Cretaceous cold start: the Jurassic-Cretaceous transition in the Rollrock Section, Sverdrup Basin, Canadian Arctic Archipelago

The Rollrock Section on Ellesmere Island, exposing >500 m of fine-grained sediments, is one of the most complete rock archives of the Jurassic-Cretaceous transition in the Arctic, and provides a unique opportunity to study the stratigraphy and palaeoclimate of this interval in detail. The lower half of the succession (Ringnes Formation) is dominated by mudstones, with fine-grained sandstones and siltstones forming the top of the unit. A rapid transition to mudstone deposition and the sudden occurrence of abundant dropstones mark the onset of the Deer Bay Formation, which grades into the sand-dominated Isachsen Formation at the top. The succession likely was deposited on the outer shelf of the Sverdrup Basin. Dropstones occurring throughout the Deer Bay Formation are evidence of either coastal ice rafts formed during winter, or proper icebergs. Glendonites from ten horizons in the upper half of the unit also document cold, Arctic conditions during the deposition of the Deer Bay Formation. We sampled the Rollrock Section at 1.5 m intervals for microfossil, palynomorph and geochemical analysis. Additionally, macrofossils were collected from sideritic concretion horizons in the upper 350 m of the succession. Ammonites, bivalves and belemnites collectively document early Tithonian to Valanginian ages: the early Tithonian Buchia rugosa Zone; the mid Tithonian Dorsoplanites maximus Zone; the Buchia terebratuloides Zone marking the Jurassic-Cretaceous boundary interval; the early Berriasian Chetaites sibiricus and Hectoroceras kochi zones; and a Valanginian succession above 410 m. Dinoflagellate cyst assemblages suggest an Oxfordian age for the basal part of the section and broadly confirm the macrofossil ages from higher-up. They further document significant environmental perturbations during the Cretaceous interval. These data improve the biostratigraphy of the Sverdrup Basin succession as a whole, and pave the way for establishing the presence of Arctic cold snaps and ice caps over the Jurassic-Cretaceous transition.