Timing and extent of maximum transgression across Laurentia during Late Ordovician: New evidence from Slave Craton, Canadian Shield

Conodonts from the Slave Craton of the Canadian Shield provide solid evidence for estimating the timing and extent of the maximum transgression across Laurentia during the Late Ordovician. Upper Ordovician limestone xenoliths and a continuous limestone interval have been recovered from the Middle Jurassic Jericho kimberlite pipe piercing into the central Slave Craton of the Canadian Shield, an area that lacks Phanerozoic sedimentary cover nowadays. All fourteen limestone xenolith samples contain diverse conodonts, among which seventeen species belonging to twelve genera are recognized. This fauna is represented by Plegagnathus dartoni (Stone and Furnish), indicating an early Richmondian (Late Ordovician) age. In general, the fauna is characteristic of deposition in a shallow and open marine environment. This newly discovered fauna, in addition to those previously found on Canadian Shield and vicinity, provides reliable evidence that 1) the Ordovician inundation on the now-exposed Slave Craton by shallow seas occurred in the early Richmondian; 2) the previous recognized transgression during the second phase of Taconian orogeny, the Taconic tectophase, in the southeastern Laurentia in the early Chatfieldian (Late Ordovician) was only the initiation of the Taconic transgression; 3) the Taconic transgression reached its maximum extent and much of Laurentia was submerged in the early Richmondian.