REVISED STRATIGRAPHY OF THE “ARNHEIM” FORMATION (UPPER ORDOVICIAN, UPPER KATIAN) OF TENNESSEE AND ECOLOGICAL IMPACTS OF THE RICHMONDIAN INVASION IN THE NASHVILLE DOME

The “Arnheim” Formation of the Nashville Dome was correlated with the Arnheim Fm of the Cincinnati Arch despite considerable faunal differences between the two. The large number of Richmondian taxa present in the “Arnheim” of Tennessee (e.g., Grewingkia canadensis, Strophomena planumbona, Eochonetes clarksvillensis, Hiscobeccus capax) has led some authors to suggest, instead, a correlation with the Waynesville Formation rather than the Arnheim. Integrated stratigraphy (lithostratigraphy, biostratigraphy, chemostratigraphy, and sequence stratigraphy) indicates that the “Arnheim” Formation of the Nashville Dome represents both the Arnheim Formation and part of the Waynesville Formation, of the Cincinnati Arch (third order sequences C4 and C5). Lithologically similar, massive silty calcareous mudstones (or marls) lower, middle, and upper portions of the called “Arnheim” are separated by ferruginous, phosphatic hardgrounds and pebble beds that appear to mark condensed intervals recorded at the bases of sequences (C5A, C5C) of the classic Cincinnatian. Moreover, the overlying “Sequatchie tongue”, which is variably truncated beneath the Fernvale Limestone appears to record the upper Waynesville or Blanchester Member (C6A), and its sharp lower contact may be the mid-Richmondian unconformity.

The upper or Waynesville-equivalent portion of the Tennessee “Arnheim” Formation records the southern expression of the “Clarksville Phase” of the Richmondian Invasion, which accounts for the distinct faunas of the lower and upper “Arnheim”. Parallel to the patterns seen in the Cincinnati Arch, this biotic immigration event was a coordinated invasion, which had considerable and lasting impacts on the fauna of the Nashville Dome. Fossil assemblages of the “Arnheim” were analyzed from a community ecology perspective using cluster analysis, guild analysis, occupancy modeling, rarefaction, and detrended correspondence analysis. The results demonstrate that the Clarksville Phase is clearly abrupt and had similar ecological impacts in the Cincinnati Arch and Nashville Dome. The widespread and geologically abrupt appearance of this suite of taxa suggests a eustatic/climatic driver and is attributed to a glacioeustatic transgression and/or warming transgression connecting previously isolated basins.