ABSTRACT

The Paleozoic section at Windmill Window along the northern front of the Simpson Park Range consists almost wholly of carbonate rocks ranging in age from Late Ordovician to Middle Devonian and includes the following formations listed in ascending order: Hanson Creek Formation, Roberts Mountains Formation, Windmill Limestone, Rabbit Hill Limestone, McColley Canyon Formation, and Denay Limestone. The uppermost beds of the Roberts Mountains Formation are shown to be of Early Devonian age, thus the Siluro-Devonian boundary falls within an essentially unfossiliferous sequence in this formation previously regarded as wholly Silurian. The Windmill Limestone, proposed as a new formation, contains interbedded graptolitic and shelly faunas, the brachiopods proving to be of Early Devonian, late Gedinnian or Siegenian age. The monograptids are assigned to M. hercynicus, also of late Gedinnian and possibly Siegenian age. The brachiopods of the Windmill Limestone constitute a distinctive fauna that occurs at a consistent stratigraphic interval within the faunal sequence and thus is given zonal status as the Quadrithyris zone. The overlying Rabbit Hill Limestone similarly contains a distinct, sequentially consistent fauna and is given zonal status as the Spinoplasia zone. The Spinoplasia zone is overlain conformably by the Trematospira zone at the base of the McColley Canyon Formation. The middle beds of the McColley Canyon Formation are unfossiliferous, but the highest beds belong to the "Spirifer" pinyonensis zone which in turn is unconformably overlain by the lower part of the Denay Limestone with the Leptathyris circula zone fauna. Faunas from the uppermost beds of the Roberts Mountains Formation, from the Quadrithyris zone, and from the "S." pinyonensis zone have their faunal counterparts in Arctic Canada.