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Frosty times in the Sverdrup Basin: the Jurassic–Cretaceous ‎transition in the Rollrock Section, Canadian Arctic Archipelago
Simon Schneider‎, Simon Kelly, Jörg Mutterlose, Peter Hülse, Berta Lopez-Mír

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-06 05:20 PM – 05:40 PM
Last modified: 2018-11-24


The Rollrock Section on northern Ellesmere Island (Canadian Arctic Archipelago, Nunavut) exposes an over 500 m thick, continuous succession of Late Jurassic to Early Cretaceous sediments, and was regarded as the most important Jurassic–Cretaceous transition section of the Canadian Arctic by Jeletzky (1984). The succession is assigned to the Ringnes and Deer Bay formations, and is dominated by mudstones, with minor siltstones and subordinate fine-grained sandstones. The Deer Bay Formation grades into the sand-dominated Isachsen Formation at the top. The sediments were deposited in the Sverdrup Basin, most likely on a moderately shallow shelf in relatively proximal position; they correspond to the climax and post-climax intervals of a major syn-rift stage (Hadlari et al. 2016).

The Rollrock Section was logged and sampled by the first author, with the help of field assistant Alex Chavanne, during five days in July 2015. Mudstone samples were collected at 1.5 m intervals, and sideritic mudstone concretions, which occur in discrete horizons, were carefully examined for macrofossils. The base of the Ringnes Formation is not exposed, but the lower 251 m of the logged interval are assigned to this unit. Its uppermost 20 m are dominated by fine-grained sandstones and siltstones. A rapid change back to mudstone deposition and the sudden occurrence of abundant dropstones mark the onset of the Deer Bay Formation. Similar dropstones, i.e. up to 100 mm sized dark chert and milky quartz pebbles, occur throughout the Deer Bay Formation. Together with common glendonites, which are confined to ~10 horizons in the upper half of the unit, these dropstones document cold, Arctic conditions during the deposition of the Deer Bay Formation. It is open to debate whether ice rafts formed seasonally or by calving of glaciers.

Fossil bivalves, mostly Buchia spp., were found in four concretion horizons of the upper Ringnes Formation (192, 196, 197 and 203 m) and in seven horizons of the Deer Bay Formation (307, 328, 333.5, 355, 356 and 363.5 m). Additionally, six levels yielded ammonites (297, 307, 355, 356, 357 and 363.5 m; Fig. 1). In the upper half of the unit, belemnites were collected from three horizons (410.5, 425 and 470 m).

Collectively, these fossils document early Tithonian to Valanginian ages. Based on the occurrence of Buchia rugosa, the 192, 196 and 197 m horizons are assigned to the early Tithonian Buchia rugosa Zone. The co-occurrence of Dorsoplanites maximus (Fig. 1) and D. sachsi in the 307 m horizon indicates the mid Tithonian Dorsoplanites maximus Zone.

A monospecific Buchia terebratuloides shell pavement at 333.5 m places this horizon in the latest Tithonian to earliest Berriasian; this is the fossil horizon closest to the Jurassic-Cretaceous boundary. The ammonites Praetollia maynci at 355 m, Pseudocraspedites at 356, 357 and 363.5 m, and Borealites at 363.5 m, together with Buchia okensis at 355, 356 and 363.5 m, collectively indicate an early Berriasian age for this interval. Finally, belemnites determined as Arctoteuthis cf. porrectiformis, collected at 410.5 and 425 m, are Valanginian in age.

The new macrofossils significantly improve the dating of the Rollrock Section underlining its importance for interpreting the Sverdrup Basin succession as a whole. Furthermore, the well-dated macrofossil horizons provide constraints for future analyses of microfossils, palynomorphs and geochemistry, which are underway.