Lower Devonian-Middle Devonian Boundary in Central Nevada: GEOLOGICAL NOTES

Abstract

New fossil collections from beds immediately overlying the long established Spirifer pinyonensis zone show that there is a complete faunal break between these horizons in the northern Roberts Mountains and in the northern Simpson Park Range, central Nevada. Review of other Lower and Middle Devonian sections in the Great Basin shows that the unconformity postulated from the faunal break separates quartz sandstone at the base of the Simonson Dolomite from the cherty argillaceous of the Sevy Dolomite. This member is equated with the uppermost argillaceous beds of the McColley Canyon Formation and it bears the Spirifer pinyonensis fauna at all of the reported fossiliferous localities. The argillaceous beds are shown to thin eastward and wherever the unit is not recognized in eastern Nevada and in Utah it is believed to be absent beneath the unconformity. The unconformity is believed to be regional, separating Middle or Upper Devonian beds from Lower Devonian or older beds.