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, southern Free State, and in the eastern KwaZulu-Natal provinces of South Africa. Outcrops and exposures are also found in several localities in Lesotho such as Qacha's Neck, Hill Top, Quthing, and near the capital, Maseru. The Elliot Formation is further divided into the lower (LEF) and upper (UEF) Elliot formations to differentiate significant sedimentological differences between these layers. The LEF is mainly Late Triassic (Norian-Rhaetian) in age while the UEF is mainly Early Jurassic (Hettangian-earliest Pliensbachian) and is tentatively regarded to preserve a continental record of the Triassic-Jurassic boundary in southern Africa. This geological formation is named after the town of Elliot in the Eastern Cape, and its stratotype locality is located on the Barkly Pass, 9 km north of the town.

Geology

The Elliot Formation unconformably overlies the Molteno Formation and is conformably overlain by the Clarens Formation. Due to the reddish colour of the rocks, the Elliot Formation is colloquially referred to as the "Red Beds" in older geologic literature.

The Elliot Formation is dominated by mudstones and siltstones that can be finely laminated. However, the internal structures in the mudstones are often not visible due to locally poor laminations. Calcareous nodules are also found in the mudstone layers and become more frequent up section into the UEF. The mudstones range in colour from greyish purple red in the LEF and turn a more brick red colour with more mature palaeosols in the UEF. Localized intraformational pebble conglomerates that comprise intrabasinal clasts that comprise mud chips, quartzite pebbles, pedogenic nodules, and fossil bone fragments only occur in the UEF. The lower and upper Elliot formations both contain sandstones but they vary in their internal geometries. The sandstones of the LEF mainly comprise laterally accreting channel deposits that are multi-story and contain trough, low angle, and planar, cross-bedding. Ripple cross laminations with good horizontal lamination are also present. In the UEF, sandstone beds are single story and mainly reflect downstream accretion channel geometries and are more tabular in appearance. Common internal sedimentary structures of UEF sandstones are planar, low angle cross-bedding, horizontal and ripple-cross laminations.

The LEF was deposited in a fluvio-lacustrine environment where rivers were more perennial and formed meandering channel geometries, as evidenced by the presence of lateral accretion. However, this depositional environment changed at the onset of the UEF deposits where evidence of shallower river channels, longer periods of floodplain stasis (mature palaeosols) and flash flood events (pedogenic nodule conglomerates) shows that the climate became more arid.

Correlation

The Elliot Formation is currently considered to correlate chronostratigraphically with geological formations of the Bodibeng Sandstone of the Tuli Basin in Botswana, the Omingonde Formation of the Etjo Basin in Namibia, and the Chinle Formation of the Colorado Plateau in Utah, United States.

Age

U-Pb radiometric dating using zircon grains has refined the duration of the Elliot Formation. Zircons of the Lower Elliot Formation are entirely Late Triassic, ranging from around 220 to 205 million years old (Norian to Rhaetian stages). Zircons of the Upper Elliot are mostly Early Jurassic, ranging from around 202 to 192 million years old (latest Rhaetian to Sinemurian stages). The oldest zircons of the overlying Clarens Formation are about 187 million years old (mid-Pliensbachian stage).

The entire Elliot Formation also preserves a record of magnetostratigraphy, with at least nine reverse-normal polarity chron pairs (EF1r to EF9n). The lower-upper Elliot boundary is in the middle of chron EF6n. The Elliot Formation's magnetostratigraphy can be partially correlated with similar records in the Newark Basin (northeast United States), Chinle and Moenave formations (southwest United States), and Los Colorados Formation (Argentina).

Paleontology

The Elliot Formation is well known for its diverse fossils of dinosaur and other vertebrates. The formation is divided into two biozones, the Scalenodontoides Assemblage Zone and Massospondylus Assemblage Zone. The Scalenodontoides Assemblage Zone corresponds to the Lower Elliot Formation. Formerly known as the 'Euskelosaurus' Range Zone, its index taxa include the cynodont Scalenodontoides and the sauropodomorphs Blikanasaurus and Melanorosaurus. The Massospondylus Assemblage Zone extends from the Upper Elliot Formation into the Clarens Formation. Its index taxa include the sauropodomorph Massospondylus, the ornithischian Lesothosaurus, and the crocodylomorph Protosuchus.

Sauropodomorphs are particularly diverse in the Elliot Formation. The most common dinosaur is a sauropodomorph species, Massospondylus carinatus. Fossilised Massospondylus eggs, some with the fossilized remains of embryos intact, have been recovered from UEF deposits in the Golden Gate Highlands National Park.

The mudstones of the LEF sometimes yield petrified wood, fossil plant matter, crustaceans, fishes, and turtles while the sandstones of the upper Elliot Formation more often contain various trace fossils. These include vertebrate trackways of basal ornithischian dinosaurs found in the Leribe, Mafeteng, and Mohales Hoek Districts of Lesotho. Possible trackways of the dicynodont Pentasaurus have been found on Morobong Hill in the Mohales Hoek District of Lesotho.

Dinosaurs

Ornithischians

Sauropodomorphs

Theropods

Large theropod-like teeth have been found in the Lower Elliot Formation. Various exposures of the formation (primarily in Lesotho) preserve informative interactions between theropod trackmakers and soft sediment.

Pseudosuchians

Other reptiles

Synapsids

A possible late-surviving diademodontid cynodont may be present in the Upper Elliot Formation.

Other vertebrates