Abstract
Abstract New conodont results from the Sappington Formation in southwestern Montana, U.S.A., revise its age from primarily Devonian to Mississippian, provide new evidence for the biozones of Siphonodella praesulcata and Siphonodella sulcata in relation to other Mississippian species, and document the extension of a key Devonian miospore, Retispora lepidophyta, into the Mississippian, questioning its utility as a Devonian index fossil.
The shallow marine Sappington Fm consists of 6 units: lower and upper black shales comprising Unit 1 and Unit 6 respectively, and clastic middle Units 2 – 5. The latest Devonian conodont Siphonodella praesulcata Sandberg was previously found in Units 2, 3, and 5 and used to infer that the Devonian - Mississippian boundary was at the top of the Sappington Formation Unit 5. The latest Devonian miospore Retispora lepidophyta was previously found in Unit 4 and also used to infer the position of the Devonian - Mississippian boundary.
In this study, close-interval sampling and processing for both conodonts and palynomorphs was conducted at 33 localities in an attempt to precisely locate the Devonian-Mississippian boundary. The first record of identifiable conodonts from Unit 4 is documented in this study. An early Mississippian conodont with nodose ornamentation and rostral ridges, Siphonodella cf. hassi Ji, was found in lower Unit 4 within unique, fossiliferous channel sandstone at Antelope Valley. This study also documents the early Mississippian conodont Siphonodella bransoni Ji in middle Unit 5 at Horseshoe Canyon West and upper Unit 5 at Devils Fence West Unit 5. The early Mississippian conodont Siphonodella cooperi Hass was found in oolite in lower Unit 5 at Moose Creek. As a result of these new data, Unit 4 and Unit 5 are assigned to the Mississippian Siphonodella cooperi Zone.
The documentation of early Mississippian conodonts in Unit 4 and Unit 5 prove that the Devonian-Mississippian boundary in the Sappington Formation occurs below Unit 4, and could be within Unit 3. The conodonts Siphonodella praesulcata and Siphonodella sulcata were found together in the same Unit 3 sample from Beaver Creek East and in several localities in Unit 5. Siphonodella sulcata Huddle has been used to delineate earliest Mississippian strata and its presence is consistent with other Mississippian conodonts found in Units 4 and 5; however it is recently thought by some workers to be a morphotype of Siphonodella praesulcata. The actual range of Siphonodella sulcata is uncertain. This study highlights the need to use large sample sizes and sets in order to capture the complete suite of conodonts. Studies based on small sample sizes (core for instance) should be interpreted carefully.
Retispora lepidophyta was found beginning in Unit 1 of the Sappington Fm in this study. A diverse miospore assemblage containing Retispora lepidophyta was found throughout the shale of Unit 4 and in shale found in Unit 5. Rare Verrucosisporites nitidus was also found in Units 4 and 5. Although in Europe and eastern North America these species together delineate the uppermost Devonian with the extinction of Retispora lepidophyta at the Devonian-Mississippian boundary, Retispora lepidophyta was found in early Mississippian strata in the correlative Bakken Formation in a previous study. Controversy remains as to whether or not Retispora lepidophyta is in situ or is reworked into younger sediments. If this species is in situ, then its range must be extended upwards into the middle Tournaisian stage of the Mississippian for western North America, thereby making it less useful as an indicator of the Devonian-Mississippian boundary. The age of late Devonian glacial pulses based on the presence of Retispora lepidophyta is also questioned.
The underlying Three Forks Formation contained conodonts such as Palmatolepis glabra pectinata and Pa. glabra distorta that were recovered from the Lower Trident Limestone unit, representing the first record in Montana of recovery from this unit. Conodont lag deposits discovered at the base of the Trident Green Shale unit were very fragmented. More work on these units may provide definitive biozonations.