The Controversial Origin of Ferruginous “Coprolites”

Ferruginous bromalites (coprolites and cololites) occur in enormous quantities in the Upper Cretaceous Whitemud Formation of Saskatchewan, Canada, and in Miocene deposits in Madagascar and southwest Washington, USA. The origins of these specimens have been the subject of diverse and often conflicting interpretations. This paper includes some discussion of other localities, but the main focus is on specimens from Wilkes Formation at Salmon Creek, Lewis County, Washington State, USA. This locality is notable because the geologic setting and paleoenvironment are well-established, and the purported bromalites can be observed in situ, providing stratigraphic and taphonomic information that is not available for the Canada and Madagascar locations. Past research at Salmon Creek has a curious history. Supporters of the coprolite interpretation have relied on Salmon Creek specimens collected by others. In contrast, field-based investigators have concluded that the extruded objects are probably pseudofossils. Was the origin of these objects biotic excretion or abiotic extrusion? Past evidence is not sufficient for resolving this issue. New information strengthens the biogenic interpretation, bu tthese ferruginous specimens remain as a geologic enigma.