EVIDENCE FOR INCISEMENT OF THE DEVONIAN CLIFTY FORMATION INTO THE ORDOVICIAN EVERTON FORMATION IN NORTHWEST, ARKANSAS: IMPLICATION FOR SEA-LEVEL AND PALEOENVIRONMENTAL INTERPRETATIONS

In outcrops along the southern portion of Beaver Lake (Northwest Arkansas), the Middle Devonian Clifty Formation disconformably overlies the Middle Ordovician Everton Formation. The Everton Formation is a mixed carbonate and clastic unit interpreted as representing shallow marine environments on the southern cratonic margin of Laurentia. The Everton Formation is identified from a conodont fauna that includes Paraprioniodus costatus and Multioistodus subdentatus. This fauna is indicative of the Whiterockian Everton Formation in Arkansas. Recent fieldwork revealed incisement by a coarse-grained sandstone into the top of the Everton Formation. Precise age of the overlying clastic unit is not known due to its poor fossil content, but is presumed to be Clifty Formation based on its textural/mineralogical similarity to known outcrops of Clifty Formation in northwest Arkansas. Incision into the Everton Formation supports the idea that regional base level at the time of Clifty deposition was at least 4m lower than the top of the Everton Formation. This supports previous interpretations based on facies analysis that the Clifty Formation represents a range of depositional settings from subaerial to shallow marine environments. The paleoshore of the southern craton margin during Clifty Formation deposition must have been in the vicinity of the Beaver Lake study areas.