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. d¹³C_org, d³⁴S_py) 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