Description
The silicified macrofossils from a section of Roberts Mountains Formation near Beatty, Nevada have been used to determine geologic age and depositional environments. The 1074-foot (327 m) section was divided into six informal lithologic units. The first five units consist of laminated, platy-weathering limestones, fossiliferous, mottled, graded and nongraded limestones, and thick-bedded, fossiliferous limestones. The sixth unit is 272 ft (83 m) of light-gray, fossiliferous to unfossiliferous dolostone. Most of the rock types are fine-grained, and reddish-brown weathering chert occurs throughout the section. Macrofossils include abundant brachiopods, tabulate and solitary rugose corals, crinoid debris, and rare gastropods, stromatoporoids, and dasycladacean algae. Brachiopods and corals are used as age indicators. Evidence of fossil transport does not preclude the associations being correlated to faunal assemblages determined for the corals and brachiopods of the central Nevada localities of the Roberts Mountains Formation and other time-equivalent rocks of North America. The brachiopod genera are interpreted to represent the B and C faunas of late Wenlockian and Ludlovian age. The poorly preserved, small to medium-sized, solitary Rugosa fall within early Wenlockian Zone B and an unnamed zone of late Wenlockian through Ludlovian age. The brachiopod and coral occurrences are also compared with graptolite and conodont data. The upward changes in the macrofossil associations represent a shallowing sequence from outer-shelf basin to foreslope environments. The smaller, thin-shelled brachiopods of the B fauna are replaced by the shallower C fauna containing large, thick-shelled pentameroids adapted to the more turbulent water of upper foreslope above wave base. This C fauna is comparable to the Pentamerus Community. The sequence of changes in size and shape of both rugose and tabulate corals, parallels the brachiopod morphological changes for adaptation from a quiet to more turbulent water environment. Graded, fossiliferous beds alternating with thinly-laminated, fine-grained lime mudstone are interpreted as turbidites interrupting the quiet water, outer-shelf basin and lower foreslope sedimentation. The thinly-laminated lime mudstone does not appear upsection in rocks indicating the shallower environment of the upper foreslope and shelf margin. No true barrier reefs are found in this section, only loosely aggregated carbonate buildups.
