CORRELATING SAPPINGTON FORMATION SEQUENCES INTRABASINALLY AND AS PROXIES TO LATE DEVONIAN GLACIATION: UTILIZING PALYNOSTRATIGRAPHY

Since 1985, the Late Devonian part of the late Paleozoic ice age (LPIA) was ignored, considered "insignificant," or too local to be of global consequence. Caputo (1985) demonstrated presence of continental ice sheets in the Parnaïba Basin, Brazil. Since that time, paleogeographic scale of the glaciation has expanded significantly (Bolivia and Perú: Vavrdová et al., 1991; Isaacson et al., 1999, 2008; several Brazilian basins: Caputo et al., 2008; North Africa: Lang et al., 1991; and the Appalachians: Brezinski et al., 2008, and others). Dating of the event has been through occurrence of the miospore, Retispora lepidophyta, a "Strunian" (latest Famennian) index fossil that does not extend into the Lower Mississippian (Tournaisian). It has been suggested that coincident with the glaciation was a significant sealevel drop that produced intracratonic shallow marine, euxinic basins with deposition of high-TOC black shales (e.g., Bakken, Woodford, and Bedford shales). Further, it has been suggested that associated global drawdowns produced extensive brecciation in lower Famennian and older carbonates through phreatic zone dissolution, along with lacunae, iron oolites, evaporites, and other features. A question remains whether the latest Devonian glacials represent deposition during actual glacial ice retreats, and the advances are not easily found in the rock record.

As it has been demonstrated in Carboniferous time, proxy beds and sequences relating to the LPIA have been described in North America (i.e., cyclothems and carbonate parasequences). The Sappington Formation in Montana is coeval to the Devonian glacials. Inasmuch as there have been depositional models and sequences proposed for this unit, its accurate biostratigraphic correlations need development. Conodont studies have been useful, but newer work in Europe on taxa within the praesulcata Zone reveals that careful re-study in North American sequences is needed. A palynological study by di Pasquo et al. (2012) shows promise for an accurate biozonation of the Sappington Fm. and its subsurface correlatives. Preliminary later Famennian palynozones were established, and further palynological work (in progress) will not only delineate lacunae in the Sappington Fm., but it will give accurate data on timing(s) of the Gondwana and Appalachian glaciations.