Kirkwood Formation

The Kirwood Formation was formed along the southern section of South Africa during the break up of Gondwana. Dating it has proven tricky, with some regarding it as a Late Jurassic (~145 Ma).[4][5] or Early Cretaceous deposit (~135 Ma).[6][7][8] 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 conditions at or near sea level, such as variegated mudstone (an iconic feature), medium-grained lithic 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.[9][10]

The Kirkwood is the second formation of the Uitenhage Group, positioned between the overlying Sundays River Formation and the underlying Enon Formation. 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, siltstones and sandstones. The third, the Bethelsdorp Member, resembles the Colchester in containing dark-grey shales and sandstone, and contains marine microfossils.[11] The upper section of the member is overlain by mudstone and siltstone.[12]

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 dinosaurs, and several sauropod species in which the Kirkwood is particularly rich.[13] These species include undetermined Diplodocinae, Brachiosauridae, and Eusauropoda. One of the first sauropod species discovered in the Kirkwood was Algoasaurus.[14] A partial skull and teeth of a stegosaur, Paranthodon, has also been discovered.[15] Mesozoic-aged amber has also been recovered from the Kirkwood.[16] The most well-known find from the Kirkwood is Nqwebasaurus, a basal ornithomimosaur.[17] Fragmentary remains of various reptile,[18] frog, insect, and mammal fossils have also been found, including fish scales and freshwater bivalves.[19] In addition several plant species such as bryophytes, ferns, conifers, cycads, and bennettitaleans have been discovered, including silicified fossil tree trunks in the sandstone sections which show evidence of being burned.[20][21]