# 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 (including the upper Clarens Formation desertic interbeds), 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 analisis 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 (1971) 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 (Archea).
According to Barrett, "...the basalt-dominated Mawson Formation and tholeiitic flows (Kirkpatrick Basalt)...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 (180 Ma) 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 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

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
| 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 | |

### Crustacea

| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
| Carapacestheria | C. disgregaris C. balli | Carapace Nunatak Mackay Glacier | All the Sections | Carapaces | A Freshwater member of Diplostraca (Spinicaudatan). Related to the modern Cyzicus mexicanus and recovered in siliclastic interbeds, representing the most common fossil animal in the unit. | |
| Cornia | C. sp. 1 C. sp. 2 | Blizzard Heights | All the Sections | Carapaces | A Freshwater member of Diplostraca (Spinicaudatan). Represents the only Jurassic Records of the genus, know mostly from Permian and Triassic deposits, being a possible relict genus. Specimens recovered show different variations in coloration, what can indicate effects of hydrothermal influence on either the living animal or the dead carapace. | |
| Darwinula | D. spp. | Carapace Nunatak Mackay Glacier | All the Sections | Valves | Common Early Jurassic Freshwater ostracod. The specimens of this genus cannot be identified to species level, yet bear resemblance with specimens from the same age of South Africa, as well as Triassic specimens from India. |
| Isopodichnus | I. isp. type A I. isp. type B | Carapace Nunatak | All the sections | Braided Structures | Freshwater tubular braided Structures. Interpreted as traces of crustaceans searching for food in the lacustrine bottom | |
| Lepidurus | L. stormbergensis | Carapace Nunatak | All the Sections | Complete Specimens | A Freshwater member of Notostraca. Represented by specimens much bigger than forms (20 mm compared with smaller 10–12 mm breadth) from South Africa | Extant Specimen of the Genus |
| Lioestheria | L. doumanii | Blizzard Heights Brimstone Peak | All the Sections | Carapaces | A Freshwater member of Diplostraca (Spinicaudatan). Correlated with coeval East African and Indian lioestheriids | |
| Protamphisopus | P. wianamattensis | Carapace Nunatak | All the Sections | Complete Specimens | A Freshwater member of Isopoda. Shows affinities with specimens from the Upper Triassic of New South Wales | Extant Phreatoicus typicus, closely related with Protamhisopus wianamatthensis |
| Scoyenia | S. isp. | SW Gair Mesa | All the Sections | Burrows | Burrow fossils in lacustrine environment, probably made by arthropods | |
| Syncarida | Inderminate | Carapace Nunatak | All the Sections | Complete Specimens | A Freshwater member of Syncarida. | Extant example of Syncarid |

### Arachnida

| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
| Oribatida | Indeterminate | Storm Peak | Middle Section | Excavated areas filled with coprolites | Mite traces, incertae sedis inside Oribatida. The tunnels where recovered in wood stems, fern rhizomes and petioles. | example of Oribatida mite |

### 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 |
| 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 (Schizophoridae, Catiniidae) and some adephagian beetles (Hygrobiidae, Amphizoidae) 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 |

### Fish

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Archaeomaenidae | Indeterminate | Storm Peak | Middle Section | One patch of scales Coprolite | A Freshwater member of Archaeomaenidae. |
| Oreochima | O. ellioti | Storm Peak Blizzard Heights Carapace Nunatak | Middle Section | Various specimens Isolated Scales | A Freshwater archaeomaenid. One of the few fishes from this family recovered outside Australia, represents a genus that likely lived linked with Hydrothermal settings and was very proliferous on the local lacustrine systems. Represents a rather small genus. |

### Fungi

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Ceratocystis? | Indeterminate | Carapace Nunatak | Middle Section | Hyphae | Parasitic Fungus, probably of the family Ceratocystidaceae. Infestation traces and fungal parasitic interaction on several plants. The morphology shown by this hypae and the colonization pattern in the woods resemble that of the extant Verticicladiella wageneri. |
| Fungi | Indeterminate | Carapace Nunatak | Middle Section | Hyphae | Parasitic Fungus of uncertain relationships. Infestation traces of thick-walled hypae where recovered on Brachyphyllum-type foliage locally |
| Fungi Traces? | Indeterminate | Storm Peak | Middle Section | Galleries? in Valves | Galleries of an infesting organism in conchostracan valves |

### 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 (2 to 6 m) 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. | Extant Araucaria. Callialasporites may come from a related plant |
| Baculatisporites | B. comaumensis | Carapace Nunatak | Spores | Affinities with the family Osmundaceae in the Polypodiopsida. Near fluvial current ferns, related to the modern Osmunda regalis. | Extant Osmunda specimens; Todisporites probably come from similar genera or maybe a species from the genus |
| 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. | Extant Pityrogramma specimens; Contignisporites probably come from similar genera or maybe a species in the genus |
| 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. | Extant Cyathea; Cyathidites probably come from similar genera |
| 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. | Extant Ginkgo, the only surviving member of the Ginkgoaceae. Monosulcites pollen is similar to the pollen of this extant species. |
| Inaperturopollenites | I. limbatus | Carapace Nunatak | Pollen | Affinities with Cupressaceae. | |
| Ischyosporites | I. crateris | Carapace Nunatak Storm Peak Coalsack Bluff | Spores | Uncertain peridophyte affinities | |
| Neoraistrickia | N. spp. | Carapace Nunatak | Spores | Affinities with the Selaginellaceae. Herbaceous lycophyte flora, similar to ferns, found in humid settings. | Extant Selaginella, typical example of Selaginellaceae. Genera like Neoraistrickia probably come from a similar or a related Plant |
| 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. | Extant Picea. Pinuspollenites maybe come from a related plant |
| 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 | Extant Podocarpus. Podosporites maybe come from a related plant |
| 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. | Extant Sphagnum specimens; Sculptisporis probably come from similar genera |
| 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. | Extant Lophosoria specimens; Trilobosporites probably come from similar genera |
| 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 assamblage of fossil trunks, with diameters between 8 and 23 cm and possible arthropod tunnels, are know from Suture Bench.
| Genus | Species | Location | Material | Notes | Images |
| Agathoxylon | A. pseudoparenchymatosum A. 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. currumilii B. 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 (Closely related with the extant genera Dennstaedtia, Lindsaea, and Odontosoria). | Coniopteris specimen |
| 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. | Example of extant relative of Marchantites, Marchantia |
| 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. | Example of Otozamites specimen |
| 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. | Example of Pagiophyllum specimen |
| 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 | Example of Zamites specimen |