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Sedimentology and Stratigraphy of the Old Fort Point Formation (Neoproterozoic), Windermere Supergroup, western Canada: an example of a deep-marine, organic-rich regional marker horizon.

Mark D. Smith

University of Ottawa, Department of Earth Sciences

Ottawa, Ontario, Canada

[email protected]

The ~608 Ma Old Fort Point Formation (OFP) is a fine-grained siliciclastic + carbonate unit in the Neoproterozoic Windermere Supergroup, exposed locally over an area of 35,000 km2 of the southeastern 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 – 350 m thick and were deposited within basin-slope to basin-floor environments. Stratigraphically-upward, the OFP comprises three separate units: 1) siltstone that grades upward to a carbonate + marl rhythmite (trangression); 2) sulfidic, carbonaceous mudstone (condensed section and highstand); and 3) mudstone and siltstone, arenaceous limestone, coarse-grained carbonate-cemented sandstone, breccia and conglomerate (regression). Preliminary work shows that the organic-rich mudstone unit of the OFP is characterized by distinctive chemical (e.g. Al2O3, Mo, V, Sb) and isotopic (e.g. d13Corg, d34Spy) signatures unlike other fine-grained facies from the Windermere Supergroup, southeastern Canadian Cordillera. The OFP is proposed to represent an important regional deep-marine stratigraphic marker horizon. Its deposition resulted from a major glaciogenic eustatic sea-level rise which terminated coarse siliciclastic input into the basin and facilitated the subsequent development of anoxic bottom-water conditions.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid