Macrofauna and biostratigraphy of the Rollrock Section, northern Ellesmere Island, Canadian Arctic Islands – a comprehensive high latitude archive of the Jurassic–Cretaceous transition
Introduction
The Jurassic–Cretaceous transition interval (Tithonian to Valanginian) is widely known as a phase of environmental perturbation and associated biotic turnover (see Tennant et al., 2017 for a summary). However, the magnitudes of extinction, recovery and ecosystem change remain poorly known (e.g. Hallam, 1986, Tennant et al., 2017). Notable is a global eustatic sea-level drop of 40–50 m close to the system boundary (latest Tithonian), followed by a second drop of similar magnitude during the latest Berriasian to mid Valanginian, resulting in the lowest sea-level of the entire Cretaceous Period (Haq, 2014). Both sea-level drops were associated with significant cold intervals, but whether the low temperatures corresponded to phases of polar glaciation is still a matter of debate (e.g. Price, 1999, Price et al., 2013, Haq, 2014, O'Brien et al., 2017, Vickers et al., 2019).
Logically, one would turn to high latitude records when searching for evidence of cold snaps and ice caps, or more generally for an archive of pronounced climate fluctuations. The Mesozoic sedimentary succession of the Sverdrup Basin in Arctic Canada is one of the best candidates for investigating these topics (Kemper, 1987, Galloway et al., 2020). Today, only the Wandel Sea Basin in northern Greenland has exposures of the Jurassic–Cretaceous boundary interval that lie at a higher latitude (e.g. Håkansson et al., 1981). However, the palaeo-latitude of the Sverdrup Basin during the Jurassic-Cretaceous transition was lower than today, but still Arctic to sub-Arctic, approximately between 60° and 70° N (van Hinsbergen et al., 2015). Moreover, cold climatic conditions during the deposition of the Tithonian to Valanginian Deer Bay Formation of the Sverdrup Basin are suggested by the occurrence of glendonites (Kemper and Schmitz, 1975, Kemper and Schmitz, 1981; Grasby et al., 2017) and abundant dropstones (Embry, 1991).
In order to investigate palaeoecology and palaeoclimate in detail, a robust stratigraphy is first needed for the study interval. In 2015, we thus logged and sampled a more than 500 m thick, continuously exposed succession of Upper Jurassic to Lower Cretaceous mudstones, cropping out on the northern flank of the Rollrock River Valley, northern Ellesmere Island. This outcrop extends laterally over more than five kilometres (Fig. 1, Fig. 2) and exposes the mudstone-dominated Ringnes and Deer Bay formations, grading into sandstones of the Isachsen Formation at the top. Based on its macrofossil content, the Rollrock Section was regarded as the biostratigraphically most important Jurassic–Cretaceous transition section of the Canadian Arctic by Jeletzky (1984). Furthermore, the succession contains glendonites in several horizons and abundant dropstones over a large interval, and thus is likely to provide a direct record of Late Jurassic to Early Cretaceous Arctic cooling.
Discovered and briefly described by Wilson (1976), the Rollrock Section was logged and sampled by A. F. Embry and N. S. Ioannides in 1977 (personal communication A. F. Embry, 2017). Their results are expressed in the 1:250,000 scale geological map of the area, issued by the Geological Survey of Canada (Map no. 1886A, Tanquary Fiord; Mayr and Trettin, 1996), and the macrofossils they collected were published by Jeletzky (1984). However, no detailed account of the succession is available, and the collected palynology data were never published.
Herein, we present a comprehensive log and brief sedimentological description of the succession exposed in the Rollrock Section, along with a lithostratigraphic framework. We further provide systematic descriptions of the macrofauna, together with an updated ammonite and bivalve biostratigraphy, which correlates the Jurassic-Cretaceous boundary succession of the Sverdrup Basin with the entire Boreal Realm.
Section snippets
The Sverdrup Basin
The Sverdrup Basin is located in the Queen Elizabeth Islands, Nunavut and the Northwest Territories, and covers an area of approximately 300,000 km2 (e.g. Embry and Beauchamp, 2008, Pugh et al., 2014, Fig. 1). Basin development commenced in the Mississippian on Neoproterozoic to Devonian strata, and terminated with the onset of the Eurekan Orogeny in the latest Cretaceous (e.g. Hadlari et al., 2016).
The Sverdrup Basin is filled with up to 13 km of Carboniferous to Eocene sedimentary strata,
Material and methods
The Rollrock Section was logged and sampled during five days in July 2015. The strata dip gently (17° on average) towards northwest (dip direction is 321° on average), which is approximately perpendicular to the slope. A telescopic walking pole, calibrated to 1.5 m length, was used for measuring thickness. We chose one of the easternmost ridges for logging, since it provided relatively easy access and had less scree cover in its lowest part (base near 81.61172°N, −75.58489°W; top near
Sedimentology
The lower 230 m of the Rollrock Section expose a succession of dark, thinly bedded or laminated clayey siltstones and mudstones, with rare intercalations of siltstone or fine-grained sandstone (Fig. 6). From 190 m upward, several horizons of sideritic concretions occur, some of which have yielded bivalves, a lobster and fossilised wood. Above 230 m, a 20 m thick interval of siltstones and fine-grained sandstones follows. At 251 m, the change back to mudstone sedimentation is sharp and sudden.
Lithostratigraphy
The Ringnes Formation has its type section on central Amund Ringnes Island, and was defined as a succession of dark grey to black, very silty to slightly sandy shale with abundant, randomly distributed, giant, up to 5 m long, ellipsoidal, yellowish weathering, sideritic mudstone concretions (Balkwill et al., 1977, Balkwill, 1983). These characteristic concretions served to distinguish the unit from the McConnell Island Formation below and the Deer Bay Formation above. Sparse macrofossils
Systematic palaeontology
All fossils from the Rollrock Section are part of the Nunavut Collections of the Canadian Museum of Nature, Ottawa, Canada, and stored under registration numbers with the prefixes NUIF (for invertebrate fossils) and NUPB (for plant remains). Numbers NUIF 2979–3019, NUIF 3030–3046, NUIF 3061–3064, NUIF 3134–3140, NUIF 3158, NUIF 3165–3167 and NUPB 571, 572 and 575 are assigned to fossils from the Rollrock Section. Registration numbers for individual specimens are provided in the material
Biostratigraphy
As a result of the dearth of macrofossils in the Upper Jurassic to Lower Cretaceous interval in the Sverdrup Basin, biostratigraphic correlation with the remainder of the Boreal Realm is relatively poorly constrained. Dinoflagellate cysts and foraminifera offer only limited insight. This is because the intra-basin zonation schemes developed by Davies (1983) and Wall (1983), respectively, have not been correlated to other parts of the Arctic, and the precision of the ages determinable from these
Conclusions
The Rollrock Section on northern Ellesmere Island provides the most comprehensive outcrop record of the Jurassic-Cretaceous transition interval in the Sverdrup Basin of the Canadian Arctic, and is globally one of the northernmost exposures of such rocks. More than 500 m of a continuously exposed mudstone-dominated succession are almost evenly split into the lower, Oxfordian to lower Tithonian Ringnes Formation and the overlying, lower Tithonian to middle Valanginian Deer Bay Formation, grading
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Working in the Arctic would not be possible without manifold support. Our sincere thanks go to the following people in Canada who helped to make our research a success: Sylvie LeBlanc (Department of Culture and Heritage, Iglooik, Canada); Jane Chisholm (Parks Canada, Iqaluit, Canada); John Innis (Universal Helicopters); the rangers of Parks Canada at Quttinirpaaq National Park (Ellesmere Island, Canada); the Polar Continental Shelf Programme team at Resolute (Cornwallis Island, Canada);
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