LOWER SILURIAN CONODONTS OF CENTRAL NEVADA AND THEIR STRATIGRAPHIC SIGNIFICANCE

Conodont elements are sparse to abundant in samples from the Lower Silurian Hanson Creek Formation, Martin Ridge beds and basal Roberts Mountains Formation in Eureka County, Nevada. Conodont genera represented include Oulodus, Walliserodus, Distomodus, Dapsilodus, Decoriconus, Panderodus, Pseudooneotodus, Belodella, Ozarkodina, Aspelundia, Aulacognathus, Astropentagnathus and Pterospathodus. The Martin Ridge beds in Copenhagen Canyon are mostly within the D. kentuckyensis Biozone and within the Cystograptus vesiculosus-Coronograptus cyphus graptolite Biozones. The upper 2.0 meters of the Martin Ridge beds are within the D. staurognathoides and P. eopennatus Biozones and within the Spirograptus turriculatus graptolite Biozone. Conodonts are abundant and diverse in this interval. Apparatuses of Aulacognathus, Distomodus, and Astropentagnathus display great variability, the pectiniform elements in particular. Not only is it possible to recognize Au. bullatus, D. staurognathoides, and As. irregularis, but there are likely several additional species of those genera also represented, as well as species referred to Pedavis and new genera in Murphy, Dunham, Berry, and Matti (1979). The basal portion of the Roberts Mountains Formation in Copenhagen Canyon and Birch Creek is within the Pterospathodus eopennatus Biozone, probably still within the S. turriculatus Biozone, and definitely below the Oktavites spiralis Biozone. Pectiniform elements of P. eopennatus are more common in the Roberts Mountains Formation than they are in the uppermost Martin Ridge beds, with both sinistral and dextral Pa elements occurring together in samples 1.0 to 2.5 meters above the base. Other conodonts represented in the basal Roberts Mountains Formation include Au. bullatus, Au. kuehni, D. staurognathoides, As. irregularis and Apsidognathus tuberculatus. Conodont distribution in the upper Martin Ridge beds, upper beds of the Hanson Creek Formation and in the basal beds of the Roberts Mountains Formation indicates that the unconformity at the base of the Roberts Mountains Formation is within the Pterospathodus eopennatus Biozone, which suggests that only a brief hiatus in sedimentation occurred between deposition of the uppermost Martin Ridge or Hanson Creek beds and basal Roberts Mountains beds in that area, verifying the same conclusion reached in Murphy and others (1979).