THE STRATIGRAPHIC POSITION OF THE EARLIEST PENNSYLVANIAN STRATA IN THE MORROWAN TYPE AREA AND ITS RELATIONSHIP TO THE MISSISSIPPIAN-PENNSYLVANIAN BOUNDARY

The Cane Hill Member, Hale Formation, is the lowest unit in the Morrowan type area of northwest Arkansas. It unconformably overlies Mississippian strata referred to the Pitkin Formation or the underlying Fayetteville Formation, where the Pitkin is truncated by the erosion surface. At many localities, the unconformity is directly marked by a conglomerate, one to three feet thick, assigned to the Cane Hill. The conglomerate contains limestone pebbles derived from the underlying Pitkin Formation. Cane Hill strata above the conglomerate are composed of shale and siltstone that accumulated on tidal flats and in tidally-dominated subtidal environments. A conglomeratic lens, 26 feet above the basal conglomerate, has yielded ammonoid and conodont assemblages assigned to the Retites semiretia and Rhachistognathus primus biozones respectively. The Cane Hill interval below the fossiliferous lens, including the basal conglomerate, is extensive and well exposed, but has yielded no biostratigraphically significant fossils. An exposure created by a recent excavation in the environs of Fayetteville, Arkansas, provides a Cane Hill-Fayetteville contact. The basal, limestone pebble conglomerate of the Cane Hill Member is two feet thick at the northern end of the exposure and thins southward to approximately four inches. As the conglomerate thins it becomes progressively overlain to the south by a unit of crinozoan grainstone over a distance of 300 feet. The grainstone achieves a thickness of five feet and comprises two beds separated by a thin, pebble conglomerate. The grains are well sorted and accumulated in a shallow environment where wave and current activity was significant. The conglomerate within this interval suggests that sedimentation was briefly interrupted by an erosion event. The grainstone has yielded the first biostratigraphically significant conodont fauna from the basal Cane Hill Member in close proximity to the Mississippian-Pennsylvanian boundary represented by specimens of Adetognathus lautus and A. gigantus. This suggests that the Mississippian-Pennsylvanian boundary may fall below the appearance of Declinognathodus noduliferous in a succession similar to that at the Nevada Test Site.