Abstract

A few, thin, Mississippian siliciclastic limestone beds, interbedded with ammonoid (uppermost Eumorphoceras Zone)-bearing shales within the South Syncline Ridge section on the Nuclear Test Site in southern Nevada, contain an abundant, low-diversity assemblage of late Serpukhovian/late Chesterian calcareous foraminifers dominated by the archaediscaceans Neoarchaediscus altiluminis, Brenckleina rugosa, Eosigmoilina robertsoni, and Betpakodiscus of the group B. attenuatus. These limestone beds were deposited in a shallow-water, clastic facies of the Scotty Wash Formation and, based on common conodont occurrences, correlate southeast to the Bird Spring Formation below the Mississippian-Pennsylvanian GSSP at Arrow Canyon, Nevada. The South Syncline Ridge foraminifers are comparable to those found in coeval beds at Arrow Canyon and represent the only other known foraminiferal assemblage to exist in association with uppermost Eumorphoceras Zone ammonoids in North America outside of Arkansas in the southern Midcontinent. Reconciliation of regional conodont and ammonoid zonations shows that the range of eosigmoiline foraminifers (E. robertsoni and B. rugosa), now generally considered an upper Serpukhovian index, extends from a position either just below or at the lower-upper boundary of the Serpukhovian Stage into the lower part of the Bashkirian Stage in North America; their upper range falls within the lower part of the Homoceras ammonoid zone beginning in the upper part of the Serpukhovian Stage. Discussion of the foraminiferal taxa includes support for retaining the genus Betpakodiscus rather than synonymizing it under Tubispirodiscus, as proposed by some specialists during the past few years.

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