A new Burgess Shale-type locality in the'thin' Stephen Formation, Kootenay National Park, British Columbia: stratigraphic and paleoenvironmental setting

摘要

A new Burgess Shale-type locality in the Middle Cambrian Stephen Formation was excavated in August 2008 by a team organized by the Royal Ontario Museum. This locality lies below Stanley Glacier in Kootenay National Park, approximately 40 km SE of Walcott's Quarry in the Stephen Formation on Fossil Ridge, near Field, British Columbia. Region- ally, the Stephen Formation has two different expressions, which historically have been termed the"thick" and"thin" Stephen, the former now called the Burgess Shale Formation by some workers. Respectively, these units have been interpreted to represent deposition on a drowned carbonate platform and in an adjacent basin, which was separated from the platform by a submarine escarpment, known regionally as the Cathedral Escarpment. Whereas the Walcott Quarry and other fossil-bearing localities in the area of Field occur in the"thick" Stephen Formation, directly adjacent to the escarpment, the new Burgess Shale-type locality occurs in the"thin" Stephen Formation, deposited either above an escarpment which has no local expression, or on a ramp where no escarpment was present. The sediments which contain the exceptional biota at the Stanley Glacier locality are similar to those of the Burgess Shale in cm to μm scale attributes; however, the stratigraphic and paleoenvironmental setting of the two localities is different. The"thick" Stephen Formation is approximately 300 m in thickness and is shale dominated in the Field area, whereas the Stephen Formation at Stanley Glacier is 32.5 m thick, and is composed of six shale-wackestone parasequences, which range from 1.5 to 15 m in thickness. At Stanley Glacier, the Stephen Formation is similar in thickness and lithology to other"thin" Stephen sections described from the region but differs in that no evidence of grading, scour, hummocky cross-stratification or cross-bedding is present, indicating that the entire Stephen Formation at Stanley Glacier was deposited below storm wave base. The Stanley Glacier fossil assemblage occurs in the upper of two members of the Stephen Formation and thus is younger in age than those of the classic Burgess Shale lo