Correlating Sappington Formation sequences intrabasinally and as proxies to Late Devonian Glaciation: Utilizing Palynostratigraphy.

ISAACSON, P.; GRADER, GEORGE; DI PASQUO, M.M.; WARREN, A.M.; RODRIGUEZ, A.P.

Bozeman

Encuentro; GSA 2014 Joint Rocky Mountain/Cordilleran Section Meeting; 2014

GSA

Proposed in 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 continental ice sheets in the Parnaïba Basin, Brasil. Since that time, paleogeographic scale of the glaciation has expanded significantly (Bolivia and Perú: Vavrdová et al., 1991; Isaacson et al., 1999, 2008; several Brasilian 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 its range 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 this drawdown produced extensive brecciation in Frasnian 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 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 show that this zone needs careful re-study in North American sequences. A palynological study by di Pasquo et al. (2012) shows promise for an accurate biozonation of the Sappington Fm. and its subsurface correlatives. R lepidophyta was found, and preliminary palynozones were established. 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.