Paleoenvironments of an Earliest Cambrian (Tommotian) Shelly Fauna in the Southwestern Great Basin, U.S.A.

ABSTRACT A pre-trilobite shelly fauna was recently discovered at Mt. Dunfee, near Gold Point in Esmeralda County, Nevada. These fossils occur in the lower to middle portions of the lower member of the Deep Spring formation, approximately 1,000 m below the first trilobite body fossils and 400 m below the first arthropod trace fossils. Constituents of the fauna include Coleoloides sp., Sinotubulites sp., Salanytheca sp., and an unidentified skeletal fossil. Ichnofossils consist of Paleophycus or Planolites and traces resembling Neonereites, Scolicia, and Bergaueria. Four lithofacies are observed in the lower member of the Deep Spring Formation at Mt. Dunfee: 1) a fossiliferous limestone facies (Facies A); 2) a red, glauconitic, peloidal limestone facies (Facies B); 3) a mixed carbonate and siliciclastic facies (Facies C); and 4) a sandstone-shale facies (Facies D). These facies were deposited within a shallow, subtidal marine setting. The nearshore zone was dominated by siliciclastic sedimentation with alternating episodes of energetic and quiescent depositional conditions (Facies D). The ichnofauna is well developed in the finer-grained sediments of this facies. Further offshore, on an open shelf, carbonate mud was the dominant sediment (Facies A). This shallow carbonate shelf was subjected to relatively low-energy deposition, with the exception of occasional storm pulses. Locally, sediments consisting predominately of glauconitic peloids were deposited (Facies B). Between the nearshore siliciclastic zone and the carbonate shelf, carbonate and siliciclastic sediments (Facies C) were deposited within a mixing zone. Mixing occurred during storm events or along the diffuse margins of laterally migrating environments. The Mt. Dunfee fauna appears to have been restricted to a warm, low-energy, shallow-water carbonate-shelf environment in which siliciclastic influx was minimal (Facies A). The preferred environment of the fauna is typical of the environments in which other pre-trilobite shelly faunas first appeared worldwide.