Uppermost Devonian Ostracodes from the Western Canadasystematic, biostratigraphy and palaeoecology

Abstract

A diversified Ostracod fauna has been found inthe upper Frasnian, Famennian and Strunian rock sequences of Western Canada. In Alberta, the rock units are Wabamun, Winterburn (subsurface) and Palliser (Rocky Mountains) which indicate a carbonate sedimentation in a vast epicontinental sea. Towards the southwestern District of Mackenzie, the rock units Kotcho, Tetcho, Trout River, Kakisa, Redknife, Jean-Marie and Fort Simpson are calcareous, marly, argillaceous and sometimes silty strata. Throughout the present-day Mackenzie regions, the upper Devonian sequences are detritic and its sedimentation area is believed to be deeper. For this study, sampling concerns 15 sites (cores, cuttings, and sections) in the different facies of Alberta and Northwest Territories. 138 Ostracod species are described. These belong to 70 genera which are already known. 36 new species are introduced, but are close to several russian and North American species. 24 have been previously described in Russia, North America but few in Europe. 78 are with open nomenclature, of which 28 are compared with species mainly known in Russia and North America. Most of the species with open nomenclature are regarded as new ones. The studied ostracod species are distributed among the taximonic groupes as follows: 29 Beyrichicopina 21 Kloedenellocopina 1 Paraparchitacea 2 autres Palaeocopida 8 Barychilinidae+Cavellinidae 6 Metacopina 66 Bairdiacea 3 Entomozoidae 2 Cypridinacea The distinctive characters of genera belonging tothe superfamily Kloedenellacea are given. Generic relationships within the family Barychilinidae are suggested. These systematic results, joined to the study of the Big Valley Formation (Lethiers, 1978), allow to propose a zonation with 20 localities of sampling situated from the Saskatchewan to the District of Mackenzie, on a distance of about 1250 miles. Among the 169 species gathered in this way, 82 are selected to establish 7 zones. In the upper part of the Frasnian, the DFR6 zone completes the precedent zonation of this stage (Braun, 1967). The DFA1, DFA2, DFA3 and DFA4 zones divide the famennian rocks into four biostratigraphic units. The DST1 and DST2 zones are related to sediments of Strunian age. In every drill and section, the ostracod fauna isanalysed allowing for the facies and the zonation. Some ostracod species, the stratigraphic level of which are known, can be used for correlation between the zonation, the rock units and the european subdivisions. The Kakisa and Redknife Formations including the Jean-Marie Member are essentialy of upper Frasnian age. The Trout River and Tetcho Formations are mainly related to the lower Famennian. The Kotcho Formation can be present from the upper Famennian to the Strunian. The Palliser and Wabamun units from Alberta essentialy belong to the Famennian. The Big Valley Formation concerns the uppermost Famennian and the Strunian. Most of these rocks units seem to be diachronic. An irregular erosional limitoccurs either in the upper part of the Famennian or in the Strunian. A hiatus exits between the subsurface upper Famennianand middle Frasnian sequences in the Root River region. Thus the correlation of the lithologic units mentioned above is stated precisely or corrected. 112 species are known in the uppermost FamennianlowerStrunian rocks that are recognized on a distance of about 1250 miles, from the Saskatchewan to Northwest Territories. Three marine environments are distinguished, and are related to the Big Valley Formation, the top of the Wabamum-Palliser Formations and the upper part of the Kotcho Formation and equivalents. The most abundant specimens are found either in marly facies or in alternated limestones and shales. It can be explained by good palaeoecological factors combined with more chances for the conservation. The Entomozoid Ostracods are facies-dependent microfossils in connexion with the fossilization. In the total ostracod fauna, the species of Bairdiacea are the most. important. They prefer an open marine environment, with a calcareous and oxygenated sedimentation on the infralittoral and circalittoral zones. Most Metacopina are localized in the Big Valley facies probably for a climatic reason. The Kloedenellacea have numerous adaptative possibilities. It seems that the species of the genus Cryptophyllus prefer an environment close to the littoral zone and with restricted characters. The compared studies of the localized species, the endemic species and the species which are common to 2 or 3 facies, is a way to decipher the palaeocological charaters. Some endemic species will be colonizing roots during the lower mississippian time. Some genera such as Evlavonia, Buregia and Barychilina specialy have environment-dependent species. The palaeoecological characters clearly differentiate the Pig Valley and Wabamum-Palliser environments whereas there is a similarity between the Wabamum from the Northern Alberta and the equivalent levels from Northwest Territories. There was probably faunal communications between the Big Valley and Kocho environments towards the Northeastern Alberta. A species localized in an environment given from the epicontinental seas of Western Canada nervertheless can have a world-wide distribution.