Permian Radiolaria of the Fantasque Fm., Ursula Creek, Canada.

Paul B. Wignall 1 and Peter A. Cejchan 2
1 School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK, wignall{at}earth.leeds.ac.uk
2 Laboratory of Paleobiology and Paleoecology, Institute of Geology, Academy of Sciences of the Czech Republic, Prague, CZ, cej{at}gli.cas.cz

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Abstract

A rich and diverse radiolarian biota of Late Permian Changxingian Stage Fantasque Formation has been documented from the Ursula Creek section (British Columbia, Canada). The radiolarians come from two samples of chert located near the P/T boundary and represent a pre-extinction assemblage.

Geological background

A continuous Middle Permian to Middle Triassic section is well displayed in a near-vertically dipping section on the shores of Lake Williston, British Columbia, a few kilometers west of the mouth of Ursula Creek (Fig. 2). The P–Tr transition is recorded in the cherts of the Fantasque Formation and the overlying shales of the Grayling and Toad formations. Relatively little work has been undertaken on the section, although Wang et al. (1994) recorded a negative spike in {delta}13Corg and thus were able to define the location of the P–Tr boundary, confirmed in Wiognall and Newton, 2003, by conodont evidence. The Ursula Creek section reveals the progressive decline of seafloor oxygen values in the Changxingian Stage (loss of bioturbation, authigenic U enrichment, appearance of pyrite framboid populations), followed by the persistent development of euxinic conditions in the latest Changxingian and throughout the Early Triassic; an event that coincides with the disappearance of a siliceous sponge fauna and the loss of diverse radiolarian populations.

map 1

FIGURE 1. Global paleogeography during the Permian-Triassic boundary interval (based from data in Stampfli et al., 1991, Scotese and Longford, 1995, and Kobayashi, 1999).

map 2

FIGURE 2. Location of the study site at Ursula Creek, British Columbia, Canada.

Chronostratigraphy

The cherts of the Fantasque Formation are widespread in western Canada and generally are regarded as Wordian-Capitanian age, based on their conodont fauna (Henderson, 1997). They form part of a regionally widespread, but poorly dated, chert depositional event (Murchey and Jones, 1992; Beauchamp and Baud, 2002). At Ursula Creek, limestone concretions from near the top of the Formation (Fig. 11) have yielded Mesogondolella sheni, an index for the late Changxingian (Henderson and Mei, 2000). This conodont also has been recovered from the topmost chert beds, indicating a Changxingian age for the upper Fantasque Formation. Thus, the Fantasque Formation at Ursula Creek either may record a prolonged phase of condensed chert deposition (from the Wordian to Changxingian stages), or may be younger than elsewhere in the region. The former alternative is preferred because a single horizon of chert deposition is seen in sections throughout western and Arctic Canada (Murchey and Jones, 1992; Beauchamp, 1994).

stratigraphic section

FIGURE 3. Stratigraphic section of Upper Permian–Lower Triassic strata, Ursula Creek, British Columbia. See Figure 12 for sediment fabric key. The Lower Triassic strata consist of finely laminated, anoxic facies, thin examples of which also occur in the Upper Permian.

Facies Analysis

The Fantasque Formation consists primarily of radiolarian and spiculitic chert, with minor dark-gray shale interbeds. Decimeter-thick beds of chert separated by centimeter-thick partings of shale are typical, although thicker beds of chert and shale also are present (Fig. 11). The contact between the Fantasque and Grayling Formations is sharp and marks the loss of cherts, which are replaced by dark-gray shales displaying papery fissility and, at higher levels in the formation, less-fissile shaly mudstones. Interbedded with the shales are distinctive, laminated, orange-brown weathering beds ranging from 1–35 cm thick. Henderson (1997) suggested these beds were siltstones that marked the culmination of small-scale coarsening-upwards units, but thin-section and SEM analyses reveal them to be comprised of a mosaic of small dolomite rhombs and organic-rich laminae. Two heavily recrystallised filamentous limestones occur towards the top of the formation.

stratigraphic section

FIGURE 4. Lithological and 13Corg changes in the vicinity of the P–Tr boundary at Ursula Creek. Carbon isotope data from Wang et al., 1994

Radiolaria

Radiolaria are abundant in cherts of the Fantasque Formation, with the most-diverse assemblages (as determined in thin sections [P.B.W] and HF-acid etching residues [P.A.C.]) occurring towards the top of the formation (Fig. 11), where the assemblage includes representatives of the Entactinaria and Grandetorturiidae (Fig. 15). Only indeterminate, small chalcedony-filled spheres (radiolaria?) are present in the top 75 cm of the Fantasque Formation, a few of which range up into the laminated dolomite bed 75 cm above the base of the Grayling Formation. As noted above, small, simple radiolaria (spheroids) reappear in the Toad Formation.

radiolarians in thin sections

FIGURE 5. Photomicrographs of radiolaria from the Fantasque Formation (sample 85 cm from the top of the Formation) showing representatives of the Grandetorturiidae (on the left), the Entactiniidae, probably Copicyntra, in the middle, and an indeterminate spumellarian on the right. All to the same scale; the spumellarian is 0.4 mm in diameter.

Other biota

Well-preserved monaxon spicules of siliceous sponges are common in the uppermost meters of the Fantasque Formation, and megascleres up to 1 cm in length are common locally. In detail, the sponge fauna is common in the bioturbated beds and absent from the laminated strata. The Lower Triassic formations of Ursula Creek lack macrofossils, except for the rare presence of well-preserved fish (cf. Bobastraenia) in the upper Grayling Formation.

Plates

radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian radiolarian




... to be continued ...

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