Age of the Ogilvie Formation (Devonian), Northern Yukon: Based Primarily on the Occurrence of Brachiopods and Conodonts

Abstract

The Ogilvie Formation, formerly dated as Eifelian and Givetian, is locally confined to the Early Devonian, Emsian. Regionally the top of the Ogilvie is markedly diachronous ranging in age from Emsian to Givetian. The Ogilvie Formation conformably overlies the Late Silurian to Early Devonian Gossage and/or early Emsian Michelle and Cranswick Formations. The stratigraphic thickness of the Ogilvie varies from 200–3500 ft (61–1068 m) over relatively short geographic distances. The ages of the lower and upper beds of the Ogilvie are based on brachiopods, conodonts, corals, and trilobites. Locally, the lower part of the overlying unnamed shale unit is regarded as a lateral equivalent to parts of the younger Ogilvie carbonate sections. The Eifelian age of the basal beds of the unnamed shale unit is derived from a few strategically located dacryoconarid tentaculite collections. The Ogilvie Formation is a shallow water carbonate shelf deposit, locally reefal, which had its most extensive development in the later part of the Emsian. From the later part of the Emsian through Givetian, the carbonate shelf was gradually inundated by shales until in the late Givetian only a very small area of carbonate deposition remained in the western Yukon (Mount Burgess).The brachiopod affinities lie with the Cordilleran Subprovince of the Old World Province and with forms of similar age described from central Nevada. The conodonts and trilobites are correlative with forms from Arctic Islands, eastern Alaska, and Nevada. The Emsian tetracoral fauna is similar to that of the Emsian of eastern Australia whereas the Eifelian fauna is similar to that of other parts of northwestern Canada.Five informal conodont faunal units, ranging in age from Emsian through Eifelian, are proposed for the Michelle–Ogilvie sequence. In addition, the P. varcus Zone (Givetian) is recognized at the top of the Ogilvie at one section.