Geology and Palaeoecology of Ancient Seeps at the Mount Stephen Trilobite Beds, Burgess Shale, Yoho National Park, British Columbia  

The Mount Stephen Trilobite Beds occur within the Campsite Cliff Shale Member of the Burgess Shale, which overlies the Yoho River Limestone Member.  The topographically uppermost reaches of the beds are the Upper Trilobite Beds (UTBs). These are remarkable for their density of fossils, mostly trilobites, which occur in an 80 m wide, 8 m thick, diamond-shaped lithosome visible from the Kicking Horse Valley.  Lateral to this lithosome, fossil abundance wanes dramatically and lithological character changes.  About 100 m upslope, the Burgess Shale terminates against the Cathedral Escarpment.  The UTBs occur near a change in direction of the Escarpment from SW to SE.  This ‘corner’ marks the intersection of two faults that controlled the shape of the Escarpment surface locally, and focused brines and clinochlore mud that were expelled onto the seafloor.  A 2.5 m-wide vertical mud/brine conduit is exposed above the UTBs.  H₂S in the brines promoted microbial activity, with an attendant increase in the abundance of animals compared to background faunal density, as occurs around modern marine brine seeps. 

A newly discovered section at the UTBs reveals stacked facies which, in ascending order, are: (1) a thin facies of mostly non-fossiliferous, black clinochlore; (2) a thin, densely fossiliferous (nearly exclusively Ogygopsis), low-diversity, black-weathering facies; (3) a thin, red-weathering, high-diversity, less densely fossiliferous facies; and (4) a thick, green-weathering, high-diversity, even less densely fossiliferous facies.  Facies 1, only a few centimetres thick below the UTBs, thickens to about 2 m adjacent to the Escarpment.  Here, the lowest beds contain euhedral dolomite rhombs floating in clinochlore matrix.  Facies 2, 3 and 4 are interpreted as successively distal facies from the fluid source.  The Lower Trilobite Beds are mostly grey- to tan-weathering, more fissile than the UTBs, and represent a more distal facies relative to the brine source.  Major-element analyses show anomalous seep-related MgO concentrations of up to 27.0 wt% in the clinochore compared to typically < 2.0 wt% in the more peripheral beds. 

The underlying Yoho River Limestone also exhibits seep-related structures including: carbonate mud mounds and 0.5 m-wide, vertical, calcite pipes.  Bedding plane exposures of the uppermost beds show extensive orthogonal sets of dilational cracks.  Vast bedding surfaces are covered with  ‘ripple’ marks of varying wavelength (20–24 cm; 2–3 cm amplitude).  These are unlikely to be a result of contour currents or shallowing because none shows cross-bedding.  We suggest these may instead represent surficial wrinkling resulting from flowing mud. 

The brines and clinochlore were likely derived from serpentinization of mantle-derived lherzolitic peridotite source materials above subducting ocean floor off the north (now west) coast of Laurentia.  Mud volcano-dominated seafloor environments of the eastern Mediterranean and Mariana forearc are suitable modern analogues for the Trilobite Beds on Mount Stephen.