COMPARISON OF FOSSIL AND MATRIX COMPOSITION WITHIN THE WALCOTT QUARRY OF THE BURGESS SHALE, BRITISH COLUMBIA, CANADA

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.

Previous results indicate that there is no direct relationship between the muscovite-chlorite ratio in a particular fossil, and the bulk chemistry of the rock that contains it. This maybe due to the fine scale at which compositional variation occurs within individual laminae in the Burgess shale relative to the scale of sampling for whole-rock chemistry. Further analytical work at the scale of individual laminae was required and accomplished by using an electron microprobe analysis. The LOI values were individually correlated with the K2O/Al2O3 ratio for the matrix and the ms/ms+chl ratio of fossil. The results showed that in both cases the LOI values were inversely proportional with the matrix and fossil composition. Thus, 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. Furthermore, the factor that controlled sheet silicate replacement of fossils diagenesis and metamorphism must have been bulk-rock chemistry rather than pore-water composition.