MICROVERTEBRATE CORRELATION OF LOWER DEVONIAN STRATA, CENTRAL-EASTERN IDAHO: EXTENDING BIOSTRATIGRAPHIC POTENTIAL TO MARGINAL-MARINE PALEOENVIRONMENTS

Biostratigraphic, lithostratigraphic and facies analyses of Lower Devonian strata in central-eastern Idaho show great promise for correlating nonmarine-to-shallow-marine deposits with open-marine deposits using microvertebrates and conodonts, respectively.

Facies and stratigraphic analysis of an east-to-west transect, from the northern Lemhi Range to Spar Canyon, shows a basinward cross-section of the Cordilleran miogeoclinal shoreline of Late Silurian (Pridolian)-Middle Devonian (Eifelian) age. Interbedded and mixed siliciclastic and carbonate facies grade westerly from thin, valley-fill deposits (estuarine (?) paleoenvironment) to thick, laterally-continuous deposits (storm-influenced shallow-marine paleoenvironments). The presence of the thelodonts, Nikolivia elongata and Turinia pagei, and tesserate heterostracan elements similar to Aporemaspis pholidata and Lepidaspis serrata, indicate a Lochkovian age for sandstone facies (shallow marine to transitional paleoenvironments) and allows correlation with conodont-bearing carbonate facies (basinward, open-marine paleoenvironment). The microvertebrates are found with a macrovertebrate assemblage including pteraspids, acanthodians, osteostracans and placoderms. The macrofauna cannot be indentified beyond a generic level, and the majority appear to be new species. The microfauna is similar to one previously described from the Windmill Limestone, central Nevada, in which Nikolivia elongata is also associated with elements from tesserate heterostracans. The presence of associated conodonts of the delta zone places the Nevada fauna in the middle Lochkovian. This study demonstrates the potential for using the combination of conodont and microvertebrate biostratigraphy for regional correlation of Lower Devonian strata, especially across nonmarine-to-marine facies transitions.