I welcome Leslie and Bergström's comment on our recent article (Pope and Steffen, 2003); it allows me the opportunity to clarify and reiterate some points in that article. The comment makes a number of points that I have labeled 1–6, and I address them in the general order they occur in the comment.
1. As noted in the erratum published in Geology (v. 31, p. 656), we inadvertently transposed the position of the Turinian and Chatfieldian stages in Figures 2 and 4 in our original article. Similarly, the Polk Creek Shale is equivalent to the Sylvan Shale and should have been listed in the position of the Womble Shale on our original Figure 2. These were unfortunate oversights, and I regret the errors.
2. Simpson Group rocks are interpreted to occur in the subsurface of eastern Texas and they outcrop in the Arbuckle Mountains of Oklahoma, immediately below the Viola Group (Suhm, 1997; Denison, 1997).
3. We did not mean to imply that early Hirnantian strata are recognized in the Maravillas or Late Ordovician eastern Midcontinent rocks. However, the new biostratigraphy of the Maravillas Formation (Goldman et al., 1995) was from one section (Picnic Grounds) that does not include a 20–30 m thick cherty shale that McBride (1989) demonstrated occurred above the cherty carbonates; thus, we still contend the upper age of the Maravillas is uncertain. Similarly, sporadic outcrops of Hirnantian iron ooidstones (e.g., Keel, Noix, Queenston Formation) overlying more widespread Late Ordovician carbonates in the Midcontinent (Van Houten, 1990) indicate there is some deposition in North America during this period.
4. We used Hein and Parrish (1987) as our starting point for the age constraints on Late Ordovician cherty carbonates and cherty shale units outside of North America that were used in Figure 4. We updated this citation through an extensive GEOREF search that did not provide the newer age correlations cited by Leslie and Bergström. We welcome these new dates and agree that some of them, if substantiated, likely have nothing to do with enhanced upwelling during a prolonged Late Ordovician glaciation.
5. Figure 4 in the original article shows units with depositional chert so the Woods Hollow Shale was omitted because it does not contain any chert, whereas the Fort Peña Formation was included because it does have subtidal chert, which we interpreted to possibly form by upwelling. Similarly, a number of Early, Middle, and Late Ordovician chert-rich carbonates (e.g., Jefferson City and Plattin) units were not listed on Figure 4 because their chert formed in peritidal settings through a variety of processes (i.e., evaporation, late diagenetic replacement by groundwater or burial fluids, silica migration from bentonites, etc.) that have nothing to do with oceanic upwelling.
6. Leslie and Bergström's contention that the Montoya Formation, specifically the Aleman Formation and its correlative chert-rich carbonates in Texas, Oklahoma, and the Great Basin are confined to the Richmondian stage stands in stark contrast to published conodont data from these units indicating they were deposited during the Maysvillian to Richmondian (Sweet, 1979, 2000; Poole et al., 1995). For example, the Aleman Formation contains conodont elements of both the Amorphognathus superbus and Amorphognathus ordovicicus zones and spans a portion of conodont Faunas 11 and 12 (Sweet, 1979), which Leslie and Bergström concede occur during the Maysvillian (see Fig. 1 of their comment). Thus, until new data on these units is published, we will continue to contend these units were deposited over a longer period of time than suggested by Leslie and Bergström.
The larger issue, which Leslie and Bergström do not dispute, is that no matter if these chert-rich units are only Richmondian or span from the Edenian to Richmondian, they formed over a broad area by upwelling. If we are correct, and this upwelling was enhanced by ocean circulation driven by thermohaline currents caused by glaciation, then the enigmatic Late Ordovician glaciation was not limited to the Hirnantian (e.g., Brenchley et al., 1994).