BIOSTRATIGRAPHY OF DEVONIAN?MISSISSIPPIAN SAPPINGTON FORMATION IN SOUTHWESTERN MONTANA, U.S.A.

DI PASQUO, MERCEDES; BEVERLY RICE; GRADER, GEORGE; TED DOUGHTY; PETER ISAACSON; LEONARDO SILVESTRI

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Congreso; Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

New conodont results from the Sappington Formation in southwestern Montana revise its age from primarily Devonian to Mississippian and question the utility of using global species like Siphonodella praesulcata and Siphonodella sulcata and the palynomorph Retispora lepidophyta for delineating the Devonian/Mississippian boundary. This boundary has been placed at the top of the upper Sappington Formation (Unit 5) for over 40 years based on the latest Devonian conodont Siphonodella praesulcata and the latest Devonian miospore Retispora lepidophyta found in the underlying Unit 4. An unconformable contact with the overlying Cottonwood Canyon/Lodgepole Formation is confirmed by conodonts of the Early Mississippian crenulata Zone with several missing zones at the contact. In this study, 33 localities were studied using large sample sizes and close sample spacing. Diverse miospore assemblages bearing Retispora lepidophyta were retrieved throughout the Sappington Formation in the lower Shale (U1), the middle shale (U4) and from rare shales occurring in the upper clastics (U5). In contrast, several localities yielded Early Mississippian conodonts and primary observations are: (1) siphonodellids Siphonodella sulcata and Siphonodella praesulcata were found coexisting in U3 from Beaver Creek East and in U5 at Snake Pit, Horseshoe Canyon West and Vermont, all barren of palynomorphs. Previous studies only found Siphonodella praesulcata without Siphonodella sulcata in U3 and U5 at Lick Creek and other localities in the Basin; (2) an Early Mississippian conodont with nodose ornamentation and rostral ridges, identified as Siphonodella cf. S. hassi, occurs with Retispora lepidophyta in a unique, fossiliferous lower U4 channel sandstone at Antelope Valley; (3) an Early Mississippian conodont Siphonodella bransoni occurs in the middle of U5 at Horseshoe Canyon West and near the top of U5 at Devils Fence West; neither outcrop contains palynomorphs; (4) an Early Mississippian conodont Siphonodella cooperi was found in oolitic grainstones within lower U5 at Moose Creek, above Retispora lepidophyta-bearing U4. These new data show that Siphonodella praesulcata and Siphonodella sulcata coexist with Early Mississippian conodonts and further work is needed to determine if they are morphotypes of the same species or can still be used to indicate the Devonian/Mississippian boundary as previously thought. As well, a controversy remains as to whether or not Retispora lepidophyta is in situ everywhere in the Northern Rockies since it has now been documented in Early Mississippian strata in the Sappington, Bakken, and Banff formations. Globally, it is frequently found reworked from Late Devonian deposits, triggered by global regression of glacio-eustatic origin confirmed by diamictites in South America, Africa, and eastern North America. Considering all this information, the boundary in the Sappington Formation must be below U4 and could be as old as U3, but it would be difficult to identify without the diagnostic Tournaisian conodonts found using large sample sizes. These results are consistent with previously published Early Mississippian conodonts from the top of the Middle Bakken in East-Central Alberta.*Financial support: NGS grant 9808-15, UofI teaching assistantship 2016-2018 (B.R.), PRISEM Geoconsulting, LLC and Hess, Corp. and CONICET PIP0305 2011-2013, PIP0812 2015-2017.