The Kota Formation is a geological formation in India. The age of the Kota Formation is uncertain; it is commonly considered to date to the Early Jurassic, but some studies have suggested it may extend into the Middle Jurassic or even later. It conformably overlies the Lower Jurassic Upper Dharmaram Formation and is unconformably overlain by the Lower Cretaceous Gangapur Formation. It is split into a Lower Member and Upper Member. The Lower Member is approximately 100 m thick while the Upper Member is 490 m thick. Both subunits primarily consist of mudstone and sandstone, but near the base of the upper unit there is a 20-30 metre thick succession of limestone deposited in a freshwater setting.
Stratigraphy The lower boundary of the Kota Formation is made of pebbly sandstone, covering the topmost clay seen in the Dharmaram Formation. The Kota Formation has been traditionally divided into 2 main members, the Lower and Upper members, yet more recent work have redivided it into 3. The Lower member can be seen at locations such as Adamilli, Kamavarapukota and Sudikonda, being made of sandstones, with clay clasts, with greater or lower stratification. The Middle Member is well developed along the Continental Gondwana basin, specially towards the northwestern part, and is made of medium to fine white sandstone with clay and concretionary limestone, suggesting the development of paleosols associated with alluvial floodplains. The last member is mostly made of broad sandstone sheets with large clay casts associated with fluvial channels, and has an extension that can be easuly seen on several continuous kilometers. The Uppermost section of the unit is mostly made of limestones and is overlain on an angular unconformity by the Gangapur Formation.
Age The age of the Kota Formation is controversial. There are no magmatic rocks or volcanic ash beds associated with the Kota Formation, which means that its age cannot be determined directly through radiometric dating. The maximum age of the Kota Formation is constrained by the underlying Upper Dharmaram Formation, which is Early Jurassic, probably Hettangian or Sinemurian, in age. Various researchers have attempted to date the Kota Formation using biostratigraphy. Krishnan (1968), Jain (1973), and Yadagiri and Prasad (1977) favored an Early Jurassic age based on the fish fauna. Govindan (1975) suggested a Middle Jurassic age based on ostracods. In 2006, Bandyopadhyay and Sengupta argued that the fish fauna suggested a Toarcian age for the Upper Kota Formation, possibly extending into the Aalenian, and in turn estimated the Lower Kota to be Sinemurian to Pliensbachian in age. Guntupalli V. R. Prasad, along with various coauthors, has argued for a younger age. In 2001, Vijaya and Prasad proposed based on palynological evidence that the Kota Formation was deposited between the Callovian age of the Middle Jurassic and the Barremian age of the Early Cretaceous. In 2002, Prasad and Manhas argued that the mammal genus Dyskritodon, known only from the Kota Formation and the Early Cretaceous of Morocco, provides evidence for a young age for the Kota Formation. In 2020, Prasad and Parmar argued that the similarity of the dinosaur fauna of the Kota Formation to that of the Middle Jurassic of the United Kingdom supported a Middle Jurassic age for the Kota Formation.
Paleoenvironment
The Kota Formation represents mostly a Continental succession related to a continental rift basin, the Pranhita-Godavari Gondwana Basin of peninsular India. The associated facies of sandstone and limestones are likely related to playa-type lake, with nearby fluvial currents, part of low gradient hanging wall alluvial fans, being deposited on it´s margin. There have been records of freshwater lue green algal stromatolites and oncolites, suggested to be deposited on low energy and low bathymetry lacustrine settings. More recent works have proven the basin hosted in the Early Jurassic a freshwater carbonate wetland marked by the presence of limestones. The environmental model proposed include a depositional cycle marked by several facies types, A for the sublittoral zones of shallow water bodies, followed by palustrine environments, including surfaces with abundance of influence of both plants and animals, specially rhizobrecciation indicating active colonization of the margins by plants, having a similar deposition to the modern Las Tablas de Daimiel wetlands. Associated with the lacustrine facies have recovered microbial bioherms and lacustrine spring mounds, shallow ephemeral ponds with carbonated mud and Phyllopods, pedogenic calcrete under arid seasons and short-lived distributary channels. The depositional setting may have been partially sheltered from the input of siliciclastic materials, except on flooding seasons. Microbial biomats likely developed on shallow waters, while rhizoliths increased it´s presence of abandoned channel fills and pedogenic facies indicate drought seasons. The carbonate layers with iron-rich grains and mudstones suggest a transition zone between a lake margin and wetland in a rift valley, in a marsh that experienced periodic influxes of iron-rich and barium-rich waters through faults. These waters briefly increased acidity but were neutralized by mixing with alkaline surface water, allowing carbonate formation. Iron-oxidizing microbes likely helped deposit iron oxides, cementing the sediment. During high water flow, these deposits were eroded and transported to deeper waters, forming iron-rich coatings. Thin sandy layers suggest that freshwater influxes occurred, forming carbonate bodies near water discharge points. As the water returned to being more alkaline, carbonates could form again, supporting freshwater Bivalvia, similar to modern geothermal areas of New Zealand.
Fossil content
Ostracoda
Phyllopoda
Insecta
Fish
Amphibia
Mammaliaforms
Lepidosauromorpha
Testudinata
Crocodylomorpha Atoposaurid crocodiles are known from the unit, yet is not clear from what locality.
Pterosauria
Dinosaurs Indeterminate ankylosaurian remains have been unearthed in the Kota Formation.
Megaflora
See also List of dinosaur-bearing rock formations
Notes
References
