Paper No. 9
Presentation Time: 8:00 AM-12:00 PM
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. δ13Corg, δ34Spy) 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.