SYNGENETIC, DEEP-SOURCED, CLINOCHLORE MUD DIAPIRS IN THE MIDDLE CAMBRIAN BURGESS SHALE

An unusual feature of the middle Cambrian Burgess Shale and other units in the Chancellor Group and the adjacent platform is the occurrence of Mg-rich clay lithosomes. XRD shows this material to be nearly pure clinochlore, with minor rutile and quartz. Clinochlore is produced from hydrothermal alteration of mafic minerals. Its occurrence in the Burgess Shale is not explicable by conventional distal shelf sedimentation. Examples of reworking and rare occurrences of fossils in bedded clinochlore indicate it was syngenetically erupted and not a result of emplacement during deep-burial diagenesis. Geochemical analyses of the clinochlore return MgO values up to 31 wt%, compared to background shale values of about 2 wt%. Dolomite commonly occurs with the clinochlore as isolated “floating” rhombs, or may volumetrically exceed clinochlore in some samples. 

Earlier studies proposed that the clinochlore lithosomes represent precipitates from seafloor brine seeps, the clinochlore having formed later from low-grade metamorphism of a Mg-Al-rich protolith. However, the geometries and textures of the clinochlore lithosomes indicate that brine seeps were part of a broader suite of tectonic and hydrothermal processes that included penecontemporaneous faulting and mud volcanism. Much of the clinochlore was likely deposited as brine-carried mud evulsed onto the Cambrian seafloor. A protolith is unnecessary, as clinochlore occurs in some modern mud volcano deposits, although at low concentrations. Conduits for mud volcanism occur at major faults, notably the Fossil Gully Fault Zone and faults at the Cathedral “escarpment”, which host clinochlore dikes that cross-cut stratigraphy. Magnesium for the clinochlore (and dolomite, talc and magnesite) deposits was likely sourced from serpentinization of oceanic crust subducted below the now western edge of Laurentia during northward drift from high latitudes in the southern hemisphere. At some localities in the Chancellor Group, mud dikes hosted in faults and associated strata-bound material are Ba-rich (not Mg-rich) and in some instances show no exotic geochemical signature. These occurrences suggest that mud volcanism in the Chancellor Basin was complex, with mud derived from differing sources or that evolved compositionally via fractionation in underlying mud chambers.