|dc.description.abstract||During the Upper Cambrian there were three mass
extinctions, each of which eliminated at least half of the trilobite
families living in North American shelf seas. The Nolichucky
Formation preserves the record of one of these extinction events
at the base of the Steptoean Stage.
Sixty-six trilobite collections were made from five sections
In Tennessee and Virginia. The lower Steptoean faunas are
assigned to one low diversity, Aphelaspis-dominated biofacies,
which can be recognized in several other parts of North America.
In Tennessee, the underlying upper Marjuman strata contain two
higher diversity biofacies, the Coosella-Glaphyraspis Biofacies and
the Tricrepicephalus-Norwoodiid Biofacies. At least four different
biofacies are present in other parts of North America: the
Crepicephalus -Lonchocephalus Biofacies, the Kingstonia Biofacies,
the Cedaria Biofacies, and the Uncaspis Biofacies.
A new, species-based zonation for the Nolichucky Formation
imcludes five zones, three of which are new. These zones are the
Crepicephalus Zone, the Coosella perplexa Zone, the Aphelaspis
buttsi Zone, the A. walcotti Zone and the A. tarda Zone.
The Nolichucky Formation was deposited within a shallow
shelf basin and consists largely of subtidal shales with stormgenerated
carbonate interbeds. A relative deepening is recorded
In the Nolichucky Formation near the extinction, and is indicated
In some sections by the appearance of shale-rich, distal storm
deposits above a carbonate-rich, more proximal storm deposit
sequence. A comparable deepening-upward sequence occurs near
the extinction in the Great Basin of southwestern United States
and in central Texas, and this suggests a possible eustatic control.
In other parts of North America, the extinction IS recorded
In a variety of environmental settings that range from near-shore
to slope. In shelf environments, there is a marked decrease in
diversity, and a sharp reduction in biofacies differentiation.
Although extinctions do take place in slope environments, there IS
no net reduction in diversity because of the immigration of
several new taxa.||en_US