The Omingonde Formation is a Late Permian (Wuchiapingian) to Middle Triassic (Ladinian) geologic formation, part of the Karoo Supergroup, in the western Otjozondjupa Region and northeastern Erongo Region of north-central Namibia. The formation has a maximum thickness of about 600 metres (2,000 ft) and comprises sandstones, shales, siltstones and conglomerates, was deposited in a fluvial environment, alternating between a meandering and braided river setting. The Omingonde Formation is correlated with a series of formations in northwestern Argentina and the Paraná Basin in southeastern Brazil, deposited in a larger basinal area, 120 million years before the break-up of Pangea. The lower part of the formation has yielded fossils of a gorgonopsian and dicynodont, and can be assigned to the upper part of the Endothiodon Assemblage Zone. The upper part of the formation has provided fossils of several therapsids, amphibians, and ichnofossils 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 600 metres (2,000 ft), deposited in the Waterberg Basin where it overlies the Damara basement, and is unconformably overlain by the Early Jurassic Etjo Sandstone. 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. 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 350 metres (1,150 ft) thick conglomeratic red-bed sequence that probably correlates with the Omingonde Formation. The formation comprises red and white colored clastics including conglomerates, sandstones and siltstones. Many units are channelized and show paleosol 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. 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 80 to 150 metres (260 to 490 ft) 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.
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