The Gai-As Formation is an

Early Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

Middle Permian The Guadalupian is the second and middle series/epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico and Texas, and dates between 272.95 ± 0. ...

( Kungurian to Wordian) geologic

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondar ...

correlated with the Ecca Group and designated "Ecca" Group, because it does not belong to the

Karoo The Karoo ( ; from the Afrikaans borrowing of the South Khoekhoe !Orakobab or Khoemana word ''ǃ’Aukarob'' "Hardveld") is a semi-desert natural region of South Africa. No exact definition of what constitutes the Karoo is available, so its ext ...

, in the southwestern Kunene Region and northern Erongo Region of northwestern

Namibia Namibia (, ), officially the Republic of Namibia, is a country in Southern Africa. Its western border is the Atlantic Ocean. It shares land borders with Zambia and Angola to the north, Botswana to the east and South Africa to the south and ea ...

. The Gai-As Formation represents the second-oldest sedimentary unit of the Huab Basin, overlying the Huab Formation. The formation was deposited in a

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

to lacustrine setting. The Gai-As Formation is correlated with a series of formations in the

Pelotas Pelotas () is a Brazilian city and municipality (''município''), the third most populous in the southern state of Rio Grande do Sul. It is located 270 km (168 mi) from Porto Alegre, the state's capital city, and 130 km (80.8&nb ...

and Paraná Basins in southeastern Brazil, deposited in a larger basinal area, 150 million years before the break-up of Pangea. The abundance of '' Glossopteris'' and ''

Mesosaurus ''Mesosaurus'' (meaning "middle lizard") is an extinct genus of reptile from the Early Permian of southern Africa and South America. Along with it, the genera '' Brazilosaurus'' and ''Stereosternum'', it is a member of the family Mesosauridae a ...

'' fossils in the underlying Huab Formation are characteristic of the

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...

n correlation across present-day South America, Africa, Antarctica and Australia. The Gai-As Formation has provided fossil

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

s and an indeterminate stereospondylid.

Description

The Gai-As Formation is named after the ruins of a colonial German police station located at a spring in the central Huab area.Wanke, 2000, p.11 The formation is a lithological unit with an approximate maximum thickness of , deposited in the Huab Basin where it overlies the Huab Formation, separated by a significant hiatus,Wanke, 2000, p.25 and is unconformably overlain by the Etjo Sandstone,Horsthemke et al, 1990, p.68 while later authors assign a separate formation to the upper of the Gai-As Formation, named Doros Formation. The formation comprises

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

s and

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

s deposited in a lacustrine

environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...

. U/Pb SHRIMP dating of zircons from two of several fallout

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

beds in the Gai-As Formation above the stereospondyl find gave weighted mean values of 272 ± 1.8 Ma and 265 ± 2.5 Ma. The Gai-As Formation is the most widespread depositional unit within the Huab Basin. The approximately thick succession is easily distinguished from all the other units by its characteristic reddish to violet color. The Gai-As Formation gradationally overlies the lacustrine deposits of the Huab Formation in the western part of the basin, but in the east succeeds the

deposits of the Huab Formation with a sharp contact. The basal beds of the formation are ubiquitously represented by fine-grained distal deposits and imply a much larger extent of the formation due to the absence of sediments from marginal environments.Horsthemke et al, 1990, p.73 The lower part of the succession is formed by approximately of thick reddish-violet, slightly

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

s. In the lower half of this mainly pelitic succession, intercalations of dark brown, very often concretionary calcareous layers up to thick, occur. These layers contain siltstone beds which locally exhibit flaser bedding, oscillation ripples and hummocky cross-bedding. The calcareous layers also contain small fossiliferous lenses with fishbones. The upper half of the basal shaly section contains a few layers, thick, which are whitish and analcime-rich and occasionally thin layers of green

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

. The basal beds grade into a thick succession of silty to fine-grained, and eventually medium- to coarse grained, sandstone. Sections of the uppermost part show various types of intercalations: silt- and sandstone layers grade laterally into sequences of interbedded shale and sandstone with occasional intercalations of stromatolitic layers, carbonate horizons with

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

s, bone beds of fish scales and white layers rich in authigenic

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...

. The shale frequently exhibits desiccation cracks, whereas root marks are abundant in sandstone.

Halite Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, p ...

crystal moulds have been found locally. The white tuff layers within the Gai-As Formation contain very well-preserved former glass shards, now mainly replaced by

barite Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...

, are rich in potassium feldspar and have a porcelain-like appearance.Werner, 2006, p.14

Depositional environment

The Gai-As Formation as a whole displays an upward coarsening sequence which starts with distal shallow lacustrine shales. The occurrence of flaser bedding, oscillation ripples and hummocky cross-stratification in silty intercalations indicates that the sediment, surface must have been above the storm wave base. The remarkably high content of analcime (30 volume percent) in some layers may be indicative of contemporaneous volcanic activity. Early diagenetic formation of analcime from volcanic glass in an alkaline environment has been described. High alkalinity of the lake water may also have been the reason for the almost complete lack of fossils in this part of the section. The overlying succession of sandstones, siltstones, stromatolites and emerged horizons reflects the upward shallowing of the formation. Besides extensive sand flats, large playas, several square kilometres in size, must have existed during the final stages of deposition. The playa deposits are represented by shaly sediments with intercalated feldspar-rich layers (), stromatolite horizons () and fish scale bone beds (). Fossiliferous stratiform carbonates ( thick) with molluscs of the '' Terraia altissima'' type are transitional to the sand-flat facies which is characterised by sandy, flat and lenticular deposits and occasionally contain abundant root marks. The lowermost part of the formation indicates a fluvio-lacustrine environment.Holzförster et al, 2000, p.286

Fossil content

The following fossils have been reported from the formation:Huab Basin amphibian locality
at

Fossilworks Fossilworks is a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database The Paleobiology Database is an online resource for information on the distribution and classification of fossil animals ...

.orgKlein Gai-As, lower Gai-As Formation, Huab Basin
at

.orgKlein Gai-As, upper Gai-As Formation, Huab Basin
at

.org ;Amphibians * Stereospondyli indet. ;Bivalves * '' Cowperesia emerita'' * '' Huabiella compressa'' * '' Terraia cf. altissima'', ''T. cf. curvata''

Correlations

The lower part of the formation is correlated with the Serra Alta Formation and the upper part with the Teresina Formation of the Paraná and Pelotas Basins in

Rio Grande do Sul Rio Grande do Sul (, , ; "Great River of the South") is a Federative units of Brazil, state in the South Region, Brazil, southern region of Brazil. It is the Federative_units_of_Brazil#List, fifth-most-populous state and the List of Brazilian st ...

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

.Horsthemke et al, 1990, p.74 Due to increased subsidence in the central Paraná Basin, thicknesses of the correlative units are considerably greater than that of the Gai-As Formation, attaining a maximum of .Horsthemke et al, 1990, p.75 In the

Karoo Basin The Karoo Supergroup is the most widespread stratigraphy, stratigraphic unit in Africa south of the Kalahari Desert. The group (stratigraphy), supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Pennsylvan ...

of southern Namibia and the Eastern,

Northern Northern may refer to the following: Geography * North, a point in direction * Northern Europe, the northern part or region of Europe * Northern Highland, a region of Wisconsin, United States * Northern Province, Sri Lanka * Northern Range, a ra ...

and

Western Cape The Western Cape is a province of South Africa, situated on the south-western coast of the country. It is the fourth largest of the nine provinces with an area of , and the third most populous, with an estimated 7 million inhabitants in 2020 ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...

, the formation is time-equivalent with the Collingham,

Ripon Ripon () is a cathedral city in the Borough of Harrogate, North Yorkshire, England. The city is located at the confluence of two tributaries of the River Ure, the Laver and Skell. Historically part of the West Riding of Yorkshire, the city ...

and Fort Brown Formations. The fossil assemblages of '' Glossopteris'' and ''Mesosaurus'' that occur in the underlying Huab Formation are known from other parts of

; the Vryheid Formation of South Africa and coal deposits of the Lower Permian in Australia.Horsthemke et al, 1990, p.69

References

Bibliography

* * * * {{cite LSA , last=Werner , first=Mario , year=2006 , title=The stratigraphy, sedimentology, and age of the Late Palaeozoic Mesosaurus Inland Sea, SW-Gondwana - New implications from studies on sediments and altered pyroclastic layers of the Dwyka and Ecca Group (lower Karoo Supergroup) in southern Namibia (PhD thesis) , url=https://opus.bibliothek.uni-wuerzburg.de/opus4-wuerzburg/frontdoor/deliver/index/docId/1918/file/00-Complete_thesis-STD.pdf , publisher= Julius-Maximilians-Universität Würzburg , pages=1–428 , accessdate=2018-08-26