Ichnofossils of the Carbonate Platteville Formation (Middle Ordovician) in Southeastern Minnesota

Abstract

The carbonates of the Platteville Formation (Tippecanoe sequence) were deposited in the epeiric sea which occupied the Hollandale Embayment during the middle Ordovician. The ichnofossils found within the Platteville are numerous, diverse, and moderately well preserved. The entire assemblage conforms generally with the Cruziana ichnofacies. A total of 20 ichnogenera have been recognized: Arenicolites, Bifungites, Chondrites, Conostichus, Cylindrichnus, Diplichnites, Fustiglyphus, Gyrolithes, Lingulichnus, Muensteria, Nereites, Palaeophycus, Planolites, Rhizocorallium, Rusophycus, Subphyllochorda, Teichichnus, Thalassinoides, (?)Torrowangea and Trypanites. Two additional genera, Diplocraterion and Skolithos are found in the subjacent St. Peter and Glenwood Formations. The most characteristic biogenic feature of the Formation is the high degree of bioturbation, which effectively destroyed earlier burrows. The isochronous Deicke (Carimona) bentonite horizon is typified by seven traces: Arenicolites, Bifungites, Chondrites, Lingulichnus, Planolites, Thalassinoides, and Problematica type I. This association indicates shallow marine deposition estimated to be 1 O to 20 meters in depth, under essentially uniform environmental conditions. The Deicke bentonite is estimated to have been compressed from an original minimal thickness of 27 cm to the present 7.0 cm average thickness based on the compression of Chondrites and Planolites burrows within the ash layer. Shale interbeds are estimated to have been 3 to 5 times their present thickness based on compressed Chondrites burrows. The periodic influx of terrigenous sediments may have had an adverse effect upon some members of the benthic community, resulting in nonbioturbated laminated shale horizons. The presence of Chondrites among laminated shales and some lime-mudstones, and the absence of any other traces, may indicate that interstitial waters had a low oxygen content. Orientation of orthoconic cephalopods within the Platteville shows a strong northwest-southeast trend. It is suggested that these animals were oriented after death with the long axes of the shells parallel to a southeast current flow from the Transcontinental Arch across the embayment.