BIOSTRATIGRAPHY OF THE EARLY CAMBRIAN MONTEZUMAN STAGE IN WESTERN NORTH AMERICA

In western Nevada, unusually thick and complete sections of the Lower Cambrian include the stratotype section for the Montezuman Stage, a Laurentian stage incorporating the two earliest North American trilobite zones, the Fallotaspis and Nevadella zones. In the type area, the stage encompasses 537 m of fine siliciclastics with some carbonate units in the upper third and with two significant sequence boundaries. The base of the stage, and of the Fallotaspis Zone, is at a thin interval with cf. Eofallotaspis, but the earliest trilobite occurrence in the area is an interval of cf. Repinaella, 57 to 93 m below. Above cf. Eofallotaspis, fallotaspidid trilobites are common for over 100 m. In the transgressive sediments above the first sequence boundary, Fallotaspis occurs with Bradyfallotaspis, Paranevadella, cf. Cirquella succeeded by the earliest nevadiids.This fauna has been recognized as far north as the Mackenzie Mountains of northwestern Canada and may extend into Mexico. Nevadia, Esmeraldina and other holmiids occur in a relatively dysaerobic environment. Ethmophyllum appears in thin carbonates as the sequence shallows. Carbonate is abundant following the second sequence boundary. Algal-archaeocyathan bioherms occur with interreef Nevadia and fewer Bradyfallotaspis, holmiids, ptychoparioids, Kootenia?, and Cirquella. Similar faunas occur in northwestern Canada, but there pagetids, important for international correlation, are also present. Still higher, an inner detrital interval has Nevadia followed by Nevadella occurring with helicoplacoids, brachiopods (including Mickwitzia), hyoliths and Chancelloria. This fauna is known from Death Valley to northwestern Canada. The base of the succeeding stage, the Dyeran is designated as the first occurrence of a new wanneriid genus, the beginning of the Bonnia-Olenellus Zone, following the disappearance of the nevadiid trilobites. The Montezuman Stage is suggested to correlate with most of the Atdabanian and the oldest part of the Botomian stages of Siberia.