FIRST PALYNOLOGICAL RECORD FROM THE WINNIPEG SHALE (UPPER ORDOVICIAN) IN THE NORTHERN BLACK HILLS OF SOUTH DAKOTA, U.S.A.

JAMES MARTIN; DI PASQUO MERCEDES

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Congreso; Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

The marine Winnipeg Formation exposed in the northern Black Hills of South Dakota is composed of the Icebox and Roughlock members, which represent the southern extension of transgressive episodes documented in the northern Great Plains (Williston Basin) of the USA and Canada. The typically dark green to black shales of the Icebox grade up into argillaceous carbonates of the Roughlock. The first palynologic analysis of the Icebox and lower Roughlock members includes a well-preserved and diverse palynoflora from 14 unweathered samples collected from Spearfish Canyon (U.S. Highways 14 and 85). The lower 10 samples were taken approximately every meter from the nine meters of Icebox shales and three additional samples from thin transitional layers into the calcareous shale of the Roughlock (1 m), from which another sample was collected. The 14 assemblages are variably composed of chitinozoans (25 species) and acritarchs (23 species), with fewer cryptospores, cyanophycean Gloecapsomorpha prisca, and five scolecodonts. Frequency trends from the last four samples, corresponding to the transition of Icebox to Roughlock, show chitinozoans dominating the lower sample with a lesser proportion of acritarchs that dominate in the next two samples. Gloecapsomorpha prisca, an Ordovician colonial marine microorganism, was found intermittently through the entire interval but is especially abundant in the sample from the Roughlock; hence interpreted as blooms related to environmental changes. Although many chitinozoans and acritarchs are long-ranging Ordovician species, the chitinozoans Calpichitina lenticularis, Euconochitina (Jenkinochitina) tanvillensis and Belonechitina robusta, documented from the base up to the mid-upper Icebox, support a correlation of our studied interval with the mid Caradocian (equivalent to North American Trentonian and current Sandbian?Katian stages) homonymous palynozones of North Gondwana. The acritarchs Elektoriskos aktinotos, Peteinosphaeridium accintulum and Sylvanidium paucibrachium, known from the Ashgillian of Missouri, and Peteinosphaeridium septuosum from the Caradocian and Ashgillian of Kansas, Missouri and Oklahoma, and Belonechitina punctata (recorded in the upper Icebox), reinforce this age. This is in concordance with previous studies of conodonts from upper Icebox and Roughlock at Icebox Gulch and Whitewood Creek in the Black Hills, where lower and mid Caradocian species were documented (Belodina (Panderodus) compressa, Chirognathus duodactylus (= delicatula), Distacodus insculptus, Falodus prodentatus, Oulodus serratus, Plectodina dakota, Plectodina tenuis, Ptiloconus gracilis, Scyphiodus primus, and several species of Panderodus). A preliminary paleobiogeographic affinity indicates a close relationship to Northern Gondwanan, Baltic and Laurentian chitinozoan assemblages during the late Ordovician.*Financial support: CONICET (Fulbright Scholarship 2009, PIP 0812 2015-2017).