The distribution and possible biostratigraphic significance of the ichnogenus Oldhamia in the shales of the Blow Me Down Brook Formation, western Newfoundland

Abstract

The Blow Me Down Brook Formation is a siliciclastic unit in the Humber Arm allochthon of western Newfoundland that was deposited on the ancient North American continental slope and rise during the early stages of the development of the Iapetus Ocean. This unit consists primarily of sand-rich turbidites that appear to be barren of body fossils. The shales, however, contain trace fossil assemblages dominated by the ichnogenus Oldhamia and also include Planolites, Gordia, and other indistinguishable simple forms. Oldhamia is considered as an index fossil for the Early Cambrian, and the validity of this age assignment is reviewed.Oldhamia traces have been found in the Blow Me Down Brook Formation at 27 localities along the length of the allochthon and occur beneath each of the four ophiolitic massifs. Together, the assemblages from this unit contain five Oldhamia ichnospecies including O. radiata, O. antiqua, O. smithi, O. flabellata, and the newly reported O. curvata. Different ichnospecies are dominant in the Oldhamia assemblages at the 27 localities and these differences appear to be systematic. In the northernmost localities of the Blow Me Down Brook Formation simple radial forms predominate, including O radiata and O. smithi, whereas in the southernmost localities dendritic forms such as O. flabellata are common. This north to south distribution from radial to dendritic forms may represent an evolutionary trend of feeding optimization.