MINERALOGICAL AND LITHOLOGICAL FEATURES ASSOCIATED WITH BRINE SEEPS IN THE BURGESS SHALE, WITH IMPLICATIONS FOR THE INTERPRETATION OF THE KINZERS SHALE, PA

The Burgess Shale is anomalous in the abundance of fossils of soft-bodied fauna that it hosts, as well as in the quality of their preservation. The unit also exhibits a set of lithologically and mineralogically anomalous features. Lenses of thinly bedded strata composed predominantly of clinochlore occur in proximity to carbonate escarpments in the Burgess Shale Formation, and the younger Duchesnay Unit. These Mg-rich beds (up to 31 wt% MgO) are interpreted to be metamorphosed accumulations of Mg-rich minerals (brucite and Mg-smectite) that precipitated from warm brines that seeped from carbonate escarpments and accumulated in local seafloor depressions. Low-diversity, high-abundance fossil assemblages commonly rim the brine pools, as is the case around Recent deep-sea brine pools. Dolostone adjacent to exhalative strata commonly contains distinctive, prismatic, doubley-terminated quartz crystals up to 25mm in length that are commonly hollow and zoned. Identical quartz alteration is found adjacent to talc and magnesite ore bodies along a >100km length of the Cathedral Escarpment. Such quartz porphyroblasts can be generated by the presence of a Mg-Al-bearing solution which favors precipitation of crystalline quartz rather than microcrystalline, or opaline forms of silica. Another unusual mineralogical feature of the fossil-bearing region along the Cathedral Escarpment is the abundance of early diagenetic herringbone calcite cement in open spaces and fractures within the Cathedral carbonates. Restricted almost exclusively to the Precambrian, this distinctive form of calcite cement is produced when calcite grows in the presence of a calcite inhibitor ion such as Fe2+ or Mg2+. Accordingly, a diverse set of mineralogical, lithological, and paleontological features in the Burgess Shale can be unified and tied genetically to the presence of Mg-rich brine seeps. A direct association between seep activity and BST preservation has not yet been documented at any other Laurentian localities. However, herringbone calcite and hollow quartz megacrysts occur in carbonates associated with the Kinzers Formation of southeastern PA, and its BST preservation, thereby suggesting a brine association at this locality as well.