Heliolitine corals of the topmost Douro and overlying formations (Upper Silurian), Canadian Artic Islands

Journal of Paleontology, Nov 1998 by Dixon, Owen A

Fauna 3 heliolitines have been found at eight localities on four islands in the Boothia Uplift region (Fig. 1; Appendix). On Somerset Island, the Douro Formation is gradationally overlain by mainly peritidal carbonates and silty carbonates of the Somerset Island Formation (Fig. 2), a shallowing-upward sequence that culminates in continental clastic wedge facies derived from the emergent Boothia Uplift. South of Pressure Point on northwestern Somerset Island, a small heliolitine coral assemblage!(five specimens) was collected from a subtidal limestone unit containing small mudmounds, in the basal 15 m of the Somerset Island Formation. A large assemblage (119 specimens) comes from near Cunningham Inlet from a fault-bounded sequence of subtidal limestones at a higher level, not yet determined precisely, in the Somerset Island Formation. Unique pentamerid brachiopod shell concentrations (Jones, 1978, 1979) are associated with the heliolitine assemblage near Cunningham Inlet. Although initially included in the Read Bay Formation, these beds are regarded here as part of the lower member of the Somerset Island Formation that Miall and Gibling (1978) interpreted as becoming predominantly limestone, eastward on Somerset Island.

On southwestern Devon, southern Cornwallis, and Griffith islands, the Douro is conformably overlain by the Barlow Inlet Formation (Fig. 2), a sequence comprising predominantly shallow water, restricted to open marine, rimmed shelf carbonates. Fauna 3 heliolitines are particularly common in the Gascoyne Inlet area of southwestern Devon Island. Assemblages were collected from within and near microbial mudmounds in the top 10 m of the Douro Formation (18 specimens) and through the basal 185 m of the Barlow Inlet Formation (171 specimens); the buildups and local stratigraphy of this boundary sequence have been documented in Dixon and Graf (1992). A small assemblage (10 specimens), also from the top of the Douro Formation, comes from a large mudmound inland from Griffin Inlet on southwestern Devon Island. An assemblage of 20 heliolitines was collected from beds surrounding large mudmounds at the top of the Douro Formation, immediately beneath the Barlow Inlet Formation, near Cheyne Point on Griffith Island. Locally on Cornwallis Island a thin lower member (mainly marine deltaic mudrocks) separates the predominantly carbonate upper member of the Barlow Inlet Formation from the underlying Douro Formation (Fig. 2); a unique monospecific assemblage (eight specimens) of Fauna 3 heliolitines was found flanking a small mudmound near the base of the upper member at Goodsir Creek on eastern Cornwallis Island.

The shelf carbonate facies of the Barlow Inlet Formation pass laterally northward on Devon Island into basinal shales of the Devon Island Formation (Fig. 2). The latter rest conformably but abruptly on the Douro Formation and in an area inland from Dragleybeck Inlet on western Devon Island where they drape over dozens of large microbial mudmounds (Sweet, 1995). Fauna 3 heliolitines (58 specimens) were collected from these topmost Douro mudmounds and their flankbeds, but none was found in capping beds of the Devon Island Formation. On Cornwallis Island both the Douro and Barlow Inlet formations pass laterally northward into slope and basinal facies of the Cape Phillips Formation; a few poorly preserved heliolitines have been recovered from distal carbonate shelf deposits near the transition (Fig. 2), north of Snowblind Bay.