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The Virgin Hills Formation is a geologic formation dating to the Late Devonian period, specifically the Frasnian and Famennian stages. It is located in the Kimberley of Western Australia. The formation represents fore-reef[2] facies deposited as part of an extensive reef system that fringed the south-west Kimberley Block in Western Australia. Its rocks record a staggering diversity of animals, particularly ammonoids, conodonts and trilobites, and it additionally records the Kellwasser extinction event at the Frasnian-Famennian border, showcasing the disappearance of much of the indigenous life, before their replacement by newcomers from outside the Canning Basin.[1][3][4]
Each species is sorted into time-correlated conodont zones, which is a common method of establishing specific biostratigraphic zones in many formations.[5][6][7] As one will see in the table below, the zones with only number codes are Frasnian in age, and the ones with only conodont species names are Famennian in age, although both do have conodont names.
It is important that one does not tie conodont zones to localities as indivisible units, and instead as two separate parts that are be used together. This is because, one, some localities, such as Windy Knolls in both zones 11 and 13a, can occur in non-contiguous conodont zones. Two, as mentioned before, conodont zones are time-based and are used globally, while the localities are stratigraphically based and specific to the Virgin Hills Formation. If in the Localities column and a cell for example says 'McWhae Ridge (Base, etc.,)', that means one of the localities is an unspecified part of the McWhae area. This also applies if it simply says 'McWhae Ridge' without any specific localities, and this also applies to Bugle Gap, which has the locality 'Windy Knolls', and can also be referred to without a specified locality.[8]
In regards to ammonoids, they have been designated to a more precise biochronological order in the form of zones and subzones, most of which are named after one of the ammonoids found there, although many of them are not endemic to that particular zone. This spans the entire Canning Basin, including the Napier and Gogo Formations, but in the Virgin Hills Formation the zones begin at UD I-E1 (UD standing for Upper Devonian, though it is optimal to omit this part) and conclude at III-C. These have been mapped to conodont zones, making it possible to convert ammonoid zones to conodont zones.. However, vice versa is not fully possible, as conodont zones are generally broader and can span multiple ammonoid zones. Note that [ur] means low diversity upper range zone, which is defined by the extinction of several species). A collapsed table below displays this biostratigraphical mapping.[5]
Correlation of Ammonoid & Conodont Zonation[5][11]
1234567Petersen, Morris S. (1975). Upper Devonian (Famennian) Ammonoids from the Canning Basin, Western Australia. Journal of Paleontology, 49(S8), 1–55.https://doi.org/10.1017/S002233600006176X
↑Becker, Ralph & House, M.R. (2009). Devonian ammonoid biostratigraphy of the Canning Basin. Geological Survey of Western Australia, Bulletin, 145, 415-439.
12McNamara, K. J., Feist, R. & Ebach, M. C. 2009. Patterns of evolution and extinction in the last harpetid trilobites during the Late Devonian (Frasnian). Palaeontology 52(1): 11-33
↑Feist, R. & McNamara, K. J. 2007. Biodiversity, distribution and patterns of extinction of the last odontopleuroid trilobites during the Devonian (Givetian, Frasnian). Geological Magazine 144(5): 777-796
12Korn D. & Bockwinkel J. 2022. The early gephuroceratid ammonoids from the Roteisenstein Formation of Dillenburg (Cephalopoda, Ammonoidea). European Journal of Taxonomy 823: 10–47. https://doi.org/10.5852/ejt.2022.823.1811
