Stratigraphic column in southern Victoria Land Antarctica
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.[6] 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.[7] 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.[8] 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.[8] 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.[9] 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.[10]
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”.[2] 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.[11] 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.[11] 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.[1] 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.[2][1] 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.[12] 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.[13]
The Mawson Formation was developed in a similar way to modern Waimangu Volcanic Rift Valley of New Zealand, with Lakes Influenced by Hydrothermal vents. The Carapace Paleolake itself developed like the Lake Magadi in the Kenyan Rift Valley
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.[14] 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.[14] 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.[14] 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.[15]
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).[16]
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.[17][18]
Fossil content
There abundant Fossils of microorganisms, as members of the group Archea and other who take advantage of the hydrothermal activity[16][6][19] 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.[20]
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
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.[16][6]
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.[25]
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.[16][6]
Fossil insect wings not described to the genus level are known from the formation.[13] The overall record of local insects include up to 50 specimens all recovered in lacustrine deposits.[31]
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.
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.[35]
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.[36]
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 PteridaceaeContignisporites cooksoni are also common palynological residues in local samples.[38]
Affinities with the family Podocarpaceae. Occasional bryophyte and lycophyte spores are found along with consistent occurrences of Podosporites variabilis
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.[41] A large assamblage of fossil trunks, with diameters between 8-23 cm and possible arthropod tunnels, are know from Suture Bench.[13]
A conifer pollen cone of uncertain Relationships. Chimaerostrobus is reminiscent of extant Araucariaceae and several extinct taxa such as Kobalostrobus and Voltziales.[47]
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
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.
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
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.[50]
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.[50]
^Barrett, P.J. (1991). Tingey, Robert (ed.). The Devonian to Jurassic Beacon Supergroup of the Transantarctic Mountains and correlatives in other parts of Antarctica, in The Geology of Antarctica. Oxford: Clarendon Press. pp. 122–123, 129, 145. ISBN0198544677.
^Tingey, R.J. (1991). Tingey, Robert (ed.). Mesozoic tholeiitic igneous rocks in Antarctica: the Ferrar (Super) Group and related rocks, in The Geology of Antarctica. Oxford: Clarendon Press. pp. 159–160. ISBN0198544677.
^ abcdPlumstead, E. P. (1955). "Fossil Floras of Antarctica, with an appendix on Antarctic fossil wood by Richard Krüusel". Trans–Antarctic Expedition. 9 (1): 1–154.
^ abcPlumstead, E.P (1964). "Palaeobotany of Antarctica". Proceedings of the 1st International Symposium on Antarctic Geology, Cape Town. Amsterdam, North Holland. 1 (2): 637–654.
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