The Old Fort Point Formation: Redefinition and formal subdivision of a distinctive stratigraphic marker in the Neoproterozoic Windermere Supergroup, southern Canadian Cordillera

Abstract

Abstract The southern Canadian Cordillera strata of the Neoproterozoic Windermere Supergroup form an areally extensive outcrop belt of deep-marine sedimentary rocks. Within this generally monotonous pile of siliciclastic and minor carbonate rocks, the Old Fort Point Formation forms a lithologically and geochemically distinctive unit that serves as a key regional stratigraphic marker. New sedimentologic and stratigraphic work demonstrates that it is lithologically distinctive, mappable and correlatable on a regional scale and deserves formal recognition. The Old Fort Point Formation comprises three lithostratigraphic members that have a consistent stratigraphic relationship across the basin and can easily be distinguished from lithofacies in the enveloping strata of the Windermere Supergroup. The basal Temple Lake Member is composed primarily of siltstone to mudstone that grades upward into rhythmically interstratified limestone-siltstone. The middle Geikie Siding Member is a thin, organic-rich mudstone-pelite. The Whitehorn Mountain Member is the uppermost unit and varies locally and regionally in thickness and lithology, including diamictite, breccia to conglomerate, mudstone to siltstone, subarkose, quartzarenite, calcareous arenite, arenaceous limestone, and limestone. This unique lithostratigraphic unit is here formally named the Old Fort Point Formation and other site-specific names should be discontinued. The use of the name Old Fort Point Formation is an attempt to simplify part of a complicated and informally defined stratigraphic nomenclature currently in use for rocks in the Neoproterozoic Windermere Supergroup, southern Canadian Cordillera.