Palynology of the Devonian-Mississippian transition in western Montana: Three Forks, Sappington and Bakken formations

DI PASQUO, M.M.; GRADER, G.; ISAACSON, P.

Lexington

Encuentro; 45 Annual Meeting of AASP (The Palynological Society) and CIMP (Commission Internationale de la Microflore du Paléozoïque Subcommissions),; 2012

Universtity of Kentucky

Late Devonian-Early Mississippian strata east of the Antler continental borderlands in the incipient Central Montana Trough (CMT) are coeval with the Exshaw of Alberta and Bakken formations of the Williston intracratonic basin. Carbonates and siliciclastics of the Middle Famennian Three Forks Formation and latest Famennian-Tournaisian Sappington Formation are characterized by a facies change and pinch-out associated with high-amplitude eustatic base-level changes and widespread marine disconformities (i.e. 3rd-order sequences or 2nd-order icehouse episodicity occurring over several Ma). Like the Bakken, Sappington facies represent distally-thinning and -fining shoreface though offshore clastic-dominated ramp facies (black shale, burrowed siltstone and sandstone). Whereas South American and eastern North American palynology from the middle Famennian Tournaisian interval (~6 Ma) has been well studied, spore and pollen assemblages from Montana are not known. Black and dark green shales from CMT outcrops show promising initial results and work has been extended to well cores in NW Montana. Two palynologically productive samples from the Trident Member at Milligan Canyon on the Lombard Thrust and six from Logan Gulch on the Horseshoe Hills in Montana yielded low diverse palynoassemblages, with different preservations (e.g., orange well-preserved, dark brown and black poorly preserved palynomorphs). The Milligan assemblages (MEAs) and the older three samples from Logan (LAs1) are mostly composed of cosmopolitan, long ranging phytoplankton species that are recorded from the Frasnian to the Strunian (e.g., Cymatiosphaera perimembrana Staplin, Elektoriskos dolos Wicander and Loeblich, Gorgonisphaeridium ohioense (Winslow) Wicander, Gorgonisphaeridium absitum Wicander, Gorgonisphaeridium plerispinosum Wicander, Maranhites britoi Stockmans and Willière, Michrystridium adductum Wicander, Solisphaeridium astrum Wicander, Stellinium comptum Wicander and Loeblich, Unellium piriforme Rauscher, Unellium lunatum (Stockmans and Willière) Eisenack et al.). Some species have a more restricted stratigraphic range (Ammonidium garrasinoi Ottone from Givetian to late Frasnian, Gorgonisphaeridium evexispinosum Wicander from the Famennian, Unellium elongatum Wicander and Pterospermella latibalteus Wicander from mid-late Famennian, Exilisphaeridium simplex Wicander from ?Strunian, and Leiotriletes struniensis Moreau-Benoit from Strunian palynofloras). Hence, these marine assemblages (MEAs and LAs1) are likely pre-Strunian Famennian in age. Four samples processed from very carbonaceous lower Sappington black shales at Milligan East were barren and one from the very thin, basal brown shale at Logan (LAs2) yielded only leiosphaerids with an internal folding named here “monosacoid” due to its similarity with the Plicatipollenites and Potonieisporites monosaccate pollen grains. Two samples processed from USGS Bakken cores from NW Montana (Big Sky 1, lower Bakken at 9926 and 9920 ft), yielded two different assemblages. The older one is composed of a fairly well preserved assemblage with the same species recorded in MEAs and LAs1. Hence, a correlation between them is here proposed. The other assemblage yielded less diverse acritarchs (Gorgonisphaeridium ohioense, G. absitum, Hemiruptia legaultii Ottone) and abundant leiosphaerids of different sizes (ca. 50 up to 400 µm) and morphologies (i.e., without folding, with random folding, with monosacoid folding). Although this assemblage has no biostratigraphical useful species, its stratigraphic position and the presence of leiosphaeirids with monosacoid folding suggest correlation to the LAs2, in a lagoonal depocentre. From the basal part of the Lodgepole depositional system (upper Sappington shale) at Logan, dark brown shales yielded badly preserved, dark brown to black trilete spores with quite abundant Botryococcus braunii Kützing. A Tournaisian age is given to the upper Sappington (LAs3) based on few biostratigraphically useful species such as Waltzispora polita (Hoffmeister et al.) Smith and Butterworth, Leiotriletes sphaerotriangulus (Loose) Potonié and Kremp, ?Grandispora echinata Hacquebard, and Punctatisporites glaber (Naumova) Playford. The ages observed in this study agree with previous conodont work for the Three Forks Formation (Trident Member - middle Famennian marginifera/trachytera Zones), lower Sappington (expansa Zone), and for the upper Sappington black shale (Tournaisian duplicata/sandbergi Zones). The Strunian praesulcata Zone was previously defined in shales of the Unit 4 interbedded within the middle Sappington (reservoir) in the BridgerRange in Montana. A Retispora lepidophyta assemblage was found in the same unit. The latter was not observed here, but more detailed studies will be developed. Evaporitic, shallow through deep marine rocks of the Three Forks and Sappington formations in western Montana represent local intrashelf troughs inboard of active Devonian-Mississippian arches along the Montana-Idaho border. Few of these units nor easily recognizable Sappington facies occur on these uplifted and down-dropped paleohighs that were rapidly buried by active Antler sedimentation. On the other hand, remarkably similar and widespread latest Famennian stacking patterns across the “more stable” part of the Montana craton (and other parts of the world) suggest primary global controls during abrupt late Famennian change in climate/eustasy, arborescence, and transition to a Carboniferous icehouse.