A TRILOBITE BIOSTRATIGRAPHY FOR THE MONTEZUMAN STAGE (LOWER CAMBRIAN) IN WESTERN LAURENTIA

The International Commission on Stratigraphy is proceeding to select a global system of series and stages for the Cambrian which will supersede the nomenclature for individual continents. However, early Cambrian trilobites are severely endemic and a local biostratigraphy will be needed to extend the internationally accepted stage boundaries around a continent. For Laurentia, the Montezuman and Dyeran stages were proposed by Palmer in 1998 to represent the trilobite-bearing portion of the Lower Cambrian. The Montezuman as designated in western Nevada included the previously defined but broad Fallotaspis and Nevadella zones. The base of the succeeding Dyeran Stage is drawn at the appearance of Olenellus Zone trilobites in the same area.

Recent studies of the relatively continuous section through the Montezuman Stage in the Esmeralda Basin of western Nevada allow us to suggest a more detailed trilobite biostratigraphy. 1) The “Fallotaspis” Zone begins at the first appearance of Archaeaspis n. sp. and extends through 137 m with an explosion of new species in the upper part of the zone. Trilobites from this zone are also found in northern Sonora region of Mexico, southern California, and in a thin interval in the Cassiar Range and Mackenzie Mts. of northwestern Canada. 2) The Montezumaspis parallela Zone spans 26 m and has not been recognized beyond the Esmeralda Basin. 3) The Esmeraldina rowei Zone extends for 127 m and has also been noted in Sonora, the Cassiar Range and Mackenzie Mts. 4) The Grandinasus patulus Zone occurs in the top 23 m of the Campito Formation and is recognized only in the Esmeralda Basin. 5) The Avefallotaspis maria Zone spans the entire lower member of the Poleta Formation with up to 18 species, but identifiable trilobites are absent in the carbonate grainstones at the top of the zone. This zone is widespread occurring in Sonora, southern California, northeastern Washington and southern British Columbia, the Cassiar Range and Mackenzie Mts. 6)The Nevadia addyensis Zone is in the lower 57 m of the middle Poleta. It is also known from California, northeast Washington and southern B. C. 7) Nevadella eucharis Zone is the youngest zone, about 34 m thick and is also present in southern California and southern B.C.