CLASSIFICATION OF ORDOVICIAN CHAZYAN SHELLY AND GRAPTOLITE SEQUENCES FROM CENTRAL NEVADA
The Antelope Valley Limestone, nearly 1000 feet thick in the Toquima Range in central Nevada, contains typical zones of the “Whiterock Stage.” Trilobites and cephalopods in the lower Antelope Valley seem older than those in the middle (Crownian) and upper (Valcourian) type Chazyan and are like those in the Table Head Limestone of Newfoundland; upper Antelope Valley-Whiterock faunas resemble some in the higher Chazyan stages. Thus, the lower Antelope Valley-Whiterock has the stratigraphic position of the lower type Chazyan Dayan Stage. The Petes Summit Formation in the Clipper Canyon Sequence of argillites, cherts, quartzites, and volcanic rocks has a succession of graptolite faunas through some 600 feet. Assemblages near the base resemble those in the Didymograptus extensus and Didymograptus hirundo zones of the Arenigian Series and those in the upper Deepkill of New York; they may extend into the lower Llanvirnian Zone of Didymograptus bifidus. Well above these in the Petes Summit is an exceptionally rich assemblage with Pterograptus elegans and forms associated in the upper Llanvirnian Zone of Didymograptus murchisoni. The highest faunules in the Petes Summit have Glyptograptus teretiusculus, suggestive of the Llandeilan Series. The succession supports the view that the Deepkill and Levis Didymograptus bifidus-bearing beds are Arenigian, older than the named zone in Europe. As Table Head faunas lie below graptolites like those low in the Petes Summit, the lower Antelope Valley Limestone and Whiterock Stage seem to be highest Arenigian, and the higher part Llanvirnian and Llandeilan.