The Omingonde Formation is an

Early Early may refer to: Places in the United States * Early, Iowa, a city * Early, Texas, a city * Early Branch, a stream in Missouri * Early County, Georgia * Fort Early, Georgia, an early 19th century fort Music * Early B, stage name of Jamaican d ...

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...

(

Anisian In the geologic timescale, the Anisian is the lower stage (stratigraphy), stage or earliest geologic age, age of the Middle Triassic series (stratigraphy), series or geologic epoch, epoch and lasted from million years ago until million years ag ...

Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic ...

) geologic formation, part of the

Karoo Supergroup The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a per ...

, in the western

Otjozondjupa Region Otjozondjupa is one of the fourteen regions of Namibia. Its capital is Otjiwarongo. The region further contains the municipalities of Okahandja and Grootfontein and the towns Okakarara and Otavi. , Otjozondjupa had 97,945 registered voters. Geogr ...

and northeastern

Erongo Region Erongo is one of the 14 regions of Namibia. The capital is Swakopmund. It is named after Mount Erongo, a well-known landmark in Namibia and in this area. Erongo contains the municipalities of Walvis Bay, Swakopmund, Henties Bay and Omaruru, as ...

of north-central

Namibia Namibia, officially the Republic of Namibia, is a country on the west coast of Southern Africa. Its borders include the Atlantic Ocean to the west, Angola and Zambia to the north, Botswana to the east and South Africa to the south; in the no ...

. The formation has a maximum thickness of about and comprises

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

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. Although its permeabil ...

s and conglomerates, was deposited in a

fluvial A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...

environment, alternating between a

meandering A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave bank ( cut bank or river cliff) and deposits sediments on an inn ...

and

braided river A braided river (also called braided channel or braided stream) consists of a network of river channel (geography), channels separated by small, often temporary, islands called ''braid bars'' or, in British English usage, ''aits'' or ''eyots''. ...

setting. The Omingonde Formation is correlated with a series of formations in northwestern

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

and the

Paraná Basin The Paraná Basin (, ) is a large cratonic sedimentary basin situated in the central-eastern part of South America. About 75% of its areal distribution occurs in Brazil, from Mato Grosso to Rio Grande do Sul states. The remainder area is distribu ...

in southeastern Brazil, deposited in a larger basinal area, 120 million years before the break-up of

Pangea Pangaea or Pangea ( ) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia (continent), Siberia during the Carboniferous period ...

. The formation has provided fossils of several therapsids,

amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

s and

ichnofossil A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...

s and belongs to the ''Cynognathus'' Assemblage Zone. The Omingonde Formation preserves the most diverse fauna of Middle Triassic cynodonts in the world.

Description

The Omingonde Formation is named after a water well in Mount Etjo, Namibia. The formation is a lithological unit with an approximate maximum thickness of , deposited in the Waterberg Basin where it overlies the Damara

basement A basement is any Storey, floor of a building that is not above the grade plane. Especially in residential buildings, it often is used as a utility space for a building, where such items as the Furnace (house heating), furnace, water heating, ...

, and is unconformably overlain by the

Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic� ...

Etjo Sandstone.Wanke et al., 2000, p.294 The formation is widespread in the Otjiwarongo, Grootfontein and Omaruru Districts. Main outcrops are south of the Waterberg thrust near Mount Etjo, Omatakos and north of the Waterberg Plateau.Pickford, 1995, p.59 The Omingonde Formation splits naturally into four units, each comprising several upward-fining cycles reflecting distinct architectural characteristics. The lower two units coincide with the Lower and Middle Omingonde Formation, the upper two units make up the Upper Omingonde Formation. Thicknesses and architectural style of the units vary laterally and are influenced by syn-sedimentary tectonic activity of the Waterberg-Omaruru Lineament. The full range of facies is best developed in the Mt. Waterberg area, whereas farther west in the Erongo Region more proximal facies are favored. This proximal facies association has been termed the Krantzberg Formation in the western, northern and eastern Erongo area, and interfingers with the Lions Head Formation towards the southeastern Erongo Region. North of the Ameib Line, which is part of the Waterberg-Omaruru Fault zone, no rocks of Karoo age are preserved until the Otjongundu Basin, which hosts a thick conglomeratic red-bed sequence that probably correlates with the Omingonde Formation.Wanke, 2000, p.40 The formation comprises red and white colored clastics including conglomerates,

s and

s. Many units are channelized and show

paleosol In Earth science, geoscience, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The definition of the term in geology and paleontology is slightly different from its use in soil science. In geo ...

development. Three major facies are developed in the lower unit, including channelized, trough cross-bedded conglomerates, followed by sandy mudstones, which are interbedded with more massive, pebbly mudstones. The conglomerates are matrix-supported and grade vertically and laterally into sandstones. Their clasts derived dominantly from metasedimentary and granitic rocks. The channels are usually well confined and amalgamation occurs rarely. The sandy mudstones are reddish in colour, they contain lenticular interbeds of fine grained sandstones. They are characterised by a climbing-ripple lamination. Furthermore, the mudstones contain thin interlayers of blocky calcrete and carbonate nodules with some rhizoliths. Massive pebbly mudstones are restricted to positions very proximal to the Waterberg-Omaruru Fault. They are characterised by a massive matrix-supported texture and a low structural and compositional maturity of both, matrix and clasts.Wanke, 2000, p.41 The Upper Omingonde Formation is best developed in the Mt. Waterberg area, but no occurrences have been found in the Goboboseb-Otjongundu region. The succession begins with a laterally amalgamated channel facies alternating with a sandy mudstone facies. In the lower units, the channel bodies are wide and the degree of lateral amalgamation is similar to the Middle Omingonde Formation, but vertical stacking is less pronounced and decreases systematically upwards. In the upper part of the formation, channels are isolated and only a few meters wide. Up-section, the maturity of channel fills successively increases until well rounded medium to coarse grained sandstones dominate. The architecture remains cyclic with the channel facies alternating with laminated fine grained sandstones and bioturbated mudstones.Wanke, 2000, p.42

Depositional environment

The Omingonde Formation comprises chiefly braided

deposits,Wanke et al., 2000, p.293 which record semi-arid climatic conditions prevailing until the Upper Omingonde Formation. The latter marks a temporary shift to a wetter climate until semi-arid conditions re-established with the succeeding Etjo Formation. The degree of channel amalgamation and palaeosol development corresponds with sediment discharge rates: More confined meandering channels and extensive palaeosol developments are associated with lower discharge rates rather than amalgamated braided channel systems.Wanke, 2000, p.43 The channel geometries and the low degree of amalgamation in the Lower Omingonde Formation indicate an overall meandering river system. Sandy mudstones were generated during crevasse splays and associated flood stages. Intervals of non-deposition are indicated by immature paleosols, which are represented by scarce rhizoliths and blocky calcretes. In proximal positions to the Waterberg-Omaruru Fault pebbly mudstone units reveal mud flow deposits related to an enhanced fault generated relief. The Middle Omingonde Formation has been interpreted as a mobile braided stream system due to the well developed trough cross-bedding and upward-fining character of poorly confined channel bodies, together with their sheet-like appearance and ubiquitous vertical and lateral amalgamation. In addition, significant vertical accretion is indicated by the preservation of overbank fines, that abundantly show pedogenic modification. The proximal deposits of the Erongo area show textural and grain size characteristics of debris flows that amalgamated to form fault-bounded alluvial fan aprons. These fans give evidence for persistent syn-sedimentary activity of the Waterberg Fault system. The Upper Omingonde Formation reflects a progressive change from a braided river system to a more meandering river system with decreasing discharge rates. Temporarily wetter climates are indicated by bioturbated mudstones, that formed in seasonal lakes. Although a hiatus to the overlying Etjo Formation is proposed, the facies of the Upper Omingonde Formation represents a transitional stage to the facies of the Lower Etjo Formation. The syn-sedimentary tectonic control on Omingonde deposition is recorded by both thickening of strata towards the Waterberg-Omaruru Fault and by the pronounced shift of the adjacent depocenter, the latter suggesting left-lateral oblique slip movement.Wanke et al., 2000, p.295

Fossil content

The formation belongs to the ''Cynognathus'' Assemblage Zone as identified by Keyser in 1973;Pickford, 1995, p.53 and preserves the most diverse fauna of Middle Triassic cynodonts in the world; the formation is a biostratigraphic link among Middle Triassic faunas from South Africa, Tanzania (

Manda Formation The Manda Formation (also known as the Manda Beds) is a Middle Triassic (Anisian?) or possibly Late Triassic (Carnian?) Formation (stratigraphy), geologic formation in Tanzania. It preserves fossils of many terrestrial vertebrates from the Triassi ...

), Zambia, Argentina, Brazil, and Antarctica.Abdala et al., 2009, p.837 The cynodonts of the Triassic era are important markers to correlate terrestrial formations. ''Trirachodon'' fossils have also been found in the Driekoppen Formation of South Africa.Botha & Chinsamy, 2004, p.620 The following fossils have been reported from the formation:Etjo Mtn general, btwn lower + middle arenaceous layers
at

Fossilworks Fossilworks was a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. History Fossilworks was cr ...

.orgN slopes, Etjo Mtn, btwn lower + middle arenaceous layers
at

.orgEtjo Mtn general, below lowest arenaceous layer
at

.orgN slopes, Etjo Mtn, below lower arenaceous layer
at

.orgwestern Etjo Mtn, above upper arenaceous layer
at

.orgN slope, Etjo Mtn, upper arenaceous layer
at

.orgwestern Etjo Mtn, above lower arenaceous layer
at

.orgEtjo Nord, EN-3
at

.orgOmingonde 96, OM-3
at

.orgOmingonde 96, OM-2
at

.orgOmingonde 96, OM-5
at

.org ;Amphibians * Mastodonsauridae indet. Reptiles * ''Etjosuchus'' ''recurvidens'' ;Therapsids ;Ichnofossils * '' Saurichnium anserinum'', '' S. damarense'', '' S. parallelum'', '' S. tetractis'' * '' Tetrapodium elmenhorsti''

Correlations

The formation is correlated with the lower part of the

Santa Maria Formation The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It is primarily Carnian in age (Late Triassic), and is notable for its fossils of cynodonts, " rauisuchian" pseudosuchians, and early dinosaurs and othe ...

of the

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 s ...

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

.Bertoni & Holz, 2006, p.275 The

Chañares Formation The Chañares Formation is a Carnian-age geologic formation of the Ischigualasto-Villa Unión Basin, located in La Rioja Province, Argentina. It is characterized by drab-colored fine-grained volcaniclastic claystones, siltstones, and sandstones ...

of the Ischigualasto-Villa Unión Basin in northwestern

is also time-equivalent with the Omingonde Formation,Colombi et al., 2013, p.32Marsicanoa et al., 2015, p.511 and similar fauna assemblages as the Omingonde Formation have been reported from the Río Seco de la Quebrada Formation of Argentina.Previtera et al., 2016, p.199 In the

Karoo Basin The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a per ...

of southern Namibia and the

Eastern Eastern or Easterns may refer to: Transportation Airlines *China Eastern Airlines, a current Chinese airline based in Shanghai * Eastern Air, former name of Zambia Skyways *Eastern Air Lines, a defunct American airline that operated from 192 ...

Northern Northern may refer to the following: Geography * North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating ...

and

Western Cape The Western Cape ( ; , ) is a provinces of South Africa, province of South Africa, situated on the south-western coast of the country. It is the List of South African provinces by area, fourth largest of the nine provinces with an area of , an ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

, the formation is time-equivalent with the

Burgersdorp Burgersdorp is a medium-sized town in Walter Sisulu in the Joe Gqabi District Municipality of the Eastern Cape province of South Africa. In 1869 a Theological Seminary was established here by the '' Gereformeerde Kerk'', but in 1905 it was mov ...

and

Elliot Formation The Elliot Formation is a geological formation and forms part of the Stormberg Group, the uppermost geological group that comprises the greater Karoo Supergroup. Outcrops of the Elliot Formation have been found in the northern Eastern Cape, so ...

.Bordy & Eriksson, 2015, p.311 In

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

, the formation correlates with the

Fremouw Formation The Fremouw Formation is a Triassic-age rock formation in the Transantarctic Mountains of the Ross Depenency of Antarctica. It contains the oldest known fossils of tetrapods from Antarctica, including synapsids, reptiles and amphibians. Fossiliz ...

References

Bibliography

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