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

Journal of Paleontology, Nov 1998 by Dixon, Owen A

ABSTRACT-Heliolitine corals occur in Upper Silurian strata of the topmost Douro, lower Barlow Inlet, and lower Somerset Island formations in the Boothia Uplift region. The fauna includes Heliolites (nine species), Propora (two species), the stelliporellid genera Podollites (three species) and Stelliporella (one species), and the plasmoporid Squameolites (one species). Species present but not previously reported from the region include Heliolites arcuatus Bondarenko, Heliolites uksunayensis Mironova, Stelliporella ilensis Bondarenko, Podollites assitus new species, Podollites parryi new species, Podollites plectilis new species, Propora tubulata (Lonsdale) and an unidentified species of Propora. All species occur in reef and reef-associated facies of open carbonate shelf and ramp settings. In restricted nonreefal carbonate shelf settings, diversity attenuates to a minimum, exemplified by numerically abundant monospecific assemblages of Heliolites uksunayensis. Six of the 16 morphotaxonomic units recognized compare with species reported previously in Silurian faunas of Kazakhstan and Mongolia. Combinations of characters in the species attributed to Podollites support revision of the genus, possibly to include Denvatolites and Tarbagatailites, in a group morphologically intermediate between Stelliporella and Heliolites.

INTRODUCTION

LUDLOW-AGE HELIOLITINE corals occur abundantly in three discrete stratigraphic intervals in the Boothia Uplift region of the southern Canadian Arctic archipelago (Fig. 1). For convenience these are termed "Faunas 1, 2, and 3" in this paper. Fauna 1 (Dixon, 1989) occurs in the lower half of the Douro Formation (Fig. 2), in strata termed the "Lower Reefal Zone" by Narbonne and Dixon (1984), and in nonreefal beds closer to the boundary of the Douro with the underlying Cape Storm Formation. Fauna 2 (Dixon, 1979, 1993, 1996) occurs in the "Upper Reefal Zone" in the upper half of the Douro Formation, and in laterally equivalent nonreefal beds in some areas. Fauna 3, the subject of this new paper, occurs in the topmost Douro and immediately overlying parts of the Barlow Inlet and Somerset Island formations (Fig. 2). In all three faunas, the most diverse assemblages are closely associated with organic buildups: those of Faunas 1 and 2 occur in and around lithistid sponge/microbial mudmounds, first described on Somerset Island (Narbonne, 1981; Narbonne and Dixon, 1984), and those of Fauna 3 are associated with similar mudmounds, mudmound complexes, and stromatoporoid reefs, principally on southwestern Devon Island (e.g., Dixon and Graf, 1992).

Although these faunas all occur within the relatively short time span of the late Ludlow, each differs somewhat in taxonomic composition: a few species occur in all three faunas, whereas others occur in only one or two of the faunas. Despite their taxonomic differences, the faunas show analogous stratigraphic and/or paleogeographic patterns of diversity change, a reflection of distinctive paleoenvironmental distributions. The faunas differ in terms of maximum species diversity, and each has a different geographic locus of maximum diversity in the Boothia Uplift region, suggesting appreciable paleogeographic change through Ludlow time. The faunas occur in a rock sequence showing evidence of profound changes of relative sea level. Previous studies have shown that Boothia Uplift tectonism mitigated or even reversed more regional pulses of relative sea level rise (ideas summarized in Trettin et al., 1991, p. 182). As common and environmentally sensitive elements of benthic assemblages in this Ludlow sequence, the heliolitine corals offer a variety of evidence that contributes to the understanding of stratigraphic sequence and paleoenvironmental development in the region.

The main emphasis of this paper is taxonomic, with the objective of recognizing, describing, and illustrating the species that make up Fauna 3 in carbonate shelf/ramp facies in the region. Taxa not previously recorded in the Arctic are documented thoroughly below, while those previously recorded in Faunas 1 and 2 (Dixon, 1979, 1989, 1996) are described briefly, primarily augmenting the earlier descriptions with the help of the new, larger morphological database derived from Fauna 3. A paper in preparation will provide a key to the taxonomy of the entire suite of Upper Silurian heliolitine corals in Faunas 1, 2, and 3, with an overview of their stratigraphic and geographic distribution in the region, and their biogeographic affinities.

Heliolitine corals have also been found in other regions of the Arctic, in flank beds of pinnacle reefs and other basinal buildups, such as those described by DeFreitas et al. (1993) and DeFreitas and Dixon (1995). These assemblages are known to include Llandovery, Wenlock, Ludlow, and Pridoli representatives, but are as yet unstudied.

COLLECTIONS, STRATIGRAPHY AND PALEOGEOGRAPHY

Pertinent to previously reported stratigraphic occurrences of heliolitine corals in the region, the Upper Silurian Read Bay Formation, Members A, B, C, and D of older literature, has become the Read Bay Group, with its members assigned formation names: Member A is now the Douro Formation; Members B and C, the lower and upper members of the Barlow Inlet Formation; and Member D, the Sophia Lake Formation.