A MIDDLE CAMBRIAN SMALL SHELLY FAUNA FROM CARBONATE MEMBERS OF THE BURGESS SHALE FORMATION, BRITISH COLUMBIA

The middle Cambrian Burgess Shale is a basinal succession of mostly calcareous shales and carbonates deposited adjacent to a carbonate platform margin. The shale faunas are well studied but not the carbonate faunas. Here we report on small shelly fauna (SSF) in the Yoho River Limestone (YRL) and Wash Limestone (WL) members of the Burgess Shale Formation in Yoho National Park. The fossils, mostly silicified and less commonly phosphatic, were recovered by bulk dissolution using 10% acetic acid. The assemblages are low diversity and include: rhynconelliformean brachiopods (Kutorginida, Naukatida and others); linguliformean brachiopods; monoplacophorans (Helcionellida); Stenothecoida; phosphatic tubes, and echinoderm thecal plates. Comparisons with shelly fauna of the shale members show marked differences: abundant elements of the SSF carbonate assemblages such as stenothecoids, naukatid brachiopods, and some species of helcionellids, are unknown or rare in the shale units. Conversely, shelly components common in the shale members, including hyolithids and trilobites, are so far, unknown from the SSF assemblages, although at least trilobites are reported elsewhere in the carbonate members.

The YRL and WL members are complex units that include conglomerates, grainstones, wackestones, thin-bedded and massive micritic beds, carbonate mounds, and, at some localities in the YRL, ripple marks and dilational cracks. These units are wedge-shaped, thinning basinward, and are laterally restricted to the type area around Field, B.C. Some beds in these units may represent mass flow deposits from platformal environments, while others, including those with carbonate mounds, were clearly deposited in situ. Sedimentologic work has yet to determine whether the SSF-bearing beds are allochthonous. If the SSF assemblages represent platform communities transported down paleoslope, then it is unlikely that the taxonomically differing shelly assemblages of the shale members include allochthonous platform elements as some authors have suggested. If autochthonous, then the SSF assemblages signal a biofacies change compared to the shale assemblages, perhaps associated with shallowing paleoenvironmental conditions arising from localized tectonic uplift.