites variabilis
Protohaploxypinus	P. sp.	Carapace Nunatak Storm Peak Coalsack Bluff	Pollen	Affinities with the family Pinaceae in the Pinopsida. Conifer pollen from medium to large arboreal plants.
Puntactosporites	P. scabratus	Carapace Nunatak	Spores	Uncertain peridophyte affinities
Retitriletes	R. austroclavatidites	Carapace Nunatak	Spores	Affinities with the Lycopodiaceae.
Sculptisporis	S. moretonensis	Carapace Nunatak	Spores	Affinities with the family Sphagnaceae. "Peat moss" spores, related to genera such as Sphagnum that can store large amounts of water.
Striatella	S. seebergensis	Carapace Nunatak	Spores	Affinities with the Pteridaceae in the Polypodiopsida. Forest ferns from humid ground locations.
Todisporites	T. minor	Carapace Nunatak	Spores	Affinities with the Pteridaceae in the Polypodiopsida. Forest ferns from humid ground locations.
Trilobosporites	T. antiquus	Carapace Nunatak Storm Peak Coalsack Bluff	Spores	Affinities with the genus Dicksoniaceae in the Polypodiopsida. Tree fern spores.
Verrucosisporites	V. varians	Carapace Nunatak	Spores	Uncertain peridophyte affinities
Vitreisporites	V. signatus	Carapace Nunatak	Pollen	Affinities with the Caytoniaceae

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. A large assemblage of fossil trunks, with diameters between 8 and 23 cm and possible arthropod tunnels, are known from Suture Bench.

Genus	Species	Location	Material	Notes	Images
Agathoxylon	A. pseudoparenchymatosumA. sp.	Carapace Nunantak Mt. Fazio, Mesa Range Haban Spur, Mesa Range	Fossil Wood	Affinities with Araucariaceae or Cheirolepidiaceae inside Pinales.
Brachyoxylon	B. sp. cf. B. currumiliiB. sp.	Carapace Nunantak Mt. Fazio, Mesa Range	Fossil Wood	Affinities with Araucariaceae or Cheirolepidiaceae inside Pinales.
Brachyphyllum	B. spp.	Carapace Nunatak	Several isolated Branched Shoots	A member of the family Cheirolepidiaceae. Associated with Classostrobus cones.
Chimaerostrobus	C. minutus	Carapace Nunatak	Single Pollen Cone	A conifer pollen cone of uncertain Relationships. Chimaerostrobus is reminiscent of extant Araucariaceae and several extinct taxa such as Kobalostrobus and Voltziales.
Cladophlebis	C. oblonga	Carapace Nunantak	Isolated Pinnae	A Fern of the family Osmundaceae. Some specimens where reworked from the Hanson Formation to the Mawson Formation. Linked with the tree fern genus Osmundacaulis
Classostrobus	C. elliotii	Carapace Nunatak	Five permineralized pollen cones	A member of the Cheirolepidiaceae. More than five Brachyphyllum-type leaves where found in close association with these cones.
Coniopteris	C. hymenophylloides	Carapace Nunantak	Isolated Pinnae	A Fern of the family Polypodiales inside Polypodiidae. Common cosmopolitan Mesozoic fern genus. Recent research has reinterpreted it a stem group of the Polypodiales.
Dictyozamites	D. sp. cf. minisculus	Carapace Nunatak	Leaflets	A cycadophyte of the family Bennettitales.
Elatocladus	E. confertus	Carapace Nunatak	Branched Shoots	A member of the family Cupressaceae. Related to specimens found in the Middle Jurassic of Hope Bay, Graham Land. Probably represent belong to the Conifer Austrohamia from the Lower Jurassic of Argentina and China.
Marchantites	M. mawsonii	Carapace Nunantak	Isolated Thalli	A liverwort of the family Marchantiales. Some specimens where reworked from the Hanson Formation to the Mawson Formation. This liverwort is related to modern humid-environment genera.
Mixoxylon	M. jeffersonii	Mt. Fazio, Mesa Range	Fossil Wood	Spermatophyte Wood, probably related to Bennettitales or Cycadales and previously know only from Cretaceous strata, suggesting the Antarctic Floral Biome appeared already in the Jurassic
Nothodacrium	N. warreni	Carapace Nunatak Storm Peak	Cutinised and fertile material	A member of the family Voltziales. A genus with Resemblance with the extant Dacrydium that was referred to Podocarpaceae, yet a more recent work found it to be just a convergently evolved relative of Telemachus.
Otozamites	O. antarcticus	Carapace Nunatak	Leaflets	A cycadophyte of the family Bennettitales.
Pagiophyllum	P. spp.	Carapace Nunantak	Single Branched Shoot	A member of the Pinales of the family Araucariaceae. Representative of the presence of arboreal to arbustive flora.
Podostrobus	P. warrenii	Carapace Nunatak	Single cone	A member of the family Voltziales. Originally assigned to the Cheirolepidiaceae, was later suggested to share affinities with the Podocarpaceae, and then found to be a member of Voltziales. Likely represents the cone of the same conifer that produced the Nothodacrium foliage, as convergently resembles cones from extant Microcachrys and Dacrydium.
Protocupressinoxylon	P. spp.	Coombs Hills Storm Peak Mt. Fazio, Mesa Range	Fossil Wood	A member of the family Cupressaceae.
Polyphacelus	P. stormensis	Storm Peak	Silicified rachides	A Polypodiopsidan of the family Dipteridaceae. Closely related to Clathropteris meniscoides.
Zamites	Z. spp.	Carapace Nunatak	Leaflets	Spermatophyta incertae sedis