SHEET-SILICATE REPLACEMENT OF SKELETAL FOSSILS IN THE WALCOTT QUARRY OF THE BURGESS SHALE, BRITISH COLUMBIA, CANADA

The fossil-bearing Burgess Shale Formation of the Rocky Mountains of southern British Columbia is famous both for its faunal diversity and exceptional preservation of soft-bodied, non-mineralizing organisms. One of the interesting features of the fossils is that they are composed largely of sheet silicates, both chlorite and muscovite. Common fossils such as trilobites and brachiopods also display significant replacement by muscovite and chlorite but also have greater thickness and well documented primary shell structures. Such fossils can be easily studied using petrographic analysis. Accordingly, the main purpose of this study is to further understand the general character and controls of sheet silicate fossil replacement in Burgess Shale fossils through the examination of skeletal fossils. The Walcott Quarry Shale Member of the Burgess Shale Formation is composed of calcareous siltstone and mudstone with discrete limestone beds. These beds were metamorphosed in greenschist facies and contain a typical low-grade metapelitic mineral assemblage of chlorite-muscovite-quartz-carbonate. The relative abundance of each of these minerals varies from bed to bed but can be correlated with major element chemistry. The only abundant Al-bearing minerals are sheet silicates, and the only abundant K-bearing mineral in muscovite. Accordingly, the ratio K2O/Al2O3 reflects the abundance of muscovite relative to chlorite. Carbonate content is reflected in LOI values. Mineral composition of identifiable skeletal fossils was documented. In carbonate-rich rocks the fossils were composed predominantly of carbonate. Fossils in shaley and silty rocks had a far higher sheet silicate content, and the relative percentage of chlorite to muscovite generally correlates with the chlorite to muscovite ratio in the matrix. Thus the factor that controlled sheet silicate replacement of fossil diagenesis and metamorphism must have been bulk-rock chemistry rather than pore-water composition.