GEOCHEMISTRY OF THE NEOPROTEROZOIC OLD FORT POINT FORMATION: A DEEP-MARINE CAP-CARBONATE EQUIVALENT IN THE WINDERMERE SUPERGROUP, SOUTHERN CANADIAN CORDILLERA

The Neoproterozoic (~608 Ma) Old Fort Point Formation (OFP) is a fine-grained siliciclastic + carbonate unit in the Windermere Supergroup, exposed locally over an area of 35,000 km² of the southern Canadian Cordillera. The unique physical and chemical attributes of the OFP distinguish it from the thick succession of coarse-grained turbidites typical of the Windermere Supergroup. Exposures of the OFP range from 50 – 175 m thick and were deposited within basin-slope to basin-floor environments. Stratigraphically-upward, the OFP comprises three separate units: 1) fine-grained siltstone that grades upward to a carbonate + marl rhythmite (trangression); 2) sulfidic, carbonaceous pelite (condensed section and highstand); and 3) fine-grained mudstone and siltstone, coarse-grained carbonate-cemented sandstone, breccia and conglomerate (regression). Preliminary work shows that the pelite unit of the OFP is characterized by distinctive chemical (e.g. Al, Mo, V, Sb) and isotopic (e.g. δ¹³C_org, δ³⁴S_py) signatures unlike other pelites from the Windermere Supergroup in the southern Canadian Cordillera. δ¹³C isotope analyses from the limestone units are uniformly depleted, similar to cap carbonates seen globally at this time period. Deposition of the OFP occurred during a shutdown of coarse clastic input into the basin followed by the onset of anoxic bottom-water conditions below a postulated pycnocline. Local anoxic conditions are supported by relatively enriched concentrations of redox sensitive trace-elements and organic carbon contents. The deep-marine OFP is interpreted as the depositional response to a major post-glacial eustatic rise and may provide a unique window into the geochemical evolution of the Neoproterozoic Earth System.