The Kirkwood Formation is a geological

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 ...

found in the Eastern and Western Cape provinces in South Africa. It is one of the four formations found within the

Uitenhage Group The Uitenhage Group is one of three geological groups, which comprise the onshore and offshore post-Karoo middle to lower Upper Mesozoic geological rock units in South Africa. Stratigraphically, the Uitenhage Group overlies the Suurberg Group an ...

of the Algoa Basin – its type locality – and in the neighbouring Gamtoos Basin. Outcrops of the Kirkwood are also found along the Worcester-Pletmos, Herbertsdale-Riversdale, Heidelberg-Mossel Bay, and Oudtshoorn-Gamtoos basin lines. At these basins the Kirkwood Formation underlies the Buffelskloof Formation and not the Sundays River Formation.

Geology

The Kirwood Formation was formed along the southern section of South Africa during the break up of

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 ...

. Dating it has proven tricky, with some regarding it as a Late Jurassic (~145 Ma) or Early Cretaceous deposit (~135 Ma). However, dating the Kirkwood's abundant volcanic ash layers by U-Pb methods is currently underway. The Kirkwood Formation is composed of sedimentary rocks deposited under

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 ...

conditions at or near sea level, such as variegated

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. ...

(an iconic feature), medium-grained

lithic Lithic may refer to: *Relating to stone tools **Lithic analysis, the analysis of stone tools and other chipped stone artifacts **Lithic core, the part of a stone which has had flakes removed from it **Lithic flake, the portion of a rock removed to ...

sandstone (often being charcoal-rich), and sporadic conglomerates. At its base, the Kirkwood Formation includes a significant deposit of fine-to-medium grained, poorly-sorted (often quartzitic) estuarine sandstone with subordinate dark grey to brow shale layers. This deposit was described as the Swartkops Member. The lower portion of the remainder of the Formation is composed predominantly of siltstones, but includes a smaller sandstone layer (described as part of the Bethelsdorp Member). The upper portion of the Formation is composed predominantly of fine, clay-rich mudstones, with multiple smaller layers of estuarine sandstone. The Kirkwood is the second

of the Uitenhage Group, positioned between the overlying Sundays River Formation and the underlying

Enon Formation The Enon Formation is a geological formation found in the Eastern and Western Cape provinces in South Africa. It is the lowermost of the four formations found within the Uitenhage Group of the Algoa Basin, its type locality, where it has been me ...

. Only three geological members have been described within it: the Swartkops Member which contains estuarine sandstone deposits and lacks fossils. The Colchester Member is the most fossil-rich containing terrestrial and lacustrine fossils and is composed of dark grey

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 ...

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 sandstones. The third, the Bethelsdorp Member, resembles the Colchester in containing dark-grey shales and sandstone, and contains marine microfossils. The upper section of the member is overlain by mudstone and siltstone.

Paleontology

The Kirkwood Formation is the most fossil-rich formation of Late Jurassic - Early Cretaceous age known in South Africa. It has yielded disarticulated remains of theropod and

ornithopod Ornithopoda () is a clade of ornithischian dinosaurs, called ornithopods (), that started out as small, bipedal running grazers and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world ...

dinosaurs, and several

sauropod Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their bo ...

species in which the Kirkwood is particularly rich, and includes indeterminate members of Diplodocidae, Dicraeosauridae and Brachiosauridae, as well as ''

Algoasaurus ''Algoasaurus'' (; "Algoa Bay reptile") is a genus of sauropod dinosaur from the Berriasian- early Valanginian-age Early Cretaceous Upper Kirkwood Formation of Cape Province, South Africa, specifically near a town called Despatch. Discovery an ...

'', a eusauropod of uncertain affinities. A partial skull and teeth of a stegosaur, '' Paranthodon'', has also been discovered."Table 16.1," in Weishampel, et al. (2004). Page 345 In addition to ''Paranthodon'', an

ankylosaur Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. ...

vertebra has also been described. Fossils belonging to a

dryosaurid Dryosaurids were primitive iguanodonts. They are known from Middle Jurassic to Early Cretaceous rocks of Africa, Europe, and North America. Phylogeny Until recently many dryosaurids have been regarded as dubious ('' Callovosaurus'' and '' Kan ...

ornithopod were unearthed in three expeditions between 1995 and 1999. In 2022, these remains were described as a new genus and species, ''

Iyuku raathi ''Iyuku'' (meaning "hatchling", pronounced "eye-yoo-koo") is a genus of iguanodontian dinosaur from the Early Cretaceous Kirkwood Formation of South Africa. The type species is ''Iyuku raathi''. Discovery and naming ''Iykuku'' is known from a ...

''. ''Iyuku'' is primarily known from remains belonging to hatchling and juvenile individuals. Indeterminate members of

Rhynchocephalia Rhynchocephalia (; ) is an order of lizard-like reptiles that includes only one living species, the tuatara (''Sphenodon punctatus'') of New Zealand. Despite its current lack of diversity, during the Mesozoic rhynchocephalians were a diverse g ...

and Squamata have also been discovered. Mesozoic-aged amber has also been recovered from the Kirkwood. The most well-known find from the Kirkwood is ''

Nqwebasaurus ''Nqwebasaurus'' (; anglicized as or ) is a basal coelurosaur and is the basal-most member of the coelurosaurian clade Ornithomimosauria from the Early Cretaceous of South Africa. The name ''Nqwebasaurus'' is derived from the Xhosa word "Nqweb ...

'', a basal ornithomimosaur. Fragmentary remains of various reptile, frog, insect, and mammal fossils have also been found, including fish scales and freshwater

bivalves 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 ...

. In addition several plant species such as bryophytes, ferns,

conifers Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All extan ...

, cycads, and bennettitaleans have been discovered, including silicified fossil tree trunks in the sandstone sections which show evidence of being burned.

References

{{Geology of South Africa, formations Geologic formations of South Africa Lower Cretaceous Series of Africa Cretaceous South Africa Berriasian Stage Hauterivian Stage Valanginian Stage Sandstone formations Shale formations Mudstone formations Siltstone formations Fluvial deposits