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Aztec Siltstone
Aztec Siltstone is a geological formation in Victoria Land, Antarctica. It is part of the Taylor Group which preserves fossils dating back to the Middle-Late Devonian boundary. The formation is the uppermost formation in the group and overlays the Beacon Heights Orthoquartzite and underlies the Weller Sandstone. It preserves an alluvial plain environment with a seasonal climate, a large amount of fish fossils including both placoderms and acanthodians have been found in the formation. Discovery and NamingThe Aztec Siltstone was originally defined in 1963 as a sequence made up of green and red siltstone between the Beacon Heights Orthoquartzite and the Weller Sandstone. Like its name suggests, the type section of the formation is located on the Aztec Mountain.[1] Though found on other places in the continent, the material at the formation was the first in situ fish material found in Antarctica; these were found during the Trans-Antarctic Expedition that took place between 1955-1957. Over the decades, the Aztec Sandstone has become one of the most diverse vertebrate assemblages from the Middle to Late Devonian.[2] Lithology The Aztec Siltstone varies in depth with the strata of the formation with the sections at Mount Richie reaching depths of 220 m while those seen at Maya Mountain only being a thickness of 40 m. There is a general thinning of the layers of the formation from south or north which is most likely caused by ice-sheets present in the area during the Late Carboniferous to Early Permian. Overlying of the Aztec Siltstone at the base of the Weller Coal Measure, the Metshel Tilite disconformity is present. The contact is gradational due to the lower beds of the disconformity being composed of locally eroded parts of the Aztec Siltstone. Unlike its upper contact, the lower between the Aztec Siltstone and Beacon Heights Orthoquartzite is conformable. This gradational contact is indicated by the finer beds which are very characteristic of the Siltstone and is around 5-10 m thick.[1] SandstoneIt is largely made up of fine to medium grain sandstones which are white or grey in color, with these rocks making up 50% of the volume of the formation. These beds range between 0.1-15 m thick and are evenly distributed throughout the formation. The different grained sandstones are laid in cycles and are most commonly tablet or lens-shaped. The sandstone blocks can span for hundreds of meters with thicker bodies made up of multiple beds even being able to span for kilometers. Sandstones rich in quartz are only present in small amounts and are associated with quartz pebbles. Though the sandstones aren't red, there is evidence like the presence of bleached sections of sandstone near red claystones, that suggests that the red pigment of the sandstone may have been chemically removed.[1][3] Siltstone and ClaystoneSiltstones and claystones also make up a large amount of the lithology, with the red beds making up 30% of the fine-grained parts of the formation. Just like the sandstone beds, these deposits are commonly lens-shaped and can span between 3 to 500m. The size of these beds is mostly determined by the depth which are much more variable than what is seen in the sandstone beds; these range between a few centimeters and 15 m in depth. Though these beds are originally described as being green and red in color, it is more accurate to say that they are greenish-grey and redish-green in color. The green color of the lithology is commonly a secondary color with remnant red pigment being found within it. The colors of these beds aren't present in any sort of order with zones of green being present within red beds. Though uncommon, there are grey sandstones also present.[3] There are conglomerates made up of clasts of these rocks in course-member units which range between a few millimeters. They likely represent underwater erosion of finer-grained lithologies right before the coarse-members were deposited. Due the conglomerates' non-rounded shapes, its likely they traveled a maximum of around a few hundred meters from where they originated from.[1] LimestoneThe least common sediment that makes up the lithology is limestones, mostly being present as cement within sandstones. There is a single occurrence of limestone with spherical to subspherical ooliods present, being in a thin, lens-shaped Portal Mountain. These ooliods are considered to have been formed in shallow calcium carbonate-rich lakes, specifically the high-energy wave zones. DatingThough consistently dated to the Middle to Late Devonian, the exact dating of the formation has slightly changed since the original description. This is due to the small amount of trace and plant fossils along with the conchostracans found at the Aztec Siltstone not being datable. It was originally suggested to be Upper Devonian in 1921 only to be dated to the upper Middle Devonian in 1969. One of the more recent suggestions was by Richie in 1975, which placed the formation between the Famennian to potentially the Strunian age. [1]The Strunian being a name for the uppermost segment of the Famennian which ranges from around 363 to 360 mya.[4] PaleobiotaAcanthodii
Chondrichthyes
Ichnotaxa
Osteichthyes
Placodermi
Plantae
Thelodonti
Paleoenvironment The Aztec Siltstone, as a whole, represents a low-lying alluvial plain. The floodplain would have been made up of a large amount of channels ranging between a few cm and around 0.5 m in width as indicated by a number of "scoured" surfaces. Along with these smaller streams, there would have been larger channels which would have been over 10 m wide and had depths ranging from 0.5-3 m. Throughout these channels, there is an extensive presence of mud cracks with indicate subaerial exposure that would have been common throughout the floodplain. These cracks are most similar to those seen in sandy ephemeral streams along with a low amount of vegetation near the body of water. However, due to the presence of tubes suggested to be created by roots along with plant spores found associated with them, there was some sort of vegetation present. The spores assemblage present at the formation is dominated by Geminospora, it is similar to what are seen in other late Devonian formations seen in Australia.[11] Another feature of the floodplain would have been the presence of oxbow lakes and pluvial ponds as shown by the presence of a large amount of small, symmetrical ripples in certain parts of the formation. These lesser parts of the floodplain would have had a depth of around 70 cm. [1][14] The chemistry of the formation show soils that suggest that the area had a seasonal, semi-arid climate being similar to those seen in certain portions of Australia. This is suggested by the presence of analcime, which may form as a result of saline alkaline soils caused by that type of climate, along with calcrete. Due to the presence of calcrete specifically, it is estimated that the area that makes up the Aztec Siltstone would have received less that 50 cm of precipitation during the year. After the alluvial plane dried up, there would have been a period of time of around 4000 to 10000 years where the sediments would have been exposed to the air.[14] References
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