Description
Interbedded Cambrian limestones and dolostones of the Bonanza King Formation near Pahrump, Nevada are divided into two formal lithologic units (the Papoose Lake Member and Banded Mountain Member), having a combined thickness of 960 m. Each member was divided into three distinct lithologic units (subunits). Typical rock-types occurring in the Bonanza King Formation are: mottled biogenic mudstones, dolomitic algal boundstones, lime-wackestones, and trilobite lime-packstones. Included in this stratigraphic succession are eight faunal zones representing the late Middle Cambrian through early Late Cambrian time. Ehmania, Syspacheilus, Coosella, Llanoaspis, Maryvillia, Proagnostus, Aphelaspis, Dicanthopyge, Labiostria, and Dunderbergia are observed trilobite genera characteristic of these zones. Zones in ascending order are: Glossopleura, Bathyuriscus-Elrathina, Bolaspidella, Cedaria, Crepicephalus, Aphelaspis, Dicanthopyge and the Prehousia Assemblage Zones. Environmental constraints restricted fauna from the Glossopleura (Papoose Lake Member) and Bolaspidella (middle subunit of the Banded Mountain Member) Zones creating stratigraphic intervals comprised of shoal and shelf-lagoon algal associated rocks. These stratigraphic intervals that are associated with these zones occur before and after the observed faunal occurrence characteristic of the Bathyuriscus-Elrathina Zone. Variable shelf-subsidence controlled and caused cyclic depositional environments and associated biofacies resulting in stratigraphic cycles ( "grand cycles" ). Analysis of these stratigraphic cycles reveal three distinct depositional environments and associated biofacies: l) a restricted inner-shelf biofacies represented by Ehmania and associated fauna of the Bathyuriscus-Elrathina Assemblage Zone that occur in the basal subunit of the Banded Mountain Member; 2) an outer-shelf biofacies that includes trilobites of the Cedaria, Crepicephalus, Aphelaspis, Dicanthopyge and Prehousia Assemblage Zones that occur in the upper subunit of the Banded Mountain Member; 3) a shoal and shelf-lagoon biofacies characterized by thick and thinly laminated algal boundstones and mottled biogenic mudstones that occur in the Papoose Lake Member and the middle subunit of the Banded Mountain Member. Thorough dolomitization affected algal associated rocks typical of the shoal environment. Preferential uptake of ambient divalent cations (magnesium and calcium) by the algal sheath material may have been the precurser to dolomitization in later diagenesis. Cedaria and Crepicephalus Zone polymeroid fauna occurring in the upper subunit of the Banded Mountain Member correlate with polymeroid fauna included in the Cedaria-Cedarina, Coosella, and Maryvillia Zones of the Riley Formation in Texas. Intercontinental biostratigraphical implications are best achieved with the less provincial agnostoids. Agnostoid trilobites occurring in the upper subunit of the Banded Mountain Member (Proagnostus modestus) correlate with subzones within the Swedish Olenus Zone. However, striking similarities exist between polymeroid trilobites occurring in China and the upper subunit of the Banded Mountain Member. Included in the upper subunit of the Banded Mountain Member is an observed faunal discontinuity. This biomere boundary area separates Crepicephalus and Aphelaspis fauna and represents the replacement of the Crepicephalus by the Aphelaspis fauna.
