Age: Lower Cambrian
The preservation of the soft-bodied Cambrian biota within the Sirius Passet fauna has led to inevitable comparisons with the Burgess Shale. However, the Sirius Passet fauna is found in rocks which are actually more closely contemporaneous with the Chengjiang fauna of China. The Burgess Shale is some ten or fifteen million years younger, 505 Ma compared with 518 Ma (Martin et al. 2000). Biostratigraphic dating involving the trilobite Buenellus higginsi suggests the fauna is of Nevadella Biozone age (North American usage) (Budd, 1999).
The Sirius Passet fauna is contained the Buen Formation (Budd, 1999), the lower parts of which consist mainly of cross-bedded sandstones. The fossils come from well-laminated mudstones at the very bottom of the formation. The Buen Formation is part of a shelf sedimentary sequence (Budd, 1999), and the mudstones of the Sirius Passet fauna were deposited in an "outer shelf to slope-basin" setting (Butterfield, 1995), a deeper water facies than the Burgess Shale sediments (Clowes [www] 2002).
The mudstones are dominated by clay minerals, but the presence of chlorite, muscovite and chloritoid indicates that the rock unit has been subjected to the elevated pressures and temperatures associated with the greenschist metamorphic grade (Babcock [www] 2002), [between 150°C and 250°C and at low pressure].