UPPER DEVONIAN (LATE FRASNIAN) BRACHIOPOD FAUNA OF THE TWIN FALLS FORMATION (GRUMBLER GROUP), NORTHERN TERRITORIES, WESTERN CANADA

Late Frasnian age (Late Devonian) carbonate platform deposits of the Twin Falls, Tathlina, and Redknife formations (upper Grumbler Group) yield diverse, locally abundant, and well preserved rhynchonelliform brachiopod faunas. Most taxa present in these faunas are widespread elements of a cosmopolitan fauna seen throughout Laurussia and carbonate platforms of tropical northern Gondwana. These rocks are exposed along the Hay, Kakisa, Birch, Trout and Liard rivers in the Northern Territories of western Canada. This region was located at an equatorial position on the western continental margin of Laurussia at that time. Sea level rise in the Late Frasnian drowned the Middle Frasnian Alexandra Reef system and initiated Twin Falls to Redknife (Jean Marie Member) formations carbonate ramp deposition during the interval of Frasnian conodonts Zones 11 and 12. The Jean Marie Member carbonate ramp system was drowned by rapid relative sea level rise in the upper part of Frasnian Zone 12 coinciding with the Lower Kellwasser Extinction event in the Late Frasnian, expressed regionally by extinction of 11 of 19 species of brachiopods of the Jean Marie Member fauna.

The fauna of the Twin Falls Formation outlined here is associated with Frasnian Zone 11 conodonts). The fauna from the lower Twin Falls above Alexandra Falls on Hay River, below Lady Evelyn Falls on Kakisa River and nearby borrow pit quarries on the east side of Highway 1-Mackenzie Highway includes: Schizophoria iowaensis, Douvillina n. sp., Douvillinaria perversa, Strophodonta (S.) n. sp., Nervostrophia sp., Floweria sp. cf. prava, Productella sp., Regelia occidentalis, Cyrtospirifer thalattodoxa, C. whitneyi, Conispirifer? sp., Rigauxia sp. cf. orestes, Crytina lapidea, Pseudoatrypa devoniana, Desquamatia (Seratrypa) cillipes, Atryparia (Costratrypa) sp. cf. varicostata, Iowatrypa deflecta, Devonatrypa rubromitra, Cranaena sp., Gypidula sp. cf. cornuta, and Petrocrania sp.. The upper Twin Falls fauna is known from exposures on Hay River southwest of Alexandra Falls, and roadcuts west of the Highway 3 intersection south of the Mackenzie River. It includes: Strophodonta (S.). n. sp., Devonoproductus sp., Pseudoatrypa devoniana, D. (S.) cillipes, Devonatrypa canadensis, I. deflecta, Cyrtospirifer whitneyi, Theodossia keeni, and Gypidula sp.