Trace fossils and regional significance of a Middle Devonian (Givetian) disconformity in southwestern Ontario

The Hungry Hollow Formation on the southeastern margin of the Michigan Basin is interpreted as a composite stratigraphic unit representing the lateral equivalents of two depositional cycles in the northern Appalachian Basin. The basal beds (? = Pompey and upper Levanna gressive hemicycle of the upper Skaneateles Formation in New York) are separated by disconformity surfaces both from the underlying Arkona Shale and from the upper part of the Hungry Hollow (=upper part of Centerfield Limestone Member). These basal beds were deposited on eroded, overcompacted claystones of the Arkona Shale, contain a diverse suite of remanie sediments, fill deep, delicately sculpted furrows of Cruziana transversa n. ichnosp., C. reticulata n. ichnosp., and Rusophycus bilobatum (Vanuxem, 1842). This lower disconformity may be recognizable regionally over the Northern Midcontinent. Overlying beds feature the lowest local occurrence offshore-aspect conodont fauna of the lower Polygnathus varcus Subzone. The underlying Arkona Shale yields Icriodus-dominated (nearer-shore aspect) faunas apparently representing Polygnathus xylusensensis Zone assemblages. The upper part of the Hungry Hollow Formation and overlying lower part of the Widder Formation represent an offlap-onlap cycle within the lower Polygnathus varcus Subzone that is comparable to that of the upper part of the Centerfield Limestone. Cruziana and Rusophycus from the Hungry Hollow were not originally produced as "undertracks" but were formed at the sediment-water interface on erosion-resistant, overconsolidated muds. The producer of Cruziana reticulata n. ichnosp. appeared in the benthic community field area before the excavator of C. transversa n. ichnosp. and of Rusophycus bilobatum (possibly the trilobite Dipleura). The degree of limb swing required to produce the reticulate scratch patterns in C. reticulata n. ichnosp. suggests the evolution of specialized coxa-sternite and inter-podomere hinges that were not previously considered to be present in trilobites. Comparable reticulate scratch patterns occur as early as the earliest Cambrian and are apparently related to excavation on disconformity-related mud firmgrounds