The paleobathymetric and paleogeographic distribution of Middle Ordovician trilobites within a portion of the Valley and Ridge of East Tennessee

Over 500 trilobite specimens were collected for this investigation from five stratigraphic sections within the study area (Evan's Ferry, Cedar Springs, St. Clair, Silver City and Raccoon Valley). These sections are all underlain by the post-Knox unconformity and include the part of the Middle Ordovician below the Moccasin Formation. The stratigraphic occurrences of the genera when plotted on a stratigraphic cross section of the study area, reveal distinct patterns of associations or groups (i.e., assemblages) which occur in and dominate distinct stratigraphic intervals deposited in specific depth zones.

Five distinct trilobite assemblages characterized at the generic level are present in the study area: the Illaenid, Asaphid, Pliomerid, Harpid and Raphiophorid-Remopleurid assemblages. The Illaenid Assemblage generally occurs in and dominates (with respect to total trilobite numbers) the lithologies in supratidal (+l0 - +5 meters), intertidal (+5 - -5 meters), shallowest subtidal (-5 - -10 meters) and shallow subtidal (-10 - -30 meters) depth zones. Asaphid and Pliomerid assemblages dominate the deep subtidal (-30 - -50 meters) depth zone. The Harpid Assemblage dominates the shallow water depth zones (+10 - -50 meters) of the terrigenous clastic influenced platform environments. The Raphiophorid-Remopleurid Assemblage dominates the terrigenous deep water environments of the slope (-50 - -100 meters) and basin (greater than -100 meters) depth zones.

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iv Four (Evan's Ferry, Cedar Springs, St. Clair and Silver City) of the five sections represent a transect from a shallow carbonate shelf environment to a deep terrigenous basin environment. The Evan's Ferry and Cedar Springs sections represent near shore shallow carbonate platform environments. Cedar Springs, however, was deposited in a setting closer to the shelf edge. The St. Clair and Silver City sections mainly represent environments of deposition in the slope and basin environments. The fifth section, Raccoon Valley, was deposited in an environment much like that of the Evan's Ferry section. At the beginning of the Middle Ordovician, this area was characterized by shallow carbonate shelf environments and the Illaenid Assemblage. As the basin continued to subside the Silver City and St. Clair area was characterized by deep subtidal environments. The seas began to transgress the previously exposed Knox surface to the northwest and these areas became shallow subtidal peribasinal environments. The illaenids migrated northwestward into these areas. Asaphids and pliomerids began to dominate deep subtidal environments of the Silver City and St. Clair sections. As the basin continued to subside the peribasinal shallow subtidal environments continued to shift northwestward. Finally, much of the northwestern portion of the study area was characterized by deep subtidal environments, the culmination of the northwestward transgression of the seas in the study area during the Middle Ordovician. At this time the spatial extent of the illaenids was apparently reduced in the study area because of the restriction of shallow subtidal environments which were their preferred habitat. Maximum extent of the deep subtidal assemblages (Asaphid and Pliomerid) coincided with this transgression.

An invasion by the Raphiophorid-Remopleurid Assemblage accompanied the influx of terrigenous clastics across the basin from the exposed surface to the southeast. At this time the Asaphid and Pliomerid assemblages began to shift northwestward along with the deep subtidal environments. At Cedar Springs the Raphiophorid-Remopleurid Assemblage makes a brief appearance in the Benbolt Formation. With a continued filling of the basin with clastics, the study area began to shallow. This shallowing allowed the illaenids in the northwest portion of the study area to expand southeastward into the new shallow subtidal environments. The Asaphid and Pliomerid assemblages disappeared from the study area as did the deep subtidal environments. The Harpid Assemblage began to migrate from the shallow platform clastic environments of the southeast across the basin and began to replace the Raphiophorid- Remopleurid Assemblage in the shallowing terrigenous environments. Elements of the Harpid Assemblage apparently competed with the Illaenid Assemblage in the shallow platform environments which contained increasing amounts of terrigenous sediment.