Pope and Steffen (2003) described an interesting new model suggesting relations between the Gondwana glaciation and late Middle (Chatfieldian) through Late Ordovician (Cincinnatian) upwelling of cold water and chert formation. They used the distribution of Middle and Late Ordovician chert-bearing units as evidence to support their conclusion of glaciation lasting 10–14 m.y. rather than 1 m.y. or less. However, the listed age of many of their chert-bearing units is incorrect, and this directly bears on their conclusions. Some of the incorrect formation ranges may be due to the fact that the authors have followed imprecise secondary references but this can hardly explain why they place the North American standard Chatfieldian Stage (Leslie and Bergström, 1995) below, rather than above, the Turinian Stage in both their Figures 2 and 4. Further, the base of the Maravillas Formation is now firmly dated as Richmondian, not Middle Ordovician (Goldman et al., 1995) as are the upper part of the Viola Group (Goldman and Bergström, 1997) and the Aleman Formation. The Womble Shale has nothing to do with the Sylvan Shale—it underlies the Bigfork Chert and is of Middle Ordovician (Turinian) age (Finney, 1986), and the Simpson Group is not recognized in the Bigfort-Womble distribution area. No early Hirnantian strata have been recognized in the Maravillas Formation of West Texas or in the eastern Midcontinent. Their Figure 4 shows numerous errors in the ranges of the stratigraphic units. Hence, the Anthill Shale is Early Ordovician (Cooper, 1979). The virtually unfossiliferous Mallacoota Beds appear to range from the Early through the Late Ordovician (Webby et al., 1981), but the only fossils found are of Middle Ordovician age. The age of the unfossiliferous Girilambone Group is taken to be Cambrian or Early Ordovician (Webby et al., 1981). The Tallebung Formation is late Middle to early Late Ordovician in age. The Portixeddu Formation of Sardinia represents a narrow interval near the Caradocian-Ashgillian boundary (Leone et al., 1991) with conodonts (Ferretti and Serpagli, 1999) referable to the Amorphognathus superbus zone (upper Maysvillian or lowermost Richmondian). The Postolonnec-Kermeur succession in France ranges from the Early Ordovician to about halfway up in the Late Ordovician (Hammann et al., 1982). The unit below the Maravillas Formation in West Texas is not the Fort Peña Formation but the Turinian Woods Hollow Shale, which is underlain by the early Middle Ordovician Fort Peña Formation (Berry, 1960). Recent studies support the classic idea that the Ely Springs, Saturday Mountain, and Whittaker Formations, as well as an unnamed Sonora unit, are likely to represent only the Richmondian Stage. Figure 1 summarizes the stratigraphic unit ranges compared to those given in Pope and Steffen (2003). It shows that the ranges of these chert-bearing units are far more scattered than previously indicated, which makes the use of the presence of chert as support for an extended, late Middle–Late Ordovician glacial period more tenuous. We also note that abundant chert is also present in Laurentia in shallow-water sediments with tropical faunas that are unlikely to have been affected by cold-water upwelling, for instance, the Lower Ordovician Jefferson City and Middle Ordovician Plattin of Missouri.
Widespread, prolonged late Middle to Late Ordovician upwelling in North America: A proxy record of glaciation?: Comment and Reply: COMMENT
Stephen A. Leslie, Stig M. Bergström; Widespread, prolonged late Middle to Late Ordovician upwelling in North America: A proxy record of glaciation?: Comment and Reply: COMMENT. Geology ; 31 (1): e28–e29. doi: https://doi.org/10.1130/0091-7613-31.1.e28
Download citation file: